Synthesis of Solid Catalysts with Spatially Resolved Acidic and Basic

Research Article Cite This: ACS Catal. 2018, 8, 2870−2879

pubs.acs.org/acscatalysis

Synthesis of Solid Catalysts with Spatially Resolved Acidic and Basic Molecular Functionalities Zhihuan Weng,‡ Tianyi Yu, and Francisco Zaera* Department of Chemistry and UCR Center for Catalysis, University of California, Riverside, California 92521, United States ABSTRACT: A synthetic procedure was developed to prepare dual acid−base catalysts on a single platform. Mesoporous SBA-15 was derivatized using known grafting chemistry via the following steps: (1) addition of Boc-protected 3-aminopropyltriethoxysilane; (2) controlled use of UV/ozonolysis to selectively remove the exterior groups; (3) decoration of the exterior sites freed in Step 2 with 3-mercaptopropyltriethoxysilane; (4) selective oxidation of the mercaptan groups to sulfonic acid; and (5) pyrolytic deprotection of the amine. The resulting catalysts contain sulfonic acid functionality on the external surfaces and entrance of the pores and amino groups deep inside those pores, at coverage ratios that can be controlled by tuning the exposure time during the UV/ozonolysis step. The samples were fully characterized by 29Si and 13C solid-state NMR, infrared absorption spectroscopy, acid−base titrations, and N2 adsorption isotherms measurements, and they were successfully tested for the promotion of a cascade Henry reaction. Optimum performance was seen with the catalyst having an overall 1:2 acid:base molar ratio. KEYWORDS: bifunctional catalyst, cascade reaction, grafting, UV/ozonolysis, acid−base catalyst

1. INTRODUCTION Ideally, the promotion of the synthesis of specific chemicals should be done in one single process, with one catalytic system and in one vessel, even if that synthesis requires multiple steps. The design of catalytic processes in tandem, to perform a sequence of cascade reactions, is particularly valuable for the synthesis of fine chemicals, because such combined systems reduce the number of isolation and purification steps required. A lesser number of steps leads to a reduction in waste generation and to savings in energy, either because of the elimination of heat-based processing (i.e., distillation, recrystallization, etc.) and/or thanks to the reduced requirement of solvents in purification steps.1 Unfortunately, combining catalytic functionalities such as acids and bases in solution is often limited by chemical incompatibilities between the different components. Three general strategies can be followed to prevent chemical incompatibility. First, the reactions and catalysts can be chosen such that either each step is orthogonal in terms of their mechanism2−4 or the pathways are inextricably linked (i.e., subsequent reactions can only occur from the product of the initial step, as in cascade or domino reactions).5,6 This is a valuable approach, but it is one that is somewhat restrictive and that may not be implementable in some cases, especially if specific selective and irreplaceable steps are required in complex synthetic schemes. Alternatively, engineering controls such as specific flow designs, with sequential stages each designed to carry out a specific step of the overall synthetic sequence, the use of physical mixtures of catalysts, or the use of multivessel reactors can be employed to physically separate any incompatible components in each step.7,8 There are several © XXXX American Chemical Society

advantages to this type of design, but the catalysts may still interfere with each other even if they are in separate solid phases (for instance, we have found that amine-derivatized titania nanoparticles deactivate separate Au/TiO2 catalysts if both are mixed in the same solution), and the engineering approach may not resolve the issue of the survival of unstable intermediates from one step to the next. The third concept is the creation of heterogeneous systems that afford the sequestration of individual catalytic motifs to prevent them from interacting with each other. Reported examples of this approach include the selective construction of polymer scaffolds with variable catalytic functionalities9 and the attachment of catalysts to resins.10 There are many recent citations of heterogeneous tandem and cascade catalysis in the literature that follow this third approach, many involving acid−base combined chemistry.1,11−26 These report different and interesting synthetic methodologies for the addition of the several desired functionalities to a porous solid, but they do not yet offer a universal protocol for doing this. In addition, only a handful of them address the issue of the spatial distribution of the different added groups,27−31 an important consideration when dealing with cascade processes involving unstable intermediates. It is desirable to expand the library of synthetic routes by which such multifunctional catalysts can be made. Here we introduced a novel methodology that relies on known grafting chemistry to add molecular structures to solid surfaces and a combined Received: December 21, 2017 Revised: February 20, 2018

2870

DOI: 10.1021/acscatal.7b04413 ACS Catal. 2018, 8, 2870−2879

Research Article

ACS Catalysis

Figure 1. Synthetic strategy used in this research for the making of dual acid−base catalysts.

(25 mg) with a HCl solution (5 mL, 0.0224 M), filtering the solid and quantitative collecting the solution, and titrating the filtrate with a NaOH solution (0.01 M), using phenolphthalein as the indicator. The acidic groups were estimated by first reacting the solid with a known excess of Na2CO3 (4 mL, 0.01 M solution) and treating the filtered solution with a fixed excess of HCl (5 mL, 0.0244 M solution) before titrating with NaOH (0.01 M), again using phenolphthalein as the indicator. With this methodology, the surface coverages of the acidic and basic sites could be determined with better than 5% accuracy, the main limitations being the errors in weighting the catalysts and measuring the titrating volumes because of the small quantities used for the measurements. Also, because these two types of titration procedures do not interfere with each other, they could be used to estimate both acidic and basic sites in the bifunctional catalysts. The surface areas, pore volumes, and pore size distributions were estimated from N2 adsorption−desorption isotherms obtained using a commercial 2000e Model 25 Nova Win vacuum volumetric gas sorption system. The data were analyzed by using the Barrett−Joyner−Halenda (BJH) isotherm equations on both adsorption and desorption branches of the data. The catalytic reactions were carried out as follows: the catalyst (25 mg) was first heated at 50 °C for 2 h under vacuum to remove any absorbed water. Nitromethane (3 mL, ∼50 mmol) and benzaldehyde dimethyl acetal (0.25 mmol) were added under a nitrogen atmosphere, and the mixture was stirred at 90 °C, still under nitrogen, for 5 h. Aliquots (0.1 mL) were taken at different time intervals to monitor the reaction progress; they were analyzed by gas chromatography, using a HP-50 column, to determine the yields of benzaldehyde and βnitrostyrene (2-nitrovinylbenzene). It should be noted that on silica surfaces there is always some residual adsorbed water left behind, even after heating under vacuum as it was done here. Nevertheless, that does not seem to affect the Henry reaction, which has in fact been reported to take place even in aqueous media.38,39 Moreover, our final product, β-nitrostyrene, requires a subsequent dehydration step, and that also appears to be promoted readily by our catalyst.

ultraviolet-radiation/ozonolysis treatment for the controlled and selective removal of some of the tethered functionalities in specific regions of the solid. The resulting catalysts were thoroughly characterized to corroborate the success of the synthetic approach, and their catalytic performances were tested for a prototypical Henry conversion. The details of this work follow.

