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Amine-pillared Nanosheet Adsorbents for CO2 Capture Applications
A Thesis Presented
By
Hui Jiang
to
The Department of Chemical Engineering
In partial fulfillment of the requirements
For the degree of
Master of Science
In the field of
Chemical Engineering
Northeastern University
Boston, Massachusetts
Dec. 11th, 2014
1
ACKNOWLEDGEMENT
I thank my advisor, Dr. Choi for providing me an opportunity to work in his research lab-
the Nanomaterials Laboratory for Advanced Catalysis and Separations. He has also been
inspiring during my graduate career, providing excitement in regard to teaching. This dissertation
would not have been possible without his critical feedback, guidance, and assistance.
I also thank and my lab partners, Christopher Cogswell, Dinara Andirova, Xiaodan Zhao,
Yu Lei, who provided insight in my experiments and helped review this work. A special thanks
to Christopher Cogswell. As a second year PhD candidate and my laboratory partner, his novel
ideas enhanced the quality of this project. During my studies, I have been truly inspired by his
ideas and have benefited from his help. I also thank to all the undergraduates who have worked
on this project.
In addition, a thank you to my committee members, Professor Willey and Professor Wei
helped review this work and provided great suggestions; a thank to Mr. Rob Eagan for his
assistance in setting up machinery and ensuring that our laboratory has always run smoothly. I
would also like to thank Dr. Murkerjee for the use of his XRD and FTIR facilities, as well as Mr.
William Fowle for help with SEM analysis. Life outside work has been enjoyable too during the
last few years at Northeastern University.
Finally, I would like to thank my mother. Without her selfless love, I could not have done
with my graduate studies in the USA.
2
ABSTRACT
Amine-functionalized solid adsorbents have gained attention within the last decade for
their application in carbon dioxide capture, due to their many advantages such as low energy cost
for regeneration, tunable structure, elimination of corrosion problems, and additional advantages.
However, one of the challenges facing this technology is to accomplish both high CO2 capture
capacity along with high CO2 diffusion rates concurrently. Current amine-based solid sorbents
such as porous materials similar to SBA-15 have large pores diffusion entering molecules;
however, the pores become clogged upon amine inclusion.
To meet this challenge, our group’s solution involves the creation of a new type of
material which we are calling-amino-pillared nanosheet (APN) adsorbents which are generated
from layered nanosheet precursors. These materials are being proposed because of their unique
lamellar structure which exhibits ability to be modified by organic or inorganic pillars through
consecutive swelling and pillaring steps to form large mesoporous interlayer spaces. After the
expansion of the layer space through swelling and pillaring, the large pore space can be
functionalized with amine groups. This selective functionalization is possible by the choice of
amine group introduced. Our choice, large amine molecules, do not access the micropore within
each layer; however, either physically or chemically immobilized onto the surface of the
mesoporous interlayer space between each layer.
The final goal of the research is to investigate the ability to prepare APN adsorbents from
a model nanoporous layered materials including nanosheets precursor material MCM-22(P) and
nanoporous layered silicate material AMH-3. MCM-22(P) contains 2-dimensional porous
channels, 6 membered rings (MB) openings perpendicular to the layers and 10 MB channels in
the plane of the layers.1 However, the transport limiting openings (6 MB) to the layers is smaller
3
than CO2 gas molecules.2,3
In contrast, AMH-3 has 3D microporous layers with 8 MB openings
in the plane of the layers, as well as perpendicular to the layers, which are larger than CO2
molecules. Based on the structure differences between nanosheets precursor material MCM-22(P)
and nanoporous layered silicate material AMH-3, the latter might be more suitable for CO2
capturer application as an APN candidate material. However, none of the assumptions above
have been approved experimentally.
In this study, the influence of the amine loading on adsorption capacity and kinetics of
adsorption for the mixed porosity material pillared MCM-22 (P) (also called MCM-36) is studied
systematically, in order to determine a potential route to achieve a final material with both high
amine loading and high adsorption capacity. We first synthesized MCM-22(P), followed by
swelling and pillaring to create MCM-36. Polymeric amines such as polyethylenimine (PEI) are
used as an organic component of the supported amine adsorbents, with varying polymer loadings
within the adsorbents used. The kinetics and diffusion properties of carbon dioxide capture on a
MCM-36 pillared material impregnated with amine containing Polyethylenimine polymers has
been investigated. It was determined that the introduction of amine polymer cannot be used to
improve the capture capacity of the support over that of the bare material, due to the fact that
with the addition of a high loading of amine polymer the large pore diffusion channels become
impossible for carbon dioxide molecules to diffuse through. This sets an upper limit to the
capture capacity of polymer impregnated MCM-36 for carbon dioxide which does not surpass
that for the initial bare material, and greatly reduces the utility of using this sort of amine-solid
adsorbent for carbon capture plans in the future.
4
TABLE OF CONTENTS
ACKNOWLEDGEMEN ..............................................................................................................1
ABSTRACT ...................................................................................................................................2
TABLE OF CONTENTS .............................................................................................................4
LIST OF FIGURES ......................................................................................................................6
LIST OF TABLES ........................................................................................................................8
1.0 INTRODUCTION ...................................................................................................................9
2.0 CRITICAL LITRATURE REVIEW ..................................................................................13
2.1. Layered MCM-22 Precursor ............................................................................................13
2.2. MCM-22(P) Synthesis .....................................................................................................14
2.3. Swelling and Pillaring of MCM-22(P) ...........................................................................17
2.4. Amine Grafting on Support Materials ............................................................................20
3.0 EXPRIMENTAL .....................................................................................................................5
3.1. Synthesis of Nanosheets Precursor Material MCM-22(P) ..............................................23
3.2. Calcination to Form Zeolite Structure-MCM-22 ............................................................23
3.3. Swelling of Nanosheets Precursor ..................................................................................23
3.4. Pillaring of Nanosheets Precursor ..................................................................................24
3.5. Preparation of Amine-Pillared Nanosheets (APNs) Adsorbents ....................................24
3.6. Characterization ..............................................................................................................25
3.6.1. Powder X-ray Diffraction (XRD) ................................................................................25
3.6.2. NOVA Analysis for Pore Surface and Pore Size Distribution ....................................25
3.6.3. Fourier Transform Infrared (FTIR) Analysis ..............................................................27
3.6.4. Electron Microscopy ....................................................................................................28
3.6.5. Thermogravimetric Analysis (TGA) ...........................................................................28
4.0 RESULTS AND DISCUSSION ...........................................................................................29
4.1. Synthesis Results of Nanosheets Precursor Material MCM-22(P) .................................29
4.1.1. Initial Synthesis Basic on Previous Work ...................................................................29
4.1.2. Influence of Water .......................................................................................................29
4.1.3. Influence of Content of Hexamethyleneimine (HMI) in the Reaction Mixture ..........32
4.1.4. Influence of SiO2/Al2O3 Ratio .....................................................................................34
5
4.1.5. Summary of Synthesis Results ....................................................................................36
4.2. Calcination to Form Zeolite Structure-MCM-22 ............................................................36
4.3. Swelling and Pillaring Results of Nanosheets Precursor ................................................37
4.4. Amino-Pillared Nanosheets (APNs) Adsorbents ...........................................................40
4.5. Physical Properties .........................................................................................................41
4.6. CO2 Capture Capacity ....................................................................................................44
5.0 CONCLUSIONS ...................................................................................................................48
6.0 RECOMMENDATIONS ......................................................................................................48
REFERENCES ............................................................................................................................49
6
LIST OF FIGURES
Figure 1: Class one adsorbents are prepared through molecular basket method. ................10
Figure 2: Class two adsorbents are prepared through the reaction of amine-containing
silanes. ...........................................................................................................................................10
Figure 3: Class three adsorbents prepared through in situ polymerization. .........................10
Figure 4: Depiction of the unit cell of MWW family crystals such as MCM-22(P). ..............14
Figure 5: Expanded X-ray powder diffraction pattern of MCM-22(P) and swollen MCM-
22(P). ............................................................................................................................................19
Figure 6: Schematic for preparation silica-pillared MCM-22(P). ...........................................20
Figure 7: Typical BET plot. ........................................................................................................26
Figure 8: XRD patterns of (a) original recipe; (b) 10wt% increased water. .........................30
Figure 9: XRD of the MCM-22-P crystal, from Corma et al.. .................................................30
Figure 10: XRD patterns of MCM-22(P) samples for (a) 10wt% increased water; (b)
15wt% increased water; (c) 20wt% increased water. .............................................................31
Figure 11: SEM images of the MCM-22(P) samples were synthesized at different water
concentrations (a) 10wt% increased water; (b) 15wt% increased water; (c) 20wt%
increased water.............................................................................................................................32
Figure 12: XRD patterns of as-synthesized MCM-22(P) samples with different HMI
contents (a) 10wt% increased HMI; (b) 15wt% increased HMI; (c) 20wt% increased HMI.
