Published on Web 07/11/2003
Design of Heterogeneous Catalysts via Multiple Active Site Positioning in Organic-Inorganic Hybrid Materials Ve´ ronique Dufaud*,† and Mark E. Davis* Contribution from the DiVision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received February 10, 2003; E-mail:
[email protected] Abstract: Catalytic materials bearing multiple sulfonic acid functional groups and positioned at varying distances from one another on the surface of mesoporous solids are prepared to explore the effects that the spatial arrangement of active sites have on catalytic activity and selectivity. A series of organosiloxane precursors containing either disulfide or sulfonate ester functionalities (synthons of the eventual sulfonic acid groups) are synthesized. From these molecular precursors, a variety of organic-inorganic hybrid, mesostructured SBA-15 silica materials are prepared using a postsynthetic grafting procedure that leads to disulfide and sulfonate ester modified silicas: [Si]CH2CH2CH2SS-pyridyl, 2‚SBA, [Si]CH2CH2CH2SSCH2CH2CH2[Si], 3‚SBA, [Si]CH2CH2(C6H4)(SO2)OCH2CH3, 4‚SBA, and [Si]CH2CH2(C6H4)(SO2)OC6H4O(SO2)(C6H4)CH2CH2[Si], 6‚SBA ([Si] ) (tSiO)x(RO)3-xSi, where x ) 1, 2). By subsequent chemical derivatization of the grafted species, thiol and sulfonic acid modified silicas are obtained. The materials are characterized by a variety of spectroscopic (13C and 29Si CP MAS NMR, X-ray diffraction) and quantitative (TGA/DTA, elemental analysis, acid capacity titration) techniques. In all cases, the organic fragment of the precursor molecule is grafted onto the solid without measurable decomposition, and the precursors are, in general, attached to the surface of the mesoporous oxide by multiple siloxane bridges. The disulfide species 2‚SBA and 3‚SBA are reduced to the corresponding thiols 7‚SBA and 8‚SBA, respectively, and 4‚SBA and 6‚ SBA are transformed to the aryl sulfonic acids 11‚SBA and 12‚SBA, respectively. 7‚SBA and 8‚SBA differ only in terms of the level of control of the spatial arrangement of the thiol groups. Both 7‚SBA and 8‚SBA are further modified by oxidation with hydrogen peroxide to produce the alkyl sulfonic acid modified materials 9‚SBA and 10‚SBA, respectively. The performances of the sulfonic acid containing SBA-15 silica materials (with the exception of 12‚SBA) are tested as catalysts for the condensation reaction of phenol and acetone to bisphenol A. The alkyl sulfonic acid modified material 10‚SBA derived from the cleavage and oxidation of the dipropyl disulfide modified material 3‚SBA is more active than not only its monosite analogue 9‚ SBA, but also the presumably stronger acid aryl sulfonic acid material 11‚SBA. It appears that a cooperative effect between two proximal functional groups may be operating in this reaction.
Introduction
An ongoing objective in the preparation of solid catalysts is the creation of structural uniformity. By analogy to soluble catalysts, structural homogeneity may lead to high selectivity. Clearly, results from zeolite-based catalysts suggest that there can be a strong correlation between mesoscopic/macroscopic uniformity and high selectivity. A particularly attractive class of solids for investigating structure-property relationships are organic-inorganic hybrid materials.1 One type of these hybrid solids utilizes inorganic materials to provide surface area and porosity and has organic functional groups attached to the surface. The organic groups can be randomly distributed or organized in some fashion, and are the active sites for catalytic reactions. Of current interest are (i) the creation of multiple organic groups in precise arrangements to study the effects of cooperativity, and (ii) the use of multiple organic group types to catalyze multistep reaction pathways. † Present address: Laboratoire de Chimie, Ecole Normale Supe ´ rieure de Lyon, 46 alle´e d’Italie, 69364 Lyon cedex 07, France.
