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Jonathan De Vydt
catalystsof the next generation of cooperative heterogeneousGreen aldol condensation: synthesis, testing and design
Academic year 2014-2015Faculty of Engineering and Architecture
Chairman: Prof. dr. Isabel Van DriesscheVakgroep Anorganische en Fysische Chemie
Chairman: Prof. dr. ir. Guy MarinDepartment of Chemical Engineering and Technical Chemistry
Master of Science in Chemical EngineeringMaster's dissertation submitted in order to obtain the academic degree of
Counsellor: Ir. Jeroen LauwaertSupervisors: Prof. dr. ir. Joris Thybaut, Prof. Pascal Van Der Voort
..
Jonathan De Vydt
catalystsof the next generation of cooperative heterogeneousGreen aldol condensation: synthesis, testing and design
Academic year 2014-2015Faculty of Engineering and Architecture
Chairman: Prof. dr. Isabel Van DriesscheVakgroep Anorganische en Fysische Chemie
Chairman: Prof. dr. ir. Guy MarinDepartment of Chemical Engineering and Technical Chemistry
Master of Science in Chemical EngineeringMaster's dissertation submitted in order to obtain the academic degree of
Counsellor: Ir. Jeroen LauwaertSupervisors: Prof. dr. ir. Joris Thybaut, Prof. Pascal Van Der Voort
Acknowledgement Nu deze opleiding op zijn einde loopt, wil ik graag even de tijd nemen om enkele mensen te
bedanken die me geholpen hebben tijdens deze periode. Deze thesis was een ware uitdaging.
Voornamelijk de vele arbeidsintensieve momenten in het labo, werden in het tweede semester
steeds langer. Maar wat eens zo veraf bleek, is nu binnen handbereik. Het studentenleven kan
met een goed, toch wel trots, gevoel afgesloten worden.
In de eerste plaats bedank ik prof. dr. ir. Guy B. Marin voor de mogelijkheid die ik heb gekregen
om mijn masterproef op het Laboratory for Chemical Technology uit te voeren.
Mijn promotor prof. dr. ir. Joris W. Thybaut wil ik bedanken voor de raad en de opvolging tijdens
deze thesis. Het was verrijkend om tijdelijk deel uit te maken van de CaRE-groep. Via de CaRE
lunch meetings kwam ik in contact met de ruime wereld van Catalytic Reaction Engineering. Een
interessante ervaring!
Daarnaast wens ik ook prof. dr. Pascal Van Der Voort te danken voor de gekregen kans om me
verder te verdiepen in het onderzoeksdomein van periodieke mesoporeuze organosilica’s.
Mijn begeleider, Jeroen Lauwaert, verdient een heel grote “dankuwel”. Hij was steeds
beschikbaar indien ik vragen had. Ook als er problemen waren in het labo, stond hij steeds
paraat. Onder zijn begeleiding heb ik veel bijgeleerd. Tevens wil ik hem bedanken voor het
geduld bij het nalezen van dit werk.
Er zijn nog zovelen die direct of indirect betrokken zijn bij de ontwikkeling van deze thesis.
Zo ook een woord van dank aan Els De Canck, Judith Ouwehand en Dolores Esquivel voor hun
hulp tijdens de functionalisatie en karakterisatie van de gesynthetiseerde materialen. Tom
Planckaert wil ik bedanken voor de vele elementaire analyses en XRD metingen. Ook de
technische staf van het LCT, in het bijzonder Erwin Turtelboom, bedank ik voor het in orde
brengen van de waterkoeling in het labo.
Ik wil ook mijn klasgenoten, en dan speciaal de mensen uit het thesislokaal, bedanken voor de
vele verhalen en discussies. Het was fijn deze momenten met hen te kunnen delen.
Ten slotte gaat een oprecht woord van dank uit naar mijn ouders voor de onophoudelijke steun
die ze geven. Dat ik deze masterproef en de daarbijhorende opleiding tot een goed einde heb
kunnen brengen, is zeker ook aan hen te danken!
FACULTY OF ENGINEERING AND ARCHITECTURE
Department of Chemical Engineering and Technical Chemistry
Laboratory for Chemical Technology
Director: Prof. Dr. Ir. Guy B. Marin
Laboratory for Chemical Technology • Technologiepark 914, B-9052 Gent • www.lct.ugent.be
Secretariat : T +32 (0)9 33 11 756 • F +32 (0)9 33 11 759 • [email protected]
Laboratory for Chemical Technology
Declaration concerning the accessibility of the master thesis Undersigned, Jonathan De Vydt
Graduated from Ghent University, academic year 2014-2015 and is author of the
master thesis with title:
Green aldol condensation: synthesis, testing and design of the next generation of
cooperative heterogeneous catalysts
The author gives permission to make this master dissertation available for consultation
and to copy parts of this master dissertation for personal use.
In the case of any other use, the copyright terms have to be respected, in particular with
regard to the obligation to state expressly the source when quoting results from this master
dissertation.
(Date) 26/05/15
(Signature)
Summary Green aldol condensation: synthesis, testing and design of the next generation of cooperative
heterogeneous catalysts
Author: Jonathan De Vydt
Supervisors: Prof. dr. ir. Joris Thybaut, Prof. Pascal Van Der Voort
Coach: Ir. Jeroen Lauwaert
Faculty of Engineering and Architecture
Academic year 2014-2015
Abstract: In this work, the synthesis of a new type of Periodic Mesoporous Organosilica (PMO)
materials is described. These structured organosilicas have been synthesized by the acid-
catalyzed hydrolysis and condensation of bridged precursors. These materials are functionalized
with cysteine using a thiol-ene click reaction. Afterwards, the cooperative catalysts are tested in
the aldol condensation of 4-nitrobenzaldehyde with acetone. It was found that the materials
with ethane bridges are more active than the ones with benzene groups in the support. In the
second section of this work, the effect of silanol groups on the hydrophobicity of the material is
tuned by adding tetraethyl orthosilicate (TEOS) to the precursor mixture. Materials with both
hydrophobic and hydrophilic blocks are obtained. The effect of the support’s hydrophobicity is
assessed by determining the catalyst activity in the presence of water. The outcome of these
experiments is that the TE-materials have promising results. It is an interesting result to see
that, even in the presence of water, the most hydrophobic material (TE1-C) is able to preserve
its activity. This indicates that the hydrophobic pore walls impede the water molecules to enter
the pores.
Keywords: aldol condensation, cysteine, furfural, thiol-ene click reaction, effect of hydrophobicity
Green aldol condensation: synthesis, testing and
design of the next generation of cooperative
heterogeneous catalysts
Jonathan De Vydt
Coach: Ir. Jeroen Lauwaert
Supervisors: Prof. dr. Pascal Van Der Voort, Prof. dr. ir. Joris W. Thybaut
Abstract: In this work, the synthesis of a new type of Periodic
Mesoporous Organosilica (PMO) materials is described. These
structured organosilicas have been synthesized by the acid-
catalyzed hydrolysis and condensation of bridged precursors.
These materials are functionalized with cysteine using a thiol-ene
click reaction. Afterwards, the cooperative catalysts are tested in
the aldol condensation of 4-nitrobenzaldehyde with acetone. It
was found that the materials with ethane bridges are more active
than the ones with benzene groups in the support. In the second
section of this work, the effect of silanol groups on the
hydrophobicity of the material is tuned by adding tetraethyl
orthosilicate (TEOS) to the precursor mixture. Materials with
both hydrophobic and hydrophilic blocks are obtained. The
effect of the support’s hydrophobicity is assessed by determining
the catalyst activity in the presence of water. The outcome of
these experiments is that the TE-materials have promising
results. It is an interesting result to see that, even in the presence
of water, the most hydrophobic material (TE1-C) is able to
preserve its activity. This indicates that the hydrophobic pore
walls impede the water molecules to enter the pores.
Keywords: aldol condensation, cysteine, furfural, thiol-ene click
reaction, effect of hydrophobicity
I. INTRODUCTION
The aldol condensation is an important reaction to create
new C-C bonds and yield larger and more complex molecules.
The aldol condensation might have a bright future in the
discipline of green, renewable and sustainable energy, e.g. in
the valorization of biocomponents such as glycerol and
furfural.
Figure 1: Aqueous-phase conversion of sugars and derivatives into
liquid hydrocarbon fuels [1]
Even with the shale gas boom in recent years, one cannot
deny that the diminishing reserves of these non-renewable
resources cause a lot of concerns. In this regard, biomass can
serve as a sustainable source for our industrialized society.
Furfural is an important intermediate during the conversion of
biomass and can be used as reactant in the aldol condensation
with acetone, see Figure 1, to produce C8 and C13 alkanes
which can be employed as biofuel [1, 2].
In industry, the product stream of furfural contains water.
Therefore for an industrial application of the aldol
condensation with furfural, it would be beneficial for the
feasibility if the reaction can proceed at a significant rate even
in the presence of water.
In this regard, Periodic Mesoporous Organosilicas can be a
good solution. Due to their large pore sizes and surface areas,
these mesoporous silica materials provide sufficient space to
incorporate multiple functional groups while the stability of
the matrix is ideal for many types of reaction. Thus, if one is
able to synthesize a material with hydrophobic pores, the
water could be excluded from entering, leading to a situation
in which only the reactants are able to physisorb in the pores.
II. PROCEDURES
A. Catalyst synthesis
An overview of the synthesis and functionalization of the
Periodic Mesoporous Organosilica (PMO) materials is shown
in Figure 2.
The co-condensation of 1,2-bis(triethoxysilyl)ethane
(BTEE) and 1,4-bis(triethoxysilyl)benzene (BTEB) with
vinyltriethoxysilane (VTES) is performed in the presence of a
structure directing agent, Brij 76. The synthesis mixture is
stirred at 50°C during 24 hours and subsequently aged at 90°C
for 24 hours. After filtration of the precipitate, the material is
refluxed three times with acidified ethanol at 80°C during 24
hours to remove the surfactant as much as possible.
B. Functionalization
After the synthesis of these PMO materials, the amino acid
is introduced into the pores via a post-functionalization. In a
UV reactor, cysteine is grafted onto the vinyl group of the
VTES precursor via a thiol-ene click reaction.
First 0.50 g photo-initiator (Irgacure 2959) is dissolved in
10 mL demineralized water and the long vertical Schlenk
flask is placed into a supersonic bath. Meanwhile 0.53 g
cysteine is added to a separate glass bottle with 10 mL
demineralized water. The mixture is heated using a heat gun
until a clear solution is obtained. Next, the cysteine and 0.50 g
of the organosilica are brought together in the Schlenk flask.
The stopcock is used to place the mixture under an inert
atmosphere with helium. After that, the mixture is placed in
the UV reactor and stirred during 24 hours for the thiol-ene
click reaction to take place.
Figure 2: Synthesis and functionalization of Periodic Mesoporous
Organosilicas
C. Catalyst characterization
Nitrogen adsorption-desorption measurements are carried
out at 77K using a Tristar 3000 gas analyser of Micromeritics.
The specific surface area and pore volume are determined
using the Brunauer–Emmett–Teller (BET) method. The
average pore size of the organosilica is obtained using the
Broekhoff and de Boer (BdB-FHH) method, with the
modification of Frenkel, Halsey and Hill.
The amine loading is determined using elemental (CHNS)
analysis. These experiments are performed on a Thermo Flash
2000 elemental analyser using V2O5 as catalyst.
The structure of the mesoporous materials is determined
with the use of X-ray diffraction. The X-ray diffraction
patterns were recorded on a ARL X'TRA Diffractometer of
Thermo Scientific which is equipped with a Cu Kα tube and a
Peltier cooled lithium drifted silicon solid stage detector.
The presence of amine groups after grafting is demonstrated
by means of Diffuse Reflectance Infrared Fourier Transform
(DRIFT) spectroscopy. These measurements are performed on
a Nicolet 6700 of Thermo Scientific with a nitrogen cooled
MCT-A detector. The spectra are obtained using a Graseby
Specac diffuse reflective cell, operating in vacuo at 120°C.
D. Catalytic experiments
The experiments were performed in a 25 mL two-neck
round-bottom flask equipped with a condenser and a septum.
The reaction mixture was prepared separately by mixing the
desired amounts of acetone (50 vol%), n-hexane (co-solvent,
50 vol%), 4-nitrobenzaldehyde (0.03 mmol/mL) and methyl
4-nitrobenzoate (internal standard, 0.022 mmol/mL).
Afterwards 8 mL of the reaction mixture was injected into the
flask which contains the catalyst (4 mol% with respect to the
4-nitrobenzaldehyde concentration) and the flask was
immediately placed in an oil bath at 45°C. The moment the
flask was placed in the oil bath was taken as the start of the
reaction. The reaction was monitored for 4 hours by taking
a sample of the reaction mixture (about 100 µL) every 30
minutes. Typically 0.9 mL of acetone was used to wash the
syringe needle and to transfer the sample to a vial. Afterwards
the catalyst was separated from the sample by means of
centrifugation. Finally, the samples were analyzed using a
reversed-phase high-performance liquid chromatograph (RP-
HPLC). The components were identified using a UV-detector
with a variable wavelength. Quantification of the different
components in the reaction mixture was performed by relating
the peak surface areas to the amount of internal standard,
methyl 4-nitrobenzoate [3].
E. Vapour-Liquid equilibrium
Because the molar ratio of the reactants has an influence on
the reaction rate, it is important to know the exact composition
of the liquid phase after the vapour-liquid equilibrium has
been established. Thus, it is necessary to check how much of
each component has to be added to the Parr reactor, in order to
obtain the desired concentrations in the liquid phase. An
existing code based on the Non-Random Two-Liquid (NRTL)
and Hayden-O’Connell (HOC) methods has been modified
which allows for an initial estimation of the vapour-liquid
equilibrium. This code accounts for the thermodynamic non-
ideality of both the liquid as well as the gas phase via so-
called activity coefficients and fugacity coefficients.
III. EXPERIMENTAL RESULTS
A. Valorization of furfural via the aldol condensation
In order to obtain higher conversions for the aldol
condensation of furfural with acetone, higher reaction
temperatures were investigated. When higher temperatures
(>60°C) are applied, one has to consider the vapour-liquid
equilibria of the reactants. In particular for acetone, with a
boiling point of 56°C, one has to take into account that a
fraction of this reactant will be in the vapour phase.
The results from a simulation with the NRTL-HOC code
indicate that this system deviates a few percentages from an
ideal mixture. As was expected, only a fraction of the acetone
evaporates at higher temperatures. For toluene and furfural it
is correct to assume that everything remains in the liquid
phase. If a reaction temperature of 90°C is considered, about
1.55 % of the acetone will evaporate, which corresponds with
a pressure increase up to a total pressure of 3.7 bar.
For the aldol condensation of furfural with acetone at 90°C,
the decline of the turnover frequency as a function of the
volume percentage of water in the reaction mixture indicates
that a small amount of 1 vol% or 2 vol% water is already
sufficient to lower the catalytic activity with 23.43 %,
respectively 57.17 %.
Figure 3: TOF ratio as a function of the volume % of water in the
reaction mixture
0.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 10 12
TO
F r
ati
o
Water (vol %)
B. Catalyst characterization
Characterization of the organosilicas affirmed the successful
synthesis and functionalization of the materials. With nitrogen
physisorption a type IV isotherm was obtained, indicating that
the material is mesoporous. After functionalization, a small
decrease in surface area and total pore volume - resulting from
the loss in free volume - was observed.
X-ray diffraction patterns confirm that for most materials a
hexagonal ordered mesoporous structure is maintained before
and after the functionalization. However, from the X-ray
diffractograms, it was found that the washing procedure at
120°C is too harsh for the "modified SBA-15" materials (T1).
Adapting the washing procedure to a milder temperature of
40°C and a shorter time (2h - 3h) results in a material which
properties are better preserved. Although, the mesoscopic
ordering is still lost and an amorphous solid was obtained.
Elemental analysis determined the amine loading to be in
the range of 0.05 – 0.15 mmol/g.
The DRIFT spectra demonstrated the presence of a primary
amine after functionalization with cysteine.
C. Aldol condensation of 4-nitrobenzaldehyde with acetone
The functionalized materials were used as catalyst in the
aldol condensation of 4-nitrobenzaldehyde with acetone at
45°C. To investigate the influence of the hydrophobicity of
the catalyst support on the catalytic activity, the reaction was
performed both in the absence and presence of water.
Although both catalyst have similar amine loadings, E1-C
(0.0592 mmol/g) and B10-C (0.0692 mmol/g) have very
different reaction rates. The turnover frequency of E1-C is
about one order of magnitude higher than B10-C.
For a possible explanation it is necessary to take into
consideration that the concentration of reactants in the pores
may be different from the concentration of the reaction
mixture. The properties of the pores play an important role on
the physisorption of the reactants into the pores. It is not
unlikely that for the material B10-C the environment of the
pores is more in favour of 4-nitrobenzaldehyde to enter due to
some interactions with the benzene groups in the support. This
can be justified with the "like likes like" principle.
As a consequence of the relative higher concentration of
4-nitrobenzaldehyde in the pores, this reactant reacts with the
primary amine and may lead to the formation of a stable
Schiff base, which eliminates active sites. This inhibition of
the reaction kinetics was also observed by Kandel et al. [4]
An overview of the turnover frequencies for the catalysts
comprising of both hydrophobic and hydrophilic blocks (TE-
and TB-materials) is given in Figure 4.
The lower turnover frequencies obtained from the catalytic
experiments indicate that no linear relationship applies for the
activity of the catalysts. Adding three different precursors to
the precursor mixture complicates the synthesis because every
precursor has its own hydrolysis and condensation rates.
When the difference between these rates is too large, the
tendency towards homocondensation reactions increases. This
will be a problem during the co-condensation because the
homogeneous distribution of the organic functionalities in the
framework cannot be guaranteed.
As a consequence, it is possible that the different precursors
are clustered in the support. And after functionalization, the
amine functionalities are too close to each other and are not
fully promoted by neighbouring silanol groups. Leading to
lower TOF-values than would be expected from linear
interpolation.
Figure 4: Turnover frequency as a function of the molar % of TEOS
in the catalysts. Blue: TE-materials; Red: TB-materials. Dashed lines
indicate the theoretical TOF values in case of linear interpolation.
D. Effect of water in the reaction mixture
The influence of the hydrophobicity of the catalyst support
on the catalytic activity in the presence of water is
investigated. This is an important property because for a
possible application in industry, e.g. in the valorization of
biocomponents such as glycerol and furfural, the reactants
will most likely be in an aqueous solution. Thus the activity of
these bifunctional catalysts in the presence of water will have
a major impact on the potential feasibility of these materials.
The turnover frequencies of the TE-materials both for the
reference reaction and the reaction in presence of 1 vol%
water are compared in Table 1. For the most hydrophobic
material, TE1-C, the catalytic activity remains at the same
level. It is an interesting result to see that, even in the presence
of water, TE1-C is able to preserve its activity. This indicates
that the hydrophobic pore walls impede the water molecules
to enter the pores. Thus the reaction continues as it would in
the situation without water. Interestingly, while in the
reference reaction the turnover frequency of TE1-C was still
lower than the one of T1-C-S2, this catalyst now becomes
more active in the presence of water. Materials with more
TEOS in the pore wall are more susceptible to the water and
therefore the effect of water is more pronounced.
The turnover frequencies of the TB-materials in the absence
or presence of water are shown in Table 2. In comparison to
the TE-materials, the presence of water has a much larger
effect on the catalytic activity. The TOF values for the aldol
condensation in the presence of 1 vol% water drop
significantly. All TB-materials exhibit a turnover frequency
around 9 x 10-6
s-1
.
An explanation why the ratio of the TB-materials is much
lower than the TE-materials has to do with the Snyder polarity
index. Water is one the most polar solvents, and has a Snyder
polarity index of 9, while benzene has a lower value of 3. The
polarity of an ethane is in the range of 0.1 to 0.3. Comparing
the polarity index of ethane and benzene, it can be expected
that the ethane groups in the silica support have a stronger
repulsive effect on water. Thus, the TE-materials are capable
to exclude water to a greater extent from the pores than the
TB-materials.
0.0E+00
2.0E-05
4.0E-05
6.0E-05
8.0E-05
1.0E-04
1.2E-04
1.4E-04
1.6E-04
1.8E-04
2.0E-04
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0
TO
F (
1/
s)
TEOS (mol %)
Table 1: Comparison of TOF values for the TE-materials
Material TOFreference reaction (s-1
) TOFreaction with water (s-1
) Ratio
E1-C 1.77 x 10-4
1.86 x 10-4
1.049
TE1-C 7.16 x 10-5
7.70 x 10-5
1.075
TE2-C 4.41 x 10-5
3.73 x 10-5
0.846
T1-C-S2 8.67 x 10-5
5.15 x 10-5
0.594
Table 2: Comparison of TOF values for the TB-materials
Material TOFreference reaction (s-1
) TOFreaction with water (s-1
) Ratio
B10-C 1.77 x 10-5
1.30 x 10-5
0.721
TB1-C 2.00 x 10-5
9.71 x 10-6
0.486
TB2-C 3.22 x 10-5
9.04 x 10-6
0.281
TB3-C 4.45 x 10-5
9.54 x 10-6
0.214
T1-C-S2 8.67 x 10-5
5.15 x 10-5
0.594
E. Regeneration
Finally, the reusability of the TE- and TB-materials was
investigated. First, assuming that no inactive intermediates are
formed and thus no important changes occur to the
heterogeneous catalyst, the material is simply washed with a
small amount of deionized water and afterwards with a larger
amount of acetone to improve the drying of the sample. When
the regenerated materials were tested in the aldol
condensation of 4-nitrobenzaldehyde with acetone at 45°C,
the turnover frequencies dropped significantly.
A second regeneration method was tested in which the spent
catalyst was thoroughly rinsed with ethyl acetate, deionized
water, and ethanol for several times and dried in vacuum at
40°C. This method was described by An et al. [5] and worked
fine for materials with alternating hydrophobic and
hydrophilic blocks in the pore wall. However, when these
regenerated materials were tested in the aldol condensation,
similar TOF values were obtained indicating that the inactive
intermediates were still present on the catalytic surface.
DRIFT spectra of the TE1-C material confirmed the presence
of nitro groups on the catalyst after reaction. Nonetheless, no
indication of an increased amount of imine was measured.
It has to be mentioned that the catalyst only loses its activity
during the first catalytic run. After that, the material retains a
constant catalytic activity, see Figure 5.
Figure 5: Conversion as a function of time for the TE1-C material.
Blue: reference reaction; Red: regeneration 1; Green: regeneration 2
IV. CONCLUSIONS & FUTURE WORK
The synthesis of Periodic Mesoporous Organosilicas (PMO)
containing vinyltriethoxysilane (VTES) was performed. These
materials were functionalized with cysteine via a thiol-ene
click reaction. Characterization of the organosilicas affirmed
the successful synthesis and functionalization of the materials.
Afterwards, the cooperative catalysts were tested for the aldol
condensation of 4-nitrobenzaldehyde with acetone. It was
found that the materials with ethane bridges are more active
than the ones with benzene groups in the support.
To investigate the influence of the catalyst support's
hydrophobicity on the catalytic activity, the reaction was also
performed in the presence of water. Especially, the catalysts
with ethane groups in the pore wall show promising results.
For the most hydrophobic material, TE1-C with 64.7 mol%
ethane, the catalytic activity remains at the same level. It is
interesting to see that, even in the presence of water,
TE1-C is able to preserve its activity. This indicates that the
hydrophobic pore walls impede the water molecules to enter
the pores.
The regenerated catalysts exhibit lower turnover frequencies
after the first catalytic run. However, in a second and third
catalytic run, the TOF-value doesn’t change anymore. There
are indications that the initial drop in activity is a consequence
of one-time only pore blocking. Afterwards, the active sites
that aren't blocked, remain accessible for subsequent
reactions, resulting in a steady catalyst's activity. It could be
useful to investigate the exact cause of this deactivation.
Different regeneration procedures have to be tested in order to
achieve again the original activities.
In this work, differences in surface arrangements due to
clustering of the precursors may have interfered with a correct
comparison between the catalysts’ activity. It might be useful
for further investigations on the effect of the catalyst support’s
hydrophobicity to carefully control the arrangement of the
active sites with respect to each other. A material with
alternating hydrophobic and hydrophilic blocks could provide
such possibility.
Finally, it is recommended to examine the functionalization
of these organosilicas with secondary amines containing an
alcohol group close to the amine. Because literature suggests
that intermolecular amine-silanol or intramolecular amine-
alcohol cooperativity is better than intramolecular amine-
carboxylic acid cooperativity [6].
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
0 50 100 150 200 250 300
Co
nve
rsio
n (
%)
Time (min)
TOF = 7.16 10-5 s-1
TOF = 1.90 10-5 s-1
TOF = 1.95 10-5 s-1
ACKNOWLEDGEMENTS
The author would like to acknowledge the helpful hand and
suggestions of Els De Canck, Dolores Esquivel and Judith
Ouwehand of the Center for Ordered Materials, Organo-
metallics & Catalysis. The author also would like to thank
Tom Planckaert for the characterization of many samples via
elemental analysis and X-ray diffraction.
REFERENCES
1. Serrano-Ruiz, J.C. and J.A. Dumesic, Catalytic
routes for the conversion of biomass into liquid
hydrocarbon transportation fuels. Energy &
Environmental Science, 2011. 4(1): p. 83-99.
2. Chheda, J.N. and J.A. Dumesic, An overview of
dehydration, aldol-condensation and hydrogenation
processes for production of liquid alkanes from
biomass-derived carbohydrates. Catalysis Today,
2007. 123(1-4): p. 59-70.
3. Ouwehand, J. and J. Lauwaert, Internal discussion.
2015.
4. Kandel, K., et al., Substrate inhibition in the
heterogeneous catalyzed aldol condensation: A
mechanistic study of supported organocatalysts.
