Cooperative Catalysis with Acid–Base Bifunctional Mesoporous Silica

Jun 11, 2012 - Laura Espinal , Martin L. Green , Daniel A. Fischer , Dean M. DeLongchamp , Cherno Jaye , Jarod C. Horn , Miles A. Sakwa-Novak , Watcha...
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Cooperative Catalysis with Acid−Base Bifunctional Mesoporous Silica: Impact of Grafting and Co-condensation Synthesis Methods on Material Structure and Catalytic Properties Nicholas A. Brunelli, Krishnan Venkatasubbaiah, and Christopher W. Jones* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia, 30332-0100 S Supporting Information *

ABSTRACT: The structural and cooperative catalytic characteristics of acid and base cofunctionalized mesoporous silica synthesized through grafting and co-condensation methods are investigated. It is shown that incorporation of the mutually reactive amine and carboxylic acid functional groups is aided by a protecting group in the grafting method. Using a thermally cleavable protecting group on the carboxylic acid organosilane, the differential effect of silanol removal and acid group functionalization on catalytic activity is studied. For samples prepared here by both the co-condensation and grafting procedures, the removal of silanols and the introduction of the carboxylic acid has a negative impact on activity of the catalyst in aldol condensations under the conditions used here. These results demonstrate that a weaker Brønsted acid silanol is more effective in cooperatively catalyzing the aldol condensation in combination with an amine base than the stronger carboxylic acid for all the materials prepared in this study. KEYWORDS: nitroaldol, Henry reaction, bifunctional catalyst, aldol, organic−inorganic hybrid functionalization (i.e., < 1 mmol g−1) on silica substrates that have not been treated (dehydroxylated) at high temperatures (>700 °C), the surface will contain the organosilane in addition to a significant amount of silanol groups that can act as weak acid or hydrogen-bonding partners. This approach has been frequently used to create amine sites for catalytic applications23 or for construction of supported metal complexes.24,25 Grafting of a second catalytic component may require one of the reactive components to have a protecting group during the grafting process to prevent self-assembly and mutual quenching (e.g., in the case of acid and base sites). Although many protecting groups can later be removed with either acid or base,26,27 the mutual presence of an acid and base component and the stability of the silica matrix can severely complicate this strategy for protecting group cleavage. A more compatible protecting group would be one that is thermally cleavable. This method was demonstrated previously for a material containing a single organic functional molecule17,28 and more recently with incorporating a second organic group containing a thiol.17 The second organic component complicates this strategy since the heat treatment should not alter or destroy the second functional group. Spatially isolating the two functional groups26,27 would prevent the interaction of the two functional groups, and is more suitable for studying cascade reactions and not suitable for studying cooperative reactions. An alternative strategy for incorporation of two functional groups is the co-condensation method. In this approach, the

1. INTRODUCTION Coupling reactions that are typically acid or base-catalyzed, such as aldol, nitroaldol, and Knoevenagel reactions, can be accelerated through the addition of a second component that acts cooperatively. For these reactions, the two catalytic components can separately activate the two different substrates,1,2 ultimately yielding a lower energy reaction pathway. Often, the components are an acid and a base: the acid will activate one component for electrophilic addition while the base will increase the nucleophilicity of the other substrate.2−5 Although efficient dual activation promotes the reaction, the acid and base catalytic components can also interact in a nonproductive manner based on their relative strength and proximity, quenching the activating behavior of the individual sites, and rendering the catalyst inactive if the acid and base sites completely and strongly self-associate.6,7 The interaction between the acid and base can be limited through attachment of the components to a rigid matrix, such as mesoporous silica.6−18 The large pore volume and surface area of mesoporous silicas provide ample room to incorporate multiple functional groups while the matrix stability is ideal for many types of reactions.19−22 The functional groups can be placed onto the surface of the material through either the cocondensation or grafting route. Attachment of an organosilane onto a mesoporous silica material is commonly achieved through grafting an organosilane, often a trialkoxysilane, to the silica surface after synthesis of the silica is complete. The alkoxide bonds are hydrolyzed upon reaction with the surface silanols and/or adsorbed water, ultimately resulting in surface attachment of the organosilane via condensation with the silanols. At low degrees of © 2012 American Chemical Society

