Optimizing Acid−Base Bifunctional Mesoporous Catalysts for the

Nov 12, 2008 - broad range of such multifunctional hybrid inorganic- ..... of APTS in 200 mL of ethanol under reflux at ∼78 °C for 6 h. The solutio...
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Langmuir 2008, 24, 14306-14320

Optimizing Acid-Base Bifunctional Mesoporous Catalysts for the Henry Reaction: Effects of the Surface Density and Site Isolation of Functional Groups Krishna K. Sharma, Robert P. Buckley, and Tewodros Asefa* Department of Chemistry, Syracuse UniVersity, Syracuse, New York 13244 ReceiVed September 12, 2008 We report on the effects of the surface density and the spacing between grafted organoamines (and residual ungrafted silanols) of amine-functionalized mesoporous materials on their (cooperative) catalytic activity in the Henry reaction. The spacing between the bifunctional groups (amines and silanols), their site isolation, and their surface density were controlled by one-step or two-step grafting of a series of organosilanes containing linear alkylamine, alkyldiamine, alkyltriamine, and meta- and para-substituted aromatic amines onto mesoporous silica in ethanol and/or toluene. The grafting in ethanol produced site-isolated, flexible alkylamines, alkyldiamines, and alkyltriamines of different tether lengths and rigid meta- and para-substituted aromatic amines and high surface area materials, whereas the grafting in toluene resulted in closely spaced organoamines and materials with lower surface areas. The spacing between the organoamine groups was probed by complexing cupric ions with the amines and by measuring the electronic spectra of the complexes. The materials’ catalytic activities were dependent not only on the degree of site isolation of the amine groups and the surface areas of the materials, but also on the relative spacing between the functional groups and their surface density. Samples grafted with monoamine groups in ethanol and samples grafted with diamine or triamine groups in toluene for 5 h gave ∼100% conversion in 16 min of the Henry reaction between p-hydroxybenzaldehyde and nitromethane. However, the corresponding monoamine-grafted sample in toluene and diamine- and triamine-grafted samples in ethanol gave ∼100% conversion after 1 h. On the basis of turnover number (TON) and TON per surface area, the samples containing optimum concentrations of ∼0.8 - 1.5 mmol of grafted organoamines/g, which we dubbed as the critical density of organic grafted groups, gave the highest catalytic efficiencies. These samples have the most favorable amine-silanol cooperative catalytic activity. Furthermore, samples functionalized with rigid meta-substituted aminophenyl groups in ethanol showed higher catalytic efficiency than the corresponding sample containing the amine groups at the para-position, possibly due to the close proximity of the bifunctional groups in the former. The capping of ungrafted silanols with noncatalytic organosilanes in toluene resulted in reduction of catalytic activities, confirming the involvement of silanols.

Introduction Over 80% of synthetic materials produced by chemical transformations involve at least one catalytic step in the course of their synthesis.1 Improving the efficiency of catalysts increases the yield of the products, reduces the number and amount of unwanted byproducts, or lowers the energy and costs involved in several industrial catalytic processes.2 One approach to improve catalysts involves the coplacement of two or more catalytic functional sites on a solid nanoporous substrate to produce bifunctional or multifunctional heterogeneous catalysts.3 By using various synthetic methods, a broad range of such multifunctional hybrid inorganicorganic catalysts have been successfully synthesized in the * To whom correspondence should be addressed. E-mail: tasefa@ syr.edu. (1) Marcilly, C. J. Catal. 2003, 216, 47–62. (2) (a) Wasilke, J.-C.; Obrey, S. J.; Baker, R. T.; Bazan, G. C. Chem. ReV. 2005, 105, 1001–1020. (b) Tietze, L. F. Chem. ReV. 1996, 96, 115–136. (c) Guisnet, M.; Gnep, N. S.; Alario, F. Appl. Catal., A 1992, 89, 1–30. (3) (a) Setoyama, T. Catal. Today 2006, 116, 250–262. (b) Baeza, A.; Casas, J.; Najera, C.; Sansano, J. M.; Saa, J. M. Angew. Chem., Int. Ed. 2003, 42, 3143– 3146. (c) Adams, R. D. J. Organomet. Chem. 2000, 600, l–6. (d) Mori, K.; Kondo, Y.; Morimoto, S.; Yamashita, H. Chem. Lett. 2007, 36, 1068–1069. (e) Climent, M. J.; Corma, A.; Iborra, S.; Mifsud, M. J. Catal. 2007, 247, 223–230. (f) Della, P. C.; Falletta, E.; Rossi, M.; Gargano, M.; Giannoccaro, P.; Ciriminna, R.; Pagliaro, M. Appl. Catal., A 2007, 321, 35–39. (g) Guizzetti, S.; Benaglia, M.; Pignataro, L.; Puglisi, A. Tetrahedron: Asymmetry 2006, 17, 2754–2760. (h) Motokura, K.; Fujita, N.; Mori, K.; Mizugaki, T.; Ebitani, K.; Htsukawa, K.; Kanedar, K. Chem.sEur. J. 2006, 12, 8228–8239. (i) Motokura, K.; Fujita, N.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Tetrahedron Lett. 2005, 46, 5507–5510. (j) Thadani, A. N.; Rawal, V. H. Org. Lett. 2002, 4, 4321–4323. (k) Matsunaga, S.; Ohshima, T.; Shibasaki, M. AdV. Synth. Catal. 2002, 344, 3–15.

