Comparative Study of Nb-Incorporated Cubic Mesoporous Silicates as

Dec 29, 2014 - Unlike the other catalysts, both the cyclohexene conversion and epoxide selectivity go through a maximum in the case Nb-SBA-16 material...
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Comparative Study of Nb-Incorporated Cubic Mesoporous Silicates as Epoxidation Catalysts Anand Ramanathan,† Hongda Zhu,†,‡ Rajamanickam Maheswari,†,∥ Prem S. Thapa,§ and Bala Subramaniam*,†,‡ †

Center for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence, Kansas 66047, United States Department of Chemical and Petroleum Engineering, §Microscopy and Analytical Imaging Laboratory, The University of Kansas, Lawrence, Kansas 66045, United States ∥ Department of Chemistry, Anna University, Chennai 600025, India ‡

ABSTRACT: Niobium in varying amounts was successfully incorporated into SBA-16 material via a one-pot direct synthesis technique. Complementary small angle X-ray spectroscopy (SAXS), wide-angle X-ray diffraction (XRD), N2 sorption isotherms, and high resolution transmission electron microscopy (HR-TEM) imaging results confirm the structural features and integrity of the prepared materials. The Nb-SBA-16 materials possess high surface areas (829−1060 m2/g) and pore volumes (0.69−0.82 cm3/g) that decrease with increasing niobium content. Niobium exists mostly as tetrahedral NbO4 and oligomeric NbO4 species. NH3-TPD results reveal that Nb insertion imparts Lewis acidity to the SBA-16 supports, with the acidity increasing with Nb loading. The Nb-SBA-16 materials show good cyclohexene epoxidation activity with H2O2 as oxidant, with the epoxide selectivity decreasing at higher Nb loadings due to ring-opening and other side reactions of the epoxide, attributed to increased acidity. The Nb-SBA-16 materials generally show superior performance compared to either Nb-KIT-6 or Nb-KIT-5 materials at lower Nb loadings and similar reaction conditions. Unlike the other catalysts, both the cyclohexene conversion and epoxide selectivity go through a maximum in the case Nb-SBA-16 materials, suggesting that the surface Nb oxide species that dictate the overall acidity are different in the three catalysts. We recently reported the synthesis of Nb-KIT-6,16 a threedimensional mesoporous silicate that showed significant activity for ethylene epoxidation with aqueous H2O2 as oxidant.17 For cyclohexene epoxidation, we recently reported that interconnected 3D mesostructured silicates such as SBA-16 and KIT-5 outperform two-dimensional (2D) mesostructured SBA-15, confirming that the activity is structure-dependent.18 While the synthesis of Nb-SBA-16 material has previously been reported, the nature of Nb species or its catalytic activity is unknown.19 Prompted by the growing evidence of the activity dependence of Nb-incorporated mesoporous silicates on the structure of the support, we report herein comparative epoxidation activities of niobium incorporated SBA-16 type silicates (with various Nb loadings), Nb-KIT-5 and Nb-KIT-6 with cyclohexene as model substrate and H2O2 as oxidant.

1. INTRODUCTION Olefin epoxidations have been extensively studied because of the importance of the epoxide as an intermediate for synthesizing valuable products such as plasticizers, perfumes, and epoxy resins. Both homogeneous and heterogeneous catalytic systems involving transition metals such as W, Mo, Ti, and Mn have been investigated for the epoxidation of various olefins.1−5 Recently, niobium containing heterogeneous catalysts have been reported as efficient and stable for epoxidation of various olefinic substrates.6−11 Among the mesoporous silicates investigated as supports, mesoporous silicates, MCM-41 and SBA-15, have received increased attention. Niobium was incorporated into these mesoporous silicates by techniques such as impregnation, grafting, and addition of niobium salts during preparation of the mesoporous silicates itself. Impregnation of Nb species in MCM-41 and MCM-48 type silicates led to the formation of isolated NbO4 species that are shown to be active for cyclohexene epoxidation.12 The higher activity in the case of MCM-41 support compared to the three-dimensional MCM-48 was attributed to a more suitable environment for the formation of active site in MCM-41 (hexagonal structure).12 When Nb species are grafted onto a silica support, the formation of fourcoordinate oxo-Nb(V) species in close proximity to other Nb(V) centers was reported.13 Both these species are shown to be active for epoxidations of cyclohexene and 1-methylcyclohexene. 13 Further, mesoporous Nb 2 O 5 by itself (i.e., unsupported) was also shown to catalyze cyclohexene oxidation by H2O2 yielding 1,2-cyclohexanediol as major product.14,15 © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of Nb-SBA-16. The Nb-SBA-16 materials were synthesized following a similar procedure reported for W-SBA-16,18 using tetraethyl orthosilicate (TEOS 98%, Acros Organics) and NbCl5 (Strem Chemicals) as silica and niobium sources, respectively. The as-synthesized product was hot-filtered without washing, dried Special Issue: Scott Fogler Festschrift Received: November 5, 2014 Revised: December 27, 2014 Accepted: December 29, 2014

