structure and reactivity of sol–gel sulphonic acid silicas

Applied Catalysis A: General 228 (2002) 127–133

Structure and reactivity of sol–gel sulphonic acid silicas

Karen Wilson∗, Adam F. Lee1, Duncan J. Macquarrie, James H. ClarkDepartment of Chemistry, Green Chemistry Group, University of York, Heslington, York YO10 5DD, UK

Received 24 September 2001; received in revised form 8 November 2001; accepted 8 November 2001

Abstract

A range of mesoporous solid sulphonic acid catalysts have been prepared from a mercaptopropyl-trimethoxysilane (MPTS)precursor by sol–gel synthesis. The creation of surface sulphonic acid functionality via thiol oxidation has been followedby XPS and Raman spectroscopy. It is possible to continuously vary the sulphonic acid loading from 1 to 12 wt.% whilemaintaining pore volume and mesostructure. The resulting materials exhibit high thermal stability and acid strength acrossthe composition range and show good activity and selectivity in esterification and condensation reactions. © 2002 ElsevierScience B.V. All rights reserved.

Keywords: Solid acid catalysts; Organically modified silica; Clean technology; Green chemistry; Sulphonic acid; Mesoporous silica

1. Introduction

The syntheses of many fine and speciality chemicalsoften relies on homogeneous mineral acids, bases ormetal salts, which are frequently used in stoichiomet-ric amounts. Tightening legislation on the treatmentand disposal of excessive toxic waste, produced dur-ing the separation and neutralisation of products fromthese reaction media, is driving industry to considercleaner technologies, including the use of heteroge-neous catalysis. While zeolites are widely used as solidacid catalysts in gas phase chemistry, their use in liquidphase organic synthesis is limited by their small poresizes (<8 Å) which make them unsuitable for reactionsinvolving bulky substrates. However, recent develop-ments in materials chemistry have led to the discoveryof the M41S family of mesoporous molecular sieves

∗ Corresponding author. Fax:+44-1904-434546.E-mail address: [email protected] (K. Wilson).

1 Present address: Department of Chemistry, University of Hull,Cottingham Road, Hull HU6 7RX, UK.

[1] offering pore sizes in the range 20–100 Å and,thus, new avenues for liquid phase solid acid catalysis.

The subsequent report of neutral templating meth-ods using long chain alkylamines to prepare hexag-onal mesoporous silicas (HMS) [2] has attractedmuch interest since the template removal processby solvent extraction is simple and environmentallybenign. In addition, the use of a neutral templat-ing process also opens possibilities for incorporatingorgano-functionalised silanes into the silica frame-work during the sol–gel process [3,4]. These directpreparation routes offer the opportunity to incorporatehigher loadings of organic groups than those attain-able via post-grafting techniques [4]. Through this ap-proach a range of materials containing phenyl, cyanovinyl, amine, thiol, carboxylic acid, or sulphonicacid groups can be prepared [5,6]. Derivatisation ofselected materials can also subsequently be used toprepare immobilised organo-metallic complexes [7].

Organically modified silicas are widely used inseparation science [8], and the techniques for theirpreparation are well documented. For example silica

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128 K. Wilson et al. / Applied Catalysis A: General 228 (2002) 127–133

functionalised with sulphonic acid or thiol groups isfrequently used for binding metal ions [9,10]. Thischemistry has recently been exploited to prepare apure Brönsted sulphonic acid-functionalised meso-porous silica [11–13]. However, these studies alsoemployed a final H2SO4 acidification step [11,12]which generates undesirable aqueous acid waste.Such materials are interesting alternatives to commer-cially available sulphonated resins, Amberlyst-15 andNafion-H (sulphonated polystyrene and perfluorinatedsulphonic acid resins, respectively) which suffer fromlow surface areas and thermal stability [14–16].