Journal of Catalysis, 2012. 291: p. 63-68.
5. An, Z., et al., L-Proline-Grafted Mesoporous Silica
with Alternating Hydrophobic and Hydrophilic
Blocks to Promote Direct Asymmetric Aldol and
Knoevenagel-Michael Cascade Reactions. Acs
Catalysis, 2014. 4(8): p. 2566-2576.
6. Lauwaert, J., et al., Spatial arrangement and acid
strength effects on acid–base cooperatively catalyzed
aldol condensation on aminosilica materials. Journal
of Catalysis, 2015. 325(0): p. 19-25.
Table of Contents i
Table of Contents
Table of Contents i
List of Figures iii
List of Tables v
Nomenclature vi
Chapter 1 Introduction 1
1.1 Aldol condensation 4
1.1.1 General mechanisms 4
1.1.1.1 Base-catalyzed aldol condensation 5
1.1.1.2 Acid-catalyzed aldol condensation 6
1.1.2 Types of aldol condensation 8
1.1.3 Applications 10
1.2 Catalysis: homogeneous vs heterogeneous 12
1.3 Mesoporous materials 14
1.3.1 Silica materials 14
1.3.2 Functionalized silica materials 14
1.3.2.1 Post-synthesis functionalization 16
1.3.2.2 One-pot synthesis 17
1.3.2.3 Periodic mesoporous organosilicas – PMOs 18
1.3.2.4 PMO synthesis parameters 21
1.4 Cooperative catalysis 24
1.4.1 Effect of the acidity of the acid group 25
1.4.2 Effect of the base functionality type 32
1.4.3 Effect of the distance between the base and acid functionality 38
1.4.4 Effect of the reaction medium 42
1.4.5 Effect of the hydrophobicity of the support 44
1.5 Scope of the thesis 45
Chapter 2 Materials and methods 47
2.1 Materials 47
2.2 Elemental Analysis 47
2.3 Nitrogen physisorption 48
2.4 X-Ray Diffraction 50
2.5 Diffuse Reflective Infrared Fourier Transform (DRIFT) Spectroscopy 52
2.6 Aldol condensation of furfural with acetone 53
Table of Contents ii
2.7 Aldol condensation of 4-nitrobenzaldehyde with acetone 53
2.8 High Performance Liquid Chromatography (HPLC) 54
2.9 NRTL-HOC code 55
Chapter 3 Valorization of furfural via the aldol condensation 59
3.1 Aldol condensation of furfural with acetone 60
3.2 Optimization of the reaction conditions 61
3.3 Effect of water in the reaction mixture 63
Chapter 4 Synthesis and characterization of cooperative acid-base PMOs materials 65
4.1 Synthesis procedure 65
4.2 Functionalization 67
4.2.1 Thiol-ene “click” chemistry 67
4.2.2 Recipe 68
4.3 First set of materials 70
4.3.1 Molar composition of the PMO materials 70
4.3.2 Characterization 71
4.3.2.1 DRIFT 71
4.3.2.2 Nitrogen physisorption and elemental analysis 72
4.3.2.3 XRD 73
4.4 Reference work 75
4.5 Second set of materials 76
4.5.1 Determination of the theoretical amine loading 76
4.5.2 Molar composition of the PMO materials 77
4.5.3 Characterization 77
4.5.3.1 DRIFT 78
4.5.3.2 Nitrogen physisorption and elemental analysis 78
4.5.3.3 XRD 81
Chapter 5 Catalytic experiments 85
5.1 Comparison of ethane and benzene bridged organosilica supports 85
5.1.1 Catalysts containing both hydrophobic and hydrophilic blocks 86
5.2 Effect of water in the reaction mixture 87
5.2.1 Catalysts containing both hydrophobic and hydrophilic blocks 88
5.3 Regeneration 90
Chapter 6 Conclusions and Future Work 93
Chapter 7 References 97
Appendix A 101
Appendix B 104
List of Figures iii
List of Figures
Figure 1: Conversion of biomass to refined products [1] ................................................................................. 2
Figure 2: Aldol condensation of 5-hydroxymethylfurfural (HMF) with acetone [3] ............................. 3
Figure 3: The aldol condensation [6] ......................................................................................................................... 4
Figure 4: Base-catalyzed aldol condensation [6] .................................................................................................. 5
Figure 5: Base-catalyzed dehydration of an aldol [6] ......................................................................................... 6
Figure 6: Acid-catalyzed keto-enol tautomerism [6] .......................................................................................... 7
Figure 7: Acid-catalyzed aldol condensation [6] .................................................................................................. 7
Figure 8: Acid-catalyzed dehydration of an aldol [6] .......................................................................................... 8
Figure 9: Aldol condensation between ethanal and propanal [6] ................................................................. 9
Figure 10: Formation of a cyclopentenone by an aldol cyclization [6] ....................................................... 9
Figure 11: Formation of a cyclohexenone by an aldol cyclization [6] ...................................................... 10
Figure 12: Robinson annulation reaction [6] ...................................................................................................... 10
Figure 13: Four possible stereoisomers of the aldol product [5] ............................................................... 11
Figure 14: Structures of mesoporous silicas: a) MCM-41, b) MCM-48 and c) MCM-50 [27] ........... 14
Figure 15: Overview of the different synthesis methods for obtaining hybrid materials [27] ...... 15
Figure 16: Post-functionalization of a silica material by anchoring R-Si-(OR')3 to the silanol
groups. R is an organic functionality, R' is an alkyl group [27] ................................................................... 16
Figure 17: Functionalization via the co-condensation of an organosilane with TEOS [27] ............. 17
Figure 18: Synthesis of a PMO material via the condensation of a bissilane in the presence of a
surfactant and an acidic or base environment [27] .......................................................................................... 19
Figure 19: Overview of precursors that have been used in PMOs. Terminal Si = Si(OR)3 with R
equal to –CH3 or –C2H5 [27]......................................................................................................................................... 20
Figure 20: Structure of the different PMO precursors used during the synthesis [43] 1:
bis(triethoxysilyl)ethane, 2: bis(triethoxysilyl)methane, 3: bis(triethoxysilyl)ethene, 4: 1,4-
bis(triethoxysilyl)benzene .......................................................................................................................................... 21
Figure 21: Interactions between the inorganic species (I) and the head group of the surfactant
(S) with consideration of the possible synthetic pathway in acidic, basic or neutral media [27] 23
Figure 22: Aldol condensation of 4-nitrobenzaldehyde with acetone [45] ............................................ 25
Figure 23: Proposed catalytic cycle [46] ............................................................................................................... 26
Figure 24: Possible pathway of proton transfer assisted by silanol groups [47] ................................ 28
Figure 25: TOFs of the catalysts (type A, rhombus) and the HMDS-treated catalysts (type B,
square) as a function of the molar silanol-to-amine ratio [55] ................................................................... 29
Figure 26: Proposed reaction mechanism for the aldol condensation of 4-nitrobenzaldehyde
with acetone in the presence of the promoting silanol groups [55].......................................................... 30
Figure 27: Schematic representation of the synthesis of SBA-X with X referring to the functional
group, i.e. 'I' for the iodo-group, 'A' for the secondary amine, 'AL' for the alcohol group, 'CA' for
the carboxylic acid and 'PA' for the phosphoric acid [56] ............................................................................. 31
Figure 28: Formation of a Schiff base between 4-nitrobenzaldehyde and the aminopropyl group
of AP-MSN [47] ................................................................................................................................................................ 34
Figure 29: Proposed reaction mechanism for the aldol condensation of 4-nitrobenzaldehyde
with acetone [58] ............................................................................................................................................................ 34
Figure 30: Nucleophilic addition of acetone to a primary amine [58] ..................................................... 35
List of Figures iv
Figure 31: Percentage of amines promoted as a function of the silanol-to-amine ratio; Diamonds:
catalysts functionalized with primary amine; Squares: catalysts functionalized with secondary
amine [58] .......................................................................................................................................................................... 37
Figure 32: Proximal-C-A-SBA-15 and maximum-C-A-SBA-15 [49] ............................................................ 38
Figure 33: Proposed reaction mechanism for the aldol condensation reaction on the solid
support [49] ....................................................................................................................................................................... 39
Figure 34: Aminosilanes grafted onto SBA-15 with controlled linker lengths [53] ............................ 40
Figure 35: Conversion of 4-nitrobenzaldehyde in the aldol condensation with acetone at 50 °C
for the catalysts with different alkyl linkers [53] .............................................................................................. 40
Figure 36: Overall reaction pathways for 4-nitrobenzaldehyde and acetone in the presence of
AP-MSN: inhibition (red) and aldol reaction (black) [48] .............................................................................. 43
Figure 37: Presentation of a mesoporous silica with alternating hydrophobic and hydrophilic
blocks in the pore wall and the different interfaces inside the channels [61] ....................................... 44
Figure 38: Different types of adsorption isotherms [63] ................................................................................ 48
Figure 39: Type IV isotherm of SBA-15 [32] ........................................................................................................ 49
Figure 40: Diffraction of X-rays [65] ....................................................................................................................... 51
Figure 41: Scheme of the geometry of the XRD device [66] .......................................................................... 51
Figure 42: Representation of a hexagonal (P6mm) structure ...................................................................... 52
Figure 43: Aqueous-phase conversion of sugars and derivatives into liquid hydrocarbon fuels
[73] ........................................................................................................................................................................................ 59
Figure 44: Aldol condensation of furfural with acetone [26]........................................................................ 60
Figure 45: Conversion as a function of time for the aldol condensation of furfural with acetone
(blue) and 4-nitrobenzaldehyde with acetone (red) ........................................................................................ 61
Figure 46: Turnover frequencies for the aldol condensation of furfural with acetone at 90 °C as a
function of the molar ratio of the reactants (furfural/acetone) .................................................................. 62
Figure 47: TOF ratio as a function of the volume % of water in the reaction mixture ....................... 63
Figure 48: General overview of the synthesis ..................................................................................................... 65
Figure 49: Precursors added during synthesis. a) VTES: vinyltriethoxysilane b) BTEE: 1,4-
bis(triethoxysilyl)ethane c) BTEB: 1,4-bis(triethoxysilyl)benzene d) TEOS: tetraethyl
orthosilicate ....................................................................................................................................................................... 66
Figure 50: Reaction mechanism for the hydrothiolation of a C=C bond in the presence of a photo-
initiator and the appropriate UV radiation. In the propagation circle R stands for cysteine .......... 67
Figure 51: Structure of the components used during the post-functionalization a) Irgacure 2959
b) Cysteine ............................................................................................................................................ 68
Figure 52: Functionalization of the synthesized PMO materials ................................................................. 69
Figure 53: Structure of the functionalized PMO material ............................................................................... 69
Figure 54: Co-condensation of BTEE with VTES or BTEB with VTES ....................................................... 70
Figure 55: DRIFT spectrum of E10 (blue) and E10-C (red) ........................................................................... 72
Figure 56: XRD diffractogram of E10 (blue) and E10-C (red) ...................................................................... 73
Figure 57: Conversion as a function of time ......................................................................................................... 75
Figure 58: Comparison of the turnover frequencies at the same conditions ......................................... 75
Figure 59: DRIFT spectrum of TB1 (blue) and TB1-C (red) .......................................................................... 78
Figure 60: Comparison between the pore size distribution before and after functionalization and
washing procedures. Red: 120 °C, Green: 40 °C ................................................................................................. 80
Figure 61: XRD diffractogram of E1 (blue) and E1-C (red) ............................................................................ 81
Figure 62: XRD diffractogram of T1 (blue) and T1-C (red) ........................................................................... 82
List of Tables v
Figure 63: Results of the aldol condensation of 4-nitrobenzaldehyde and acetone at 45 °C, Blue:
E1-C; Green: B10-C ......................................................................................................................................................... 85
Figure 64: Turnover frequency as function of the molar percentage of TEOS in the catalysts.
Blue: TE-materials; Red: TB-materials. Dashed lines indicate the theoretical TOF values in case of
linear interpolation ........................................................................................................................................................ 87
Figure 65: Turnover frequency as function of the molar percentage of TEOS in the catalysts.
Blue: TE-materials; Red: TB-materials. Dashed lines indicate the theoretical TOF values in case of
linear interpolation ........................................................................................................................................................ 88
Figure 66: Drift spectrum of TE1-C (red) and TE1-C-REGEN (green) ...................................................... 91
Figure 67: Conversion as a function of time for the TE1-C material. Blue: reference reaction; Red:
regeneration 1; Green: regeneration 2 .................................................................................................................. 92
Figure 68: Results of the aldol condensation of 4-nitrobenzaldehyde with acetone at 45 °C, Blue:
TE1-C; Green: TE2-C ....................................................................................................................................................106
Figure 69: Results of the aldol condensation of 4-nitrobenzaldehyde with acetone at 45 °C, Blue:
TB1-C; Red: TB2-C; Green: TB3-C ..........................................................................................................................107
List of Tables
Table 1: List of frequently used surfactants [44] ............................................................................................... 22
Table 2: Results of the catalytic tests [46] ............................................................................................................ 26
Table 3: Effect of SBA-15 as an additive on the amine-catalyzed aldol reaction [57] ........................ 32
Table 4: Turnover frequencies obtained with both the unpromoted base as the corresponding
cooperative acid-base catalysts [56, 58] ............................................................................................................... 36
Table 5: Catalytic properties [49] ............................................................................................................................ 38
Table 6: Solvent effect on the catalysis [45] ........................................................................................................ 42
Table 7: Components from the reaction mixture with their retention times, wavelength for
maximal absorption and calibration factors ....................................................................................................... 55
Table 8: Molar composition of the PMO materials ............................................................................................ 71
Table 9: Properties of the PMO materials ............................................................................................................. 72
Table 10: Physicochemical properties of the synthesized PMO materials ............................................. 74
Table 11: Molar composition of the organosilicas ............................................................................................ 77
Table 12: Properties of the organosilicas before and after functionalization ....................................... 79
Table 13: Properties of the T1 material before and after functionalization .......................................... 80
Table 14: Physicochemical properties of the synthesized organosilicas ................................................ 82
Table 15: Comparison of the turnover frequencies for the TE-materials ............................................... 89
Table 16: Comparison of the turnover frequencies for the TB-materials ............................................... 90
Table 17: Parameters of the NRTL obtained from Aspen Plus ..................................................................101
Table 18: Added amount of precursors ...............................................................................................................104
Table 19: Evolution of the physisorpted amine concentrations ...............................................................105
Nomenclature vi
Nomenclature Roman Symbols
a Mass of the adsorbent [g]
A Cross-section of one adsorbate molecule [m²]
a0 Width of the unit cell [nm]
AP-MSN Amine-functionalized mesoporous silica nanoparticle
APTES 3-aminopropyltriethoxysilane
BdB-FHH Method of Broekhoff and de Boer, with the modification of Frenkel, Halsey
and Hill
BET Theory of Brunauer, Emmett and Teller
BJH Method of Barrett, Joyner and Halenda
BTEB 1,4-bis(triethoxysilyl)benzene
BTEE Bis(triethoxysilyl)ethane
BTEENE Bis(triethoxysilyl)ethene
BTME Bis(trimethoxysilyl)ethane
c BET constant
CTAB Cetyltrimethylammonium bromide
d Interplanar distance between two lattice planes [nm]
dp Average pore diameter [nm]
DRIFT Diffuse Reflective Infrared Fourier Transform
e Wall thickness of the material [nm]
F127 Pluronic F127
HMDS 1,1,1,3,3,3-hexamethyldisilazane
HMF 5-hydroxymethylfurfural
HOC Hayden-O'Connell model
Irgacure 2959 2-Hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone
IUPAC International Union of Pure and Applied Chemistry
MAPTMS N-methylaminopropyltrimethoxysilane
MCM Mobil Composition of Matter
MPTMS 3-mercaptopropyltrimethoxysilane
MSN Mesoporous silica nanoparticle
MVK Methyl vinyl ketone
Nomenclature vii
n Order of the reflection
NA Avogadro constant
NMR Nuclear Magnetic Resonance
NRTL Non-Random Two-Liquid model
OTAC Octadecyltrimethylammonium chloride
P123 Pluronic P123
PMO Periodic Mesoporous Organosilicas
PMS Periodic Mesoporous Silicas
RP-HPLC Reversed-phase high-performance liquid chromatograph
SBA Santa Barbara Amorphous
SDA Structure determining agent
SBET Specific surface area [m²/g]
TB Organosilica containing VTES, TEOS and BTEB
TE Organosilica containing VTES, TEOS and BTEE
TEOS Tetraethyl orthosilicate
TMOS Tetramethyl orthosilicate
TOF Turnover frequency [s-1]
V Adsorbed volume [cm³]
Vmon Volume corresponding to monolayer adsorption [cm³]
Vp Pore volume [cm³/g]
VTES Vinyltriethoxysilane
XRD X-ray diffraction
z Relative pressure (p/p°)
Greek symbols
ε Molar absorptivity at a given wavelength
θ Scattering angle [°]
λ Wavelength of the X-rays [nm]
Introduction 1
Chapter 1
Introduction
The aldol condensation may have good prospects in the discipline of green, renewable and
sustainable energy, e.g. in the valorization of glycerol and furfural. Fossil fuels, such as coal, oil
and natural gas provide more than 75 % of the world's energy. Even with the shale gas boom in
recent years, one cannot deny that the diminishing reserves of these non-renewable resources
cause a lot of concerns (e.g. rising oil prices, the increasing energy demand, limited supply of
fuels, etc.) [1]. In this regard, biomass can serve as a sustainable source for both the organic
carbon as for the energy in our industrialized society. Moreover, the production of energy from
biomass has the potential to generate lower greenhouse gas emissions, because the CO2 released
during energy conversion can be utilized for biomass regrowth. The Roadmap for Biomass
Technologies has predicted that by 2050, biofuels could provide 27 % of total transportation fuel
and 25 % of chemicals will be produced from biomass. At the same time reducing the total
amount of greenhouse gas emission in the energy sector by 80 % in 2050 (compared to the
levels in 1990) [2].
Figure 1 illustrates the conversion of biomass, and particularly lignocellulosic biomass, into
bioproducts and energy. In general, a variety of fuels and chemical intermediates can be
produced from carbohydrates by employing various types of reactions including hydrolysis,
dehydration, aldol condensation, reforming and oxidation. Which can be combined to produce
an even wider range of products [1].
Unlike petroleum which contains limited functionality, biomass-derived carbohydrates contain
an excess of functional groups (especially groups containing oxygen) which can be used as fuels
and chemicals. The challenge in this field is to develop methods to control the functionality in
the final product [1].
Introduction 2
Figure 1: Conversion of biomass to refined products [1] The aldol condensation and subsequent hydrogenation are key intermediate steps to form larger
organic molecules using biomass-derived molecules. The production of heavier liquid-phase
alkanes from carbohydrates involves a series of reaction steps starting with an acid hydrolysis of
polysaccharides such as (hemi)cellulose and starch to produce monosaccharides such as
glucose, fructose and xylose. These carbohydrates can further undergo an acid-catalyzed
dehydration to form carbonyl-containing furan compounds such as 5-hydroxymethylfurfural
(HMF) and furfural. It is important to note that furfural and HMF cannot undergo self-
condensation, because they do not have an α-H atom that is required for the reaction. However,
both furfural and HMF have aldehyde groups that can condense with other molecules that can
form carbanion species. As shown in Figure 2, acetone forms an intermediate carbanion species
that can cross-condense with HMF in the presence of a base catalyst to form C9 species, which
can subsequently react with a second HMF molecule to form a C15 species. As a result of their
non-polar structure, these aldol adducts have a low solubility in water and thus precipitate out
of the aqueous phase. Next, the C=C and C=O bonds are saturated by hydrogenation in the
presence of a metal catalyst (usually Pd). Finally, these molecules are converted into liquid
alkanes (C9 - C15) by aqueous-phase dehydration/hydrogenation over a bifunctional catalyst
containing an acid and metal site [1, 3].
Introduction 3
Figure 2: Aldol condensation of 5-hydroxymethylfurfural (HMF) with acetone [3]
Chapter 1 describes the different mechanisms of the aldol condensation and its applications.
The advantages of heterogeneous catalysts are briefly mentioned. Next, the properties of
mesoporous materials and the synthesis of periodic mesoporous organosilicas are discussed.
The last section of the introduction is about recent developments in cooperative catalysis.
An overview of the different effects on the catalyst’s activity is given.
The techniques and procedures which were employed for the characterization of the
synthesized periodic mesoporous organosilicas and the analysis of the catalytic experiments are
described in chapter 2.
In chapter 3, the results of the valorization of furfural via the aldol condensation are presented.
A comparison with the aldol condensation of 4-nitrobenzaldehyde with acetone is made.
The synthesis, functionalization and characterization of cooperative heterogeneous catalysts are
extensively discussed in chapter 4.
In chapter 5, catalytic experiments are performed with the synthesized organosilicas. The
influence of the hydrophobicity of the catalyst support on the catalytic activity in the presence of
water is investigated.
Chapter 6 summarizes the conclusions of this work. Also future work is proposed to continue
the research on this topic.
Introduction 4
1.1 Aldol condensation
The aldol addition reaction is an acid- or base-catalyzed coupling reaction of a ketone or
aldehyde. The product of the aldol addition reaction, a β-hydroxy carbonyl compound, is called
an aldol because it contains both the functional group of an aldehyde and an alcohol.
Under certain conditions, the reaction product may undergo dehydration leading to an
α,β-unsaturated aldehyde or ketone. This variant is called the aldol condensation [4]. Even if the
dehydration is not spontaneous, it can usually be easily performed by heating the mixture,
because the new double bond is connected to the C=O bond. Condensation reactions combine
two (or more) molecules, often with the loss of a small molecule such as water or an alcohol.
This leads to a method which can be used for preparing α,β-unsaturated aldehydes (enal) and
ketones (enone) [4-6]. The reaction is shown in Figure 3.
The aldol condensation is a frequently used reaction in organic chemistry to connect two smaller
carbonyl groups to each other and is thus a convenient way to increase the carbon chain length.
It is an important reaction to create new C-C bonds and yield larger and more complex
molecules.
Figure 3: The aldol condensation [6] Because the product of an aldol condensation is again an aldehyde or ketone, a possible
troublesome side reaction is the further condensation of the aldol, enal or enone.
1.1.1 General mechanisms
Up to a few years ago, most aldol condensations have been homogenously catalyzed. These
homogenous catalysts exhibit an excellent selectivity and, due to a good understanding of the
reaction mechanism, allow an easy identification of the intermediates.
Although acid-catalyzed aldol reactions (e.g. by means of sulfuric acid) are known [7], the most
common form of the reaction uses a base. During the first years, sodium hydroxide and sodium
carbonate were commonly used in this reaction. However recently, stronger bases, such as
alkoxides (RO-) or amides (R2N-) are also often used. Also weakly base amines have been used as
Introduction 5
catalysts in the aldol condensation. Depending on the nature of the catalyst, different reaction
mechanisms are obtained.
1.1.1.1 Base-catalyzed aldol condensation
The mechanism of a strong base-catalyzed reaction involves the formation of an enolate ion
(equilibrium between two ions), followed by a nucleophilic addition of this enolate to the
carbonyl group of another aldehyde or ketone. In a third step, protonation of the formed
alkoxide anion leads to the formation of the aldol product and the recovery of the basic catalyst
(see Figure 4). Finally, dependent on the acidity of the α proton of the aldol product, dehydration
can occur. Abstraction of the α proton gives an enolate that can eliminate hydroxide ion to give a
more stable, conjugated product [4, 6]. The dehydration mechanism is shown in Figure 5.
Figure 4: Base-catalyzed aldol condensation [6]
Introduction 6
Figure 5: Base-catalyzed dehydration of an aldol [6] The hydroxide ion is not strong enough to convert substantially all of an aldehyde or ketone
molecule to the corresponding enolate ion. This means that the equilibrium of step 1 in Figure 4
is shifted to the left. Nevertheless, enough enolate ion is present for the reaction to proceed [5].
The location of this equilibrium can be influenced by the solvent. Protic solvents, such as water
or ethanol, are acidic enough to donate protons and react with the enolate anion, thus shifting
the equilibrium to the left. In an aprotic solvent, such as acetone or acetonitrile, the equilibrium
lies more to the right. But in the next steps of the mechanism the enolate ion reacts with another
carbonyl to form an alkoxide, followed by a protonation to give the aldol product. By removal of
the enolate ion, the equilibrium of step 1 is slightly shifted to the right [5].
1.1.1.2 Acid-catalyzed aldol condensation
It is also possible to carry out the aldol condensation under acidic conditions. In a first step, the
enol is formed by an acid-catalyzed keto-enol tautomerism as is shown in Figure 6. Hereby, a
proton is moved from the α carbon to oxygen by first protonating oxygen and then removing a
proton from carbon.
Introduction 7
Figure 6: Acid-catalyzed keto-enol tautomerism [6] Next, the enol form of the aldehyde or ketone serves as a weak nucleophile to attack a
protonated carbonyl group. Deprotonation of the resonance-stabilized intermediate gives the
aldol product (see Figure 7).
Figure 7: Acid-catalyzed aldol condensation [6] Finally, this aldol product can be transformed into a conjugated α,β-unsaturated aldehyde or
ketone. The mechanism is similar to the dehydration reaction of alcohols. Dehydration results
from E1 elimination of the protonated alcohol functionality as can be seen in Figure 8. First,
protonation converts the hydroxyl group of the aldol product to a good leaving group. After the
water leaves, a carbocation is formed. Followed by the removal of a proton, gives the new C=C
double bond.
Introduction 8
Figure 8: Acid-catalyzed dehydration of an aldol [6]
1.1.2 Types of aldol condensation
The aldol reaction can be divided into two categories. First there is the selfcondensation, this is a
reaction between two molecules of the same aldehyde or the same ketone. The second group are
bundled under the name crossed aldol condensation. These reactions involve two different
aldehydes or two different ketones, or a combination of an aldehyde and a ketone.