........................................................................................................................................................33
Figure 13: SEM images of as-synthesized MCM-22(P) samples with different HMI contents
(a) 10wt% increased HMI; (b) 15wt% increased HMI; (c) 20wt% increased HMI. ............33
Figure 14: XRD patterns of as-synthesized MCM-22(P) samples with different SiO2/Al2O3
ratios (a) and (b) SiO2/Al2O3=97; (c) and (d) SiO2/Al2O3=54. ..................................................35
Figure 15: SEM images of as-synthesized MCM-22(P) samples with different SiO2/Al2O3
ratios (a) and (b) SiO2/Al2O3=97; (c) and (d) SiO2/Al2O3=54. ..................................................35
Figure 16: XRD patterns of samples: (a) uncalcined product; (b) calcined product. ...........37
Figure 17: XRD patterns of MCM-22(P) (a) before swelling and (b) after swelling. ............37
Figure 18: SEM images of MCM-22(P) (a) before swelling and (b) after swelling................38
Figure 19: XRD pattern of pure CTAB powder. ......................................................................38
Figure 20: XRD patterns of MCM-22(P) (a) before swelling and (b) after swelling. ............39
7
Figure 21: XRD patterns of MCM-22(P) (a) before swelling; (b) after swelling and (c) after
pillaring. ........................................................................................................................................40
Figure 22: FTIR spectra of (a) MCM-36 substrates; (b) PEI functionalized (16 wt%)
MCM-36. .......................................................................................................................................41
Figure 23: Nitrogen adsorption-desorption isotherms of (a) MCM-36, (b) MCM-22, (c) 4
wt% PEI functionalized MCM-36 and (d) 18 wt% PEI functionalized MCM-36. ...............42
Figure 24: Weight change of bare MCM-36 on CO2 capture test by TGA. ..........................44
Figure 25: Influence of PEI loading on CO2 adsorption. .........................................................47
Figure 26: Adsorption equilibrium half time various PEI loading. ........................................47
Figure 27: Mole ratio of CO2 adsorbed/ amine loaded various PEI loading. .........................48
8
LIST OF TABLES
Table 1: Physical properties of MCM-22, MCM-36 and PEI functionalized MCM-36
samples ..........................................................................................................................................43
9
1.0 INTRODUCTION
Since the Industrial Revolution, burning fossil fuels has become a large anthropogenic
point source for carbon dioxide emissions. For instance, according to the report from Earth
Policy Institute, from the year 1712 to 2012, atmospheric carbon dioxides concentration has
increased about 36%. As one of the greenhouse gases, the increase of CO2 concentration has
caused climate change. During this same period, the world has warmed approximately 0.8
degrees Celsius since the Industrial Revolution. Currently, the most widely accepted strategy for
carbon mitigation is removing CO2 from flue gas via liquid amine-based system, such as
monoethanolamine(MEA)4. However, these aqueous amine-based CO2 capture method requires
high energies for heating H2O in the amine solution during the regeneration process5. Moreover,
there are some additional problems with using liquid amine-based systems, for example,
equipment damage caused by the corrosive nature of aqueous amines and amine degradation via
evaporation5.
Loading amines on solid nanoporous materials, such as carbon nanotubes, metal organic
frameworks (MOFs), and porous silica, has received attention due to lower regeneration energy
and other advantages such as less damage to equipment and lower amine degradation5. Among
these nano-porous materials, the majority of study focuses on the use of mesoporous silica,
examples including amine-functionalized SBA-156,7
, MCM-418,9
etc.
Amine-modified solid adsorbents can be organized into three classes according to their
preparation methods5. Class one adsorbents are prepared through the molecular basket method,
more specifically, amino polymers such as polyethyleneimine (PEI) are physically impregnated
on the surface of nanopores of the support materials (Fig. 1). Class two adsorbents are prepared
10
through the reaction of amine-containing silanes with the silica support’s surface silanol group
via covalent linkage (Fig. 2). Class three adsorbents are prepared through in situ polymerization
of an amine-containing monomer reactive species (Fig. 3).
Figure 1. Class one adsorbents are prepared through molecular basket method.
Figure 2. Class two adsorbents are prepared through the reaction of amine-containing silanes.
Figure 3. Class three adsorbents prepared through in situ polymerization.
For each class, there are advantages and disadvantages. For example, class one
adsorbents have large amine loading since nearly the entire pore spaces of the support is filled
with amino polymers, thus they have tend to have high CO2 adsorption capacity. On the other
11
hand, since the most pore spaces are occupied by amino polymers, there is low CO2 diffusion
through the material. Also, since there is no covalent bounding between amine polymers and the
surface of the support materials, under regeneration conditions, amino polymers in class one
adsorbents may be easily leached as well. Class two adsorbents have enhanced stability due to
the covalent linkage and faster kinetics since only the pore walls are functionalized with amine-
containing silanes and the center of the pores is still open for gas diffusion. But due to the lower
amine loading, adsorption capacity is generally lower than for class one adsorbents. Similarly as
for class one adsorbents, class three adsorbents have high adsorption capacity because of the
large number of amine sites in aminopolymers; however, at the same time, these have blocked
pores and low diffusion rates. Similarly to class two adsorbents, class three adsorbents have
enhanced stability due to the covalent linkage.
According to the facts above, no single solid amine adsorbents combines both higher
capture capacity and fast sorption kinetics so far. In this study we propose a new class of solid
amine adsorbents, amino-pillared nanosheet (APN) adsorbent, which might meet the key features
of CO2 adsorbents. The APN adsorbents can be prepared from a starting nanoporous layered
material, such as zeolitic material MCM-22 precursor, layered silicate material AMH-3. By
introducing silica pillars into the gallery space of nanoporous layered material, the gallery space
is pillared to facilitate loading of amine-containing molecules or polymers. The final structure of
APN adsorbents consist of nanoprous layers supported by silica pillars, in which the interlayer
spaces are functionalized by amine-containing moieties.
Amino-pillared nanosheet adsorbents are considered as promising adsorbents for CO2
capture because of the separate meso- and micro- pore systems. The surface of single layers
includes surface silanols, onto which amine-containing molecules or polymers can be
12
impregnated in the mesopore galleries. Since the micropores within the layers do not include
surface silanols are smaller than amine-containing molecules, amine groups can be occluded. On
the other hand, the diameter of the micropores is bigger than the kinetic diameter of CO2;
therefore these micropores can be only used for CO2 diffusion. Because of the different functions
of the separate pore systems, CO2 molecules can access the amine sites in interlayer mesopore
galleries through micropores.