2. EXPERIMENTAL DETAILS Most of the chemicals used, including the SBA-15, were purchased from Sigma-Aldrich (research purity) and used as supplied. 3-tert-Butyloxycarbonylaminopropyltriethoxysilane (3TBS) was prepared according to the method described by Medhi et al.:32 3-Aminopropyltriethoxysilane (3.1 g, 14.2 mmol) and di-tert-butyldicarbonate (3.5 g, 15.9 mmol) were mixed in 12 mL of ethanol, and the mix was stirred overnight at room temperature. The solvent was removed under vacuum, and the residual liquid was distilled to afford 3TBS as a colorless liquid (bp 93 °C at 0.05 Torr). 1H NMR (δppm, 400 MHz, CDCl3): 0.62 (m, 2H, CH2Si), 1.24 (t, 9H, OCH2CH3), 1.44 (s, 9H, C(CH3)3), 1.60 (m, 2H, CH2CH2CH2), 3.12 (m, 2H, CH2N), 3.81 (q, 6H, OCH2), 4.75 (s, 1H, NH). The 29Si and 13C cross-polarization magic-angle spinning (CP/MAS) NMR data were acquired on a Bruker Avance 600 spectrometer equipped with a 4 mm H/X CP-MAS probe operating at 119.2 MHz.33−35 All spectra were recorded employing a spinning speed of 10 kHz. For the 29Si CP/ MAS NMR spectra, a cross-polarization contact time of 2 ms, a 1 H decoupling bandwidth of 80 kHz, and a recycle time of 3 s were used. Data were acquired as 12 000 coadded 2048 complex data point FIDs with a 100 kHz sweep width. Post acquisition processing consisted of exponential multiplication with 200 Hz of line broadening and zero filling to 4096 data points. Chemical shifts were referenced to an external DSS sample. Infrared absorption (IR) spectra were acquired in transmission mode using a Bruker Tensor 27 Fourier-transform infrared (FTIR) spectrometer.29,33,34,36 The solids were pressed into pellet form, and placed at the focal point of the sample compartment. The data correspond to averages of 1024 scans, taken at 4 cm−1 resolution and referenced to spectra for pure SBA-15. The surface concentrations of the sulfonic acid and amine groups were quantified by acid−base back-titrations.33,37 The amine coverages were estimated by neutralizing the catalysts

3. RESULTS 3.1. Synthesis of Dual Acid−Base Catalysts. Both acidic (sulfonic acid) and basic (amine) moieties where tethered to a SBA-15 mesoporous support40 according to the procedure 2871


Research Article

ACS Catalysis shown in Figure 1, relying on well-known grafting chemistry based on alkyltrialkoxysilane linkers.33−35,41−44 The synthetic steps were the following: 1. Tethering of 3TBS to the surfaces of SBA-15 (Step 1, to make NH-Boc-SBA-15): SBA-15 (1 g) was calcined under vacuum at 200 °C for 2 h, after which 25 mL of anhydrous toluene and 0.5 g of 3TBS were introduced into the flask. The condensation was conducted under a N2 atm at 110 °C for 48 h, and it was followed by filtering, washing with toluene, and drying under vacuum overnight. A large excess of 3TBS was used to ensure the derivatization of all available silanol groups on the silica surface. 2. Selective removal of −NH-Boc groups from the outside and pore-entrance surfaces (Step 2, to make UV/O3(NH-Boc-SBA-15)): The NH-Boc-SBA-15 samples were exposed to a combination of ultraviolet (UV) radiation and ozone for various hours (typically 3 h, unless otherwise indicated), using a Spectrolinker XL-1500 UV cross-linker instrument equipped with both its original λ (wavelength) = 254 nm lamp and an additional λ = 185 nm radiation source (to produce ozone from molecular oxygen).45,46 The UV power was set to 10 000 μW/cm2. 3. Tethering of −SH groups to the outside and poreentrance surfaces (Step 3, to make SH-(UV/O3-NHBoc-SBA-15)): 3-Mercaptopropyltriethoxysilane (0.1 g) was mixed with 0.3 g of UV/O3-(NH-Boc-SBA-15) under a N2 atmosphere for 24 h at 80 °C. The resulting solid was collected by filtering, and washed with toluene and dried under vacuum overnight. Again, an excess of 3mercaptopropyltriethoxysilane was used to ensure the derivatization of all newly freed silanol groups. 4. Oxidation of −SH groups in SH-(UV/O3-NH-Boc-SBA15) to −HSO3 (Step 4, to make HSO3-(UV/O3-NHBoc-SBA-15)): 0.3 g of SH-(UV/O3-NH-Boc-SBA-15) was stirred in 10 mL of a 30 wt % H2O2 aqueous solution at room temperature in air for 24 h. The solid was filtered and washed several times with deionized water, and the resulting solid was added to a 50 mL 0.01 M HCl solution for 12 h. The solid product was collected by filtering, washed with deionized water until the pH value of the filtrate reached a value close to 7, and dried overnight under vacuum. 5. Deprotection of −NH2 groups in HSO3-(UV/O3-NHBoc-SBA-15) (Step 5, to make the final HSO3-(NH2SBA-15)): The carbamate deprotection was performed by thermal treatment of HSO3-(UV/O3-NH-Boc-SBA-15) at 150 °C under vacuum for 6 h. 3.2. Catalyst Characterization: 29Si CP/MAS NMR and Acid−Base Titrations. The success and efficiency of the tethering Steps 1 and 3 and the preservation of the tethered groups throughout the rest of the synthetic process were evaluated by using 29Si CP/MAS NMR. Typical spectra obtained after each step are reported in Figure 2. The peaks in those data not only attest to the existence of the tethered species on the surface of the SBA-15 pores but also provide information about the type of bonding involved, and can be used in a semiquantitative way to estimate relative changes in

Figure 2. 29Si cross-polarization magic-angle-spinning (CP/MAS) NMR spectra for our SBA-15-based catalysts after each synthetic step, as indicated in Figure 1. The peak assignment is provided on the right.

