Amine-Functionalized Ordered Mesoporous Silica Transesterification

Oct 9, 2009 - Texas A&M UniVersity, 3122 TAMU, College Station, Texas 77843-3122. The synthesis, characterization, and catalytic behavior of several ...
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Ind. Eng. Chem. Res. 2009, 48, 10375–10380

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Amine-Functionalized Ordered Mesoporous Silica Transesterification Catalysts Victor Varela Guerrero and Daniel F. Shantz* Texas A&M UniVersity, 3122 TAMU, College Station, Texas 77843-3122

The synthesis, characterization, and catalytic behavior of several amine-functionalized ordered mesoporous silica (OMS) materials are reported. All samples show catalytic activity for the conversion of glyceryl tributyrate with methanol to methyl esters in high yield (>60%). The reactivity of the amines approximately correlates with the pK values of the free amines. The effect of the amine identity, loading, and substrate pore size is also reported. To determine if leached amine was responsible for the observed reactivity, the hot filtrate method was employed. Those experiments show that upon removal of the OMS material the reaction conversion does not increase, indicating surface-tethered amines are primarily responsible for the observed reactivity. The stability of the most promising catalyst, 3-(anilinopropyl)-MCM-41 was analyzed via recycle studies. Leaching of the amine is observed via a decrease in conversion with multiple recycles and a concomitant change in sample color. Capping the residual silanols groups with hexamethyldisilazane leads to an increase in the catalyst stability with a decrease in activity. The work indicates that these solids are interesting alternatives to conventional homogeneous catalysts for this class of chemistry. 1. Introduction The development of ordered mesoporous materials was viewed with great enthusiasm in the area of heterogeneous catalysis.1-4 It was initially hoped that these materials would represent large-pore analogues of zeolites, in that they would possess thermal stability and strong acid sites, enabling cracking of large substrates not possible with zeolites.5 While this has not turned out to be the case, ordered mesoporous materials in general, and ordered mesoporous silicas (OMS) in particular, remain a topic of research. The picture emerging is that suitably functionalized OMS materials will represent both interesting model catalysts for fundamental studies, and in some cases offer new possibilities in heterogenizing homogeneous catalysts.6 In this regard numerous laboratories have reported the synthesis and catalytic testing of OMS materials with organic groups attached.7,8 This literature has expanded enormously in recent years, and several reviews are available for the interested reader.9-11 In general terms it seems reasonable to categorize these investigations into three groups. The first has been to understand the reactivity of amines on mesoporous silicas. Many laboratories, including those of Brunel, Lin, Katz, and Davis have investigated the activity of amines on OMS phases for a variety of reactions including Aldol chemistry, Michael additions, etc.12-27 The second body of work has focused on prolinederivatized and alkaloid-derivatized surfaces with the aim of making solid base catalysts for the production of chiral molecules.10,28,29 The third body of work has investigated sulfonic-acid functionalized silicas.30,31 A few laboratories have also investigated the synthesis of OMS materials containing multiple functional groups and their catalytic efficacy.19,32-34 Transesterification chemistry has attracted considerable interest in the past few years in the context of biodiesel production via the formation of methyl esters from triglycerides (Scheme 1).35-37 While this reaction can be performed using acids or bases as catalysts, sodium hydroxide is preferred as the kinetics of the reaction are faster in basic than acidic media. Thus the current method of choice for this chemistry is the use of either * To whom correspondence should be addressed. E-mail: shantz@ chemail.tamu.edu. Tel.: (979) 845-3492. Fax: (979) 845-6446. Address: Artie McFerrin Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, TX 77843-3122.