Here, we build upon these studies by investigat-ing the detailed structural and reactive properties ofsolid sulphonic acid catalysts prepared via a one-potsol–gel route. We show that the loading of sulphonicacid active sites can be readily tuned with consequentenhancement of the resulting catalytic performance inesterification and also condensation chemistry.

2. Experimental

A range of templated, sulphonic acid-functionalisedsilicas were prepared by a co-condensation sol–gelroute utilising the following procedure. Molar ratios oftetraethoxysilane (TEOS) and mercaptopropyl-trime-thoxysilane (MPTS), over the range 19:1–1:1 (whereTEOS+ MPTS= 0.4 mol), were added to a solutionof dodecylamine (0.11 mol) in an ethanol (4 mol) andwater (11.4 mol) templating solution. The mixturewas then aged for 24 h whilst stirring, with the result-ing solid being filtered prior to removal of the dode-cylamine template by soxhlet extraction in ethanol.Finally, the powders were dried for 8 h at 70◦C.These thiol-functionalised silicas were then oxidisedat 25◦C using a 10-fold excess (based on the theo-retical S loading) of 50% H2O2 in water. The finalmaterial was washed with water and then ethanol,prior to drying at 70◦C for 8 h. The resulting materi-als were stored under ambient conditions and did notdegrade over several months.

Porosity and surface area measurements wereperformed following the N2 adsorption on a Mi-cromeretics ASAP 2010 instrument. Surface areaswere calculated using the BET equation over thepressure range (P/P0) 0.02–0.2, where a linear rela-tionship was maintained, while pore size distributions

(PSDs) were calculated using the BJH model. Scontent was determined by elemental analysis usinga Fisons EA1108 CHN/S analyser. Raman spectrawere recorded using a 60 mW, 514.5 nm laser and8 cm−1 detector resolution. XPS measurements wereperformed using a Kratos AXIS HSi instrumentequipped with a charge neutraliser and Mg K� X-raysource. Spectra were recorded at normal emissionusing an analyser pass energy of 20 eV, and X-raypower of 225 W. Evolved gas thermal analysis wasperformed using a NETZCH thermal analyser cou-pled to a Bruker Equinox 55 FT-IR. A heating rampof 10◦C min−1 was applied to the sample which washeld under a constant N2 flow of 100 ml min−1 tocarry the off gasses into the FT-IR gas cell.

Reactions were performed in a Radleys carouselreactor, with samples being taken periodically foranalysis using a Shimadzu GC17A gas chromato-graph fitted with an autosampler and a DB5 capillarycolumn. The liquid phase esterification of butan-1-olesterification was performed at 60◦C using 40 mmolacetic acid and 10 mmol butan-1-ol. Claisen–Schmidtcondensation of acetophenone and benzaldehyde wasalso performed in the liquid phase at 150◦C using20 mmol of both reactants. In all cases, 50 mg of cat-alyst and 0.5 g of tetradecane (internal standard) wereused. Initial rates of reaction were determined whilethe conversion of alcohol was<15%, with at leastfour data points being available in each plot. Catalystselectivity and overall mass balances (closure was>98% in all cases) were determined using appropriatereactant and product response factors.

3. Results and discussion

The success of the sol–gel templating route in gen-erating mesoporous materials was first verified acrossthe MPTS range. Fig. 1 shows that the surface area isapproximately constant over the range 1–4% S, how-ever, further increases in the MPTS content reducethe surface area. Corresponding trends are seen in thePSD shown in Fig. 2. For low loadings, a well-definedPSD is observed at 16.5 Å, however as the MTPScontent is increased the average pore diameter falls,reaching∼6 Å for the highest loading sample. De-spite these changes even the highest loading materialretains a high surface area in excess of 200 m2 g−1.

K. Wilson et al. / Applied Catalysis A: General 228 (2002) 127–133 129

Fig. 1. Average pore size and surface area of sol–gel preparedsulphonic acid silica as a function of S loading (wt.%).

