Applied Catalysis A: General 358 (2009) 232–239

Perfluorinated alkylsulfonic acid functionalized periodic mesostructuredorganosilica: A new acidic catalyst

David Dube a, Micha Rat a, Wei Shen a, Bendaoud Nohair a, Francois Beland b, Serge Kaliaguine a,*a Department of Chemical Engineering, Laval University, Pavillon Pouliot, Quebec City, QC, Canada G1K 7P4b SiliCycle Inc., 114-1200 Ave. Saint-Jean-Baptiste, Quebec City, QC, Canada G2E 5E8

A R T I C L E I N F O

Article history:

Received 21 November 2008

Received in revised form 10 February 2009

Accepted 12 February 2009

Available online 21 February 2009

Keywords:

Fluorinated alkylsulfonic acid

functionalized periodic mesostructured

organosilica

Perfluorinated alkylsulfonic acid silane

Acidic catalyst

Periodic mesostructured organosilica

A B S T R A C T

Fluorinated alkylsulfonic acid functionalized periodic mesostructured organosilica was one-pot

synthesized using 1,2-bis(trimethoxysilyl)ethane (BTME) and a perfluorinated alkylsulfonic acid silane

(PSTE) under acidic conditions using Pluronic 123 as surfactant. Wormhole mesostructures with surface

areas up to 500 m2/g and narrow pore size distribution (around 4.1 nm) were obtained. This work thus

provides an example of one-pot synthesis for new functionalized PMO acid catalysts. The physico-

chemical properties were monitored by N2 adsorption, XRD, thermogravimetric analysis, 13C, 19F and 29Si

MAS NMR and proton conduction. Liquid phase self-condensation of heptanal was performed at 75 8C in

the presence of these catalysts, and results were compared with those obtained with several other

heterogeneous hydrophobic acid catalysts.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Periodic mesostructured organosilicas (PMOs) with organicgroups bridged in the framework have opened a new area indevelopment of heterogeneous catalysts [1–3]. Since the discoveryof PMOs, various organosilica precursors with bridging organicgroups, such as methane, ethane, ethylene, benzene, thiophene andbiphenyl, have been incorporated in the framework [4–9]. Recently,self-assembly of a trimethylene-bridged cyclic silesquioxane pre-cursor to yield the well-ordered trimethylene-bridged 3-ring PMOhas been performed by Landskron and Ozin [10]. Che et al. [11] haveproposed a new templating route for preparing hybrid mesoporousmaterials using anionic surfactants. They used aminosilane orquaternized aminosilane as co-structure-directing agent (CSDA)which interacts electrostatically with the anionic surfactants toproduce well-ordered mesostructured materials.

Efforts have been made to create solid acid catalysts withstrong Bronsted acidity, similar to zeolites, but with larger porediameters accommodating larger substrates. Diaz et al. [12] havegrafted mercaptopropyltrimethoxysilane on MCM-41 and MCM-48. A propylsulfonic acid moiety was then produced by theoxidative reaction of the thiol functional group with H2O2. Thismethod was further applied to other mesostructured materials

* Corresponding author. Tel.: +1 418 656 2708; fax: +1 418 656 3810.

E-mail address: serge.kaliaguine@gch.ulaval.ca (S. Kaliaguine).