sodium hydroxide or sodium methylate. Recently there has been interest in developing heterogeneous catalysts to produce biodiesel.35,38,39 Their benefits include simplification of the separation of the reaction products, reuse of the catalyst, and possibly low sensitivity to free fatty acids (FFAs) and water. In the current work we report on the synthesis of a series of amine-functionalized OMS materials and their ability to catalyze this reaction, using glyceryl tributyrate as a model substrate. 2. Experimental Section 2.1. Materials. Sodium hydroxide (NaOH, 99%) and ethanol (ACS reagent grade) were obtained from Mallinckrodt chemicals. Sulfuric acid (H2SO4, 95-99%), hydrochloric acid (HCl, 35 wt %), aminopropyltriethoxysilane (APTES, 94%), and tetraethoxysilane (TEOS, >99%) were obtained from Fluka. Sodium silicate (27 wt % SiO2), N,N-dimethylaminopropyltrimethoxysilane (96%), N-(3-(trimethoxysilyl)propyl)aniline (99%), N-(3-(trimethoxysilyl) propyl)ethylenediamine (97%), (3-(methylamino)propyl) trimethoxysilane (97%), and glyceryl tributyrate (GTB, 99%) were obtained from Aldrich. N-butylaniline (97%) was purchased from Sigma. Methanol and toluene (ACS reagent grader) were obtained from Fisher Scientific. Toluene was dried using an MBraun solvent system and transferred using standard Schlenk procedures. Pluronic P123 (EO20PO70EO20, MW ) 5800) was obtained as a gift from BASF. Cetyltrimethyl ammonium bromide (CTAB, high purity grade) was obtained from AMRESCO. d-Chloroform (CDCl3, 99.8%) was obtained from Cambridge Isotope Laboratories. Inc. 2.2. OMS Synthesis and Functionalization. MCM-41 was synthesized using the method reported by Edler and White.40,41 A 7.9 g portion of sodium silicate was added to a propylene beaker, then 45.4 mL of deionized water was added to this along with 0.27 g of NaOH. Then 7.8 mL of 1 M H2SO4 was added slowly. A 7.8 g portion of CTAB was added to the vessel, and the mixture was stirred for an additional 10-15 min. The vessel was then put into the oven at 100 °C for 24 h under static conditions. After 24 h, the sample was removed from the oven, allowed to cool so it could be handled, and titrated with 1 M H2SO4 dropwise to adjust the pH to approximately 10. The sample was then placed back in the oven. This titration was

10.1021/ie9003915 CCC: $40.75  2009 American Chemical Society Published on Web 10/09/2009

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Scheme 1. Transesterification of Glyceryl Tributyrate

Scheme 2. Aminosilanes Used in Postsynthetic Grafting

Scheme 3. Molecular Structures of GTB and the Methyl Ester Product, and Chemical Shifts of the Different Protons as Determined by 1H NMR

repeated at 24 h intervals two additional times. After 96 h at 100 °C the reaction mixture was cooled, filtered, rinsed with copious quantities of deionized water, and dried. The solid product was calcined to remove the CTAB used in the synthesis. The calcination procedure was as follow. The temperature was increased from 25 to 500 °C at rate of 1 °C/min and then held at 500 °C for 8 h. SBA-15 was synthesized using a method comparable to that reported previously.3 Pluronic P123 (4.0 g) was dissolved in 60 mL of 4 M HCl and 85 mL of deionized water by stirring for 5 h at room temperature. Then 8.5 g of TEOS was added to that solution, and the mixture was stirred for 24 h at 35 °C. The mixture was then aged at 80 °C for 24 h under static conditions. The solid product was filtered, washed with copious quantities of deionized water, and air-dried overnight. The solid product was calcined to remove the polymer included in the as made sample. The calcination procedure was as follows: the sample was heated from room temperature to 100 °C at a rate of 1 °C/min; held at 100 °C for 1 h; increased from 100 to 500 °C at a rate of 1 °C/min and then held a 500 °C for 5 h. Amine-functionalized MCM-41 and SBA-15 were prepared via postsynthetic grafting. As a representative example the conditions for the 3-(anilinopropyl)-MCM-41 at 0.8 mmequiv/g loading are as follows: a 193 µL aliquot of 3-(anilinopropyl)trimethoxysilane was added to 1 g of calcined MCM-41 in 100 mL of anhydrous toluene under argon. This mixture was stirred for 24 h in a closed container at room temperature. The product was collected by filtration, washed with 50 mL of methanol and 1 L of deionized water, and air-dried overnight. Scheme 2 shows the organosilanes used in the postsynthetic grafting. Samples were prepared with target silane loadings of 0.2, 0.4, and 0.8 mmequiv silane/g OMS. Throughout the paper the sample nomenclature will be x-OMS-y, where x is the number indicated in Scheme 2 for the ligand, OMS denotes which silica (MCM-41 or SBA-15) was used as substrate, and y is the target loading in mmol/g. 2.3. Analytical. Powder X-ray diffraction (PXRD) measurements were performed using a Bruker AXS D8 powder diffractometer with Cu KR radiation over a range of 1-10° 2θ. Infrared spectroscopy measurements were performed using a Nexus 670 FT-IR spectrometer from Thermo Nicolet. Thermal gravimetric analyses (TGA) were performed using a TG 209C Iris instrument from Netzsch over a temperature range of 25-500 °C using oxygen and nitrogen as carrier gases and a temperature ramping rate of 2 °C min-1. Elemental analysis was performed by Galbraith Laboratories. Nitrogen adsorption experiments were performed on a Micromeritics ASAP 2010