past few decades.4,5 Compared to their monofunctional counterparts, these multifunctional catalysts are capable of catalyzing and cocatalyzing reactions more effectively, activating reactants to undergo reactions more efficiently,6 or performing tandem reactions.2a,7 Among these, acid-base bifunctional catalysts, which can be synthesized by the coplacement of acid and base functional groups on solid materials such as silica gels as reported by Katz and co-workers,8 have been drawing considerable attention in recent years as they are capable of performing acid-base cooperative catalysis. (4) (a) Zhang, C. X.; Laine, R. M. J. Am. Chem. Soc. 2000, 122, 6979–6988. (b) Chung, Y. M.; Rhee, H. K. Catal. Lett. 2002, 82, 249–253. (c) Wan, Y.; Zhang, D. Q.; Zhai, Y. P.; Feng, C. M.; Chen, J.; Li, H.-X. Chem.sAsian J. 2007, 2, 875–881. (d) Elias, X.; Pleixats, R.; Man, M. W. C.; Moreau, J. J. E. AdV. Synth. Catal. 2007, 349, 1701–1713. (e) Bootsma, J. A.; Shanks, B. H. Appl. Catal., A 2007, 327, 44–51. (f) Blanco, B.; Moreno-Manas, M.; Pleixats, R.; Mehdi, A.; Reye´, C. J. Mol. Catal. A 2007, 269, 204–213. (g) Blanco, B.; Brissart, M.; Moreno-Manas, M.; Pleixats, R.; Mehdi, A.; Reye´, C.; Bouquillon, S.; Henin, F.; Muzart, J. Appl. Catal., A 2006, 297, 117–124. (h) Taylor, I.; Howard, A. G. Anal. Chim. Acta 1993, 271, 77–82. (i) Voss, R.; Thomas, A.; Antonietti, M.; Ozin, G. A. J. Mater. Chem. 2005, 15, 4010–4014. (j) Rosenholm, J. M.; Linde´n, M. Chem. Mater. 2007, 19, 5023–5034. (k) Papp, A.; Miklos, K.; Forgo, M.; Molnar, A. J. Mol. Catal. A 2005, 229, 107–116. (5) Zeidan, R. K.; Davis, M. E. J. Catal. 2007, 247, 379–382. (6) Kanai, M.; Kato, N.; Ichikawa, E.; Shibasaki, M. Synlett 2005, 1491– 1508. (7) Louie, J.; Bielawski, C. W.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 11312–11313. (8) (a) Bass, J. D.; Katz, A. Chem. Mater. 2006, 18, 1611–1620. (b) Bass, J. D.; Anderson, S. L.; Katz, A. Angew. Chem., Int. Ed. 2003, 42, 5219–5222. (c) Notestein, J. M.; Iglesia, E.; Katz, A. J. Am. Chem. Soc. 2004, 126, 16478–16486. (d) Notestein, J. M.; Andrini, L. R.; Kalchenko, V. I.; Requejo, F. G.; Katz, A.; Iglesia, E. J. Am. Chem. Soc. 2007, 129, 1122–1131.

10.1021/la8030107 CCC: $40.75  2008 American Chemical Society Published on Web 11/12/2008

Optimizing Acid-Base Catalysts for the Henry Reaction

Mesoporous materials, which are synthesized by supramolecular self-assembly of various metal oxide precursors,9 have been effectively used as solid supports or hosts for a variety of catalytic groups10 due to their well-ordered nanometer pore structures and exceedingly high surface areas. 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containing amine/silanol,22 amine/ureidopropyl,23 and amine/ sulfonic acid functional groups.24,25 Some of these materials have proved to catalyze the Henry, Knoevenagel condensation, and Michael addition reactions.22,23,26 Bifunctional amine/silanol mesoporous materials containing the amine groups spatially spaced over the mesopore walls have also been synthesized via the grafting method.27 However, many of these previously reported synthetic methods to produce bifunctional materials as well as acid-base bifunctional catalysts involve multistep, costly synthetic procedures, result in materials having a very high concentration of organoamines and low surface areas, or produce a low concentration of randomly distributed acidic and basic sites whose acid-base cooperative activities are often poor.28 For instance, the multistep synthetic approach to bifunctional amine/silanol mesoporous materials developed by McKittrick et al.27 employed lengthy steps of synthesis and grafting of organosilanes containing bulky imine groups onto mesoporous silica, followed by their hydrolysis to result in spaced organoamines.29 Furthermore, the bifunctional acid-base catalysts reported by Katz and co-workers8,22 were carried out on silica gels, which have low surface areas and polydisperse pore size distributions.30 While the method of coassembly of two terminal organosilanes also results in bifunctional catalysts as reported by Zeidan et al.24 and Alauzun et al.,25 it produces an intrinsically low concentration of randomly distributed acidic and basic groups.31 On the other hand, while bifunctional zeolites32 including acid-base bifunctional zeolite catalysts33 for efficient cooperative reactions34 and tandem, one-pot catalytic reactions35 have long been known, their small pore diameters compared to those of mesoporous materials have often limited their use for catalyzing a broad range of reactants. We have recently reported a simple synthetic approach to efficient bifunctional mesoporous catalysts for the Henry reaction by one-step grafting of site-isolated organoamine groups onto the surface silanols of mesoporous silica (MCM-41).36 The synthesis involved the grafting of (3-aminopropyl)trimethox(21) (a) Sakthivel, K.; Notz, W.; Bui, T.; Barbas, C. F. J. Am. Chem. Soc. 2001, 123, 5260–5267. (b) Saito, S.; Yamamoto, H. Acc. Chem. Res. 2004, 37, 570– 579. (22) Bass, J. D.; Solovyov, A.; Pascall, A. J.; Katz, A. J. Am. Chem. Soc. 2006, 128, 3737–3747. (23) Huh, S.; Chen, H.-T.; Wiench, J. W.; Pruski, M.; Lin, V. S.-Y. Angew. Chem., Int. Ed. 2005, 44, 1826–1830. (24) (a) Zeidan, R. K.; Hwang, S. J.; Davis, M. E. Angew. Chem., Int. Ed. 2006, 45, 6332–6335. (b) Gelman, F.; Blum, J.; Avnir, D. Angew. Chem., Int. Ed. 2001, 40, 3647–3649. (25) Alauzun, J.; Mehdi, A.; Reye, C.; Corriu, R. J. P. J. Am. Chem. Soc. 2006, 128, 8718–8719. (26) Choudary, B. M.; Kantam, M. L.; Sreekanth, P.; Bandopadhyay, T.; Figueras, F.; Tuel, A. J. Mol. Catal. A 1999, 142, 361–365. (27) McKittrick, M. W.; Jones, C. W. Chem. Mater. 2005, 17, 4758–4761. (28) Zeidan, R. K.; Davis, M. E. J. Catal. 2007, 247, 379–382. (29) McKittrick, M. W.; Jones, C. W. J. Am. Chem. Soc. 2004, 126, 3052– 3053. (30) Notestein, J. M.; Katz, A. Chem.sEur. J. 2006, 12, 3954–3965. (31) Zeidan, R. K.; Hwang, S.-J.; Davis, M. E. Angew. Chem., Int. Ed. 2006, 45, 6332–6335. (32) (a) Bhavani, A. G.; Pandurangan, A. J. Mol. Catal. A 2007, 267, 209–217. (b) Woltz, C.; Jentys, A.; Lercher, J. A. J. Catal. 2006, 237, 337–348. (c) Huybrechts, W.; Vanbutsele, G.; Houthoofd, K. J.; Bertinchamps, F.; Narasimhan, C. L. S.; Gaigneaux, E. M.; Thybaut, J. W.; Marin, G. B.; Denayer, J. F. M.; Baron, G. V.; Jacobs, P. A. A.; Martens, J. A. Catal. Lett. 2005, 100, 235–242. (d) Weitkamp, J.; Ernst, S.; Kumar, R. Appl. Catal. 1986, 27, 207–210. (e) Liu, S. T.; Wang, L.; Ohnishi, R.; Ichikawa, M. J. Catal. 1999, 181, 175–188. (33) (a) Kanai, M.; Kato, N.; Ichikawa, E.; Shibasaki, M. Synlett 2005, 1491– 1508. (b) Tai, J.; Davis, R. J. Catal. Today 2007, 123, 42–49. (c) Tai, J.; Davis, R. J. Catal. Today 2007, 123, 42–49. (d) Kumarraja, M.; Pitchumani, K. J. Mol. Catal. A 2006, 256, 138–142. (e) Nie, Y. T.; Niah, W.; Jaenicke, S.; Chuah, G. K. J. Catal. 2007, 248, 1–10. (34) (a) Woltz, C.; Jentys, A.; Lercher, J. A. J. Catal. 2006, 237, 337–348. (b) Miyaji, A.; Ohnishi, R.; Okuhara, T. Appl. Catal., A 2004, 262, 143–148. (35) (a) Nie, Y. T.; Chuah, G. K.; Jaenicke, S. Chem. Commun. 2006, 790– 792. (b) Trasarti, A. F.; Marchi, A. J.; Apesteguia, C. R. J. Catal. 2007, 247, 155–165.

