Water Resistant, Catalytically Active Nb and Ta Isolated Lewis Acid

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Water Resistant, Catalytically Active Nb and Ta Isolated Lewis Acid Sites, Homogeneously Distributed by Direct Synthesis in a Beta Zeolite Avelino Corma,*,† Francesc X. Llabre´s i Xamena,† Carmelo Prestipino,‡ Michael Renz,† and Susana Valencia† Instituto de Tecnologıá Quı´mica (UPV-CSIC), UniVersidad Politećnica de Valencia, AVda. de Los Naranjos s/n, 46022 Valencia, Spain, and European Synchrotron Radiation Facility (ESRF), BP 220, 38043 Grenoble, France ReceiVed: March 17, 2009; ReVised Manuscript ReceiVed: April 29, 2009

Nb and Ta solid Lewis acids able to work in the presence of water have been prepared by incorporating these elements in the framework of zeolite Beta by a direct synthesis procedure. Characterization of the samples by extended X-ray absorption fine structure and X-ray absorption near edge structure indicate that the calcined samples contain tetracoordinated Nb5+ (or Ta5+) with three siloxane bridges and one NbdO (or TadO) bonds. The presence of Lewis acidity has been characterized by IR of adsorbed pyridine and cyclohexanone. The Nb- and Ta-Beta samples are catalytically active for the Meerwein-Ponndorf-Verley reduction of cyclohexanone with 2-butanol and the etherification of 2-butanol with p-methoxybenzalcohol. Both reactions can be successfully combined in a domino reaction to obtain the ether starting with p-methoxybenzaldehyde. The catalytic activity of Nb- and Ta-Beta has been compared with that of the homogeneous counterparts (NbCl5 and TaCl5) reported in the literature for the intramolecular carbonyl-ene reaction of citronellal to isopulegol, being the turnover number (TON) and the diastereoselectivity of the solids much higher than those of the homogeneous catalysts. Introduction Crystalline inorganic porous materials with well-defined single isolated sites, can be outstanding catalysts when considering activity, selectivity, stability, and recyclability.1 Probably the most popular example of this class is titanium silicalite-1 (TS-1).2 TS-1 in combination with hydrogen peroxide has allowed the industrial implementation of environmentally friendly technologies, such as phenol hydroxylation and cyclohexanone ammoximation.2 Owing to the relatively small pore diameter of the 10-membered ring TS-1 (5.5 Å), this zeolite catalyst cannot be used for reaching bulky substrates and efforts have been made to synthesize large pore 12-membered ring Ti zeolites3 and Ti-containing mesoporous materials.4 Important progress in the development of this kind of catalyst was made with the synthesis of large pore Ti-Beta,5 Ti-MWW,6 Ti-BEC,7 and Ti delaminated zeolites.8 Ti-Beta catalysts synthesized in fluoride medium are characterized by a very low concentration of silanol defect groups, and they have been shown to be more active and selective than the analogous Ti-Beta prepared in basic medium, which contains a much higher amount of defects,5 for reactions in where water is present. Another four-valent metal, i.e., tin, was successfully incorporated into the framework of pure silica zeolite Beta and provided unique catalytic activity for the Baeyer-Villiger oxidation with hydrogen peroxide9,10 and Meerwein-Ponndorf-Verley reductions11 among others.12 Later, zirconium Beta was synthesized following the fluoride synthesis route, with similar activity as Sn-Beta for the Meerwein-Ponndorf-Verley reduction and the intramolecular carbonyl-ene reaction.13 † Instituto de Tecnologıá Quı´mica (UPV-CSIC), Universidad Politećnica de Valencia. ‡ European Synchrotron Radiation Facility.

Having in mind the success of titanium, tin, and zirconium as solid Lewis acid catalysts, it is of interest to investigate the possibilities for synthesizing large pore zeolites containing framework metals with a high charge to radius ratio, such as Nb and Ta, and to study their coordination and catalytic activity as Lewis acid sites. The incorporation of Nb and Ta in the silicalite structure has been studied,14-16 but as far as we know, there are no reports on their incorporation in the framework of large pore zeolites, and more specifically in zeolite Beta. Here we have explored that possibility by carrying out the synthesis of Nb- and TaBeta zeolites by working in fluoride media, since the probabilities for metal incorporation in fluoride should be higher than those in OH-media. Indeed, the fluoride route guarantees the availability of the metal precursors during zeolite synthesis, whereas in a basic medium the precursor is often precipitated as metal oxide that has to be redissolved to be incorporated into a zeolite framework. A second advantage of solid Lewis acid synthesis in fluoride medium is the minimization of silanol defect sites in the zeolite, producing more hydrophobic samples that allow reaction with less polar substrates, while minimizing readsorption of more polar products. Herein, we report the synthesis of Beta zeolites with Nb and Ta. The incorporation of the metals into the zeolite structure has been proven by X-ray absorption spectroscopies (EXAFS and XANES), while pyridine and cyclohexanone adsorption followed by IR spectroscopy show the presence of the Lewis acid sites. Finally, the Lewis acid sites generated are catalytically active for Meerwein-Ponndorf-Verley (MPV), etherification and in a reduction etherification cascade reaction. Finally, it will be shown that Nb- and Ta-Beta give higher TON and diastereoselectivities than homogeneous Nb and Ta catalysts for the intramolecular carbonyl-ene reaction of citronellal to isopulegol.