A

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Figure 1. (A) SAXS patterns and (B) wide angle XRD of Nb-SBA-16 samples.

Table 1. Physico-chemical Characteristics of Nb-SBA-16 Samples SBA-16 (Si/Nb)a

Si/Nbb

Nb wt %

a0c (nm)

SBETd (m2/g)

Ampe (m2/g)

Vtpf (cc/g)

Vmpg (cc/g)

dP, NLDFTh (nm)

total acidity (mmol NH3/g)

Nb-SBA-16(100) Nb-SBA-16(40) Nb-SBA-16(20) Nb-SBA-16(10)

91 38 21 11

1.6 3.9 6.9 12.3

16.3 16.1 16.1 15.6

1060 1004 902 829

556 480 394 322

0.82 0.80 0.73 0.69

0.25 0.21 0.18 0.14

10.1 10.1 10.1 10.0

0.12 0.18 0.23 0.26

a Molar ratio in the synthesis gel. bActual molar ratio in sample determined by ICP-OES. ca0 = unit cell parameter, dSBET = specific surface area determined using Brunauer−Emmett−Teller (BET) equation from adsorption isotherm at P/P0 between 0.05 and 0.30, eAmp= micropore area estimated from t-plot method, fVtP = Total pore volume at 0.98 P/P0, gVmP = micropore volume estimated from t-plot method, hdP,NLDFT = determined using NLDFT kernel developed for silica exhibiting cylindrical/spherical pore geometry

at 100 °C, and calcined in flowing air at 550 °C for 5 h. The resulting solids are denoted as Nb-SBA-16(X) where X represents the molar Si/Nb ratio in the synthesis gel. The catalysts for comparison, Nb-KIT-616 and Nb-KIT-5,20 were prepared as previously reported. Detailed characterization of the supports (SBA-16, KIT-5, and KIT-6) may also be found in previous publications.16,18,20 Briefly, small angle X-ray spectroscopy (SAXS) (2θ = 0.5−2.5°) and powder X-ray diffraction (XRD) (2θ = 10−80°) patterns were recorded on a S-MAX 3000 instrument and Phillips X’pert X-ray diffractometer respectively with Cu Kα radiation (λ = 0.1548 nm). Elemental analysis was performed on a Horiba Jobin Yvon JY 2000 (ICPOES) instrument after digesting the catalyst samples in a HF and H2SO4 mixture. Nitrogen adsorption−desorption isotherms were measured at −196 °C on a Quantachrome NOVA 2000e sorption analyzer. Diffuse reflectance UV−vis spectra were recorded with a PerkinElmer Lambda 850 spectrometer equipped with diffuse reflectance integrating sphere, with Spectralon as the reference. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (with energy dispersive X-ray (EDX) spectra) were obtained on a Technai F20 G2 X-Twin Field Emission STEM instrument operating at 200 kV and a Versa 3D dual beam Scanning Electron Microscope/Focused Ion Beam (FEI, Hillsboro, OR, U.S.A.) with a silicon drift EDX detector, respectively. Temperature-programmed reduction (H2-TPR) spectra and temperature-programmed desorption of ammonia (NH3-TPD) spectra were generated with a Micromeritics Autochem 2910 instrument equipped with a thermal conductivity detector (TCD). 2.2. Cyclohexene Epoxidation. The epoxidation of cyclohexene (≥99.0%, Sigma-Aldrich) was performed in a two-neck round-bottomed flask equipped with a condenser in one neck and a stopper in the other neck with access for periodically sampling the product mixture. The catalysts were pretreated for 5 h at 550 °C in flowing air. In a typical run, 30