The reaction rate of the selfcondensation of two ketones is lower than that of two aldehydes due
to sterical hindrance. When the enolate of one aldehyde (or ketone) adds to the carbonyl group
of a different aldehyde or ketone, the result is called a crossed aldol condensation. If the
compounds used in the reaction are not selected carefully, a mixture of four products will be
formed. However, if one of the reactants does not have an α hydrogen, only two aldols are
possible, and in many cases the crossed product is the main one. This crossed-aldol reaction is
often called the Claisen-Schmidt reaction [6, 8]. As an example, the aldol condensation between
ethanol and propanal is shown in Figure 9.
Introduction 9
Figure 9: Aldol condensation between ethanal and propanal [6] In general, the reactions in the addition phase are reversible. The equilibrium constant for the
dehydration phase is usually favourable because of the conjugated α,β-unsaturated carbonyl
system that is formed. When the reaction conditions are sufficient to cause dehydration, the
overall reaction can go to completion, even if the equilibrium constant for the addition step is
unfavourable (e.g. selfcondensation of ketones) [4].
Aldol reactions are often used to close five- and six-membered rings. This intramolecular aldol
condensation is well known under both basic [6] or acid [9] conditions. Because of the favorable
entropy, such aldol cyclization generally take place without much difficulty. Aldol cyclizations of
rings larger than six and smaller than five are less common because larger and smaller rings are
less favored due to their larger ring strain (mixture of angle strain and torsional strain). Figure
10 shows how a 1,4-diketone can condense and dehydrate to give a cyclopentenone. While
Figure 11 shows how a 1,5-diketone gives a cyclohexenone [5, 6].
Figure 10: Formation of a cyclopentenone by an aldol cyclization [6]
Introduction 10
Figure 11: Formation of a cyclohexenone by an aldol cyclization [6] An important extension of the intramolecular aldol condensation is the Robinson annulation
reaction, which has been often used in the synthesis of steroids and terpenes [10]. The
mechanism is shown in Figure 12 and begins with the Michael addition of the cyclohexanone
enolate to methyl vinyl ketone (MVK), forming a δ-diketone. The enolate of the methyl ketone
attacks the carbonyl of the cyclohexanone. Finally, the aldol product dehydrates to give a
cyclohexenone .
Figure 12: Robinson annulation reaction [6]
1.1.3 Applications
The aldol condensation is used for the synthesis of organic compounds, particularly specialty
chemicals such as perfumes and synthetic flavours. Many aldehydes have very typical odours.
For example, benzaldehyde has a characteristic almond-like odour and is at low levels used as an
artificial flavouring in many nutrients [11]. Perfumers have an almost unlimited number of raw
materials available to create fragrances. These raw materials can be grouped into three classes:
natural essential oils, synthetic chemicals and semi-synthetic chemicals derived by chemical
processing of natural products. A number of important perfume chemicals are manufactured on
a large scale by a variety of condensation reactions, including the aldol condensation. Five
widely used products made via aldol condensation are pseudomethylionone, α-amylcinnamic
aldehyde, α-hexylcinnamic aldehyde, alkyl-α-methyldihydrocinnamic aldehyde and cinnamic
aldehyde. The reactions are catalyzed by alkali under condition which minimize self-
condensation of the more reactive aldehyde, i.e. propionaldehyde in the preparation of the
unsaturated intermediates for Cyclamen aldehyde and acetaldehyde for cinnamic aldehyde [12].
Introduction 11
The aldol condensation can be used to synthesize a larger molecule with a specific
functionalization from smaller reagents. Therefore, the reaction is also widely applied in the
pharmaceutical industry, as well as for the preparation of coatings, plastics and many other fine
chemicals [13].
The wide synthetic applicability of the aldol reaction can be attributed to the ability to achieve
both versatility in reactants and control of regiochemistry and stereochemistry. The term
directed aldol addition is applied to reaction conditions that are designed to achieve specific
regio- and stereochemical outcomes. Control of the product structure requires that one reactant
acts exclusively as the electrophile and the other exclusively as the nucleophile [4].
The reaction can be made regioselective by preforming the reactive nucleophilic enolate and
ensuring that the addition step is fast. The aldol reaction creates two new stereogenic centers,
and, in the most general case, there are four stereoisomers of the aldol product, which are
presented in Figure 13 [4, 5].
Figure 13: Four possible stereoisomers of the aldol product [5] Aldol condensations can also be made enantioselective by using chiral enol derivatives, chiral
aldehydes or ketones, or both. In this case only one of the four isomers of the products will be
dominant. Chiral amino acids, such as proline, can be used for this reaction [5].
Introduction 12
1.2 Catalysis: homogeneous vs heterogeneous
Catalysis is an important field in chemistry. Today over 90 % of all industrial chemicals are
produced with the aid of catalysts [14]. Because of environmental issues, energy supply and
pollution control, catalysis appears to be even more important than before. The IUPAC
(International Union of Pure and Applied Chemistry) has defined a catalyst as "a substance that
increases the rate of a reaction without modifying the overall standard Gibss energy change in
the reaction" [15]. The catalyst is both a reactant and a product of the reaction, meaning that the
catalyst is restored after each catalytic cycle. Also, the catalyst does not influence the
thermodynamical equilibrium composition after the reaction has terminated [14, 16].
In the 1990s the 12 principles of green chemistry have been introduced, a comprehensive
set of guidelines including the atom economy, the use of reusable catalysts and the minimization
of waste [17]. Within green chemistry, there is an increasing interest in developing novel
materials that can be applied as heterogeneous catalysts for chemical processes. Thus replacing
effective but hazardous chemicals as H2SO4, HF and AlCl3. The essence behind green chemistry is
that instead of using every possible chemical process, only those that are environmentally
benign should be used. The E-factor, a quantitative parameter that can be used to evaluate the
environmental acceptability of a process, was proposed by Sheldon in 1992 and reports the
amount of unit waste produced per unit product. Ideally this factor should be equal to zero,
but an analysis of the waste generation for each chemical industry sector, highlights the
need for improvements especially in fine chemicals (E-factor: 5-50) and pharmaceuticals
(E-factor: 25-100) [16-19].
The concern about renewables, CO2 levels and sustainability lead to a new chemical field
oriented at biomass feedstocks which is still very dependent upon new catalysts to make it
competitive with petroleum based feedstocks [14]. More efficient catalytic processes require
improvements in the catalytic activity and selectivity. Both aspects can be improved by a
tailored design of catalytic materials with the desired structures and the desired distribution of
the active sites. Materials such as zeolites, metal organic frameworks and mesoporous materials
offer such possibilities [16].
The first zeolite, stilbite, was discovered by Axel Fredrik Cronstedt in 1756 who found that the
mineral loses water rapidly on heating and thus seems to boil. The name zeolite comes from the
Greek words ζέω (zéō = to boil) and λίθος (líthos = stone). But it is only since 1955 that the
development of synthetic zeolites lead to the discovery of many new and shape selective
catalytic processes [14].
Introduction 13
Since the end of the twentieth century, societal concerns have become an important motivation
for intensive research to convert the current by-products to useful products and to treat all
kinds of wastes in order to preserve and protect the environment. A challenge for our century is
the control of technological processes in the chemical, petrochemical and pharmaceutical
industries to develop atom-economical, environmental friendly processes with as little as
possible by-products [16].
Nowadays, the substitution of the homogeneous by the more environmentally friendly
heterogeneous process is highly desirable. Because of environmental concerns, heterogeneous
counterparts for the homogeneous catalysts are being investigated. The use of insoluble solid
catalysts presents several advantages. For one, the reaction temperature can be raised because
solvent is not necessarily required and consequently, aldol condensations in the gas phase can
be performed. Secondly, solid catalysts result in less energy intensive separation processes and
problems related to the disposal of strong acid or basic solutions can be avoided. The
regeneration and recycling of used catalyst enhance the overall efficiency of the process. Also,
the reactions can be carried out without the reactor being exposed to concentrated solutions of
corrosive acids or bases [20].
In the past two decades, a variety of heterogeneous catalysts - including zeolites [21, 22], anion-
exchange resins [23], Mg-Al layered double hydroxides [24, 25] and metal organic frameworks
[26] - have already been tested for the aldol condensation. But most advantages can perhaps be
achieved with mesoporous materials. By combining the properties of organic and inorganic
building blocks within a single material it is possible to combine the enormous functional
variation of organic chemistry with the advantages of a thermally stable and robust inorganic
substrate. The symbiosis of organic and inorganic components can lead to materials whose
properties differ considerably from those of their individual, isolated components. Through the
incorporation of the desired functional groups in a mesoporous structure, a solid heterogeneous
catalyst is created [27]. These materials will be discussed in the next chapter.
Introduction 14
1.3 Mesoporous materials
It is only since 1980 that serious attempts have been made to create "zeolite-like" materials with
ordered and uniform pores which are greater than two nanometers. These larger pores would
allow the host-guest chemistry of large molecules and polymers (= guest) to realize specific
interactions with the support (= host), but also increase the rate of diffusion of these molecules
in the pores. Larger pores would be a huge advantage in for example the fine chemical and
pharmaceutical industries as bulky products do not fit in smaller pores [28, 29].
1.3.1 Silica materials
During the last decades, different types of mesoporous materials (pore diameter between 2 and
50 nm) are synthesized, but one of the most important discoveries was the development of
ordered periodic mesoporous silicas (PMS) with a particular arrangement (MCM-41: hexagonal,
MCM-48: cubic, MCM-50: laminar, see Figure 14) of the mesopores. In 1992, scientists at Mobil
Oil Corporation discovered the direct synthesis of mesoporous materials, the Mobil Catalytic
Materials or MCM [30]. In the same year, researchers at the University of Santa Barbara also
developed mesoporous silica materials (SBA-15) by using a triblock copolymer (P123 or F127)
as surfactant [31, 32].
Figure 14: Structures of mesoporous silicas: a) MCM-41, b) MCM-48 and c) MCM-50 [27]
1.3.2 Functionalized silica materials
After the development of the MCM and SBA-materials, the interest to incorporate organic groups
in the periodic mesoporous materials has increased in order to enrich them with different
properties. These periodic mesoporous silica materials, with high internal surface areas up
to about 1000 m²/g and pore diameters up to 6 nm, are the ideal starting point for the
incorporation of many and large organic groups. The integration of functional organic groups in
Introduction 15
the inorganic structures has led to the discovery of organic-inorganic hybrid materials with well-
defined porous structures and unique properties [33-35].
The synthesis of these hybrid materials can proceed via three routes and an overview of the
three different synthesis methods is shown in Figure 15. Every synthesis method will be
discussed in more detail.
1. A post-synthesis functionalization takes place through the anchoring of terminal organic
groups on the pore walls by making use of the reactivity of the silanol groups in the
material. (grafting method; paragraph 1.3.2.1)
2. The condensation of a tetraalkoxysilane with an terminal trialkoxyorganosilane of the
type R-Si-(OR’)3 is a one-pot synthesis, in which R is an organic functionality, and R’ is an
alkyl group (co-condensation method; paragraph 1.3.2.2)
3. The direct condensation of a poly(trialkoxysilyl) bridged organic component, eg.
(R’O)3-Si-R-Si-(OR’)3. (Periodic Mesoporous Organosilicas – PMOs; paragraph 1.3.2.3)
Figure 15: Overview of the different synthesis methods
for obtaining hybrid materials [27]
Introduction 16
1.3.2.1 Post-synthesis functionalization
The term grafting refers to a post-
synthesis modification of the interior
surface with organic groups. This
procedure involves the condensation
of an organosilane, typically of the type
R-Si-(OR’)3, with the free silanol groups
present on the pore walls (Figure 16).
This procedure is also called post-
functionalization. These materials are
eventually made from an inorganic
surface on which an organic layer is
grafted [27, 33].
Figure 16: Post-functionalization of a silica
material by anchoring R-Si-(OR')3 to the
silanol groups. R is an organic functionality,
R' is an alkyl group [27]
Because of the significant quantity of possible organosilanes one can, by variation of the organic
group R, prepare a large set of organic functionalized silica materials with different chemical and
physical properties. The amount of organic groups is limited by the number of free silanol
groups on the pore walls.
A second advantage to this method is that the original mesostructure of the silica material
almost remains unchanged when a controlled functionalization of the organic groups occurs.
As a third advantage one can, by using bulky R-groups, selectively anchor these organosilanes to
the pore-opening. This allows them to block or completely shut-off the pores in such a way that
the air is trapped inside the pore. This property is of great importance for obtaining materials
with a low dielectric constant or also known as low-k materials (i.e. an important application of
PMO materials) [32, 33].
If during the beginning of the synthesis, the organosilanes bind mainly to the pore mouth, the
further spreading of the following molecules can be obstructed. This leads to a non-
homogeneous distribution of the organic groups in the pores and a lower degree of occupation.
A second disadvantage of this post-functionalization is that by introducing organic groups at the
surface, the pore volume will logically decrease. Moreover, this post-synthesis functionalization
requires of course an additional synthesis step [27, 32].
Introduction 17
1.3.2.2 One-pot synthesis
The co-condensation method is, in contrast to the anchoring of organosilanes (see paragraph
3.2.1), a functionalization method in which all the precursors are already together at the
beginning of the reaction, as shown in Figure 17. The hybrid material is prepared by the co-
condensation of tetra-alkoxysilanes, Si(OR)4, usually tetraethyl orthosilicate (TEOS) or
tetramethyl orthosilicate (TMOS), with terminal tri-alkoxyorganosilanes R-Si-(OR’)3 in the
presence of a structure determining agent (SDA) [32, 33].
Figure 17: Functionalization via the co-condensation of an organosilane with TEOS [27] Because the organic groups are added during the beginning of the synthesis and are thus already
part of the pore walls, the chance of pore-blocking is smaller. One obtains a more homogeneous
distribution of the organic functions in comparison with the materials via post-synthesis
functionalization [36, 37]. However, it should be mentioned that the homogeneous distribution
of the organic functions is highly dependent on the hydrolysis and condensation rates of the
silica and the organosilica precursors. If the difference in relative velocities is too large, it may
give rise to the separate condensing of the two components. This process is also referred to as
self-condensation and prevents the homogeneous distribution of the organic groups [32, 33].
A second disadvantage to this method is the effect of the organic groups on the degree of
mesoscopic ordering of these materials. In general, the ordering decreases with an increasing
concentration of organosilanes and in the worst case will lead to completely unstructured
materials. Therefore, no more than 40 mole percent of the organic functions is usually added.
Typical values are in the majority of cases even lower, around 15 – 25 mole percent [27, 34].
Introduction 18
This limitation may be due to a number of aspects: the organic functions are likely to interfere
with the formation of the silica structure and reduce the formation of micelles, especially when it
concerns bulky R-groups [38].
An increasing concentration of organosilanes will also have a decrease of the pore diameter,
pore volume and specific surface area as a result. Finally, a last disadvantage which must be
taken into account is that during the removal of the surfactant, the organic groups functions may
not be damaged or destroyed. For these reasons, a calcination step is not advisable and generally
only extraction methods are used [27, 32].
1.3.2.3 Periodic mesoporous organosilicas – PMOs
Although the previous synthesis methods are widely used in order to obtain some important
hybrid materials, both the post-synthesis and the one-pot synthesis have advantages as well as
disadvantages. As a solution to these drawbacks, a third alternative route was developed in
order to obtain organic-inorganic ordered mesoporous materials with a high concentration of
organic groups, wherein the organic and inorganic units are uniformly distributed in the
structure. These Periodic Mesoporous Organosilica materials, often abbreviated as PMO,
may be formed through the direct condensation of bridged organosilanes, for example
(R’O)3-Si-R-Si-(OR’)3, in the presence of a structure determining agent [27, 32, 33].
With this synthesis method, the organic units are directly incorporated into the three-
dimensional silica structure via two covalent bonds. This is not the case with the organic
functionalized silica materials which have been obtained via post-functionalization or direct
synthesis. In these last two methods, a combination of a tetra-alkoxysilane and a terminal tri-
alkoxysilane of the type R-Si-(OR’)3 is used. During the synthesis the alkyl group (OR’)3 co-
condense with each other, while the organic functionality R is oriented into the pore.
The first materials synthesized in this way (see Figure 18) have a homogeneous distribution
of the organic groups, a very large internal surface area and an improved thermal stability.
PMOs are characterized by periodically ordered pores and a very small pore size distribution.
These materials are also characterized by both the large amount and the uniform distribution of
the organic groups in the pore walls; large internal surface areas; thick pore walls; large pores
and high pore volumes [27, 39].
Introduction 19
Figure 18: Synthesis of a PMO material via the condensation of a bissilane
in the presence of a surfactant and an acidic or base environment [27]
Because the organic functions are embedded in the walls of the channels, the pores remain fully
open and they are easily accessible for further manipulations (see paragraph 4.2.1) of the
chemical and physical properties without losing the stability of the porous structure [33].
With the help of this third route, one can circumvent problems such as the non-uniform
distribution and the influence of the organic groups on the mesoscopic ordering, the blocking of
the pores, different hydrolysis and condensation rates of the precursors and possible other
disadvantages of the grafting and co-condensation procedure [36].
The first PMO materials were synthesized in 1999 by three independent research groups.
Stein et al. [40] used two precursors with different bissilanes, namely bis(triethoxysilyl)ethane
(BTEE) and bis(triethoxysilyl)ethene (BTEENE). As a surfactant, cetyltrimethylammonium
bromide (CTAB) was used. In the same year, Inagaki et al. [41] succeeded to convert
bis(trimethoxysilyl)ethane (BTME) to an organic-anorganic hybrid material in base conditions
and in the presence of octadecyltrimethylammonium chloride (OTAC) as a structure
determining agent. Finally, Ozin et al. [42] used a combination of bis(triethoxysilyl)ethane
with TEOS, which was dissolved in a solution of CTAB, ammoniumhydroxide and water to obtain
a periodic mesoporous organosilica.
During the first years, only a few precursors (methane, ethane, ethene and benzene bissilanes)
were used frequently used. But nowadays, there is a lot of research for introducing all kinds of
bridged organic components. Figure 19 gives a small overview of organic precursors that have
been used in PMO materials [27, 32, 33].
Introduction 20
Figure 19: Overview of precursors that have been used in PMOs.
Terminal Si = Si(OR)3 with R equal to –CH3 or –C2H5 [27]
Starting the PMO synthesis from these bissilanes (precursors) one can synthesize materials with
specific functions [32, 33]. Although the list of organic precursors is not exhaustive, only with a
limited number of these precursors an ordered mesoporous hybrid material can be prepared
which only consists of that precursor. This is because the pore walls of the PMO material must
possess a certain rigidity. Therefore, short chains or conjugated systems give better results
If the material does not have the desired structure, one usually chooses a co-condensation with a
rigid organic precursor with an ethane or benzene function as organic bridge. These are called
bifunctional PMO materials [27, 33, 34]. The bridged organic components should not be too
bulky either. If the distance between two inorganic units (Si-R-Si) in the pore wall becomes too
large, there is a risk that the pore walls will collapse. A final constraint is the interaction between
the surfactant and the precursor. To avoid a possible phase separation with the surfactant, one
can adjust one of the following parameters: acidity, temperature, concentration of both
components, type of surfactant, environmental conditions and additives [33, 36].
Introduction 21
1.3.2.4 PMO synthesis parameters
- Precursor:
In the last years many syntheses have been discussed in the literature. The great majority of
reports describe the synthesis and characterization of one specific PMO. Often, the reported
synthetic procedure does not produce ordered organosilicas when different precursors are
substituted. This can be explained by the kinetics of the precursor since every precursor has its
own specific hydrolysis and condensation rates.
A general procedure to prepare a wide variation of PMO materials with different organic
functionalities was described by Burleigh et al. [43] in 2004. A typical synthesis procedure starts
with adding concentrated HCl to deionized water to obtain the initial aqueous solutions. Brij-76
surfactant was added to the solution upon stirring, and the mixtures were heated at 50 °C for 12
hours. The precursors (Figure 20) were then added to the resulting clear solution. The synthesis
mixtures were covered and stirred at 50 °C for 12 hours, followed by heating at 90 °C under
static conditions for an additional 24 hours.
Figure 20: Structure of the different PMO precursors used during the synthesis [43]
1: bis(triethoxysilyl)ethane, 2: bis(triethoxysilyl)methane,
3: bis(triethoxysilyl)ethene, 4: 1,4- bis(triethoxysilyl)benzene
The versatility of this synthesis can be attributed to a combination of key factors that affect the
surfactant template approach to synthesizing porous materials. These include the precursor
molecular structure and reactivity, interactions between the surfactant and the hydrolyzed
precursors, the overall stability of Brij-76 micelles and the acidic reaction conditions [43].
- Surfactant:
An overview of some frequently used surfactants during the synthesis of PMO materials is given
in Table 1. PMOs synthesized using an ionic surfactant as a structure determining agent often
have pore diameters which are restricted to the range between 2 to 5 nm. This limitation was
overcome by using non-ionic triblock copolymers of the type EOxPOyEOx (EO = poly(ethylene
oxide) and PO = poly(propylene oxide)). Characteristic to using P123 or F127 as template is that
mesoporous structures can be obtained under various pH values. This allows for the synthesis of
PMO materials with pore diameters bigger than 5 nm [44].
Introduction 22
Table 1: List of frequently used surfactants [44]
Abbreviation Name Type interaction Structure
Ionic: pore diameter: 2 - 5 nm
CTAC/CTAB Cetyltrimethylammonium
chloride/bromide S+I-
of
S+X-I+
OTAC Octadecyltrimethylammonium
chloride
Non-ionic: pore diameter: > 5 nm
Brij-56 Polyoxyethylene(10) cetyl ether
S°I°
of
S°H+X-I°
Brij-76 Polyoxyethylene(10) stearyl ether
P123 Pluronic P123
F127 Pluronic F127
The alkylethylene oxide surfactant Brij-76 is an ideal choice for a PMO template. In addition,
Brij-76 is inexpensive, non-toxic and biodegradable. It also has a better template stability during
the synthesis owing to the relatively low solubility of Brij-76 in ethanol [43].
- Acidity
The acidity of the reaction mixture influences the interactions between the precursor and the
surfactant. It also has an impact on the hydrolysis and condensation rate of the precursor. This
parameter is very important for the ordering in the PMO material. Figure 21 shows the possible
interactions between the head groups of the surfactant and the inorganic species, taking into
account whether these interactions take place in a basic (21a and 21c), acid (21b and 21d) or
neutral (21e and 21f) media [27].
Introduction 23
Figure 21: Interactions between the inorganic species (I) and the head group of the surfactant (S) with consideration of the possible synthetic pathway
in acidic, basic or neutral media [27]
- Temperature
The reaction temperature can play an important role. Each synthesis will feature an optimal
temperature which promotes the formation of larger pores. Within a certain procedure, the
various synthesis steps can take place at different temperatures. For example, the synthesis can
start at room temperature, but in a second step require 80- 100 °C. Another important factor is
the stirring time, which determines how long the temperature is maintained. Thanks to stirring,
a better distribution of heat is obtained, this also has an effect on the pore size distribution.
Introduction 24
1.4 Cooperative catalysis
In the last decade, incorporating multiple types of active sites into solid materials has become an
area of interest in the synthesis of catalysts. The design of multifunctional materials capable of
carrying out heterogeneous catalysis through the incorporation of organic functional groups has
been growing in attention. In organisms, the aldol reaction is performed by aldolases, which
activate donor ketones with the amino group of lysine, to give enamines. Next, the enamines
attack aldehyde acceptors and are then hydrolysed to release the product. Enzymes, such as
aldolases, are interesting examples of multifunctional catalysts as they are capable of handling
multistep reactions. Enzymes can immobilize functional groups, which are mutually
incompatible and would not be tolerated together in solution, in a certain way that maintains
their independent functionality [45-47].
Surface-modified mesoporous materials with various active sites have been widely investigated
in recent years. Monofunctionalized mesoporous silica catalysts have been demonstrated to
have unique properties. But for many applications, bifunctional mesoporous catalysts are much
more wanted because the combined functionalities may act in a cooperative way to improve the
reactivity of the catalyst. Unravelling of reaction mechanisms in heterogeneous catalysis poses
additional challenges due to the different environments that the active sites can encounter on a
solid support, interfacial phenomena, competitive adsorption and kinetics of mass transfer [47-
49].
Many research groups have been devoting their time to this cooperative catalysis [45-56]. They
found out that the catalytic activity of a heterogeneous catalyst is dependent on many factors,
such as the acidity of the acid group, the type of the amine, the distance between both the base
and acid functionality. Also the reaction medium plays an important role during the reaction.
During the last years, different reaction mechanisms have been proposed. These aspects will be
discussed in the following chapter.
Most of them use SBA-15 (Santa-Barbara Amorphous) as a heterogeneous support. SBA-15 is
a well-ordered 2d-hexagonal mesoporous silica. Due to their large pore sizes and surface areas,
these mesoporous silica materials provide sufficient space to incorporate multiple functional
groups while the stability of the matrix is ideal for many types of reaction. Thus, making these
materials very useful as model catalyst for research purposes. Attachment of an organosilane
onto a mesoporous silica material is commonly achieved through grafting or via co-
condensation. Both methods have been discussed in the previous chapter.
Introduction 25
Coupling reactions that are typically acid or base-catalyzed, such as the aldol reaction, can be
accelerated through the addition of a second component that acts cooperatively. The two
catalytic components can separately activate the two different substrates, yielding a lower
energy reaction pathway [50]. In the context of catalysis, the term cooperativity refers to a
system where at least two different catalytic entities act together to increase the rate of a
reaction beyond the sum of the rates achievable from the individual entities alone [51].