surface concentrations. The peak assignment, which nicely follows what has been reported previously for similar systems,47−49 is provided on the right-hand side of Figure 2. It can be seen that a significant fraction of silanol (Si−OH) groups in SBA-15, over 30%, are consumed during the tethering of the Boc-protected amine groups in the first step (the T/Q area ratios used to estimate these values are provided in Table 1). Incomplete consumption of silanol groups is expected, as it is known that only about half of the terminal silanol groups are available on the surface for reaction.50 It is also evident from the 29 Si CP/MAS NMR data that bonding to the surface can occur via one, two, or three Si−O−Si links, but that the dominant modality (more than two-thirds of the total) is with two bonds. This is also common, and has been seen in other cases.33−35,47,48,51 What is perhaps more significant in this study is that the 29Si CP/MAS NMR signals follow, and provide a semiquantitative measure for, the addition or elimination of surface tethered groups after each step of the synthesis. The values for the T peak areas, the signal from the tethered species, are summarized in Table 1. They are reported both as a ratio relative to the signals from the nonderivatized Si−OH groups (the Q peaks) and as a percentage of the initial surface Si−OH coverages. Absolute values for the surface coverages of the basic (−NH2) and acidic (−HSO3) sites, determined via acid−base titrations, are also reported in Table 1. The NMR and titration data agreed quite well for the samples with acidic or basic sites, specifically after Steps 1, 2, and 5 (after Steps 3 and 4 the basic sites are Boc-protected, so they cannot be titrated): they show how the total density of tethered groups decreases upon the UV/O3 treatment in Step 2, by about a third, and again after thiol oxidation, Step 4 (by about a quarter). On the other hand, an increase is clearly seen after Step 3, upon the addition of the thiol units. The titration experiments afford the additional parsing of these coverages between the acidic and basic sites; it yielded a 1:2 acid:base ratio in the final sample for this case, the catalyst that showed the highest activity. That ratio can be tuned by varying the time of UV/O3 exposure in Step 2, as discussed later; it was 3 h in this sample. One note of caution is warranted here: because the signal in CP/MAS NMR relies on polarization transfer from hydrogen atoms to the atoms being probed, the technique is in general not quantitative; the analysis 2872


Research Article

ACS Catalysis

Table 1. 29Si CP/MAS NMR Peak Areas and Surface Coverages of Acidic and Basic Sites, Measured by Titration Experiments, for Samples Obtained after Each Step of the Catalyst Synthesis, as Indicated in Figure 1

the figure. It is also interesting to note that the spectrum for that sample matches quite closely the reference data obtained with the pure, untethered NH-Boc-silane (in a CDCl3 solution, bottom trace). The same NH-Boc-SBA-15 13C CP/MAS NMR peaks are seen in the trace obtained after UV/O3 (Step 2, third trace from the bottom in Figure 3), indicating that the structure of the tethered species remains intact and that only their coverage is diminished (the peak intensities are lower). In fact, after extensive UV/O3 exposures, all 13C CP/MAS NMR peaks disappear, indicating that the treatment results in the full removal of the tethered groups, including the propyl linker (data not shown). The addition of thiol moieties (Step 3, second trace from top) leads to a spectrum that also looks almost unchanged, as most of the structure of the newly tethered species is similar to that of the Boc-protected amine (they both share the same propyltriethoxysilane groups). Only the peak at 43 ppm shifts (slightly) and becomes broader, presumably because the signal for the carbon in position 5 is similar but not identical for the amine versus thiol surface groups. By contrast, the spectrum for the final HSO3-(NH2SBA-15) sample, after Step 5 (top trace) shows some significant differences, including the following: (1) the main peaks associated with the Boc protecting group, namely, the signal at 158 ppm for the carboxylic group of the amido link (i.e., peak 8) and those for the tert-butyl group at 27 and 79 ppm (i.e., peaks 4 and 7) are gone; and (2) a new peak appears at 53 ppm, associated with the methylene carbon next to the new acidic functionality. 3.4. Catalyst Characterization: IR. Further information on the nature of the tethered species was obtained by using IR spectroscopy. Typical data are shown in Figure 4. Here, two panels are displayed, providing IR traces for the samples obtained after key synthetic steps during the making of the HSO3-(NH2-SBA-15) (left) sample as well as for the production of the reversed NH2-(HSO3-SBA-15) catalyst, where the thiol/acidic moieties were added first so that the final solid contains acidic sites on the inside and basic sites on the outside and pore-entrance surfaces. In both cases, several peaks can be seen associated with the tethered functionalities, among which those associated with the Boc protecting group are the most prominent. In Figure 4, we have highlighted the very intense and broad peak seen at about 1700 cm−1 due to the carboxylic group, but also evident are the features at 1371 and 1398 cm−1 due to the symmetric (umbrella) deformation vibrations of the terminal methyl groups. All those signals

provided in Table 1 should therefore be taken with a grain of salt. Nevertheless, similar analysis has been used in the past successfully for the characterization of different types of silica and aluminosilicate samples.47,52 Also, our analysis relies on relative signal intensities, which minimizes some of the problems associated with CP/MAS NMR data quantitation. 3.3. Catalyst Characterization: 13C CP/MAS NMR. The integrity of the tethered organic moieties was checked with 13C CP/MAS NMR and IR. The data from a typical 13C CP/MAS NMR characterization sequence is shown in Figure 3. The peak assignments is straightforward,17,32,37 as illustrated for the case of the NH-Boc-SBA-15 intermediate (Step 1) by the numbers placed next to the peaks seen in the spectra (second from bottom), which match the numbering providing at the top of

Figure 3. 13C CP/MAS NMR spectra from our SBA-15-based catalysts after each synthetic step, as indicated in Figure 1. The peak assignment for the first sample, NH-Boc-SBA-15, is provided by the carbon-atom numbering shown at the top. A spectrum for untethered NH-BocSilane in a CDCl3 solution is also provided as reference (bottom, green, trace), to highlight the preservation of the integrity of the molecular structure upon tethering. 2873


Research Article

ACS Catalysis

how this treatment slowly removes the Boc-protected amine tethered groups from the surface. On the one hand, it is clear that the intensities of the peaks associated with the tert-butyl groups below 1400 cm−1 (and between 2800 and 3000 cm−1), the carboxylic moieties around 1700 cm−1, and the methylene units of the propyl linker between 1400 and 1500 cm−1, all decrease with increasing UV/O3 exposure time, to the point of disappearing almost completely after a 16 h treatment. Particularly significant is the parallel evolution of the intensities of the peaks corresponding to the Boc group (represented in Figure 5 by the feature at 1370 cm−1, due to a methyl umbrella mode) and the linker (followed by the signal at 1458 cm−1 for the scissoring mode of its methylene moieties), because that indicates that the UV/O3 treatment removes the entire tethered units from the surface, the Boc-protected basic functionality plus the propyl surface linker. In addition, it is also seen that the removal of these −NH-Boc groups is compensated by the creation of new free silanol groups, which are detected by the broad signal between about 2700 and 3800 cm−1. To note here is the detection of isolated Si−OH groups, indicated by the sharp peak at 3742 cm−1: these are the new surface sites used to tether the thiol functionality in Step 3 (which is oxidized in Step 4 to produce the −HSO3 acidic sites). Quantification of this time evolution of the surface groups seen by IR is provided in the right frame of Figure 5. As mentioned above, optimal catalytic performance was obtained with the sample UV/O3treated for approximately 3 h, after which about one-third of the Boc-protected amine surface groups are removed from the surface and turned into silanol sites. Those become available for further tethering. The final composition of that sample was −HSO3:−NH2 = 1:2 (Table 1). 3.5. Catalyst Characterization: N2 Adsorption Isotherms. Further catalyst characterization was carried out by recording N2 adsorption isotherms. Thanks to the well-defined structure of the pores in mesoporous materials such as SBA-15, the pore diameter distributions can be used to detect substitutions on the surfaces of such pores. Additional data can be extracted in terms of total surface area and pore volume as well. The data obtained from our studies are displayed in Figure 6, and the results are summarized in Table 2. It can be seen that our samples all show Barrett−Joyner−Halenda (BJH) type IV isotherm behavior, as is characteristic of ordered mesoporous materials.53,54 There are significant losses of area and volume upon the initial tethering of the Boc-protected