0926-860X/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2009.02.015

(FSM-16 [13] and SBA-15 [12,13]). Lin and co-workers [14]reported a new co-condensation method to generate carboxylicand sulfonic acid functionalized mesostructured silica MCM-41type nanosphere materials while controlling the surface con-centration of the organic functional groups. The method involvesthe utilization of disulfide-containing organosilanes, such as 3-[30-(trimethoxysilyl)propyldisulfanyl]propionic acid and 2-[3-(trimethoxysilyl)propyldisulfanyl]ethanesulfonic acid sodiumsalt, to electrostatically match with the cationic template (CTAB)in a base catalyzed reaction of TEOS. The presence of electron-withdrawing fluorine atoms in the structure significantlyincreases the acid strength of the terminal sulfonic acid group.Alvaro et al. [15,16] presented the preparation of hybrid MCM-41and SBA-15 silicas functionalized with perfluoroalkylsulfonic acidgroups by a single step reaction between the surface OHs of themesostructured materials and 1,2,2-trifluoro-2-hydroxy-1-tri-fluoromethylethane sulfonic acid b-sultone (THTSAS). Theseheterogeneous catalysts yielded a better catalytic activity inthe etherification of octanoic acid with ethanol (C8 acid conver-sion of 88% at 60 8C) than Nafion1 (C8 acid conversion of 57%under the above conditions). Moreover, the acylation of anisolewas also possible, with high selectivity for 4-methoxyacetophe-none. Hybrid material with maximum acid capacities of 0.5 mmolH+/g was obtained. Following the same idea Shen et al. [38]introduced THTSAS in PMO ethane silica and obtained a sulfurloading up to 0.8 mmol S/g. On the other hand, Harmer et al.[17] and Macquarrie et al. [18] presented a non-oxidative direct

D. Dube et al. / Applied Catalysis A: General 358 (2009) 232–239 233

synthesis of mesostructured silica-perfluorosulfonic acid materi-als with strong Bronsted acid sites. The materials were preparedusing a perfluorosulfonic acid silane and TEOS using the sol–gelco-condensation route. The complete incorporation of bothsilanes gave a well-ordered material with a loading of 0.2 mmolS/g. The catalyst displays a high selectivity for the reactionbetween 2-methylfuran and acetone to yield the bis-furan.

Acid functional groups can also be directly incorporated in PMOs.Acid silane precursors may be introduced in the PMO synthesismixture and co-condensed with the silsesquioxane. All thesetechniques allow to create acid sites in a less polar organicenvironment while increasing the diffusivities of reactants andproducts owing to the large pore diameters of the sulfonic acidfunctionalized PMOs so produced [19–24]. Kondo and co-workers[25,26] have proposed a chemical modification technique to create anew type of PMO acid catalysts. In this procedure, ethylene sites atthe surface of an ethylene-bridged PMO are converted to phenylenesulfonic acid groups in a two-step process. The procedure involves aDiels-Alder reaction with benzocyclobutene followed by sulfonationin the presence of H2SO4. Dube et al. [27] have prepared a sulfonicacid functionalized PMO (ethylene bridged) by a two-step chemicalmodification technique. First, the ethylene bridges were arylatedwith benzene using AlCl3 as catalyst. This material was furthertreated with H2SO4 for the sulfonation of the phenyl moietiesyielding the sulfonic acid functionalized PMO.

Several reactions have been tested with different sulfonic acidmesostructured silicas or organosilicas such as esterification ofglycerol with fatty acid [12,28], esterification of acetic acid withethanol [24], synthesis of 2-ethoxytetrahydropyran from 3,4-dihydro2H-pyran and ethanol [29], synthesis of bisphenol A bycondensation of phenol with acetone, alkylation of phenol with 2-propanol [23], etherification of 1-butanol [19], heptanal self-condensation [27], liquid phase acetalization of heptanal by 1-butanol [30] and alkylation of isobutane/1-butene [38]. Amongthese reactions all the water producing ones should benefit fromthe increased hydrophobicity of the acid functionalized PMOs. Inthat case, Morales et al. [32] proposed a new method using highresolution solid-state 2D heteronuclear correlation (HETCOR) NMRspectroscopy under fast magic angle spinning to provide highlyresolved spectra between protons and low-gamma nuclei (13C and29Si) to provide detailed structural characterization. The resultsestablished that the incorporation of organosilicas moities into thelattice reduces the water adsorption by sulfonic acid sites duringthe etherification of vanillyl alcohol with 1-hexanol.

Self-condensation of carbonyl compounds is an importantreaction in synthesis of fine chemicals because it leads to C–C bondformation. This reaction can be catalyzed by acids or bases. Most ofthe studies in the literature have reported the self-condensationover solid base catalysts [31,33,34].