micropore system using approximately 0.06 g of sample. The samples were degassed under vacuum at room temperature for 2 h then at 100 °C for 24 h before analysis. The surface area and mesopore volume were determined using the Rs-method.42,43 The mesopore size distributions were calculated from the adsorption branch of the isotherms using the Barret-JoynerHalenda (BJH) method with a modified equation for the statistical film thickness. Solution 1H NMR spectra were measured on a 300 MHz Varian mercury spectrometer. The 1H 90° pulse length was 6 µs, the recycle delay was 6 s, and 16 FIDs were recorded per spectrum. 2.4. Catalytic Testing. Catalysts were tested for activity in the transesterification of glyceryl tributyrate with methanol. Catalytic testing was performed as follows: to a 50 mL flask equipped with a stir bar and condenser, glyceryl tributyrate (GTB) and methanol were added in a 1:10 weight ratio (0.5 g GTB/5 g MeOH). A 0.175 g portion of catalyst was then added, which corresponds to an approximate GTB/amine ratio of 12:1 for the highest amine loading. The reaction, unless noted, was run at reflux (∼65 °C). For initial assessment of activity at the end of the reaction period the flask was placed in an ice bath to quench the reaction, and the solids were separated by centrifugation and the liquid phase isolated. For kinetics measurements of conversion versus time, aliquots were taken out at the time intervals noted. The conversion of the glyceryl tributyrate to methyl ester was determined from the relative intensities of the methyl ester protons (δ ) 3.6 ppm, group A in Scheme 3) and methylene protons adjacent to the ester group (δ ) 2.2 ppm, group 3 and group B in Scheme 3). 1H NMR analysis of these reaction mixtures has been reported previously.44 For the recycle studies the recovered catalyst was rinsed with 20 mL of methanol three times, dried overnight at 40 °C, and the testing repeated. For the hot filtration test,45,46 at the time denoted the sample was filtered while hot, collecting the solid catalyst. The solution recovered was then heated back to reflux conditions to assess if any homogeneous reactions were taking place. 3. Results and Discussion 3.1. Catalyst Characterization. Figure 1 shows a representative powder X-ray diffraction (PXRD) and nitrogen adsorption isotherm for a parent MCM-41 material before functionalization. One can observe four Bragg reflections in the pattern, consistent with previous work and indicating the materials are highly ordered. Analysis of the nitrogen adsorption isotherm indicates the material has a mesopore volume of 0.67 cm3/g and a nominal

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Figure 1. (Left) Powder X-ray diffraction and (right) nitrogen adsorption results for a representative parent MCM-41 sample. Inset shows the high angle data magnified by a factor of 5. On the adsorption figure open and solid circles are used for the adsorption and desorption branches, respectively. Table 1. Summary of Nitrogen Adsorption Data of the Amine-Functionalized Silicas Investigated sample ID

SR-s (m2/g)

Vmeso (Rs) (cm3/g)

Dp (BJH) (nm)

MCM-41 1-MCM-41-0.2 1-MCM-41-0.4 1-MCM-41-0.8 2-MCM-41-0.2 2-MCM-41-0.4 2-MCM-41-0.8 3-MCM-41-0.2 3-MCM-41-0.4 3-MCM-41-0.8 4-MCM-41-0.2 4-MCM-41-0.4 4-MCM-41-0.8 5-MCM-41-0.2 5-MCM-41-0.4 5-MCM-41-0.8 SBA-15 1-SBA-15-0.8 2-SBA-15-0.8 3-SBA-15-0.8 4-SBA-15-0.8 5-SBA-15-0.8

836 896 616 641 1164 597 1025 759 710 523 833 545 690 838 759 630 660 436 452 401 425 506

0.67 0.68 0.46 0.36 0.82 0.41 0.67 0.58 0.48 0.34 0.62 0.35 0.54 0.62 0.52 0.38 0.49 0.43 0.39 0.25 0.29 0.47