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ysilane with MCM-41 in various polar-protic and dipolar-aprotic solvents, which resulted in site-isolated organoamine groups as well as many residual ungrafted silanols.36 The site isolation of the grafted organoamine groups was proved by complexing Cu2+ with the grafted organoamines and by probing the electronic spectra of the resulting Cu2+-amine complexes. The materials containing site-isolated grafted organoamine groups gave mononuclear CuNO5 complexes and red-shifted absorption maxima compared to the samples containing more densely populated organoamines. Until these recent reports, most grafting syntheses to amino-functionalized mesoporous catalysts were done by stirring mesoporous silica in excess organoaminosilanes in nonpolar solvents such as toluene and cyclohexane, often under reflux37 and sometimes without reflux.38 In some cases, a low concentration of aminosilanes in toluene with reflux39 or without reflux40 was also employed. Although the grafting of organosilanes in toluene results in a high concentration of grafted organic groups, it produces more densely populated organoamines, low surface area materials, and fewer residual silanols, leading to poor catalytic properties.36 While researchers have long pointed out that the grafting of organosilanes in various solvents on silicon or glass substrates produces different types of grafted groups,41 similar experiments on mesoporous substrates, especially for catalysis, have not been well investigated. In particular, the work of Dressik et al. showed that the grafting of aromatic organosilanes such as (p,m-chloromethylphenyl)trichlorosilane and 1-(dimethylchlorosilyl)-2-(p,m-chloromethylphenyl)ethane onto fused silica slides produced different densities of surface organic groups depending on the types of solvents used.41 For instance, the silica slides they obtained from aromatic-based solvents (e.g., toluene) had surface organic grafted group densities nearly half of those slides obtained from alkyl-based solvents (e.g., hexane) due to the inclusion of a geometrically well-matched solvent (toluene) in the aromatic self-assembled monolayer (SAM) during the film deposition in the former. In this paper, we report on the effects of the surface density and relative separation between the organoamine (and silanol) functional groups in amino-functionalized mesoporous catalysts on the degree of the functional groups’ (cooperative) catalytic activities in the Henry reaction. A series of samples containing flexible organomonoamine, organodiamine, and organotriamine groups as well as rigid meta- and para-substituted aminophenyl groups with different degrees of site isolation were synthesized by one-step or two-step grafting of their corresponding organosilanes onto mesoporous silica in ethanol and/or toluene. The degree of site isolation of the grafted organoamine groups in the (36) (a) Sharma, K. K.; Asefa, T. Angew. Chem., Int. Ed. 2007, 46, 2879– 2882. (b) Sharma, K. K.; Anan, A.; Buckley, R. P.; Ouellette, W.; Asefa, T. J. Am. Chem. Soc. 2008, 130, 218–228. (37) (a) Wang, X.; Lin, K. S. K.; Chan, J. C. C.; Cheng, S. J. Phys. Chem. B 2005, 109, 1763–1769. (b) Wang, X.; Lin, K. S. K.; Chan, J. C. C.; Cheng, S. Chem. Commun. 2004, 2762–2763. (c) Macquarrie, D. J.; Maggi, R.; Mazzacani, A.; Sartori, G.; Sartorio, R. Appl. Catal., A 2003, 246, 183–188. (d) Cauvel, A.; Renard, G.; Brunel, D. J. Org. Chem. 1997, 62, 749–751. (e) Kubota, Y.; Nishizaki, Y.; Ikeya, H.; Saeki, M.; Hida, T.; Kawazu, S.; Yoshida, M.; Fujii, H.; Sugi, Y. Microporous Mesoporous Mater. 2004, 70, 135–149. (38) (a) Luechinger, M.; Prins, R.; Pirngruber, G. D. Microporous Mesoporous Mater. 2005, 85, 111–118. (b) Knofel, C.; Descarpentries, J.; Benzaouia, A.; Zelenak, V.; Mornet, S.; Llewellyn, P. L.; Hornebecq, V. Microporous Mesoporous Mater. 2007, 99, 79–85. (39) Hicks, J. C.; Dabestani, R.; Buchanan, A. C.; Jones, C. W. Chem. Mater. 2006, 18, 5022–5032. (40) Rosenholm, J. M.; Linde´n, M. Chem. Mater. 2007, 19, 5023–5034. (41) (a) Dressick, W. J.; Chen, M.-S.; Brandow, S. L. J. Am. Chem. Soc. 2000, 122, 982–983. (b) Dressick, W. J.; Chen, M. S.; Brandow, S. L.; Rhee, K. W.; Shirey, L. M.; Perkins, F. K. Appl. Phys. Lett. 2001, 8, 676–678. (c) Chen, M. S.; Brandow, S. L.; Schull, T. L.; Chrisey, D. B.; Dressick, W. J. AdV. Funct. Mater. 2005, 15, 1364–1375. (d) Martin, B. D.; Brandow, S. L.; Dressick, W. J.; Schull, T. L. Langmuir 2000, 16, 9944–9946. (e) Brandow, S. L.; Schull, T. L.; Martin, B. D.; Guerin, D. C.; Dressick, W. J. Chem.sEur. J. 2002, 8, 5363–5367.