Received: October 25, 2011 Revised: June 5, 2012 Published: June 11, 2012 2433

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functional groups are introduced in the initial stages of the synthesis of the mesoporous material.29 The viscous and acidic nature of the synthesis gel is believed to permit the two organic groups to cohabitate without quenching. A significant drawback to this approach is the inability to remove the template completely and therefore, accurately assess the organic loading from thermogravimetric data.30,31 The template cannot be removed postsynthetically through a high-temperature calcination step, as is done with the unfunctionalized material, as the calcination would also combust or oxidize the desired organosilane groups along with the template. Rather, the template is typically extracted with a solvent.32 Many methods have been reported with different degrees of success for template extraction, but complete template extraction is not often demonstrated.33,34 The residual template from the cocondensation procedure complicates the estimation of the organic loading from thermogravimetric analysis, which is the typical method used to assess the loading of organic active sites on the silica surface. The co-condensation method has been reported to allow the incorporation of a primary amine with different acid groups (i.e., phenyl sulfonic acid, phosphoric acid, and carboxylic acid) in addition to surface silanols, without protecting either component.16,35,36 The activity of the multifunctional materials increased with increasing pKa of the organic acid component, meaning the material containing the carboxylic acid and amine was demonstrated to be the most active in the aldol condensation, of the cofunctionalized materials evaluated.16,35 As the cooperative interactions between the surface silanols (pKa ≈ 7)37−39 and amines have also been demonstrated to lead to enhanced activity relative to the amine alone,28 the relative impact of cooperative catalysis with amine-silanol pairs vs. amine-carboxylic acid pairs (pKa ≈ 5) remains unclear. In considering the two synthesis routes  grafting and cocondensation  for the synthesis of acid−base bifunctional mesoporous silicas, one can identify several advantages and disadvantages of each approach. The co-condensation synthesis, which has been more often used, has been reported to incorporate both the acid and base sites into the solid without complete quenching of each functional group. It is a relatively straightforward synthetic approach,6,18 however, it can be difficult to quantify the amount of organic active sites present (e.g., by elemental analysis) if the functional group contains no elements that are unique from the template, because of the inability to completely extract the template.40 (Elemental analysis can be used to identify N species associated with base sites and P or S species associated with phosphoric or sulfonic acid sites. However, identification of carboxylic acid sites is complicated by the presence of the same atoms in both the acid site and the residual surfactant (C,H,O).) Rather, the literature concerning template extraction from mesoporous silica using various solvents most often shows that complete surfactant removal is not achieved (vide supra). Furthermore, organosilanes may be incorporated/imbedded into the pore walls in some cases, leaving inaccessible functional groups.31,40 The grafting approach has the advantage that the loading of the acid and base components can be directly estimated based upon TGA analysis because the two components can be added separately, in two grafting steps. However, if unprotected organosilanes are used, the acid and base sites may strongly selfassociate and quench each other. This disadvantage can be overcome by using at least one protected organosilane during the grafting process.41

Of these issues, the residual template left after extraction using a co-condensation synthesis presents a significant challenge. For mesoporous materials made using polymer templates, numerous postsynthetic extraction procedures have been reported to remove considerable amounts of template material, but extraction has not been conclusively shown to be complete. The inability to achieve complete template extraction has been related in some cases to strong binding in the micropores of the material.42 The micropore volume can be eliminated through aging the silica mixture at a temperature of 130 °C, as evidenced by the characterization of the textural properties of the material.43,44 In this work, we investigate the difference in catalyst performance for the aldol condensation of 4-nitrobenzaldehyde with acetone (Scheme 1) using base or acid/base bifunctional Scheme 1. Aldol Condensation with 4-Nitrobenzaldehyde and Acetone

silica-based catalytic materials prepared through grafting and co-condensation. For the co-condensed materials, we find that increasing the aging temperature improves the template extraction, but does not permit complete extraction of the polymer template under any conditions tested. For the grafted materials, we demonstrate that the sequential grafting of an amine followed by a newly synthesized organosilane containing a carboxylic acid protected with a thermally cleavable group. The thermal deprotection is confirmed through CP MAS 13C NMR. Finally, the observed differences in reaction rate using the array of new catalysts are described, and it is determined that introduction of the carboxylic acid group decreases the rate of reaction relative to the amine functionalized material under the conditions used here, suggesting that cooperative aminesilanol pairing is more effective than amine-carboxylic acid pairing using the synthetic and catalytic conditions used here.

2. EXPERIMENTAL METHODS 2.1. Chemicals. Pluronic P123 EO-PO-EO triblock copolymer (poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol); Mn ≈ 5800 Sigma-Aldrich), tetraethyl orthosilicate (TEOS, 98%, Sigma-Aldrich), hydrochloric acid (HCl, conc. 37%, J.T. Baker), acetone (ACS grade, BDH), ethanol (reagent grade, BDH), chloroform-d (CDCl3, 99.8%, Cambridge Isotope Laboratories), hexane (Sigma-Aldrich), 3-aminopropyltrimethoxysilane (APTMS, 97%, Sigma-Aldrich), and diethylene glycol dibutyl ether (DGDE, 98%, Sigma-Aldrich) were used as received. Toluene (Sigma-Aldrich, anhydrous) and methanol (Sigma-Aldrich, anhydrous) were stored in the glovebox prior to use and used according to standard Schlenk conditions. The tert-butyl methacrylate (Sigma-Aldrich) was distilled prior to use and stored in a refrigerator. 2.2.1. Synthesis of tert-butyl 2-methyl-3-(triethoxysilyl)propanoate. All glassware was dried in an oven before use. The tert-butyl group methacrylate triethoxy silane was produced via a hydrosilylation reaction (Scheme 2). The tert-butyl group methacrylate (2.0 g, 15.6 mmol; vacuum-distilled and refrigerated prior to use) was added to a 100 mL two-neck round-bottom flask equipped with a condenser and a rubber septum. With nitrogen flowing, the round-bottom was placed in an ice bath, and a micropipet was used to add 100 μL of Karstedt’s complex slowly to prevent excess heating that could lead to polymerization. The headspace was purged with nitrogen for 10 min before the dropwise addition of 4.5 mL of triethoxy silane 2434

