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Synthesis-structure-function relationships of silicasupported niobium(V) catalysts for alkene epoxidation with HO 2
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Nicholas Earl Thornburg, Scott L Nauert, Anthony B Thompson, and Justin M Notestein ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01796 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016
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Synthesis-structure-function relationships of silica-supported niobium(V) catalysts for alkene epoxidation with H2O2 Nicholas E. Thornburg, Scott L. Nauert, Anthony B. Thompson,† and Justin M. Notestein* Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Technological Institute E136, Evanston, Illinois 60208, United States ABSTRACT: Many industrially significant selective oxidation reactions are catalyzed by supported and bulk transition metal oxides. Catalysts for the synthesis of oxygenates, and especially for epoxidation, have predominantly focused on TiOx supported on or co-condensed with SiO2, while much of the rest of Groups 4 and 5 have seen less research. We have recently demonstrated through periodic trends using a uniform molecular precursor that niobium(V)-silica catalysts reveal the highest activity and selectivity for efficient utilization of H2O2 for epoxidation across all of Groups 4 and 5. In this work we graft a wide range of Nb(V) precursors, spanning surface densities of 0.07–1.6 Nb groups nm-2 on mesoporous silica, and characterize these materials with UV-visible spectroscopy and Nb K-edge XANES. Further, we apply in situ chemical titration with phenylphosphonic acid (PPA) in the epoxidation of cis-cyclooctene by H2O2 to probe the numbers and nature of the active sites across this series and in a set of related Ti-, Zr-, Hf-, and Ta-SiO2 catalysts. By this method, the fraction of kinetically-relevant NbOx species ranges from ~15% to ~65%, which correlates with spectroscopic evaluation of the NbOx sites. This titration leads to a single value for the average turnover frequency, on a per active site basis rather than a per Nb atom basis, of 1.4 ± 0.52 min-1 across the 21 materials in the series. These quantitative maps of structural properties and kinetic consequences link key catalyst descriptors of supported Nb-SiO2 to enable rational design for next-generation oxidation catalysts.
KEYWORDS: niobium, heterogeneous catalysis, supported catalyst, supported oxide, hydrogen peroxide, epoxidation, active site, calixarene
1. INTRODUCTION Supported metal oxides are important catalysts for selective oxidation processes in production of both commodity and fine chemicals.1-3 Olefin epoxidation is a critical synthetic reaction widely studied for these and other applications,4-5 with recent emphasis placed on understanding supported metal oxide catalysts that utilize H2O2 as oxidant.6-7 Hydrogen peroxide offers high active oxygen content with environmentally benign byproduct H2O.8 Unlike organic hydroperoxides, H2O2 does not require selling or disposing of an alcohol co-product.8-9 Reactivity and selectivity for H2O2 activation by heterogeneous catalysts is heavily influenced by the supported metal oxide structure, which itself is a consequence of synthesis method, metal loading (i.e. surface density) and the silica support.2, 10 As we and others have previously shown, the nature of the metal precursor used in synthesis strongly influences the relative extents of M–O–Si vs. M–O–M linkages, even at very low metal surface density.11-13 For example, substituting traditional metal chloride precursors with bulky and chelating metal precursors such as calixarene coordination complexes14 results in grafting to SiO2 surfaces at self-limiting surface densities, facilitating and enforcing high dispersions of
cations that result in a preponderance of undercoordinated, highly active oxide sites.11, 15-17 Titanium(IV) oxide supported on or co-condensed with SiO2 (Ti-SiO2) are workhorse catalysts for alkene epoxidation and have been the subject of extensive investigation.18-22 Some of us have recently demonstrated through periodic trends in epoxidation with H2O2 across all of Groups 4 and 5 that silica-supported niobium(V) (Nb-SiO2) catalysts can offer >2x faster initial rates and higher pathway selectivities than a structurally-analogous Ti-SiO2 catalyst.11 In this prior study, grafting bulky Nbcalixarene coordination complexes23 was critical to achieving undercoordinated, highly active Nb-SiO2, leading to a more active and selective catalyst on a per-Nb atom basis compared to a traditionally-synthesized catalyst from grafted NbCl5. When comparing different formulations of supported oxides, key questions remain about whether highperforming catalysts have a greater number of active sites overall or, in contrast, intrinsically more-active sites than analogous, correspondingly slower catalysts. Some of us have recently probed this question for TiOx-SiO2 materials with a technique that uses phenylphosphonic acid (PPA) titration under liquid-phase epoxidation reaction conditions as a simple technique for counting kinetically
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relevant Ti sites of low average coordination number.24 The PPA titrant selectively and irreversibly poisoned highly undercoordinated Lewis sites on Ti-SiO2 without binding irreversibly to SiO2, preventing activation of H2O2 and providing local information of the number of active structures. Here, this technique is expanded to the rest of Groups 4 and 5, with particular emphasis on Nb-SiO2 catalysts synthesized at different surface densities and with different precursors. Relative to Ti-SiO2, Nb-SiO2 materials are scarcely reported in the epoxidation literature,25-35 and in general, few reports systematically investigate precursor effects.35-41 In this report, we synthesize a series of 21 NbOx-SiO2 catalysts spanning final surface densities of 0.07-1.6 Nb groups nm-2, derived from a variety of molecular Nb precursors across three different incorporation methods. Structural and electronic properties are characterized with diffuse reflectance UV-visible (DRUV-vis) and X-ray absorption near-edge structure (XANES) spectroscopies, while reactivities are probed with cis-cyclooctene epoxidation with aqueous H2O2 oxidant. Further, in situ titration with phenylphosphonic acid (PPA) during epoxidation is applied to assess quantitative trends in active NbOx site populations and to evaluate catalyst synthesis efficiencies. These techniques connect Nb catalyst synthesis methods to key descriptors of electronic structure and catalytic reactivity, thus providing a comprehensive report of quantitative synthesis-structure-function relations of these highly dispersed niobium(V)-silica catalysts that outperform industrially-significant Ti-SiO2 catalysts.