3.7 3.7 3.4 3.3 3.4 3.4 3.1 3.5 3.4 3.1 3.7 3.2 3.4 3.7 3.2 3.1 7.9 7.9 7.9 7.4 7.4 7.9

pore diameter of 3.7 nm. This is also consistent with previous work.41 All functionalized samples were analyzed by diffraction and nitrogen adsorption. The diffraction results indicate there is no modification of the silica matrix due to grafting. The adsorption results are summarized in Table 1; in general the pore volume decreases with increased amine loading. The most notable exception is the samples containing 2 and 4 where the 0.4 loading samples appear to have the smallest pore volumes. The explanation for this result is unclear; one possibility is that preferential grafting near the pore opening has taken place in these samples and some of the pores are inaccessible. Why this would happen in the midrange grafting level and not the highest level though is unclear. The diffraction patterns and adsorption isotherms for all samples are included in the Supporting Information. The data for the SBA-15 samples is also given in Table 1 and shows comparable surface areas and pore volumes at the loading of 0.8 mmequiv/g. The organic content was investigated using TGA measurements along with elemental analysis (EA) of selected samples. The TGA work focused on the high loading (0.8 mmequiv/g) samples, as these samples were the most catalytically active (see below), and the data are simpler to interpret. The details of the TGA and EA results are given in the Supporting

Figure 2. GTB conversion at 6 h for the amine-MCM-41 samples as a function of nominal amine loading.

Information. In general for the 0.8 mmequiv/g samples the TGA indicates loadings between 0.9 and 1.1 mmequiv/g. That the numbers are systematically high is not surprising, as the analysis does not account for the possibility of silanols reacting at elevated temperatures and leading to the release of water. Carbon NMR (not shown) shows that there are no alkoxy groups remaining on the silane, ruling out that possible contribution. Elemental analysis (Si, C, N) data were obtained for the 5 and 2 MCM-41 and SBA-15 samples at high loading. The results from elemental analysis are typically in line with the TGA; the differences between the two are typically between 10-20%. This is consistent with previous work in our lab. On whole the data indicates that the ligands are present on the silica in loadings comparable to the expected values. 3.2. Catalytic Properties. 3.2.1. Initial Assessment of Catalytic Activity. Initial work focused on ascertaining if the amine-OMS materials were active for the transesterification of glyceryl tributyrate (GTB). Figure 2 shows the GTB conversion at 6 h for the samples described above. As can be observed from the figure, all experiments indicate catalytic activity, with the conversion increasing with increased amine loading. One can also observe that the 5-MCM-41 samples are the most catalytically active and that the 4-MCM-41 sample at high loadings displays relatively lower activity compared to the rest. This initial result suggests that the materials are catalytically active; the possibility of amine leaching and corresponding homogeneous catalysis is discussed in more detail below (vide

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infra). Control reactions performed with just MCM-41 (i.e., no amines grafted) led to no observable methyl ester products. Also no mono- or disubstituted product are observed; that is, once the triglyceride begins to react it appears to be converted completely to 3 equiv of methyl ester and 1 equiv of glycerol. While it is tempting to use this information to invoke which step in the mechanism may be rate determining it is not warranted. Given the large excess of methanol in these mixtures it is difficult to observe protons from hydroxyl groups (i.e., glycerol or mono-/disubstituted glycerides) as they are expected to be in the chemical shift region near the methanol -OH resonance. The methodology employed here is determining the relative amount of methyl ester formed as compared to the amount of GTB observed by 1H NMR. To observe the effect of the catalyst pore size, a series of amine-functionalized SBA-15 materials were also synthesized and tested. The conversions range between 32% for 1-SBA-15 to 41% for 5-SBA-15. Thus the conversion (at a nominal amine loading of 0.8 mmequiv/g) is lower for all SBA-15 samples when compared to the MCM-41 samples, and the differences in conversion appear less dependent on the amine identity. One possible explanation for this is that a fraction of the amines are grafted into the SBA-15 micropores and thus are not accessible.47 To test this idea a sample of SBA-15 was calcined at 900 °C to remove the micropores.48 Ligand 5 was then grafted at a loading of 0.8 mmequiv/g and the material was tested. This material, which had little/no microporosity, gave a conversion of 43%. Thus it appears that the reaction is at least somewhat dependent on the pore size of the substrate, as 3.7 nm pore size MCM-41 gives a much higher conversion than 7.5 nm SBA15. An additional control was performed by grafting ligand 5 onto amorphous silica gel (nominal pore size 8 nm). For that sample the conversion observed after 6 h was 12%. Finally for both samples, SEM (not shown) indicates that the particles obtained from both the MCM-41 and SBA-15 prep are irregular micrometer-sized aggregates. Thus it seems unlikely that the differences in conversion above could be ascribed to particle size effects. Based on the results above the remainder of the paper will focus on the MCM-41 materials with ligand 5. However, before reporting kinetics measurements of the materials, four additional samples were prepared and tested to assess the effect of sample variability on reactivity. The average conversion for the four samples after 6 h was 88% with a standard deviation of 5%. Note that it is reasonable to attribute a variation on the order of (2% to the integration of the NMR spectra based on our experience. Moreover at higher conversions this error is likely to be higher, given that small relative changes in the integrated intensity will lead to larger errors. 3.2.2. Kinetics Studies and Catalytic Stability. Having established the activity of these materials, conversion versus time data was measured as a function of temperature for 5-MCM-41 (Figure 3). As expected, the conversion increases with increasing temperature over the range of 303-338 K. There is a pronounced jump in the conversion between 3 and 4 h. One possibility is that contributions from homogeneous catalysis are being observed because of amine leaching. Measurements of the short time behavior (i.e., conversion versus time out to 180 min) were also performed (not shown) and are essentially identical to the results presented in Figure 3. To probe the issue of amine leaching and how it impacts reactivity the hot filtrate test was performed. In this experiment the solid is removed by filtration after a given time, and the remaining liquid heated back to reaction conditions.45,46 If further conversion is observed the implication is that it is due