Our measurements on the low loading materials arein accord with previous reports [13], and provide ameasure of the ‘accessible’ pore dimension avail-able for reactants. Such materials possess a degreeof crystallinity with a unit cell parameter of∼3.9 nmas determined by XRD [13], significantly larger thanthat determined by N2 physisorption. This observeddiscrepancy might reflect contributions from porewall thickness and a reduction in pore volume by thetethered organic functional groups.

Fig. 2. PSD of sol–gel prepared sulphonic acid silica as a functionof S loading (wt.%).

Fig. 3. Correlation of actual and theoretical S loading in sulphonicacid materials.

The catalytic performance of these materials hingesupon the successful incorporation of S functional-ity within the silica framework. Elemental analysis(Fig. 3) shows the bulk S content as a function ofthe nominal loading assuming complete S uptake. Itis evident that the S loading increases linearly withMPTS concentration across the entire TEOS:MPTScomposition range demonstrating efficient (>90%)thiol inclusion during the sol–gel preparation. Fig. 4shows that surface S levels determined by XPS mirrorthis bulk trend indicating these materials possess a

Fig. 4. S(2p) XPS spectra of mercaptopropyl silica (11.62 wt.%S) before and after oxidation. Inset shows correlation betweensurface and bulk S content for the range of catalysts prepared.


uniform composition. The successful oxidation of re-duced S species associated with surface bound MPTSis also apparent from the S XP spectra. The residual,unoxidised S signal likely reflects a contribution fromsubsurface S incorporated within the Si frameworkdetected due to the long inelastic mean free path ofthese photoelectrons.

The nature of these surface S species was furtherexplored using Raman spectroscopy which is moresensitive than conventional FT-IR towards S–H vi-brations [17]. Prior to the oxidation step, all thematerials exhibited a strong peak at 2585 cm−1 char-acteristic of the thiol group in mercaptopropyl silica[17]. Complete loss of this S–H mode was observedfollowing oxidation with H2O2 (Fig. 5a). This loss isaccompanied by the growth of two new bands at 1040and 1100 cm−1, which are attributed to the symmetricand asymmetric vibrational modes of SO3

− (Fig. 5b).It should be noted that the Si–O and Si–OH modesobscure these S=O stretching vibrations using con-ventional DRIFTS, however, the present Raman mea-surements enable us to directly identify the creation ofsurface sulphonic acid groups. In addition, the reten-tion of modes at 1250 and 1300 cm−1, assigned toCH2–S and CH2–Si wagging modes, confirm the in-tegrity of the alkyl linker between the sulphonic acidand silicon centres. Furthermore, the S–C stretch-ing mode is also observed at 650 cm−1. Importantly,we observe no vibrational modes over the range500–550 cm−1 [18] suggesting that under our reactionconditions no undesirable disulphide species wereformed (Fig. 5c).

Comparison of XPS and Raman results for the high-est loading material reveals a small amount of unoxi-dised thiol remains which is Raman invisible. This dis-crepancy most likely reflects the different sensitivity ofthe two techniques towards thiol groups. S–H modesexhibit weak dipoles making them difficult to detectby vibrational techniques, while S(2p) photoelectronshave an escape depth of∼15 Å. Subsurface thiolgroups located within the silica framework will thusbe XPS visible, while their interactions with siloxaneor silanol groups may reduce their vibrations to belowthe Raman detection limit. This highlights the needfor complementary catalyst characterisation tools.

The acidity of these surface sulphonic acid groupswas confirmed via DRIFTS measurements of adsorbedpyridine (Fig. 6). Independent of S loading, peaks are

Fig. 5. Raman spectra of mercaptopropyl silica (11.62 wt.% S)before and after oxidation: (a) S–H stretching region; (b) S–Ostretching region; (c) S–S stretching region.


Fig. 6. DRIFT spectrum showing pyridine titration of sulphonicacid silica (11.62 wt.% S).

observed at 1495, 1545, 1595 and 1645 cm−1, charac-teristic of the pyridinium ion formed by the interactionof pyridine with Brönsted acid sites. The peaks visi-ble at 1405 and 1440 cm−1 can be assigned to pyri-dine physisorbed on the silica surface. The surfaceacid strength as determined by Hammett indicators isaround,H0 = −3 for all sulphonic acid loadings.