2. EXPERIMENTAL METHODS 2.1 Catalyst Synthesis. All grafted catalysts were synthesized using standard Schlenk line techniques under N2 or in a controlled Ar atmosphere glovebox. Toluene solvent was distilled over CaH2 under N2, degasified by standard methods and stored inside the glovebox. Niobium(V) precursors (NbCl5 (“Cl”), Nb(C5H5)Cl4 (“Cp”), Nb(N(CH3)2)5 (“DMA”), and Nb(OC2H5)5 (“OEt”)) and other metal precursors (TiCl4(THF)2, ZrCl4(THF)2, HfCl4, and TaCl5) were used as received from Strem Chemicals and stored inside the glovebox. p-tert-Butylcalix[4]arene (95%) was used as received from Sigma Aldrich and stored inside the glovebox. Niobium(V) oxalate hydrate, C10H5NbO20·nH2O (“Ox”), was used as received from Alfa Aesar for templated co-condensation (“SBA-15”) and for incipient wetness impregnation (“IWI”) synthesis methods. Niobium content in the hydrated salt was determined by elemental analysis via inductively coupled plasma optical emission spectroscopy (ICP-OES; see Section 2.2). Ultrapure H2O (18 MΩ·cm) was used for cocondensation and impregnation procedures. For all materials except SBA-15 catalysts, a mesoporous silica gel (Selecto, 32-63 µm, 570 m2 g-1 BET surface area, 0.95 cm3 g-1 pore vol.) was partially dehydroxylated at 300 °C for 10 h under dynamic vacuum (> Zr > Hf when these oxides are used as highly dispersed Lewis acid catalysts for alkene epoxidation with H2O2.11 We first explore surface density effects of the high performing grafted Nb oxide by spanning loadings of 0.07-1.53 mmol Nb g-1 (0.07-1.6 Nb atoms nm-2; nearly equivalent values as mmol g-1 for the surface area of this SiO2 support). Catalysts are synthesized by a single grafting cycle of either Nb(V)calixarene (“Cx”) or Nb(V) chloride (“Cl”) precursors onto a conventional silica gel, with calcination at 550°C in air as a final step (Scheme 1). Nb(V)-calixarene coordination complexes are synthesized by reacting stoichiometric amounts of NbCl5 and p-tert-butylcalix[4]arene, previously demonstrated by us to yield highly active Nb oxide sites for epoxidation.11 Some of us have previously shown the uncalcined supported metal-calixarene complexes to be active epoxidation catalysts,11, 15-16 although these materials are not further studied here. Nb-calixarene precursors are geometrically-limited at a maximum loading of ~0.20 Nb groups nm-2 based on the calixarene’s bulky size, as some of us have previously observed;11, 16 above this surface density, the grafting efficiency drops off rapidly (Figure 1). In contrast, NbCl5 grafts quantitatively up to loadings of ~1 Nb groups nm-2, and we observe a maximum surface density of ~1.6 Nb groups nm-2 from a single grafting cycle of NbCl5.
Optical edge energy from DRUV-vis spectroscopy provides qualitative insight into niobium oxide surface speciation.11, 38, 40, 55 Note that all of the UV-visible and X-ray absorption spectroscopy was performed on dehydrated catalysts. Generally, the DRUV-vis edge decreases in energy with increased Nb chloride loading, while edge energies of catalysts from Nb-calixarene are practically insensitive to loading (Table 1, Figure 2). Crucially, even within very low surface densities of ≤0.20 Nb nm-2, Cx-derived catalysts are ~0.5 eV higher in edge than corresponding Cl-derived materials. Following assignments from related supported VOx catalysts,56 Cx-derived Nb catalysts display O2-Nb5+ ligand-to-metal charge transfer (LMCT) bands between 5.1-5.4 eV tentatively assigned to Nb=O in an isolated Nb(V) center38; in contrast, Cl-derived catalysts exhibit some extent of Nb–O LMCT character between 3.6-4.8 eV, attributable to small NbOx aggregates. Surface density indeed influences the resulting structure, but the nature of the Nb precursor also plays an important role. Table 1. Physicochemical summary of Nb-SiO2 catalysts from grafted Nb-calixarene and NbCl5 Catalyst
Nb loadinga (mmol Nb g-1)
Initial rateb (min-1)
Edgec (eV)
C.N.d
Nb-0.07-Cx
0.07
0.80
4.1
e
Nb-0.10-Cx
0.10
0.76
4.0
4.4
Nb-0.13-Cx
0.13
0.88
4.0
4.3
Nb-0.20-Cx
0.20
0.98
4.0
4.5
Nb-0.07-Cl
0.07
0.013
3.4
e
Nb-0.10-Cl
0.10
0.059
3.3
5.8
Nb-0.14-Cl
0.14
0.15
3.6
5.5
Nb-0.20-Cl
0.20
0.29
3.6
5.7
Nb-0.49-Cl
0.49
0.20
3.6
6.0
Nb-0.92-Cl
0.92
0.18
3.5
5.6
Nb-1.53-Cl
1.53
0.12
3.4
5.9
a
From ICP-OES of calcined catalysts. Initial rate = mmolepoxide mmoltotal Nb-1 min-1, computed for the first 15 min of reaction from linear regression through product concentration vs. time plots. c From x-intercept of the indirect Tauc plot [F(R)·hν]1/2 vs. hν (eV), where F(R) is Kubelka-Munk pseudoabsorbance.49-52 d Interpolated from XANES pre-edge feature fitting with 4-, 5-, and 6coordinate Nb oxide standards.11 Dehydrated catalysts. See Figures 3 and S1. e C.N. indeterminate from low signal-to-noise in XANES spectra. b
Figure 1. Grafting efficiencies of Nb-calixarene (red) and NbCl5 (blue). Horizontal lines indicate geometric grafting limits of precursors. Dashed line represents complete grafting. Data points enlarged to show error from ICP-OES (y-axis, ±0.02 Nb groups nm-2) and from physical transfers of precursor and SiO2 during synthesis (x-axis, ±~0.05 mmol Nb g-1).