Figure 4. Transmission infrared absorption (IR) spectra from the solid samples obtained after key steps of the synthesis of our catalysts. Traces are shown for the making of two types of samples, the main HSO3-(NH2-SBA-15) catalyst (left panel), and a second NH2-(HSO3SBA-15) catalysts where the addition of the acidic and basic functionalities was reversed.

disappear once the samples are heated in vacuum to remove the deprotection group, as expected (the remaining broad peak at ∼1640 cm−1 is associated with the silicon oxide solid). Because of the prominence of the Boc peaks in the IR spectra, this technique is ideal to study the effect of the UV/ ozonolysis treatment on our catalysts. Figure 5 shows a sequence of IR spectra taken for NH-Boc-SBA-15 samples as a function of the time of exposure used. The data clearly show

Figure 5. Left: IR spectra from NH-Boc-SBA-15 samples after UV/O3 treatment (Step 2). The data are reported as a function of the time of exposure to this treatment, to highlight the progressive removal of the tethered Boc-protected amine groups from the surface. New silanol sites are created instead, as indicated by the peak at 3742 cm−1. Right: Time evolution of the silanol (3742 cm−1), −NH-Boc (1370 cm−1), and propyl linker (1458 cm−1) surface coverages calculated from the IR data.

Figure 6. N2 adsorption isotherms (left) and calculated pore diameter distributions for our SBA-15-based catalysts after each synthetic step, as indicated in Figure 1. 2874


Research Article

ACS Catalysis Table 2. N2 Adsorption Isotherms Data Summarya step

sample

A/m2 g−1

VT/cm3 g−1

/nmb

0 1 2 4 5

SBA-15 NH-Boc-SBA-15 UV/O3-(NH-Boc-SBA-15) HSO3-(UV/O3-NH-Boc-SBA-15) HSO3-(NH2-SBA-15)

653 393 420 397 497

1.13 0.74 0.73 0.72 0.87

6.92 5.92 6.03 5.92 6.13

a A = area; VT = total pore volume; = average pore diameter. bCalculated using the Barrett−Joyner−Halenda (BJH) equation on the desorption branch.

Table 3. Deacetylation Plus Henry Cascade Reaction Sequence Used To Test Our Acid−Base Dual Catalysts and Performance versus Reference Monofunctional Acidic or Basic Catalysts

entry

catalyst

time/h

%conversion

% yield of 2

% yield of 3

1 2 3 4

NH-Boc-SBA-15 NH2-SBA-15 HSO3-SBA-15 HSO3-(NH2-SBA-15)

16 2 13

99.1 98.7

99.1 0.3

0.0 98.4

entry 4). Two more points should be made at this stage of our discussion. First, there are no free silanol surface groups in any of our catalysts, because those are all saturated with the tethered acidic and/or basic functionalities. Moreover, blank experiments with pure SBA-15 indicated that those sites alone cannot promote either step of the reaction sequence studied here (the Henry coupling was tested with benzaldehyde instead of benzaldehyde dimethyl acetal as the main reactant).58 Second, we did not see any evidence for the need of close acid− base cooperative activity, as reported for related coupling systems.59 Next, the performance of the catalyst was optimized in terms of the ratio of acidic (HSO3−) to basic (NH2−) sites present on the surface, which could be controlled by tuning the time of exposure of the NH-Boc-SBA-15 sample to the UV/O3 treatment (Step 2), as indicated in Figure 5. The results from such test, where the UV/ozonolysis treatment was varied from 2 to 16 h, are summarized in entries 1 to 5 in Table 4. Short exposures result in catalysts with an insufficient concentration of acidic sites, making the catalysis slow: the catalyst made with 2 h UV/O3 exposure takes approximately 60 h to reach full benzaldehyde dimethyl acetal conversion. Long exposures, by contrast, remove most of the basic sites, so the resulting catalysts have enough acidic sites to rapidly convert all the initial benzaldehyde dimethyl acetal to benzaldehyde, but not to the final product; virtually no β-nitrostyrene was detected in those cases. The ideal treatment involves UV/O3 exposure times of 3 to 4 h; the resulting catalysts from such treatment can promote the reaction to completion within half a day or so. To further check this catalyst optimization result, a brief kinetic study was carried out. The data for the evolution of the reactant (benzaldehyde dimethyl acetal, BDA, blue solid circles and coarse-dashed line), intermediate (benzaldehyde, green open squares and solid line), and product (β-nitrostyrene, red solid diamonds and fine-dashed line) versus reaction time are shown in the left panel of Figure 7. The symbols correspond to the raw data, whereas the lines are the result of a simulation using kinetic model with two sequential first-order reactions

amine groups, and the average pore diameter is reduced by approximately 1 nm, suggesting that the newly tethered groups stick out of the surface by approximately 5 Å. Both the area and the average pore diameter increase upon UV/O3 treatment (Step 2), as some of the tethered material is removed, decrease again upon the addition of the acidic functionality (Step 4), and finally go up again as the Boc-protecting groups are removed from the amine moieties (Step 5). The changes are in general not as marked in the pore volume, possibly because of the relatively low density of surface sites, but a significant volume increase is nevertheless seen once the (bulky) Boc groups are removed. For reference, the external surface area of the original SBA-15 mesoporous material was estimated, by using the t-plot method,53,54 to be 30 m2/g, or about 3% of the total. It is also worth pointing out that the narrow distribution of the pore diameter in SBA-15 is retained all throughout this synthesis. 3.6. Catalytic Performance. The dual HSO3-(NH2-SBA15) acid−base catalysts made by our synthetic procedure were tested for the two-step conversion of benzaldehyde dimethyl acetal to β-nitrostyrene, following an initial acid-catalyzed deacetylation reaction to produce benzaldehyde and a subsequent addition of nitromethane via a base-catalyzed Henry reaction. This process, shown at the top of Table 3, has been often used as a prototypical probe for cascade acid− base catalysis.28,55−57 Our first task here was to make sure that both acidic and basic functionalities are necessary. The conversion of benzaldehyde dimethyl acetal + nitromethane mixtures was tested with three reference catalysts containing one single functionality: Boc-protected (NH-Boc-SBA-15) and Boc-unprotected (NH2-SBA-15) amine groups, and sulfonic acid (HSO3-SBA-15) (entries 1 to 3 in Table 3, respectively). Clearly, none of those catalysts promote the complete conversion to β-nitrostyrene: the amine-based catalysts cannot convert the dimethyl acetal reactant at all, and the sulfonic acid catalyst is quite efficient in promoting the first step, to benzaldehyde, but cannot help with the nitromethane addition. Only the catalyst containing both acidic and basic sites is capable of carrying out the conversion to completion (Table 3, 2875