Figure 3. Conversion versus time for 5-MCM-41 (0.8 mmequiv/g) in the temperature range of 303-338 K. Lines between points are meant as a guide to the eye.

Figure 4. Conversion versus time for 5-MCM-41 from hot filtrate experiments where catalyst is removed after 4 h.

to homogeneous species. Figure 4 shows the results when the catalyst is removed after 4 h. As can be seen from this figure the conversion is essentially constant after catalyst removal. For the run where the catalyst is removed after 5 h (not shown) the conversion stays constant at 40% after the catalyst removal. The results in Figure 4 indicate that the role of homogeneous species in the conversions observed in Figures 2 and 3 is minimal (the conversions in those instances are >80% after 6 h). Hot filtrate experiments where the catalyst was removed after 2 and 3 h (not shown) are consistent with the results in Figure 4. Thus on the basis of this data, most if not all the conversion obtained from these materials is due to surface-tethered amines. To investigate homogeneous chemistry effects further, reactions were performed using N-butylaniline as a catalyst, which should be a reasonable homogeneous catalyst analogue of 5-MCM-41. The plot of conversions versus time is shown in the Supporting Information, and a much lower conversion of 12% after 6 h is observed. Thus consistent with the results above, the surface-tethered amines are much more catalytically active than the homogeneous N-butylaniline. The catalyst stability was also investigated. Figure 5 shows the conversion after 6 h versus reaction cycle and a clear drop in conversion can be observed. The fifth reaction cycle (not shown) gave no conversion. Consistent with that the sample of 5-MCM-41, which is pink due to the presence of the aniline ligand, is

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Acknowledgment Financial support for V.V.G. via a CONACyT fellowship is gratefully acknowledged. Supporting Information Available: Adsorption isotherms and powder X-ray diffraction for all samples reported, TGA and elemental analysis summary, conversion versus time data for the reaction catalyzed with N-butylaniline. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited

Figure 5. Conversion after 6 h as a function of reaction cycle for 5-MCM41 (filled circles) and HMDS-treated 5-MCM-41 (open circles).