mL of acetonitrile, 2.5 mmol of cyclohexene, and 50 mg of NbSBA-16 catalyst were charged into the flask containing a magnetic stirrer. The flask with its contents was attached to the condenser, purged with nitrogen flow, and immersed in an oil bath to equilibrate to desired temperature. Then, 5 mmol of H2O2 (50 wt % in water, Fisher Scientific) was added to the flask. Samples were analyzed on an Agilent 7890 GC equipped with a ZB-5 column and a FID. Parallel batches were run to estimate experimental uncertainties in conversion and selectivity values. The maximum relative standard deviations for conversion and selectivity values are 4.8% and 4.6%, respectively. The H2O2 concentration in the reaction mixture was determined by titrating with 0.1 N ceric (IV) sulfate in an acidic medium of diluted sulfuric acid, in the presence of ferroin indicator.21 For catalyst recyclability study, the catalyst was filtered after each batch run, washed with acetone, dried, and calcined at 550 °C for 5 h. The following definitions are used in assessing the performance of the tested catalysts: Xcyclohexene = Soxide =

UH2O2 =

0 − ncyclohexene) (ncyclohexene 0 ncyclohexene

noxide 0 − ncyclohexene) (ncyclohexene

noxide (n H0 2O2

− n H2O2)

× 100

× 100

× 100

where Xcyclohexene denotes substrate conversion, Soxide denotes selectivity toward cyclohexene oxide, UH2O2 denotes utilization of H2O2 toward epoxide formation, nA denotes the moles of species A at the sampled batch time (A = cyclohexene or H2O2), and n0A denotes the moles of species A at the beginning of reaction. B

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Figure 2. (A) Nitrogen isotherm and (B) pore size distribution of Nb-SBA-16 samples.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. SAXS spectra of Nb-SBA16 samples exhibited the typical intense peak at 2θ degree of

Figure 5. Diffuse reflectance UV−vis of Nb-SBA-16 samples compared with Si-SBA-16.

noted with an increase in Nb content. The unit cell parameter (a0) decreased rather abruptly at the highest Nb loading (Table 1) suggesting the presence of extra-framework niobium species at the higher loadings. However, powder XRD patterns of NbSBA-16 in the high angle showed evidence of the presence of only mesoporous silica (peak centered around 2θ = 23°) with no extra-framework crystalline Nb2O5 species (Figure 1B). ICP-OES analysis of the Nb-SBA-16 materials confirmed that the Si/Nb ratios are close to those used in the synthesis gel (Table 1). The integrity of ordered cubic arrangement of Nb-SBA-16 samples was further verified from N2 physisorption isotherms (Figure 2A). The textural properties of the Nb-SBA-16 samples are given in Table 1. Typical type IV sorption isotherms with H2-type hysteresis loop were observed for all Nb-SBA-16 samples confirming the cage type mesoporous character of these materials. Whereas the specific surface area (SBET) and total pore volume (Vtp) decreased with an increase in the Nb content, no significant change in the mesopore cage diameter (determined using NLDFT method) was noted (see Figure 2B and Table 1). High-resolution TEM images of Nb-SBA-16 (Figure 3) further confirm long-range cubic ordering in these samples similar to those observed typically for SBA-16 type materials. The SEM image and elemental mapping of Nb-SBA16(10) sample (Figure 4) reveal fairly uniform dispersion of the Nb species. In addition, niobia clusters are also clearly seen (circled region in Figure 4). The Nb content was estimated to be approximately 14.5 wt %, slightly greater than the value (∼12.3 wt %) obtained by ICP. Figure 5 displays the diffuse reflectance UV−vis spectra of Nb-SBA-16 samples. While no absorption band was noticed for the parent Si-SBA-16 sample, significant absorption peaks

Figure 3. TEM images of Nb-SBA-16 samples.