1.4.1 Effect of the acidity of the acid group
Last years, attention has been focused on combinations of organic functional groups (amines,
thiols and sulfonic acid groups). In 2006, Zeidan et al. [45] reported the synthesis of a solid
material that contains both organic acid and base groups. A sulfonic acid and amine were
incorporated into SBA-15. The immobilized catalyst was tested for activity in the aldol
condensation of 4-nitrobenzaldehyde with acetone at 50 °C (see Figure 22).
Figure 22: Aldol condensation of 4-nitrobenzaldehyde with acetone [45] The material exhibited a remarkable reactivity that is not achievable with the same groups in
solution. The bifunctional catalyst gave a total conversion of 62 % after 20 hours. While the
individual SBA-15 materials, functionalized with either only a sulfonic acid or amine group, gave
significantly lower conversions of 16 %, respectively 33 %. Interesting, a physical mixture of
acid-functionalized SBA-15 and amine-functionalized SBA-15 showed an intermediate level of
conversion, i.e. 44 %, indicating the importance of both functionalities being present at the same
time. With the homogeneous analogues being added separately, only conversions lower than
8 % were achieved as no cooperative effects are possible. When the homogeneous sulfonic acid
and amine were simultaneously added, the reaction showed no conversion because these
functionalities neutralize each other, forming a salt that has no catalytic reactivity [45].
The observed enhancement of reactivity is likely due to interactions between neighbouring acid
and base sites [45]. The equilibrium between acids and bases when immobilized together is not
well defined but likely is very important in understanding the catalysis properties of such
materials. The proposed mechanism for acid/base cooperative catalysis is shown in Figure 23
and demonstrates the potential importance of acid/base interactions [46].
Introduction 26
Figure 23: Proposed catalytic cycle [46] According to the mechanism in Figure 23, regeneration of the active catalytic species relies on
the rate of proton exchange between the acid and base groups, which would be facilitated by
surface silanols and water when immobilized on mesoporous supports.
Zeidan et al. [46] also investigated the effect of the equilibrium between acid and base groups by
varying the pKa of the acid group. When examining this equilibrium, they reasoned that a careful
choice of acids to pair with the amino group could allow for a change in the equilibrium toward
an increased number of free acid/base sites and potentially lead to higher catalytic activity if less
strong acids with a higher pKa were chosen. Three different acids were immobilized on the SBA-
15 and tested in the aldol condensation of 4-nitrobenzaldehyde with acetone. The results of
these catalytic tests are given in Table 2. In the case of sulfonic acids and amines, the equilibrium
would be expected to be heavily shifted towards the inactive, neutralized ion pair.
Table 2: Results of the catalytic tests [46]
Acid catalyst pKa A [%] B [%] Total conversion [%]
Sulfonic acid -2 45 17 62
Phosphoric acid 3 62 16 78
Carboxylic acid 5 75 24 >99
The conversion increases significantly with decreasing acid strength. The carboxylic acid/amine
material converts the 4-nitrobenzaldehyde nearly quantitatively and gives a constant selectivity
Introduction 27
throughout the course of the reaction. By proper pairing of weaker acid groups with amine
groups, it is possible to synthesize a catalytic material with excellent conversion toward the
aldol reaction, indicating that the equilibrium is shifted towards the non-neutralized material
and gives rise to enhanced reactivity [46].
In 2012, Brunelli et al. [50] prepared a set of functionalized mesoporous silica materials to
investigate the role of the synthesis method and the impact of the interaction of amines, silanols,
and carboxylic acid groups on the catalytic rate during the aldol condensation of 4-
nitrobenzaldehyde and acetone. They concluded that the synthesis of a carboxylic acid
organosilane is best performed with a protecting group, such as a tert-butyl group that can be
thermally cleaved, to prevent hydrolysis of the ethoxysilyl groups during the synthesis and to
permit grafting onto the surface in the presence of an organic base. The grafting was carried out
in both toluene and methanol. There are indications that methanol as grafting solvent would
promote better spacing upon surface functionalization than toluene [52]. Thus obtaining a more
active catalyst because of an increase in the cooperativity achieved by preventing amine
aggregation on the surface of the silica support.
The reference material (SBA-15 functionalized with 0.5 mmol/g aminopropyl) reached 80 %
conversion after 5 hours. Introduction of the protected carboxylic acid in the grafted materials
decreased the activity of the catalyst to only 50 % conversion after 5 hours. It is assumed that
the second organic group occupied silanol groups adjacent to the amine sites, inhibiting the
beneficial cooperative interaction between amines and silanols. After the material was heated,
the thermal deprotection step introduces a more acidic carboxylic acid site than the silanol it
removes. The activity further decreased to only 35 % conversion after 5 hours. These results
suggest that acidic protons in very close proximity to amines lead to protonated amines that are
significantly less active for the aldol condensation. Brunelli and co-workers concluded that the
acid strength and the proximity determine the equilibrium concentration of inactive amines, and
thus the corresponding activity of the catalyst [50].
Removal of the silanols and the introduction of a carboxylic acid has a negative impact on the
activity of the catalyst. A consensus is forming that weaker acid sites result in the greatest
enhancement of the reaction rate in base-catalyzed coupling reactions such as the aldol
condensation [53]. Of particular importance is the acid-base cooperative catalytic interactions of
amines and silanols that was first demonstrated by Kubota et al. [54] in 2003 for a soluble amine
mixed with a mesoporous silica.
The role of silanols in the reaction can be explained by the fact that carbonyl compounds adsorb
on the surface of the support via hydrogen bonding. NMR measurements suggest that silanol
groups play a key role in bringing all reactants and the catalytic group together for the reaction
Introduction 28
to take place. Silanol groups assist the catalytic activity of immobilized amines by offering
binding sites for the reactants in close proximity to the amines, providing pathways for proton
transfer throughout all the steps of the reaction, and facilitating the departure of water during
the formation of intermediates (see Figure 24). The formation of the intermediate enamine
involves a series of proton transfers. The mildly acidic silanol groups could assist theses
transfers by aligning with acetone and amine groups in six-membered ring-like arrangements
[47].
Figure 24: Possible pathway of proton transfer assisted by silanol groups [47] More recently, Lauwaert et al. [55] investigated the effect of the silanol-to-amine ratio on the
aldol condensation. As already mentioned, the catalytic activity of amines in aldol condensations
can be improved by incorporating weak acid sites. This promoting effect is in particular
interesting because the silica materials already intrinsically possess weakly acidic silanol
groups. In this work, (3-aminopropyl)triethoxysilane (APTES) was chosen as a precursor for the
active amine groups. Two types of materials were synthesized. Via the grafting of APTES, a
material with both amine and free silanol groups is obtained, leading to a bifunctional catalyst
(type A). For the second type, the bifunctional material is treated with 1,1,1,3,3,3-
hexamethyldisilazane (HMDS). It is assumed that through this treatment with HMDS all the
surface silanols are removed, resulting in a monofunctional catalyst (type B) [55].
The catalysts' performance was tested for the aldol condensation of 4-nitrobenzaldehyde with
acetone at 55 °C. The evolution of the turnover frequencies (TOFs) obtained with the different
catalysts as a function of their silanol-to-amine ratio is shown in Figure 25.
Introduction 29
Figure 25: TOFs of the catalysts (type A, rhombus) and the HMDS-treated catalysts (type B, square) as a function of the molar silanol-to-amine ratio [55]
Silanol groups are not necessary to catalyse the aldol condensation because they are not an
actual, catalytically active site, but they do act as a promoter to the reaction. This can be seen
from the HMDS-treated catalysts (type B) all exhibiting a similar catalytic activity, while the
TOFs obtained with the bifunctional catalysts (type A) are higher and exhibit an S-shaped
relation with the molar silanol-to-amine ratio. The observed TOF increases rapidly for a molar
silanol-to-amine ratio in the range from 0.5 to 1.1 and reaches a plateau around 1.7. A further
increase in the silanol-to-amine ratio does not lead to increase in the catalytic activity. This
suggests that when an excess of silanol groups are present, no amines are left which are not
promoted [55].
According to the reaction mechanism in Figure 26, the activity of all amines is promoted if all
amines have one silanol as neighbour. This should result in the catalytic activity reaching a
plateau at a molar silanol-to-amine ratio of one. But as can be seen in Figure 25, the plateau is
only reached for a slightly higher silanol-to-amine ratio. This indicates that at a molar silanol-to-
amine ratio of one, not every amine has a silanol next to its location and the actual arrangement
on the surface deviates from an ideal draughtboard pattern. This is explained because hydrogen
bonding can already occur between two amines in the catalyst synthesis mixture. If sufficient
amines are present in the synthesis mixture, they can be considered to move in a clustered
manner during the grafting procedure and will be positioned next to each other on the surface.
This model was confirmed by experimental data [55].
To obtain a better understanding of the heterogeneously catalyzed reaction and in particular,
the surface intermediates that are potentially formed, Lauwaert et al. [55] investigated the
homogeneous reaction between acetone and n-propylamine by using in situ Raman
spectroscopy. Based upon the experimental observations, a reaction mechanism is proposed for
Introduction 30
the primary-amine-catalyzed aldol condensation in the presence of silanol groups. The
promoting effect can be assigned to the hydrogen-bridge interactions between the carbonyl
moieties in the reactants and the silanol groups. As a result, the reactants become more
susceptible to nucleophilic reactions and the formation of intermediates (1b) and (3b) is
facilitated [55]. The promoting effects of the silanol groups are indicated by (1a) and (3a) in
Figure 26.
Figure 26: Proposed reaction mechanism for the aldol condensation of 4-nitrobenzaldehyde with acetone in the presence of the promoting silanol groups [55]
In 2015, Lauwaert et al. [56] synthesized different acid-base catalysts by functionalizing an
SBA-15 support with (3-iodopropyl)trimethoxysilane. The iodo-group is subsequently replaced
with the desired functional groups, i.e. a secondary amine and an acid site separated by one
carbon atom (see Figure 27). In this work, the effect of the acid strength of the promoting site is
investigated while carefully controlling the arrangement of the sites because the iodosilane
cannot form hydrogen bonds, it does not have any tendency to be grafted in a clustered manner.
The low average site density indicates that the amines will most likely interact with the acid
groups on the same linker and potentially also with surrounding silanols, but not with the acid
groups on a neighbouring linker.
Introduction 31
Figure 27: Schematic representation of the synthesis of SBA-X with X referring to the functional group, i.e. 'I' for the iodo-group, 'A' for the secondary amine, 'AL' for the alcohol group,
'CA' for the carboxylic acid and 'PA' for the phosphoric acid [56] To investigate the intramolecular acid strength effects on the cooperativity, the activity of the
newly synthesized materials were assessed through aldol condensation experiments with 4-
nitrobenzaldehyde and acetone at 45 °C. Incorporating an intramolecular alcohol on the amine
site increases the TOF, indicating that in this case, the active and promoting sites are in a more
favourable configuration to catalyze the aldol condensation (compared to a structure in which
only intermolecular surface silanols promote the activity of an amine site). This observation can
be attributed to the joint movement of the amine and promoting alcohol site when they are
located on the same linker. Due to this joint movement, the amine and alcohol can bend together
toward a neighbouring silanol on the silica surface, resulting in two promoting sites in the
vicinity of the amine active site, i.e., the intramolecular alcohol function and the intermolecular
surface silanol. Subsequently, the two reactants can simultaneously be activated by the
formation of a hydrogen bond instead of the consecutive activation when there is only one
promoting site in the vicinity of the amine and lead to an increase in catalytic activity.
However, changing the alcohol to a stronger acid site such as a carboxylic acid or a phosphoric
acid decreases the TOF, confirming that stronger acid sites have a negative impact on the activity
of the amines. These results suggest that the optimal promoting sites for the aldol condensation
are H-bond donors, such as alcohols and silanols, and not strong acid sites.
Introduction 32
Lauwaert and co-workers concluded that materials combining the amine with an alcohol exhibit
the highest TOFs, indicating that bringing an alcohol in close proximity to the amine is beneficial
for the cooperativity between the two sites, on the condition that the inclusion of such
intramolecular alcohol functions does not significantly increase steric hindrance [56].
1.4.2 Effect of the base functionality type
In 2010, Kubota et al. [57] used the same SBA-15 material as silica support for the
immobilization of primary, secondary and tertiary amines. In a post-synthetic modification, the
mesoporous silica SBA-15 was dissolved in toluene and after adding the corresponding
aminoalkyl groups, the mixture was stirred under reflux for 2 hours. The catalytic activity of
these materials was tested in the aldol condensation of 4-nitrobenzaldehyde with acetone at
30 °C. Both the primary (TOF = 4.2 h-1) and secondary (TOF = 26.0 h-1) amine-immobilized
SBA-15 achieved nearly 100 % conversion after 6 hours. The very high activity displayed by the
secondary amine-immobilized SBA-15 material is probably due to the facile formation of
enamine, which is a key reaction intermediate. However, the reaction catalyzed by a tertiary
amine-immobilized SBA-15 resulted in almost no reaction.
This group also investigated the effect of SBA-15 as an additive on the amine-catalyzed aldol
reaction. When only the homogeneous, organic bases were used, the reactions barely reached a
few percentages of conversion after 6 hours. Surprisingly, the activity increased significantly by
simply adding SBA-15 to the secondary amine, forming a physical mixture of the organic base
and the mesoporous silica. For the primary amine, the activity did not change much when
SBA-15 was added; while the use of tertiary amines, either with or without SBA-15, led to almost
no product [57].
Table 3: Effect of SBA-15 as an additive on the amine-catalyzed aldol reaction [57]
Catalyst Additive Time [h] Total conversion [%]
Primary amine None 6 1
SBA-15 6 5
Secondary amine None 6 1
SBA-15 1 26
SBA-15 6 >99
SiO2 1 18
Tertiary amine None 6 0
SBA-15 6 1
None SBA-15 6 0
Introduction 33
On the other hand, in the absence of any amine functionality SBA-15 was totally inert, which
indicates that high activity is only realized when the reaction is carried out in the presence of
both the amine and silicate additive. Thus, besides the presence or absence of SBA-15, the
activity of this catalytic system is dependent on the type of amine used.
The reaction was also performed with a mixture of the secondary amine and amorphous SiO2.
These results indicate that a periodical structure has advantages over an amorphous structure.
Due to a preferable arrangement and direction of the silanol group on the surface of SBA-15, the
rate-enhancement ability per one surface silanol in an ordered mesoporous silica is higher than
that in amorphous silica. To verify that the silanol group of the SBA-15 material is involved in
the mechanism, end-capping of the silanol groups by trimethylsilylation was carried out. Adding
this mesoporous material to a reaction mixture with a secondary amine caused a significant
decrease in the reaction rate. After 6 hours the conversion was around 30 % (compared to more
than 99 % conversion with the non-trimethylsilylated SBA-15). This indicates the importance of
an acid-base cooperative system [57].
By comparing the results of the amine-SBA-15 mixed system and amine-immobilized SBA-15
after 30 minutes of reaction, the effect of immobilization could be investigated. For the primary
amine, the conversion increased from 3 % in the mixed system to 24 % in the immobilized
system, while for the secondary amine, the conversion increased significantly from 15 % in the
mixed system to 92 % in the immobilized system. This indicates that immobilization results in
much better enhancement of the reactivity than simple physical mixing. Also the reusability of
the amine-immobilized mesoporous silica was tested. Therefore, the mixture was filtered and
the catalyst was washed thoroughly with benzene and recovered. X-Ray Diffraction confirmed
that the framework structures of the catalysts didn’t change after the reaction. The recycled
amine-immobilized materials retained their very high conversions and a constant yield of the
aldol products [57].
In the case of a primary amine-functionalized mesoporous silica nanoparticle (AP-MSN),
Kandel et al. [47] observed an inhibition of the reaction kinetics at high concentrations of 4-
nitrobenzaldehyde. They suggested that the catalytic sites of the material could be blocked by
the formation of a stable Schiff base (see Figure 28), which not only eliminates active sites but
also blocks diffusion in the pores.
Introduction 34
Figure 28: Formation of a Schiff base between 4-nitrobenzaldehyde and the aminopropyl group of AP-MSN [47]
By comparing the infrared spectra of the functionalized material before and after the reaction,
they confirmed the formation of an imine intermediate, which was stable even after washing and
drying the material. This suggested a chemical transformation of the aminopropyl group rather
than only physisorption of 4-nitrobenzaldehyde to the surface of the particles. Comparison of
the nitrogen content of the material before and after the reaction by elemental analysis revealed
that approximately 70 % of amine groups formed the imine (the amine loading varied from 1.0
mmol/g before reaction to 1.7 mmol/g after the formation of the stable intermediate).
In 2014, Lauwaert et al. [55] proposed a reaction mechanism for the primary-amine-catalysed
aldol condensation between 4-nitrobenzaldehyde and acetone. Figure 29 represents a further
elaborated version of this reaction mechanism which was used to determine the effect of the
amine structure and base strength [58].
Figure 29: Proposed reaction mechanism for the aldol condensation of 4-nitrobenzaldehyde with acetone [58]
Introduction 35
When no promoting silanol groups are available, the first step of the reaction mechanism is the
formation of an enamine (1) through a nucleophilic addition of acetone to the amine active site.
This happens in two steps: first a carbinolamine is formed which, subsequently, dehydrates with
formation of the enamine. In the case of a primary amine the carbinolamine can also dehydrate
with formation of an imine (3) which inhibits the further reaction (see Figure 30). For the actual
aldol condensation to take place, the enamine reacts with 4-nitrobenzaldehyde yielding an
iminium ion (2). The primary reaction product (aldol) is formed by a water-assisted desorption
of this iminium ion. Finally, the aldol product releases water and forms the secondary reaction
product (ketone). In the presence of promoting silanol groups, the nucleophilic reactions will be
facilitated due to the formation of hydrogen-bridge interactions between carbonyl moieties and
the silanol groups, represented by the species indicated by (1a), (2a), (3a) and (4a) [58].
Figure 30: Nucleophilic addition of acetone to a primary amine [58] In the present work an acetone excess has been used. As a result, the direct interaction between
4-nitrobenzaldehyde and the primary amine is suppressed. Therefore, the formation of the
inhibiting imine (3) from 4-nitrobenzaldehyde can be considered limited. In order to investigate
the effect of the catalyst properties on the aldol condensation, all catalysts were subject to
identical operating conditions. As can been seen in Table 4, pronounced differences in turnover
frequency are observed as a function of the base that was grafted on the silica.
Introduction 36
Table 4: Turnover frequencies obtained with both the unpromoted base as the corresponding cooperative acid-base catalysts [56, 58]
Amine active site Type of amine TOFunpromoted (s-1) TOFpromoted (s-1)
Aminopropane Primary 2.0 *10-4 7.80 *10-4
Methylaminopropane Secondary 1.0 *10-3 3.30 *10-3
Ethylaminopropane Secondary 3.61*10-5 1.41*10-4
Cyclohexylaminopropane Secondary 9.6 *10-6 3.1 *10-5
Phenylaminopropane Secondary - 7.0 *10-6
Diethylaminopropane Tertiary 1.2 *10-5 3.9 *10-5
Primary amines exhibit a reasonable turnover frequency. While both cyclohexyl and phenyl
substituents lead to lower values of the turnover frequency; methylaminopropane resulted in
the highest turnover frequencies, clearly indicating the important effect of the substituent on
secondary amines. In case of a secondary amine the formation of any imine is prevented due to
the lack of a second hydrogen on the nitrogen, resulting in a higher reaction rate for this type of
amine. By changing the substituent of a secondary amine from a methyl to an ethyl group, the
TOF is lowered with about one order of magnitude. This suggest that steric hindrance matters
even for relatively small substituents. With tertiary amines extremely low TOF values were
observed. Because for tertiary amines, the formation of the carbinolamine intermediate is no
longer possible due to the absence of hydrogen atoms on the nitrogen atom, explaining the low
activity of the tertiary amine [56, 58]
The data reported in Table 4 clearly show how the activity exhibited by amine groups is
enhanced by the presence of silanols, which again confirms the cooperativity between the acid
and base sites. In their previous work Lauwaert and co-workers [55] showed that this
cooperative effect is also depended on the arrangement of the active site on the catalyst surface.
As already explained, primary amines tend to be positioned in a clustered manner on the silica
surface due to hydrogen bonding in the synthesis mixture. On the other hand, the precursor
leading to a secondary amine doesn't have this tendency and shows no significant deviations
from ideality. Leading to a secondary amine being grafted in a random manner on the silica
surface. As we can see in Figure 31, this results in a higher percentage of promoted secondary
amines in the silanol-to-amine ratio of 0 to 1.7 in comparison to the primary amines.
Introduction 37
Figure 31: Percentage of amines promoted as a function of the silanol-to-amine ratio; Diamonds: catalysts functionalized with primary amine;
Squares: catalysts functionalized with secondary amine [58] At high conversions, a selectivity towards the secondary ketone product of about 16 % is
obtained with the aminopropane active site while a selectivity of about 10 % is obtained with
the methylaminopropane active site. Hence, the nature of the active site seems to influence the
obtained product distribution.
Although many primary, secondary and tertiary amines have similar base strengths, these
amines exhibit significant differences in catalytic activities for the aldol condensation.
Cyclohexylaminopropane has the highest base strength and it is unable to form the inhibiting
imine, but the active site showed a low activity. This can be explained by steric hindrance
induced by the bulky cyclohexyl group close to the amine, increasing the activation energy of the
reaction. They concluded that a secondary amine with a small substituent avoiding steric
hindrance seems to be the optimal amine type to catalyze the aldol condensation between 4-
nitrobenzaldehyde and acetone [58].
Introduction 38
1.4.3 Effect of the distance between the base and acid functionality
Although efficient dual activation promotes the reaction, the acid and base catalytic components
can also interact in a non-productive manner based on their relative strength and proximity,
quenching the activating behaviour of the individual sites; and rendering the catalyst inactive if
the acid and base sites strongly self-associate [50, 59].
Yu et al. [49] reported the synthesis of bifunctional catalysts with controlled spatial arrangement
of the functional groups by controlling the steric hindrance. The authors tried to immobilize
different amine groups of lysine on mesoporous silica. The acid-base distance can be adjusted
within a range of one to five carbon atoms, to prevent the mutually destruction of the acidic site
and basic site. This resulted in two materials: proximal-C-A-SBA-15 and maximum-C-A-SBA-15,
which are shown in Figure 32.
Figure 32: Proximal-C-A-SBA-15 and maximum-C-A-SBA-15 [49] The synthesized materials were tested in the aldol condensation of 4-nitrobenzaldehyde with
acetone at 50 °C and their catalytic properties are shown in Table 5.
Table 5: Catalytic properties [49]
Catalyst a A [%] B [%] Total conversion b [%] TON c
Proximal-C-A-SBA-15 71 19 90 7.4
Maximum-C-A-SBA-15 57 17 74 6.0
a 0.5 mmol 4-nitrobenzaldehyde in 10 mL acetone stirred at 50°C for 20h. b Conversion calculated by 1H
NMR spectroscopy analysis. c Number of moles of aldehyde converted per 1 mol of -COOH.
While containing almost the same amount of lysine, proximal-C-A-SBA-15 showed a higher TON
value than maximum-C-A-SBA-15, indicating that the rate per active site was enhanced by the
proximal acid-base distance. The activity of the catalysts decreased as the acid-base distance
increased, suggesting that cooperative surface catalysis relies on the two functional groups
being close enough to each other on the surface of the mesoporous material to interact with
each other and with the reacting molecules. A cooperative mechanism was suggested by Yu and
Introduction 39
coworkers. In Figure 33, the dual activation of electrophiles and nucleophiles at the acidic sites
and the basic sites is elaborated. The aldehyde gets activated by the acidic sites, and acetone is
deprotonated to generate imine formation by the basic sites, which then attacks the electron-
deficient aldehyde to generate the aldol product [49]. Compared to the mechanism proposed by
Zeidan and co-workers (Figure 23) the latter seems to be too simplistic, given the new
information. By including possible surface intermediates and cooperative interactions, this
mechanism is more comprehensive, more complete. Note that in Figure 33 the molecule
assigned with the asterisk is missing three carbon atoms to be correct.
Figure 33: Proposed reaction mechanism for the aldol condensation reaction on the solid support [49]
Introduction 40
While the importance of tuning the acid strength has been demonstrated, the impact of the
length and flexibility of amines in their cooperative interactions with the silanols had not yet
been studied. Brunelli et al. [53] investigated the effect of the spatial separation of the amines
and silanols by systematically varying the alkyl chain length of the organosilane grafted onto a
well-defined SBA-15 mesoporous silica support. Using a set of materials functionalized with
alkyl linkers varying from methyl to pentyl (see Figure 34), they revealed that the cooperativity
between the amine and silanols could be tuned by controlling the linker length [53, 60].
Figure 34: Aminosilanes grafted onto SBA-15 with controlled linker lengths [53] The materials were tested for their catalytic efficiency in the aldol condensation of 4-nitro-
benzaldehyde with acetone at 50 °C. The linker length had a significant impact on the observed
reactions rates, as can be seen in Figure 35. The observed rate increased with the number of
carbons up to the propyl chain. Increasing the linker length to butyl or pentyl, resulted in
identical catalytic rates, suggesting that amine-silanol quenching reactions do not occur and that
a propyl linker is sufficiently long to allow for effective cooperative reactions [53].
Figure 35: Conversion of 4-nitrobenzaldehyde in the aldol condensation with acetone at 50 °C for the catalysts with different alkyl linkers [53]
Introduction 41
The material with the shortest chain (C1) showed an interesting behaviour. The conversion
rapidly increased to 10 % within 1 hour, after which the conversion increased slightly over time.