10.1021/jp902375n CCC: $40.75  2009 American Chemical Society Published on Web 06/08/2009

Beta Zeolites with Nb and Ta Experimental Part Zeolite Synthesis. The syntheses of the metal-containing zeolite Beta materials were performed in fluoride media starting from gels of the following composition

SiO2:xM:0.54TEAOH:0.54HF:yH2O where x was 0.01 and 0.02 and y was varied between 4 and 7. The reactants were Nb(V) and Ta(V) ethoxides, tetraethylorthosilicate as the silica source, tetraethylammonium hydroxide (TEAOH, 35 wt % in water), and HF (50 wt % in water). Dealuminated zeolite Beta seeds were also employed for the synthesis. The crystallization was carried out in Teflon-lined stainless steal autoclaves at 140 °C under rotation for different periods of time. After the desired time, the autoclaves were cooled down to room temperature and the zeolites were recovered by filtration, washed with water, and dried at 100 °C overnight. The as-synthesized metal containing zeolite Beta materials were calcined in static air at 580 °C for 3 h in an oven in order to remove the occluded organic. Reference materials were prepared by impregnating pure silica Beta with ethanolic solutions of the Nb(V) and Ta(V) ethoxides followed by drying at 100 °C overnight and calcination at 580 °C for 3 h. The Si to metal molar ratios of the final impregnated samples were around 100. Zeolite Characterization. Crystallinity and phase identification of the materials was determined by powder X-ray diffraction (XRD) in a Philips X’Pert MPD diffractometer equipped with a PW3050 goniometer (Cu KR radiation, graphite monochromator) provided with a variable divergence slit. Infrared (IR) measurements were performed in a Nicolet 710 FTIR spectrometer using vacuum cells. The spectra in the framework vibration region were done using the KBr pellet technique, whereas experiments with adsorption of pyridine and cyclohexanone as probe molecules were performed in selfsupported wafers of 10 mg cm-2 that were degassed overnight under vacuum (10-4 to 10-5 Pa) at 400 °C. The spectra were recorded, and then pyridine or cyclohexanone was admitted and, after equilibration, the samples were outgassed for 1 h at increasing temperatures (150/250/350 °C for pyridine and 25/ 50/100/200 °C for cyclohexanone). After each desorption step, the spectrum was recorded at room temperature and the background subtracted in the pyridine case. The metal content of the calcined samples was determined by chemical analysis (Varian 715-ES ICP-optical emission spectrometer) after dissolution of the solids in a HNO3/HF solution. X-ray absorption experiments on Nb K and Ta L edges were performed at the BM29 beamline17 at the European Synchrotron Radiation Facility (ESRF). The monochromator was equipped with two Si(111) flat crystals and harmonic rejection was achieved using Si-coated mirrors placed after the monochromator. In order to monitor for possible drift of the energy, a Nb or a Ta foil was measured simultaneously with the sample. For the XANES part of the spectra, a sampling step of 0.5 eV has been applied, while for the EXAFS part the sampling step adopted was 0.025 Å-1. Four spectra were recorded for each sample, with a sampling rate of 1 s per step. EXAFS analysis was performed using IFEFFIT code.18 Phase and amplitude were calculated by FEFF 8.20A19 and compared with model compounds (Nb2O5 and Ta2O5). All the samples were measured at room temperature using a metallic cell allowing in situ hightemperature treatments and gas dosage. The Nb- and Ta-

J. Phys. Chem. C, Vol. 113, No. 26, 2009 11307 TABLE 1: Summary of the Synthesis and Characterization Results of the Metal-Containing Zeolite Beta Samples Prepared by Direct Synthesis M

Si/M (gel)

crystallinity (%)

Nb Nb Ta Ta

100 50 100 50

100 95 90 50b

Si/M (solid)a

SBET (m2g-1)

Vmicropore (cm3g-1)

109 53 78

470 510 461

0.21 0.20 0.19

a Molar ratio determined by chemical analysis. b After 30 days of crystallization time.

containing zeolites were studied before and after calcination of the template and degassing under vacuum at 400 °C to remove adsorbed water molecules. These samples will be hereafter denoted as-synthesized, calcined, and calcined-degassed, respectively. Catalytic Experiments. Meerwein-Ponndorf-Verley Reductions (MPV) of Cyclohexanone (1a). A stock solution of 855 mg of cyclohexanone (1a) and 33.4 g of 2-butanol (2b) was prepared. A 3.42 g portion of this stock solution (85 mg (0.87 mmol) of cyclohexanone and 3.34 g (45.1 mmol) of 2-butanol) was placed in a round-bottom flask, and 50 mg of catalyst was added. The reaction mixture was stirred magnetically and heated to 100 °C for 8 h. The progress of the reaction was monitored by gas chromatography (HP-5 column, 15 m, 0.32 mm, 0.5 µm with an adequate temperature program). Etherification of p-Methoxybenzyl Alcohol (3b) with 2-Butanol (2b). Four millimoles of p-methoxybenzyl alcohol (3b) was dissolved in 3.00 g of 2-butanol (2b), a 50 mg sample of catalyst was added, and the reaction mixture was stirred magnetically and heated to 100 °C for 1 h. Aliquots were taken periodically and analyzed by GC analysis. Cascade Reaction of p-Methoxybenzaldehyde (3a) with 2-Butanol (2b). A 1.3 mmol portion of p-methoxybenzaldehyde (3a) was dissolved in 3.00 g of 2-butanol (2b), and 50 mg of catalyst was added. The reaction mixture was stirred magnetically and heated to 100 °C for 2 h. The progress of the reaction was monitored by gas chromatography. Intramolecular Carbonyl-ene Reaction of Citronellal (4) to Isopulegol (5). Four millimoles of citronellal was dissolved in 3.00 g of acetonitrile. One hundred milligrams of Nb-Beta or Ta-Beta was added while stirring the reaction mixture that was heated to the reaction temperature. Aliquots were withdrawn periodically and analyzed by gas chromatography. Results and Discussion Synthesis and Structural Characterization of the Nb- and Ta-Beta Zeolites. When Nb was incorporated into the synthesis gel, it was possible to obtain highly crystalline zeolite Beta samples with Si/Nb ratios of 100 and 50 (Table 1). Samples with higher Nb incorporation could not be prepared. In the case of Ta, good crystallinities were obtained during the synthesis of Ta-Beta with Si/Ta ) 100. However, for a lower Si/Ta ratio (50) the crystallinity of the resultant material was 50% after 30 days crystallization at 140 °C. In any case, the introduction of Nb and Ta in the synthesis gel slows down the rate of crystallization of the Beta zeolite as it also occurs with Ti or Sn-Beta.5,9 The synthesis results are summarized in Table 1, where it is shown that well crystallized metal-containing zeolite Beta materials with micropore volumes of 0.19 - 0.21 cm3g-1 have been obtained for Si/Nb ratios of 50 and 100 and Si/Ta of 100, in good agreement with the crystallinity observed by XRD (Figure 1).