Figure 4. SEM image and elemental map of Nb-SBA-16(10) sample.

0.77° corresponding to (111) reflection and weak peaks around 1.34° and 1.56° corresponding to (200) and (211) reflections, respectively (Figure 1A). These diffraction lines are characteristic of cubic cage type Im3m group.22 A marginal decrease in the intensity of (111) peak with a shift toward higher angle was C

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Figure 6. Temperature-programmed (a) desorption of ammonia and (b) reduction of Nb-SBA-16 samples.

Figure 7. Effects of (a) temperature, (b) catalyst amount, (c) H2O2 concentration, and (d) catalyst recyclability on cyclohexene epoxidation over Nb-SBA-16(10) catalyst. Reaction condition: Nb-SBA-16(10) = 50 mg, cyclohexene = 2.5 mmol, H2O2 = 5 mmol, t = 4 h. T = 50 °C for runs represented in b−d. The maximum relative standard deviations for conversion and selectivity (normalized with their mean values plotted in the figures) are 4.8% and 4.6%, respectively.

suggesting an increase of total acidity with Nb content similar to those observed for Nb-KIT-6 samples.16 Temperatureprogrammed reduction of Nb-SBA-16 samples (Figure 6B) shows evidence of a growing reduction peak at approximately 1000 °C for higher Nb loadings signifying the presence of reducible NbOx species. 3.2. Nb-SBA-16 Catalyzed Cyclohexene Epoxidation. The liquid phase reaction of cyclohexene with aqueous H2O2 as oxidant yields several products, including cyclohexene oxide, hexanedial, cyclohexenone, cyclohexenols, and cyclohexane diols. Temperature effects during a 4-h batch run were studied with Nb-SBA-16(10) catalyst (Figure 7a). As the temperature is increased from 50 to 80 °C, cyclohexene conversion increases from 42 to 65%. However, a decrease in cyclohexene oxide

between 190 and 300 nm with increasing intensity were observed for the Nb-SBA-16 samples. This absorption band is a mixture of two to three peaks (one near 200 nm and another between 235 and 240 nm) similar to that observed for Nb-KIT6 samples.16 The absorption band between 195 and 220 nm is typically associated with O → NbO 4 charge transfer transitions16,23−26 and the band between 230 and 250 nm is assigned to oligomeric NbO4 tetrahedra with low coordination number.16,27 Consistent with wide angle XRD analysis, no characteristic peak for either bulk or crystalline Nb2O5 was observed in any of the tested Nb-SBA-16 samples. The NH3-TPD spectra of the Nb-SBA-16 samples are shown in Figure 6A and the corresponding total acidities are listed in Table 1. A gradual increase in NH3 desorption was noticed D

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Figure 8. Effects of Nb loading (a and b) and total acidity (c and d) on cyclohexene epoxidation catalyzed by Nb-SBA-16(10). Reaction conditions: cyclohexene = 2.5 mmol, H2O2 = 5 mmol, T = 50 °C, t = 4 h. The maximum relative standard deviations for conversion and selectivity (normalized with their mean values plotted in the figures) are 4.8% and 4.6%, respectively.