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materials was then elucidated. Finally, the materials’ catalytic properties in the Henry reaction and their structure-property correlations were investigated. The samples grafted with monoamine in ethanol and with diamine and triamine groups in toluene have resulted in ∼100% conversion in 15 min of the Henry reaction between p-hydroxybenzaldehyde and nitromethane. The corresponding samples grafted with monoamine in toluene and diamine and triamine in ethanol produced ∼100% conversion only after 1 h. On the basis of the turnover number (TON) per surface area (SA), the diamine- and triamine-grafted samples in toluene showed higher catalytic efficiency than the corresponding samples grafted in ethanol and the sample grafted with monoamine in ethanol. Furthermore, the samples that showed higher catalytic efficiency, conversion (%), and TON/SA appeared to have optimum concentrations of ∼0.8-1.5 mmol of grafted organoamines/g, which we dubbed as the critical density of organic grafted groups (CDOGG). The higher catalytic efficiencies exhibited by these materials were attributed to the presence of the most favorable amine-silanol cooperative catalytic activity in them. This cooperative catalysis by the amine and silanol groups was also found to be dependent on the relative distance between them. For example, the samples grafted with the longer, flexible 3-[(2-aminoethyl)amino]propyl propyldiamine, in which back-flipping of the chain is possible, and the shorter 3-aminopropyl groups that are close to silanols exhibited higher catalytic efficiency, while the samples grafted with rigid meta- and parasubstituted aminophenyl groups showed weaker catalytic activity. The amine and silanol groups can be in close proximity and perform favorable cooperative activity in the longer, flexible 3-[(2-aminoethyl)amino]propyl and in the shorter 3-aminopropyl groups compared to the rigid aminophenyl groups. Furthermore, the meta-substituted aminophenyl sample grafted in ethanol, whose amine groups are in closer proximity to the silanols, exhibited slightly higher catalytic efficiency than the corresponding para-substituted aminophenyls. The involvement of silanols in the cooperative catalytic activity was confirmed by further capping them in the functionalized materials with 3-cyanopropyl groups, as reported by Katz et al.,22 which resulted in lower catalytic efficiency and TON/SA.

Experimental Section Materials and Reagents. Nitromethane, p-hydroxybenzaldehyde, p-nitrobenzaldehyde, cetyltrimethylammonium bromide (CTAB), ninhydrin monohydrate, tetraethyl orthosilicate (TEOS), (3-aminopropyl)trimethoxysilane (APTS), (3-cyanopropyl)triethoxysilane (CPTS), and [3-[(2-aminoethyl)amino]propyl]trimethoxysilane (AAPTS) were all obtained from Sigma-Aldrich and were used as received without further purification. Anhydrous toluene, methanol, and ethanol (Fisher Scientific) were also used as received. [3-[[2[(2-Aminoethyl)amino]ethyl]amino]propyl]triethoxysilane (AAAPTS), (p-aminophenyl)trimethoxysilane (p-APS), and (m-aminophenyl)trimethoxysilane (m-APS) were purchased from Gelest, Inc. Synthesis of Mesoporous Silica. A mesostructured material was prepared as reported previously.42 Briefly, 5.49 mmol of CTAB was mixed with 26.67 mmol of Millipore water and 7 mL of 2.0 M NaOH solution. The solution was stirred at 80 °C for 30 min. After addition of 50.6 mmol of TEOS, the solution was stirred for 2 h at 80 °C. The solution was then filtered while it was still hot, and the solid was washed with Millipore water and ethanol. The resulting solid was dried in air for 4 h. The surfactant from the mesostructured material was extracted by stirring 1 g of the material in a solution of 0.5 mL of HCl and 150 mL of ethanol at 50 °C for 5 h. The solution was filtered, and the surfactant-extracted material was washed with Millipore water and ethanol. The solid material was then air(42) Huh, S.; Chen, H.-T.; Wiench, J. W.; Pruski, M.; Lin, V. S.-Y. J. Am. Chem. Soc. 2004, 126, 1010–1011.