These findings are further corroborated with X-ray absorption near-edge structure (XANES) spectroscopy at the Nb K-edge. The diagnostic pre-edge feature at ~18,995 eV marks 1s-3p electronic transitions and serves as a proxy for structural information (Figure 3).38, 57-58 We have previously reported a fitting procedure for assigning a formal Nb coordination number to catalysts Nb-0.20-Cx and Nb0.20-Cl by linearly correlating the pre-edge feature height to that of Nb oxide standards of known coordination number (Tables 1 and S1, Figures 3 and S1).11 In brief, a more intense pre-edge height corresponds to a lower
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average coordination number (C.N.) for the Nb center. Extending to the present Nb-SiO2 series, XANES estimates Cl-derived materials to have C.N. between 5 and 6 – even at low loadings – while Cx-derived catalysts are all ~4.5. These coordination numbers refer to the catalyst state when dehydrated for spectroscopy. This average value necessarily incorporates differences in total C.N., effects of disorder, as well as relative populations of Nb–O and Nb=O substructures.11 Unfortunately, we are unable to obtain publication-quality EXAFS data for these catalysts, which may assist further in making structural assignments. Further, the XANES spectra of the lowestloaded Nb-0.07-Cx and Nb-0.07-Cl give low signal-tonoise, and C.N. are thus indeterminate.
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Catalyst reactivity is assessed in the epoxidation of ciscyclooctene with aqueous H2O2 at 65°C. This reaction is intrinsically selective to form the epoxide, as has been observed by us24, 59 and others60-61, with selectivities to cyclooctane trans-diol 5 even when dehydrated and edge < 4.0 eV. Across all grafted and impregnated materials, we again conclude Nb-calixarene precursors yield Nb-SiO2 catalysts with the greatest preponderance of undercoordinated sites, and these are predictably good catalysts. Unfortunately, the low-loaded SBA-15 materials give low signalto-noise for XANES, making C.N. assignments indeterminate, though the high edge energies of these catalysts suggest this co-condensation technique also yields highly dispersed Nb centers, consistent with other reports for this class of materials.42-43, 66 However, initial rates per total Nb for the Nb-SBA-15 catalysts are much lower than would be expected from their DRUV-vis edge energies.
This would be consistent with some Nb sites being fluidinaccessible within the SBA-15 framework. Several commercially-available precursors (e.g. Cp, OEt, and DMA) give good rates at low loadings, but these would not necessarily have been predicted from spectroscopy results shown in Figure 5. Table 2. Physicochemical summary of Nb-SiO2 catalysts from various precursors and methods Catalyst
Nb loadinga (mmol Nb g-1)
Initial rateb (min-1)
Edgec (eV)
C.N.d
Nb-0.20-Cx
0.20
0.98
4.0
4.5
Nb-0.20-Cl
0.20
0.29
3.6
5.7
Nb-1.53-Cl
1.53
0.12
3.4
5.9
Nb-0.23-Cp
0.23
0.71
3.9
5.8
Nb-1.61-Cp
1.61
0.13
3.4
6.0
Nb-0.21-DMA
0.21
0.79
3.6
5.4
Nb-1.57-DMA
1.57
0.14
3.5
5.8
Nb-0.20-OEt
0.20
0.61
3.7
5.6
Nb-0.97-OEt
0.97
0.28
3.5
6.0
Nb-0.08-SBA-15e
0.08
0.35
4.4
f
e
0.10
0.50
4.3
f
Nb-0.10-SBA-15
Nb-0.21-Ox-IWI
0.21
0.33
3.5
5.1
Nb-0.54-Ox-IWI
0.54
0.086
3.5
5.8
a
From ICP-OES of calcined catalysts. Initial rate = mmolepoxide mmoltotal Nb-1 min-1, computed for the first 15 min of reaction from linear regression through product concentration vs. time plots. c From x-intercept of the indirect Tauc plot [F(R)·hν]1/2 vs. hν (eV), where F(R) is Kubelka-Munk pseudoabsorbance.49-52 d Interpolated from XANES pre-edge feature fitting with 4-, 5-, and 6coordinate Nb oxide standards.11 See Figures 3 and S1. Dehydrated catalysts. e BET data: Nb-0.08-SBA-15, 1260 m2 g-1, 6.4 nm avg. pore dia., 0.04 Nb nm-2; Nb-0.10-SBA-15, 1180 m2 g-1, 6.4 nm avg. pore dia., 0.05 Nb nm-2. See Figure S3. f C.N. indeterminate from low signal-to-noise in XANES spectra. b
Figure 5. Correlations of (a) initial rate (mmolepoxide mmoltotal Nb min-1) from cis-cyclooctene epoxidation vs. optical edge energy (eV) from DRUV-vis, (b) initial rate vs. average coordination number (when dehydrated) from Nb K-edge XANES, and (c) optical edge vs. coordination number. Precursor legend: Cx (red), Cl (dark blue), Cp (green), DMA (orange), OEt