selectivity, from 42 to 22%, was noticed in the same T range. Increasing the catalyst amount from 25 to 150 mg increased cyclohexene conversion from 36 to 58%, almost linearly, at 50 °C (Figure 7b). This increase in conversion was accompanied by a decrease in the epoxide selectivity from 49 to 21%, and a concomitant increase in the ring-opening product, cyclohexane diol, presumably caused by the increased acidity at the higher loadings. These results guide further optimization studies aimed at maximizing the epoxide yield. The variation of feed H2O2/cyclohexene mole ratio was investigated over Nb-KIT-6 (10) at 50 °C. As inferred from Figure 7c, both cyclohexene conversion and the epoxide selectivity increased with an increase in H2O2 concentration. However, H2O2 utilization toward epoxide formation was found to be similar (20−25%), based on measurement of H2O2 concentration via titration with ceric sulfate. Catalyst stability was assessed during three recycle runs (Figure 7d). The catalyst was filtered following each run, washed with acetone, dried, and calcined. No significant deactivation is observed after three recycles with only mild variations in the epoxide selectivity. When the Nb content in the catalyst was varied from 1.6 to 12.3 wt %, both the cyclohexene conversion and the epoxide selectivity pass through a maximum between 4 and 7 wt % Nb (Figure 8). While NbO4 centers that favor cyclohexene epoxidation increase with Nb loading, so does the acidity of the catalyst samples. The increased acidity at higher loadings could catalyze ring-opening, rearrangement and addition reactions of cyclohexene oxide, decreasing its yield.13 A performance comparison of several niobium-incorporated cubic mesoporous silicates with similar Si/Nb ratios was done under identical reaction conditions (Figure 8). The activity for

converting cyclohexene followed the trend Nb-SBA-16 > NbKIT-6 > Nb-KIT-5. A maximum epoxide selectivity of 65% at 50% cyclohexene conversion was noticed for Nb-SBA-16, whereas a similar selectivity was achieved over Nb-KIT-6 at a conversion of 35%. On the other hand, only 36% cyclohexene conversion with maximum epoxide selectivity of 54% was achieved over Nb-KIT-5. Comparison of the variations of cyclohexene conversion and epoxide selectivity with Nb loadings (Figure 8a and b) and the corresponding total acidity of the Nb catalyst samples (Figure 8c and d), indicates that NbSBA-16 provides higher conversion and selectivity at relatively low Nb loadings (and therefore acidity). The cyclohexene conversion, as well as epoxide selectivity, goes through a maximum at increased metal loadings in the case of the NbSBA-16 catalyst. In sharp contrast, the cyclohexene conversion tends to level off at increased metal loadings in the case of both Nb-KIT-5 and Nb-KIT-6 catalysts. These trends may be linked to different extents of H2O2 decomposition at the higher Nb loadings/acidities in these catalysts. The variations in acidity are manifested in the epoxide selectivity as well, which goes through a maximum at higher metal loadings in the case of NbSBA-16 and Nb-KIT-6 catalysts. It should be noted that unlike Nb-KIT-5 or Nb-KIT-6, the Nb-SBA-16(10) showed clear evidence of reducible NbOx species at higher Nb loadings. Such species not only favor H2O2 decomposition (resulting in low cyclohexene conversion) but also promote ring-opening reaction of the epoxide. These results suggest that the nature of the oxides and the acidity stemming from these sites on the various supports might be different, resulting in differences in the catalyst activity and selectivity. This aspect merits further fundamental investigations. E