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Scheme 2. Hydrosilylation Reaction for Production of tButylmethacrylate Triethoxy silane

reports, scaling the ingredients accordingly. Briefly, 24.0 g of the template polymer (P123; Mn ≈ 5800) was dissolved in a mixture of 120 mL concentrated hydrochloric acid and 636 mL distilled water in a 2 L Erlenmeyer flask heated to 40 °C in a silicone oil bath. Tetraethylorthosilicate (TEOS; 52.6 g) was added to the mixture with stirring for 20 h. The magnetic stir bar was removed, and the mixture was heated to 100 °C for 24 h. The resulting solid was cooled and filtered using a water aspirator, washing with copious amounts of water, and allowing the material to dry overnight. The dried material was calcined in a temperature-programmed furnace under flowing air, according to the following program: (1) ramp to 200 at 1.2 °C min−1, (2) soak for 1 h, (3) ramp to 550 at 1.2 °C min−1, (4) soak for 4 h, (5) cool to room temperature. The material was further dried at reduced pressure (10 mTorr) with heating to 200 °C overnight. 2.2.4. Generalized Grafting Procedure, SBA-G-X. The calcined SBA-15 was dried overnight at reduced pressure (10 mTorr) and heated to 100 °C in a round-bottom flask. After removing from the vacuum line, the round-bottom was capped with a septum, and degassed with nitrogen for 30 min. A mixture was made of dry toluene and the organosilane to graft onto the surface, maintaining the ratio of silica to toluene as constant and adding the desired amount of organosilane (25 mL toluene:1 g silica). The mixture was injected via syringe into the round-bottom with the powder while stirring. Stirring proceeded for 24 h at room temperature before removing the septum, adding 20 μL of distilled water per gram of silica, and heating the round-bottom to 80 °C with a condenser for an additional 24 h. The resultant material was cooled and filtered, washing with 100 mL of toluene, 100 mL of hexanes, and 100 mL of ethanol sequentially. The material was dried overnight under reduced pressure (10 mTorr) at 100 °C. The materials were labeled according to the type of organosilane, the theoretical loading (typically, 0.5 mmol/g), and the order of addition of the functional groups. The abbreviations for the different organosilanes were as follows: aminopropyltrimethoxy silane (AP), 3-(triethoxysilylpropyl) tert-butyl group carbmate (AC), and tbutyl 2-methyl-3-(triethoxysilyl)propanoate (E). Thus, a material prepared through grafting aminopropyltrimethoxysilane, followed by t-butyl 2-methyl-3-(triethoxysilyl)propanoate would be labeled (SBAG-AP0.5-E0.5). 2.2.5. Thermal Cleavage of Tert-Butyl Protecting Group,SBA-G-XDeprotect. The material was placed into a 100 mL round-bottom flask with a septum. Nitrogen was passed through the sample, using a bubbler to exhaust the flow for 30 min prior to heating. The material was heated to 270 °C (the temperature was measured external to the flask using an Omega HH501DK temperature readout with a K-type thermocouple) under the nitrogen flow to cleave the tert-butyl protecting group from the carboxylic acid.

(3.27 g, 28.2 mmol) via syringe to the vigorously stirred mixture. The mixture was heated to 80 °C, and the inert gas line was removed to permit oxygen into the system. The temperature was maintained for 12 h before cooling the mixture to room temperature. The solution was fractionally distilled, removing the excess triethoxysilane before collecting the fraction boiling in the 60 to 75 °C temperature range at a vacuum level of 30 mTorr. This procedure yielded 2.4 g (51% yield) of product. 1H NMR (CDCl3): δ 0.72 and 1.09 (m, 2H, Si-CH2), 1.22 (m, 12H, CH3−CH2 and CH3−CH), 1.43 (s, 9H, CH3−C), 2.50 (m, 1H, CH), and 3.80 (m, 6H, CH2−O); 13C NMR (CDCl3): δ 14.8 (s, Si−C), 18.2 (s, −CH2−O), 19.6 (s, CH3−CH), 27.9 (s, CH3−C), 35.3 (s, CH), 58.3 (CH3−CH2), 79.5 (s, C-(CH3)3), 176.8 (s, CO2− C). 2.2.2. Co-condensation Synthesis of Mesoporous Silica, SBA-CoAP0.5-TX (X = 100, 130, 150). The co-condensation procedure was repeated according to previous reports.35,36 Pluronic P123 (4.0 g; poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol); Mn ≈ 5800) was dissolved in 20 mL of concentrated hydrochloric acid and 106 mL of distilled water with stirring in a 250 mL Erlenmeyer flask at 40 °C. Upon polymer dissolution, 7.71 g of tetraethylorthosilicate (TEOS; 37 mmol) was added with continued stirring. After 45 min,31,45 the organosilane was added to the synthesis mixture (3aminopropyltrimethoxysilane (174 μL; 1 mmol) and/or sodiated carboxytriol silane (786 μL of a 25 wt % aqueous solution; 1 mmol)). The mixture was maintained at 40 °C for 20 h before transferring the gel equally to 4 Teflon liners (45 mL capacity). The Teflon liners were sealed inside digestion bombs before aging the mixture statically at a fixed temperature (T, i.e., 100, 130, and 150 °C; the aging temperature was varied using only 3-aminopropyltriethoxysilane) for 24 h. The resultant material was filtered, washing with copious amounts of distilled water (4 × 500 mL). The template was removed through stirring the mesoporous silica in refluxing ethanol for 24 h (1.0 g washed silica per 400 mL ethanol). The material was filtered, washed with copious amounts of ethanol (4 × 500 mL), dried overnight with a water aspirator, and subsequently dried under reduced pressure (10 mTorr) at 90 °C overnight. 2.2.3. Bare Mesoporous Silica Synthesis for Grafting, SBA-15. The mesoporous silica was synthesized in a manner analogous to previous

Table 1. Physical and Chemical Characteristics of the Different Functionalized Mesoporous Materials SAphysi (m2/g of silica)a

total pore volume (cm3/g of silica)

pore diameter (nm)