This suggests a relatively rapid reaction of the aldehyde with the amine sites on the silica
surface. But the single methylene linker does not permit sufficient flexibility of the amine site
to allow the reaction to proceed through a silanol-activated pathway. Instead, the reaction
proceeds much slower, as it would for a base-only catalyzed reaction at a rate comparable with
homogeneous 3-propylamine [53, 60].
With an intermediate linker length (C2), the linker enabled a portion of the amines to act
cooperatively with the silanol groups. The increase in catalytic activity with the longer linkers
suggests that a certain degree of flexibility of the alkyl linker is required before the amine and
silanols can efficiently cooperate [60].
These results are in contrast with the work of Yu et al. [49] for the heterogeneous carboxylic
acid and amine system, where the additional flexibility associated with longer chains between
the functional groups resulted in decreased activity. This difference in behaviour may suggest
that the silanols on the surface of the silica are sufficiently weak (pKa around 7.1), such that they
do not protonate the amines, unlike the stronger carboxylic acid (pKa around 5). Therefore,
silanols may behave more like a hydrogen-bonding partner than an acid in these catalysts [60].
Another important aspect of the amine-silanol cooperativity is the curvature of the surface. With
an average carbon-carbon bond length of 0.154 nm, the aminosilanes will approximately reach
0.5 nm into the pore. This means that for the SBA-15, with a pore diameter of 6.5 nm, the surface
appears relatively flat on the length scale that the aminosilane can extend. So by changing the
pore diameter, a significant change in the cooperativity between the amine and silanol groups
should be expected. Brunelli et al. [60] synthesized a mesoporous material with a much smaller
pore diameter (2.3 nm) and noticed that the catalytic cooperativity of the intermediate linker
(C2) increased when placed inside the small pore diameter material and was equivalent to that
of the C3 linker in the larger pore diameter catalyst. Changing the curvature of the pores brought
the aminosilane into closer proximity with the surface silanols, changing the flexibility
requirements of the linker to achieve high catalytic activity [60].
Introduction 42
1.4.4 Effect of the reaction medium
It is well-known that the selection of solvents can have an important effect on the rates of
reactions. Such an effect is explained by the contribution of solvation energy to the total free
energy of the systems and by stabilization of the transition states with a subsequent reduction in
the free energy of activation. The solvent effects can change preferences for various possible
pathways over the potential energy surface of the reaction system [48].
Zeidan et al. [45] suggested that the acid and base groups in the mesoporous material would be
in equilibrium between the free acid and base and the ion pair that results from neutralization;
and that the solvent in which the reaction is performed, would have an important effect on this
equilibrium. To examine the effect of solvent polarity on conversion, the reactions were carried
out in a variety of co-solvents of which the results are summarized in Table 6. In polar, protic
solvents, such as water and methanol, the equilibrium lies towards the ion pair because proton
exchange is rapid and the protic solvent will stabilize the ion pair the most. Leading to only a
small conversion around 30 % after 20 hours. The conversion more than doubles to about 70 %
upon moving to polar, aprotic solvents, such as diethyl ether and chloroform. Non-polar, aprotic
solvents, such as hexane and benzene, cause slower exchange of the protons, thus moving the
equilibrium in favour of the free acid and base; giving further improvements in conversion to
about 90 % in the case of hexane.
Table 6: Solvent effect on the catalysis [45]
Cosolvent (1:1) A [%] B [%] Total conversion [%]
Water 27 7 34
Methanol 14 9 23
Diethyl ether 52 13 65
Chloroform 60 9 69
Hexane 75 13 88
Benzene 62 12 74
Mesoporous silica nanoparticles (MSNs) functionalized with primary amines are poor catalysts
for the aldol reaction in hexane because of the formation of an imine intermediate. This low
activity of AP-MSN was overcome by using a secondary amine-functionalized material (MAP-
MSN) where the methyl group on the aminopropyl prevented the formation of imine and gave a
3-fold increase in the apparent rate constant in hexane [48].
Introduction 43
However, switching the reaction medium between hexane and water led to a reversal in
activities between the primary and secondary amine-functionalized material. These differences
in behaviour are not the result from the solvents directing each catalytic reaction through
different pathways. In order to provide a more in-depth explanation, Kandel et al. [48]
investigated the different effects of the solvent properties on the catalytic activity.
Corresponding to the findings by Zeidan and co-workers [45], the polarity of the solvent had a
negative effect on the activity of bifunctionalized materials. As polarity increases, proton
transfer between the acidic silanols and the basic amine can take place, reducing the availability
of the deprotonated amine required to perform enamine catalysis. Because the reaction
catalyzed by AP-MSN in water doesn't follow this trend, the enhanced activity of the catalyst in
this solvent cannot be due to polarity.
The solvent can also have an effect on the different equilibria involved in the reaction. Water can
be considered as a reagent in the last step (part 3b of Figure 26), where it combines with the
enamine intermediate to form the aldol product. Thus, in the overall conversion (as shown in
Figure 36), water is a product only in the inhibition route (red) but is not part of the net
reactants or products of the aldol route (black). Using water as a the reaction solvent, shifts the
overall equilibrium toward the formation of the aldol product and minimizes the inhibition
pathway. Kandel and co-workers concluded that the catalytic activity of both functionalized
materials in water should be a balance between the inhibitory effects of polarity and the
promoting effect of the solvent on equilibria [48].
Figure 36: Overall reaction pathways for 4-nitrobenzaldehyde and acetone in the presence of AP-MSN: inhibition (red) and aldol reaction (black) [48]
Nonetheless, it has to be mentioned that in the above conclusion it was assumed that the aldol is
the main product of the reaction. If the most abundant product were the enone or enal, water
would be formed as a by-product of the reaction and as a consequence would play no role at all
in shifting the equilibrium. However, analysis of the product distribution revealed that the aldol
was the major product for the reactions with selectivities around 80 % to 90 %.
Introduction 44
1.4.5 Effect of the hydrophobicity of the support
In 2014, Zhe An et al. [61] synthesized mesoporous silicas with alternating hydrophobic and
hydrophilic blocks in the pore wall by tailoring the molar ratio of 1,4-bis(triethoxysilyl)benzene
(BTEB) and tetraethylsilicate (TEOS). Both materials also contained 10 mol% of 3-
mercaptopropyl-trimethoxysilane (MPTMS). The materials were abbreviated as HHB-B8T2 and
HHB-B5T5. For BxTy, x/y represents the molar ratio of BTEB and TEOS. To compare the
catalytic activity of these materials, they also prepared mesoporous silicas with a hydrophobic
surface (containing only BTEB and MPTMS) and mesoporous silicas with a hydrophilic surface
(containing only TEOS and MPTMS). The mercaptopropyl group was later used to functionalize
these materials with L-proline (using chloromethylstyrene as the linker) via a thiol-ene click
reaction (see Chapter Materials & methods).
The catalytic asymmetric aldol reaction of 4-nitrobenzaldehyde with cyclohexanone at 25 °C was
performed. When cyclohexanone was used as solvent, the mesoporous silica with hydrophobic
surface gave a 58 % yield in 24 hours. With HHB-B8T2-Pro and HHB-B5T5-Pro the reaction
resulted in a yield of respectively 42 % and 27 %. For the material with hydrophilic surface, no
reaction was observed (< 3 % in 48 hours). The catalytic reaction was then performed in water.
For the hydrophobic surface, the yield increased to 96 % in 24 hours. HHB-B8T2-Pro and HHB-
B5T5-Pro respectively gave 83 % and 48 % yield. Also the hydrophilic surface afforded a higher
yield of 18 % in 24 hours.
Figure 37: Presentation of a mesoporous silica with alternating hydrophobic and hydrophilic blocks in the pore wall and the different interfaces inside the channels [61]
Therefore, the catalytic activity relies both on the surface properties of the catalyst solids as well
as on the reaction medium. The superior activity of hydrophobic or hydrophobic and hydrophilic
Introduction 45
alternating catalyst to hydrophilic catalyst can be attributed to the synergistic effects of the
hydrophobic blocks in their channels. In neat condition, these channels advanced the access of
organic reactants to the catalytic sites, hereby accelerating the reaction. In aqueous medium, see
Figure 37, the mesoporous channels were able to attract the organic reactants from the water
and constrain them, increasing the reactant concentration around the catalytic sites [61].
1.5 Scope of the thesis
The aldol condensation of furfural with acetone will be performed in cooperation with the thesis
of Elien Laforce. In comparison with 4-nitrobenzaldehyde, furfural doesn’t contain an electron-
withdrawing group, resulting in a lower reaction rate. Higher reaction temperatures are
required to obtain significant conversions. In industry, the product stream of furfural contains
water. Therefore the hydrophobicity of the catalyst support will be tuned during the synthesis.
Recent research indicated that periodic mesoporous organosilicas (PMOs) have a greater
activity in the aldol condensation than functionalized silica materials. In this thesis, a series of
PMO materials will be synthesized using vinyltriethoxysilane (VTES), tetraethyl orthosilicate
(TEOS), 1,2-bis(triethoxysilyl)ethane (BTEE) and 1,4-bis(triethoxysilyl)benzene (BTEB) as
precursors. Different ratios of these precursors are added during the synthesis to vary the
hydrophobicity of the support.
Afterwards, the vinyl group of VTES will be used to functionalize the PMO materials with
cysteine via a thiol-ene click reaction. After sufficient washing procedures to remove any
remainings of physisorpted cysteine, the functionalized PMO materials are characterized via
elemental analysis to determine the amine loading. The mesoporous structure is verified via
nitrogen physisorption and X-Ray diffraction.
The aldol condensation of 4-nitrobenzaldehyde with acetone will be performed to compare the
catalytic activity of these bifunctional materials. The reaction will also be performed in the
presence of water to investigate the influence of the hydrophobicity of the support on the
catalyst's activity. The used catalyst will be recovered to test the reusability of these
heterogeneous catalysts.
Introduction 46
Materials and methods 47
Chapter 2
Materials and methods
This chapter describes the techniques and procedures which were employed for the
characterization of the synthesized Periodic Mesoporous Organosilicas and the analysis of the
catalytic experiments. Also the specifications of the used devices will be mentioned.
2.1 Materials
1,2-bis(triethoxysilyl)ethane (abcr, 97 %), 1,4-bis(triethoxy-silyl)benzene (abcr, 95 %),
2-Hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, Sigma Aldrich, 98 %),
4-nitrobenzaldehyde (Acros Organics, 99 %), Acetone (Acros, 99.6 %, ACS reagent), Acetone
(Chem-Lab NV, 99+%), Acetonitrile (Acros, for HPLC), Brij S10 (Sigma Aldrich, Mn ~711),
Disolol (Chem-Lab NV, 97 %, 3 % IPA), Ethanol (Fiers, 96 %, 2 % MEK + 2 % IPA), Furfural
(Sigma Aldrich, 99 %), KCl (Roth, >99.5 %), L-cysteine (Acros Organics, 99+ %), methyl 4-
nitrobenzoate (Sigma Aldrich, >99 %), n-Hexane (Acros Organics, pure), Tetraethyl orthosilicate
(abcr, 98 %), Toluene (Acros Organics, 99.8 %), Vinyltriethoxysilane (abcr, 98 %).
2.2 Elemental Analysis
Elemental analysis, also called CHNS-analysis, is a qualitative and quantitative analysis which
allows the determination of the composition (in percentages) of the elements carbon, hydrogen,
nitrogen and sulphur. This measurement provides absolute values with a very high accuracy and
precision for the main elements in organic and inorganic compounds.
First the sample is weighed into a tin bowl and is placed in a combustion furnace with an excess
of oxygen. This very high concentration of oxygen is needed for the combustion of the sample.
During the combustion, the elements which have to be analyzed are converted into gaseous
components: CO2, H2O, NOx and SO2. With the aid of the carrier gas, helium, the gas mixture is
transported to a gas chromatograph. Here, the gases are separated from each other and the
detection of the individual components can take place [62].
The elemental analysis was realized by means of a Thermo Flash 2000 device. During the
measurement, V2O5 was added as a catalyst.
Materials and methods 48
2.3 Nitrogen physisorption
Nitrogen physisorption is a technique to determine the specific surface area, the pore size, the
pore volume and pore size distribution. But it also gives insights into the shape of the pores.
An amount of nitrogen gas adsorbs at 77 K, a constant temperature at which the adsorption is
only dependent on the pressure and on the interaction between the adsorbent and the
adsorbate. Based upon the strength of this interaction, all adsorption processes can be divided
into two categories, namely physisorption and chemisorption. If the cross-section of the
adsorbed gas molecule is known and one can determine how much gas is needed to obtain a
monolayer on the surface of the sample, then one can quite easily calculate the surface area [29,
32, 63]. Plotting the amount of adsorbed nitrogen gas versus the relative pressure result in
adsorption isotherms which, according to Brunauer, Deming, Deming and Teller (BDDT), can be
divided into five types, as shown in Figure 38.
Figure 38: Different types of adsorption isotherms [63] Type I isotherms are encountered when adsorption is limited to one molecular layer. This
condition is fulfilled when chemisorption or physisorption at microporous adsorbentia occurs.
This adsorption isotherm is also often referred to as the Langmuir isotherm. Type II isotherms
are most frequently encountered when adsorption occurs on non-porous or macroporous
materials. The inflection point of the isotherm (point B) usually occurs near the completion of
the first adsorbed monolayer. With increasing relative pressure, second and higher layers are
completed until, at saturation, the number of adsorbed layers becomes infinite. Type III and
Type V isotherms are a variation on type II and type IV respectively. The adsorption is facilitated
by interactions with pre-adsorbed molecules, and hence, multilayer adsorption occurs before
monolayer adsorption is complete.
Materials and methods 49
Type IV isotherms apply to mesoporous materials. The isotherm can be divided into different
regions, as can be seen in Figure 39. First, at low relative pressures, the micropores are filled
with nitrogen. From the location of point A, the micropore volume can be determined. With
increasing relative pressure, nitrogen molecules are starting to adsorb on top of each other.
During the formation of these multilayers, the amount of adsorbed gas is more or less linearly
related to the increase of the relative pressure. This region (point A-B) of the adsorption
isotherm will be used to calculate the specific surface area via the BET theory. At even higher
relative pressures, the adsorbed molecules stack until the nitrogen molecules will condense. The
result of this capillary condensation (point B-C) is a strong increase in the amount of adsorbed
gas. The slope of this increase determines the pore size distribution. The steeper the increase,
the more pores with similar diameters are filled at the same time and thus is an indication for
the uniformity of the material. When the capillary condensation is completed, the pores are
completely filled with liquid adsorbate, so that no further adsorption is possible, and we will see
a plateau in the isotherm. The height of this plateau determines the total pore volume. A typical
hysteresis can be seen: the desorption of the adsorbate occurs at lower relative pressures than
the adsorption. This is the consequence of a difference in condensation and desorption enthalpy.
The form of the hysteresis is very important, as it predicts the shape of the pores [32, 63].
Figure 39: Type IV isotherm of SBA-15 [32] In order to determine the specific surface area of a porous material, the BET theory (Brunauer,
Emmett and Teller) and the following BET-equation [64] are used:
𝑧
(1 − 𝑧)𝑉=
1
𝑐𝑉𝑚𝑜𝑛+
(𝑐 − 1)𝑧
𝑐𝑉𝑚𝑜𝑛
Materials and methods 50
in which,
z (= p/p°) is the relative pressure;
V is the adsorbed volume at pressure p;
Vmon is the volume corresponding to monolayer adsorption;
C is a BET constant.
When the left hand side of the equation is plotted as a function of the relative pressure (in the
range 0.05 < p/p° < 0.35) a straight line is obtained. The volume corresponding to monolayer
adsorption can be determined from the slope and the intercept of this straight line. If one
assumes that the adsorbate is an ideal gas, the number of moles of adsorbate at monolayer
adsorption can be obtained from:
𝑚𝑚𝑜𝑛 = 𝑉𝑚𝑜𝑛
22400
and with this value, the specific surface area, expressed per gram of adsorbent, can be
calculated:
𝑆𝐵𝐸𝑇 = 𝑚𝑚𝑜𝑛
𝑎 𝑁𝐴 𝐴
in which,
a is the mass of the adsorbent;
NA is the number of Avogadro;
A is the cross-section of one adsorbate molecule (for N2: 0.162*10-18 m²).
The nitrogen adsorption and desorption measurements were performed at 77K on a Tristar
3000 of Micromeritics. Samples were dried at 100°C under vacuum overnight.
2.4 X-Ray Diffraction
X-Ray Diffraction (XRD) is a versatile and non-destructive technique to quickly obtain detailed
structural and phase information of materials. When these X-rays (0.01 - 10 nm) hit the sample
they are scattered in all directions. However, if the material possesses an ordered structure, a
portion of the X-rays can be reflected and undergo constructive interference [62]. This leads to
Bragg's law, which describes the condition on the scattering angle θ for the constructive
interference to be at its strongest:
𝑛𝜆 = 2𝑑 sin 𝛳
Materials and methods 51
In which,
λ is the wavelength of the X-rays;
d is the interplanar distance between two lattice planes;
θ is the scattering angle;
n is an integer called the order of the reflection.
Figure 40: Diffraction of X-rays [65] A selected wavelength from a polychromatic beam of X-rays will only be reflected for a specific
angle. By varying the detection system at an angle of 2θ, various wavelengths can be examined.
The measured intensity of the reflected X-rays is plotted as a function of twice the angle of
incidence, 2θ. The obtained diffractograms are characteristic for certain structures of the
material.
Figure 41: Scheme of the geometry of the XRD device [66] For hexagonal structures, the XRD data can be used to determine the distance between two
adjacent pores, also known as the width of the unit cell a0. And, as can be seen in Figure 42,
Materials and methods 52
in combination with the pore diameter (dp) and the distance between two lattice surfaces (d100),
the wall thickness of the material (e) can be calculated using the following formula.
𝑎0 =2𝑑100
√3
𝑒 = 𝑎0 − 𝑑𝑝
Figure 42: Representation of a hexagonal (P6mm) structure The X-ray diffraction patterns were recorded on a ARL X'TRA Diffractometer of Thermo
Scientific which is equipped with a Cu Kα tube and a Peltier cooled lithium drifted silicon solid
stage detector.
2.5 Diffuse Reflective Infrared Fourier Transform
(DRIFT) Spectroscopy
Infrared spectroscopy is primarily used for structure identification. By radiating light with
appropriate energy into a sample, the molecule will absorb a certain amount of energy and a
transition between two energy levels can be induced. The precise energy difference between
each of these levels is function of the type of chemical bond. Thus, knowledge of these
differences will therefore lead to identification of functional groups. The irradiated infrared rays
can be either absorbed or reflected. The reflection can occur in two different ways: specular
reflectance or diffuse reflectance. With DRIFT, this last type of reflection is used, wherein the
radiation enters into a theoretically infinitely thin layer, and then reflects on a few particles in
the sample. In order to increase the fraction of diffuse reflectance, the sample is finely pulverized
into smaller particles of approximately five micrometers. If the sample absorbs the infrared rays
too much, potassium bromide (KBr) is added to the sample as a non-absorbent matrix [67].
Materials and methods 53
Diffuse Reflective Infrared Fourier Transform (DRIFT) spectroscopy was performed on a hybrid
IR-RAMAN spectrophotometer, namely Nicolet 6700 (Thermo Scientific) with a MCT-A detector
which gets cooled with liquid nitrogen and uses a Graseby Specac diffuse reflective cell.
Measurements were performed under vacuum at 120 °C.
2.6 Aldol condensation of furfural with acetone
The experiments on furfural were performed by Elien Laforce. Silica gel 60 was grafted with a
methyl substituted secondary amine. It was mentioned in the literature study that the
length of the linker has an influence on the catalytic activity. Therefore, N-methylamino-
propyltrimethoxysilane (MAPTMS) was chosen as precursor because Brunelli et al. [53] proved
that a propyl chain leads to the best activity of silanol assisted amine catalysts. After calcination
at 700 °C, to remove the remaining surfactant, a silica material with approximately 1.1 silanol
groups per nm² is obtained. Next, this material is dissolved in dry toluene (99.8 %, Acros
Organics) and MAPTMS is added. The mixture is heated to 120 °C and stirred during 24 hours
under inert atmosphere. By varying the amount of precursor, catalysts with different silanol-to-
amine ratio are obtained. Following the grafting procedure, the catalyst is washed with
chloroform to remove any remainings of physisorpted organic components. Finally, the
bifunctional catalyst are dried at room temperature, under vacuum. The monofunctional
catalysts are prepared by treating the bifunctional catalysts with hexamethyldisilazane
(abcr, 98 %) after which the free silanol groups are covered with trimethylsilane and cannot
longer play a promoting role in the aldol condensation.
The samples of the aldol condensation of furfural and acetone are analysed with a gas
chromatograph. Toluene is used as internal standard. However, no hexane was added as co-
solvent because this would lead to a phase separation with furfural. In the reference reaction,
hexane is added as co-solvent because non-polar, aprotic solvents cause slower exchange of the
protons. Thus moving the equilibrium in favour of the free acid and base, resulting in higher
conversions [45].
2.7 Aldol condensation of 4-nitrobenzaldehyde with
acetone
The activity of each catalyst was assessed via the aldol condensation of 4-nitrobenzaldehyde and
acetone at 45 °C. An acetone excess is used in all experiments in order to suppress the direct
interaction between 4-nitrobenzaldehyde and the amine site which yields an inhibiting imine.
Hexane was added as co-solvent to improve the reaction rate.
Materials and methods 54
The experiments were performed in a 25 mL two-neck round-bottom flask equipped with a
condenser and a septum. First, the functionalized catalyst was added to the flask. To compare
the activities, 4 mol% with respect to the 4-nitrobenzaldehyde concentration was added to
every flask. The reaction mixture was prepared separately by mixing the desired amounts of
acetone (50 vol%), n-hexane (co-solvent, 50 vol%), 4-nitrobenzaldehyde (0.03 mmol/mL) and
methyl 4-nitrobenzoate (internal standard, 0.022 mmol/mL). Afterwards 4 to 8 mL of the
reaction mixture (depending on the available quantity of catalyst) was injected into the flask
which contains the catalyst and the flask was immediately placed in an oil bath at 45 °C. The
moment the flask was placed in the oil bath was taken as the start of the reaction. The reaction
was monitored for 4 hours by taking a sample every 30 minutes (about 100 µL) of the reaction
mixture. Typically 0.9 mL of acetone was used to wash the syringe needle and to transfer the
sample to a vial. Afterwards the catalyst was separated from the sample by means of
centrifugation. Finally, the samples were analyzed using a reversed-phase high-performance
liquid chromatograph (RP-HPLC), from Agilent (1100 series). The HPLC was operated at a
column temperature of 30°C using a gradient method with water (0.1% Trifluoroacetic acid) and
acetonitrile (HPLC grade) as solvents. In this gradient method the volumetric percentage of
acetonitrile is varied from 30% to 62% over a period of 7 minutes. The components were
identified using a UV-detector with a variable wavelength. Quantification of the different
components in the reaction mixture was performed by relating the peak surface areas to the
amount of internal standard, methyl 4-nitrobenzoate [68].
2.8 High Performance Liquid Chromatography (HPLC)
High performance liquid chromatography is a technique used to separate, identify and quantify
the components in a mixture. The mobile phase is a liquid and high pressures are needed to
guide the eluent through the column. In normal phase chromatography, the column is packed
with a hydrophilic, polar stationary phase. In reversed phase chromatography, the column is
packed with a hydrophobic, non-polar stationary phase [62]. The latter was used for the
characterization of the reaction mixture. Separation results from adsorption of hydrophobic
molecules onto the hydrophobic solid support in a polar mobile phase. A mixture of solvents,
water (0.1 % trifluoroacetic acid) and acetonitrile, is used as mobile phase with varying
composition in time to increase the efficiency of the separation. Decreasing the mobile phase
polarity by adding more organic solvent, reduces the hydrophobic interaction between the
solute and the solid support, resulting in desorption. Depending on their interaction with the
stationary phase, the compounds flowing through the column each have a specific retention time
(see Table 7) . The fractions coming off the column are detected and quantified with the use of a
Materials and methods 55
UV detector which measures the difference between the intensity of the incident light, 𝐼0, and the
light transmitted after the detector cell, 𝐼. The intensity of absorption is measured in terms of
absorbance, A, which is assumed to be proportional to the concentration of the analyte, c, and
the optical path length, L. This is combined into the Lambert-Beer law:
𝐴 = log (𝐼0
𝐼) = 𝜀 ∗ 𝐿 ∗ 𝑐
In which,
ε is a constant known as the molar absorptivity at a given wavelength.
The detection sensitivity of a component can be increased by varying the wavelength of the UV
detector. Every component has a characteristic wavelength which will be maximal absorbed.
Quantification of the different components is the reaction mixture was performed by relating the
peak surface areas to the amount of internal standard. Calibration factors are determined by
measuring calibration curves of known mixtures of the different components with the internal
standard. These calibration factors are related to the sensitivity of the detector, and thus are
dependent on the used wavelength. These wavelengths for maximal absorption and the
corresponding calibration factors can be found in Table 7. Samples of the aldol condensation of
4-nitrobenzaldehyde with acetone were analysed on a RP-HPLC, Agilent 1100 Series.
Table 7: Components from the reaction mixture with their retention times, wavelength for maximal absorption and calibration factors
Component Retention time
(min)
Wavelength
(nm)
Calibration
factor
Acetone 1.9 265 759.31
4-hydroxy-4-(p-nitrophenyl)-butan-2-one 4.2 272 1.2936
4-nitrobenzaldehyde 5.9 265 0.9711
4-(p-nitrophenyl)3-butene-2-one 7.1 305 0.5349
4-methylbenzoate 7.8 260 1
2.9 NRTL-HOC code
An existing code based on the Non-Random Two-Liquid (NRTL) and Hayden-O’Connell (HOC)
methods has been modified which allows for an initial estimation of the vapour-liquid
equilibrium. This code accounts for the thermodynamic non-ideality of both the liquid as well as
the gas phase via so-called activity coefficients and fugacity coefficients. The activity of
component 𝑖 in a liquid mixture is calculated from the mole fraction of this component in the
Materials and methods 56
mixture and an activity coefficient, 𝛾𝑖 . The fugacity of a component in a gas mixture is calculated
from its partial pressure and a fugacity coefficient, 𝜙𝑖 [69].