In this work, we propose a method to one-pot synthesizeperfluorinated alkylsulfonic acid functionalized PMO with highthermal stability. The synthesis involves an acid-catalyzed hydro-lysis and condensation of 1,2-bis(trimethoxysilyl)ethane (BTME)and a perfluorinated alkylsulfonic acid silane (PSTE) using PluronicP123 as the structure-directing agent followed by a soxhletextraction. PMO was further OH capped by the grafting on itssurface of the trimethylmethoxysilane (TMMS). The catalyticproperty in the self-condensation of heptanal which reactionproduces water was studied.

2. Experimental

2.1. Catalyst preparation

The designation of the catalysts reflects four characteristics of thematerial. Here PMO stands for ethane silica, C means OH capped, F

means fluoroalkyl and SA indicates the sulfonic acid moiety so thatPMO-CFSA0.4 designates a perfluoroalkylsulfonic acid functiona-lized ethane-silica OH capped having a loading of 0.4 mmol H+/g. Aschematic illustration of the synthetic pathway to the newfluoroalkylsulfonic acid functionalized PMO is shown in Fig. 1.

2.1.1. Preparation of the perfluorinated sulfonic acid silane (PSTE)

Perfluorinated alkylsulfonic acid silane (PSTE) was preparedusing the 3 steps synthesis procedure described by Harmer et al.[17]. First, 25 g of tetrafluoro-2-(tetrafluoro-2-iodoethoxy)etha-nesulfonyl fluoride (ICCF) and 6.68 g of allyl acetate were stirredand heated at 100 8C. 0.48 g of benzoyl peroxide was added over4 h. The solution was then distilled to yield 21.6 g of CH3COOCH2-

CHICH2(CF2)2O(CF2)2SO2F (AcOICCF), bp 87 8C at 0.1 mmHg. Theproduct was added over 1 h in a mixture of 2.97 g of isopropylalcohol, 4.53 g of acetic acid and 5.89 g of zinc powder at 90 8C for4 h. The residue was washed with NaHCO3 1 M up to pH 7,extracted with CH2Cl2 and distilled. The distillation product gave3.7 g of CH2 CHCH2(CF2)2O(CF2)2SO2F (PrCCF), (yield 30%) bp134–135 8C. Finally, 3.7 g of PrCCF, 2.66 g triethoxysilane wereadded to 5 mL of toluene and 16 drops of platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex 2% in xylene under argonfor 72 h at 25 8C. The distilled product gave 4.0 g of perfluorinatedsulfonic acid silane 1 (PSTE) (yield 70%), bp 54 8C at 0.05 mmHg.

2.1.2. Preparation of PMO functionalized with perfluorinated sulfonic

acid silane (PMO-CFSA0.4 and PMO-FSA0.4)

PMO functionalized by perfluorinated sulfonic acid silane wasprepared using 1,2-bis(trimethoxysilyl)ethane (BTME) and PSTEfollowing the PMO synthesis procedure described by Hamoudi et al.[35]. In a typical synthesis, 3.27 g of P123 (BASF) were dissolved in84.7 g of H2O and 19.7 g HCl 2 M at room temperature. After com-plete dissolution, the solution was heated at 40 8C before addition of4.26 g of BTME followed by 1.0 g of PSTE (2 min after BTME). Theresulting solution was stirred for 20 h. The resulting mixture wastransferred into an autoclave for aging at 90 8C for 24 h. The productwas filtered, washed with water and dried at 80 8C overnight.The template was removed from as-synthesized material bySoxhlet extraction with EtOH 48 h. Finally, surface OHs were cappedusing trimethylmethoxysilane (TMMS) in toluene reflux for 24 h.

