Multiple Promotional Effects of Vanadium Oxide on Boron Nitride for Oxidative Dehydrogenation of Propane

Featuring high olefin selectivity, hexagonal boron nitride (h-BN) has emerged recently as an attractive catalyst for oxidative dehydrogenation of propane (ODHP). Herein, we report that dispersion of vanadium oxide onto BN facilitates the oxyfunctionalization of BN to generate more BOx active sites to catalyze ODHP via the Eley–Rideal mechanism and concurrently produce nitric oxide to initiate additional gas-phase radical chemistry and to introduce redox VOx sites to catalyze ODHP via the Mars–van Krevelen mechanism, all of which promote the catalytic performance of BN for ODHP. As a result, loading 0.5 wt % V onto BN has doubled the yield of light alkene (C2–C3) at 540–580 °C, and adding an appropriate concentration of NO in the reactants further enhances the catalytic performance. These results provide a potential strategy for developing efficient h-BN-based catalysts through coupling gas-phase and surface reactions for the ODHP process.


Mass transfer limitation
The absence of mass transport resistances was checked by a) Weisz-Prater criterion (CWP) for internal diffusion and b) Mears' criterion (CM) for external diffusion. 1-2 a) If < 1, then internal mass transfer effects can be neglected.
[b] Taking the average mesh size 70 as an example.
Applying corresponding values into the equation gave = 7.1×10 -5 < 1. Therefore, the internal diffusion effect can be neglected. b) If < 0.15, then external mass transfer effects can be neglected.

Term Physical meaning Unit Value
Observed reaction rate [a] kmol kgcat -1 s -1 2.51×10 - 6 Bulk density of catalyst bed kg m -3 1127 Catalyst particle radius [ [a] Taking the highest C3H8 conversion attained on 0.5V/BN-T at 580 o C as the example.
[b] Taking the average mesh size 70 as an example.

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[c] As reported, the O2-ODHP proceeds through ER mechanism and MvK mechanism on BN and VOx, respectively. On BN, the reaction order is second-order dependent on the partial pressure of C3H8, while is first-order in the case for VOx. Therefore, both 1 and 2 were applied in the calculations.
Applying corresponding values into the equation gave CM = 4.5×10 -4 and 9.0×10 -4 for second-order and first-order reaction, respectively, both of which are much less than 0.15. Therefore, the external diffusion effect can be neglected.
c) The absence of heat transfer was checked by Mears' criterion.

Term Physical meaning Unit Value
Observed reaction rate [a] mol kgcat -1 s -1 2.51×10 -3 Bulk density of catalyst bed kg m -3 1127 Catalyst particle radius [ [a] Taking the highest C3H8 conversion attained on 0.5V/BN-T at 580 o C as the example.
[b] Taking the average mesh size 70 as an example.
Applying corresponding values into the equation gave 1.9×10 -4 < 0.15. Therefore, the heat transfer limitation can be neglected.

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The Raman spectrum of the spent 0.75V/BN-T, collected after O2-ODHP reaction, shows a new band at 882 cm -1 , which can be attributed to the borate species with hydroxylated nonring boron (B-OH) ( Figure 3B). 3 For comparison, BOx/SiO2 and VOx/BOx/SiO2 were prepared, and a similar band is observed on both fresh and spent catalysts, as shown in Figure S11.   Data reduction was performed using the Athena program from the Demeter software suite. 5 R space EXAFS spectra were produced taking the Fourier transform of k 2 weighed data over a k range of 3-8 Å -1 . XANES pre-edge analysis was performed using the XAS viewer GUI contained in the python package xraylarch. 6 The pre-edge peak was fit using constant width gaussians and the rising edge background was fit using a Lorentzian peak and a first order polynomial.

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Pre-edge peak areas were normalized based on the pre-edge peak area for NH4VO3, which is a V 5+ compound with a distorted tetrahedral structure. The centroid of the background subtracted pre-edge peak was calculated as the first moment of the distribution. Pre-edge peak centroids were normalized to the threshold energy for vanadium metal (5465 eV). Reported errors in normalized pre-edge peak area and pre-edge centroid are calculated from peak fits performed in xraylarch.
Results and discussion Figure S12 shows the normalized V K edge XANES for vanadium reference compounds.
In general, the pre-edge, shakedown transition energy, and main edge increase in energy with oxidation state. 7 The energy and shape of the above mentioned features also change with the coordination in compounds of the same oxidation state, which can be seen in the spectra of V2O3 and V(AcAc)3 or V2O5 and NH4VO3.   The catalyst in O2 alone and the one in C3H8/O2 have XANES spectra that almost completely overlap (Figure S13). This suggests that under reaction conditions the oxidation state of V is like that in the case of the O2-treated catalyst in the initial 30-min time-on-stream (TOS).
The catalyst treated in C3H8 alone shows a strong decrease in the intensity of the pre-edge peak and a shift to lower energy. This demonstrates a decrease in the oxidation state of vanadium in the catalyst. A subsequent treatment of C3H8-treated catalyst in O2 returns the vanadium to the more S21 oxidized state, largely matching that of the initially O2-treated catalyst. Clearly, the redox cycles present under the ODHP reaction.  For comparison, a blank test was conducted at 600 o C, and no products are observed in the gas phase, except the fraction of propane ( Figure S15A). Also, a reference spectrum was acquired at room temperature on 1.0V/BN-T prior to temperature elevation. The resultant spectrum confirms that no products exist in the gas phase ( Figure S15B). These observations imply that the presence of BN-based catalysts contribute to the all detected species in gas phase on both BN-T and 1.0V/BN-T under reaction conditions derive from the catalyst.

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performed on quartz wool-filled bed at the temperature range of 480-600 o C. 5% C3H8/He and 5% O2/He were used with a C3H8/O2 ratio of 1. Additional NO was introduced by using 0.1% NO/He, and the concentration was 350 ppm. The overall GHSV was fixed at 24000 mL g -1 h -1 . The activity performance was shown in Figure S16. Although the NO-induced enhancement in activity is significant, VOx/BN-T catalysts still show higher activity under the same reaction conditions, except at 600 o C where the gas-phase reaction dominates.