𝑎𝑖𝐿 = 𝛾𝑖𝑥𝑖 (1)
𝑓𝑖𝐺 = 𝛷𝑖𝑝𝑖 (2)
The Non-Random Two-Liquid (NRTL) method [70] is a widely accepted method to calculate
activity coefficients and other thermophysical data of compounds in liquid multi-component
mixtures without the explicit use of experimental data. This method make use of binary
interaction parameters to describe the interactions between two molecules or two functional
groups in the mixture. Equations of state such as Hayden-O’Connell (HOC) [71] are used to
calculate the fugacity coefficient of a gas phase. Several studies showed that the NRTL method in
combination with the HOC method describes the non-ideality and vapour-liquid equilibria of
mixtures of acetic acid, methanol, methyl acetate and water well [69]. The adapted code also
worked well for mixtures of acetone, furfural, toluene and water.
The NRTL model is a popular method to determine thermodynamic data for liquid mixtures. The
model is based on Scott’s two-liquid model [72] and on an assumption of non-randomness. The
activity coefficient can be determined from equation 3. The NRTL model contains three
parameters, 𝑎𝑖𝑗 , 𝑏𝑖𝑗 and 𝑐𝑖𝑗 , which describe the binary interactions between two components in
the liquid mixture and a parameter 𝛼𝑖𝑗 , which describes the non-randomness [69]. These
parameters are obtained from the commercial simulation software Aspen Plus and are given in
Appendix A.
ln 𝛾𝑖 =∑ 𝜏𝑗𝑖𝐺𝑗𝑖𝑥𝑗𝑗
∑ 𝐺𝑙𝑖𝑥𝑙𝑙+ ∑
𝑥𝑗𝐺𝑖𝑗
∑ 𝐺𝑙𝑗𝑥𝑙𝑙(𝜏𝑖𝑗 −
∑ 𝑥𝑟𝜏𝑟𝑗𝐺𝑟𝑗𝑟
∑ 𝐺𝑙𝑗𝑥𝑙𝑙)
𝑗
(3)
𝐺𝑖𝑗 = exp(−𝛼𝑖𝑗𝜏𝑖𝑗) (4)
𝜏𝑖𝑗 = 𝑎𝑖𝑗 +𝑏𝑖𝑗
𝑇+ 𝑐𝑖𝑗 ln(𝑇) (5)
Equilibrium between a liquid and a vapour mixture is established when the chemical potential of
each component 𝑖 in the liquid phase is equal to the chemical potential of that component in the
vapour phase. If the chemical potential of all components is expressed with respect to the ideal
gas phase, as is shown in equation 6, this reduces to an equality of fugacities (equation 7). The
liquid fugacity of component 𝑖, is its activity multiplied with a standard state fugacity. Using the
definitions of fugacities in equations 2 and 8, the ratio of each component in vapour and liquid
phase can be obtained from equation 9. These ratios combined with the molar balances in
Materials and methods 57
equation 10 and the trivial conditions in equations 11 and 12, results in a set of equations which
has to be solved to the unknown 𝑥𝑖, 𝑦𝑖 , 𝑛𝐿 and 𝑛𝐺 [69].
𝜇𝑖(𝑇, 𝑝) = 𝜇𝑖,𝑔𝑎𝑠° (𝑇) + 𝑅𝑇 ln (
𝑓𝑖
𝑓°) (6)
𝑓𝑖𝐿 = 𝑓𝑖
𝐺 (7)
𝑓𝑖𝐿 = 𝑎𝑖
𝐿𝑓𝑖° = 𝛾𝑖𝑥𝑖𝛷𝑖
𝑆𝑝𝑖𝑆 exp (
𝑉𝑖𝐿(𝑝 − 𝑝𝑖
𝑆)
𝑅𝑇) (8)
𝐾𝑖 =
𝑦𝑖
𝑥𝑖=
𝛾𝑖𝛷𝑖𝑆𝑝𝑖
𝑆 exp (𝑉𝑖
𝐿(𝑝 − 𝑝𝑖𝑆)
𝑅𝑇 )
𝛷𝑖𝑝
(9)
𝑛𝑖 = 𝑛𝐿𝑥𝑖 + 𝑛𝐺𝑦𝑖 (10)
𝑋 = ∑ 𝑥𝑖
𝑖
− 1 = 0 (11)
𝑌 = ∑ 𝑦𝑖
𝑖
− 1 = 0 (12)
This set of equations has to be solved in an iterative way since all 𝐾𝑖′𝑠 are function of the liquid
and gas compositions. An algorithm to solve this set of equations, is given in Appendix A.
Materials and methods 58
Valorization of furfural via the aldol condensation 59
Chapter 3
Valorization of furfural via the
aldol condensation
The aldol condensation might have a bright future in the discipline of green, renewable and
sustainable energy, e.g. in the valorization of biocomponents such as glycerol and furfural.
Hemicellulose is a major component of plant cell walls and can be extracted from a wide range
of renewable raw materials. Furfural is an important intermediate during the conversion of
biomass and is produced from the dehydration of xylose, which is one of the main components
of hemicellulose. Furfural is more available than 4-nitrobenzaldehyde and from an industrial
point of view, it's a more relevant reactant to research. For example, furfural can be used as a
solvent for extractive distillation. It can also be used as reactant in the aldol condensation with
acetone, see Figure 43, to produce C8 and C13 alkanes which can be employed as biofuel [3, 73].
Figure 43: Aqueous-phase conversion of sugars and derivatives into liquid hydrocarbon fuels [73]
Valorization of furfural via the aldol condensation 60
The reaction scheme is given in Figure 44. Furfural and acetone react together and form the
aldol product 4-(2-furyl)-4-hydroxybutan-2-on which after dehydration leads to a ketone 4-(2-
furyl)-3-buten-2-on. This ketone can undergo further condensation with furfural, forming a C13
ketone 1,4-pentadien-3-on-1,5-di-2-furanyl.
Figure 44: Aldol condensation of furfural with acetone [26]
3.1 Aldol condensation of furfural with acetone
The first experiment was performed at the same reaction conditions as the aldol condensation of
4-nitrobenzaldehyde and acetone (i.e. a temperature of 45 °C). This reaction will be more
elaborately discussed in Chapter 5. In Figure 45, the conversion as a function of time is shown
for both aldol condensations. The reaction rate of the aldol condensation of furfural with acetone
is much lower. The turnover frequency of this reaction is about one order of magnitude lower
than the aldol condensation of 4-nitrobenzaldehyde with acetone. This lower reaction rate can
be explained by the fact that the strongly electron-withdrawing nitro group of 4-
nitrobenzaldehyde makes the carbonyl function more susceptible for a nucleophilic attack to
form the aldol product. Because furfural doesn’t contain such an electron-withdrawing group,
the aldol condensation will proceed relatively slow.
Valorization of furfural via the aldol condensation 61
Figure 45: Conversion as a function of time for the aldol condensation of furfural with acetone (blue) and 4-nitrobenzaldehyde with acetone (red)
3.2 Optimization of the reaction conditions
In collaboration with Elien Laforce, the optimal reaction conditions for the catalysed aldol
condensation between furfural and acetone was investigated. The molar ratio of the reactants
(furfural/acetone) was varied between 0.004 and 0.124, while a reaction temperature between
60 °C and 100 °C was tested. These experiments were performed in a batch reactor (Parr 4560,
300 mL) in which always 1 mmol/L active sites were added via the catalyst.
In order to obtain higher conversions, higher reaction temperatures (80 °C, 90 °C and 100 °C )
were investigated. When higher temperatures (> 60 °C) are applied, one has to consider the
vapour-liquid equilibria of the reactants. In particular for acetone, which has the lowest boiling
point in the reaction mixture i.e. 56 °C, one has to take into account that a fraction of this
reactant will be in the vapour phase. Due to the higher boiling points of toluene and furfural,
110.6 °C and 161.7 °C respectively, only a very small amount will evaporate. Because the molar
ratio of the reactants has an influence on the reaction rate, it is important to know the exact
composition of the liquid phase after the vapour-liquid equilibrium has been established. Thus,
it is necessary to check how much of each component has to be added to the Parr reactor, in
order to obtain the desired concentrations in the liquid phase.
0
10
20
30
40
50
60
70
80
90
0 20 40 60 80 100 120 140 160 180
Co
nve
rsio
n (
%)
Time (min)
Furfural 4-nitrobenzaldehyde
TOF = 2.70 10-4 s-1
TOF = 2.64 10-3 s-1
Valorization of furfural via the aldol condensation 62
Therefore, an existing code based on the Non-Random Two-Liquid (NRTL) and Hayden-
O’Connell (HOC) methods has been modified which allows for an initial estimation of the
vapour-liquid equilibrium. This code accounts for the thermodynamic non-ideality of both the
liquid as well as the gas phase via so-called activity coefficients and fugacity coefficients. A
detailed description of the NRTL-HOC code can be found in Materials and methods.
The results from this simulation indicate that this system deviates a few percentages from an
ideal mixture. As was expected, only a fraction of the acetone evaporates at higher temperatures.
For toluene and furfural it is correct to assume that everything remains in the liquid phase.
If a reaction temperature of 90 °C is considered, about 1.55 % of the acetone will evaporate,
which corresponds with a pressure increase up to a total pressure of 3.7 bar. The turnover
frequencies as a function of the molar ratio of the reactants (furfural/acetone) are given in
Figure 46. A reverse S-shape curve is obtained with a maximal reaction rate at a molar ratio
(furfural/acetone) of 0.03. These TOF values illustrate that a decent conversion is obtained for
the aldol condensation of furfural with acetone at 90 °C.
Figure 46: Turnover frequencies for the aldol condensation of furfural with acetone at 90 °C as a function of the molar ratio of the reactants (furfural/acetone)
0.0
0.4
0.8
1.2
1.6
2.0
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
TO
F (
x 1
0-3
s-1
)
Ratio furfural/acetone (mol/mol)
Valorization of furfural via the aldol condensation 63
3.3 Effect of water in the reaction mixture
Also the influence of water will be assessed by experiments. This is an important element
because during the dehydration of one mole xylose, one mole of furfural is formed but also three
moles of water are obtained as by-product. Thus, in the industry the product stream of furfural
will always contain water. Therefore for an industrial application of the aldol condensation with
furfural, it would be beneficial for the feasibility if the reaction can proceed at a significant rate
even in the presence of water. If the heterogeneous catalyst can provide a good activity for the
aldol condensation in an aqueous environment, this would eliminate the need for an expensive
removal of water, to obtain pure furfural. For the aldol condensation of furfural with acetone at
90 °C, the decline of the turnover frequency as a function of the volume percentage of water in
the reaction mixture is given in Figure 47. A small amount of 1 vol% or 2 vol% water is already
sufficient to lower the catalytic activity with 23,43 %, respectively 57,17 %.
Figure 47: TOF ratio as a function of the volume % of water in the reaction mixture In this regard, Periodic Mesoporous Organosilicas can be a good solution. During the synthesis of
these materials, one can chose which organosilicas to add. Thus, if one is able to synthesize a
material with hydrophobic pores, the water could be excluded from entering, leading to a
situation in which only the reactants are able to physisorb in the pores. Due to the limited
amount of synthesized PMO materials, these materials were first tested in the aldol
condensation of 4-nitrobenzaldehyde with acetone, as a better comparison with previous
obtained data would be possible.
0.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 10 12
TO
F r
ati
o
Water (vol %)
Valorization of furfural via the aldol condensation 64
Synthesis and characterization of cooperative acid-base PMOs materials 65
Chapter 4
Synthesis and characterization
of cooperative acid-base PMOs
materials This chapter describes the synthesis, functionalization and characterization of Periodic
Mesoporous Organosilicas. These PMO materials will be used as a bifunctional catalyst in the
aldol condensation of 4-nitrobenzaldehyde with acetone (Chapter 5).
4.1 Synthesis procedure
In Figure 48, a general overview of the synthesis procedure is shown. The first step in the
synthesis is dissolving a surfactant (Brij-76) in acidified water. The solution is stirred at 50 °C
during at least 4 hours. In a second step, the organosilica precursors are added dropwise to the
clear solution. The synthesis mixture is stirred at 50 °C during 24 hours and subsequently aged
at 90 °C for 24 hours. Next, the precipitate is filtered on a glass filter and washed with water and
acetone. In the final step, the surfactant is removed via extraction with acidified ethanol. The
surfactant cannot be removed by means of calcination at 400 - 500 °C, as this would result in the
destruction of the organic functionalities. Thus, the material is refluxed three times with
acidified ethanol at 80 °C during 24 hours to remove the surfactant as much as possible.
Figure 48: General overview of the synthesis
Synthesis and characterization of cooperative acid-base PMOs materials 66
During the synthesis four different precursors, which are shown in Figure 49, have been used:
vinyltriethoxysilane (VTES), 1,4-bis(triethoxysilyl)ethane (BTEE), 1,4-bis(triethoxysilyl)benzene
(BTEB) and tetraethyl orthosilicate (TEOS).
a) b)
c) d)
Figure 49: Precursors added during synthesis. a) VTES: vinyltriethoxysilane b) BTEE: 1,4-bis(triethoxysilyl)ethane
c) BTEB: 1,4-bis(triethoxysilyl)benzene d) TEOS: tetraethyl orthosilicate In each synthesis the total amount of silicon atoms has been kept at a constant value while
varying the ratio of these precursors to obtain materials with different properties. This allows an
experimental assessment of the effect of the different types of support and its hydrophobicity on
the catalytic activity.
Because the co-condensation of VTES with BTEE or BTEB was not yet described in literature,
a specific procedure that guarantees the formation of ordered mesoporous materials was not
available. Therefore two types of surfactants have been tested in this work. First Pluronic P123
was chosen as surfactant, because it is reported that this structure directing agent lead to pore
sizes bigger than 5 nm. However, the synthesis resulted in gel-like materials, possibly indicating
that the synthesis conditions were not favourable for the interactions between P123 and the
silica precursors. Different synthesis conditions (temperature, pH, additives such as KCl to
improve the hydrothermal stability during the crystallization process) were tested but no
precipitation of a white solid was obtained. Therefore, a more general procedure described by
Burleigh et al. [43] was tested. This recipe uses Brij-76 as surfactant, which is capable of forming
ordered mesoporous materials at broader synthesis conditions. The final recipe that lead to PMO
materials is given in appendix B.
Si
O
OOSiO
O
O Si
O
O
O
Si
O
O
O Si
O
O
O
O
Si
O
OO
Synthesis and characterization of cooperative acid-base PMOs materials 67
4.2 Functionalization
After the synthesis of these PMO materials, the amino acid functionality is introduced into the
pores via a post-functionalization. In a UV reactor, cysteine is grafted onto the vinyl group of
VTES via a thiol-ene "click" reaction.
4.2.1 Thiol-ene “click” chemistry
Thiol-ene click chemistry is a twofold concept, in which the term "click chemistry" was
introduced in 2001 by Kolb et al. [74]. A "click" reaction is a stereospecific reaction that meets a
set of stringent criteria. The reaction is modular and it can thus be applied to various products.
The reaction produces high yields and if by-products are formed, they are harmless and easily
removable. From a practical point of view, these "click" reactions should be easy to carry out
with readily available starting materials and reagents, with no solvent or a solvent that is benign
or easily removed. Ideally the reaction is insensitive to the presence of oxygen and water. Also,
the product should be isolated in a simple manner [75, 76].
The term "thiol-ene" refers to the addition of a thiol group to an olefin. This hydrothiolation of a
carbon-carbon double bond is a very fast reaction and can be carried out at ambient
temperature and pressure. The thiol-ene reaction is easy and versatile to use and usually takes
place under radical conditions, photochemically induced (shown in Figure 50) or via a thermal
treatment [74, 75].
Figure 50: Reaction mechanism for the hydrothiolation of a C=C bond in the presence of a photo-initiator and the appropriate UV radiation. In the propagation circle R stands for cysteine
Synthesis and characterization of cooperative acid-base PMOs materials 68
In general, the reaction contains an initiation, propagation and termination step. Initiation
involves the treatment of a thiol group with a photo-initiator, which after radiation results in the
formation of a thiyl radical, R-S°. During the propagation, first a direct addition of the thiyl
radical to a carbon-carbon double bond takes place, hereby an intermediate carbon radical is
formed. In a second step, a chain transmission is carried out to a second thiol molecule. This
results in the desired thiol-ene product and a new thiyl radical. The reactivity of the thiol-ene
reaction is strongly dependent on the chemical structure and steric effects of the olefin and the
thiol group [75, 76].
4.2.2 Recipe
For the post-functionalization of these periodic mesoporous organosilicas with cysteine,
2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959) is used as photo-
initiator. The structure of this complex component is shown in Figure 51.
a) b)
Figure 51: Structure of the components used during the post-functionalization a) Irgacure 2959 b) Cysteine
First 0.50 g photo-initiator is dissolved in 10 mL demineralized water and the long vertical
Schlenk flask is placed into a supersonic bath. Meanwhile 0.53 g cysteine is added to a separate
glass bottle with 10 mL demineralized water. The mixture is heated using a heat gun until a clear
solution is obtained. Next, the cysteine and 0.50 g of the PMO material are brought together in
the Schlenk flask. The valve is used to place the mixture under an inert atmosphere with helium
to avoid that the photo-initiator starts reacting with oxygen. Thereafter, the mixture is placed in
the UV reactor (λ = 254 nm) and stirred during 24 hours for the thiol-ene click reaction to take
place. This can be visualized by a change in the colour of the reaction mixture from white to
yellow-brown (see Figure 52). Finally, the solid is filtered and washed several times with warm
water to remove the remaining unreacted or physisorpted cysteine. More details about the
different washing procedures and the evolution of the physisorpted amine concentrations can
be found in Appendix B.
Synthesis and characterization of cooperative acid-base PMOs materials 69
Figure 52: Functionalization of the synthesized PMO materials The structure of the functionalized PMO materials is shown in Figure 53.
Figure 53: Structure of the functionalized PMO material
Synthesis and characterization of cooperative acid-base PMOs materials 70
4.3 First set of materials
A first set of materials were prepared with VTES and BTEE or VTES and BTEB as precursors
(see Figure 54). In both cases the total amount of silicon atoms in the synthesis mixture was kept
at a constant value. The goal of these experiments was to determine the threshold of the VTES
concentration at which a further increase of VTES would lead to a loss of the mesoscopic
ordering of the material.
The exact concentrations of the precursors added in the synthesis are given in appendix B.
Figure 54: Co-condensation of BTEE with VTES or BTEB with VTES
4.3.1 Molar composition of the PMO materials
The molar composition of the different PMO materials is given in Table 8. The letter in the
abbreviation stands for the type of support: E when ethane was used, B when the benzene group
was added. The number indicates the molar percentages of VTES in the global precursor
mixture. Two of these materials, E10 and B10, have been functionalized with cysteine. These
materials are indicated with the letter C added to the abbreviation.
In the next paragraph, these materials are characterized and a comparison between the
materials is made.
Synthesis and characterization of cooperative acid-base PMOs materials 71
Table 8: Molar composition of the PMO materials
Material VTES (mol%) BTEE (mol%) BTEB (mol%)
E10 10 90 0
E20 20 80 0
E30 30 70 0
B10 10 0 90
B20 20 0 80
4.3.2 Characterization
The synthesized PMO materials were characterized with the use of Diffuse Reflective Infrared
Fourier Transform spectroscopy, nitrogen physisorption, X-ray diffraction and elemental
(CHNS) analysis.
4.3.2.1 DRIFT
In Figure 55, typical DRIFT spectra of the ethane type materials are shown. The small peak at
3728 cm-1 is very common for these organosilica materials as it is caused by the silanol groups.
The broad peak between 3700 cm-1 and 3300 cm-1 is due to the presence of adsorbed water on
the sample and overlaps with the carboxylic acid O-H stretch. The N-H stretch of the primary
amine results in a strong peak around 3400 cm-1 – 3250 cm-1. The peaks between 3000 cm-1 and
2850 cm-1 correspond to the C-H stretch. The tiny peak at 1600 cm-1 can be attributed to the
carbon-carbon double bond of E10. This peak isn't visible after the functionalization because the
vinyl group reacts with the thiol group during the thiol-ene click reaction. For the material
E10-C, the peaks of the carbonyl group (1620 cm-1) and the N-H bending of the primary amine
(1590 cm-1) are more dominant. The peaks that appear in the range 1450 cm-1 to 1250 cm-1 can
be attributed to C-H deformation caused by the surfactant which is not entirely removed from
the material. The strong peak around 1100 cm-1 - 1000 cm-1 corresponds to the siloxane bonds.
Lastly, the peaks between 1000 cm-1 and 600 cm-1 can be attributed to Si-C or Si-O-Si vibrations.
Synthesis and characterization of cooperative acid-base PMOs materials 72
Figure 55: DRIFT spectrum of E10 (blue) and E10-C (red)
4.3.2.2 Nitrogen physisorption and elemental analysis
The mesostructure of the materials is confirmed with the use of nitrogen adsorption and
desorption isotherms. The results of the nitrogen physisorption are shown in Table 9.
Table 9: Properties of the PMO materials
Material SBET (m²/g) Vp (cm³/g) a dp (nm) b
E10 983 0.86 3.19
E10-C 487 0.51 3.12
E20 915 0.61 2.68
E30 765 0.38 2.68
B10 971 0.70 3.10
B10-C 456 0.46 3.04
B20 721 0.50 2.75
a The pore volume is determined with the BJH method. b The average pore size is
determined with the BdB-FHH method [77].
As was expected, an increasing amount of VTES has a detrimental effect on the structure and
porosity of the PMO material. The degree of mesoscopic order of the material decreases with
increasing concentration of VTES in the reaction mixture. For the materials containing ethane as
05001000150020002500300035004000
Ku
be
lka
-Mu
nk
(a
.u.)
wavelength (cm-1)
E10 E10-C
Silanol
Water
Si-OSi-O-Si
N-H
C-H
C=C
C=ON-H
O-H
Synthesis and characterization of cooperative acid-base PMOs materials 73
support, the specific surface area decreases from 983 to 765 m²/g, while the pore volume drops
from 0.86 to 0.38 cm³/g. The same trend can be noticed for the materials with benzene in the
pore walls. These negative effects are also observed in the adsorption isotherms. While E10
still displays a type IV isotherm which corresponds to mesoporous materials, E30 has a type I
isotherm which are encountered when adsorption is limited to one molecular layer.
The amine loading of both functionalized materials is determined via elemental analysis and
results for E10-C and B10-C in 0.2870 mmol/g and 0.0692 mmol/g respectively. The specific
surface area and pore volume further decrease as the cysteine is grafted onto the vinyl groups
inside the pores.
4.3.2.3 XRD
The structure of the mesoporous material can be determined with the use of X-ray diffraction.
Figure 56: XRD diffractogram of E10 (blue) and E10-C (red) The XRD patterns in Figure 56 show an intensive reflection at the low 2θ values which
corresponds to the (100) plane. Usually, these PMO materials also exhibit two smaller peaks at
higher 2θ values, corresponding to the (110) and (200) planes. However, these peaks are not
always present which is the case when the planes are not properly aligned to produce a
diffraction peak. In this case only background is observed.
These peaks indicate that the material has a 2D hexagonal (P6mm) mesostructure. The XRD
diffractograms show that the hexagonal ordered mesoporous structure is maintained after the
functionalization with cysteine. A small shift of the (100) reflection towards higher 2θ values is
0.7 1.7 2.7 3.7 4.7
Inte
nsi
ty (
a.u
.)
2ϴ (degrees)
E10 E10-C
Synthesis and characterization of cooperative acid-base PMOs materials 74
observed which can be attributed to a reduction of the pore diameter of the PMO material due to
the introduction of cysteine in the pores and is confirmed by the results of nitrogen
physisorption in Table 9. As explained in Materials and methods, the XRD data can be used to
calculate the width of the unit cell and the wall thickness, these values are presented in Table 10.
Table 10: Physicochemical properties of the synthesized PMO materials
Material d100 (nm) a Unit cell (nm) dp (nm) Wall thickness (nm)
E10 5.39 6.22 3.19 3.03
E10-C 5.31 6.13 3.12 3.01
E20 5.22 6.03 2.68 3.35
E30 5.08 5.86 2.68 3.18
B10 5.32 6.15 3.10 3.05
B10-C 5.08 5.87 3.04 2.83
B20 5.13 5.92 2.75 3.17
a The d-spacing has been determined with the XRD-software WinXRD.
The d100-spacing decreases with increasing amount of VTES, this can be explained by comparing
the chemical structure of the precursors. While BTEE and BTEB have six ethoxy groups, VTES
has only three. Thus, with increasing amount of VTES in the precursor mixture, this results in a
relative smaller amount of siloxane groups in the pore wall after co-condensation. A second
effect that influences the wall thickness is the pore diameter. With increasing amounts of VTES,
the pore diameter decreases because of a loss of the mesoscopic ordering of the material.
Therefore, materials such as E20 en E30 with smaller pore diameters result in a larger wall
thickness.
It is expected that due to the grafting procedure, only the pore diameter would decrease. This
assumption holds true for the material E10. However, the smaller wall thickness of the material
B10-C, compared to the material before grafting, is something what was not anticipated. This
possibly indicates that the PMO material with benzene groups is more sensitive to the grafting
conditions.