2.1.3. Preparation of SBA-15 functionalized with perfluorinated

sulfonic acid silane (SBA-CFSA0.6 and SBA-FSA0.6)

Perfluorinated sulfonic acid silane functionalized SBA sampleswere prepared using tetraethoxysilane (TEOS) and PSTE followingthe synthesis procedure described by Hamoudi et al. [35]. In atypical synthesis, 1.97 g of P123 (BASF) were dissolved in 60 g HCl2 M at room temperature. After complete dissolution, the solutionwas heated at 40 8C before addition of 3.67 g of TEOS followed 1.0 gof PSTE (2 min after BTME). The resulting solution was stirred for20 h. The resulting mixture was transferred into an autoclave foraging at 90 8C for 24 h. The product was filtered, washed withwater and dried at 80 8C overnight. The template was removedfrom the as-synthesized material by Soxhlet extraction with EtOH48 h. Finally, surface OHs were capped using trimethylmethox-ysilane (TMMS) in toluene reflux for 24 h.

2.2. Characterization

X-ray diffraction patterns were recorded using a D4 Endeavor/Bruker AXS powder diffraction system with Cu Ka radiation(l = 1.5406 A). Diffraction patterns were recorded with scan stepof 0.028 for 2u between 0.58 and 58.

Nitrogen adsorption/desorption isotherms were measured atliquid nitrogen temperature using a QUANTACHROME NOVA 2000instrument. Before adsorption, the samples were evacuated at

Fig. 1. Schematic representation of the two-step synthesis procedure of PMO acid catalyst.

D. Dube et al. / Applied Catalysis A: General 358 (2009) 232–239234

120 8C for 6 h. Specific surface area was calculated using the BETmethod in the relative pressure range of 0.05–0.25. The porediameter was estimated from the peak position of the BJH pore sizedistribution.

The weight loss curves (TGA–MS) were recorded using a TAInstruments TGA model Q500 from ambient temperature to 700 8Cat a heating rate of 5 8C/min under helium. The samples weredehydrated at 110 8C for 2 h before TGA analysis.

Sample compositions were established by elemental analysisusing a CNS analyser CARLO ERBA model 1500. The ion exchangecapacities (corresponding to acid site concentration) of the sulfonicacid catalysts were determined using aqueous solutions of NaCl(0.1 M) and NaOH (0.005 M) [36]. In a typical experiment 0.05 g ofsolid was added to 10 g of NaCl solution. The resulting suspensionwas allowed to equilibrate and thereafter titrated by a drop wiseaddition of a NaOH solution.

Impedance spectroscopy measurements were performed overthe frequency range 1–107 Hz with oscillating voltage 100 mV,using a PC controlled SI 1260 impedance/gain-phase analyser

(Solartron) at room temperature. The impedance data werecorrected for contribution from the empty and short-circuitedcell. The proton conduction of most solid electrolytes is a waterassisted phenomenon, which is extremely sensitive to wateradsorption. The procedure for conductivity measurements used inthis work is described elsewhere in more details [37].

29Si and 13C MAS NMR spectra were obtained at room tem-perature on a BRUKER AVANCE 300 MHz. 29Si NMR was recordedat a frequency of 59.6 MHz and at 8 kHz spinning rate. 13C NMRwas recorded at a frequency of 76 MHz and at 8 kHz spinning rate.19F MAS NMR was obtained at room temperature on a VARIAN/CHEMAGNETICS INFINITY PLUS 400 WB. 19F NMR was recorded ata frequency of 375 MHz and at 10 kHz spinning rate.

2.3. Catalytic tests

2.3.1. Heptanal self-condensation

The liquid phase catalytic test was performed in a glass batchreactor equipped with an in situ ATR-FTIR probe (ASI, Mettler-

Fig. 2. XRD diffraction pattern of (a) PMO-FSA0.4 and (b) PMO-CFSA0.4.

Fig. 3. 13C MAS NMR spectra of (a) PMO-FSA0.4 and (b) PMO-CFSA0.4.

D. Dube et al. / Applied Catalysis A: General 358 (2009) 232–239 235

Toledo, USA) [19]. Linear calibration curves were established aspreviously described [19,30]. Before each reaction test, the catalystwas predehydrated for 1 h at 60 8C under vacuum. This predehy-dration operation was found to be crucial since the contact withhumid air yields drastic catalyst deactivation [19]. Typically 0.2 gof the dried catalyst was added to 50 ml of pure heptanal. In oneseries of experiments, the catalyst mass was varied from onecatalyst to the next in order to keep the same total content of acidsites in the reactor. The reaction was carried out in absence ofsolvent under autogenous pressure at 75 8C. The IR spectra of thereaction medium were recorded at specific time intervals and werelater processed using the React IR software. A calibration waspreviously realized at 75 8C with different concentrations ofheptanal using the 1720 cm�1 band. One hybrid mesostructuredmaterial (ASMES: arene sulfonic acid functionalized ethane silica[19]) and Nafion1 SAC-13 were used as reference samples.