10.1021/ja034594s CCC: $25.00 © 2003 American Chemical Society
Dual, organic functionalization of periodic, mesoporous silicas has been reported.2-6 Typically, two types of organosilanes are used simultaneously to provide surface functionalized silicas. While one of the organic groups provides for the catalytic site, the other (may be alkyl, aryl, benzyl, or vinyl) normally is not another active site and is present to mediate the surface properties, for example, hydrophobicity. These methodologies do not provide for functional group positioning and control over uniformity. Avnir and Blum have recently reported the use of multiple organic functionalities in accomplishing reaction networks within a single vessel.7,8 Mutually destructive organic function(1) Wight, A. P.; Davis, M. E. Chem. ReV. 2002, 102, 3589-3613 and references therein. (2) Macquarrie, D. J.; Jackson, D. B.; Tailland, S.; Wilson, K.; Clark, J. H. Stud. Surf. Sci. Catal. 2000, 129, 275-282. (3) Asefa, T.; Kruk, M.; MacLachan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2001, 123, 8520-8530. (4) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 2000, 12, 2448-2459. (5) Hall, S. R.; Fowler, C. E.; Lebeau, B.; Mann, S. Chem. Commun. 1999, 201-202. (6) Bhaumik, A.; Tatsumi, T. J. Catal. 2000, 189, 31-39. J. AM. CHEM. SOC. 2003, 125, 9403-9413
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Scheme 1
Scheme 2
alities, for example, acids and bases, were entrapped separately in sol-gel-derived silicas and used together within a single reaction vessel to catalyze a sequence of reaction steps that in the absence of the “sol-gel protected” active centers would not proceed because of the annihilation of the active sites. Each solid contains a single type of organic group, and no method of control of functional group positioning was used. Katz and Davis combined molecular imprinting with solgel synthesis techniques to prepare microporous, amorphous silicas with tailored microcavities containing multiple aminopropyl functional groups covalently attached to the silica.9 The controlled positioning of two and three organic groups was accomplished, and the methodology has been extended to mesoporous materials.10 Here, we report on the synthesis and characterization of organic-functionalized, mesoporous silicas. The goal of our work is to develop methodologies to prepare uniform arrangements of organic functional groups on solid surfaces to access their ability to act in a cooperative fashion when accomplishing chemical reactions. Specifically, thiol and sulfonate ester functional groups were organized onto the ordered mesoporous solid SBA-1511-14 through the use of designed organosilane coupling to surface silanol groups. The sulfur-containing moieties were converted to sulfonic acids, and the solids were tested as catalysts for the condensation reaction between acetone and phenol to produce bisphenol A. Particular attention was given to the characterization of the materials at each step of the preparation. Results and Discussion
I. Synthesis of Molecular Precursors. A route to the preparation of mesoporous silica supported sulfonic acids involves the creation of a thiol containing silica, typically a mercaptopropyl functionalized silica, that is then oxidized, in a second step, with hydrogen peroxide.15-18 A drawback of this approach when starting from a thiolsilane is the concurrent formation of disulfide groups that can produce a material that has various types of functional groups, notably partially oxidized 9404 J. AM. CHEM. SOC.
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disulfides and other sulfur species.15,18,19 To avoid these problems, we either protect the thiol functionality with a reversible blocking reagent such as a pyridyl group (precursor 2) or use a sulfonate ester precursor (precursor 4) to generate the sulfonic acid function. To produce ordered, dimeric sites for comparison to the isolated sites that are generated from precursors 2 and 4, precursors 3 and 6 were synthesized (Scheme 1). Starting from the symmetrical dipropyl disulfide (precursor 3), two alkyl sulfonic acid groups in close proximity to one another may be produced by conversion to alkyl sulfonic acid centers, and from the molecular precursor bis(arylsulfonate ester) (precursor 6), two aryl sulfonic acid sites can be generated by removal of the phenyl spacer. Disulfide Microstructures (Unsymmetrical, 2; Symmetrical, 3). The unsymmetrical propylpyridyl disulfide 2 was obtained by reacting 3-mercaptopropyltrimethoxysilane, (CH3O)3Si-(CH2)3SH, 1, with 