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(yellow), Ox-IWI (brown), and SBA-15 (light blue). Note that C.N. are indeterminate for all low loading catalysts (Nb0.07-Cx, Nb-0.07-Cl, Nb-0.08-SBA-15 and Nb-0.10-SBA-15). Dashed lines represent trends in (a) and (b) and separation of regimes in (c). In spite of surface rearrangement effects from calcination, we observe a distinct memory effect of the Nb precursor ligand. Corroboration of results from these orthogonal techniques across an order of magnitude in Nb surface density establish that both spectroscopic readouts are useful descriptors of general Nb-SiO2 Lewis acid behavior; however, both techniques are imperfect for reconciling underlying differences in reactivity in otherwise structurally similar catalysts. 3.3 Kinetic Consequences of in situ Titration. Epoxidation studies across Nb-SiO2 catalysts prepared from various Nb precursors broadly cross-correlate with spectroscopic descriptors, though key questions remain about what underlies a high vs. a low initial rate, on a per Nb atom basis. To reconcile these apparent reactivity differences across different catalyst surfaces, we further evaluate our Nb-SiO2 catalysts with an in situ titration technique. Our group has recently demonstrated phenylphosphonic acid (PPA) to be a selective, irreversible titrant for poisoning undercoordinated, highly Lewis acidic surface TiOx-SiO2 sites during cis-cyclooctene epoxidation by H2O2.24 In a typical titration, variable amounts of an aqueous PPA solution are added to parallel batches of catalyst at 65°C prior to epoxidation reaction. Final titrant loadings span 0-2 eq. PPA per total Nb charged to the reactor. Initial reactivity is evaluated as before and monotonically diminishes as a function of added PPA, as illustrated for Nb-0.20-Cx in Figure 6a. When Nb-0.20-Cx is treated with increasing equivalents of PPA in the absence of reactants and the materials are recovered for XANES, the increase in apparent C.N. tightly correlates with the loss of initial rate in analogous in situ titration experiments (Figure 6b). The rapid decrease in average coordination number shows that the titrant irreversibly binds to Nb sites of lowest C.N. first (e.g. those that are approximately 4-coordinate when dehydrated) and increases the apparent coordination number
to ~6. These undercoordinated sites are predictably most active for H2O2 activation, and they are removed from catalytic reaction by this titration. As some of us have previously reported, calixarene-Ti-SiO2 catalysts deliberately synthesized to have 5- and 6-coordinate structures proceed with an order of magnitude slower epoxidation rate than 4-coordinate surface structures.67 Analogously, a fully titrated Nb-SiO2 catalyst with average C.N. ~6 still retains some residual reactivity, though it proceeds an order of magnitude slower than the native catalyst and therefore does not contribute significantly to the overall rate. Concisely, bulk structural information from XANES corroborates metrics from in situ PPA poisoning (both loss of epoxidation rate and simply PPA added) and gives insight into which Nb sites may be most kinetically relevant. Further, catalysts Nb-0.13-Cx and Nb-0.92-Cl, which are structurally distinct by XANES and DRUV-vis, yield 31P CP-MAS NMR spectra with the same features (Figure S4) after PPA titration, consistent with our hypothesis that under reaction conditions, PPA selectively targets one type of active site regardless of the surface being probed. No loss of Nb is observed in any of these experiments (Table S2). Both catalysts exhibit a double peak at +15 ppm and +19 ppm, and neither material resembles bulk niobium phosphate (-11 ppm) (Figure S4). Following literature assignments by others for PPA adsorption on crystalline TiO2 and on TiOx-SiO2 catalysts,24, 68-70 the overlapping resonances at +15 ppm and +19 ppm may be attributable to tridentate PhP(O-M)3 and bidendate PhP(OH)(OM)2 structures (M = Nb or Si), respectively. PPA thus may be adsorbed through P–O–Nb linkages in addition to weak interactions with surface silanols, as some of us have proposed for corresponding Ti-SiO2 systems.24 Precise phosphorus coordination assignments are not yet possible for these catalysts.
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Figure 6. (a) Initial time-course data of cis-cyclooctene epoxidation with in situ PPA titration of Nb-0.20-Cx, and (b) comparison of loss of initial epoxidation rate during in situ PPA poisoning of Nb-0.20-Cx (circles, left-hand axis) with increase in coordination number in Nb-0.20-Cx from PPA titration without reactants present (yellow triangles, right-hand axis). For circle data points in (b), abscissa values represent PPA added to reactor per total Nb. For triangle data points in (b), abscissa values represent P/Nb ratios from ICP-OES of titrated materials. C.N. estimated from XANES pre-edge fitting (see Figure S1).
Figure 7. (a) Graphical procedure to estimate the fraction of kinetically relevant Nb atoms, A, from 95% initial rate loss during in situ PPA titration of Nb-0.20-Cx during cis-cyclooctene epoxidation, and (b) parity plot of Agraphical (determined from procedure illustrated in (a)) vs. Amodel. See Discussion section and Supporting Information for model details, and Table S3 for all values. Precursor legend: Cx (red), Cl (dark blue), Cp (green), DMA (orange), OEt (yellow), Ox-IWI (brown), and SBA-15 (light blue).