3. Results

3.1. Characterization

Two perfluorinated alkylsulfonic acid grafted PMO and SBA-15catalysts were synthesized and analyzed. They were preparedusing different silica sources (silsesquioxane and silane) in order toobtain two different surface hydrophobicities with perfluorinatedalkylsulfonic acid function. The textural and compositionalproperties of the catalysts are presented in Table 1. Nitrogenadsorption–desorption isotherms for PMOs and SBA-15 supportsshow type IV curves according to the IUPAC nomenclature, whichindicates the mesoporous nature of the catalysts. The specificsurface area and the pore diameter of each catalyst decrease duringthe OH capping treatment. Those changes are due to the surfacemodification by the grafting reactions (TMMS) onto the surface oftheir supports. The sulfur and proton content reflect the density ofsulfonic acid groups and the values are reported in Table 1. TheXRD data (Fig. 2) show that the mesoporous structure is preservedafter the chemical modification. Diffraction peaks in XRD patternand regular arrangement in TEM (not shown) image reveal thatPMO-FSA0.4 and PMO-CFSA0.4 synthesized materials have thecharacteristic of wormhole structure, which indicates that theordering of the acidic functionalized PMO is not affected by thegrafting of trimethylsilane (OH capping). It is found that the 100peak of PMO-FSA0.4 and PMO-CFSA0.4 samples is slightly shiftedtoward higher diffraction angles after the grafting operation, likelyassociated with the average decrease in pore diameter due to theintroduction of the trimethylsilane function.

Figs. 3 and 4 show the 13C MAS NMR spectra of both catalysts(PMO-CFSA0.4 and SBA-CFSA0.6) before and after OH cappingtreatment. According to a study by Hamoudi et al. [35], thestrongest peak at 6 ppm is attributed the ethane bridges (Fig. 3). InFigs. 3b and 4b the peak occurring at 2 ppm shows the presence ofthe trimethylsilane used as capping agent. The peaks at 12, 14 and33 ppm are assigned to the alkyl chain of the fluorinated silane onboth catalysts supported on PMO and SBA-15. The peak at 59 ppm

Table 1Physico-chemical properties of the catalysts and their precursors.

Catalyst SBET

(m2/g)

Pore

diameter (nm)

%C [S]

(mmol/g)

[H+]

(mmol/g)

PMO-FSA0.4 698 4.3 23.2 0.37 0.40

SBA-FSA0.6 661 3.9 9.7 0.59 0.61

PMO-CFSA0.4 603 4.1 25.7 0.37 0.40

SBA-CFSA0.6 432 3.8 12.6 0.60 0.62

ASMES 782 5.3 18.6 0.65 0.68

Nafion1 SAC-13 200a 10a – 0.12a –

a Values obtained from the manufacturer.

is attributed to the non-hydrolysed ethoxy and methoxy moietiesfrom the fluorinated silane, TMS and BTME [32]. Signals at 17, 71and 78 ppm which are observed in both spectra are assigned to theremnant surfactant (P123) [20].

Figs. 5 and 6 show the 19F MAS NMR spectra of the PMO-CFSA0.4 and SBA-CFSA0.6 catalysts. Both spectra presented 4peaks at �82.1 ppm (OCF2CF2SO3H), �87.3 ppm (CF2CF2O),�111.7 ppm (CF2CF2O), �117.4 ppm (CF2CF2SO3H) corresponding

Fig. 4. 13C MAS NMR spectra of (a) SBA-FSA0.6 and (b) SBA-CFSA0.6.