2,2′-dithiodipyridine (Aldrithiol) via a thiol-disulfide interchange reaction (Scheme 2).20,21 If the thiol 1 is present in excess (2.5 equiv per disulfide bond), the product is the symmetrical dipropyl disulfide 3 (Scheme 2). After (7) Gelman, F.; Blum, J.; Avnir, D. J. Am. Chem. Soc. 2000, 122, 1199912000. (8) Gelman, F.; Blum, J.; Avnir, D. Angew. Chem., Int. Ed. 2001, 40, 36473649. (9) Katz, A.; Davis, M. E. Nature 2000, 403, 286-289. (10) Ini, S.; Defreese, J. L.; Parra-Vasquez. N.; Katz, A. Mater. Res. Soc. Symp. Proc. 2002, 723, M2.3.1-M2.3.7. (11) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (12) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024-6036. (13) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275-279. (14) Kruk, M.; Jaroniec, M. Chem. Mater. 2000, 12, 1961-1968. (15) Van Rhijn, W. M.; De Vos, D.; Sels, B. F.; Bossaert, W. D.; Jacobs, P. A. J. Chem. Soc., Chem. Commun. 1998, 317-318. (16) Bradley, R. D.; Ford, W. T. J. Org. Chem. 1989, 54, 5437-5443. (17) Bossaert, W. D.; De Vos, D. E.; Van Rhijn, W. M.; Bullen, J.; Grobet, P. J.; Jacobs, P. A. J. Catal. 1999, 182, 156-164. (18) Diaz, I.; Marquez-Alvarez, C.; Mohino, F.; Perez-Pariente, J.; Sastre, E. J. Catal. 2000, 193, 283-294. (19) Lim, M. H.; Blanford, C. F.; Stein, A. Chem. Mater. 1998, 10, 467-470. (20) Guo, W.; Pleasants, J.; Rabenstein, D. L. J. Org. Chem. 1990, 55, 373376. (21) Houk, J.; Whitesides, G. M. J. Am. Chem. Soc. 1987, 109, 6825-6836.
Design of Heterogeneous Catalysts
Figure 1. 13C{1H} NMR spectrum of 2 in CD2Cl2; CP-MAS spectrum of 2‚SBA. S denotes resonances from the solvent.
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13C
NMR
Figure 3. 13C{1H} NMR spectrum of 4 in CD2Cl2; CP-MAS 13C NMR spectrum of 4‚SBA. S denotes resonances from the solvent, and * marks spinning sidebands. Scheme 3
Figure 2. 13C{1H} NMR spectrum of 3 in C6D6; CP-MAS spectrum of 3‚SBA. S denotes resonances from the solvent.
13C
NMR
reaction (less than 3 h for 2; 4 days to 1 week for 3), both disulfide compounds are obtained in nearly quantitative yield by extraction with petroleum ether to remove the pyridine-2thione. Precursors 2 and 3 were characterized by 1H and 13C NMR spectroscopy and GC/MS in the case of 3. The 13C NMR spectra of both compounds (Figures 1 and 2 for 2 and 3, respectively) exhibit in the 0-50 ppm region four signals: the resonances at around 40 and 22 ppm are attributed to the carbons of a propyl moiety situated, respectively, in R and β positions of the disulfide bond, the resonance at around 10 ppm to the carbon directly attached to the silicon atom, and the resonance at 48
ppm to the methoxy groups. In addition, downfield signals from all of the carbons of the pyridyl moiety are observed. Monosulfonate Ester Microstructure, 4. The preparation of arylsulfonic acid esters was achieved by the reaction of an alcohol with the appropriate sulfonyl chloride in the presence of an organic base.22 The sulfonyl chloride used here was the commercially available 2-(4-chlorosulfonylphenyl)ethyltrichlorosilane), (Cl)3Si-(CH2)2-C6H4-SO2Cl. The reaction to the desired sulfonic acid ester was not clean.23 Thus, the reaction of this chloride with ethanol in pyridine (Scheme 3) leads to precursor 4 in a relatively low yield (30%). Precursor 4 was characterized by 1H and 13C NMR. In particular, the 13C NMR spectrum (Figure 3) exhibits two resonances at 14.8 and 67.3 ppm characteristic of the methyl and the methylene carbons of the ethylsulfonate group, respectively, and the signals of the ethoxysilane groups at 18.4 and 58.5 ppm. The alkyl carbon linked to the aromatic ring resonates at 29.4 ppm, while the carbon that attached to the silicon atom gives a chemical shift of 12.4 ppm. All of the resonances of the carbons of the aromatic ring are also observed in the 160-120 ppm region. Disulfonate Ester Microstructure, 6. A two-step synthetic scheme was used to prepare 6 (Scheme 4). The first step involved the preparation of the aryl ester of para-vinylbenzenesulfonic acid by condensation of para-vinylbenzenesulfonic chloride with hydroquinone in the presence of pyridine to produce 5, CH2dCH-C6H4-(SO2)O-C6H4-O(SO2)-C6H4(22) Tipson, S. R. J. Org. Chem. 1944, 9, 235-241. (23) The starting 2-(4-chlorosulfonylphenyl)ethyltrichlorosilane is sold as a solution in methylene chloride with 30% free sulfonic acid and small amounts of silylsulfonic acid condensation products. The sulfonic acid may react with EtOH to form diethyl ether and water, the latter inducing hydrolysis and condensation of the chlorosilane groups. J. AM. CHEM. SOC.