Optimizing Acid-Base Catalysts for the Henry Reaction dried, resulting in mesoporous silica (MCM-41), which was used to prepare a series of organoamine-functionalized materials and other control samples below. Synthesis of Monoamine-Functionalized Mesoporous Silica in Ethanol. A monoamine-functionalized mesoporous material was synthesized by grafting APTS onto MCM-41.21 Briefly, 500 mg of MCM-41 was stirred in an excess amount, 3.68 mmol, of APTS and 200 mL of anhydrous ethanol under reflux at ∼78 °C for 6 h. The solution was filtered, and the solid was washed with dichloromethane, followed by ethanol. The solid material was allowed to dry under ambient conditions, resulting in a 3-aminopropyl-grafted sample labeled as AP-E1. Synthesis of Monoamine-Functionalized Mesoporous Silica in Toluene. Two organoamine samples were prepared by grafting APTS onto MCM-41 in toluene.21 The first sample was prepared by stirring 500 mg of MCM-41 in an excess amount, 3.68 mmol, of APTS and 200 mL of toluene at ∼78 °C. The solution was filtered, and the solid was washed with dichloromethane, followed by ethanol. The solid was allowed to dry under ambient conditions and is referred to as AP-T1. The second sample was prepared using the same procedure but under reflux at ∼112 °C for 6 h. This sample is referred to as AP-T2. Grafting of More Organosilane onto AP-E1 and AP-T1 in Ethanol or Toluene. Further grafting of two different organosilanes, APTS and CPTS, onto the functionalized sample AP-E1, which was prepared above, in ethanol and toluene was carried out. Typically, 300 mg of the AP-E1 was stirred in an excess amount, 2.23 mmol, of APTS in 200 mL of ethanol under reflux at ∼78 °C for 6 h. The solution was filtered, and the solid was washed with dichloromethane and ethanol. The solid was allowed to dry under ambient conditions, resulting in a sample labeled as AP-EE1. A second sample was prepared with the same procedure but in 200 mL of toluene at ∼78 °C, resulting in a sample labeled as AP-ET1. The grafting of 2.23 mmol of APTS at ∼78 °C onto AP-T1 for 6 h in 200 mL of toluene resulted in a third sample, AP-T3. Similarly, CPTS was grafted onto sample AP-E1 in ethanol and toluene. Typically, 300 mg of sample AP-E1 was stirred in an excess amount, 2.23 mmol, of CPTS in 200 mL of ethanol under reflux at ∼78 °C for 6 h. The solution was filtered, and the solid was washed with dichloromethane and ethanol and allowed to dry under ambient conditions. The resulting solid sample is denoted as AP-EE2. A second sample was prepared in the same way but in 200 mL of toluene at ∼78 °C and is denoted as AP-ET2. Control Samples by Stirring AP-E1 in Ethanol and Toluene. Two control samples were prepared by stirring sample AP-E1 in ethanol and toluene, in the absence of any organosilane. Typically, 300 mg of sample AP-E1 was stirred under reflux at ∼78 °C for 3 h in 200 mL of ethanol. The solution was filtered, and the solid was washed with dichloromethane and ethanol. The solid was then allowed to dry under ambient conditions, resulting in a sample denoted as AP-E1A. A second sample was prepared with the same procedure but in 200 mL of toluene at ∼78 °C, resulting in sample AP-E1B. Synthesis of Diamine-Functionalized Mesoporous Silica by Grafting in Ethanol and Toluene. Two mesoporous samples functionalized with organodiamine groups were prepared by grafting AAPTS onto MCM-41 in ethanol and toluene. Briefly, 500 mg of MCM-41 was stirred in an excess amount, 3.68 mmol, of AAPTS and 200 mL of ethanol under reflux at ∼78 °C for 6 h. The solution was filtered, and the solid was washed with dichloromethane and then with ethanol. The precipitate was dried under ambient conditions, resulting in sample AAP-E1. Similarly, the grafting of an excess amount, 3.68 mmol, of AAPTS onto 500 mg of MCM-41 in 200 mL of toluene under reflux at ∼78 °C for 6 h was carried out. The solution was filtered, and the precipitate was washed the same way as above. The resulting sample is referred to as AAP-T1. Grafting of More Organosilanes onto AAP-E1 and AAP-T1 in Ethanol and Toluene. Additional grafting of AAPTS onto samples AAP-E1 and AAP-T1, which were prepared above, in ethanol and toluene was carried out. Typically, 300 mg of AAP-E1 was stirred in an excess amount, 2.23 mmol, of AAPTS in 200 mL of ethanol