Research Article

ACS Catalysis

(TOF0) measured here, 17 h−1, is higher than what has been reported before for other bifunctional catalysts (13 h−1).17 A few additional tests were performed to further assess the properties of our catalyst. Entries 6 and 7 in Table 4 refer to catalytic runs with samples made in reverse, by tethering the acidic functionality first (to have it end deep inside of the pores, and the amine groups on the outside and pore-entrance surfaces). Perhaps surprisingly, although these NH2-(HSO3SBA-15) catalysts were also able to perform both steps in our cascade scheme, they did so at a much slower pace (even though the total surface coverage of acidic sites was higher, closer to 0.3 mmol/g); full conversion could not be reached even after 2 days of reaction. No accumulation of the benzaldehyde intermediate was ever seen either. In fact, the sample where no UV/ozonolysis was performed at all (entry 6) was more active (and equally selective) than the one where some removal of acidic groups was performed (entry 7). Clearly, in these cases, the rate-limiting step was the first, the acid-catalyzed deacetylation. It is concluded that even in reaction sequences like this, where the intermediate is stable, there is some directionality to the optimum performance of the catalyst. Further evidence for the directionality of the reaction scheme tested here comes from the data in entries 8 and 9 of Table 4, which report a comparison of the performance of the optimized bifunctional catalyst against a physical mixture of two catalysts with acid-only (HSO3-SBA-15) and base-only (NH2-SBA-15) sites, respectively, at an early stage of the conversion. It can be seen that the physical mixture is more active in terms of overall conversion but significantly less selective toward the final product than the bifunctional catalyst, in spite of the fact that the total number of both acid and base sites were matched by adjusting the masses of the two catalysts used in the former case. Clearly, the full reaction scheme, from the initial reactant to the final product, can be promoted by a combination of monofunctional catalysts, because the intermediate, benzaldehyde, is stable. The kinetics of the overall process, however, has proven to be dependent on the spatial distribution of the two types of catalytic sites. In this particular case, the performance of the bifunctional catalyst is preferred because of its significantly better selectivity (and total activity toward the final product, the yield of which was 23%, versus 8% with the physical mixture). Finally, the recyclability of the catalysts was tested. Molecularly modified oxides tend to deteriorate over time, and many anchored catalysts tend to leach out over time. Some deterioration was seen in our case as well, but the problem was not too severe. The data from three sequential catalytic runs with the optimized HSO3-(NH2-SBA-15) catalyst are provided in entries 10 to 12 of Table 4. A loss of approximately 5% in total activity is seen by the third run, without any noticeable loss in selectivity. The silane-based bonds used for tethering are quite robust and the propyl amino and sulfonic acid groups are also fairly stable, but other factors may play a role in this subtle loss in activity. More experiments need to be performed to better understand the slow deactivation of our catalysts.

Table 4. Optimization and Recycling Tests for Our Dual Acid−Base Catalysts in the Deacetylation Plus Henry Cascade Reaction Sequence entry

catalyst

time/h

%conversion

% yield of 2

% yield of 3

1

HSO3-(2h UV/O3NH2-SBA-15) HSO3-(3h UV/O3NH-SBA-15) HSO3-(4h UV/O3NH-SBA-15) HSO3-(10h UV/ O3-NH-SBA-15) HSO3-(16h UV/ O3-NH-SBA-15) NH2-(No UV/O3HSO3-SBA-15) NH2-(2 min UV/ O3-HSO3-SBA15) HSO3-(3h UV/O3NH-SBA-15) HSO3-SBA-15 + NH2-SBA-15 HSO3-(3h UV/O3NH2-SB-15) recycle 1 recycle 2

60

94.6

0.4

94.2

13

98.7

0.3

98.4

16

100.0

0.5

99.5

2

98.5

98.5

--

2

99.3

99.3

--

44

87.2

1.0

86.2

44

64.7

0.7

64.0

2

30.0

7.1

22.9

2

80.6

72.1

8.5

13

98.7

0.3

98.4

13 13

97.2 93.2

0.9 1.1

96.3 92.1

2 3 4 5 6 7

8 9 10 11 12

Figure 7. Kinetic data for a deacetylation plus Henry cascade reaction sequence promoted by our optimized HSO3-(NH2-SBA-15) catalyst. Left: Raw data of concentrations versus time. Right: Time evolution of conversion and selectivity. The lines correspond to a kinetic model consisting of two sequential first-order reactions with the indicated parameters.

(the scheme in Table 3), which yielded reaction rate constants of k1 = 1 H-mmol−1 min−1 and k2 = 30 NH2-mmol−1 min−1, respectively (these were normalized to the number of acidic and basic sites, respectively, from Table 1). The model fits the data reasonably well, and it highlights the fact that although the second step is intrinsically much faster than the first, more basic than acidic sites are needed because the steady-state concentration of the intermediate (benzaldehyde) is always low. The right panel of Figure 7 displays the time evolution of the conversion and of the selectivity to the intermediate and final products. The initial buildup of some benzaldehyde in the reaction mixture, which is then kept at a low value (∼2−3% of the reactant) throughout the course of the conversion, is a sign of a kinetically well-balanced two-step process. It is also interesting to point out that the initial turnover frequency

4. DISCUSSION In this publication, we have reported a new strategy for the synthesis of bifunctional acid−base solid catalysts. In our design, both functionalities are molecular in nature and have been tethered to the surface via well-known grafting chemistry. Additional steps were incorporated in the synthetic protocol to 2876


Research Article

ACS Catalysis

(∼250 nm) is much smaller than the average diameter of the SBA-15 particles (in the micrometer range), long times are require to induce the appropriate chemistry in the central region of the solid particles. Both explanations justify the type of distribution of acidic and basic sites within the pores of the catalyst proposed here. The spatial distribution of the two functionalities resulting from the use of UV/O3 to remove the first and make room for the second may have several uses, one of which is the ability to sequentially promote the steps in a cascade catalytic scheme. In the example reported here the intermediate product, benzaldehyde, is stable, so there may be no need for such spatial resolution. Yet, even in this case, the order of the reactions matters, as the reverse acid-on-the-inside/base-onthe-outside (NH2-(HSO3-SBA-15)) catalyst displayed a much slower promotion of the deacetylation + Henry reaction conversion than our original base-on-the-inside/acid-on-theoutside (HSO3-(NH2-SBA-15)) samples. Moreover, there may be other cases where the intermediate is unstable, and therefore no able to travel long distances; having both functionalities within the same solid particle may resolve this difficulty. It is also possible to conceive multistep processes taking place all within the environment of a single mesopore, in which case the constrained volume could be used to control selectivity in the same way as shape selectivity is used to make the para versus meta or ortho isomers of xylenes with zeolites.60−63 The important contribution here is the protocol for adding two (incompatible) functionalities to the same support with spatial resolution, which is quite general and may be extended to many applications.