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Scheme 4
CHdCH2, in 60% yield.24 The vinyl groups of 5 were then hydrosilylated with triethoxysilane in the presence of chloroplatinic acid as the catalyst and 1,2-dichloroethane as the solvent.25 Different hydrosilylation conditions were attempted,26 but the best results were obtained when carrying out the reaction at 80 °C for several days and led to only 10% yield of the fully silylated product. The desired compound was a mixture of Markovnikov and anti-Markovnikov addition products, and this mixture is collectively referred to as 6 (Scheme 4 shows only the bis-anti-Markovnikov product). The complete silylation of both double bonds was evidenced by 1H NMR spectroscopy, in particular, by the total disappearance of the signals at 5.4, 5.85, and 6.7 ppm typical of the carbons of the vinylic chain (see Supporting Information). The 13C NMR spectrum of 6 is given in Figure 4. II. Synthesis of Organic-Inorganic Hybrid Materials. Two major routes have been widely investigated to chemically modify the surface of periodic mesoporous silicas via covalently bound organic functionalities. The postsynthesis procedure involves the reaction of an organosilane directly with surface silanols of a surfactant-free mesoporous oxide and further modification or derivatization of the grafted species to create new functionalities.27-30 The second approach, the so-called onepot synthesis, combines the sol-gel31 and supramolecular templating techniques to generate, in a single step, ordered organically modified mesoporous silica-based nanocomposites.4,15,19,32-40 A number of groups report the synthesis of thiol and sulfonic acid modified ordered silicas by both the (24) Prib, O. A.; Gritsai, N. I. Ukr. Khim. Zh. 1968, 34, 368-370. (25) Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley and Sons Ltd.: New York, 1989; pp 14791526. (26) Several attempts were undertaken to increase the yield of the reaction by changing, for example, the nature of the catalyst. For the particular case of the Speier’s catalyst (chloroplatinic acid solubilized in 2-propanol), no silylated products were obtained even at higher temperatures. (27) Brunel, D. Microporous Mesoporous Mater. 1999, 27, 329-344. (28) Clark, J. H.; Macquarrie, D. J. Chem. Commun. 1998, 853-860. (29) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950-2963. (30) Fryxell, G. E.; Liu, J.; Hauser, T. A.; Nie, Z.; Ferris, K. F.; Mattigod, S.; Gong, M.; Hallen, R. T. Chem. Mater. 1999, 11, 2148-2154. (31) The sol-gel approach corresponds to the inclusion of the organosiloxane precursor in an appropriate concentration along with the siloxane precursor, generally tetraethyl orthosilicate (TEOS), during the formation of the oxide. 9406 J. AM. CHEM. SOC.