Several types of nanoporous layered materials are considered as promising materials for
APN adsorbents preparation, such as layered aluminophosphates (AIPOs), nanosheets precursor
material MCM-22(P) and nanoporous layered silicate material AMH-33. MCM-22(P) contains 2-
dimensional porous channels, 6 membered rings (MB) openings perpendicular to the layers and
10 MB channels in the plane of the layers1. However, the transport limiting openings (6 MB) to
the layers is smaller than CO2 gas molecules2,3
. In contrast, AMH-3 has 3D microporous layers
with 8 MB openings which are larger than CO2 molecules. Based on the structure differences
between nanosheets precursor material MCM-22(P) and nanoporous layered silicate material
AMH-3, the latter might be more suitable for CO2 capturer application as an APN candidate
material. However, none of the assumptions above have been approved experimentally.
In this paper, the influence of the amine loading on adsorption capacity and kinetics of
adsorption for the mixed porosity material Pillared MCM-22(P) (known as MCM-36) is studied
systematically, in order to determine a potential route to achieve a final material with both high
amine loading and high adsorption capacity.
13
2.0 Critical Literature Review
Layered inorganic precursors usually have structure with consecutively repeating
individual sheets bonded by Van der Waals interactions or hydrogen bonds along the c-axis
perpendicular plane.10,11
This lamellar layered structure is present in a large number of inorganic
materials, such as clays, silicates and zeolitic precursors. These layered materials are considered
promising starting points to create novel mesoporous adsorbents by swelling and pillaring their
interlayered space.10
This modification can be achieved by introducing organic or inorganic
compounds which act as pillars into the interlayered space of layered precursor materials. This
type of layered materials is known as a pillared material.10,12,13
2.1. Layered MCM-22 Precursor
The first layered zeolitic precursor material MCM-22(P) (Fig. 4) was first synthesized by
Mobil researchers.14,15,16,17
It is composed of 2.5 nm thick stacking layer in registry. Each layer
contains a bidimensional 10-membered ring (MB) sinusoidal channel, large 12-member ring
cups and 6 MB openings perpendicular to the layers.14,18
MCM-22(P) can be synthesized by
using hexamethylenimine (HMI) as structure directing agent (SDA) which is involved in the
formation of the interlayer space of the as-synthesized material.
Upon calcination, SDA can be removed from the interlayer space and the layers
condense together to form MCM-22, which is a 3-D framework structure material.14,18
The
pillared MCM-36, a unique layered material consisting of microporosity inside the individual
layers and slit-like mesopores in the interlayer space,19
can be obtained by swelling MCM-22(P)
followed by intercalating swollen MCM-22(P) with polymeric silica precursors.14
14
Figure 4. Depiction of the unit cell of MWW family crystals such as MCM-22(P). Here red is
oxygen and the beige are silicon or aluminum. From the International Zeolite Association.
2.2. MCM-22(P) Synthesis
MCM-22(P) can be synthesized under either static or dynamic hydrothermal
crystallization condition in Teflon-lined autoclaves.20-21
Under dynamic conditions, the synthesis
requires large and complex equipment; on the other hand static synthesis doesn’t require special
equipment but may cause a slightly narrower crystallization and requires longer synthesis time.22
15
Some literatures reported that static synthesis resulted in yielding undesired phases, including
ferrierite, mordentie, ZSM-5 and MCM-49.16,23-24
In most literatures, MCM-22(P) was
successfully prepared under static condition.22,23,25
Hydrothermal crystallization condition and the chemical composition of the synthesis gel
can affect the quality of crystals. Common range of MCM-22 synthesis temperature is between
130 and 150 °C. With synthesis temperature above 150 °C, other zeolites phases such as ZSM-5
have been observed particularly when the Al content is low (30<Si/Al<70). Under 150 °C,
MCM-22(P) can be obtained between 8 and 10 days at static synthesis condition. Corma et
al.20,26
successfully synthesized MCM-22(P) at 130 °C after 11 days. Wu et al.17
reported that the
range of 140-170 °C was the favorable temperature range for MCM-22(P) synthesis. When the
temperature was lower than 120 or higher than 180 °C, only amorphous gel was produced. The
increase of temperature can enhance the growth of MCM-22 (P) due to the enhanced solubility
of silicate species.17
Wu et al.17
and Marques et al.16
both reported that the hydroxide concentration in the
synthesis gels plays a critical role in MCM-22(P) synthesis. Higher OH-/SiO2 ratio can enhance
the initial dissolution of the silicon source, nucleation during the aging step, and promote the
crystal growth during the hydrothermal crystallization step. Wu et al.17
reported that when the
OH-/SiO2 ratio was in a range of 0.15-0.25, high crystalline samples were obtained; while
undesirable MCM-49 phase appeared when the OH-/SiO2 ratio was higher than 0.3 but only
amorphous phase was observed when the ratio was lower than 0.12. Wu et al.17
also reported that
at higher alkalinity of the gels smaller particles were formed as the final product; on the other
hand, larger crystals were observed in the final product due to the absence of dissolution process
at lower OH-/SiO2 ratio.
16
H2O/SiO2 ratio can also affect the initial dissolution of the chemicals. Marques et al.16
reported that the H2O/SiO2 ratio higher than 35 has slightly positive effect on the crystallinity of
MCM-22(P). The lower value of H2O/SiO2 ratio caused undesired MCM-49 or ZSM-5
crystalline phases. Corma et al.27
also reported the similar result about the formation of ZSM-5
with a lower amount of water.
High concentrations of structure-directing agent hexamethyleneimine (HMI) is another
key factor for the successful synthesis of MCM-22(P).28
Lawton et al.29
reported that when the
mole ratio of organic cation to inorganic cations is less than 2.0, MCM-49 occurred. Corma et
al.27
mentioned that a 20% decrease of crystallinity of MCM-22(P) was due to a decrease of the
HMI/ SiO2 ratio. Marques et al.16
reported that the HMI/ SiO2 ratio in the range between 0.5 and
0.6 can enhance the crystallinity, but a further increase of HMI doesn’t affect the crystallization
process significantly. They also found that in order to get good crystallinity of MCM-22(P)
under static synthesis condition, it was necessary to increase HMI/ SiO2 ratio from 50 to 70%
higher than those reported in the literature under stirring conditions.
Another major factor which influences the formation of MCM-22(P) is the SiO2/Al2O3
ratio in the synthesis gel. Cheng et al.23
claimed that the most favorable range of SiO2/Al2O3
ratio with the present of HMI is between 20 and 30. Delitala et al. reported that under rotating
condition, pure MCM-22(P) was obtained with the SiO2/Al2O3 ratios of 21, 30 and 46. A
ferrierite phase was observed with MCM-22(P) when the SiO2/Al2O3 ratio was 9, whereas a pure
ferrierite phase was formed under static synthesis condition. Wu et al.17
investigated the effect of
SiO2/Al2O3 ratio in the gel on the crystallization process of MCM-22(P) by varying the
SiO2/Al2O3 ratio in the range of 20 and 100. They reported that with relatively lower SiO2/Al2O3
ratio of 20 and 30, crystallization was completed after 4 days. An increased SiO2/Al2O3 ratio in
17
the gel required longer crystallization time; for example the sample with the SiO2/Al2O3 ratio of
100 reached 90% of crystallinity after 14 days. They also reported the morphology change of
MCM-22(P) with different SiO2/Al2O3 ratio. With a SiO2/Al2O3 ratio of 20, rose-like particles
with much smaller and thinner sheets were observed; whereas at SiO2/Al2O3 ratios of 30, 50 and
80, thicker round lamellar particles were obtained. However, the morphology of the sample with
the SiO2/Al2O3 ratio of 100 was detected as thin disc-like particles. About the sample with the
SiO2/Al2O3 ratio of 100, Corma et al.27
reported that the crystallinity was lower compared to
those samples with lower SiO2/Al2O3 ratio. They also observed that ZSM-5 was obtained by
increasing the SiO2/Al2O3 ratio of the reaction mixture. Cheng et al. reported an increased
SiO2/Al2O3 ratio formed two different phases, including MCM-22(P) and a layered silicate
material kenyaite.