Synthesis and characterization of cooperative acid-base PMOs materials 75
4.4 Reference work
In cooperation with the Centre for Ordered Materials, Organometallics and Catalysis (COMOC),
similar PMO materials have been tested for the aldol condensation of 4-nitrobenzaldehyde with
acetone at 45 °C. The results of this reaction and the turnover frequency of the catalysts are
shown in the following figures.
Figure 57: Conversion as a function of time
Figure 58: Comparison of the turnover frequencies at the same conditions
0
5
10
15
20
25
0 20 40 60 80 100 120 140 160 180 200
Co
nve
rsio
n (
%)
Time (min)
Cysteine (0.08 mmol/g)
Cysteine (0.41 mmol/g)
Cysteine (0.84 mmol/g)
0,0E+00
1,0E-04
2,0E-04
3,0E-04
4,0E-04
5,0E-04
6,0E-04
7,0E-04
8,0E-04
9,0E-04
Silica APTES Silica HMDS APTES PMO cysteine(0.84 mmol/g)
PMO cysteine(0.41 mmol/g)
PMO cysteine(0.08 mmol/g)
TO
F (
s-1)
Synthesis and characterization of cooperative acid-base PMOs materials 76
The obtained turnover frequencies are compared to the TOF of a primary amine which is fully
promoted by the presence of silanol groups (Silica APTES) and the TOF of a primary amine
which is unpromoted after treatment of the silanol groups with 1,1,1,3,3,3-hexamethyldisilazane
(Silica HMDS APTES) [58]. The material with a low loading of cysteine exhibits a TOF which is
lower than a primary amine which is fully promoted by surface silanols but higher than an
unpromoted primary amine. This is consistent with previous research about the acid strength of
the promoting site, which showed that carboxylic acids exert a promoting effect on the activity
of amines, even though less pronounced than the promoting effect of surface silanols [50, 56].
Figure 58 also shows that at high loadings of cysteine (0.41 mmol/g and 0.84 mmol/g) the
activity of the catalyst decreases. This is most likely because at high loadings of cysteine, the
intermolecular interactions result in a protonation of the amines by the carboxylic acid located
on a different linker. As a result, the free electron pair of the amine, necessary for the catalytic
aldol condensation cycle, is no longer available for reaction with the reactants [56, 68].
4.5 Second set of materials
Based upon these results, it was decided to synthesize new PMO materials with an approximate
amine loading in the range of 0.05 - 0.15 mmol/g. A second set of materials were prepared with
VTES, TEOS, BTEE and BTEB as precursors (with varying composition). In all cases the total
amount of silicon was kept at a constant value.
The exact concentrations of the precursors added in the synthesis are given in appendix B.
4.5.1 Determination of the theoretical amine loading
By using some empirical calculations, the molar composition of the precursor mixture was
fine-tuned. Based on their molar masses, but keeping in mind that due to the co-condensation
about 50 % of the ethoxy groups form ethanol, the molar mass of the resulting PMO materials
is calculated. Taking into account the molar percentage of VTES and assuming that the
functionalization will result in an equimolar amount of cysteine, the theoretical amine loading
can be determined. However, some of the vinyl groups of VTES will be part of the pore wall and
are not accessible for the thiol-ene click reaction. The percentage of vinyl groups in the pore wall
is estimated from the first two functionalized materials, E10-C and B10-C, of which the amine
loading was determined via elemental analysis.
As will be clarified in the next paragraphs, these calculations appeared to be quite accurate.
Synthesis and characterization of cooperative acid-base PMOs materials 77
4.5.2 Molar composition of the PMO materials
The molar composition of the different PMO materials is given in Table 11. The letter in the
abbreviation represents the type of support: T stands for TEOS, E when ethane was used and B
when the benzene group was added. The binary mixtures combining TEOS with BTEE or BTEB
are abbreviated as TE and TB respectively. And to avoid further complexity with the
abbreviations, these materials are numbered from 1 to 3.
It should be mentioned that, in a strict manner of speaking, the materials containing TEOS
cannot be labelled as PMO materials as they do not fully comply with the definition of periodic
mesoporous organosilicas in which only bridged organosilica precursors are used. It would be
more correct to use the terminology "modified SBA-15" materials or "organosilicas". In the next
paragraphs, the latter term is used as a more general name to denote the entire group of these
synthesized materials.
Table 11: Molar composition of the organosilicas
Material VTES (mol%) BTEE (mol%) BTEB (mol%) TEOS (mol%)
T1 1.0 0 0 99.0
E1 2.0 98.0 0 0
E2 3.0 97.0 0 0
TE1 3.0 64.7 0 32.3
TE2 3.0 32.3 0 64.7
TB1 7.0 0 62.0 31.0
TB2 5.5 0 47.3 47.3
TB3 4.0 0 32.0 64.0
4.5.3 Characterization
The synthesized organosilicas were characterized with the use of Diffuse Reflective Infrared
Fourier Transform spectroscopy, nitrogen physisorption, X-ray diffraction and elemental
(CHNS) analysis.
Synthesis and characterization of cooperative acid-base PMOs materials 78
4.5.3.1 DRIFT
In Figure 59, typical DRIFT spectra of the benzene type materials are shown. The small peak at
3728 cm-1 is very common for these organosilica materials as it is caused by the silanol groups.
The broad peak between 3700 cm-1 and 3300 cm-1 is due to the presence of adsorbed water on
the sample and overlaps with the carboxylic acid O-H stretch. The C-H stretch of the aromatic
benzene group results in a peak around 3100 cm-1. The strong peak at 2980 cm-1 is due to the
=C-H stretch, originating from the VTES precursor, in the TB1 material. The small peaks between
3000 cm-1 and 2850 cm-1 correspond to the C-H stretch of alkanes. For the material TB1-C, the
peaks of the carbonyl group (1720 cm-1) and the N-H bending of the primary amine (1590 cm-1)
are more visible. Around 1400 cm-1 a peak corresponding to the C-C stretch of aromatics is
observed. The other peaks that appear in the range 1450 cm-1 to 1250 cm-1 can be attributed to
C-H deformation caused by the surfactant which is not entirely removed from the material. The
strong peak around 1100 cm-1 - 1000 cm-1 corresponds to the siloxane bonds. Lastly, the peaks
between 1000 cm-1 and 600 cm-1 can be attributed to Si-C or Si-O-Si vibrations.
Figure 59: DRIFT spectrum of TB1 (blue) and TB1-C (red)
4.5.3.2 Nitrogen physisorption and elemental analysis
The mesostructure of the materials is confirmed with the use of adsorption-desorption
isotherms. The results of the nitrogen physisorption are shown in Table 12.
05001000150020002500300035004000
Ku
be
lka
-Mu
nk
(a
.u.)
wavelength (cm-1)
TB1 TB1-C
Silanol
Water
C-H
=C-H
C-H
Si-OSi-O-Si
C-C
N-H
C=O
Synthesis and characterization of cooperative acid-base PMOs materials 79
Table 12: Properties of the organosilicas before and after functionalization
Material SBET (m²/g) Vp (cm³/g) a dp (nm) b Amine loading
(mmol/g) c Before After Before After Before After
T1 860 688 1.08 0.71 4.08 5.14 0.0530
E1 916 907 0.95 0.94 3.75 3.70 0.0592
E2 928 783 0.94 0.82 3.68 3.68 0.0640
TE1 1030 589 1.07 0.64 3.75 3.49 0.0931
TE2 930 616 1.19 0.70 3.87 3.80 0.1010
TB1 898 664 0.92 0.52 3.40 3.19 0.1357
TB2 875 655 0.93 0.48 3.60 2.96 0.1456
TB3 826 401 0.84 0.23 3.54 3.18 0.0882
a The pore volume is determined with the BJH method. b The average pore size is determined with the
BdB-FHH method [77]. c Amine loading was determined via elemental analysis.
Due to the smaller mol percentage of VTES in the materials, the amount of VTES has no
significant effect on the structure and porosity of the organosilicas. For the binary mixtures,
the specific surface area decreases with increasing amount of TEOS. This is a consequence of the
smaller structure of TEOS compared to BTEE and BTEB. While the pore diameter slightly
increases as the material contains more TEOS, and thus resembling more to a SBA-15 material,
which is known to have pore diameters around 5 nm.
In general, the specific surface area and pore volume decrease as the cysteine is grafted onto the
vinyl groups inside the pores. The functionalization of TB3 resulted in some significant changes
of the properties. TB3-C has a type I isotherm which are encountered when adsorption is limited
to one molecular layer. This manifests itself in a low specific surface area of 401 m²/g and a
small pore volume of only 0.23 cm³/g.
After the functionalization, the materials are washed several times with water to remove the
remaining unreacted or physisorpted cysteine. One has to be careful with materials containing
TEOS, as the increased number of silanol groups makes the material more hydrophilic and thus
more sensitive to a treatment with water. It is known that, due to the presence of organic
functionalities in the pore wall, PMO materials are more stable than pure silica materials. The
PMO materials from paragraph 4.3 were refluxed at 120 °C. However, when the material T1,
consisting of 1 mol% VTES and 99 mol% TEOS, is washed at 120 °C, an detrimental effect on the
pores (see Figure 60) was observed. Confirming that this treatment is too harsh for SBA-15-like
materials.
Synthesis and characterization of cooperative acid-base PMOs materials 80
Figure 60: Comparison between the pore size distribution before and after functionalization and washing procedures. Red: 120 °C, Green: 40 °C
With the treatment at 120 °C, three major peaks are obtained in the pore size distribution,
possibly indicating that several layers of the pore wall have been hydrolysed. Also, as a
consequence of the hydrolysis, a fraction of the material is lost, with a remarkable decrease in
specific surface area and pore volume as result, see Table 13. The X-ray diffractogram of T1 and
T1-C, shown in Figure 62, also confirms that the periodic mesostructure is lost after the
treatment with water at 120 °C.
Table 13: Properties of the T1 material before and after functionalization
Material Number of
washes
Temp
(°C)
SBET
(m²/g)
Vp
(cm³/g) a
dp
(nm) b
Amine loading
(mmol/g) c
T1 860 1.08 4.08
T1-C 2 120 377 0.66 5.10 0.0824
T1-C-S2 5 40 688 0.71 5.14 0.0530
Adapting the washing procedure to a milder temperature of 40 °C and a shorter time (2h - 3h)
results in a material (T1-C-S2) which properties are better preserved, see Table 13. However,
still a small increase in the average pore size is observed and as can be seen in Figure 62, the
hexagonal mesostructure is again lost after repeating the washing procedure five times, leaving
a non-crystalline solid. For the binary mixtures, the temperature is increased to 85 °C to obtain a
better solubility of the cysteine.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 5 10 15 20
Po
re v
olu
me
(cm
³/g
)
Pore diameter (nm)
T1
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 5 10 15 20
Po
re v
olu
me
(cm
³/g
)
Pore diameter (nm)
T1-C T1-C-S2
Synthesis and characterization of cooperative acid-base PMOs materials 81
4.5.3.3 XRD
The structures of these organosilicas can be determined with the use of X-ray diffraction.
Figure 61: XRD diffractogram of E1 (blue) and E1-C (red) Typical XRD patterns of the organosilicas, see Figure 61, show an intensive reflection at the low
2θ values which corresponds to the (100) plane. Also two smaller peaks at higher 2θ values,
corresponding to the (110) and (200) planes are visible. These peaks indicate that the material
has a 2D hexagonal (P6mm) mesostructure. The XRD diffractograms show that the hexagonal
ordered mesoporous structure is maintained after the functionalization with cysteine. A small
shift of the (100) reflection towards higher 2θ values is observed. This change can be attributed
to a reduction of the pore diameter of the PMO material because of the introduction of cysteine
in the pores and is confirmed by the results of nitrogen physisorption in Table 12.
The XRD patterns for the other catalysts look similar to the one of E1 and E1-C. Only the
material TB3-C exhibits one broader peak and the two smaller peaks around a 2θ value of 2.7
are no longer visible. This indicates that the functionalization with cysteine influenced the
mesostructure of this material, as was already observed via nitrogen physisorption by an
significant decrease in specific surface area and pore volume.
The X-ray diffractograms of T1, T1-C and T1-C-S2, in Figure 62, confirm that the periodic
mesostructure is lost after the treatment with water, leaving an amorphous silica material.
0.7 1.7 2.7 3.7 4.7
Inte
nsi
ty (
a.u
.)
2ϴ (degrees)
E1 E1-C
Synthesis and characterization of cooperative acid-base PMOs materials 82
Figure 62: XRD diffractogram of T1 (blue) and T1-C (red) As explained in Materials and methods, the XRD data can be used to calculate the width of the
unit cell and the wall thickness, these values are presented in Table 14.
Table 14: Physicochemical properties of the synthesized organosilicas
Material d100 (nm) a Unit cell (nm) dp (nm) Wall thickness (nm)
Before After Before After Before After Before After
TE1 5.80 5.48 6.70 6.33 3.75 3.49 2.95 2.84
TE2 5.88 5.61 6.79 6.48 3.87 3.80 2.92 2.68
TB1 5.51 4.85 6.36 5.60 3.40 3.19 2.96 2.41
TB2 5.55 4.97 6.41 5.74 3.60 2.96 2.81 2.78
TB3 5.61 5.41 6.48 5.93 3.54 3.18 2.94 2.75
a The d-spacing has been determined with the XRD-software WinXRD.
The d100-spacing decreases with increasing amount of VTES (TB1 > TB2 > TB3), this can be
explained by comparing the chemical structure of the precursors. While BTEE and BTEB have six
ethoxy groups, VTES has only three. Thus, with increasing amount of VTES in the precursor
mixture, this results in a relative smaller amount of siloxane groups in the pore wall after co-
condensation. The d100-spacing also seems to be influenced by the amount of TEOS. This
precursor has an enhancing effect on the ordering of the material. With increasing amount of
0.7 1.7 2.7 3.7 4.7
Inte
nsi
ty (
a.u
.)
2ϴ (degrees)
T1 T1-C T1-C-S2
Synthesis and characterization of cooperative acid-base PMOs materials 83
TEOS, the hexagonal structure of the organosilica is promoted and thus slightly increasing the
distance between two lattice surfaces (d100).
A second effect that influences the wall thickness is the pore diameter. Which was already
determined via nitrogen physisorption. It is expected that due to the grafting procedure, only the
pore diameter would decrease. However, for the TE-materials, a small decrease in wall thickness
of around 5 to 10 % is measured. The biggest change in wall thickness is observed for the
material with the highest amount of BTEB in the precursor mixture, TB1. This decrease also
occurs for the material solely containing benzene bridges (B10-C) and is again an indication
that the benzene groups in the support are more sensitive to the grafting conditions than the
ethane groups.
Synthesis and characterization of cooperative acid-base PMOs materials 84
Catalytic experiments 85
Chapter 5
Catalytic experiments
The functionalized materials were used as catalyst in the aldol condensation of acetone with
4-nitrobenzaldehyde. To investigate the influence of the catalyst support's hydrophobicity on
the catalytic activity, the reaction was performed both in the absence and presence of water. To
evaluate the regeneration possibilities, the used catalysts were filtered after the reaction and
washed with various solvents.
5.1 Comparison of ethane and benzene bridged
organosilica supports
The materials with only VTES and BTEE or BTEB (E1-C and B10-C) were first tested as catalyst.
The conversion as a function of the time is given in Figure 63.
Figure 63: Results of the aldol condensation of 4-nitrobenzaldehyde and acetone at 45 °C, Blue: E1-C; Green: B10-C
Although E1-C (0.0592 mmol/g) and B10-C (0.0692 mmol/g) have similar amine loadings, both
catalysts have very different reaction rates. The turnover frequencies are calculated from the
slope of the trendline. This value is divided by the molar percentages of amines in the mixture
and by 60 s/min to obtain the number of molecules that can be converted per active site per unit
0
2
4
6
8
10
12
14
16
0 50 100 150 200 250 300 350
Co
nve
rsio
n (
%)
Time (min)
TOF = 1.77 10-4 s-1
TOF = 1.80 10-5 s-1
Catalytic experiments 86
of time. The turnover frequency of E1-C is about one order of magnitude higher than B10-C. For
a possible explanation it is necessary to take into consideration that the concentration of
reactants in the pores may be different from the concentration of the reaction mixture. The
properties of the pores play an important role on the physisorption of the reactants into the
pores. It is not unlikely that for the material B10-C the environment of the pores is more in
favour of 4-nitrobenzaldehyde to enter due to some interactions with the benzene groups in the
support. This can be justified with the "like likes like" principle. As a consequence of the relative
higher concentration of 4-nitrobenzaldehyde in the pores, this reactant reacts with the primary
amine and may lead to the formation of a stable Schiff base, which eliminates active sites. This
inhibition of the reaction kinetics was also observed by Kandel et al. [47].
Next, by adding TEOS to the precursor mixture, the influence of an increased number of silanol
groups will be investigated by testing the catalysts containing both hydrophobic and hydrophilic
blocks (TE and TB) in the aldol condensation of 4-nitrobenzaldehyde with acetone at 45 °C.
5.1.1 Catalysts containing both hydrophobic and hydrophilic blocks
The conversion as a function of the time is given in appendix B. An overview of the turnover
frequencies is given in Figure 64. The black dashed lines indicate the expected turnover
frequency in case linear interpolation between the catalysts E1-C, B10-C and T1-C-S2 can be
applied. The experimentally determined TOF values are for both the TE- and TB-materials lower
than expected. For the TB-materials (red curve) this difference is less noticeable because of the
low TOF values. However, for the TE-materials (blue curve) a large decline is observed when
more TEOS is added to the precursor mixture. . Surprisingly, the activity increases again when
the modified SBA-material (T1-C-S2) is considered.
The lower turnover frequencies obtained from the catalytic experiments indicate that no linear
relationship applies for the activity of the catalysts containing both hydrophobic and hydrophilic
blocks. Adding three different precursors to the precursor mixture complicates the synthesis
because every precursor has its own hydrolysis and condensation rates. When the difference
between these rates is too large, the tendency towards homocondensation reactions increases.
This will be a problem during the co-condensation because the homogeneous distribution of the
different organic functionalities in the framework cannot be guaranteed, resulting in a clustered
distribution of the organosilicas, rather than a random one. After functionalization, the amine
functionalities might be too close to each other and are not fully promoted by neighbouring
silanol groups. Leading to lower TOF-values than would be expected from linear interpolation.
Catalytic experiments 87
Figure 64: Turnover frequency as function of the molar percentage of TEOS in the catalysts. Blue: TE-materials; Red: TB-materials.
Dashed lines indicate the theoretical TOF values in case of linear interpolation A higher density of silanol groups doesn't increase the catalytic activity of the TE-materials. This
suggests that the amount of silanol groups introduced by the ethane precursor are already
sufficient to promote the active sites in the pores. For the TB-materials, it can be observed that
with an increasing amount of the benzene precursor (TB1 > TB2 > TB3), the activity of the
catalyst decreases. This can again be related to the formation of an inactive intermediate,
a Schiff base, and the consequent low activity of the benzene functionality in the aldol
condensation of 4-nitrobenzaldehyde with acetone, as was shown in Figure 63. Increasing the
amount of TEOS has a positive effect on the activity because the environment of the pores
gradually improves into a situation in which a smaller effect of the “like likes like” principle is
expected. Leading to a decreased number of inactive intermediates, and thus the aldol
condensation can proceed at a higher rate.
5.2 Effect of water in the reaction mixture
The influence of the hydrophobicity of the catalyst support on the catalytic activity in the
presence of water is investigated. This is an important property because for a possible
application in industry, e.g. in the valorization of biocomponents such as glycerol and furfural,
the reactants will most likely be in an aqueous solution. Thus the activity of these bifunctional
catalysts in the presence of water will have a major impact on the potential feasibility of these
materials. The amount of water was determined based on the results of the aldol condensation
0.0E+00
2.0E-05
4.0E-05
6.0E-05
8.0E-05
1.0E-04
1.2E-04
1.4E-04
1.6E-04
1.8E-04
2.0E-04
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0
TO
F (
1/
s)
TEOS (mol %)
Catalytic experiments 88
of furfural with acetone, performed by Elien Laforce (see Chapter 3). A small amount of 1 vol%
or 2 vol% water is already sufficient to lower the catalytic activity with 23,43 %, respectively
57,17 %. Due to the low solubility of water in the non-polar reaction mixture (hexane and
acetone), simulations were performed with Aspen Plus. Eventually, only one volume percentage
of water was added to avoid phase separation.
5.2.1 Catalysts containing both hydrophobic and hydrophilic blocks
An overview of the turnover frequencies for the aldol condensation of 4-nitrobenzaldehyde
with acetone in the presence of water is given in Figure 65. The black dashed lines indicate the
expected turnover frequency in case linear interpolation between the catalysts E1-C,
B10-C and T1-C-S2 can be applied. The experimentally determined TOF values are for both the
TE- and TB-materials lower than expected. This can again be related to complexity of the
precursor mixture. When the condensation and hydrolysis rates of the different precursors do
not match, the homocondensation reaction is favoured during the co-condensation. This results
in materials of which the amine functionalities are not completely promoted by silanol groups
due to the clustering of all the precursors.
Figure 65: Turnover frequency as function of the molar percentage of TEOS in the catalysts. Blue: TE-materials; Red: TB-materials.
Dashed lines indicate the theoretical TOF values in case of linear interpolation
0.0E+00
2.0E-05
4.0E-05
6.0E-05
8.0E-05
1.0E-04
1.2E-04
1.4E-04
1.6E-04
1.8E-04
2.0E-04
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0
TO
F (
1/
s)
TEOS (mol %)
Catalytic experiments 89
The turnover frequencies of the TE-materials both for the reference reaction and the reaction in
presence of 1 vol% water are compared in Table 15. For the most hydrophobic material, TE1-C,
the catalytic activity remains at the same level. It is an interesting result to see that, even in the
presence of water, TE1-C is able to preserve its activity. This indicates that the hydrophobic pore
walls impede the water molecules to enter the pores. Thus the reaction continues as it would in
the situation without water. Interestingly, while in the reference reaction the turnover
frequency of TE1-C was still lower than the one of T1-C-S2, this catalyst now becomes more
active in the presence of water. Materials with more TEOS in the pore wall are more susceptible
to the water and therefore the effect of water is more pronounced. For TE2-C, the catalytic
activity decreases with about 15 % but compared to T1-C-S2, the loss in activity is more than
40 %. This illustrates that the hydrophobicity of the ethane group, even if the latter is in small
concentrations, does have a large influence on the catalytic activity.
Table 15: Comparison of the turnover frequencies for the TE-materials
Material TEOS (mol%) TOFreference reaction (s-1) TOFreaction with water (s-1) Ratio
E1-C 0.0 1.77 x 10-4 1.86 x 10-4 1.049
TE1-C 32.3 7.16 x 10-5 7.70 x 10-5 1.075
TE2-C 64.7 4.41 x 10-5 3.73 x 10-5 0.846
T1-C-S2 99.0 8.67 x 10-5 5.15 x 10-5 0.594
The turnover frequencies of the TB-materials in the absence or presence of water are shown in
Table 16. In comparison to the TE-materials, the presence of water has a much larger effect on
the catalytic activity. The TOF values for the aldol condensation in the presence of 1 vol% water
drop significantly. All TB-materials exhibit a turnover frequency around 9 x 10-6 s-1. Nonetheless,
the influence of the support's hydrophobicity is clear. The most hydrophobic material, TB1-C, is
able to retain most of its activity, which is however only 50 %. And while TB1-C had the lowest
activity in the reference reaction, it is now the most active catalyst. Again, materials with more
TEOS in the pore wall are more susceptible to the water and for these materials, TB2-C and TB3-
C, the activity drops with about 75 %. The catalyst TB2-C has a lower TOF value than TB3-C.
However, according to this explanation, the reverse order should be detected. This deviation can
possibly be attributed to experimental errors.
Catalytic experiments 90
Table 16: Comparison of the turnover frequencies for the TB-materials
Material TEOS (mol%) TOFreference reaction (s-1) TOFreaction with water (s-1) Ratio
B10-C 0.0 1.77 x 10-5 1.30 x 10-5 0.721
TB1-C 31.0 2.00 x 10-5 9.71 x 10-6 0.486
TB2-C 47.3 3.22 x 10-5 9.04 x 10-6 0.281
TB3-C 64.0 4.45 x 10-5 9.54 x 10-6 0.214
T1-C-S2 99.0 8.67 x 10-5 5.15 x 10-5 0.594
A possible explanation why the ratio of the TB-materials is much lower than the TE-materials
has to do with the Snyder polarity index [78]. Water is one the most polar solvents, and has a
Snyder polarity index of 9, while benzene has a lower value of 3. The polarity of ethane is not
easy to find, because the natural state of ethane is the gas phase and thus it is not included in
lists of the solvents polarity. However, knowing that alkanes are the most non-polar solvents
and based on the polarity indices of hexane, octane and decane, one can assume that the polarity
of an ethane compound would be in the range of 0.1 to 0.3. Comparing the polarity index of
ethane and benzene, it can be expected that the ethane groups in the silica support have a
stronger repulsive effect on water. Thus, the TE-materials are capable to exclude water to a
greater extent from the pores than the TB-materials.
5.3 Regeneration
The application of heterogeneous catalysts results in less energy intensive separation processes.