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(5) Xia, Q.-H.; Ge, H.-Q.; Ye, C.-P.; Liu, Z.-M.; Su, K.-X. Advances in Homogeneous and Heterogeneous Catalytic Asymmetric Epoxidation. Chem. Rev. 2005, 105, 1603. (6) Marin-Astorga, N.; Martinez, J. J.; Borda, G.; Cubillos, J.; Suarez, D. N.; Rojas, H. Control of the Chemoselectivity in the Oxidation of Geraniol Over Lanthanum, Titanium, and Niobium Catalysts Supported on Mesoporous Silica MCM-41. Top. Catal. 2012, 55, 620. (7) Nowak, I.; Misiewicz, M.; Ziolek, M.; Kubacka, A.; Corberan, V. C.; Sulikowski, B. Catalytic Properties of Niobium and Gallium Oxide Systems Supported on MCM-41 Type Materials. Appl. Catal., A 2007, 325, 328. (8) Selvaraj, M.; Kawi, S.; Park, D. W.; Ha, C. S. A Merit Synthesis of Well-Ordered Two-Dimensional Mesoporous Niobium Silicate Materials with Enhanced Hydrothermal Stability and Catalytic Activity. J. Phys. Chem. C 2009, 113, 7743. (9) Di Serio, M.; Turco, R.; Pernice, P.; Aronne, A.; Sannino, F.; Santacesaria, E. Valuation of Nb2O5−SiO2 Catalysts in Soybean Oil Epoxidation. Catal. Today 2012, 192, 112. (10) Nowak, I.; Kilos, B.; Ziolek, M.; Lewandowska, A. Epoxidation of Cyclohexene on Nb-Containing Meso- and Macroporous Materials. Catal. Today 2003, 78, 487. (11) Tiozzo, C.; Bisio, C.; Carniato, F.; Marchese, L.; Gallo, A.; Ravasio, N.; Psaro, R.; Guidotti, M. Epoxidation with Hydrogen Peroxide of Unsaturated Fatty Acid Methyl Esters over Nb(V)-Silica Catalysts. Eur. J. Lipid Sci. Technol. 2013, 115, 86. (12) Nowak, I.; Feliczak, A.; Nekoksová, I.; Č ejka, J. Comparison of Oxidation Properties of Nb and Sn in Mesoporous Molecular Sieves. Appl. Catal., A 2007, 321, 40. (13) Tiozzo, C.; Bisio, C.; Carniato, F.; Gallo, A.; Scott, S. L.; Psaro, R.; Guidotti, M. Niobium-Silica Catalysts for the Selective Epoxidation of Cyclic Alkenes: The Generation of the Active Site by Grafting Niobocene Dichloride. Phys. Chem. Chem. Phys. 2013, 15, 13354. (14) Nowak, I.; Jaroniec, M. Hard” vs “Soft” Templating Synthesis of Mesoporous Nb2O5 Catalysts for Oxidation Reactions. Top. Catal. 2008, 49, 193. (15) Shima, H.; Tanaka, M.; Imai, H.; Yokoi, T.; Tatsumi, T.; Kondo, J. N. IR Observation of Selective Oxidation of Cyclohexene with H2O2 over Mesoporous Nb2O5. J. Phys. Chem. C 2009, 113, 21693. (16) Ramanathan, A.; Maheswari, R.; Barich, D. H.; Subramaniam, B. Niobium Incorporated Mesoporous Silicate, Nb-KIT-6: Synthesis and Characterization. Microporous Mesoporous Mater. 2014, 190, 240. (17) Yan, W.; Ramanathan, A.; Ghanta, M.; Subramaniam, B. Towards highly selective ethylene epoxidation catalysts using hydrogen peroxide and tungsten- or niobium-incorporated mesoporous silicate (KIT-6). Catal. Sci. Technol. 2014, 4, 4433. (18) Maheswari, R.; Pachamuthu, M. P.; Ramanathan, A.; Subramaniam, B. Synthesis, Characterization, and Epoxidation Activity of Tungsten-Incorporated SBA-16 (W-SBA-16). Ind. Eng. Chem. Res. 2014, 53, 18833. (19) Feliczak-Guzik, A.; Wawrzynczak, A.; Nowak, I. Studies on Mesoporous Niobosilicates Synthesized Using F127 Triblock Copolymer. Adsorption 2009, 15, 247. (20) Ramanathan, A.; Maheswari, R.; Grady, B. P.; Moore, D. S.; Barich, D. H.; Subramaniam, B. Tungsten-Incorporated Cage-Type Mesoporous Silicate: W-KIT-5. Microporous Mesoporous Mater. 2013, 175, 43. (21) Ghanta, M.; Subramaniam, B.; Lee, H. J.; Busch, D. H. Highly Selective Homogeneous Ethylene Epoxidation in Gas (Ethylene)Expanded Liquid: Transport and Kinetic Studies. AIChE J. 2013, 59, 180. (22) Kleitz, F.; Kim, T.-W.; Ryoo, R. Phase Domain of the Cubic Im3̅m Mesoporous Silica in the EO106PO70EO106−Butanol−H2O System. Langmuir 2005, 22, 440. (23) Hartmann, M. Synthesis of Niobium- and Tantalum-Containing Silicalite-1. Chem. Lett. 1999, 407. (24) Ko, Y. S.; Jang, H. T.; Ahn, W. S. Hydrothermal Synthesis and Characterization of Niobium-Containing Silicalite-1 Molecular Sieves with MFI Structure. J. Ind. Eng. Chem. 2007, 13, 764.