Fig. 5. 19F MAS NMR spectra of PMO-CFSA0.4 (*spinning side band).

Fig. 6. 19F MAS NMR spectra of SBA-CFSA0.6 (*spinning side band).

Fig. 7. 29Si MAS NMR spectra of (a) PMO-FSA0.4 and (b) PMO-CFSA0.4.

D. Dube et al. / Applied Catalysis A: General 358 (2009) 232–239236

to the perfluorinated sulfonic acid silane incorporated in the PMOand SBA-15 lattice during the one-pot synthesis [17]. Peaks at+47.0 and �135 ppm are corresponding to the non-hydrolysedsulfonyl fluoride, SO2F, and the CF2CF2SO2F respectively.

29Si MAS NMR spectra of PMO-CFSA0.4 and PMO-FSA0.4 arepresented Fig. 7. Both spectra presented three peaks at �64, �57and�49 ppm attributable to T3 [RSi(OSi)3], T2 [RSi(OSi)2OH] and T1

[RSiOSi(OH)2] resonances respectively [32,35]. For the PMO-FSA0.4, the surface ratios of the peak components associated tothese silica species are recognized to be T3:T2:T1 = 37.2:45.3:17.5.After the capping step, T2 and T1 decrease, T3 increases and theratio becomes T3:T2:T1 = 51.5:35.6:12.9. These results indicate thatthe trimethylsilane is grafted on the surface of the PMO catalystas the increase in T3 corresponds to the disappearance of someof the support OHs upon capping. There is no occurrence of Qn

[Si(OSi)n(OH)4�n] resonances which attests that essentially allsilicon atoms were covalently bound to carbon.

29Si MAS NMR spectra of SBA-CFSA0.4 and SBA-FSA0.6 arepresented Fig. 8. Both spectra presented 5 peaks at �110, �101,�90, �64 and �57 ppm attributable to Q4 [Si(OSi)4], Q3

[OHSi(OSi)3], Q2 [(OH)2Si(OSi)2], T3 [RSi(OSi)3] and T2 [RSi(O-Si)2OH] resonances respectively [32,35]. For SBA-FSA0.6, thesurface ratios of the peak components assigned to these silicaspecies are recognized to be Q4:Q3:Q2:T3 = 46.0:40.4:7.6:6.0. Aftercapping step, Q3 and Q2 decrease, Q4 and T3 increase and the ratiobecomes Q4:Q3:Q2:T3:T2 = 53.3:21.3:5.1:14.2:6.1. These resultsindicate that the trimethylsilane is grafted on the surface of the

SBA-15 catalyst as the increase in Q4 and T3 correspond to thedisappearance of some of the support OHs.

The thermogravimetric curve and the corresponding gasphase MS signal for the PMO-CFSA0.4 catalyst sample are shownin Fig. 9. The differential thermogravimetric analysis for thecatalyst shows three peaks. The peak centered at 440 8C iscorresponding to the decomposition of the sulfonic acid groupsand the perfluorinated alkyl chain. The peak at 495 8C is cor-responding to the alkyl groups. The peak at 610 8C is correspond-ing to the thermal degradation of the ethane-silica framework ofthe support.

Impedance spectroscopy was used to determine the protonconductivity of the perfluorinated alkylsulfonic acid functionalizedPMO and SBA-15. The obtained conductivity results are presentedin Fig. 10. As expected, the proton conductivity increases withwater content. The maximum value obtained for PMO-CFSA0.4 is1.3 � 10�2 and 1.1 � 10�2 S/cm for SBA-CFSA0.6 at a water contentaround 200%. Nafion1 SAC-13 presented a lower conductivity at2.8 � 10�3 S/cm.

3.2. Catalytic performance

The catalytic properties of the catalysts were evaluated in theself-condensation of heptanal at 75 8C (Fig. 11), which generateswater. The GC–MS analysis of the reaction products at the end ofeach catalytic test showed that the only product obtained wascrotonaldehyde. This analysis showed no trace of the intermediatealdol product shown in Fig. 11. Thus the secondary dehydratationof the aldol product seems to be much faster than the primaryaldol condensation. Fig. 12 shows the time dependant heptanalconversion (calculated from the heptanal FTIR signals) over

Fig. 8. 29Si MAS NMR spectra of (a) SBA-FSA0.6 and (b) SBA-CFSA0.6.