completely white after the fifth reaction cycle. Thus while there is clear leaching of the amine group under the reaction conditions used, on the basis of the results in Figure 5 the activity observed appears due to the surface-tethered amines. To attempt to improve the stability of the catalyst, the sample was treated with hexamethyldisilazane to cap residual silanols groups. While the stability of the catalyst improves, its reactivity decreases. One interpretation of this result is that the presence of the silanols increases the reaction rate as such materials are effective bifunctional acid-base catalysts. An alternative interpretation is that by rendering the surface hydrophobic with HMDS treatment, the sorptive properties of the triglyceride and methyl esters is changed. Thus the current work demonstrates that amine-functionalized mesoporous silicas, which have been previously shown to act as catalysts for many organic reactions, will effectively catalyze the conversion of triglycerides to methyl esters. It is perhaps noteworthy that the aniline-functionalized OMS materials display the highest conversion, as the pK value for this amine is below 4, whereas the other amines have much higher values between 7 and 10. The results do indicate there is some substrate sensitivity as the same ligand grafted on SBA-15 and amorphous silica gel is much less active than on a smaller pored OMS material, here MCM-41. The reason for why the small pore mesoporous silica, MCM-41, is more active than the larger pore SBA-15 is unclear. One possibility is the stronger adsorption of the triglyceride into the smaller pore silica, though we have no proof to support this hypothesis. That the conversion drops significantly upon capping with hexamethyldisilazane indicates that either the surface silanol groups help facilitate the reaction or that by rendering the surface more hydrophobic the adsorption properties of the reagents are changed. 4. Conclusions The catalytic properties of amine-functionalized OMS materials for the synthesis of methyl esters from triglycerides are reported. The best materials show high conversion at moderate temperatures (338 K) and offer potential alternatives to the focus to date, which has been on homogeneous bases such as sodium hydroxide. While slow leaching of the aminosilane group is observed, hot filtrate experiments indicate that the bulk of the reactivity observed is due to surface-tethered amine groups. Improving the stability of these surface-tethered amines is an ongoing task in our lab.

(1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A New Family of Mesoporous Molecular-Sieves Prepared with Liquid-Crystal Templates. J. Am. Chem. Soc. 1992, 114, 10834–10843. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular-Sieves Synthesized by a Liquid-Crystal Template Mechanism. Nature 1992, 359, 710–712. (3) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120, 6024–6036. (4) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Templating of Mesoporous Molecular-Sieves by Nonionic Polyethylene Oxide Surfactants. Science 1995, 269, 1242–1244. (5) Corma, A. From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis. Chem. ReV. 1997, 97, 2373–2419. (6) Ciesla, U.; Schu¨th, F. Ordered Mesoporous Materials. Microporous Mesoporous Mater. 1999, 27, 131–149. (7) Fowler, C. E.; Burkett, S. L.; Mann, S. Synthesis and Characterization of Ordered Organo-Silica-Surfactant Mesophases with Functionalized MCM41-Type Architecture. Chem. Commun. 1997, 1769–1770. (8) Macquarrie, D. J. Direct Preparation of Organically Modified MCMtype Materials. Preparation and Characterization of Aminopropyl-MCM and 2-Cyanoethyl-MCM. Chem. Commun. 1996, 16, 1961–1962. (9) Ford, D. M.; Simanek, E. E.; Shantz, D. F. Engineering Nanospaces: Ordered Mesoporous Silicas as Model Substrates for Building Complex Hybrid Materials. Nanotechnology 2005, 16, S458–S475. (10) Li, C. Chiral Synthesis on Catalysts Immobilized in Microporous and Mesoporous Materials. Catal. ReV. 2004, 46, 419–492. (11) Taguchi, A.; Schu¨th, F. Ordered Mesoporous Materials in Catalysis. Microporous Mesoporous Mater. 2004, 77, 1–45. (12) Bass, J. D.; Anderson, S. L.; Katz, A. The Effect of Outer-Sphere Acidity on Chemical Reactivity in a Synthetic Heterogeneous Base Catalyst. Angew. Chem., Int. Ed. 2003, 42, 5219–5222. (13) Bass, J. D.; Katz, A. Thermolytic Synthesis of Imprinted Amines in Bulk Silica. Chem. Mater. 2003, 15, 2757–2763. (14) Bass, J. D.; Solovyov, A.; Pascall, A. J.; Katz, A. Acid-Base Bifunctional and Dielectric Outer-Sphere Effects in Heterogeneous Catalysis: A Comparative Investigation of Model Primary Amine Catalysts. J. Am. Chem. Soc. 2006, 128, 3737–3747. (15) Chen, H.-T.; Huh, S.; Wiench, J. W.; Pruski, M.; Lin, V. S. Y. Dialkylaminopyridinde-Functionalized Mesoporous Silica Nanosphere as an Efficient and Highly Stable Heterogeneous Nucleophilic Catalyst. J. Am. Chem. Soc. 2005, 127, 13305–13311. (16) Defreese, J. L.; Katz, A. Synthesis of a Confined Class of Chiral Organic Catalysts via Bulk Imprinting of Silica. Chem. Mater. 2005, 17, 6503–6506. (17) Demicheli, G.; Maggi, R.; Mazzacani, A.; Righi, P.; Sartori, G.; Bigi, F. Supported Organic Catalysts: Synthesis of (E)-Nitrostyrenes from Nitroalkanes and Aromatic Aldehydes over Propylamine Supported on MCM-41 Silica as a Reusable Catalyst. Tetrahedron Lett. 2001, 42, 2401– 2403. (18) Huh, S.; Chen, H.-T.; Wiench, J. W.; Pruski, M.; Lin, V. S. Y. Controlling the Selectivity of Competitive Nitroaldol Condensation by Using a Bifunctionalized Mesoporous Silica Nanosphere-Based Catalytic System. J. Am. Chem. Soc. 2004, 126, 1010–1011. (19) Huh, S.; Chen, H.-T.; Wiench, J. W.; Pruski, M.; Lin, V. S. Y. Cooperative Catalysis by General Acid and Base Bifunctionalized Mesoporous Silica Nanospheres. Angew. Chem., Int. Ed. 2005, 44, 1826–1830. (20) Kubota, Y.; Goto, K.; Miyata, S.; Goto, Y.; Fukushima, Y.; Sugi, Y. Enhanced Effect of Mesoporous Silica on Based-Catalyzed Aldol Reaction. Chem. Lett. 2003, 32, 234–235.