SBA-T130 SBA-T150 SBA-Co-AP0.5-T100 SBA-Co-AP0.5-T130 SBA-Co-AP0.5-T150 SBA-Co-AP0.5-C0.5T100 SBA-15 SBA-G-AP0.5

610 380 750 600 540 820

1.29 1.18 1.32 1.27 1.33 1.39

8.3 9.5 7.9 8.3 9.7 7.9

630 560

0.95 0.85

7.3 6.2

SBA-G-AP0.5-E0.5 SBA-G-AP0.5-E0.5Deprotect SBA-G-AP0.5-HMDS

550 550

0.83 0.83

6.3 6.3

0.36

540

0.74

6.2

0.325

material

TGA amine loading (mmol/g total)b

elemental analysis amine loading (mmol/g total)c

1.44 1.02 0.71

0.30(0.27e) 0.34 0.31

0.4

0.34 ± 0.03d (0.36e)

a

Based on nitrogen physisorption. bBased on thermogravimetric analysis. cBased on elemental analysis. dThe standard deviation was found to be 8% from eight measurements of the nitrogen content of the material. eMeasured via potentiometric titration of amine sites using perchloric acid.22,47 2435

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Scheme 3a

a

Naming convention used the following abbreviations to reflect preparation methods and functionalization. The preparation methods were grafting (G) and co-condensation (Co); the temperature (T) of the aging step is noted for the co-condensation procedure. The organosilanes used for functionalization were aminopropyltrimethoxysilane (AP; amines), sodiated carboxytriol (C; carboxylic acid used for co-condensation), t-butyl 2methyl-3-(triethoxysilyl)propanoate (E; carboxylic acid protected as ester used for grafting), and t-butyl (3-(triethoxysilyl)propyl)carbamate (AC; amine protected as carbamate (Scheme S1 in the Supporting Information)). Deprotect indicates that the material was thermally deprotected at 270°C under a flowing nitrogen stream. 2.3. Materials Characterization. Thermogravimetric analysis (TGA) was performed on a Netzsch STA409. The samples were analyzed under a nitrogen (30 sccm) diluted air (90 sccm) stream, heating from 30 to 900 °C at a rate of 10 °C min−1. The amine loading was estimated from relative weight loss in the range 150−400 °C. CHN analysis was performed by Atlantic Microlab (Norcross, GA). The surface area, total pore volume, and pore size distributions were determined by N2 adsorption−desorption isotherms measured on a Micromeritics Tristar 2030 at liquid nitrogen temperatures, with surface area determined by the Brunauer−Emmett−Teller (BET) method. Total pore volume and pore size were calculated using the Broekhoff-de Boer method with the Frenkel−Halsey−Hill (BdBFHH) modification46 using the adsorption isotherm. Solution 1H NMR measurements were performed using a Mercury Vx 400 MHz with CDCl3 as solvent. Solid state CP-MAS 13C NMR was performed using a Bruker 300 MHz. FTIR spectroscopy was performed using a Bruker Vertex 80v with dual FT-IR and FT-Raman benches and KBr and CaF2 beamsplitters, respectively. Quantitative analysis of reaction kinetics and conversions were performed on a Shimadzu GC-2010 gas chromatograph with flame ionization detector (GC-FID) equipped with an SHRX5 column (15 m, 0.25 lm film thickness, 0.25 mm i.d.). Verification of organic products was performed on a Shimadzu GCMS-QP2010S gas chromatograph with mass spectrometer detector (GC−MS) equipped with a SHR5XLB column (30 m, 0.25 lm film thickness, 0.25 mm i.d.). 2.4. Catalysis. The aldol coupling reactions were performed under nitrogen in 25 mL two-neck round-bottom flasks equipped with a condenser and a septum. A stock solution consisting of 200 mL of acetone, 800 μL of diethylene glycol dibutyl ether (DGDE; internal standard), and 1.511 g of 4-nitrobenzaldehyde (10 mmol) was mixed, adding 5.05 mL of this solution via micropipet to the round-bottom, and taking 50 μL as the initial data point. The supported catalyst was added so that 0.025 mmol of amine were present in the reaction (10 mol %). Kinetic sampling was performed by periodically taking a 100 μL sample via syringe through the septum. The sample was passed through a short silica gel bed in cotton-plugged pipet to remove the catalyst, using approximately 2 mL of acetone to wash the syringe needle and to transfer the sample to a GC vial.

materials that was synthesized, while Scheme 3 and Scheme S1 in the Supporting Information explain the notation used to describe each material. 3.1. Protected Carboxylic Acid Organosilane for Grafting. Introduction of a carboxylic acid group into mesoporous silica has previously been achieved in four ways.47−49 Other than the co-condensation approach, none involve direct functionalization with a carboxylic acid organosilane because the acidic nature of the carboxylic acid can lead to hydrolysis of the ethoxysilyl groups (vide inf ra). The grafting methods include, the acidic hydrolysis of a grafted nitrile group, reactive coupling of a carboxylic acid to a species grafted onto the surface, or the acidic hydrolysis of grafted succinic anhydride. The desire to incorporate an organic base with the acid in some hybrid materials limits the utility of these strategies because the free acid can react with the base. For the latter case, in particular, the succinic anhydride was found to ring open in the presence of the amine, producing a free carboxylic acid and a carboxylate (see Figure S1 in the Supporting Information). Therefore, the synthesis of a carboxylic acid organosilane is best performed with a protecting group to prevent hydrolysis during the synthesis and to permit grafting onto the surface in the presence of an organic base. The two protecting groups considered here included a t-butyl group that could be thermally cleaved and a trimethylsilyl group that could be cleaved through hydrolysis. A trimethylsilyl moiety may appear to be a promising protecting group, however, the protecting group is highly labile and hydrolytic cleavage would result in a trimethylsilanol group that, upon hydrolysis, could react with the silanols on the surface. As the silanols can contribute to the catalytic activity, the thermally cleavable protecting group was selected instead. The organosilane synthesis proceeded smoothly via the typical hydrosilylation reaction with the protected t-butyl methacrylate serving as the olefin (Scheme 2). After reaction, the protected product was purified through fractional distillation, resulting in a highly purified product, with excess triethoxysilane and unreacted t-butyl methacrylate (used herein) being distilled away before the desired hydrosilylated product. The difference between the acrylate and methacrylate was not investigated for this application, but the methyl group on the methacrylate may