Also, the regeneration and recycling of used catalyst enhance the overall efficiency of the
process. Therefore, the reusability of these materials was investigated. Spent catalyst was
filtered from the reaction mixture on a Büchner funnel with a sintered glass disc. First, assuming
that no inactive intermediates are formed and thus no important changes occur to the
heterogeneous catalyst, the material is simply washed with a small amount of deionized water
and afterwards with a larger amount of acetone to improve the drying of the sample. When the
regenerated materials were tested in the aldol condensation of 4-nitrobenzaldehyde with
acetone at 45 °C, the turnover frequencies dropped significantly. In general, a decrease in
catalytic activity of heterogeneous materials can be attributed to several causes. First, leaching
of the active sites may occur. However, there is no reason to expect that the carbon-sulfur bond
would be cleaved during the aldol condensation. Secondly, the formation of inactive
intermediates on the surface of the catalyst can block the accessibility of the active sites. This
latter case is most likely to be the main reason for the decreased activity. The presence of
Catalytic experiments 91
intermediates was confirmed by characterizing the catalysts with DRIFT spectroscopy. In Figure
66, the DRIFT spectra of the catalyst before (TE1-C) and after (TE1-C-R1) catalytic reaction are
compared. Zooming in on the region of relevance, the peaks in the range 1550 cm-1 to 1475 cm-1
are slightly elevated which correspond to the N-O asymmetric stretch. Also, an extra peak
between 1400 cm-1 and 1300 cm-1 which can be attributed to the N-O symmetric stretch is
observed. These peaks indicate the presence of a nitro group in the catalytic material and it
suggests that 4-nitrobenzaldehyde still remains inside the pores and formed an intermediate.
Because in the case that the reactant is only physisorpted, 4-nitrobenzaldehyde would have been
washed out by the regeneration procedure. However, there is no increased intensity for the peak
around 1675 cm-1 which corresponds to the C=N stretching of the inactive imine.
Figure 66: Drift spectrum of TE1-C (red) and TE1-C-REGEN (green) A second regeneration method was tested in which the spent catalyst was thoroughly rinsed
with ethyl acetate, deionized water, and ethanol for several times and dried in vacuum at 40 °C.
This method was described by An et al. [61] and worked fine for materials with alternating
hydrophobic and hydrophilic blocks in the pore wall. However, when these regenerated
materials were tested in the aldol condensation, again very low TOF values were obtained
indicating that the inactive intermediates were still present on the catalytic surface. In Figure 67,
the conversion as a function of time for the TE1-C material is plotted. The other catalysts showed
similar trends in terms of activity.
12501300135014001450150015501600165017001750
Ku
be
lka
-Mu
nk
(a
.u.)
wavelength (cm-1)
TE1-C TE1-C-REGEN
imine
N-O
N-O
Catalytic experiments 92
It should be noted that the catalyst's activity only drops during the first catalytic reaction. When
the catalyst TE1-C was used for a second and third run, the same turnover frequencies are
obtained. This is a possible indication that a single blockage of the pores occurs during the first
catalytic reaction. Hereby enclosing a fraction of the 4-nitrobenzaldehyde, thus explaining why
only the N-O symmetric and asymmetric stretch of a nitro group is observed in the DRIFT
spectra, and no extra peak which corresponds to the C=N stretching of the inactive imine is
present. Afterwards, the active sites that aren't blocked, remain accessible for subsequent
reactions, resulting in a steady catalyst's activity.
Figure 67: Conversion as a function of time for the TE1-C material. Blue: reference reaction; Red: regeneration 1; Green: regeneration 2
Further research is required to clarify which intermediates are responsible for the blockage or
deactivation of the catalysts.
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
0 50 100 150 200 250 300
Co
nve
rsio
n (
%)
Time (min)
TOF = 7.16 10-5 s-1
TOF = 1.90 10-5 s-1
TOF = 1.95 10-5 s-1
Conclusions and Future Work 93
Chapter 6
Conclusions and Future Work
The aldol condensation might have a bright future in the discipline of green, renewable and
sustainable energy, e.g. in the valorization of biocomponents such as glycerol and furfural.
At the same reaction conditions, the turnover frequency of the aldol condensation of furfural
with acetone is about one order of magnitude lower than the aldol condensation of 4-
nitrobenzaldehyde with acetone. This lower reaction rate can be explained by the fact that
furfural doesn’t contain an electron-withdrawing group. The strongly electron-withdrawing
nitro group of 4-nitrobenzaldehyde makes the carbonyl function more susceptible for a
nucleophilic attack to form the aldol product. Higher reaction temperatures are required to
obtain significant conversions. When higher temperatures (> 60 °C) are applied, one has to
consider the vapour-liquid equilibria of the reactants. The results from a simulation with an
existing code based on the Non-Random Two-Liquid (NRTL) and Hayden-O’Connell (HOC)
methods, indicate that this system deviates a few percentages from an ideal mixture. As was
expected, only a fraction of the acetone evaporates at higher temperatures. For toluene and
furfural it is correct to assume that everything remains in the liquid phase. If a reaction
temperature of 90 °C is considered, about 1.55 % of the acetone will evaporate, which
corresponds with a pressure increase up to a total pressure of 3.7 bar.
During the dehydration of one mole xylose, one mole of furfural is formed but also three moles
of water are obtained as by-product. Thus, in the industry the product stream of furfural will
always contain water. It was found that a small amount of 1 vol% or 2 vol% water is already
sufficient to lower the catalytic activity with 23,43 %, respectively 57,17 %. In this regard,
Periodic Mesoporous Organosilicas can be a good solution. Because it is possible to tune the
hydrophobicity of the catalyst support during the synthesis, one can synthesize materials which
are able to exclude water from entering, leading to a situation in which only the reactants are
able to physisorb in the pores.
In this work several organosilicas have been synthesized. During the synthesis four different
precursors have been used: vinyltriethoxysilane (VTES), 1,4-bis(triethoxysilyl)ethane (BTEE),
1,4-bis(triethoxysilyl)benzene (BTEB) and tetraethyl orthosilicate (TEOS). In all cases the total
amount of silicon atoms in the synthesis mixture was kept at a constant value. By varying the
ratio of these precursors, materials with different properties were obtained. TEOS was added to
the precursor mixture to create a set of materials with changing hydrophobicity. After the
Conclusions and Future Work 94
synthesis of these organosilicas, the amino acid functionality was introduced into the pores via a
post-functionalization. Cysteine was grafted onto the vinyl group of VTES via a thiol-ene click
reaction which took place in a UV reactor. Next, the solid was filtered and washed several times
with warm water to remove the remaining unreacted or physisorpted cysteine.
In cooperation with the Centre for Ordered Materials, Organometallics and Catalysis (COMOC),
similar PMO materials have been tested for the aldol condensation of 4-nitrobenzaldehyde with
acetone at 45 °C. These catalytic experiments indicated that at high loadings of cysteine
(0.41 mmol/g and 0.84 mmol/g) the activity of the catalyst decreases. This is most likely
because at high loadings of cysteine, the intermolecular interactions result in a protonation of
the amines by the carboxylic acid located on a different linker. As a result, the free electron pair
of the amine, necessary for the catalytic aldol condensation cycle, is no longer available for
reaction with the reactants. Based upon these results, it was decided to synthesize new PMO
materials with an approximate amine loading in the range of 0.05 - 0.15 mmol/g.
All these organosilicas have been characterized with the use of Diffuse Reflective Infrared
Fourier Transform spectroscopy, nitrogen physisorption, X-ray diffraction and elemental
(CHNS) analysis. The DRIFT spectra confirmed the successful functionalization of the vinyl
groups with cysteine. Properties such as the specific surface area, pore volume and average pore
size were determined via nitrogen adsorption and desorption isotherms. Almost all of them
exhibit a type IV isotherm, which corresponds to mesoporous materials. The amine loading of
the functionalized organosilicas was determined via elemental (CHNS) analysis and was in the
range of 0.05 to 0.15 mmol/g. From the X-ray diffractograms, it was found that the washing
procedure at 120 °C is too harsh for the "modified SBA-15" materials (T1-C). Adapting the
washing procedure to a milder temperature of 40 °C and a shorter time (2h - 3h) results in a
material which properties are better preserved. Although, the mesoscopic ordering is still lost
and an amorphous solid was obtained.
The functionalized materials were used as catalyst in the aldol condensation of acetone with
4-nitrobenzaldehyde. The materials with only VTES and BTEE or BTEB (E1-C and B10-C) were
first tested as catalyst. Although E1-C (0.0592 mmol/g) and B10-C (0.0692 mmol/g) have
similar amine loadings, both catalysts have very different reaction rates. The turnover frequency
of E1-C is about one order of magnitude higher than B10-C. For a possible explanation it is
necessary to take into consideration that the concentration of reactants in the pores may be
different from the concentration of the reaction mixture. The properties of the pores play an
important role on the physisorption of the reactants into the pores. It is not unlikely that for the
material B10-C the environment of the pores is more in favour of 4-nitrobenzaldehyde to enter
due to some interactions with the benzene groups in the support. This can be justified with the
Conclusions and Future Work 95
"like likes like" principle. As a consequence of the relative higher concentration of 4-
nitrobenzaldehyde in the pores, this reactant reacts with the primary amine and may lead to the
formation of a stable Schiff base, which eliminates active sites and results in an inhibition of the
reaction kinetics. The catalysts containing both hydrophobic and hydrophilic blocks (TE- and
TB-materials) were tested in the same reaction. The experimentally determined TOF values are
for both the TE- and TB-materials lower than expected. Adding three different precursors to the
precursor mixture complicates the synthesis because every precursor has its own hydrolysis
and condensation rates. When the difference between these rates is too large, the tendency
towards homocondensation reactions increases. This will be a problem during the co-
condensation because the homogeneous distribution of the different organic functionalities in
the framework cannot be guaranteed, resulting in a clustered distribution of the organosilicas,
rather than a random one. After functionalization, the amine functionalities might be too close to
each other and are not fully promoted by neighbouring silanol groups. Leading to lower TOF-
values than would be expected from linear interpolation.
To investigate the influence of the catalyst support's hydrophobicity on the catalytic activity, the
reaction was also performed in the presence of water. This is an important property because for
a possible application in industry, e.g. in the valorization of biocomponents such as glycerol and
furfural, the reactants will most likely be in an aqueous solution. Thus the activity of these
bifunctional catalysts in the presence of water will have a major impact on the potential
feasibility of these materials. The turnover frequencies of the TE- and TB-materials were
compared for both the reference reaction and the reaction in presence of 1 vol% water.
Especially the materials containing ethane bridges in the catalyst support appear to be quite
promising. It was an interesting result to see that, even in the presence of water, TE1-C is able
to preserve its activity. This indicates that the hydrophobic pore walls impede the water
molecules to enter the pores. Thus the reaction continues as it would in the situation without
water. Materials with more TEOS in the pore wall are more susceptible to the water and
therefore the effect of water is more pronounced. For TE2-C, the catalytic activity decreases with
about 15 % but for the material T1-C-S2, the loss in activity is more than 40 %. In comparison to
the TE-materials, the presence of water has a much larger effect on the catalytic activity of the
TB-materials. The TOF values for the aldol condensation in the presence of 1 vol% water drop
significantly. The most hydrophobic material, TB1-C, is able to retain most of its activity, which
is however only 50 %. A possible explanation why the ratio (TOFreference reaction/TOFreaction with water)
of the TB-materials is much lower than the TE-materials has to do with the Snyder polarity
index. Water is one the most polar solvents, and has a Snyder polarity index of 9. When the
polarity index of ethane (in the range of 0.1 to 0.3) and benzene (around 3) are compared, it can
be expected that the ethane groups in the silica support have a stronger repulsive effect on
Conclusions and Future Work 96
water. Thus, the TE-materials are capable to exclude water to a greater extent from the pores
than the TB-materials.
Also the reusability of these materials was investigated. Spent catalyst was filtered from the
reaction mixture on a Büchner funnel with a sintered glass disc and regeneration procedures
with various solvents was performed. When the regenerated materials were tested in the aldol
condensation of 4-nitrobenzaldehyde with acetone at 45 °C, the turnover frequencies dropped
significantly. The formation of inactive intermediates on the surface of the catalyst is most likely
the main reason for the decreased activity. These intermediates can block the accessibility of the
active sites. However, it should be mentioned that the catalyst's activity only drops during the
first catalytic reaction. When the catalyst TE1-C was used for a second and third run, the same
turnover frequencies are obtained. This is a possible indication that an one-time only blockage of
the pores occurs during the first catalytic reaction. Afterwards, the active sites that aren't
blocked, remain accessible for subsequent reactions, resulting in a steady catalyst's activity. It
should be useful to study the cause of this deactivation by identifying the exact intermediates.
And possibly elucidate the mechanism by which these intermediates are formed so this
information can be used as a guidance for the design of a next set of cooperative catalysts.
Different regeneration procedures with other solvents have to be tested in order to regain the
original activities.
In this work, differences in surface arrangements due to clustering of the precursors may have
interfered with a correct comparison between the catalysts activity. It might be useful for further
investigations on the effect of the catalyst support’s hydrophobicity to carefully control the
arrangement of the active sites with respect to each other. A material with alternating
hydrophobic and hydrophilic blocks could provide such possibility. It was noticed that repeating
the washing procedure too much, had a negative effect on the mesoscopic ordering of the
functionalized materials. Therefore it is suggested to look for another solvent with a good
solubility for cysteine. Thus a milder temperature and a shorter contact time of the washing
procedure can be applied.
Finally, it is recommended to examine the functionalization of these organosilicas with
secondary amines containing an alcohol group close to the amine. Because literature suggests
that intermolecular amine-silanol or intramolecular amine-alcohol cooperativity is better than
intramolecular amine-carboxylic acid cooperativity. Leading to catalysts with higher turnover
frequencies.
References 97
Chapter 7
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Appendix A 101
Appendix A A1. Parameters
Table 17: Parameters of the NRTL obtained from Aspen Plus
Acetone Furfural Toluene Water
Ace
ton
e
𝑎𝑖𝑗 0.0 -10.7013 0.8404 -3.5004
𝑏𝑖𝑗 0.0 3549.04 -260.308 1347.96
𝑐𝑖𝑗 0.0 0.0 0.0 0.0
𝛼𝑖𝑗 0.0 0.3 0.3 0.3
Fu
rfu
ral
𝑎𝑖𝑗 6.5452 0.0 0.0 -2.7563
𝑏𝑖𝑗 -1015.06 0.0 27.28 1911.42
𝑐𝑖𝑗 0.0 0.0 0.0 0.0
𝛼𝑖𝑗 0.3 0.0 0.3 0.3
To
luen
e
𝑎𝑖𝑗 -0.4612 0.0 0.0 -247.879
𝑏𝑖𝑗 351.203 326.553 0.0 14579.8
𝑐𝑖𝑗 0.0 0.0 0.0 35.582
𝛼𝑖𝑗 0.3 0.3 0.0 0.2
Wat
er
𝑎𝑖𝑗 8.5012 4.2362 627.053 0.0
𝑏𝑖𝑗 -2280.09 -262.241 -27269.4 0.0
𝑐𝑖𝑗 0.0 0.0 -92.7182 0.0
𝛼𝑖𝑗 0.3 0.3 0.2 0.0
A2. Algorithm
An algorithm is given to solve this set of equations in an easy way. In steps 1 to 5 a good initial
value for the vapour fraction and the molar ratio of each component in vapour and liquid phase
is obtained. Afterwards, these initial guesses are used in steps 6 to 11 to iteratively determine
the solution of the set of equations.
Appendix A 102
Step 1: Set the vapour fraction equal to 0.5
𝜑 =𝑛𝐺
𝑛𝐿 (13)
Step 2: Calculate the molar composition of both phases using equations 14 and 15 and 𝐾𝑖 using
equation 9.
𝑥𝑖 =𝑧𝑖
1 − 𝜑 + 𝐾𝑖𝜑 (14)
𝑦𝑖 =
𝐾𝑖𝑧𝑖
1 − 𝜑 + 𝐾𝑖𝜑
(15)
Step 3: Calculate 𝑋, 𝑌 and 𝜕𝑌
𝜕𝜑 using equations 11, 12 and 16, respectively.
𝜕𝑌
𝜕𝜑=
∑ 𝑧𝑖𝐾𝑖(1 − 𝐾𝑖)𝑖
(1 − 𝜑 + 𝐾𝑖)2 (16)
Step 4: Adapt 𝜑 according to equation 17.
If 𝜕𝑌
𝜕𝜑< 0 𝜑𝑛 =
𝜑𝑛−1
1 − 𝑌
If 𝜕𝑌
𝜕𝜑> 0 𝜑𝑛 = 𝜑𝑛−1(1 − 𝑌)
(17)
Step 5: Go back to step 2 and repeat the iteration until the absolute value of both 𝑋 and 𝑌 is
smaller than 10-4.
Step 6: Calculate the molar composition of both phases using equations 14 and 15.
Step 7: Determine the non-ideality of the mixture based on the NRTL model and the HOC
equation of state and calculate the ratio of each component in vapour and liquid phase using
equation 9.
Step 8: Compare the new 𝐾𝑖𝑛 values with the previous 𝐾𝑖
𝑛−1 values using equation 18.
𝐸 =
√∑ (𝐾𝑖
𝑛 − 𝐾𝑖𝑛−1
𝐾𝑖𝑛 )
2
𝑁𝑖
𝑁
(18)
Step 9: If 𝐸 > 10−6 calculate a new value for 𝐾𝑖𝑛+1 using equation 19 and go back to step 6. If
𝐸 < 10−6 go to step 10.
𝐾𝑖𝑛+1 = √𝐾𝑖
𝑛𝐾𝑖𝑛−1 (19)
Appendix A 103
Step 10: Calculate 𝑋 and 𝑌 using equations 11, 12 and a new value for the vapour fraction, using
equation 20.
𝜑𝑛+1 = 𝜑𝑛 −
∑𝑧𝑖(1 − 𝐾𝑖
𝑛)
1 + 𝜑𝑛(𝐾𝑖𝑛 − 1)
𝑁𝑖
∑𝑧𝑖(1 − 𝐾𝑖
𝑛)2
(1 + 𝜑𝑛(𝐾𝑖𝑛 − 1))
2𝑁𝑖
(20)
Step 11: If |𝑋 − 𝑌| > 10−5 go back to step 6.
Appendix B 104
Appendix B B1. Synthesis recipe
3.0 g of Brij-76 (surfactant) is dissolved in 138 mL deionized water and 10 mL HCl (12 M) into a
flask of 250 mL. The solution is stirred at 50 °C during at least 4 hours. The precursors (total
silicon amount: 0.02538 mol) are added dropwise to the clear solution. The synthesis mixture is
stirred at 50 °C during 24 hours and subsequently aged at 90 °C for 24 hours. Next, the
precipitate is filtered on a glass filter and washed with water and acetone. In the final step, the
surfactant is removed via extraction with acidified ethanol. Therefore, 1.0 g of the synthesized
PMO material is dissolved in 50 mL ethanol and 1 mL HCl (12 M) in a flask of 100 mL with reflux.
The mixture is stirred at 80 °C during 24 hours. This surfactant extraction is repeated three
times to remove the surfactant as much as possible.
B2. Precursor mixtures
The exact concentrations of the precursors added in the synthesis of the organosilicas are given
in Table 18.
Table 18: Added amount of precursors
Material VTES (mL) BTEE (mL) BTEB (mL) TEOS (mL)
T1 0.055
5.725
T2 0.109 5.667
TE1 0.164 3.131 1.870
TE2 0.164 1.566 3.740
TB1 0.382 3.285 1.793
TB2 0.300 2.504 2.732
TB3 0.218 1.696 3.701
B3. Washing procedures
After the functionalization, the solid is filtered and washed several times with warm water to
remove the remaining unreacted or physisorpted. It is known that, due to the presence of
organic functionalities in the pore wall, PMO materials are more stable than pure silica
materials. The materials from paragraph 4.3 were refluxed at 120 °C. However, when TEOS is
Appendix B 105
added to the precursor mixture, one has to be careful because the increased number of silanol
groups makes the material more hydrophilic and thus more sensitive to a treatment with water.
For the “modified SBA-15” materials, the washing procedure was adapted to a milder
temperature of 40 °C. For the materials containing both hydrophobic and hydrophilic blocks, the
temperature is increased to 85 °C to obtain a better solubility of the cysteine. The evolution of
the physisorpted amine concentrations can be found in Table 19.
Table 19: Evolution of the physisorpted amine concentrations
Date washed Temperature (°C) Date analysed Sample Loading (mmol/g)
18/03 40 27/03 T1C-S2-Wash1 1.00499
18/03 40 27/03 T1C-S2-Wash2 0.14736
07/04 40 15/04 T1C-S2-Wash3 0.10745
17/04 40 21/04 T1C-S2-Wash4 0.09010
17/04 40 21/04 T1C-S2-Wash5 0.05297
01/04 120 03/04 TE33-C-Wash1 1.59499
01/04 120 03/04 TE33-C-Wash2 0.19861
15/04 40 21/04 TE33-C-Wash3 0.15042
16/04 85 TE33-C-Wash4
16/04 85 21/04 TE33-C-Wash5 8.46877
17/04 85 21/04 TE33-C-Wash6 0.09310
26/03 120 27/03 TE50-C-Wash1 2.54423
26/03 120 27/03 TE50-C-Wash2 1.10323
02/04 120 15/04 TE50-C-Wash3 0.51067
02/04 120 15/04 TE50-C-Wash4 0.22689
16/04 85 TE50-C-Wash5
16/04 85 21/04 TE50-C-Wash6 0.10580
17/04 85 21/04 TE50-C-Wash7 0.10887
02/04 120 03/04 TE66-C-Wash1 2.98094
02/04 120 15/04 TE66-C-Wash2 2.58385
02/04 120 15/04 TE66-C-Wash3 2.53173
16/04 85 TE66-C-Wash4
16/04 85 21/04 TE66-C-Wash5 0.13922
17/04 85 21/04 TE66-C-Wash6 0.12015
18/04 85 21/04 TE66-C-Wash7 0.10102
03/04 40 15/04 TB33-C-Wash1 3.30192
06/04 40 15/04 TB33-C-Wash2 3.05219
07/04 40 15/04 TB33-C-Wash3 2.86585
16/04 85 TB33-C-Wash4
16/04 85 21/04 TB33-C-Wash5 0.23146
17/04 85 21/04 TB33-C-Wash6 0.19883
18/04 85 21/04 TB33-C-Wash7 0.15714
Appendix B 106
23/04 85 28/04 TB33-C-Wash8 0.13051
25/04 85 28/04 TB33-C-Wash9 0.14093
03/04 40 15/04 TB50-C-Wash1 3.58992
06/04 40 15/04 TB50-C-Wash2 3.43343
07/04 40 15/04 TB50-C-Wash3 2.99893
17/04 85 TB50-C-Wash4
18/04 85 21/04 TB50-C-Wash5 0.60513
18/04 85 21/04 TB50-C-Wash6 0.17006
23/04 85 28/04 TB50-C-Wash7 0.14657
25/04 85 28/04 TB50-C-Wash8 0.14471
07/04 40 15/04 TB66-C-Wash1 3.14343
07/04 40 15/04 TB66-C-Wash2 2.68894
17/04 85 TB66-C-Wash3
18/04 85 21/04 TB66-C-Wash4 0.58999
18/04 85 21/04 TB66-C-Wash5 0.10181
23/04 85 28/04 TB66-C-Wash6 0.08753
25/04 85 28/04 TB66-C-Wash7 0.08888
B4. Catalytic experiments
The catalytic activity of the materials with both hydrophobic and hydrophilic blocks (TE- and
TB-materials) were tested in the aldol condensation of 4-nitrobenzaldehyde with acetone at
45 °C. The conversion as a function of the time is shown in Figure 68 and Figure 69.
Figure 68: Results of the aldol condensation of 4-nitrobenzaldehyde with acetone at 45 °C, Blue: TE1-C; Green: TE2-C
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0 50 100 150 200 250 300
Co
nve
rsio
n (
%)
Time (min)
TOF = 7.16 10-5 s-1
TOF = 4.41 10-5 s-1
Appendix B 107
Figure 69: Results of the aldol condensation of 4-nitrobenzaldehyde with acetone at 45 °C, Blue: TB1-C; Red: TB2-C; Green: TB3-C
B5. Overview of the Labjournal
Subject Page
Introduction 1
Synthesis of 100% BTEE PMO 3
Synthesis of E10 and E40 (recipe described by Maria Chong et al.) 5
Synthesis of E10 and E40 (repeat) 7
Synthesis of E40 (recipe described by Esquivel et al.) 9
Synthesis of E10 (recipe described by Burleigh et al.) 11
Synthesis of E20 and E30 (recipe described by Burleigh et al.) 13
Synthesis of B10 and B20 (recipe described by Burleigh et al.) 15
Functionalization of E10 and B10 with cysteine 17
Kinetic experiment with E10-C and B10-C 18
Synthesis of T1, T2, E1 and E2 19
Synthesis of T1, T2, E1 and E2 (repeat) 21
Characterisation with nitrogen physisorption 22
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 50 100 150 200 250 300
Co
nve
rsio
n (
%)
Time (min)
TOF = 2.00 10-5 s-1
TOF = 3.22 10-5 s-1
TOF = 4.45 10-5 s-1
Appendix B 108
Functionalization of T1 with cysteine 23
Functionalization of E1 with cysteine 24
Synthesis of T1, T2, TE1 and TE2 25
Characterisation with nitrogen physisorption 27
Functionalization of E2 with cysteine 28
Functionalization of T1 with cysteine (repeat) 28
Functionalization of TE2 with cysteine 29
Kinetic experiment with E1-C, E2-C and B10-C 30
Synthesis of TE3, TB1, TB2 and TB3 31
Functionalization of TE1 and TE3 with cysteine 33
Functionalization of TB1, TB2 and TB3 with cysteine 34-35
Characterization with nitrogen physisorption 35
Washing procedures 36
Kinetic experiment with TE-materials 37
Kinetic experiment with TE-materials + H2O 39
Kinetic experiment with TE-materials – Regeneration 1 41
Kinetic experiment with TE-materials + H2O – Regeneration 1 43
Kinetic experiment with TE-materials – Regeneration 2 45
Kinetic experiment with TB-materials 47
Kinetic experiment with TB-materials + H2O 49
Kinetic experiment with TB-materials – Regeneration 1 51
Kinetic experiment with TE-materials (repeat) 53
..
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