2.3. Swelling and Pillaring of MCM-22(P)
In order to create a new mesoporous system and increase the surface area of the material,
MCM-22(P) was for the first time pillared to create a new pillared material with zeolitic layers
material named MCM-36 by Mobil researchers. In general, MCM-36 can be obtained by
swelling the MCM-22(P) with a surfactant solution at high pH, then followed by pillaring with
SiO2 pillars into the interlayer space and finally the material is calcined to form MCM-36.14
The conditions ensuring successful swelling proved quite severe because of the strong
interlayer bonding. The delamination of MCM-22(P) was achieved by treatment with a cationic
surfactant solution, such as hexadecyltrimethylammonium under high pH (13.8) (controlled with
a base such as NaOH or tetraalkylammonium hydroxide). Moreover, nature of the cation
accompanying the hydroxide determines whether swelling occurs or not. Roth and Vartuli30
18
investigated sodium(Na+), TMA( (CH3)4N
+), TEA( (CH3CH2)4N
+), and TPA((CH3CH2CH2)4N
+),
only the last one allowed swelling with the surfactant. They explained this behavior as the size
of the cations and their interaction with SiO- in the MCM-22(P). The ability of these cations to
enter the interlayer space and interact with SiO- in the MCM-22(P) is showing as the following:
Na, TMA, TEA>C16TMA>TPA
With hexadecyltrimethylammoinium (C16TMA) swelling is possible when TPA is present but
not the other cations. In a recent study, Roth and co-workers31
successfully swelled MCM-22(P)
by used CTMA-OH solution, prepared from CTMA-Cl solution with anion exchange resin,
instead of using CTMA-Cl and TPA-OH.
The strong alkaline condition can enhance the swelling of MCM-22(P). Roth et al.30
explained that the high pH can promote the elimination of hydrogen bonding between silanols on
the surface of each layer via deprotonation of silanols to generate negative SiO- charge. The
newly formed negative SiO- charges on the surface between each layer push against each other
and attract the intercalated long chain surfactant cations to cause swelling.
Normally high pH and high temperature conditions can favor the swelling of the layered
structure of MCM-22(P), but it also can cause partial dissolution of the layer and pore
structure.10,32
Maheshwari et al. reported a mild condition to swell MCM-22(P) at room
temperature to prevent the disruption of the MWW layered structure. They also reported that,
under room temperature, the swollen material can be restored back to MCM-22(P) under acidic
condition, but this restoration is impossible if the swelling temperature is higher than 55 °C.18,32
The most common way to identify the successful swollen MCM-22(P) is from X-ray
diffraction patterns (Fig. 5). The successfully swelled sample should result in expansion of the
19
crystallographic unit cell in the c-direction to 50-55 Å, which corresponded to the thickness of
the intercalated long chain surfactant cations.30
In the XRD pattern, the following effects could
be expected according to the discussion above: hk1 reflections shite to lower angles or disappear;
hk0 reflections remain the same; an intense dominant peak appears around 2° and an unassigned
peak shows up at about 5.5°; the 002 reflection at 6.5° disappears; 101 and 102 reflections at
about 8 and 10° respectively merge into a broad band.18,30,33-34
Figure 5. Expanded X-ray powder diffraction pattern of MCM-22(P) and swollen MCM-22(P).30
The second step for making MCM-36 is to pillar the swollen precursor at inert
atmosphere (N2 or He) by using a SiO2 pillar-tetrathylorthosilicate (TEOS), followed by
hydrolysis and calcination.10, 18,26,33-35
During the hydrolysis process, the TEOS will not flow out
from the interlay spaces due to the hydrophobic property of TEOS.36
The pH of the solution is
adjusted to 8 by NaOH,18,34
and TEOS can be rapidly hydrolyzed36
due to the presence of NaOH.
20
In the first swelling step, the long chain surfactants-CTAB molecules which were intercalated
into the interlayers act as micelle-like template during the hydrolysis of TEOS and cause the
condensation of TEOS.36
During the calcination procedure, the condensed SiO2 pillar is
decomposed to form long polymeric chains between the interlays and the organic compounds are
simultaneously removed from these interlayer spaces to form the mesopores within the interlays
which have 3.0 to 3.5 nm diameters.14
Figure 6. Schematic for preparation silica-pillared MCM-22(P).36
2.4. Amine Grafting on Support Materials
Amine functionalized solid adsorbents organized into three classes according to their
preparation methods. First class adsorbents are prepared through molecular basket method; class
2 adsorbents are prepared through the reaction of amine-containing silanes with the silica
support’s surface silanol group via Covalent Linkage; class 3 adsorbents are prepared through in
situ polymerization of an amine-containing monomer reactive species.
Class one adsorbents are prepared through physical impregnation. Supporting amines,
usually amine-containing small molecules or polymers, are physically loaded on the support
21
surface and support pores. This is also known as “wet impregnation” method,5,8-7
because the
amine-loading process usually involves a volatile solvent.5 Adsorbents are prepared via “wet
impregnation” method generally have large amount of amine loading, thus they have high gas
adsorption capacity, but since there is no covalent bounding between amine polymers and the
surface of support materials, under regeneration condition, amino polymers might be easily
leached.37
For instance, Yue et al.7 used wet impregnation method to prepare
tetraethylenepentamine (TEPA) functionalized SBA-15 for CO2 capture application; Yan et al.6
studied CO2 capture effect on ethylenimine (PEI) functionalized SBA-15 which was prepared by
wet impregnation; Wei et al.9 reported the CO2 capture on diethylenetriamine (DETA),
triethylenetetramine (TETA) and 2-amino-2-methyl-1-propanol (AMP) impregnated as-
synthesized MCM-41.
Class two adsorbents are typically based on amine species that react with the surface
silica of support materials. In this case, the amine moieties are covalently bound to the support
surface; therefore, class 2 adsorbents have enhanced stability of amine moieties. Since only the
pore walls are functionalized by amine species, the center of the pores are still open for gas
molecule transport and therefore, class two adsorbents have fast kinetics.37,38
CO2 capture studies
have been done on class two adsorbents; for example, Chang et al.39
investigated the CO2
adsorption/desorption process on SBA-15 modified with (aminopropyl)triethoxysilane (APTS)
via linkage binding; Hung et al.40
first reported the adsorption of CO2 on amine-modified MCM-
48 which was prepared via silylation reactions between the surface silanol groups of MCM-48
and the grafting amines.
Class three adsorbents are prepared through in situ polymerization of an amine-
containing monomer reactive species, which was first reported by Choi et al.37
It is prepared
22
through polymerization of an amine-containing reactive species such as aziridine within the
pores of the mesoporous silica supports, creating covalently bound hyperbranched aminopolymer
on the surface silanols of silica substrates. Class three adsorbents have high adsorption capacity
because of the large amine sites in aminopolymers, but large aminopolymers also cause slow gas
diffusion.