3. RESULTS AND DISCUSSION An array of functionalized mesoporous silica materials was prepared to investigate the role of synthesis method and the impact of the interaction of amines, silanols, and carboxylic acid groups on the catalytic rate during the aldol condensation of acetone and 4-nitrobenzaldehyde. Table 1 describes the array of 2436

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respectively). The TGA measurement had two distinct ranges over which mass loss occurred: 200 − 270 °C that can be attributed to cleavage of the t-butyl protecting group and greater than 300 °C corresponding to the remaining organic content. The mass loss in the first range was approximately 2% whereas that of the second range was 4%; the ratio of these two mass losses is consistent with expectations based on stoichiometry. The FTIR spectrum had a peak associated with the carbonyl of the protected carboxylic acid group at 1708 cm−1. Finally, the protected methacrylate had a CP MAS 13 C NMR spectrum with peaks at 180, 84, 60, 36, 26, and 20 ppm. The peaks at 84 and 26 ppm correspond to the t-butyl protecting group, and appear at similar shifts to previously reported shifts for the t-butyl group.51 The peak at 60 ppm is indicative of residual ethoxy groups. After thermal deprotection at 270 °C, important differences could be observed in the TGA, FTIR, and CP MAS 13C NMR spectra of the material (SBA-G-E0.5-Deprotect). The TGA measurement indicated no mass loss in the range of 200−270 °C, indicating thermal cleavage of the protecting group occurred (Figure 1). A subtle shift in the carbonyl peak was observed in the FTIR at 1715 cm−1, consistent with presence of a carboxylic acid group (Figure 2). The CP MAS 13C NMR spectrum was more conclusive in demonstrating the thermal cleavage of the t-butyl group (Figure 4), because the peaks at 84 and 26 ppm disappeared completely. The thermal cleavage temperature used was higher than previously reported for the tbutyl protecting group.51

be beneficial for the thermal treatment process, as it has been found to be stable to higher temperatures than the acrylic acid group in the polymeric form.50 The purified product was grafted onto mesoporous silica (SBA-G-E0.5) to create a protected carboxylate group on the silica surface, and the resulting material was characterized using TGA, FTIR, and CP MAS 13C NMR (Figures 1, 2, and 3,

Figure 1. Thermogravimetric analysis of materials functionalized through grafting. The shaded region indicates the temperature range over which the protecting group is thermally cleaved for SBA-G-E0.5.

Figure 4. CP MAS 13C NMR of the methacrylic silane grafted onto SBA-15 and deprotected at 270 °C (SBA-G-E0.5-Deprotect).

3.2. Characterization of Base, Acid, and Acid/Base CoFunctionalized Materials. 3.2.1. Co-condensation Synthesis. The mesoporous materials made through the cocondensation procedure had a low-angle diffraction pattern consistent with the bare SBA-15 (see Figure S2 in the Supporting Information). The XRD pattern remained constant to larger angles (2θ) with increasing aging temperature for both the unfunctionalized and functionalized materials. For the bare silica material, higher synthesis temperatures (i.e., temperatures greater than 165 °C) were found to decompose the template, allowing almost complete extraction while maintaining a mesoporous silica support. Template decompositionwhile permitting facile template extractionwas not considered an optimal preparation method, particularly given the observed decrease in catalytic activity observed with materials prepared at 150 °C (see Figure S3 in the Supporting Information). The textural properties of the samples as revealed using nitrogen physisorption are listed in Table 1 (isotherms are given as Figure S4 in the Supporting Information). The physisorption measurements showed that increasing the aging

Figure 2. FTIR spectra of methacrylic acid functionalized materials with related functional groups. SBA-G-SA0.5 contained a succinic anhydride organosilane that would form if adjacent carboxylic acid groups would react.

Figure 3. CP MAS 13C NMR of the t-butyl methacrylate silane grafted onto SBA-15 (SBA-G-E0.5).