take care of the potential reaction between these two incompatible functionalities: the amine used as the basic functionality was protected until the end of the synthetic route, and the acidic functionality was initially added as a thiol and only later oxidized to sulfonic acid. Because both oxidation and deprotection steps were delayed until the two species were already bonded to the surface, reaction with each other was avoided. These elements of our synthetic design are general, and should be extendable to the preparation of other dualfunctionality catalysts. As mentioned in the Introduction, there have been other reports regarding the preparation of similar acid−base catalysts. Although each reported method has its own merits, none is universal; hence there is justification for introducing a new version. The unique addition in our scheme is the introduction of a UV/ozonolysis treatment as a way to remove some of the first functionality tethered on the surface and to make room for the second. This method provides good control of the relative coverages of the acidic and basic functions, and with that a way to optimize the performance of the catalyst for any specific application. We have shown that by fixing the UV/O3 exposure time it is possible to change the acid:base ratio to match the kinetic requirements imposed by the relative rate constants of the two steps being considered in the catalytic cascade process. Another advantage of our UV/O3 step is that it leads to a spatially resolved distribution of acidic versus basic sites. For simplicity, in Figure 1 we have sketched the two regions, with the acidic and basic moieties, as being associated with the outside and inside surfaces, respectively, but a more accurate description is to refer to the front versus back ends of the pores; it should be noted that the outside surface of mesoporous materials amounts to only a few percent of the total (as estimated by the N2 adsorption experiments). This arrangement still sets some directionality to the cascade catalytic process, as highlighted by the fact that reversing the placement of the two sites led to a much poorer performing catalyst. It should be noted that our depiction of the distribution of acidic and basic sites is somewhat speculative, as we do not have direct evidence for their location within the pores. Our justification for this claim is based on our reasoning for why the UV/O3 treatment only removes surface groups slowly and partially, provided next. Indeed, it is interesting to speculate about what is the physical property that limits the removal of tethered species from the surface of the mesoporous materials, thus affording the control of their coverages. It could be thought that ozone, which is produced in situ via the UV activation of molecular oxygen, may diffuse slowly into the pores, proceeding to react with the tethered groups as it moves deeper inside. Although possible, this explanation may not account for the long times involved in the clearing of the surfaces of the pores (several hours). Alternatively, it may be envisioned that what controls the surface cleaning process is the exponential decay of the light intensity as a function of its depth of penetration, because of absorption by the solid. As described in the Experimental Details section, two UV wavelengths were used in our treatment, but it is well known that, of the two, only the λ = 185 nm produces ozone, via activation of atmospheric oxygen.45,46 The λ = 254 nm photons, on the other hand, not only do not produce ozone, but in fact decompose any existing O3 to produce reactive oxygen atoms.45 We propose that it is this latter species (O•) that attack and remove the tethered units. Because the mean penetration depth of the light

5. CONCLUSIONS A new strategy is reported for the synthesis of solid catalysts with dual molecular functionality, specifically with sulfonic acid and amine groups, to act as acidic and basic sites, respectively. Our approach relies on: (1) the use of well-known grafting chemistry to tether molecular moieties to oxide surfaces via alkyltrialkoxysilane linkers; (2) the implementation of combined ultraviolet light plus ozonolysis treatments for the controlled partial removal of tethered fragments from the surface; and (3) the use of protecting groups to avoid reactions between incompatible functionalities, in this case the acidic and basic groups. Solid-state NMR and infrared absorption spectroscopies were combined with acid−base titrations and N2 adsorption isotherm measurements to fully characterize and corroborate the success of each of the steps associated with this synthesis. The components were assembled together in a fivestep protocol to prepare specific sulfonic acid + amine derivatized SBA-15 mesoporous materials, but the procedure is fairly general and could be adapted for the synthesis of other dual catalysts. One key advantage of our approach is that the UV/ ozonolysis treatment provides the means for the spatial separation of the acidic and basic sites, with the former placed on the outside surfaces and the front of the pores and the latter deeper inside those pores (or the other way around). Our method can also be calibrated to tune the overall ratio of the coverages of the two functionalities on the surface. In this case, such tunability afforded the optimization of a cascade catalytic process where benzaldehyde dimethyl acetal is first deacetylated with the help of the acidic sites to produce benzaldehyde and nitromethane is then added in a Henry reaction promoted by the basic sites. Surprisingly, in spite of the fact that the 2877