4. CONCLUSIONS Niobium species were successfully incorporated into SBA-16 material. SAXS, XRD, ICP-OES, N2 sorption, SEM, and TEM results confirm the structural integrity of the material and the nature of Nb incorporation. Diffuse reflectance UV−vis spectra and H2-TPR studies reveal that the Nb exists as mostly oligomeric tetrahedral NbO4 species and reducible Nb2O5 species. NH3-TPD results reveal that Nb incorporation into the Nb-SBA-16 materials imparts acidity that increases with Nb content. Batch reaction studies show that while the increased NbO4 species in the Nb-SBA-16 materials favor epoxidation at lower Nb loadings, the increased acidity at higher Nb loadings leads to unwanted side reactions such as ring opening of the epoxide and possibly H2O2 decomposition, causing the conversion and selectivity profiles to pass through maxima with Nb loading. Our results confirm the dependence of the structure of the mesoporous silicate support on catalyst activity with the Nb-SBA-16 materials showing superior performance for cyclohexene epoxidation compared to either Nb-KIT-6 or Nb-KIT-5 at lower Nb loadings.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1-785-864-2903. Fax: +1-785-864-6051. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support by the U.S. Department of Agriculture and the National Institute of Food and Agriculture through Grant No. 2011-10006-30362.

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DEDICATION Tribute to Professor Scott Fogler. We feel privileged to be invited to contribute this article to the Festschrift honoring Professor Scott Fogler, who not only is an outstanding educator and researcher but also has provided exceptional leadership and service to the chemical engineering profession. One of the authors (B.S.) is particularly grateful to have enjoyed Professor Fogler’s friendship over the years and for Professor Fogler’s enthusiastic discussions and insights into the teaching of catalysis and reactor engineering to chemical engineering students. We congratulate Professor Fogler on completing 50 meritorious years at the University of Michigan and wish him the best of health and happiness in the years to come.

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REFERENCES

(1) Kholdeeva, O. A. Recent Developments in Liquid-Phase Selective Oxidation Using Environmentally Benign Oxidants and Mesoporous Metal Silicates. Catal. Sci. Technol. 2014, 4, 1869. (2) Amini, M.; Haghdoost, M. M.; Bagherzadeh, M. Monomeric and Dimeric Oxido−Peroxido Tungsten(VI) Complexes in Catalytic and Stoichiometric Epoxidation. Coord. Chem. Rev. 2014, 268, 83. (3) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Ordered Mesoporous and Microporous Molecular Sieves Functionalized with Transition Metal Complexes as Catalysts for Selective Organic Transformations. Chem. Rev. 2002, 102, 3615. (4) Herbert, M.; Montilla, F.; Alvarez, E.; Galindo, A. New Insights into the Mechanism of Oxodiperoxomolybdenum Catalysed Olefin Epoxidation and the Crystal Structures of Several Oxo−Peroxo Molybdenum Complexes. Dalton Trans. 2012, 41, 6942. F

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Industrial & Engineering Chemistry Research (25) Hartmann, M.; Prakash, A. M.; Kevan, L. Characterization and Catalytic Evaluation of Mesoporous and Microporous Molecular Sieves Containing Niobium. Catal. Today 2003, 78, 467. (26) Srinivasu, P.; Anand, C.; Alam, S.; Ariga, K.; Halligudi, S. B.; Balasubramanian, V. V.; Vinu, A. Direct Synthesis and the Morphological Control of Highly Ordered Two-Dimensional P6mm Mesoporous Niobium Silicates with High Niobium Content. J. Phys. Chem. C 2008, 112, 10130. (27) Tanaka, T.; Nojima, H.; Yoshida, H.; Nakagawa, H.; Funabiki, T.; Yoshida, S. Preparation of Highly Dispersed Niobium Oxide on Silica by Equilibrium Adsorption Method. Catal. Today 1993, 16, 297.

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