Langmuir, Vol. 24, No. 24, 2008 14309 under reflux at ∼78 °C for 6 h. The solution was filtered, and the solid was washed with dichloromethane and ethanol. The solid was allowed to dry under ambient conditions and is named AAP-EE1. A second sample was prepared with the same procedure but in 200 mL of toluene at ∼78 °C, resulting in a sample labeled as AAP-ET1. A third sample was prepared with the same procedure in 200 mL of toluene at ∼78 °C, resulting in a sample labeled as AAP-ET2. Similarly, additional grafting of 2.23 mmol of AAPTS onto sample AAP-T1 (300 mg) in 200 mL of ethanol at ∼78 °C for 6 h resulted in a new sample, AAP-TE1. Additional grafting of 2.23 mmol of AAPTS on sample AAP-T1 (300 mg) in 200 mL of toluene at ∼78 °C for 6 h resulted in AAP-TT1. Synthesis of Triamine-Functionalized Mesoporous Silica by Grafting AAAPTS in Ethanol and Toluene. These samples were synthesized by stirring 1.47 mmol of AAAPTS with 200 mg of MCM-41 in 200 mL of ethanol or toluene under reflux at ∼78 °C for 6 h. The solutions were filtered, and the solid samples were washed with dichloromethane, followed by ethanol. The solid samples were allowed to dry under ambient conditions. The resulting samples are labeled as AAAP-E1 and AAAP-T1 for grafting in ethanol and toluene, respectively. Synthesis of p-Aminophenyl-Functionalized Samples. These samples were synthesized by stirring 0.74 mmol of p-APS with 100 mg of MCM-41 in 150 mL of anhydrous ethanol or toluene under reflux at ∼78 °C for 6 h. The solution was filtered, and the solid was washed with dichloromethane and then with ethanol. The solid samples were allowed to dry under ambient conditions and were labeled as p-APE and p-APT for the samples grafted in ethanol and toluene, respectively. Synthesis of m-Aminophenyl-Functionalized Samples. These samples were synthesized by stirring 0.74 mmol of m-APS with 100 mg of MCM-41 in 150 mL of anhydrous ethanol and toluene under reflux at ∼78 °C for 6 h. The solutions were filtered, and the solid samples were washed with dichloromethane and then with ethanol. The precipitate was allowed to dry under ambient conditions and is labeled as m-APE and m-APT for the samples grafted in ethanol and toluene, respectively. Henry Reaction. The Henry reaction was carried out by using each of the organoamine-functionalized samples obtained above as the catalysts. Typically, 20 mg of the amino-functionalized mesoporous sample was added to a solution of 1 mmol of phydroxybenzaldehyde and 10 mL of nitromethane. The reaction was stirred at 90 °C (or at 50 °C) under nitrogen, and aliquots of the reaction product were taken with a filter syringe and characterized by solution 1H NMR and GC-MS over the course of the reactions. The conversions (%) were determined by using 1H NMR spectra. Resonances in acetone-d6/p-hydroxynitrostyrene (1H NMR): δ 2.85 (1H, br s), 6.96 (2H, d), 7.71 (2H, d), 7.83 (1H, d, J ) 13.5 Hz), and 8.04 (1H, d, J ) 13.5 Hz). Resonances of p-hydroxybenzaldehyde (1H NMR): δ 2.95 (1H, br s), 7.05 (2H, d), 7.8 (2H, d), and 9.84 (1H, s). Ninhydrin Test. To estimate or compare the relative number of accessible amine groups, the ninhydrin test on the amine-functionalized samples was done as reported previously.4h,i Typically, 32 mg of the samples was mixed with 5 mL of ethanolic ninhydrin solution (0.175 M). The solution was stirred at 90 °C for 25 min. It was then cooled and centrifuged for 10 min at 6000 rpm. The absorbance of the resulting Ruhemann’s purple in the supernatant, particularly at 568 nm, was monitored. Synthesis of Imines. To further estimate the number of accessible amine groups, another experiment as reported by Rosenholm et al.4j was performed. Typically, 100 mg of the functionalized samples was stirred in excess p-nitrobenzaldehyde (0.264 mmol for AP-E1 and 0.82 mmol for AP-T1) in 100 mL of methanol for 5 h at 70 °C. Then the solution was filtered, and the solid was washed thoroughly with methanol and allowed to dry under ambient conditions. The concentration (wt %) of N on the solid sample containing imine was then estimated by elemental analysis and thermogravimetric analysis. Instrumentation. The powder X-ray diffraction was measured using a Scintag powder diffractometer. Small-angle X-ray scattering (SAXS) was measured by using synchrotron radiation at the Cornell

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Scheme 1. Synthesis Flowchart of Functionalized Mesoporous Samples Containing Various Concentrations of Organoamines by Grafting Organosilanes onto MCM-41 in Ethanol and/or Toluenea

a

The compositions shown in the structures are based on the characterization results obtained.

High Energy Synchrotron Source (CHESS) as well as by using a Bruker AXS Nanostar system. The solid-state 13C (75.5 and 300 MHz) and 29Si (59.6 MHz) NMR spectra were acquired on a Bruker AVANCE 300 spectrometer. For 13C CP-MAS NMR experiments, we employed a 7.0 kHz spin rate, a 5 s recycle delay, a 1 ms contact time, a π/2 pulse width of 5.2 µs, and 1000-3000 scans using TPPM 1H decoupling. For the 29Si CP-MAS NMR experiments, we employed a 7.0 kHz spin rate, a 10 s recycle delay, a 10 ms contact time, a π/2 pulse width of 5.6 µs, and 256-1024 scans using TPPM 1H decoupling. The 29Si MAS NMR experiments were done with a 7.0 kHz spin rate, a 100 s recycle delay, a π/6 pulse width of 1.9 µs, and 700-4000 scans using high-power CW 1H decoupling. Solution 1H NMR spectra were measured with a Bruker DPX-300 NMR spectrometer. Thermogravimetric analysis was carried out with a Q-500 Quantachrome analyzer (TA Instruments) by using N2 (99.999%) as a carrier gas with a heating ramp of 5 °C/min. GC-MS spectra was measured with an HP-5971 GC-MS spectrometer. The BET gas adsorptions were measured with Micromeritics Tristar3000 volumetric adsorption analyzers at 77 K by following previously reported procedures.43 TEM images were taken with an FEI Tecnai T12 S/TEM instrument.

Results and Discussion A series of flexible alkylamine-, alkyldiamine-, and alkyltriamine-functionalized and rigid meta- and para-substituted aminophenyl-functionalized bifunctional mesoporous materials containing different surface densities or site isolations of organoamine and silanol groups were synthesized (Scheme 1 (43) (a) Kruk, M.; Asefa, T.; Coombs, N.; Jaroniec, M.; Ozin, G. A. J. Mater. Chem. 2002, 12, 3452–3457. (b) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169–3183.

and Table 1). Their syntheses involved a one-step or two-step grafting of the corresponding organosilanes, namely, APTS, AAPTS, AAAPTS, m-APS, and p-APS, onto mesoporous silica (MCM-41) in ethanol and/or toluene (Scheme 1 and Table 1). The parent MCM-41 material was synthesized as reported previously.44 The surfactant template from the as-synthesized MCM-41 was extracted by stirring it in a dilute acid/methanol solution. The grafting of the site-isolated organoamine groups was achieved by stirring APTS, AAPTS, AAAPTS, m-APS, and p-APS with MCM-41 in ethanol at 78 °C,45 which resulted in samples AP-E1, AAP-E1, AAAP-E1, m-APE, and p-APE, respectively. The site isolation of the organoamine groups in the samples was probed by complexing cupric ions onto the amine groups and by measuring the electronic spectra of the resulting Cu2+-amine complexes (see below and ref 36b). The grafting of densely populated monoamine groups was achieved by stirring APTS with MCM-41 in toluene at 78 and 112 °C under reflux, which produced samples AP-T1 and AP-T2, respectively. Similarly, higher densities of grafted organodiamine, organotriamine, and meta- and para-substituted aminophenyl groups were produced by grafting AAPTS, AAAPTS, m-APS, and p-APS, respectively, onto MCM-41 in toluene at 78 °C, resulting in samples AAP-T1, AAAP-T1, m-APT, and p-APT, respectively. (44) Huh, S.; Chen, H.-T.; Wiench, J. W.; Pruski, M.; Lin, V. S.-Y. Angew. Chem., Int. Ed. 2005, 44, 1826–1830. (45) The grafting of APTS and AAPTS onto MCM-41 in ethanol, resulting in samples AP-E1 and AAP-E1, respectively, was reported recently by us (see ref 36). These samples were prepared here again for comparative studies with the various other organoamine-functionalized catalysts we report here to probe structure-cooperative catalytic property relationships of site-isolated bifunctional catalysts.