The thermal stability of these materials was inves-tigated by thermogravimetric analysis coupled withevolved gas FT-IR (Fig. 7a and b). This technique re-veals that in all cases the sulphonic acid groups arestable to at least 250◦C, at which point a weight losscorresponding to the liberation of SO2 occurs. A sec-ond decomposition stage at∼480◦C is associated withdecomposition of the residual alkyl fragments. The de-composition rather than desorption of alkyl sulphonicacid groups confirms the strongly bound nature ofthese groups.

Having successfully demonstrated the synthesis ofthermally stable, mesoporous, sulphonic acid silicastheir catalytic performance was assessed for the liquidphase esterification of butan-1-ol with acetic acid, areaction typically catalysed by strong Brönsted acids,e.g. H2SO4. Butan-1-ol was selected so that the reac-tion kinetics could be followed without any possibleinterference from diffusional effects arising from poresize variations with sulphonic acid loading. In allcases, the only product was butyl acetate with a >98%mass balance based on butanol conversion and acetateyield being observed. Initial reaction rates are shownin Fig. 8, which reveals a near linear increase with S

Fig. 7. (a) Differential thermal analysis of sulphonic acid silica(11.62 wt.% S). (b) Evolved gas FT-IR spectrum of decompositionproducts formed during the first weight loss.

Fig. 8. Initial reaction rates for the esterification of butan-1-olby acetic acid (�) and the Claisen–Schmidt condensation of ace-tophenone with benzaldehyde (�) over sulphonic acid catalystsas a function of S loading.


Table 1Conversions observed after 6 h reaction for the esterification of butan-1-ol by acetic acid and the Claisen–Schmidt condensation ofacetophenone with benzaldehyde over sulphonic acid catalysts as a function of S loading

Sulphonic acid loading (wt.%) Butan-1-ol conversion (%) Acetophenone conversion (%)

1.25 5 171.85 25 212.85 74 346.01 76 327.01 86 32

11.62 85 34

Scheme 1. Condensation reaction of benzaldehyde and acetophenone.

loading and thus, the number of sulphonic acid sitesup to ∼7% beyond which the rate reaches a plateau.Conversion levels after 6 h reaction are shown inTable 1. The background conversion of butan-1-olunder these conditions was observed to be<3%. Ourresults for the high loading sulphonic acid catalystscompare favourably with reports of vapour phasebutan-1-ol esterification using MCM-41 supportedheteropoly acids [19]. In the latter case, the higheroperating temperatures (∼110◦C) required duringplug flow reactor operation resulted in competing acidcatalysed dehydration and etherification side reactionswhich reduced the acetate selectivity to 80–85%.

The versatility of these catalysts was also exploredusing the Claisen–Schmidt condensation of acetophe-none with benzaldehyde (Scheme 1), which is conven-tionally acid catalysed by HCl, AlCl3 or BF3.

Again only a single product, chalcone, was ob-served for all catalysts with the initial rates increasingroughly linearly with sulphonic acid loading (Fig. 8).The corresponding acetophenone conversions after6 h are shown in Table 1. These values comparevery favourably with reports of the zeolite catalysedClaisen condensation of hydroxyacetophenone withbenzaldehyde wherein similar reaction conditionsoffer only ∼12% conversion using a much highercatalyst:substrate ratio of 10 wt.% HY zeolite [20].

The near linear dependence of reaction rate on sul-phonic acid loading for these two distinct reactions

shows that increasing the MPTS:TEOS ratio onlychanged the number and not the nature of the re-sultant active sites. The average single-site turnoverfrequencies were 0.7 ± 0.1 molecules per minute forbutan-1-ol esterification and 1± 0.2 molecules perminute for the Claisen–Schmidt condensation. De-spite the variation in pore diameter with sulphonicacid loading, mass transport limitations appear to playonly a minor role in these systems; even the highestloading catalyst shows moderate activity in dodecanolesterification (31% conversion in 6 h).