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temperature increased the average pore size from 7.9 to 9.5 nm and decreased the surface area. The loss of surface area is attributed to the decrease in micropore volume observed previously.44 The decrease in micropore volume allowed a greater percentage of the template to be extracted, as determined through thermogravimetric analysis. However, the extraction process was not complete for aging temperatures less than 150 °C, as shown in the TGA measurements (see Figure S5 in the Supporitng Information) and reported in Table 1. The functionalized materials prepared via the co-condensation method had similar structural properties as the unfunctionalized meosoporous silica (see Figure S4 in the Supporting Information). The pore size increased with aging temperature while the surface area concomitantly decreased. The actual pore size calculated from the nitrogen physisorption measurement for the functionalized material was almost identical to the corresponding unfunctionalized material at all synthesis conditions, indicating that the low degree of organic functionalization had a minimal impact on the resultant porosity of the material. Thermogravimetric analysis was used to calculate an amine loading (based on combustible organic material) of 1.44 mmol g−1 for SBA-Co-AP0.5-T100 (see Figure S5 in the Supporting Information). This loading is much higher than the theoretical loading, and corresponds to species not associated with the organosilane such as the template polymer, ethoxy groups condensed with the surface silanols, and surface silanols that condense and dehydrate at high temperatures. The presence of residual template polymer is observed in the CP MAS 13C NMR as a broad peak around 80 ppm (see Figure S6 in the Supporting Information). Similarly, CHN analysis indicated a loading of 0.30 mmol g−1 and a higher C/N ratio than would be expected for a material containing only aminopropylsilyl groups, supporting this interpretation. The accuracy of estimating the amine loading with TGA data improved with increasing aging temperature, but did not accurately reflect the actual amine content determined from CHN analysis in any case. Additionally, the loading was significantly lower than theoretically expected, precluding the estimation based on the theoretical value. Therefore, the most accurate method to determine loading was CHN analysis. As noted in more detail below, the practice of estimating organic loadings from TGA analysis can lead to significant errors in quantifying reaction rates. The inaccuracy of calculating the organosilane content from TGA presented problems for determining the loading of SBACo-AP0.5-C0.5-T100. The presence of carbonyl was confirmed with FTIR with a very small peak at 1715 cm−1 (Figure 5) and a peak appearing in the CP-MAS 13C NMR spectrum at 180 ppm (see Figure S7 in the Supporting Information). Despite the acidic aqueous synthesis conditions and low organosilane content, a second peak in the FTIR appeared at 1550 cm−1 that corresponded to a carboxylate anion (formed upon filtration and washing with distilled water), as previously reported.52 The carboxylate was confirmed in-house with spectra from a methacrylic polymer that was reacted with propyl amine (spectra not shown) and by treating the material with 2 N HCl, converting the carboxylate into carboxylic acid (1715 cm−1).52 In previous reports, carboxylic acid functionalized silica alone was not sufficiently acidic to catalyze the aldol condensation.16,35 Therefore, the exact concentration of carboxylic acid species was not determined for the co-condensed material, and

Figure 5. FTIR spectra of materials containing amine and carboxylic acid functional groups.

the catalyst loading was determined solely based on amine content determined using elemental analysis. 3.2.2. Grafting and Thermal Deprotection Synthesis. The calcined SBA-15 material had the amorphous low-angle diffraction pattern consistent with SBA-15 (see Figure S8 in the Supporting Information). The structure was robust, and the functionalization and thermal deprotection were sufficiently mild so as not to affect the overall material, as monitored from XRD. The textural properties of the material measured using nitrogen physisorption indicated a mesoporous material with a Type IV isotherm (see Figures S9 and S10 in the Supporting Information). The average pore size was 7.3 nm with a pore volume of 0.95 mL per g of silica. After organosilane functionalization, the pore volume decreased as listed in Table 1, indicating functionalization of the mesopore space. TGA measurements were used to estimate the organic loading of the grafted materials (see Figure S11in the Supporting Information). SBA-G-AP0.5 was found to have an amine loading of 0.4 mmol g−1 from TGA measurement, assuming the organic content originated from the propylamine of the organosilane with, on average, one residual methoxy.26 The actual loading was calculated from CHN analysis to be 0.34 mmol g−1, taking an average of eight analyses in which the standard deviation was 8%. The difference between TGA measurements and CHN analysis for materials prepared by grafting syntheses is small relative to the error in estimation when analyzing the co-condensed materials with TGA, and thus, elemental analysis is the preferred method for all materials. The grafting was similarly performed in methanol (SBA-GAP0.5 (Methanol)) due to previous reports indicating methanol promoted better spacing upon surface functionalization than toluene; the material functionalized in methanol was reported to provide a more active catalyst in the aldol condensation than the material functionalized in toluene, presumably because of an increase in cooperativity achieved by preventing amine aggregation on the surface of the silica.53−55 Grafting in methanol (SBA-G-AP0.5 (Methanol)) resulted in a loading from elemental analysis that was similar to the material made in toluene, SBA-G-AP0.5 (Toluene). However, the TGA result indicated a higher degree of functionalization of 0.59 mmol g−1 (see Figure S12 in the Supporting Information). The observed difference between 2438

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grafting conditions, unlike the commercially available succinic anhydride organosilane. During the grafting process, the interaction of the succinic anhydride organosilane with the amine of SBA-G-AP0.5 opened the anhydride ring. FTIR confirmed the succinic anhydride ring opened, with two peaks appearing in the spectrum at 1710 and 1550 cm−1, indicating a carboxylic acid group and a carboxylate group, respectively (see Figure S11 in the Supporting Information). 3.3.1. Aldol Condensation ComparisonAminopropylFunctionalized Materials. Each of the catalysts was evaluated in the aldol condensation of 4-nitrobenzaldehyde with acetone. SBA-G-AP0.5 grafted in toluene served as the basis for comparison for all catalysts. As shown in Figure 6, SBA-G-