Figure 8. Groups 4, 5 kinetic summary of (a) initial epoxidation rates without PPA and (b) percent active metal in calcined catalysts, determined via in situ PPA titration. Active Ti, Zr, and Hf are isolated metal sites with one hydroxyl, while active Nb and Ta are deemed isolated metal sites with a hydroxyl pair. Catalyst legend: Ti-0.20-Cx, dark blue; Ti-0.21-Cl, light blue; Zr-0.23-Cx, dark purple; Zr-0.21-Cl, light purple; Hf-0.20-Cx, magenta; Hf-0.19-Cl, light pink; Nb-0.20-Cx, dark red; Nb-0.20-Cl, light red; Ta-0.22-Cx, dark green; Ta-0.22-Cl, light green. See standard reaction conditions in text. Error bars from replicate trials. See Table S4. Unlike titration of TiOx-SiO2 systems recently reported by some of us,24 wherein PPA linearly diminishes the initial rate, here we observe an apparent supralinear decrease in initial epoxidation rate as a function of added PPA, as illustrated in Figure 6b. The supralinear drop in rate indicates either (1) that a distribution of rates exists amongst the participating active sites, or (2) that multiple sites participate simultaneously in the catalyst cycle.
In the advent of a distribution of active sites, we assume each PPA to titrate a single Nb atom, and estimate the fraction of kinetically relevant atoms, A, from the amount of PPA required to eliminate 95% of the native initial rate using a log-linear plot (Figure 7a). The determined value of A ranges from ~0.15 to ~0.65, (Figure 7b, Table S3) with the Cx and SBA-15 materials exhibiting the highest values (>0.5) among this catalyst series.
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These values of kinetically-relevant Nb atoms are lower than previously reported by some of us for high performing TiOx catalysts.24 Therefore, this titration method is applied to the series of silica-supported Group 4 and 5 oxides, for which we observed that rates of epoxidation trend as Nb > Ti > Ta >> Zr > Hf.11 Here, we re-examine this series of calcined catalysts derived from metalcalixarene or metal chloride precursors at surface densities of ~0.20 M nm-2 (Table S4). As for Nb, DRUV-vis spectra for the calcined catalysts show that materials from calixarene precursor generally have higher edge energies than the analogous metal chloride-derived materials (Table S4, Figure S5). Reactivity of each calcined catalyst was evaluated in the epoxidation of cis-cyclooctene with H2O2 at 65°C as before (Figure 8a, Table S4). Across the series, initial rates of native catalysts (i.e. no PPA titrant) derived from metalcalixarene precursors exceed those from the chloride precursor, except for the relatively inactive Zr-SiO2 catalysts. The relative rates generally trend as in the previous study, with Nb > Ta >> Ti > Hf > Zr. Titration with PPA during epoxidation is then used to estimate the fraction of active metal (Figure 8b, Table S4), using the graphical method of Figure 7a. These data show that the calixarene precursor results in a larger fraction of the metal participating in catalysis than compared to the equivalent catalyst synthesized from the chloride precursor. For the Group 4 catalysts, the differences in active catalyst fraction largely can explain the differences in rates between the two precursors – the doubling in the fraction of active Ti is accompanied by a doubling in rate, for example. However, Ta and Nb show much larger increases in rate for smaller increases in active metal, and indeed in smaller active fractions as compared to the Group 4 oxides. Overall, Nb and Ta appear to be much more ‘structure-sensitive’ than Ti, Hf, or Zr.
To determine reaction orders for this catalytic cycle, a conventional epoxidation rate law is proposed, eq. 1: ݎ = ݇′ ሾܯ௧௩ ሿሾܪଶ ܱଶ ሿ ሾ݈ܽ݇݁݊݁ሿ (1) In this and following equations, kapp collects several rate and equilibrium constants and the effect of water, which do not impact the rest of the discussion. The Mactive notation is to indicate that not all atoms of the supported oxide may be kinetically relevant. For Group 4 metal oxides such as Ti, rate-limiting peroxygen transfer has been proposed to be promoted by surface silanols or coordinated alcohol solvent,19 72 81 allowing eq. 1 to be rewritten as eq 2: ݎ = ݇′′ ሾܴܱܪሿሾܱܶ݅ܪ௧௩ ሿሾܪଶ ܱଶ ሿ ሾ݈ܽ݇݁݊݁ሿ (2) For Group 5 catalysts, individual Nb sites hydrolyze to form two NbOH bonds under reaction conditions,45-47 and eq. 1 is rewritten as eq. 3, ݎ = ݇′′′ ሾܾܱܰܪ௧௩ ሿଶ ሾܪଶ ܱଶ ሿ ሾ݈ܽ݇݁݊݁ሿ (3) This has the same general form as eq. 2, but with both NbOH on the same Nb atom, and where one of the NbOH takes the role of assisting in the rate-limiting peroxygen transfer, as shown in blue in Scheme 2, while the other forms the bound hydroperoxo. This mechanism does not need to invoke an external alcohol, potentially leading to the improved rates. A further difference is that ROH (Si–OH or alcohol solvent) will not be titrated by PPA in eq. 2, whereas all NbOH are titrated in eq. 3. We define the number of active NbOH groups as in eq. 4: ሾܾܱܰܪ௧௩ ሿ = ሾܾܱܰܪ௧௧ ሿ ܣ− ሾܲܲܣሿ (4) As for the graphical method of Figure 7, A is the (average) fraction of active Nb sites. Substituting this expression into eq. 3 gives eq. 5: ݎ = ݇ ሺሾܾܱܰܪ௧௧ ሿ ܣ− ሾܲܲܣሿሻଶ ሾܪଶ ܱଶ ሿ ሾ݈ܽ݇݁݊݁ሿ (5) which is overall second order in titrant, as observed, while still maintaining a conventional overall first order in active Nb atoms, since it is just a re-expression of eq. 1.