The successful application of this range of sul-phonic acid catalysts demonstrates the potential ofsol–gel methods to produce solid Brönsted acid cata-lysts with a higher active site density than achievablevia post-grafting methods; the latter currently offera maximum S loading of only∼2 wt.%. The presenthigh loading materials are advantageous for finechemicals synthesis wherein the requisite catalystamount can be minimised, thus reducing the necessityfor large solvent volumes to aid mixing, and productlosses within the catalyst pore structure.

4. Conclusions

We have demonstrated that sol–gel methods can beused to achieve high levels of MPTS incorporationwith >90% efficiency even at loadings of 12 wt.% S.


These materials undergo efficient surface oxidation togenerate sulphonic acid sites active in esterificationand condensation reactions. The resultant materials of-fer high thermal stability combined with tunable acidsite density for liquid phase catalysis over a wide op-erating temperature regime.

Acknowledgements

The Royal Society is gratefully acknowledged forthe provision of an equipment grant (Karen Wilson)and University Research Fellowship (Duncan J. Mac-quarrie). James H. Clark thanks the RAEng-EPSRCfor a Clean Technology Fellowship. We are also grate-ful to Mr R.B. Girling for assistance with acquisitionof Raman spectra.

References

[1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T.Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olsen, E.W.Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J.Am. Chem. Soc. 114 (1992) 10834.

[2] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865.[3] D.J. Macquarrie, J. Chem. Soc., Chem. Commun. (1996)

1961.[4] L. Mercier, T.J. Pinnavia, Chem. Mater. 12 (2000) 188.

[5] P.M. Price, J.H. Clark, D.J. Macquarrie, J. Chem. Soc., DaltonTrans. (2000) 101.

[6] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000)1403.

[7] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew Chem. Int. Ed.38 (1999) 56.

[8] J.P. Blitz, C.P. Little, Fundamentals and Applied Aspects ofChemically Modified Surfaces, Royals Society of Chemistry,Cambridge, 1999.

[9] L.A. Ciolino, J.G. Dorsey, J. Chromatogr. A 675 (1994)29.

[10] L. Mercier, T.J. Pinnavia, Environ. Sci. Tech. 32 (1998) 2749.[11] W.M. VanRhijn, D.E. DeVos, B.F. Sels, W.D. Bossaert, P.A.

Jacobs, Chem. Commun. (1998) 317.[12] W.D. Bossaert, D.E. DeVos, W.M. VanRhijn, J. Bullen, P.J.

Grobet, P.A. Jacobs, J. Catal. 182 (1999) 156.[13] I. Diaz, C. Marquez-Alvarez, F. Mohino, K. Prez-Pariente, E.

Sastre, J. Catal. 193 (2000) 283.[14] M.A. Harmer, W.E. Farmeth, Q. Sun, J. Am. Chem. Soc. 118

(1996) 7708.[15] Q. Sun, M.A. Harmer, W.E. Farneth, Chem. Commun. (1996)

1201.[16] M.A. Harmer, W.E. Farmeth, Q. Sun, Adv. Mater. 10 (1998)

1255.[17] N.B. Colthup, L.H. Daly, S.E. Wiberly, Introduction to

Infrared and Raman Spectroscopy, Academic Press, NewYork, 1964.

[18] M.W. Tsao, J.F. Rabolt, H. Schonherr, D.G. Castner,Langmuir 16 (2000) 1734.

[19] M.J. Verhoef, P.J. Kooyman, J.A. Peters, H. van Bekkum,Micro Mesoporous Mater. 27 (1999) 365.

[20] M.J. Climent, A. Corma, S. Iborra, J. Primo, J. Catal. 151(1995) 60.

structure and reactivity of sol–gel sulphonic acid silicas

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