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Figure 1. X-ray diffraction patterns of calcined metal-containing zeolite Beta samples prepared by direct synthesis from gels of different Si/M ratios (numbers in parentheses).

The crystalline metal-containing zeolite Beta samples were calcined at 580 °C for 3 h in order to remove the occluded organic material, and the high crystallinity was maintained. It has to be remarked that no extra peaks corresponding to other phases different from zeolite Beta were detected in the XRD patterns, even after the calcination processes (Figure 1). Changes that might be expected upon incorporation of large Nb and Ta ions in the framework are not clearly observed due to the low concentration of the metals and the complex XRD pattern of the intergrowing structure of zeolite Beta. The chemical analysis of the metal-containing samples indicates that all the metal present in the gel has been incorporated in the solids. Pure silica zeolite Beta materials impregnated with Nb and Ta (Si/M ) 100) were also prepared (see Experimental Part) for comparison purposes. The textural characterization and XRD patterns (not shown) indicated that the structural features of zeolite Beta were maintained after metal impregnation and calcination. The IR spectra in the framework vibration region of the synthesized metal-containing Beta zeolites after calcination are compared with the pure silica Beta and the impregnated materials in Figure 2. The results evidence the presence of a band at around 960 cm-1 in the metal-containing zeolites prepared by direct synthesis that is absent in the pure silica and impregnated samples (Figure 2a,b). This band has also been observed in different zeolites containing framework metals and has been attributed to Si-O-M vibrations.5,20,21 Although its origin is still under discussion, it is generally taken as an indication of metal incorporation into the framework. The synthesis in fluoride medium yielded Nb- and Ta-Beta samples with a low number of silanols, as evidenced by the small intensity of an IR band at 3740 cm-1 (see Figure 2c).5 The small number of silanols present indicates that the resultant samples must have low polarity. EXAFS and XANES Studies of Nb- and Ta-Containing Beta Zeolites. Nb-Containing Samples. Figure 3a shows the XANES spectra at Nb K-edge of the as-synthesized, calcined, and calcined-degassed samples. The position of the edge in these samples indicates that the oxidation state of Nb is +5 in all three cases. The edge position remains constant after template removal and it is at the same position as for the reference compound Nb2O5. However, even if the oxidation state of Nb remains unaltered, significant changes occur in the XANES after template removal and degassing. The pre-edge feature red shifts from 18993.65 to 18991.30 eV (∆E ) -2.35 eV) and its

Figure 2. IR spectra in the framework vibration region of calcined Nb (a) and Ta (b) containing zeolite Beta samples prepared by direct synthesis and compared with the impregnated ones and the pure silica material and IR spectra of the OH region (c) of calcined Nb-Beta and Ta-Beta samples compared with a pure silica Beta synthesized in OH medium.

intensity strongly increases, and the two resonances at 19005 and 19020 eV become more smoothed. By comparison with spectra of other Nb-containing compounds in literature,22,23 the presence of a sharp pre-edge peak in the spectrum of the calcined-degassed sample is ascribed to the dipole allowed transition from the 1s to the t2 level present in tetracoordinated Nb centers. In the case of the sample containing the template, the lower intensity of this pre-edge feature is indicative of a

Beta Zeolites with Nb and Ta

J. Phys. Chem. C, Vol. 113, No. 26, 2009 11309 SCHEME 1: Cartoon for the Explanation of the Results Obtained by XANES and EXAFS (cf. Figure 3 and Table 2)

Figure 3. (a) XANES spectra at the Nb K edge of Nb-containing beta samples: calcined-degassed (1) and its simulation (1s), calcined (2), and as-synthesized (3). For comparison the spectra of Nb2O5 (4) and Nb metal (5) are also reported. (b) Phase-uncorrected, k3-weighted Fourier transform of the EXAFS spectra of Nb-containing beta samples: (1) as-synthesized and (2) calcined-degassed. The insets show the corresponding unfiltered EXAFS signals.

TABLE 2: Best Fit of the EXAFS Signal Obtained for the Nb-Containig Samples R (Å) (0.02

N (10%

Nb-O Nb-O Nb-Si Nb-O (2nd shell)