Research Article

ACS Catalysis

the Pores: An Approach to Cooperative Catalytic Reactions. Angew. Chem., Int. Ed. 2010, 49, 184−187. (15) Shiju, N. R.; Alberts, A. H.; Khalid, S.; Brown, D. R.; Rothenberg, G. Mesoporous Silica with Site-Isolated Amine and Phosphotungstic Acid Groups: A Solid Catalyst with Tunable Antagonistic Functions for One-Pot Tandem Reactions. Angew. Chem., Int. Ed. 2011, 50, 9615−9619. (16) Takagaki, A.; Takahashi, M.; Nishimura, S.; Ebitani, K. One-Pot Synthesis of 2,5-Diformylfuran from Carbohydrate Derivatives by Sulfonated Resin and Hydrotalcite-Supported Ruthenium Catalysts. ACS Catal. 2011, 1, 1562−1565. (17) Yang, Y.; Liu, X.; Li, X.; Zhao, J.; Bai, S.; Liu, J.; Yang, Q. A Yolk−Shell Nanoreactor with a Basic Core and an Acidic Shell for Cascade Reactions. Angew. Chem., Int. Ed. 2012, 51, 9164−9168. (18) Gianotti, E.; Diaz, U.; Velty, A.; Corma, A. Designing Bifunctional Acid-Base Mesoporous Hybrid Catalysts for Cascade Reactions. Catal. Sci. Technol. 2013, 3, 2677−2688. (19) Ganai, A. K.; Shinde, P.; Dhar, B. B.; Sen Gupta, S.; Prasad, B. L. V. Development of a Multifunctional Catalyst for a ″Relay″ Reaction. RSC Adv. 2013, 3, 2186−2191. (20) Filice, M.; Marciello, M.; Morales, M. d. P.; Palomo, J. M. Synthesis of Heterogeneous Enzyme-Metal Nanoparticle Biohybrids in Aqueous Media and Their Applications in C-C Bond Formation and Tandem Catalysis. Chem. Commun. 2013, 49, 6876−6878. (21) Nelson, N. C.; Chaudhary, U.; Kandel, K.; Slowing, I. Heterogeneous Multicatalytic System for Single-Pot Oxidation and C−C Coupling Reaction Sequences. Top. Catal. 2014, 57, 1000−1006. (22) Kung, H.; Kung, M. Inspiration from Nature for Heterogeneous Catalysis. Catal. Lett. 2014, 144, 1643−1652. (23) Climent, M. J.; Corma, A.; Iborra, S.; Sabater, M. J. Heterogeneous Catalysis for Tandem Reactions. ACS Catal. 2014, 4, 870−891. (24) Li, H.; Fang, Z.; Smith, R. L.; Yang, S. Efficient Valorization of Biomass to Biofuels with Bifunctional Solid Catalytic Materials. Prog. Energy Combust. Sci. 2016, 55, 98−194. (25) Parlett, C. M. A.; Isaacs, M. A.; Beaumont, S. K.; Bingham, L. M.; Hondow, N. S.; Wilson, K.; Lee, A. F. Spatially Orthogonal Chemical Functionalization Of a Hierarchical Pore Network for Catalytic Cascade reactions. Nat. Mater. 2016, 15, 178−182. (26) Gaona, A.; Díaz, U.; Corma, A. Functional Acid and Base Hybrid Catalysts Organized by Associated (Organo)Aluminosilicate Layers for C−C Bond Forming Reactions and Tandem Processes. Chem. Mater. 2017, 29, 1599−1612. (27) Margelefsky, E. L.; Zeidan, R. K.; Davis, M. E. Cooperative Catalysis by Silica-Supported Organic Functional Groups. Chem. Soc. Rev. 2008, 37, 1118−1126. (28) Huang, Y.; Xu, S.; Lin, V. S. Y. Bifunctionalized Mesoporous Materials with Site-Separated Brønsted Acids and Bases: Catalyst for a Two-Step Reaction Sequence. Angew. Chem., Int. Ed. 2011, 50, 661− 664. (29) Hong, J.; Lee, I.; Zaera, F. Correlated Bifunctionality in Heterogeneous Catalysts: Selective Tethering of Cinchonidine Next to Supported Pt Nanoparticles. Catal. Sci. Technol. 2015, 5, 680−689. (30) Park, M.; Kim, J. H.; Shim, I.-K.; Choy, J.-H. Fabrication of Pore Window in Ordered Mesoporous Silica by Spatial Control of Functional Groups. Chem. Eng. J. 2014, 253, 1−7. (31) Galloway, J. M.; Kung, M.; Kung, H. H. Synthesis and Characterization of Bifunctional Surfaces with Tunable Functional Group Pairs. Surf. Sci. 2016, 648, 284−290. (32) Mehdi, A.; Reye, C.; Brandes, S.; Guilard, R.; Corriu, R. J. P. Synthesis of Large-Pore Ordered Mesoporous Silicas Containing Aminopropyl Groups. New J. Chem. 2005, 29, 965−968. (33) Hong, J.; Lee, I.; Zaera, F. Cinchona Alkaloids Tethered on Porous Silica as Enantioselective Heterogeneous Catalysts. Top. Catal. 2011, 54, 1340−1347. (34) Hong, J.; Zaera, F. Interference of the Surface of the Solid on the Performance of Tethered Molecular Catalysts. J. Am. Chem. Soc. 2012, 134, 13056−13065.

intermediate in this sequence, benzaldehyde, is stable, the kinetics of the cascade process displayed some preferred directionality: a catalyst made in the reverse sequence, with the basic sites on the outside and the acidic sites in the inside of the pores, proved to be much more inefficient. Finally, the optimized catalyst could be recycled, although a slow deterioration of its performance, namely, a drop of ∼5% in total activity, was seen after three cycles.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Francisco Zaera: 0000-0002-0128-7221 Present Address ‡

Z.W.: Department of Polymer Science and Engineering, Dalian University of Technology, Dalian 116024, P.R. China Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial assistance for this project has been provided by grants from the U.S. National Science Foundation and the U.S. Department of Energy.

■

REFERENCES

(1) Lohr, T. L.; Marks, T. J. Orthogonal Tandem Catalysis. Nat. Chem. 2015, 7, 477−482. (2) Bruggink, A.; Schoevaart, R.; Kieboom, T. Concepts of Nature in Organic Synthesis: Cascade Catalysis and Multistep Conversions in Concert. Org. Process Res. Dev. 2003, 7, 622−640. (3) Fogg, D. E.; dos Santos, E. N. Tandem Catalysis: A Taxonomy and Illustrative Review. Coord. Chem. Rev. 2004, 248, 2365−2379. (4) Wasilke, J.-C.; Obrey, S. J.; Baker, R. T.; Bazan, G. C. Concurrent Tandem Catalysis. Chem. Rev. 2005, 105, 1001−1020. (5) Tietze, L. F. Domino Reactions in Organic Synthesis. Chem. Rev. 1996, 96, 115−136. (6) Nicolaou, K. C.; Montagnon, T.; Snyder, S. A. Tandem Reactions, Cascade Sequences, and Biomimetic Strategies in Total Synthesis. Chem. Commun. 2003, 551−564. (7) Snead, D. R.; Jamison, T. F. A Three-Minute Synthesis and Purification of Ibuprofen: Pushing the Limits of Continuous-Flow Processing. Angew. Chem., Int. Ed. 2015, 54, 983−987. (8) Tsubogo, T.; Oyamada, H.; Kobayashi, S. Multistep ContinuousFlow Synthesis of (R)- and (S)-Rolipram Using Heterogeneous Catalysts. Nature 2015, 520, 329−332. (9) Lu, J.; Dimroth, J.; Weck, M. Compartmentalization of Incompatible Catalytic Transformations for Tandem Catalysis. J. Am. Chem. Soc. 2015, 137, 12984−12989. (10) Phan, N. T. S.; Gill, C. S.; Nguyen, J. V.; Zhang, Z. J.; Jones, C. W. Expanding the Utility of One-Pot Multistep Reaction Networks through Compartmentation and Recovery of the Catalyst. Angew. Chem., Int. Ed. 2006, 45, 2209−2212. (11) Mäki-Arvela, P.; Sahin, S.; Kumar, N.; Heikkilä, T.; Lehto, V.-P.; Salmi, T.; Murzin, D. Y. Cascade Approach for Synthesis of R-1Phenyl Ethyl Acetate from Acetophenone: Effect of Support. J. Mol. Catal. A: Chem. 2008, 285, 132−141. (12) Huang, Y.; Trewyn, B. G.; Chen, H.-T.; Lin, V. S. Y. One-Pot Reaction Cascades Catalyzed by Base- and Acid-Functionalized Mesoporous Silica Nanoparticles. New J. Chem. 2008, 32, 1311−1313. (13) Climent, M. J.; Corma, A.; Iborra, S. Heterogeneous Catalysts for the One-Pot Synthesis of Chemicals and Fine Chemicals. Chem. Rev. 2011, 111, 1072−1133. (14) Shylesh, S.; Wagener, A.; Seifert, A.; Ernst, S.; Thiel, W. R. Mesoporous Organosilicas with Acidic Frameworks and Basic Sites in 2878