Fig. 10. RT proton conductivity as a function of water content for (a) PMO-CFSA0.4,

(b) SBA-CFSA0.6 and (c) Nafion1 SAC-13.

Fig. 11. Schematic representation of self-condensation of heptanal over an acid

catalyst.

D. Dube et al. / Applied Catalysis A: General 358 (2009) 232–239 237

perfluorinated alkylsulfonic acid functionalized PMO and SBA-15with and without OH capping treatment with a density of acid sitesof 0.40 and 0.62 mmol H+/g. For the tests reported in Fig. 12 therespective weights of the catalysts introduced in the 60 mL reactorwere adjusted in order to keep the same number of acid sites in thedifferent tests. Curves (a) and (b) represent the conversion ofheptanal for the uncapped sample SBA-FSA0.6 and PMO-FSA0.4respectively. The conversion of SBA-FSA0.6 obtained after 3 h ofreaction is 15.3% comparatively to 17.0% for PMO-FSA.0.4. Curves(c) and (d) showed the conversion of heptanal for both cappedcatalysts. The conversion of SBA-CFSA0.6 after 3 h has reached

Fig. 9. Representative thermogravimetric derivative plot and MS signal for selected

m/z values of PMO-CFSA0.4.

Fig. 12. Heptanal conversion for the self-condensation reaction at 75 8C over (a)

0.138 g of SBA-FSA0.6, (b) 0.207 g of PMO-FSA0.4, (c) 0.138 g of SBA-CFSA0.6 and (d)

0.207 g of PMO-CFSA0.4 (for the same number of acid sites in the reactor).

19.5% relatively to 22.5% for PMO-CFSA0.4. The activity of Nafion1

SAC-13 is compared with ASMES and PMO-CFSA0.4 in Fig. 13. Theconversion of heptanal obtained is comparable to the one of PMO-CFSA0.4 after 3 h (22.5%) but initial activity is much higher forPMO-CFSA0.4.

Fig. 13. Heptanal conversion for self-condensation reaction over (a) 0.122 g of

ASMES, (b) 0.690 g of Nafion1 SAC-13 and (c) 0.207 g PMO-CFSA0.4 at 75 8C (for the

same number of acid sites in the reactor).

D. Dube et al. / Applied Catalysis A: General 358 (2009) 232–239238

4. Discussion

The comparison between S and [H+] contents in the varioussolids discussed in this work (Table 1) suggests that most of theperfluorosulfonic acid moieties are protonated even though the 19FMAS NMR (Fig. 5 and Fig. 6) indicated that some non-hydrolysedSO2F groups are still present even after the capping operation.These acid sites are especially strong as indicated by the highproton conductivity (Fig. 10) and predominantly Bronsted acids.Indeed the proton density measured from ion exchange capacitysuggests that essentially all acid sites can exchange sodium ionsfrom NaOH solution, which would not be the case for Lewis acidsites. The observed density of acid sites are consistent with theones reported for similar compounds [17,24,32,35].

The perfluorinated alkylsulfonic acid functionalized PMOsexhibited thermal stability up to 350 8C, which corresponds to thetemperature of the beginning of the sulfonic acid function and theperfluoroalkyl chain decomposition. This stability is thereforecomparable to the one observed for perfluorosulfonic acid graftedsilica [17] and arene sulfonic grafted mesostructured ethane silica[35].