As-Synthesized 1.72 0.78 2.01 3.60 3.56 2.88 3.98 4.45


1.72 1.92 3.57 3.96


Calcined-Degassed 1.73 0.70 1.89 3.30 3.53 2.47 3.92 5.03

Calcined 0.70 3.50 2.49 4.20

DW (Å2) (0.002

∆E (eV) (2.0

0.0012 0.0042 0.0045 0.0078

3.3 3.3 -2.63 3.3

0.0030 0.0060 0.0051 0.0068

1.6 1.6 -2.6 1.6

0.0035 0.0030 0.0036 0.0089

0.17 0.17 -2.9 0.17

highly distorted octahedral coordination or, more likely, of a pentacoordination of Nb. The FT transformed EXAFS signals and the corresponding fits of the as-synthesized (top) and calcined-degassed (bottom) samples are shown in Figure 3b. The insets show the corresponding unfiltered EXAFS signals. It is evident that the presence of niobium oxide clusters can be ruled out in both samples, since there is not any appreciable contribution of scattering of Nb atoms in the second shell. Note that the absence of Nb in the second shell also excludes the occurrence of Nb-O-Nb framework sites. The results obtained for the best fit of the EXAFS signal are shown in Table 2. It has to be pointed out that the use of two different Nb-O distances on the first shell has been mandatory. Only the use of a third cumulant term allowed a satisfactory convergence to be reached in the case when only one oxygen shell was used to describe the first peak in the Fourier transformed signal. The solution with two shells has been chosen, because it delivers more reliable results. The rest of the signal has been reproduced with a single distance of Si atoms and oxygen in the second shell. This latter approximation, although quite rough, does not claim

to describe precisely the local environment of Nb but rather to give a tool to monitor the evolution of the second shell. Nb ions are most probably distributed among a number of sites with slightly different geometry (i.e., different crystallographic sites or different hydration degrees, vide infra). The values given for the distances and coordination number are therefore to be considered as average values for this distribution of sites. Nevertheless, from the fitting data it can be stated that template removal and degassing of the sample cause a diminution of both the average Nb-O distance (from 1.72/2.01 to 1.73/ 1.89 Å) and the average coordination number (from 0.78/3.60 to 0.70/3.30) without strongly affecting the second coordination shell. These results agree well with the observed increment of the pre-edge feature of the XANES spectra on template removal and with the penta- and tetracoordination inferred for assynthesized and calcined-degassed samples, respectively. Taking into account all the information gathered from the XANES and EXAFS analysis, and once the occurrence of Nb-O-Nb sites is discarded, the most plausible structure for Nb sites is shown in Scheme 1. According to this model, the calcined-degassed sample contains tetracoordinated Nb5+ sites with three siloxane bridges and one NbdO bond (to preserve the electric neutrality). Upon exposure to ambient moisture, the Nb5+ ions can coordinate a water molecule, thus causing an expansion of the coordination shell, from tetra- to pentacoordination, with a concomitant increment of the average d(Nb-O) distance. This is in agreement with the Lewis acid character of the outgassed Nb-containing sample, as will be evidenced by the pyridine and cyclohexanone adsorption results. Concerning the as-synthesized sample, it must be taken into account that the pore system is blocked by TEAOH template molecules inside the channels, thus precluding water elimination by degassing. Therefore, in the as-synthesized sample the Nb-sites are expected to have a coordinated water molecule, in a situation similar to that of the calcined sample after exposure to ambient moisture. The model in Scheme 1 is also confirmed by the good agreement between the spectra of calcined-degassed sample in Figure 3 (curve 1) and the simulation of XANES spectra done using this model (curve 1s). Ta-Containing Samples. In EXAFS spectroscopy, the L3 edge for Ta is commonly used, because it presents the bigger cross section in the experimental available range of energy. However, to obtain qualitative information it is preferable the use of L1 edge for the XANES part. In fact, this edge presents the same selection rules that the K edges and generally the spectra collected present more features than those collected on L3 edges. Figure 4a shows the XANES spectra at Ta L1-edge of both as-synthesized and calcined-degassed samples. As is clearly seen, the spectra present a number of similarities as compared to the spectra observed for the Nb-containing sample. The position of the edge in both as-synthesized and calcineddegassed samples indicates that the oxidation state of Ta is +5 in both cases. The edge position remains constant after template

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Figure 4. (a) XANES spectra at the Ta L1 edge of Ta-containing beta samples: calcined-degassed (1) and its simulation (1s), and calcined (2). For comparison the spectra of Ta(OBut)5 (3), Ta2O5 (4), and Ta metal (5) are also included. (b) Phase-uncorrected, k3-weighted Fourier transform of the EXAFS spectra on L3 edge of Ta-containing beta samples: (1) as-synthesized and (2) calcined-degassed. The insets show the corresponding unfiltered EXAFS signals.

TABLE 3: Best Fit of the EXAFS Signal Obtained for the Ta-Containig Samples R (Å) (0.02

N (10%

Ta-O Ta-Si Ta-O (2nd shell)

As-Synthesized 1.96 5.54 3.34 3.53 3.70 6.70


1.92 3.31 3.71


Calcined-Degassed 1.90 4.50 3.32 3.62 3.67 5.29

Calcined 4.14 2.37 6.93

DW (Å2) (0.002

∆E (eV) (2.0

0.0041 0.0075 0.0076

7.7 12.3 7.7

0.0036 0.0047 0.011

8.2 12.8 8.2

0.0035 0.0076 0.0088

7.39 11.2 7.39

removal, and it is at the same position as that for the reference compound Ta2O5. As already observed for the Nb samples, significant changes occur in the XANES after template removal without altering the oxidation state. The pre-edge feature at 11684.33 increases its intensity, and the two resonances at 11699.69 and 11716 eV become more smoothed. The presence of a sharp pre-edge peak in the spectrum of the calcineddegassed sample is ascribed to the dipole-allowed transition from the 1s to the t2 level present in tetracoordinated centers.24 In the case of the sample containing the template, the lower intensity of this pre-edge feature is indicative of a highly distorted octahedral coordination or, more likely, of a pentacoordinated Ta site. The FT transformed EXAFS signals and the corresponding fits of the as-synthesized (top) and calcined-degassed (bottom) samples are shown in Figure 4b. The insets show the corresponding unfiltered EXAFS signals. It is evident that the presence of Ta oxide clusters can be ruled out in both samples, since there is not any appreciable contribution of scattering of Ta atoms in the second shell. The results obtained for the best fit of the EXAFS signal are shown in Table 3. Differently from Nb spectra only one oxygen shell has been used to describe the first coordination shell. That could be probably ascribed to a

Figure 5. In situ IR spectra of pyridine adsorbed onto Nb- and TaBeta zeolites. Experimental conditions: The zeolite was activated overnight at 400 °C and in a vacuum of 10-4 to 10-5 Torr. During a short period of time (10 min) pyridine was adsorbed and then desorbed successively at 150, 250, and 350 °C for 60 min at each temperature. After each desorption an IR spectrum was recorded. In the figure for each sample the difference spectrum is displayed between the corresponding spectra after desorption at 150 °C and the one of the activated zeolite without adsorbant.

minor quality of the measure or to superior difficulty to extract a good EXAFS signal from L1 edge with respect to the K-edges. The rest of the signal has been reproduced as well as for Nb with a single distance of Si atoms and oxygen in the second shell.