Research Article

ACS Catalysis (35) Hong, J.; Djernes, K. E.; Lee, I.; Hooley, R. J.; Zaera, F. Heterogeneous Catalyst for the Selective Oxidation of Unactivated Hydrocarbons Based on a Tethered Metal-Coordinated Cavitand. ACS Catal. 2013, 3, 2154−2157. (36) Weng, Z.; Zaera, F. Increase in Activity and Selectivity in Catalysis via Surface Modification with Self-Assembled Monolayers. J. Phys. Chem. C 2014, 118, 3672−3679. (37) Alauzun, J.; Mehdi, A.; Reyé, C.; Corriu, R. J. P. Mesoporous Materials with an Acidic Framework and Basic Pores. A Successful Cohabitation. J. Am. Chem. Soc. 2006, 128, 8718−8719. (38) Ballini, R.; Bosica, G. Nitroaldol Reaction in Aqueous Media: An Important Improvement of the Henry Reaction. J. Org. Chem. 1997, 62, 425−427. (39) Li, C.-J. Organic Reactions in Aqueous Media with a Focus on Carbon−Carbon Bond Formations: A Decade Update. Chem. Rev. 2005, 105, 3095−3166. (40) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores. Science 1998, 279, 548−552. (41) Vinu, A.; Hossain, K. Z.; Ariga, K. Recent Advances in Functionalization of Mesoporous Silica. J. Nanosci. Nanotechnol. 2005, 5, 347−371. (42) Corma, A.; Garcia, H. Silica-Bound Homogenous Catalysts as Recoverable and Reusable Catalysts in Organic Synthesis. Adv. Synth. Catal. 2006, 348, 1391−1412. (43) Zaera, F. Nanostructured Materials for Applications in Heterogeneous Catalysis. Chem. Soc. Rev. 2013, 42, 2746−2762. (44) Pujari, S. P.; Scheres, L.; Marcelis, A. T. M.; Zuilhof, H. Covalent Surface Modification of Oxide Surfaces. Angew. Chem., Int. Ed. 2014, 53, 6322−6356. (45) Vig, J. R. UV/Ozone Cleaning of Surfaces. J. Vac. Sci. Technol., A 1985, 3, 1027−1034. (46) Guo, L.; Zaera, F. Spatial Resolution in Thin Film Deposition on Silicon Surfaces by Combining Silylation and UV/Ozonolysis. Nanotechnology 2014, 25, 504006. (47) Caravajal, G. S.; Leyden, D. E.; Quinting, G. R.; Maciel, G. E. Structural Characterization of (3-Aminopropyl)Triethoxysilane-Modified Silicas by Silicon-29 and Carbon-13 Nuclear Magnetic Resonance. Anal. Chem. 1988, 60, 1776−1786. (48) Albert, K.; Bayer, E. Characterization of Bonded Phases by Solid-State NMR Spectroscopy. J. Chromatogr. A 1991, 544, 345−370. (49) Krupczyńska, K.; Buszewski, B.; Jandera, P. Peer Reviewed: Characterizing HPLC Stationary Phases. Anal. Chem. 2004, 76, 226A− 234A. (50) Ide, M.; El-Roz, M.; De Canck, E.; Vicente, A.; Planckaert, T.; Bogaerts, T.; Van Driessche, I.; Lynen, F.; Van Speybroeck, V.; Thybault-Starzyk, F.; et al. Quantification of Silanol Sites for the Most Common Mesoporous Ordered Silicas and Organosilicas: Total Versus Accessible Silanols. Phys. Chem. Chem. Phys. 2013, 15, 642− 650. (51) Jones, C. W.; Tsuji, K.; Davis, M. E. Organic-Functionalized Molecular Sieves as Shape-Selective Catalysts. Nature 1998, 393, 52− 54. (52) Müller, M.; Harvey, G.; Prins, R. Comparison of the Dealumination of Zeolites Beta, Mordenite, ZSM-5 and Ferrierite by Thermal Treatment, Leaching with Oxalic Acid and Treatment with SiCl4 by 1H, 29Si and 27Al MAS NMR. Microporous Mesoporous Mater. 2000, 34, 135−147. (53) Lippens, B. C.; de Boer, J. H. Studies on Pore Systems in Catalysts. J. Catal. 1965, 4, 319−323. (54) Scherdel, C.; Reichenauer, G.; Wiener, M. Relationship between Pore Volumes and Surface Areas Derived from the Evaluation of N2Sorption Data by Dr-, Bet- and T-Plot. Microporous Mesoporous Mater. 2010, 132, 572−575. (55) Palomo, C.; Oiarbide, M.; Laso, A. Recent Advances in the Catalytic Asymmetric Nitroaldol (Henry) Reaction. Eur. J. Org. Chem. 2007, 2007, 2561−2574.

(56) Guan, J.; Liu, B.; Yang, X.; Hu, J.; Wang, C.; Kan, Q. Immobilization of Proline onto Al-SBA-15 for C−C Bond-Forming Reactions. ACS Sustainable Chem. Eng. 2014, 2, 925−933. (57) Biradar, A. V.; Patil, V. S.; Chandra, P.; Doke, D. S.; Asefa, T. A Trifunctional Mesoporous Silica-Based, Highly Active Catalyst for One-Pot, Three-Step Cascade Reactions. Chem. Commun. 2015, 51, 8496−8499. (58) Collier, V. E.; Ellebracht, N. C.; Lindy, G. I.; Moschetta, E. G.; Jones, C. W. Kinetic and Mechanistic Examination of Acid−Base Bifunctional Aminosilica Catalysts in Aldol and Nitroaldol Condensations. ACS Catal. 2016, 6, 460−468. (59) Lauwaert, J.; Moschetta, E. G.; Van Der Voort, P.; Thybaut, J. W.; Jones, C. W.; Marin, G. B. Spatial Arrangement and Acid Strength Effects on Acid−Base Cooperatively Catalyzed Aldol Condensation on Aminosilica Materials. J. Catal. 2015, 325, 19−25. (60) Akhmedov, V.; Al-Khowaiter, S. Recent Advances and Future Aspects in the Selective Isomerization of High n-Alkanes. Catal. Rev.: Sci. Eng. 2007, 49, 33−139. (61) Smit, B.; Maesen, T. L. M. Towards a Molecular Understanding of Shape Selectivity. Nature 2008, 451, 671−678. (62) Bejblová, M.; Procházková, D.; Cejka, J. Acylation Reactions over Zeolites and Mesoporous Catalysts. ChemSusChem 2009, 2, 486− 499. (63) Martínez, C.; Corma, A. Inorganic Molecular Sieves: Preparation, Modification and Industrial Application in Catalytic Processes. Coord. Chem. Rev. 2011, 255, 1558−1580.

2879


Synthesis of Solid Catalysts with Spatially Resolved Acidic and Basic

Recommend Documents