Roles of Enhancement of C−H Activation and Diminution of C−O Formation Within M1‐Phase Pores in Propane Selective Oxidation

Abstract

Propane and propene oxidations on M1 phase MoVTeNb mixed oxide catalysts exhibit relatively high selectivity to acrolein and acrylic acid. We probe the ability of the reactant molecules to access the catalytic sites inside the heptagonal pores of these oxides and analyze elementary steps that limit selectivity. Measured propane/cyclohexane activation rate ratios on MoVTeNbO are nearly an order of magnitude higher than non‐microporous VOx/SiO2, which suggests significant contribution of M1 phase pores to propane activation because both molecules react via homologous rate‐limiting C−H activation. Density functional theory suggests that desired C3H8 dehydrogenation and C3H6 allylic oxidation to acrolein and acrylic acid are limited by C−H activation steps, while less valuable oxygenates form via steps limited by C−O bond formation. Calculated activation barriers for C−O formation are invariably higher than C−H activation when these activations occur inside the pores, suggesting that reactions restricted within the pores are highly selective to desired products. These results demonstrate the role of pore confinement and a framework to assess selectivity limitation in hydrocarbon oxidations involving a complex network of sequential and parallel steps.

Nb−V Mixed Oxide with a Random Assembly of Pentagonal Units: A Catalyst for Oxidative Dehydrogenation of Ethane and Propane

“…This situation leads to comparable reaction rates in ODHE and ODHP despite the different CÀ H binding energies of the alkanes, because propane has some difficulty in entering the heptagonal channel for oxidation. [41][42] In the case of HDS-8Nb4V, although ethane might be allowed to enter the heptagonal channel as easily as in the above-mentioned catalysts, ethane would not be converted inside the channel, because the heptagonal channel in HDS-8Nb4V rarely participates in the redox reaction during ethane oxidation, because M 6 + À OÀ V 4 + is unlikely to be formed inside the channel. Therefore, the reaction rate of alkanes over HDS-8Nb4V is followed in BDE of the alkane.…”

“…In accordance with these reports, HDS‐MoVO and HDS‐WVO are considered to catalyze ethane oxidation by the lattice oxygen of M 6+ −O−V 4+ inside the A‐7MR heptagonal channels, in a manner similar to that with C−MoVOs, resulting in extremely high catalytic activity for ethane oxidation. This situation leads to comparable reaction rates in ODHE and ODHP despite the different C−H binding energies of the alkanes, because propane has some difficulty in entering the heptagonal channel for oxidation [41–42] . In the case of HDS‐ 8 Nb 4 V, although ethane might be allowed to enter the heptagonal channel as easily as in the above‐mentioned catalysts, ethane would not be converted inside the channel, because the heptagonal channel in HDS‐ 8 Nb 4 V rarely participates in the redox reaction during ethane oxidation, because M 6+ −O−V 4+ is unlikely to be formed inside the channel.…”

Effects of surface species and homogeneous reactions on rates and selectivity in ethane oxidation on oxide catalysts

“…Based on these considerations, enthalpy and entropy estimates were obtained by replacing contributions from harmonic frequencies lower than 90 cm −1 with a fraction (70%) of average translational and rotational contribution per mode from relevant gas‐phase molecules. Illustrative examples or such replacement are provided in our recent work 9 …”

“…Oxidative conversions of hydrocarbons and oxygenates on reducible transition metal oxide catalysts are employed in large industrial processes for the production of valuable chemicals, and are of significant interest as alternatives to other current processes 1–10 . Ethane (C 2 H 6 ) oxidative dehydrogenation, in particular, has attracted attention for its potential utilization of shale gas and production of ethylene, which serves as a key building block for the chemical industry 1,11–13 .…”

“…Oxidative conversions of hydrocarbons and oxygenates on reducible transition metal oxide catalysts are employed in large industrial processes for the production of valuable chemicals, and are of significant interest as alternatives to other current processes. [1][2][3][4][5][6][7][8][9][10] Ethane (C 2 H 6 ) oxidative dehydrogenation, in particular, has attracted attention for its potential utilization of shale gas and production of ethylene, which serves as a key building block for the chemical industry. 1,[11][12][13] Such oxidative conversions on oxides typically involve a rate-limiting C-H activation step, [14][15][16][17][18] but the species formed in this activation can undergo parallel reactions to desired and undesired products and primary products undergo further secondary reactions.…”