J. Phys. Chem. C, Vol. 113, No. 26, 2009 11311 SCHEME 2

TABLE 4: MPV Reduction of Cyclohexanone with 2-Butanol (molar ratio 1:50) Catalyzed by Zeolite Beta Samples with Different Metals Incorporateda entry 1 2 3 4 5 6 7 8

catalyst d

Nb-Beta Ta-Betae Zr-Beta Sn-Beta Nb-Betapostg Ta-Betapostg Nb(OEt)4 Ta(OEt)4

Si/M ratiob conv. (%) select. alcohol (%) TONc 109 78 134 107 93 96

h h

60 (98) 80 (92) >98 >98 0 9 8 1

>98 (>98) >98 (96) >98 >98 9 0 24

70 68

f f 0 1

a Experimental conditions: A stock solution of 855 mg of cyclohexanone and 33.4 g of 2-butanol was prepared. 3.42 g of this stock solution (85 mg (0.87 mmol) and 3.34 g (45.1 mmol) of 2-butanol were placed in a round-bottom flask and a 50 mg sample of the catalyst was added. The reaction mixture was stirred magnetically and heated to 100 °C for 8 h. The progress of the reaction was monitored by gas chromatography (HP-5 column, 15 m, 0.32 mm, 0.5 µm, with an adequate temperature program). b Molar ratio. c Millimoles of substrate converted on an average per millimoles of metal. d Values in parentheses for reaction in presence of 90 mg of catalyst after a reaction time of 8 h. e Values in parentheses for reaction in presence of 80 mg of catalyst after a reaction time of 8 h. f Values not displayed since maximum TON was not reached (reaction stopped earlier because substrate run out). g Zeolite Beta samples prepared by postsynthesis treatment (impregnation). h The metal alkoxide was employed in the same amount as incorporated in the zeolite framework, i.e. as in entries 1 and 2, respectively.

Figure 6. In situ IR spectra of cyclohexanone adsorbed onto different Beta zeolites. Experimental conditions: The zeolite was activated overnight at 400 °C and in a vacuum of 10-4 to 10-5 Torr. During a short period of time cyclohexanone (10 min) was adsorbed and then desorbed successively at 25, 50, 100, and 200 °C for 60 min at each temperature. After each desorption an IR spectrum was recorded. In the figure are displayed the corresponding spectra after zeolite activation (straight line), after desorption at 100 °C (dash line), and after desorption at 200 °C (dash dot dot line).

In the hypothesis that the distance reported for the first shell should be examined as a weighted average between the distance of a double bond oxygen and three single bonded oxygens for Ta in different crystallographic sites, the results of the fit presents an impressive similitude with that of Nb and an analogous conclusion could be drawn. As for Nb the XANES spectra of a Ta calcined-degassed sample has been simulated (Figure 4a, curve 1s). Given the lack of direct information on bond lengths, we have chosen for this simulation the arbitrary value of 1.92 Å for single bonds and 1.75 Å for the double bond. Despite this coarse approximation the agreement between experimental and simulated spectra is good.

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Figure 7. MPV reduction of cyclohexanone with 2-butanol (molar ratio 1:50) catalyzed by Nb-Beta and Ta-Beta. Open triangles: Nb-Beta catalyst was removed by filtration after 1 h at reaction temperature and heating of the solution was continued. Open diamonds: Ta-Beta catalyst was removed by filtration after 1 h at reaction temperature and heating of the solution was continued. Reaction conditions: A stock solution of 855 mg cyclohexanone and 33.4 g of 2-butanol was prepared. 3.42 g of this stock solution (85 mg (0.87 mmol) of cyclohexanone and 3.34 g (45.1 mmol) of 2-butanol were placed in a round-bottom flask and a 90 mg sample (Nb-Beta) or a 80 mg sample (Ta-Beta) of the catalyst was added. The reaction mixture was stirred magnetically and heated to 100 °C for 8 h.

TABLE 5: Etherification of p-Methoxybenzyl Alcohol with 2-Butanol in the Presence of Different Catalystsa selectivity entry

catalyst

Si/M ratio

b

conv. (%)

c

4 (%)

dimerd (%)