TGA and EA loadings was attributed to reaction of methanol with surface silanols. The methanol was not simply physisorbed, as drying at 100 °C under reduced pressure would remove such species. Instead, it was more likely the methanol had reacted with the surface silanols creating silyl esters. Removing silanol groups could have a negative impact on catalyst performance, since it has been shown that amines and silanols work in a cooperative manner to promote reactions similar to the aldol condensation.28 The second grafting step introduced a protected carboxylic acid group to the surface (SBA-G-AP0.5-E0.5). After functionalization with t-butyl 2-methyl-3-(triethoxysilyl) propanoate, the organic content was observed to increase as measured through TGA (see Figure S11 in the Supporting Information). The cleavage of the t-butyl group was observed to occur over a temperature range of 200−270 °C, exhibiting a distinct plateau between 270 and 300 °C. The cleavage temperature was similar to that found previously for the t-butyl carbamate protecting group.51 In addition to protecting the carboxylic acid during the grafting procedure, the observed mass loss from 200 to 270 °C can be used to provide an estimate for the carboxylic acid content from grafting, which would be particularly useful when preparing a carboxylic acid functionalized material via co-condensation. Assuming the observed mass loss was only due to the t-butyl protecting group, a methacrylic acid loading of 0.4 mmol g−1 was calculated from TGA. The TGA loading for methacrylic acid was slightly higher than the actual loading of 0.38 mmol g−1, determined based on the increase in carbon content measured by EA. The material was subsequently thermally deprotected under nitrogen at 270 °C for 12 h to give SBA-G-AP0.5-E0.5Deprotect. The organic content decreased, as measured with TGA, with the difference in mass loss between the protected and unprotected molecule in the temperature range of 150 and 270 °C (see Figure S11 in the Supporting Information). The organosilane functionalization and subsequent thermal transformation were confirmed with spectroscopic methods. The IR stretch at 1708 cm−1 confirmed the presence of a carbonyl group (Figure 5). Upon thermal cleavage, a subtle shift was observed in the carbonyl stretch to 1715 cm−1. Additional confirmation of the cleavage was achieved with CP MAS 13C NMR spectroscopy (see Figures S13 and S14 in the Supporting Information); the peak associated with the t-butyl group at 26 ppm was reduced substantially after thermal cleavage. After thermal cleavage of the bifunctional material (SBA-G-AP0.5E0.5), the carboxylic acid peak was present at 1715 cm−1. A small peak at 1550 cm−1 appeared as well, consistent with carboxylate formation.52 The carboxylate peak was observed after thermal deprotection regardless of the amount of t-butyl methacrylate organosilane incorporated into the material. Overall, the thermal treatment was successful in cleaving the protecting group, but resulted in a limited amount of carboxylate formation. While carboxylate formation was not completely prevented, t-butyl 2-methyl-3-(triethoxysilyl)propanoate did allow incorporation of a carboxylic acid group with the grafting process. Without the protecting group, grafting 3-aminopropyltriethxoysilane onto a material containing the deprotected methacrylic silane (SBA-G-E0.5-Deprotect) resulted in a material with a carboxylate peak in the FTIR spectrum and little to no residual peak at 1715 cm−1 corresponding to a free carboxylic acid (see Figure S1 in the Supporting Information). The t-butyl group was also stable to

Figure 6. Conversion of 4-nitrobenzaldehyde in the aldol condensation with acetone using different amine containing catalysts with a loading of 10 mol % amine at a temperature of 50 °C.

AP0.5 efficiently catalyzed the reaction, reaching 80% conversion after 5 h (Figure 6). The rate of conversion was nearly identical for the material prepared in methanol (SBA-GAP0.5 (Methanol)). At the low organosilane loadings used to prepare these materials, the grafting solvent appeared unimportant for achieving efficient cooperative catalytic activity, unlike previously found for higher amine loadings.53−55 Removing surface silanols (SBA-G-AP0.5-HMDS) decreased the catalytic activity by a factor of approximately four (Table 2), consistent with previous reports that silanols act cooperatively with amines in the aldol condensation.28,37,38 The catalyst prepared through co-condensation (SBA-CoAP0.5-T100) catalyzed the aldol condensation with an identical rate to those prepared through grafting (Figure 6). The conversion was significantly faster than previously reported for materials prepared through co-condensation (in a similar manner to SBA-Co-AP0.5-T100).16,35,36 Given that catalysts Table 2. Comparison of the Initial Turnover Frequencies for the Different Catalysts

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material

initial TOF (h−1)

SBA-Co-AP0.5 SBA-Co-AP0.5-C0.5 SBA-G-AP0.5 SBA-G-AP0.5-E0.5 SBA-G-AP0.5-E0.5-Deprotect SBA-G-AP0.5-E4.0 SBA-G-AP0.5-HMDS

2.6 1.8 2.6 1.5 1.2 0.3 0.8

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more active than the material produced through grafting (SBAG-AP0.5-C0.5).

prepared by grafting and co-condensation were of identical reactivity in this work, the previously observed, lower catalytic activity found by others may be associated with a higher degree of organosilane functionalization (blocking cooperative silanol sites), different silanol densities, use of different analytical techniques, such as TGA, to count active sites, or other differences in the structure of these complex, multisited materials. 3.2.2. Aldol Condensation Comparisonaminopropyland Carboxylic Acid-Functionalized Materials. The sequential functionalization through grafting allowed investigation of the differential impact of silanol removal and carboxylic acid introduction on the catalytic rates. Introduction of t-butyl 2methyl-3-(triethoxysilyl)propanoate (SBA-G-AP0.5-EX; where X = 0.5, 4) in the grafted materials decreased the activity of the catalyst relative to the amine functionalized material (SBA-GAP0.5), as shown in Figure 7. The second organic group

Figure 8. Conversion of 4-nitrobenzaldehyde in the aldol condensation with acetone using different amine- and carboxylic acid-containing catalysts with a loading of 10 mol % amine at a temperature of 50 °C. The materials were prepared with the cocondensation procedure.