4. DISCUSSION
Scheme 2. Proposed catalytic cycle for alkene epoxidation by H2O2 over highly dispersed Nb-SiO2 with in situ titration with PPAa
The markedly higher catalytic rates of Nb and Ta (Figure 8a), and their supralinear response to titration (Figure 6b) and fraction of active sites (Figure 8b), suggest there may be mechanistic differences between Group 4 and Group 5 oxides for epoxidation with H2O2. Following mechanisms for epoxidation over Ti-SiO2, which are supported by DFT71-73 and spectroscopy,19, 74-75 a simplified catalytic cycle is proposed (Scheme 2) where equilibrated proton transfer from H2O2 to Nb–OH forms a Nb-OOH bond, which then undergoes rate-limiting electrophilic attack by the alkene. Such active hydroperoxo species have been shown by others to form on related undercoordinated Ti76 and Ta77 sites. Although there may be temporary reorganization of surface Nb structures due to adsorption of water and other species under reaction conditions, recalcination of the spent materials recovers their original structures (Figure S6)11, 55; in contrast, titaniasilica catalysts are known to have a strong tendency to aggregate under reaction conditions.78-80
a
Titration with PPA stoichiometrically removes active NbOH groups from catalytic turnover. Additional coordinated water solvent molecules omitted for clarity. While other kinetic models could be proposed, this model does adequately capture the initial rates for 18 different catalysts freshly prepared and with sub-
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stoichiometric PPA added (see SI discussion and Table S3); catalysts with very low rates were excluded from fitting, and no attempt was made to capture the residual 50%. Grafting most commercially available precursors such as niobium chloride or niobium ethoxide can result in catalysts with a significantly smaller fraction of active niobium. These latter catalysts were also seen to lack correlations between DRUV-vis and XANES, likely because of small amounts of larger NbOx domains, to which those spectroscopies are most sensitive. Applying PPA site titration across Group 4 and 5 supported oxide active sites demonstrates that Group 4 metal oxides tend to show a larger fraction of kineticallyrelevant sites. These sites also have activity that is approximately linearly related to the number of active sites or the fraction of those sites titrated away by PPA. This is most consistent with the existence of predominantly one
type of kinetically relevant MIV–OH site on the surface. Others have also demonstrated that this site also requires a surface Si–OH or solvent moiety to facilitate proton transfer during the epoxidation elementary step.19, 72 In contrast, active Nb and Ta appear to have a nonlinear dependence of rate on added titrant. One interpretation of these data is simply that these surfaces possess a distribution of active sites. However, we also show that this behavior can be described by a kinetic model where a Nb(OH)2 dihydroxy site is kinetically relevant. On highly dispersed cations such as discussed here, these would be most prevalent on an isolated Nb atom. Overall, the conclusion is reached that Group 5 catalysts (Nb and Ta) are intrinsically faster materials than more standard Ti-SiO2 catalysts, in agreement with previous reports by some of us.11, 15 Particularly, Nb is a good catalyst since it gives excellent rates and can be synthesized to give stable, highly undercoordinated sites in relatively high abundance. Nb also has very low tendency for unselective radical oxidation pathways, as we have previously demonstrated.11 NbOx-SiO2 materials have received significantly less attention in the literature as oxidation catalysts,26-27, 36 especially compared to workhorse Ti-SiO2 catalysts. We feel these high-performing NbOx-SiO2 catalysts warrant significantly more investigation from the catalysis community, for both fundamental science and industrial application.
ASSOCIATED CONTENT Supporting Information. Detailed synthesis procedures; kinetic discussion and fitting procedure; representative Nb K-edge XANES spectra, pre-edge fitting procedure, peak heights and edge energies of all Nb catalysts; low-angle pXRD spectra and N2 physisorption data of SBA-15 catalysts; 31 P CP-MAS NMR spectra and elemental analysis of substrates; kinetic parameters from experimentation and modeling for each NbOx-SiO2 catalyst; physicochemical data and DRUV-vis spectra of Group 4, 5 metal oxide catalysts and of select spent and recalcined Nb-SiO2 catalysts; H2O2 decomposition rates for select catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author * Email:
[email protected] Present Address †
Savannah River National Laboratory, Savannah River Site, Aiken, SC 29808, USA.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT N.E.T. and J.M.N. acknowledge financial support from the Dow Chemical Company. S.L.N. and A.B.T. acknowledge financial support from NSF Grants DGE-1324585 and CBET0933667, respectively. Material characterization was performed at the IMSERC facility with financial support from
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NSF Grant DMR-0521267 and at the Quantitative BioElement Imaging Center, and at Northwestern University, and at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) at the Advanced Photon Source at Argonne National Laboratory (DOE Contract No. DE-AC0206CH11357). N.E.T. thanks Dr. Todd Eaton for his helpful discussions, Lauren McCullough for her assistance with chloride determination, and Dr. Qing Ma for his assistance with X-ray absorption spectroscopy experiments. Powder X-ray diffraction was performed at the J. B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University.
REFERENCES 1.
2.
3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
21. 22.
23.