23
3.0 EXPERIMENTAL
3.1. Synthesis of Nanosheets Precursor Material MCM-22(P)
The MCM-22 precursor was synthesized based on other’s previous works4. The molar
composition of the reactant gel was 9.2 Na2O: 53.8 HMI: 1.0 Al2O3:100.2 SiO2: 5021.1 H2O.To
prepare the reactant gel, 0.24 g sodium aluminate (Sigma-Aldrich) and 0.83 g sodium hydroxide
(Acros) were dissolved in 119.2 g DI water. Then the solution was added into 7.87 g fumed silica
(Acros). After that, 7.00 g HMI (hexamethylenimine, Aldrich) was added to the mixture. The
mixture was allowed to stir for 6 h at room temperature. After 6 h stirring, the gel was transferred
to 75 ml Teflon-lined autoclaves, and the autoclaves were placed into oven under 130 C for
hydrothermal synthesis. After 9 days, the obtained product MCM-22(P) was washed by DI water
until the pH decreased to 9.5, then the sample was filtered and dried at room temperature
overnight.
3.2. Calcination to Form Zeolite Structure-MCM-22
MCM-22 was prepared from MCM-22(P) according to Corma et al. Typically dried
MCM-22(P) was calcined at 580 C for 3 hours in air to remove the organic template and yield
MCM-22, with a 5 K/min ramp up and down rate performed to reach the calcination temperature.
3.3. Swelling of Nanosheets Precursor
According to previous report18
, MCM-22(P) was swollen with CTAB at room
temperature. Typically, 4 g of a wet cake of MCM-22(P) was mixed with 16 g DI water then
sonicated for 15 min. 22.64 g CTAB (cetyltrimethylammonium bromide, Amresco) was mixed
with 32.20 g DI water as a swelling agent then sonicated for 15 min. After sonication, MCM-
24
22(P) solution was added into CTAB gel. After that, 49.40 g of an aqueous solution of 25wt%
TPAOH (tetrapropylammonium hydroxide, TCI) was mixed with MCM-22(P) and CTAB
solution to result a typically high pH about 13.2. The mixture was allowed to stir for 24 h at
room temperature, after which the samples were washed by methanol (Fisher), collected by
filtering and dried at room for overnight.
3.4. Pillaring of Nanosheets Precursor
Nanosheet material MCM-36 was synthesized by starting with pillaring of MCM-22(P).
Pillaring was carried out with about 0.5 g of dried swollen MCM-22(P) mixed with about 3.0 g
TEOS (tetraethyl orthosilicate, Aldrich) at a weight ratio of 1:6, stirred for 24 h at 80 C under
nitrogen atmosphere. The solid was washed by DI water, collected by filtering and dried in the
oven at 85 C for 1 h. The dried solid was hydrolyzed with water at a weight ratio of 1:10 and the
pH of the mixture was adjusted to 8 with NaOH. After 5 h mixing at 40 C, the solid was washed
by DI water, then filtered, and dried under room temperature for overnight. The dried solid was
calcined at 450 C under air atmosphere34
.
3.5. Preparation of Amino-Pillared Nanosheets (APNs)
Amine-pillared nannosheets sorbents were prepared via molecular basket method. More
specifically, pillared MCM-22(P) was impregnated with liquid organic polymer-poly
(ethyleneiminie) (PEI) via the wet impregnation method. According to the procedure reported by
Yan et al, typically 0.6g of PEI (Sigma, average MW ~800 by LS, average MN ~600 by GPC,
branched type) was dissolved in 4.8g of methanol (Fisher), and 0.15g of MCM-36 solid (which
had been heated at 100 C under vacuum for 24 hours before reaction) was added into the solution.
25
The mixture was stirred at room temperature for 6 h, then filtered and dried at 100 °C for 3 h
under vacuum.
3.6. Characterization
3.6.1. Powder X-ray diffraction (XRD)
Powder X-ray diffraction (XRD) is a powerful technique which is used to characterize
the crystal structure and its interplanar spacing distance (d) between parallel planes of atoms or
ions. In this study all materials crystal structures were determined by powder X-ray diffraction
(XRD). XRD patterns were collected on a Rigaku Ultima IV X-RAY diffractometer using Cu Kα
radiation to determine MCM-22(P), MCM-22, swollen MCM-22(P), MCM-36 and amine-
modified MCM-36. Data were collected over the range of 5-50° with a step size of 0.05° 2θ and
a step time of one second.
3.6.2 NOVA Analysis for Pore Surface and Pore Size Distribution
NOVA analysis is used to determine the BET (Brunauer-Emmett-Teller) surface area,
pore size and pore distribution of solid porous materials. This is possible by making use of the
Brunauer-Emmett-Teller (BET) theory, and the resulting BET equations. Brunauer, Emmett, and
Teller extend Langmuir’s monolayer adsorption theory to multi-layer adsorption, assuming that
the uppermost molecules in adsorbed stacks are in dynamic equilibrium with vapor. This gives a
final BET equation from which can be used to determine the specific surface area of the solid
material. Where P is the equilibrium pressure at the temperature of adsorption, P0 is the
saturation pressure at the temperature of adsorption, C is the BET constant, Wm is the weight
adsorbed in a completed monolayer, and W is the weight adsorbed.
26
1
𝑊[𝑃𝑃0
− 1]=
1
𝑊𝑚𝐶+
𝐶 − 1
𝑊𝑚𝐶(
𝑃
𝑃0)
A straight line will yield by plotting the 1/W[(P/P0)-1] versus P/P0 usually in the range 0.05≤
P/P0 ≤ 0.35 (Fig. 7).
Figure 7. Typical BET plot.
From the straight line, the slope s and the intercept can be obtained as below respectively,
𝑠 =𝐶 − 1
𝑊𝑚𝐶
𝑖 =1
𝑊𝑚𝐶
Solving the preceding equations, Wm, 6 membered rings (MB) openings perpendicular to the
layers, the weight adsorbed in a monolayer can be determined.
The total surface area can be calculated from the following equation,
𝑆𝑡 =𝑊𝑚�̅�𝐴𝑥
�̅�
27
where Ax is the cross-sectional adsorbate area, �̅� is the adsorbate molecular weight, and �̅� is
Avogadro’s number. The BET surface area can be determined by dividing St by the sample
weight.
The NOVA machine works by adsorbing nitrogen gas onto a solid support at 77 K by
immersing the sample chamber in liquid nitrogen. This is done because the adsorption cross
section and molar volume of the nitrogen gas is known at this temperature. By measuring the
pressure over saturation pressure with the total volume adsorbed, one can use the BET equations
to determine the surface area of the material.
Adsorption-desorption isotherms for N2 on MCM-22, MCM-36 and amine modified
MCM-36 samples were measured using NOVA 2200e surface area and pore size analyzer
(Quantachrome Instruments). Prior to the measurements, samples were degassed at 393 K for 16
h under vacuum. The samples were then immersed in liquid nitrogen at 77 K during N2
adsorption-desorption measurements.
3.6.3 Fourier Transform Infrared (FTIR) Analysis
FTIR, or Fourier Transform Infrared spectroscopy, is a technique which was used to confirm
formation of chemical bonds. When IR radiation is passed through a sample, a part of the
infrared radiation is absorbed by the sample and some of it is passed through. As a result, a
spectrum which represents the molecular absorption and transmission can be obtained, creating a
molecular fingerprint of the sample. From the absorption spectrum, potential chemical bonds in
the material can then be determined, based on tabulated standard patterns. In this study, FTIR
was used to determine the success of amine functionalization.
28
3.6.4 Electron Microscopy
Scanning electron microscope (SEM) was used to observe overall topological qualities of
the crystals. By scanning the sample with a focused beam of electrons, different kind of signals
can be produced and detected. The single contained information about the surface topography of
the sample which can be transformed into an image.
3.6.5 Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA), which is a thermal analysis method, measures the
amount of weight change of a material as a function of time or temperature under a certain gas
atmosphere. It is a typical technique to detect the thermal stability and composition of a material.