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grafting17,41 and the co-condensation methods.15,17,18,40,42 Most studies used the commercially available mercaptopropyltrimethoxysilane and, in a subsequent oxidation step, transformed the thiol to a sulfonic acid. Stucky and co-workers, however, report the addition of hydrogen peroxide with the mercaptopropyltrimethoxysilane or the 2-(4-chlorosulfonylphenyl)ethyltrichlorosilane) in the reaction mixture to oxidize in situ the thiol functionalities.4,40 We have explored these different methods, but in this report we concentrate on the postsynthetic modification of the mesoporous silica and subsequent chemical modification of these surface functional groups. We will report results from the cocondensation synthesis strategy at a later time. II. A. Grafting Reaction. SBA-15 type silicas were used as supports and were prepared by the acid catalyzed, nonionic assembly pathway described elsewhere.11-14 The structure directing agent (Pluronic 123) was removed by calcination in air at 500 °C, and the organic-free mesoporous silica was rigorously dried under a flow of helium at 200 °C prior to the grafting reaction. The molecular precursors, 1-6, were then reacted with the SBA-15 silica in toluene under reflux conditions. The unreacted precursors were removed by Soxhlet extraction with methylene chloride overnight, and the collected organic-inorganic hybrid materials are denoted 1‚SBA-6‚SBA. Quantitative determinations of the organic content of the hybrid mesoporous silicas were performed by thermogravimetric analysis (TGA) in air and by elemental analysis. The results are summarized in Table 1. The TGA data for several hybrid materials are given in the Supporting Information. Generally, three weight loss regions were observed. A first weight loss occurred at temperatures up to about 100 °C and was endothermic. This weight loss can be assigned to the desorption of water. A second weight loss followed at temperatures ranging from 150 to 650 °C, and this loss presumably arises from the decomposition of the organic and the desorption of its fragments (depending on the nature of the organic fragments, several desorption peaks could be observed in this region). The third significant weight loss peak occurred at temperatures above 650 °C and is due to the release of water formed from the condensation of silanols in the silica structure. For the purpose of reporting organic content in Table 1, the weight loss between 150 and 650 °C is taken as an estimate of the total amount of the organic. The functional group loadings for 1‚SBA-6‚SBA are found to be 0.1-1.0 mmol/g of dry SiO2 as determined by TGA and 0.2-0.9 mmol/g of dry SiO2 as determined by sulfur analysis. The TGA data can overestimate the organic content because some weight loss below 650 °C can be due to silanol (32) Burkett, S. L.; Sim, S. D.; Mann, S. J. Chem. Soc., Chem. Commun. 1996, 1367-1368. (33) Fowler, C. E.; Burkett, S. L.; Mann, S. J. Chem. Soc., Chem. Commun. 1997, 1769-1770. (34) Lim, M. H.; Blanford, C. F.; Stein, A. J. Am. Chem. Soc. 1997, 119, 40904091. (35) Lim, M. H.; Stein, A. Chem. Mater. 1999, 11, 3285-3295. (36) Macquarrie, D. J. J. Chem. Soc., Chem. Commun. 1996, 1961-1962. (37) Macquarrie, D. J.; Jackson, D. B. J. Chem. Soc., Chem. Commun. 1997, 1781-1782. (38) Macquarrie, D. J.; Jackson, D. B.; Tailland, S.; Utting, K. A. J. Mater. Chem. 2001, 11, 1843-1849. (39) Richer, R.; Mercier, L. Chem. Mater. 2001, 13, 2999-3008. (40) Melero, J. A.; Stucky, G. D.; Van Grieken, R.; Morales, G. J. Mater. Chem. 2002, 12, 1664-1670. (41) Van Rhijn, W. M.; De Vos, D. E.; Bossaert, W. D.; Bullen, J.; Wouters, B.; Grobet, P.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1998, 117, 183-190. (42) Diaz, I.; Marquez-Alvarez, C.; Mohino, F.; Perez-Pariente, J.; Sastre, E. J. Catal. 2000, 193, 295-302.
Design of Heterogeneous Catalysts
Figure 4.
13C{1H}
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NMR spectrum of 6 in CD2Cl2; CP-MAS
13C
NMR spectrum of 6‚SBA.
Table 1. Organic and Acid Group Content of SBA-15 Type Silica Materials Containing Disulfide, Thiol, and Sulfonic Acid Fragments quantitative analysis of loading (mmol/g SiO2)a sample
precursor
functional group
S wt %
derived from sulfur analysis
measured by TGA/DTAb
measured by acid capacity titration
1‚SBA 2‚SBA 3‚SBA 4‚SBA 6‚SBA 7‚SBA 8‚SBA 9‚SBA 10‚SBA 11‚SBA 12‚SBA
1 2 3 4 6 2 3 2 3 4 6
[Si]-propyl-SH [Si]-propyl-SS-pyridyl [Si]-propyl-SS-propyl-[Si] [Si]-CH2CH2aryl-SO3Et [Si]-CH2CH2aryl-SO3-aryl-SO3-arylCH2CH2-[Si] [Si]-propyl-SH [Si]-propyl-SH HS-propyl-[Si] [Si]-propyl-SO3H [Si]-propyl-SO3H HO3S-propyl-[Si] [Si]-CH2CH2aryl-SO3H [Si]-CH2CH2aryl-SO3H HO3S-arylCH2CH2-[Si]
1.4 4.1 3.3 1.2 0.5 2.0 2.7 1.4 1.7 0.76