TONe

1 Nb-Betaf 109 15 (99) 81 (96) 19 (4) (529) 2 Ta-Betaf 78 45 (99) 89 (96) 10 (3) (389) 3 Sn-Betaf 107 20 (99) 81 (96) 16 (4) (521) 4 Zr-Beta 134 19 85 14 5 Nb-Betapostg 93 2 0 85 6 Ta-Betapostg 96 1 0 0 7 Nb(OEt)4 h 0 (0) 8 Ta(OEt)4 h 0 (0) a Reaction conditions: 4 mmol of p-methoxybenzyl alcohol was dissolved in 3.00 g of 2-butanol, a 50 mg sample of catalyst was added, and the reaction mixture was stirred magnetically and heated to 100 °C. Aliquots were taken periodically and analyzed by GC analysis. The conversions displayed were obtained after 5 min of reaction time. Selectivity: The selectivity seems to be quite different for the Beta samples. This is misleading since the byproduct, the bis(p-methoxybenzyl) ether, is only formed at the beginning of the reaction in 2-4% yield. At low conversion, this amount is more significant than at the end of the reaction. For instance, in the Sn-Beta case a selectivity of 81% is displayed and the reaction reached a selectivity >95% at full conversion.26 The formation of the bis(p-methoxybenzyl) ether at the beginning of the reaction may be promoted by a faster diffusion of the p-methoxybenzyl alcohol, i.e., by better adsorption properties of this compound. Later on the reaction coordinate, an excess of 2-butanol, can be expected at the reaction center for statistical reasons since 2-butanol is employed in excess. b Molar ratio. c To the desired butyl aryl ether product. d To the bisaryl ether. e Millimoles of substrate converted on an average per millimole of metal. f Values in parentheses after 3 h of reaction time. g Zeolite Beta samples prepared by postsynthesis treatment (impregnation). h The metal alkoxide was employed in the same amount as incorporated in the zeolite framework, i.e. as in entries 1 and 2, respectively.

In conclusion, the EXAFS and XANES results show that in Nb- and Ta-Beta samples it was not possible to detect Nb or Ta oxide species (no M-O-M units were detected), while the most plausible structure for the metals correspond to isolated tetrahedrally coordinated species with three siloxane bridges and one MdO bond. These well-isolated Nb and Ta species have accessible empty orbitals able to accept a pair of electrons and consequently should behave as “single-isolated-well-defined” Lewis acid sites within a microporous hydrophobic matrix. We will test now the presence of these Lewis acid sites by pyridine adsorption, as well as by adsorption of cyclohexanone which is a molecule that upon adsorption on the Lewis acid site can give the MPV reactions as we will see later. Characterization of Lewis Acid Sites by IR Spectroscopy of Adsorbed Pyridine and Cyclohexanone. IR spectroscopy of adsorbed pyridine is a well-established method for the characterization of Bro¨nsted and Lewis acid sites of solid materials, and in Figure 5 the IR spectra recorded after pyridine adsorption at room temperature and desorption at 150 °C are displayed. The presence of a band at 1550 cm-1 is characteristic for protonated pyridine (Bro¨nsted acid sites), and a band at 1450

cm-1 is characteristic of pyridine coordinated to Lewis acid sites. Results from Figure 5 indicate that Nb- and Ta-Beta samples show strong bands corresponding to pyridine coordinated to Lewis acid sites. Meanwhile the samples prepared by impregnation of Nb and Ta ethoxides on pure silica Beta zeolite show practically no band at 1450 cm-1 and therefore a much lower Lewis acidity than the directly synthesized Ta- and Nb-Beta zeolites. When cyclohexanone was adsorbed on Nb- and Ta-Beta zeolite, the carbonyl band of the cyclohexanone was shifted to lower wavenumbers with respect to the position of the band when adsorbed on the pure silica sample or on the pure silica sample impregnated with Nb and Ta ethoxides (see Figure 6). Thus after adsorption of cyclohexanone on the Nb- and TaBeta zeolites, we see a band at approximately 1670 cm-1, which is close to the position of the carbonyl band when adsorbing cyclohexanone on Sn-Beta (1665 cm-1). For the former metal zeolites a second band in this region can be found around 1655 cm-1. Both bands are well-defined in the case of Nb indicating two different, well-defined species. For Ta-Beta the bands are


J. Phys. Chem. C, Vol. 113, No. 26, 2009 11313

Figure 8. Etherification of p-methoxybenzyl alcohol with 2-butanol in the presence of Nb-Beta and Ta-Beta. Open triangles: Nb-Beta catalyst was removed by filtration after 6 min at reaction temperature and heating of the solution was continued. Open diamonds: Ta-Beta catalyst was removed by filtration after 3 min at reaction temperature and heating of the solution was continued. Reaction conditions: 4 mmol of p-methoxybenzyl alcohol was dissolved in 3.00 g of 2-butanol, a 50 mg sample of catalyst was added, and the reaction mixture was stirred magnetically and heated to 100 °C.

TABLE 6: Domino Reaction of Anisaldehyde with 2-Butanol in the Presence of Different Catalysts; MPV Reduction of the Aldehyde to Anis Alcohol with Subsequent Etherificationa selectivityb entry

catalyst

M/Si ratio

conv. (%)

3b (%)

4 (%)

1 2 3 4

Nb-Betac Ta-Betac Sn-Beta Zr-Beta

109 78 107 134

30 (81) 33 (82) 33 87

75 (73) 0 0 14

25 (27) 95 (98) 100 86

a Reaction conditions: 1.3 mmol of anisaldehyde was dissolved in 3.00 g of 2-butanol, a 50 mg sample of catalyst was added, and the reaction mixture was stirred magnetically and heated to 100 °C. Aliquots were taken periodically and analyzed by GC analysis. The conversions displayed were obtained after 2 h of reaction time. b Normalized to 100%. c Values in parentheses after 24 h of reaction time.

broadened as has been observed before for Sn-MCM-41, obtained either by direct synthesis or by grafting tin-precursors.25 In summary, the IR spectroscopy of adsorbed cyclohexanone also indicates the presence of Lewis acid sites in the Ta- and Nb-Beta zeolites, which are able to strongly interact with the carbonyl group, precluding the possibility that these materials can be catalytically active for Lewis acid catalyzed reactions, such as the Merwein-Pondorf-Verley reactions that involve hydrogen transfer to carbonyl groups. Catalytic Activity. Meerwein-Ponndorf-Verley Reduction of Cyclohexanone. Cyclohexanone (1a) was reacted with 2-butanol (2b) in a 1:50 molar ratio at 100 °C (eq 1 in Scheme 2). The conversions and selectivities obtained with the different catalyst samples are displayed in Table 4. As could be expected from the IR results after adsorption of cyclohexanone, good catalytic activity was obtained with Nb- and Ta-Beta zeolites, being cyclohexanol (1b) the only product observed (cf. Table 4, entries 1 and 2). Practically total conversion of cyclohexanone was achieved with 1.5-1.9 mol % of metal, with turnover numbers (TONs) of almost 70 for both metal zeolite catalysts. For comparison purposes, the impregnated Beta samples were also tested. Notice that to avoid agglomerations of the metals during impregnation, they were incorporated in low concentrations. Thus, when the impregnated samples were tested for their catalytic activity in the MPV reduction of cyclohexanone with