In this study, TGA (TA Instrument Q500) is used for carbon dioxide capture test for
MCM-22, MCM-36 and amine functionalized MCM-36. In this instrument, carbon dioxide (high
purity 99.999%) is introduced into the sample chamber after the removal of water and any other
species which was adsorbed in the material from ambient storage conditions. More specifically,
desorption was the first step under 120 degrees Celsius in nitrogen for 3 hours to remove
previously adsorbed species, followed by cooling back to 25 degrees Celsius. Following this
pure carbon dioxide gas was flown into the furnace for 5 hours. This data was used to obtain the
total capture capacity of carbon dioxide, the initial capture rate, and the equilibrium time for each
sample. TGA was also used to test amine loading on MCM-36 samples. The total amine content
of each impregnated sample was obtained by heating the samples at a ramp rate of 10 degrees
Celsius per minute up to 800 degrees Celsius and then held for 40 min. This was compared with
the bare material weight percent curve to determine the total polymer loading.
29
4.0 RESULTS AND DISCUSSION
4.1. Synthesis Results of Nanosheets Precursor Material MCM-22(P)
4.1.1 Initial synthesis basic on previous work
The initial MCM-22(P) was synthesized based on the method described by Corma et al20
.
In general, the gel was made from 0.361 g sodium aluminate (Sigma-Aldrich), 1.24 g sodium
hydroxide (Acros), 155.75g DI water, 11.8 g fumed silica (Acros) and 9.80 g HMI
(hexamethylenimine, Aldrich). After 6 h stirring, the gel was transferred to 95 and 115 ml
Teflon-lined autoclaves, then the autoclaves were placed into oven under 130 C for 11 days.
However, after 11 days, this gel yielded amorphous phase (Fig. 8 a).
4.1.2 Influence of water
It is known that water plays an important role in hydrothermal synthesis of single crystals,
because it is dependent on the solubility of minerals in water16
. As a solvent, water can enhance
dissolution of other chemicals, diffusion of molecules, and increase the solubility of reagents.
Furthermore, zeolite crystallization is usually done at autogenous pressure at high temperature
generated by water. To study the effect of water, in this study, we increased 10wt% of DI water
in the synthesis gel. After 11 days, XRD pattern of the final product was collected (Fig. 8 b) and
it is in agreement with those reported in the literature26
for MCM-22(P) (Fig 9).
30
Figure 8. XRD patterns of (a) original recipe; (b) 10 wt% increased water.
Figure 9. XRD of the MCM-22-P crystal, from Corma et al.
For further study of the influence of water on MCM-22(P), we increased 15wt% and
20wt% of DI water in the synthesis gel. Fig. 10 is the XRD patterns of as-synthesized MCM-
22(P) samples with different H2Owt%, showed typical XRD peaks assigned to MCM-22(P),
31
which are very similar to those reported in other literatures. Morphologies of the MCM-22(P)
samples were synthesized at different water concentrations are also provided as SEM images, as
shown in Fig. 11. It is demonstrated that the morphology changed with the different percentage
of H2O. Bigger donuts like particles are observed at higher water percentage, on the other hand,
at higher water level, relatively smaller rose like particles are observed.
The change of the crystalline morphology at different water concentration suggested that
overall crystal size decreases with increasing water content in the mother gel. It is proposed that
the increase of water can enhance dissolution and increase the solubility of chemicals; this might
lead to more nucleus formation during the aging step, produced smaller particles as the final
product.
Figure 10. XRD patterns of MCM-22(P) samples for (a) 10wt% increased water; (b) 15wt%
increased water; (c) 20wt% increased water.
32
Figure 11. SEM images of the MCM-22(P) samples were synthesized at different water
concentrations (a) 10wt% increased water; (b) 15wt% increased water; (c) 20wt% increased
water.
4.1.3 Influence of content of hexamethyleneimine (HMI) in the reaction mixture
The success of MCM-22(P) synthesis requires a high content of the organic template-
hexamethyleneimine (HMI)17
. Furthermore, the amount of organic structural-directing agent in
the reaction mixture is expected to affect the crystal morphology of MCM-22(P)24,16
. In order to
determine the effect of HMI content on the final crystal morphology, three samples with
different HMI content in the mother gel were synthesized where the amount of HMI in the
reactant gel was increased from 10wt% to 20wt%. All samples were synthesized after 9 days.
The XRD patterns and SEM images of these samples are showing in Fig. 12 and Fig. 13
respectively.
33
Figure 12. XRD patterns of as-synthesized MCM-22(P) samples with different HMI contents (a)
10wt% increased HMI; (b) 15wt% increased HMI; (c) 20wt% increased HMI.
Figure 13. SEM images of as-synthesized MCM-22(P) samples with different HMI contents (a)
10wt% increased HMI; (b) 15wt% increased HMI; (c) 20wt% increased HMI.
XRD profiles are very similar between the three samples, assigned to MCM-22(P),
suggested that crystalline phase of MCM-22(P) were detected after 9 days. A decrease of the
crystallization time might be that the increase of HMI content leads to an increase of alkalinity of
the synthesis mixture and thus results an increase of the crystallization rate in a shorter period24
.
With higher HMI content (20wt%), the intensity of XRD peaks decreased along with more broad
34
patterns, which suggests the presence of smaller particles. SEM photos show that the increase of
HMI content in the reaction mixture led to a decrease in overall crystal. It was suggested that
increasing the amount of organic structural-directing agent in the reaction mixture may cause
more nucleation during the initial aging step, thus higher HMI content in the synthesis
compositions accelerated the crystallization, produced smaller MCM-22(P) crystals24
(for 10wt%
increased HMI sample, the average particle size is 3 µm in diameter and 1.0 µm in thickness; the
average particle size for 15wt% and 20wt% increased HMI samples are 2 µm in diameter and
0.25 µm in thickness).
4.1.4 Influence of SiO2/Al2O3 ratio
The SiO2/Al2O3 ratio in the initial reaction mixture plays an important role in the
synthesis of MCM-22(P)15-17,
23,27,28
. Wu et al17
reported that the morphology of MCM-22(P)
crystals changed with the SiO2/Al2O3 ratio. Cheng et al23
reported that with HMI lower
SiO2/Al2O3 ratio is favorable for MCM-22(P) synthesis and can limit the formation of impurity
crystal.
To investigate the effect of SiO2/Al2O3 ratio on the crystallization of MCM-22(P), the
SiO2/Al2O3 ratio was decreased to 50, about half of the original SiO2/Al2O3 ratio by adding less
amount of fumed silica to the synthesis mixture. The XRD patterns and SEM images of MCM-
22(P) with various SiO2/Al2O3 ratios are showing in Fig. 14 and Fig. 15 respectively. XRD
pattern showed MCM-22(P) was synthesized at lower SiO2/Al2O3 ratio. SEM images
demonstrated that at lower SiO2/Al2O3 ratio, better morphology is observed, but the morphology
of the crystals are not as homogeneous as the sample was synthesized at higher SiO2/Al2O3 ratio,
including both rose like particles and thinner disc like particles.
35
Figure 14. XRD patterns of as-synthesized MCM-22(P) samples with different SiO2/Al2O3 ratios
(a) and (b) SiO2/Al2O3=97; (c) and (d) SiO2/Al2O3=54.
Figure 15. SEM images of as-synthesized MCM-22(P) samples with different SiO2/Al2O3 ratios
(a) and (b) SiO2/Al2O3=97; (c) and (d) SiO2/Al2O3=54.
36
4.1.5 Summary of synthesis results
Increasing water content in the synthesis mixture can facilitate the generation of MCM-
22(P) crystal nuclei and leads to the rapid growth of crystals, as well as increasing HMI content.