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Table 1. Synthesis and Structural and Composition Data for Representative Monoamine-, Diamine-, Triamine-, and meta- and para-Substituted Aminophenyl-Functionalized Mesoporous Materials and Other Control Samples Synthesized by One-Step or Two-Step Grafting of the Corresponding Organosilanes onto Parent Materials in Ethanol and Toluene samplea

substrate + organosilane, solvent (grafting temp)b

unit cellc (Å)

BET surface area (m2/g)

BJH pore diamd (Å)

wall thicknesse (Å)

MCM-41 AP-E1 AP-T1 AP-T2 AP-T3 AP-EE1 AP-ET1 AP-EE2 AP-ET2 AP-E1A AP-E1B AAP-E1 AAP-T1 AAP-EE1 AAP-ET1 AAP-ET2 AAP-TE1 AAP-TT1 AAAP-E1 AAAP-T1 p-APE p-APT

MCM-41 + APTS, ethanol (78 °C) MCM-41 + APTS, toluene (78 °C) MCM-41 + APTS, toluene (112 °C) AP-T1 + APTS, toluene (78 °C) AP-E1 + APTS, ethanol (78 °C) AP-E1 + APTS, toluene (78 °C) AP-E1 + CPTS, ethanol (78 °C) AP-E1 + CPTS, toluene (78 °C) AP-E1 + ethanol (78 °C) AP-E1 + toluene (78 °C) AAPTS, ethanol (78 °C) AAPTS, toluene (78 °C) AAPE1 + AAPTS, ethanol (78 °C) AAPE1 + AAPTS, toluene (80 °C) AAPE1 + AAPTS, toluene (120 °C) AAPT1 + AAPTS, ethanol (78 °C) AAPT1 + AAPTS, toluene (80 °C) AAAPTS, ethanol (78 °C) AAAPTS, toluene (78 °C) MCM-41 + p-APS, ethanol (78 °C) MCM-41 + p-APS, toluene (78 °C)

44.3 44.4 45.6 44.4 45.0 44.2 44.0 44.5 44.1 44.0 43.8 44.2 43.6 44.1 43.6 43.7 44.0 43.6 44.0 44.1 43.4 43.2

1148 992 546 311 128 756 594 687 631 689 798 839 717 826 731 717 730 549 745 541 809 684

25.6 24.2 23.6 22.1 22.4 24.0 24.0 24.0 23.1 23.0 23.9 24.3 22.2 25.2 23.6 23.5 23.8 22.8 23.6 24.1 23.8 21.8

18.7 20.2 22.0 22.3 22.6 20.2 20.0 20.5 21.0 21.0 19.9 19.9 21.3 18.9 20.0 20.2 20.2 20.8 20.4 20.0 19.6 21.4

a Representative mesoporous samples prepared and studied. b The functionalized mesoporous samples were synthesized by grafting the given organosilane onto the given parent materials in the given solvent at the given temperature for 5 h. c Unit cell of the mesostructured sample calculated from the sample’s d-spacing value on its X-ray diffraction (XRD) pattern (unit cell ao ) 2d100/31/2 for hexagonal P6mm mesostructures). d Obtained from the adsorption branch on the N2 gas adsorption isotherm. e Wall thickness ) unit cell - pore diameter.

Furthermore, control samples labeled as AP-EE1 and AP-ET1 were synthesized by grafting more APTS onto a functionalized sample, AP-E1, in ethanol and toluene, respectively. Similarly, CPTS was grafted onto AP-E1 in ethanol and toluene, resulting in samples AP-EE2 and AP-ET2, respectively. The latter four samples, AP-EE1, AP-ET1, AP-EE2, and AP-ET2, were expected to have lower densities of silanols compared to their parent material, AP-E1, because of the second grafting. Additional control samples were obtained by simply stirring AP-E1 in ethanol and toluene without the presence of the organosilanes, which resulted in samples AP-E1A and AP-E1B, respectively. Furthermore, by grafting more AAPTS onto AAP-E1 in ethanol at 78 °C, in toluene at 78 °C, and in toluene at 112 °C, samples AAP-EE1, AAP-ET1, and AAP-ET2, respectively, were synthesized. Similarly, by grafting additional AAPTS onto AAP-T1 in ethanol and in toluene at 78 °C, samples AAP-TE1 and AAPTT1, respectively, were obtained. The synthetic procedures and structural data for the samples are given in Scheme 1 and Table 1. The structures of the parent mesoporous silica (MCM-41) and the organoamine-functionalized mesoporous samples were characterized by powder XRD and SAXS (Figures 1 and S1, Supporting Information). The XRD and SAXS patterns of all the functionalized samples as well as MCM-41 showed a sharp (100) Bragg reflection and at least two additional peaks corresponding to (110) and (200) Bragg reflections, which indicated that all the samples have highly ordered hexagonal, P6mm, mesostructures. From the d-spacing values on the XRD pattern, the unit cell dimensions of the materials were calculated to be ∼43.2-45.6 Å (unit cell ao ) 2d100/31/2 for P6mm mesostructures) (Table 1). As can also be seen in Figures 1 and S1 (Supporting Information), only a minor reduction in the intensities of the Bragg reflections was observed in the functionalized samples compared to their parent material, MCM-41. This indicates that the grafting of the flexible as well as rigid organoamine groups onto MCM-41, both in ethanol and in toluene, did not cause changes in the unit