2-butanol, they show practically no activity (Table 4, entries 5 and 6). The metal precursors used in the zeolite synthesis, i.e., niobium ethoxide and tantalum ethoxide, on their own do not yield the MPV product or only in traces (Table 4, entries 7 and 8). Both results demonstrate the catalytic benefit of having the Nb and Ta isolated sites in the framework of the Beta zeolite. Potential metal leaching during the reaction was tested as follows. The catalyst was filtered out from the hot solution after 1 h reaction time when the conversion was still relatively low (25-40%). Then the filtrate was heated at reaction temperature and no further advance of the reaction was detected (Figure 7). The result confirms that the active catalytic species are metal centers incorporated into the zeolite framework and not any species leached out from the structure into homogeneous phase. The results obtained with Nb- and Ta-Beta have been compared with those obtained previously on Sn- and Zr-Beta (cf. Table 4). It can be seen that the last two catalysts are more active than the former, while the same high selectivity is observed in all cases. However, in a former theoretical study it was claimed that the relative rate of different metals in zeolites for different Lewis acid catalyzed reactions depends on the coordination of the metal and the specific spatial orientations of the empty orbitals of the metal when the metal is constrained within the framework.26 Taking this into account, it is not possible to extrapolate the reactivity of a given catalyst for one rection to other reactions, and consequently we have studied the catalytic behavior of Nb- and Ta-Beta for a second reaction, such as alcohol etherification. Etherification of 2-Butanol and p-Methoxybenzyl Alcohol. The etherification of the 2-butanol (2b) with p-methoxybenzyl alcohol (3b), to obtain p-methoxybenzyl sec-butyl ether (4) can be catalyzed by Lewis acidic sites (eq 2 in Scheme 2).27 Thus, considering that both Nb- and Ta-Beta show Lewis acidity, we could expect them to be catalytically active for etherification reactions. Moreover, and owing to their hydrophobicity, the zeolites should tolerate the water formed during the process. Indeed, conversions of 15 and 45% were obtained with NbBeta and Ta-Beta after 5 min of reaction time, respectively (cf. Table 5, entries 1 and 2), and full conversion was obtained after 3 h of reaction time. When the catalytic activity of Ta- and Nb-Beta for the etherification reaction is compared that with Sn- and Zr-Beta, the results in Table 5 show that Ta-Beta is at least twice more active than any of the others, while the activity of Nb-Beta is close to that of Sn- and Zr-Beta.

11314


Corma et al.

TABLE 7: Cyclization of Citronellal into Isopulegol in the Presence of Different Niobium and Tantalum Species entry 1 2 3 4 5 6 7 a

catalyst b

NbCl5 Nb-Beta Nb-Beta TaCl5b TaCl5b Ta-Beta Ta-Beta

substr./metal ratio (mol/mol) 10 267 267 10 1 196 196

temp (°C) roomtemp. roomtemp. 80 roomtemp. roomtemp. roomtemp. 80

react. time (h) 1.3 6 6 4 0.5 6 6

conv. (%) 100 46 89 57 100 51 96

select. 6 (%) c

55 71c 77c 42d 41d 82c 85c

TONa (mol/mol) 10 123 238 6 1 100 188

Moles of substrate converted per mol of metal. b From ref 28. c In acetonitrile solvent. d In dichloromethane as solvent.

A potential leaching experiment and its influence on reactivity was performed as described previously for the MPV reduction. Again, the reaction did not progress after filtration and removing the solid catalyst (cf. Figure 8), indicating that it only occurs at the metal centers incorporated into the zeolite framework. Furthermore, the precursors of the metal center, i.e., Nb(OEt)4 and Ta(OEt)4, on their own did not show any catalytic activity for the etherification reaction (cf. Table 5, entries 7 and 8). In summary, incorporation of Nb and Ta into the zeolite Beta structure may be interesting alternatives to the classical tetravalent metal sites for performing etherification reactions. Furthermore, the hydrophobicity of the zeolite allows the reaction to be completed without the necessity to remove continuously the water formed, as is the usual practice. Catalytic Activity for a Domino Sequence of MeerweinPonndof-Verley Reduction and Etherification. A domino reaction involving the MPV reduction of p-methoxybenzaldehyde (3a) with 2-butanol (2b) to the corresponding primary alcohol, i.e., p-methoxybenzyl alcohol (3b), followed by its etherification with an excess of 2-butanol (see eq 3 in Scheme 2) has been studied with Ta- and Nb-Beta, and the results have been compared with those obtained with other zeolitic solid Lewis acids such as Sn- and Zr-Beta (see Table 6). The results obtained are in good agreement with those observed for the isolated reactions, i.e., Zr is the most active catalyst for the MPV reaction, but its activity for etherification is somewhat lower than others. On the other hand, Ta- and Sn-Beta are the most active for etherification, being Nb-Beta the slowest etherficiation catalyst. Indeed, results for the domino coupling reaction in Table 6 show how in Zr-Beta the etherification of alcohols is the controlling step of the process and p-methoxybenzyl alcohol (3b) is accumulated in the reaction media. The accumulation of 3b is even more notorious for Nb-Beta. On the other hand, in the case of Ta- and Sn-Beta, practically the only product obtained is the desired ether. These results and the theoretical work reported previously,25 indicate that in the case of domino reactions catalyzed by Lewis acid sites, zeolites should offer the possibility to maximize the final yield by introducing more than one type of site in the framework. Indeed, a catalyst with Zr and Sn in the Beta should give the highest yield of the desired ether (see Table 6). Comparison of the Catalytic Activity of Nb-Beta and TaBeta with Homogeneous Nb and Ta Catalysts. After it was established that the incorporation of isolated niobium and tantalum sites into the Beta zeolite network generates Lewis acid catalysts, the performance of these materials was compared with that of soluble niobium and tantalum species for a reaction in where Nb and Ta have been presented as suitable homogeneous catalysts.28 It has been reported that NbCl5 as well as TaCl5 catalyzes the intramolecular carbonylene reaction of citronellal to isopulegol at room temperature, with NbCl5 more active than TaCl5 (eq 4). For this reaction