Lower SiO2/Al2O3 ratio is favorable for the synthesis of MCM-22(P) but leads to heterogeneous
morphology and crystal size. By controlling the composition of synthesis mixture, basically three
different kind of crystalline morphology with different sizes can be obtained. Since for the bigger
donuts like particles, the swelling might be different to occur, and for thinner disk like particles
structure disruption might be caused by intruding long chain swelling and pillaring agent, the
rose like particles will be chosen as the final MCM-22(P) for making MCM-36. Thus, as the
final synthesis recipe, the SiO2/Al2O3 ratio is maintained as 100, but increased 15wt% more DI
water and 10wt% more HMI compared to the original synthesis recipe.
4.2. Calcination to Form Zeolite Structure-MCM-22
Upon calcination, structure directing agents HMI can be eliminated from the interlayer
space of MCM-22(P). At the same time, surface Si-OH groups are condensed to form Si-O-Si
interlayer bridges10,14
. This 3D MWW zeolitic material was patented as MCM-22, which as
discussed in Section 2.1.
Calcination was carried out under the condition was described in experimental section 3.2.
The XRD pattern of calcined sample is given in Fig. 16 b and the XRD pattern of uncalcined
sample is showing in Fig. 16 a as a reference. The XRD pattern of calcined sample is in
agreement with those reported in the literature.10,14,27
37
Figure 16. XRD patterns of samples: (a) uncalcined product; (b) calcined product.
4.3. Swelling and Pillaring Results of Nanosheets Precursor
The swelling experiment was carried out based on the method described in the
experiment section. According to previous report18
, MCM-22(P) was swollen with CTAB at
room temperature. The initial swelling was not washed by methanol, but only washed by DI
water and filtered. After dried in room temperature, the sample was characterized by XRD and
SEM. The XRD pattern and SEM images are showing in Fig. 17 and Fig. 18.
Figure 17. XRD patterns of MCM-22(P) (a) before swelling and (b) after swelling.
38
Figure 18. SEM images of MCM-22(P) (a) before swelling and (b) after swelling.
As can be seen from the XRD pattern of swollen MCM-22(P) in Fig. 16 b, the pattern
doesn’t agree with the character of swollen sample described earlier in section 2.2 (Fig. 5), but
some intense peaks were observed after swelling. The SEM image of swollen sample showed
that the rose like particles could not be observed, but large agglomeration of the particles were
detected. Further evidence showed that, these intense peaks were verified as swelling agent
CTAB powder (the XRD pattern of pure CTAB powder is in Fig 19.)
Figure 19. XRD pattern of pure CTAB powder.
In order to eliminate unreacted CTAB from swollen sample, methanol was introduced to
wash the sample. The dried sample was analyzed by XRD and the result is showing in Fig. 20.
39
The XRD pattern of the swollen sample agrees well with the character of swollen sample
described earlier. The XRD pattern clearly showed that after swelling, hk0 reflections such as
(220) and (310) reflections remained the same; the (002) reflection at 6.5 theta disappeared; (101)
and (102) reflections at 8 theta and 10 theta turned into a broad band because the layer registry in
the c direction got lost after swelling33
.
Figure 20. XRD patterns of MCM-22(P) (a) before swelling and (b) after swelling.
Pillaring of the swollen sample was performed by following the procedure reported by
Masheshwari et al which was described in Section 3.3. XRD powder patterns of pillaring sample
in Fig. 21 c correspond well to those reported in the literature34,33,10,32
and the prepared solids
have been identified as MCM-36 materials.
40
Figure 21. XRD patterns of MCM-22(P) (a) before swelling; (b) after swelling and (c) after
pillaring.
4.4. Amino-Pillared Nanosheets (APNs)
By changing the concentrations of PEI in methanol solution, six samples with different
amount of PEI loading were prepared. TGA was used to determine the total loading of amine on
the sample surface. According to TGA results, the amine loading on the samples are varied from
3.97 wt% to 18.35 wt% (Table 1).
FTIR is used to determine the success of amine functionalization. The IR spectra of
MCM-36 and PEI functionalized (16wt%) MCM-36 are shown in Fig. 22. All samples show the
characteristic peaks below ~1250 cm-1 of an aluminosilica porous solid. The MCM-36 substrate
(Fig. 22 a) showed an FT-IR broad band around 3424 cm-1
which could be attributed the O-H
stretching vibrations of the hydrogen-bonded silanol group and adsorbed water molecules. The
band appearing at 1631 cm-1
was assigned to adsorbed water. All PEI modified MCM-36
samples exhibited similar spectra. For example for PEI functionalized (16wt%) MCM-36 (Fig.
41
22 b), after PEI loading, the O-H bending and adsorbed water bands disappeared, indicating that
the amine groups of PEI and the surface hydroxyl groups of MCM-36 substrates formed Si-O-
N+H3R and/or Si-O
-N
+H2R groups
6. Then broad band at 3375 cm
-1 could be attributed to the N-H
bending vibrations. The new bands in the ~2800-3000 cm-1
regions were due to the CH2 bending
vibrations. There are also three other IR bands appeared between ~1400 and 1700 cm-1
, which
could be characteristic of primary amines (NH2) and secondary amines (NRH) in PEI6.
Figure 22. FTIR spectra of (a) MCM-36 substrate; (b) PEI functionalized (16 wt%) MCM-36.
4.5. Physical Properties
NOVA analysis is used to determine the BET (Brunauer-Emmett-Teller) surface area,
pore size and pore distribution of MCM-22, MCM-36 and PEI functionalized MCM-36.
1631 3424
a
b 3375 2949
2833
42
Nitrogen adsorption-desorption isotherms of MCM-36 and MCM-22 are shown in Fig. 23 a and
b. The MCM-22 isotherm displays a Type I isotherm with a slight hysteresis, indicating high
micropore content. The isotherm of MCM-36 is Type II isotherm which typically indicates a
material with mixed micro- and mesoporosity11
. An increase in the adsorption volume up to P/P0
of ̴ 0.3 due to the presence of mesopores which were created by pillaring in MCM-36 sample32
.
Nitrogen adsorption-desorption isotherms of 4 wt% PEI and 18 wt% PEI loaded MCM-36
samples are shown in Fig. 23 c and d. The isotherms change significantly after introduced amine
molecules on bare MCM-36 samples. The overall Type II like isotherm of 4 wt% PEI content
MCM-36 sample is due to the microporous region inside the layers, which should not be
significantly changed by the introduction of PEI into the interlayer space. As more polymer
chains are introduced, the isotherm eventually become those for a primarily non-porous material
with slight hysteresis as seen for the 18 wt% PEI content MCM-36 sample, suggesting some
mesopore access. This result claims that higher amine loadings will eventually completely block
the pore access in the material.
Figure 23. Nitrogen adsorption-desorption isotherms of (a) MCM-36, (b) MCM-22, (c) 4 wt%
PEI functionalized MCM-36 and (d) 18 wt% PEI functionalized MCM-36.