For the bifunctional materials, carboxylate formation was difficult to prevent. Most likely, the immobilized amine sites caused the condensation of the organosilane on sites adjacent to the amine, as organic bases such as triethylamine are often added to assist silanol deprotonation during the grafting process.56,57 FTIR confirmed the proximal functionalization, as a carboxylate is formed upon thermal cleavage of the t-butyl group (see Figure S15 in the Supporting Information). Limiting proximal functionalization was attempted through use of a protected amine as well as the protected carboxylic acid. For the amine, the thermally cleavable t-butyl group carbamate organosilane was grafted onto mesoporous silica. Analysis by CP MAS 13C NMR confirmed the functionalization of the silica (see Figure S16 in the Supporting Information) and subsequent thermal deprotection (see Figure S17 in the Supporting Information). The deprotected material (SBA-G-AC0.5-Deprotect) was tested in the aldol condensation. This material was less active in the aldol condensation than the unprotected amine material (SBA-G-AP0.5), most likely due to incomplete deprotection despite the high deprotection temperature (i.e., 270 °C − higher temperatures may cause amine decomposition) (see Figure S18 in the Supporting Information). Additionally, the protecting group on the amine did not prevent carboxylate formation when both silanes materials were grafted onto the same material and simultaneously deprotected, as observed in FTIR (see Figure S15 in the Supporting Information). Therefore, the protection of both organic functional groups was deemed unnecessary. The relatively high degree of functionalization (i.e., 0.3−0.5 mmol g−1) most likely contributed to the observed carboxylate formation in both the grafted and co-condensed materials discussed. An alternative strategy of using very low organic loadings of 0.1 mmol g−1 was used previously.58 The lower surface functionalization resulted in a material with one organic group per three square nanometers of surface area. The material containing the carboxylic acid and amine organosilanes (SBA-G-AP0.1-C0.1-random; see see Scheme S1 in the Supporting Information) had similar catalytic activity as the

Figure 7. Conversion of 4-nitrobenzaldehyde in the aldol condensation with acetone using different amine- and carboxylic acid-containing catalysts with a loading of 10 mol % of amine at a temperature of 50 °C.

presumably occupied silanol groups adjacent to the amine sites, inhibiting the beneficial cooperative interaction between amines and silanols. We surmise that the higher t-butyl group methacrylate loaded material (SBA-G-AP0.5-E4) effectively capped many silanols adjacent to the amines with the second organosilane and hindered reactant accessibility to the amines, as the reaction rate was lower than a HMDS-treated material, which had silanols capped with a smaller organic group. After the material was heated to expose the carboxylic acid group, the thermal deprotection step (producing SBA-G-AP0.5E0.5-Deprotect) introduces a more acidic carboxylic acid site than the silanol it removes.39 Catalytic testing demonstrated that the activity of SBA-G-AP0.5-E0.5-Deprotect catalyst further decreased relative to protected carboxylic acid material (SBA-G-AP0.5-E0.5). Similarly, the co-condensed material containing both functional groups (SBA-Co-AP0.5-C0.5T100) was less reactive than the co-condensed material containing only the amine (SBA-Co-AP0.5-T100) when the loading was determined from nitrogen content, but the bifunctional catalyst (SBA-Co-AP0.5-C0.5-T100) was similar in activity to the previously reported bifunctional material produced through co-condensation (Figure 8).35 The cocondensed material (SBA-Co-AP0.5-C0.5-T100) was slightly 2440

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aminopropyl organosilane functionalized material (SBA-GAP0.1). The low degree of functionalization allowed the aminopropyl group to act cooperatively with the silanols, as if the carboxylic acid was not present. Forcing the amine closer to the carboxylic acid through using a bifunctional organosilane (SBA-G-AP0.1-C0.1-paired, see see Scheme S2 in the Supporting Information) resulted in a material that was less active than aminopropyl functionalized material, despite the low organic loading used in both cases.58 With the more acidic sulfonic acid group paired with an amine, the catalyst was found to be inactive. Combining this information with the new information presented in the present work suggests that acidic protons in very close proximity to amines result in protonated amines that are significantly less active for the aldol condensation. The acid strength and the proximity determine the equilibrium concentration of inactive amines, and thus the corresponding activity of the catalyst. It appears the best combination for catalysis of the aldol condensation of the systems studied is an amine that can interact with neighboring silanol groups, and substitution of silanol groups for carboxylate groups or stronger acids decreases the activity.

ASSOCIATED CONTENT

S Supporting Information *

Sample naming convention, FTIR spectra, reaction kinetics plots, nitrogen physisorption isotherms, TGA results, NMR spectra, and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org/.

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REFERENCES

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4. CONCLUSIONS A novel route to functionalize mesoporous silica with carboxylic acids was demonstrated with the synthesis of t-butyl 2-methyl3-(triethoxysilyl)propanoate. After grafting onto silica, thermal cleavage of the protecting group was demonstrated to produce a carboxylic acid functionalized material. The newly synthesized organosilane provided a mild grafting route for incorporation of two organic functional groups through sequential grafting of precursors onto a porous silica host. The protected organosilane permitted demonstration of the differential effect of silanol removal and introduction of a carboxylic acid group. For both the co-condensation and grafting methods, the removal of silanols and introduction of the carboxylic acid had a negative impact on the activity of the catalyst. These results demonstrate that a weaker Brønsted acid (e.g., a silanol, which may be better considered as a hydrogen bonding partner) is more effective in cooperatively catalyzing the aldol condensation than a strong acid under the conditions used here, as was previously demonstrated for a related reaction.59

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to the Department of Energy Office of Basic Energy Sciences for financial support of this work through Catalysis Contract DEFG02-03ER15459. 2441

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