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(21) Lee, A.; Kim, W.; Lee, J.; Hyeon, T.; Kim, B. M. Heterogeneous Asymmetric Nitro-Mannich Reaction Using a Bis(oxazoline) Ligand Grafted on Mesoporous Silica. Tetrahedron: Asymmetry 2004, 15, 2595–2598. (22) Macquarrie, D. J.; Maggi, R.; Mazzacani, A.; Sabater, M. J.; Sartori, G.; Sartori, R. Understanding the Influence of the Immobilization Procedure on the Catalytic Activity of Aminopropylsilicas in C-C Forming Reactions. Appl. Catal., A 2003, 246, 183–188. (23) Shimizu, K.-I.; Hayashi, E.; Inokuchi, T.; Kodama, T.; Hagiwara, H.; Kitayama, Y. Self-Aldol Condensation of Unmodified Aldehydes Catalyzed by Secondary-Amine Immobilized in FSM-16 Silica. Tetrahedron Lett. 2002, 43, 9073–9075. (24) Shimizu, K.-I.; Suzuki, H.; Hayashi, E.; Kodama, T.; Tsuchiya, Y.; Hagiwara, H.; Kitayama, Y. Catalytic Direct 1,4-Conjugate Addition of Aldehydes to Vinylketones on Secondary-Amines Immobilized in FSM16 Silica. Chem. Commun. 2002, 1068–1069. (25) Yokoi, T.; Yoshitake, H.; Tatsumi, T. Synthesis of AminoFunctionalized MCM-41 via Direct Co-condensation and Postsynthesis Grafting Methods Using Mono-, Di-, and Triamino-Organoalkoxysilanes. J. Mater. Chem. 2004, 14, 951–957. (26) Yoshitake, H.; Koiso, E.; Tatsumi, T.; Horie, H.; Yoshimura, H. Preparation and Characterization of Polyamine-Functionalized Mesoporous Silica. Chem. Lett. 2004, 33, 872–873. (27) Cauvel, A.; Renard, G.; Brunel, D. Monoglyceride Synthesis by Heterogeneous Catalysis Using MCM-41-type Silicas Functionalized with Amino Groups. J. Org. Chem. 1997, 62 (3), 749–751. (28) Corma, A.; Iborra, S.; Rodriguez, I.; Iglesias, M.; Sa´nchez, F. MCM41 Heterogenized Chiral Amines as Base Catalysts for Enantioselective Michael Reaction. Catal. Lett. 2002, 82, 237–242. (29) Dhar, D.; Beadham, I.; Chandrasekaran, S. Proline and Benzylpenicillin Derivatives Grafted Into Mesoporous MCM-41: Novel OrganicInorganic Hybrid Catalysts for Direct Aldol Reaction. Proc. Indian Acad. Sci. (Chem. Sci.) 2003, 115, 365–372. (30) Alvaro, M.; Corma, A.; Das, D.; Forne´s, V.; Garcı´a, H. SingleStep Preparation and Catalytic Activity of Mesoporous MCM-41 and SBA15 Silicas Functionalized with Perfluoroaklysulfonic Acid Groups Analogous to Nafion. Chem. Commun. 2004, 965–957. (31) Melero, J. A.; Stucky, G. D.; van Grieken, R.; Morales, G. Direct Synthesis of Ordered SBA-15 Mesoporous Materials Containing Arenesulfonic Acid Groups. J. Mater. Chem. 2002, 12, 1664–1670. (32) Dufaud, V.; Davis, M. E. Design of Heterogeneous Catalysts via Multiple Active Site Positioning in Organic-Inorganic Hybrid Materials. J. Am. Chem. Soc. 2003, 125, 9403–9413. (33) Zeidan, R. K.; Dufaud, V.; Davis, M. E. Enhanced, Cooperative, Catalytic Behavior of Organic Functional Groups by Immobilization. J. Catal. 2006, 239, 299–306. (34) Zeidan, R. K.; Hwang, S.-J.; Davis, M. E. Multifunctional Heterogeneous Catalysts: SBA-15-Containing Primary Amines and Sulfonic Acids. Angew. Chem., Int. Ed. 2006, 45, 6332–6335.