Bartholomew, C. H.; Farrauto, R. J., Fundamentals of Industrial Catalytic Processes. 2nd ed.; Wiley: Hoboken, NJ, 2006. Clerici, M. G.; Kholdeeva, O. A., Liquid Phase Oxidation via Heterogeneous Catalysis: Organic Synthesis and Industrial Applications. Wiley: Hoboken, NJ, 2013; p 526. Thomas, J. M.; Thomas, W. J., Principles and Practice of Heterogeneous Catalysis. 2nd ed.; Wiley VCH: New York, NY, 2015; p 768. Jorgensen, K. A., Chem. Rev. 1989, 89, 431-458. Meunier, D.; de Mallmann, A.; Basset, J.-M., Top. Catal. 2003, 23, 183-189. Wong, O. A.; Shi, Y., Chem. Rev. 2008, 108, 3958-3987. Brutchey, R. L.; Lugmair, C. G.; Schebaum, L. O.; Tilley, T. D., J. Catal. 2005, 229, 72-81. Sheldon, R. A., J. Mol. Catal. 1980, 7, 107-126. Thomas, J. M.; Raja, R.; Lewis, D. W., Angew. Chem. Int. Ed. Engl. 2005, 44, 6456-82. Notari, B.; Willey, R. J.; Panizza, M.; Busca, G., Catal. Today 2006, 116, 99-110. Thornburg, N. E.; Thompson, A. B.; Notestein, J. M., ACS Catal. 2015, 5, 5077-5088. Prieto-Centurion, D.; Notestein, J. M., J. Catal. 2011, 279, 103110. Eaton, T. R.; Campos, M. P.; Gray, K. A.; Notestein, J. M., J. Catal. 2014, 309, 156-165. Homden, D. M.; Redshaw, C., Chem. Rev. 2008, 108, 50865130. Morlanés, N.; Notestein, J. M., J. Catal. 2010, 275, 191-201. Notestein, J. M.; Iglesia, E.; Katz, A., J. Am. Chem. Soc. 2004, 126, 16478-16486. de Silva, N.; Hwang, S.-J.; Durkin, K. A.; Katz, A., Chem. Mater. 2009, 21, 1852-1860. Clerici, M. G.; Bellussi, G.; Romano, U., J. Catal. 1991, 129, 159167. Clerici, M. G.; Ingallina, P., J. Catal. 1993, 140, 71-83. Marchese, L.; Gianotti, E.; Dellarocca, V.; Maschmeyer, T.; Rey, F.; Coluccia, S.; M. Thomas, J., Phys. Chem. Chem. Phys. 1999, 1, 585-592. Gao, X.; Wachs, I. E., Catal. Today 1999, 51, 233-254. Capel-Sanchez, M. C.; Blanco-Brieva, G.; Campos-Martin, J. M.; de Frutos, M. P.; Wen, W.; Rodriguez, J. A.; Fierro, J. L. G., Langmuir 2009, 25, 7148-7155. Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C.; Re, N.; Rizzoli, C.; Chiesi-Villa, A., J. Am. Chem. Soc. 2000, 122, 36523670.
Page 12 of 14
24. Eaton, T. R.; Boston, A. M.; Thompson, A. B.; Gray, K. A.; Notestein, J. M., ChemCatChem 2014, 6, 3215-3222. 25. Nowak, I., Catal. Today 2012, 192, 80-88. 26. Nowak, I.; Ziolek, M., Chem. Rev. 1999, 99, 3603-3624. 27. Tanabe, K., Catal. Today 1990, 8, 1-11. 28. Aronne, A.; Turco, M.; Bagnasco, G.; Ramis, G.; Santacesaria, E.; Di Serio, M.; Marenna, E.; Bevilacqua, M.; Cammarano, C.; Fanelli, E., Appl. Catal. A Gen. 2008, 347, 179-185. 29. Feliczak, A.; Walczak, K.; Wawrzyńczak, A.; Nowak, I., Catal. Today 2009, 140, 23-29. 30. Feliczak-Guzik, A.; Nowak, I., Catal. Today 2009, 142, 288292. 31. Di Serio, M.; Turco, R.; Pernice, P.; Aronne, A.; Sannino, F.; Santacesaria, E., Catal. Today 2012, 192, 112-116. 32. Coelho, J. V.; Oliveira, L. C. A.; Moura, F. C. C.; de Souza, P. P.; Silva, C. A.; Batista, K. B.; Silva, M. J. d., Appl. Catal. A Gen. 2012, 419–420, 215-220. 33. Tiozzo, C.; Bisio, C.; Carniato, F.; Marchese, L.; Gallo, A.; Ravasio, N.; Psaro, R.; Guidotti, M., Eur. J. Lipid Sci. Technol. 2013, 115, 86-93. 34. Turco, R.; Aronne, A.; Carniti, P.; Gervasini, A.; Minieri, L.; Pernice, P.; Tesser, R.; Vitiello, R.; Di Serio, M., Catal. Today 2015, 254, 99-103. 35. Gao, X.; Wachs, I. E.; Wong, M. S.; Ying, J. Y., J. Catal. 2001, 203, 18-24. 36. Nowak, I.; Kilos, B.; Ziolek, M.; Lewandowska, A., Catal. Today 2003, 78, 487-498. 37. Gallo, A.; Tiozzo, C.; Psaro, R.; Carniato, F.; Guidotti, M., J. Catal. 2013, 298, 77-83. 38. Tiozzo, C.; Bisio, C.; Carniato, F.; Gallo, A.; Scott, S. L.; Psaro, R.; Guidotti, M., Phys. Chem. Chem. Phys. 2013, 15, 13354-62. 39. Gallo, J. M. R.; Pastore, H. O.; Schuchardt, U., J. Non-Cryst. Solids 2008, 354, 1648-1653. 40. Carniti, P.; Gervasini, A.; Marzo, M., J. Phys. Chem. C 2008, 112, 14064-14074. 41. Feliczak-Guzik, A.; Wawrzyńczak, A.; Nowak, I., Microporous Mesoporous Mater. 2015, 202, 80-89. 