cell dimension of the mesostructures. The slight decrease in the intensity of the Bragg reflections of samples grafted in toluene, such as AP-T1 and AP-T2, compared to the samples grafted in ethanol, such as AP-E1 and AP-EE1, was most likely due to the minor reduction in electron contrast between the channel pores and channel walls in the former and/or the increased strain on the silica walls due to the presence of silicon in the mesopores.46 This was caused by the grafting of more organoamine groups in the toluene-grafted samples than the ethanol-grafted samples (see below). The slight noticeable reduction in the unit cell dimension of AP-T2 compared to AP-T1 and AP-E1 was, however, most likely a result of the minor shrinkage of the mesostructures, which was caused by the condensation of residual surface silanols in samples such as APT2 and AAP-ET2, as the latter samples were grafted at relatively higher temperature (112 °C), compared to the grafting temperature used for all the other samples, which was 78 °C. The reduction in the unit cell dimension or the shrinkage of mesoporous channels due to the condensation of surface silanols at higher temperature is common.47 The presence of mesostructures in the materials was further confirmed by their TEM images, which showed nanorod-shaped particles containing highly hexagonally ordered mesoporous structures (Figure 2). Our observation was consistent with results reported previously for similar mesoporous materials obtained with supramolecular self-assembly of tetraalkoxysilanes in aqueous sodium hydroxide solution.48 The N2 sorption isotherms were type IV for all the samples (Figure 3), which also indicated the presence of well-ordered (46) (a) Minakata, S.; Tsuruoka, R.; Komatsu, M. J. Am. Chem. Soc. 2008, 130, 1536–1537. (b) MacLachlan, M. J.; Ginzburg, M.; Coombs, N.; Raju, N. P.; Greedan, J. E.; Ozin, G. A.; Manners, I. J. Am. Chem. Soc. 2000, 122, 3878–3891. (c) Coleman, N. R. B.; O’Sullivan, N.; Ryan, K. M.; Crowley, T. A.; Morris, M. A.; Spalding, T. R.; Steytler, D. C.; Holmes, J. D. J. Am. Chem. Soc. 2001, 123, 7010–7016. (47) Kruk, M.; Asefa, T.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2002, 124, 6383–6392.

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Figure 2. TEM images of (A) MCM-41 and (B-F) various representative organoamine-functionalized samples: (B) AP-E1, (C) AP-T1, (D) AP-T2, and (E) AAP-E1. (F) High-resolution TEM image of sample AP-E1.

Figure 1. (A) Powder XRD patterns of MCM-41 compared with organomonoamine-functionalized mesoporous samples synthesized by grafting various organoaminosilanes in ethanol and/or toluene, AP-E1, AP-T1, AP-T2, AP-EE1, AP-ET1, AP-EE2, and AP-ET2. (B) XRD patterns of MCM-41 compared with organodiamine-functionalized samples AAP-E1 and AAP-T1. (C) XRD patterns of organotriaminefunctionalized samples AAAP-E1 and AAAP-T1.

mesoporous structures in the materials. The BET surface areas of the samples, however, varied significantly from sample to sample and depended on the synthesis conditions employed. The surface areas of MCM-41, AP-E1, AP-T1, and AP-T2 were 1148, 992, 546, and 311 m2/g, respectively, while the surface areas of AAP-E1 and AAP-T1 were 839 and 717 m2/g, respectively. The surface areas of the samples grafted with triamine groups, AAAPE1 and AAAP-T1, were 745 and 541 m2/g, respectively. Since (48) Huh, S.; Wiench, J. W.; Yoo, J. C.; Pruski, M.; Lin, V. S.-Y. Chem. Mater. 2003, 15, 4247–4256.

all of the materials exhibited ordered mesostructures, as judged from their XRD and SAXS patterns (Figures 1 and S1, Supporting Information), the decrease in surface area and pore diameters in the organoamine-grafted samples compared to MCM-41 was attributed primarily to the presence of grafted groups in their mesopores.15f,g This was further confirmed by removing the grafted organoamine groups via calcination at 550 °C for 5 h, which gave back the high surface areas and pore volumes (Figure 4). For example, sample AP-T3, which has a low surface area of 128 m2/g and a low pore volume of 0.093 cm3/g, regained a higher surface area of 804 m2/g, a higher pore volume of 0.5 cm3/g, and a higher pore diameter of 2.7 nm after calcination and removal of the organic groups (Figure S2 and Table S1, Supporting Information). Upon grafting of organoamine groups onto the resulting calcined sample, the surface area, pore volume, and pore diameter decreased significantly again (see Figure 4 and Table S1, Supporting Information). Further calcination of the resulting sample regenerated the higher surface area, pore diameter, and pore volume. A similar decrease in surface areas upon grafting the organic groups was also observed in the other control samples such as AP-EE1, AP-ET1, AP-EE2, and APET2 (Table 1 and Figure 3). The variations in the values of the surface areas and pore diameters in the materials were attributed to the differences in the surface density and types of grafted groups. The surface areas of the samples containing the longer organodiamine and organotriamine groups in the mesopores appear to have slightly lower surface areas than the corresponding samples containing the shorter organoamines. It is also worth mentioning that the grafting of the organic groups in toluene resulted in a greater decrease in surface area and pore volume than the grafting of the corresponding organic groups in ethanol (Table 1). This is consistent with the fact that the grafting in toluene, particularly at a higher temperature, produces a higher

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Figure 3. N2 gas adsorption isotherms of (A) MCM-41 compared with various samples grafted with monoamine in ethanol or toluene in one step or two steps and corresponding control samples, (B) samples grafted with organodiamine in ethanol (AAP-E1) compared with corresponding control samples, (C) samples grafted with organodiamine in toluene (AAP-T1) compared with corresponding control samples, and (D) samples grafted with organotriamine in ethanol and toluene.

density of grafted organic groups (see below). The BJH pore diameter of MCM-41 was ∼2.6 nm, whereas the pore diameter was slightly reduced to ∼2.2-2.4 nm for the organo-functionalized samples (Table 1 and Figure S2, Supporting Information). Interestingly, the pore diameter of AP-T3, which was grafted with APTS in toluene at 78 °C for 5 h, twice, decreased to that of micropores (