(35) Di Serio, M.; Ledda, M.; Cozzolino, M.; Minutillo, G.; Tesser, R.; Santacesaria, E. Transesterification of Soybean Oil to Biodiesel by Using Heterogeneous Basic Catalysts. Ind. Eng. Chem. Res. 2006, 45 (9), 3009– 3014. (36) Lin, X. H.; Chuah, G. K.; Jaenicke, S. Base-Functionalized MCM41 as Catalysts for the Synthesis of Monoglycerides. J. Mol. Catal A: Chem. 1999, 150 (1-2), 287–294. (37) Ma, F.; Hanna, M. A. Biodiesel Production: A Review. Bioresour. Technol. 1999, 70, 1–15. (38) Di Serio, M.; Tesser, R.; Pengmei, L.; Santacesaria, E. Heterogeneous Catalysts for Biodiesel Production. Energy Fuels 2008, 22, 207– 217. (39) Gryglewicz, S. Rapeseed Oil Methyl Esters Preparation Using Heterogeneous Catalysts. Bioresour. Technol. 1999, 70, 249–253. (40) Edler, K. J.; White, J. W. Further Improvements in the Long-Range Order of MCM-41 Materials. Chem. Mater. 1997, 9, 1226–123. (41) Branton, P. J.; Sing, K. S. W.; White, J. W. Adsorption of Carbon Tetrachloride and Nitrogen by 3.4 nm Pore Diameter Siliceous MCM-41. J. Chem. Soc., Faraday Trans. 1997, 93, 2337–2340. (42) Jaroniec, M.; Kruk, M.; Olivier, J. P. Standard Nitrogen Adsorption Data for Characterization of Nanoporous Silicas. Langmuir 1999, 15, 5410– 5413. (43) Rouquerol, F.; Roquerol, J.; Sing, K., Adsorption by Powders and Porous Solids; Academic: San Diego, CA, 1999. (44) Morgenstern, M.; Cline, J.; Meyer, S.; Cataldo, S. Determination of the Kinetics of Biodiesel Production Using Proton Nuclear Magneetic Resonance Spectrscopy. Energy Fuels 2006, 20, 1350–1353. (45) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. On the Nature of the Active Species in Palladium Catalyzed Mizoroki-Heck and SuzukiMiyaura CouplingssHomogeneous or Heterogeneous Catalysis, a Critical Review. AdV. Synth. Catal. 2006, 348, 609–679. (46) Sheldon, R. A.; Wallau, M.; Arends, I.; Schuchardt, U. Heterogeneous Catalysts for Liquid-Phase Oxidations: Philosophers’ stones or Trojan Horses. Acc. Chem. Res. 1998, 31, 485–493. (47) Yoo, S.; Lunn, J. D.; Gonzalez, S.; Ristich, J. A.; Simanek, E. E.; Shantz, D. F. Engineering Nanospaces: OMS/Dendrimer Hybrids Possessing Controllable Chemistry and Porosity. Chem. Mater. 2006, 18, 2935–2942. (48) Imperor-Clerc, M.; Davidson, P.; Davidson, A. Existence of a Microporous Corona around the Mesopores of Silica-Based SBA-15 Materials Templated by Triblock Copolymers. J. Am. Chem. Soc. 2000, 122, 11925–11933.

ReceiVed for reView March 9, 2009 ReVised manuscript receiVed September 24, 2009 Accepted September 27, 2009 IE9003915