the diastereoselectivity is an important issue, since it is an intermediate for the preparation of menthol.29 Then diastereoselectivities in the order of 50-60% have been reported for NbCl5 and TaCl5 catalysts, while Nb- and Ta-Beta give chemoselectivities of up to 85% (cf. Table 7). A possible explanation for the increased diastereoselectivity with Nb- and Ta-Beta would be related to the restricted space around the metal sites in the zeolite framework, in good agreement with earlier observations showing that a limited space around a Lewis acid center leads to a preferred formation of the isopulegol isomer.28 A second factor that can influence diastereoselectivity will be related with the ionic radius of the metal incorporated into the zeolite. Indeed, when the metal atom is significantly larger than silicon and/or the metal oxygen bonds are significantly longer than a silicon oxygen bond, there are limitations to accommodate the metal atom in the framework and it is slightly displaced toward the zeolite channel, restricting even more the available space around the active center. The effect may be responsible for the differences in diastereoselectivities when different metals are incorporated into the zeolite framework, as is the case of niobium (77%), tantalum (85%), tin (83-85%), and zirconium (87-91%).30 Conclusions Nb and Ta have been incorporated into Beta zeolite as single isolated Lewis acid sites. When these samples were analyzed by EXAFS and XANES spectroscopy, in both cases, a model involving the metal atom coordinated to three silyloxy groups and one MdO gave the best simulation spectrum. The samples show Lewis acidity by IR spectroscopy of adsorbed pyridine (band at 1450 cm-1) and cyclohexanone (shift of the carbonyl band). Nb- and Ta-Beta are active catalysts for the MeerweinPonndorf-Verley (MPV) reduction of cyclohexanone with 2-butanol and for the etherification of p-methoxybenzalcohol with 2-butanol. No metal leaching has been detected during the reaction. When the etherification is carried out together with the MPV reduction in a cascade type reaction, starting with p-methoxybenzaldehyde, Ta-Beta shows very good activity and selectivity to the final product. When the catalytic behaviors of the solid Lewis acids are compared with soluble Nb and Ta salts (NbCl5 and TaCl5) for the cyclization of citronellal (intramolecular carbonyl-ene reaction), the solids give much higher TON and better diastereoselectivities than the homogeneous catalysts. Acknowledgment. The authors thank CICYT (MAT-20063798164) and Prometeo 2008 for financial support. F.X.L.X. thanks the Spanish Ministry of Science and Education for a “Ramoń y Cajal” contract.

Beta Zeolites with Nb and Ta References and Notes (1) Corma, A.; Garcia, H. Chem. ReV. 2003, 103, 4307–4365. (2) Perego, C.; Carati, A.; Ingallina, P.; Mantegazza, M. A.; Bellussi, G. Appl. Catal. 2001, 221, 63–72. (3) Sheldon, R. A.; Wallau, M.; Arends, I. W. C. E.; Schuchardt, U. Acc. Chem. Res. 1998, 31, 485–493. (4) Corma, A.; Navarro, M. T.; Pe´rez-Pariente, J. J. Chem. Soc., Chem. Commun. 1994, 147–148. (5) Blasco, T.; Camblor, M. A.; Corma, A.; Esteve, P.; Guil, J. M.; Martıńez, A.; Perdigoń-Meloń, J. A.; Valencia, S. J. Phys. Chem. B 1998, 102, 75–88. (6) Wu, P.; Tatsumi, T. Chem. Commun. 2001, 10, 897–898. (7) Moliner, M.; Serna, P.; Cantin, A.; Sastre, G.; Diaz-Cabanãs, M. J.; Corma, A. J. Phys. Chem. C 2008, 112, 19547–19554. (8) Corma, A.; Dıáz, U.; Domine, M. E.; Forne´s, V. Angew Chem., Int. Ed. 2000, 39, 1499–1501. (9) Corma, A.; Nemeth, L. T.; Renz, M.; Valencia, S. Nature 2001, 412, 423–425. (10) Renz, M.; Blasco, T.; Corma, A.; Fornes, V.; Jensen, R.; Nemeth, L. Chem.sEur. J. 2002, 8, 4708–4717. (11) Corma, A.; Domine, M. E.; Valencia, S. J. Catal. 2003, 215, 294– 304. (12) Corma, A.; Renz, M. Chem. Commun. 2004, 550–551. (13) Zhu, Y.; Chuah, G.; Jaenicke, S. J. Catal. 2004, 227, 1–10. (14) Prakash, A. M.; Kevan, L. J. Am. Chem. Soc. 1998, 120, 13148– 13155. (15) Hartmann, M. Chem. Lett. 1999, 5, 407–408. (16) Ko, Y. S.; Ahn, W. S. Microporous Mesoporous Mater. 1999, 30, 283–291.

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