a
b
c
d
43
Table 1. Physical properties of MCM-22, MCM-36 and PEI functionalized MCM-36 samples
Sample Name BET Surface Area
(m2/g)
BJH desorption
pore volume
(cc/g)
BJH Pore Radius
(Angstroms)
Amine Content
(wt %)
MCM-22 463 0.144 18.2 0
MCM-36 765 0.199 20.4 0
3.97wt% PEI-
MCM-36
14.1 0.0249 18.1 3.97
4.99wt% PEI-
MCM-36
21.7 0.0618 18.1 4.99
5.12wt% PEI-
MCM-36
28.8 0.0495 16.1 5.12
8.79wt% PEI-
MCM-36
18.6 0.132 20.3 8.79
16.6wt% PEI-
MCM-36
5 0.0982 18.1 16.6
18.35wt% PEI-
MCM-36
4.2 0.051 16.1 18.35
A summary of physical properties of different adsorbents are listed in Table 1. As can be
seen from Table 1, MCM-36 has 765 m2/g BET surface area which is the largest surface area
among all PEI functionalized adsorbents and MCM-22. A significant decrease of the BET
surface areas and pore volumes of the PEI functionalized samples were observed as the amount
of PEI loaded into the pores is increased. This can be explained that the addition of amines to the
surface immediately decreases the external surface area available for molecule diffusion into the
material, and this results the significant decrease of the BET surface area on amine grafted
samples. Among the PEI loaded samples, it can be seen that the surface areas and pore volumes
of these samples appears to increase initially with a small amount of amine loaded on MCM-36,
and then it appears to decrease with higher amine loadings. Eventually it reached the minimum
value at the highest loadings. It is hypothesized that the interpenetration of impregnated polymer
molecules may be increasing the amount of available surface areas with increasing polymer
44
content until eventually amine impregnation completely blocks the pore space. BJH pore volume
in Table 1 further supporting this hypothesis. For instance, when the PEI loading increased to 8.8
wt%, the BJH pore volume of the resulting sample increased to 0.132 cc/g, which is a maximum
value among PEI loaded samples. This may be due to the newly formed pore structure achieved
by the polymer ligands of impregnated amines which become interpenetrated with each other in
the pore space of MCM-36, rather than the original pores in the host. As more PEI is added, the
amine guest may aggregates on external surface of the host, which results the blockage of pores,
and eventually it reached to the point that the composite displayed non-porous character.
4.6. CO2 Capture Capacity
CO2 adsorption capacities of MCM-36 and PEI-impregnated MCM-36 samples with
different levels of amine loading were tested by TGA under the experimental condition which
was described in Section 3.6.5 earlier.
Figure 24. Weight change of bare MCM-36 on CO2 capture test by TGA
45
Fig. 24 shows the weight change of bare MCM-36 by CO2 capture. All PEI loaded samples
showed the similar curves for CO2 capture by TGA. Fig. 25 shows the normalized CO2 capture
capacity (CO2 capture capacity of PEI loaded MCM-36 divided by CO2 capture capacity of bare
MCM-36) versus the PEI weight percent loaded obtained via TGA analysis. The bare MCM-36
has the highest CO2 adsorption capacity, about 1.03 mmol/g. All PEI modified MCM-36 samples
showed significant reduction in adsorption capacity of the composite which is most likely due to
the blockage of the pores of the host. The capture capacity and amine loading displace a curve
shape as can be seen in Fig. 25. CO2 capture capacity increases slightly as the pores are filled
with amine molecules, then decreases again significantly for the highest loadings achieved. It is
suggested there is an optimal loading amount of PEI dispersed in the MCM-36 support8. With
lower PEI loadings, the amine polymer may coat the external surface of the host material, and
due to blockage of pore systems caused by amine polymer, adsorbate molecules are greatly
hindered to access to the interlayer space of the support material, which caused a decrease of
diffusion of CO2. As more PEI polymer is loaded, the amount of chemisorption sites increases
which is leading to an increase in the capture capacity observed until reached to an optimal
loading point (8.8 wt% PEI loading). Further increase leaded to a significant decrease which was
caused by large aggregated amine polymer.
The normalized adsorption equilibrium half time (the half time for the adsorbents to reach
adsorption equilibrium), which is the half time of PEI loaded MCM-36 divided the half time of
bare MCM-36, showed a similar trend (Fig. 27). Half the time to reach equilibrium, defined here
as a weight percent versus time slope of less than 0.0001 mg/min in 100 minutes was found to be
between 58.5 and 33.5 minutes for the bare MCM-36 samples. This represents adsorption of CO2
onto the material with no pore blockage or diffusion limitations. Upon amine impregnation the
46
adsorption half time increase significantly, up to approximately 720 minutes for the 8.8 weight %
amine loaded sample which also had the highest CO2 capacity among all amine loaded samples.
This result suggests that carbon dioxide can diffuse through the large pore space; however this
diffusion has limitation. Further amine loading resulted a decrease of the adsorption half time to
approximately 560 minutes for the 16.6 and 18.4 wt% PEI modified MCM-36 samples, however
these samples showed greatly decreased carbon dioxide capture capacities, suggesting that the
capture capacity is decreased for these samples due to the inability of the carbon dioxide to
adequately diffuse into the pore space.
The ratio of mole CO2 adsorbed to mole amine loaded in the host gave further evidence
about the relationship between amine content and CO2 capture capacity. According to the
following equations, ammonium carbamates could be formed between the reaction of amine and
CO2.6
𝐶𝑂2 + 2𝑅𝑁𝐻2 → 𝑅𝑁𝐻𝐶𝑂𝑂− + 𝑅𝑁𝐻3+ (1)
𝐶𝑂2 + 2𝑅2𝑁𝐻 → 𝑅2𝑁𝐻𝐶𝑂𝑂− + 𝑅2𝑁𝐻2+ (2)
Since 1 mole CO2 reacts with 2 moles of amines, the ratio (mole CO2 adsorbed/ mole amine
loaded) larger than 0.5 indicates that both support material MCM-36 and functionalized PEI
phases should contribute to CO2 adsorption. The relationship between amine loading and mole
CO2 adsorbed/ mole amine loaded ratio in Fig. 26 shows that when the amine loading is lower
than 8.8 wt%, the mole ratio of adsorbed CO2 to loaded amine is larger than 0.5 which means
CO2 molecules can still diffuse through the large pore space.41
The ratio for 5.0 wt% PEI loaded
sample was 0.88 implies at this amine loading point, both MCM-36 phase and amine phase
contributed towards CO2 capture. As the amine content reached to 8.8 wt%, the ratio decreased
47
to 0.61, which is closer to 0.5, suggested that further amine loading increased the amount of
chemisorption sites but hindered CO2 diffusion. The ratios for 16.6 and 18.4 wt% PEI loaded
samples are 0.12 and 0.13, respectively, which means further amine loading has no synergy
effect for CO2 capture.
Figure 25. Influence of PEI loading on CO2 adsorption.
Figure 26. Change of adsorption equilibrium half time with various PEI loading.
48
Figure 27. Mole ratio of CO2 adsorbed/ amine loaded of the various PEI loading.
49
5.0 Conclusions
This work has shown that MCM-36 was prepared from nanosheet precursor material
MCM-22(P), and functionalized by PEI polymer. The bare MCM-36 and functionalized MCM-
36 with various amine contents were tested for CO2 adsorption. The results showed that the bare
MCM-36 has the largest BET surface area (765 m2/g) and pore volumes (0.199 cc/g) as
compared to the amine loading 5wt%, allows for up to 0.8 mmol CO2/mmol amine but a larger
amount of amine loading, 10wt% or more, inhibits CO2 adsorption because of blockage of the
mesopores of MCM-36. This suggests that because the transport limiting openings (6 MB) to the
layers of MCM-36 is smaller than CO2 gas molecules, upon impregnation of the amine species
on MCM-36 can block the interlayer and causes the CO2 gas molecules to only be able to access
a limited number of the amine groups introduced via the impregnation of the polymer species.
While these results approve that MCM-36 cannot be greatly improved as a CO2 adsorbent via
polymer-amine impregnation, it is believed that if c-axis diffusion of carbon dioxide could occur
it may be possible to increase amine loading without decreasing the diffusion properties of the
material, allowing for increased capture over the bare derivative.
6.0 Recommendations
This work has provided direction in successful construction of a conventional supported
amine adsorbent, approved c-axis opening for diffusion of carbon dioxide is a key point for
making Amine-pillared Nanosheet adsorbents. Thus, the next step in the work is to look at the
layered material has the transport openings to the layers which are larger than CO2 gas molecules,
such as AMH-3 for APN adsorbents preparation.
50
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