The OH capping procedure does not seem to induce drasticchanges of the solid supports. There are no essential changes of theXRD patterns upon grafting TMMS on both PMO and SBA catalysts(Fig. 2). Only a moderate decrease in specific surface area and porediameter is observed in this case (Table 1). This capping is intendedto enhance the pore surface hydrophobicity which is reflected byboth the 13C MAS NMR (Figs. 3 and 4) and 29Si MAS NMR dataindicating a diminution of the surface silanol density (Figs. 7 and 8).The time dependence of the heptanal conversion presented inFigs. 12 and 13 allows to compare both the specific activities and therate of deactivation of the catalysts under study. Obviously, theinitial reaction rate is drastically increased after the OH cappingoperation (compare curves (c) and (d) with (a) and (b) at time zero inFig. 12). A possible explanation would be that the long alkylper-fluorosulfonic chain of the grafted acid would induce someflexibility of the functional group and allow the acid site to interactwith the surrounding surface silanols (note that the PMO-FSA0.4sample is not free of OHs as indicated by the NMR spectra in Fig. 7).These interactions would deactivate the site in the same manner aswater adsorption. Capping the OHs would decrease these interac-tions therefore enhancing the site activity in heptanal self-condensation. The difference in initial rate between PMO-FSA0.4and SBA-FSA0.6 (curves (b) and (a) in Fig. 12) would be also ex-plained owing to the more hydrophobic nature of the PMO surface.

It is also apparent that the rate of deactivation of the cappedcatalysts is higher. Comparing for example curves (a) and (c) inFig. 12 it is clear that the capped catalyst SBA-CFSA0.6 is essentiallydeactivated after 10–15 min whereas SBA-FSA0.6 is still activeafter 3 h. This may be related to the rate at which water isgenerated in both experiments. Interestingly PMO-CFSA0.4 stillshows some activity after the initial 10–15 min period which maybe related to a higher rate of water desorption induced by the OHcapping. This relationship between the rate of deactivation andwater retention is in full agreement with the observations ofMorales et al. [32] who studied sulfonic acid functionalized SBA-15silicas and organosilicas by 2D HETCOR MAS NMR technique. Theauthors concluded from solid state two-dimensional 13C{1H} and29Si{1H} heteronuclear correlation NMR that the incorporation ofhydrophobic ethylsiloxane groups in mesoporous silica frameworkinduced reduced interaction of adsorbed water with the silica inthe immediate vicinity of the SO3H sites.

The higher initial activity in the heptanal self-condensationobtained with PMO-CFSA0.4 in comparison to the Nafion1 SAC-13 samples (Fig. 13) can be related to a higher heptanal dif-fusivity in the hydrophobic environment of the capped catalystcomparatively to the hydrophilic silica support. The waterproduced could also be easily transferred out of the catalystparticle in the large hydrophobic mesopores and thus would notcontribute as extensively to the acid site deactivation.

5. Conclusion

The synthesis of perfluorinated alkylsulfonic acid functiona-lized PMOs by a one-pot procedure followed by OH capping stepwas described. The perfluorinated alkylsulfonic acid wassynthesized using 1,2-bis(trimethoxysilyl)ethane (BTME) asframework precursor and a perfluorinated sulfonic acid silane(PTSE) as acidic function source under acidic conditions, usingPluronic 123 as structure directing agent. Surface OHs were thenfunctionalized by grafting trimethoxymethylsilane (TMMS). Theresulting solid acid catalysts show high thermal stability (up to350 8C), acid site density (up to 0.40 mmol H+/g) and RT protonconductivity (up to 1.3 � 10�2 S/cm). The new perfluorinatedalkylsulfonic acid functionalized PMOs exhibit a high catalyticactivity in self-condensation of heptanal, owing to their high acidsite strength and the presence of both hydrophobic ethane-bridged framework and trimethylsilane function. Indeed thespecific catalytic activity of the perfluorinated alkylsulfonic acidsites in hydrophobic environments in such material as PMO-CFSA0.4 was shown to be much higher than the commercialNafion1 SAC-13. This underlines the potential assets of PMOs,high strength acid functions and high thermal stability ascatalysts supports in water generating reactions whenever theacid sites are deactivated by water.

Acknowledgements

The authors thank NSERC for financial support. The authors aregrateful to G. Lemay, Dr. S. Mikhailenko and B. Levasseur forassistance in the experimental work. We thank Pr. Yining Huangfor the 19F MAS NMR measurements at The University of WesternOntario.

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