42. Kilos, B.; Nowak, I.; Ziolek, M.; Tuel, A.; Volta, J. C., Transition metal containing (Nb, V, Mo) SBA-15 molecular sieves —synthesis, characteristic and catalytic activity in gas and liquid phase oxidation. In Stud. Surf. Sci. Catal., J. Čejka, N. Ž.; Nachtigall, P., Eds. Elsevier: 2005; Vol. 158, Part B, pp 1461-1468. 43. Trejda, M.; Tuel, A.; Kujawa, J.; Kilos, B.; Ziolek, M., Microporous Mesoporous Mater. 2008, 110, 271-278. 44. Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D., Science 1998, 279, 548-552. 45. Jehng, J.-M.; Wachs, I. E., Catal. Today 1990, 8, 37-55. 46. Jehng, J. M.; Wachs, I. E., J. Phys. Chem. 1991, 95, 7373-7379. 47. Chen, Y.; Fierro, J. L. G.; Tanaka, T.; Wachs, I. E., J. Phys. Chem. B 2003, 107, 5243-5250. 48. Wang, W.-G.; Zheng, Q.-Y.; Huang, Z.-T., Synth. Commun. 1999, 29, 3711-3718. 49. Gao, X.; Bare, S. R.; Fierro, J. L. G.; Banares, M. A.; Wachs, I. E., J. Phys. Chem. B 1998, 102, 5653-5666. 50. Barton, D. G.; Shtein, M.; Wilson, R. D.; Soled, S. L.; Iglesia, E., J. Phys. Chem. B 1999, 103, 630-640. 51. Argyle, M. D.; Chen, K.; Resini, C.; Krebs, C.; Bell, A. T.; Iglesia, E., J. Phys. Chem. B 2004, 108, 2345-2353. 52. Tauc, J.; Menth, A., J. Non-Cryst. Solids 1972, 8–10, 569-585. 53. Lapina, O. B.; Khabibulin, D. F.; Romanenko, K. V.; Gan, Z.; Zuev, M. G.; Krasil’nikov, V. N.; Fedorov, V. E., Solid State Nucl. Magn. Reson. 2005, 28, 204-224. 54. Cummings, J. P.; Simonsen, S. F., Am. Mineral. 1970, 55, 9097.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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55. Tiozzo, C.; Palumbo, C.; Psaro, R.; Bisio, C.; Carniato, F.; Gervasini, A.; Carniti, P.; Guidotti, M., Inorg. Chim. Acta 2015, 431, 190-196. 56. Kim, H.-S.; Stair, P. C., J. Phys. Chem. A 2009, 113, 4346-4355. 57. Yamamoto, T., X-Ray Spectrom. 2008, 37, 572-584. 58. Yoshida, H.; Tanaka, T.; Yoshida, T.; Funabiki, T.; Yoshida, S., Catal. Today 1996, 28, 79-89. 59. Morlanés, N.; Notestein, J. M., Appl. Catal. A Gen. 2010, 387, 45-54. 60. Neumann, R.; Levin-Elad, M., J. Catal. 1997, 166, 206-217. 61. Fraile, J. M.; Garcı́a, J. I.; Mayoral, J. A.; Vispe, E., J. Catal. 2001, 204, 146-156. 62. Burcham, L. J.; Datka, J.; Wachs, I. E., J. Phys. Chem. B 1999, 103, 6015-6024. 63. Somma, F.; Puppinato, A.; Strukul, G., Appl. Catal. A Gen. 2006, 309, 115-121. 64. Zhuravlev, L. T., Colloids Surf. A 2000, 173, 1-38. 65. Anwander, R., Chem. Mater. 2001, 13, 4419-4438. 66. Olejniczak, M.; Ziolek, M., Microporous Mesoporous Mater. 2014, 196, 243-253. 67. Notestein, J. M.; Andrini, L. R.; Kalchenko, V. I.; Requejo, F. G.; Katz, A.; Iglesia, E., J. Am. Chem. Soc. 2007, 129, 1122-1131. 68. Brodard-Severac, F.; Guerrero, G.; Maquet, J.; Florian, P.; Gervais, C.; Mutin, P. H., Chem. Mater. 2008, 20, 5191-5196. 69. Lafond, V.; Gervais, C.; Maquet, J.; Prochnow, D.; Babonneau, F.; Mutin, P. H., Chem. Mater. 2003, 15, 4098-4103.
70. Di Valentin, C.; Costa, D., J. Phys. Chem. C 2012, 116, 28192828. 71. Neurock, M.; Manzer, L. E., Chem. Commun. 1996, 1133-1134. 72. Sinclair, P. E.; Catlow, C. R. A., J. Phys. Chem. B 1999, 103, 1084-1095. 73. Catlow, C. R. A.; French, S. A.; Sokol, A. A.; Thomas, J. M., Philos. Trans. R. Soc. London, Ser. A 2005, 363, 913-936. 74. Bravo-Suárez, J. J.; Bando, K. K.; Lu, J.; Haruta, M.; Fujitani, T.; Oyama, T., J. Phys. Chem. C 2008, 112, 1115-1123. 75. Thomas, J. M.; Sankar, G., Acc. Chem. Res. 2001, 34, 571-581. 76. Brutchey, R. L.; Ruddy, D. A.; Andersen, L. K.; Tilley, T. D., Langmuir 2005, 21, 9576-9583. 77. Ruddy, D. A.; Tilley, T. D., J. Am. Chem. Soc. 2008, 130, 1108811096. 78. Gianotti, E.; Bisio, C.; Marchese, L.; Guidotti, M.; Ravasio, N.; Psaro, R.; Coluccia, S., J. Phys. Chem. C 2007, 111, 5083-5089. 79. Fraile, J. M.; Garcı́a, J. I.; Mayoral, J. A.; Vispe, E., Appl. Catal. A Gen. 2003, 245, 363-376. 80. Guidotti, M.; Pirovano, C.; Ravasio, N.; Lazaro, B.; Fraile, J. M.; Mayoral, J. A.; Coq, B.; Galarneau, A., Green Chem. 2009, 11, 1421-1427. 81. To, J.; Sokol, A. A.; French, S. A.; Catlow, C. R. A.; Sherwood, P.; van Dam, H. J. J., Angew. Chem. Int. Ed. 2006, 45, 16331638.
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