We highlight the evolution and tunability of catalytic function of transition metal carbides under oxidative and reductive environments for selective deoxygenation reactions.

High aromatics yield (>90%, benzene and toluene) was obtained from vapor phase hydrodeoxygenation (HDO) of phenolic compound mixtures containing m-cresol, anisole, 1,2-dimethoxybenzene, and guaiacol over molybdenum carbide catalysts (Mo2C) under atmospheric pressure at 533-553 K, even with H2 to phenolic compound molar ratios ~3,300. Toluene selectivity increased proportionately (4%-66%) to m-cresol content in HDO of phenolic compound mixtures (molar composition: 0%-70%) at quantitative conversion. Phenol selectivity increased with decreasing conversion, implying that the aryl-methoxyl bond in guaiacol is cleaved first, before the aryl-hydroxyl bond. Low selectivity to cyclohexane and methylcyclohexane (<10%) across the conversions investigated (18-94%) demonstrates that undesired successive hydrogenation reactions of aromatics over Mo2C were inhibited, presumably due to in situ oxygen modification, as inferred from titration studies of aromatic hydrogenation reactions using methanol and water as titrants.

The overall chloriding effectiveness factor (Z*), defined as the ratio of ethyl chloride concentration in parts per million to the sum of ethylene and ethane concentration in mole percent multiplied by a weighting factor to account for their efficacy in removing chlorine-adatoms from the surface, was used as a parameter to account for the effects of chlorine on the kinetics of ethylene epoxidation on a highly promoted 35 wt % Ag/α-Al O catalyst. An increase in O order (≈0.7 to 1) and a decrease in C H order (≈0.5 to <0) with increasing Z* (Z*=2.1, 3.4, 5.2, and 8.9) was observed implicating kinetic relevance of O activation on chloride-promoted silver catalysts. Carbon dioxide co-feed (1-5 mol %) was found to promote ethylene oxide selectivity as CO co-feed reversibly inhibits CO synthesis rates (-0.6 order) more than ethylene oxide synthesis rates (-0.49 order) at all Z* values. Ethylene oxide and CO rates were found to be invariant with ethylene oxide (0-0.5 mol %) and acetaldehyde (0-1.7 ppm) co-feeds, suggesting that there is minimal product inhibition under reaction conditions. A model involving a common reaction intermediate for ethylene oxide and carbon dioxide synthesis and two types of atomically adsorbed oxygen species-nucleophilic and electrophilic oxygen-is proposed to plausibly describe the observed reaction rate dependencies and selectivity trends as a function of the chloriding effectiveness.

Mesoporous metal carbides are of particular interest as catalysts for a variety of reactions because of their high surface areas, porous networks, nanosized walls, and unique electronic structures. Here, two ordered mesoporous metal carbides, Mo2C and W2C, were synthesized using a nanocasting approach coupled with a simultaneous decomposition/carburization process under a continuous methane flow. The as-synthesized mesoporous Mo2C and W2C have three dimensionally ordered porous structures, large surface areas (70–90 m2 g–1), and crystalline walls. In vapor-phase anisole hydrodeoxygenation (HDO) reactions, they exhibited turnover frequencies of approximately 9 and 2 × 10–4 mol molCO –1 s–1, respectively, at relatively low reaction temperatures (423–443 K) and ambient hydrogen pressures. Notably, the ordered mesoporous W2C catalyst showed a greater than 96% benzene selectivity in anisole HDO, the highest benzene selectivity reported to date.

C 4 −C 6 olefin β-scission rate constants were inferred from experimental studies at 773−813 K and <15% conversion by considering every C 4 −C 6 olefin isomer and all available β-scission modes: 2°to 2°(C), 1°to 3°a nd 3°to 1°(E), 1°to 2°and 2°to 1°(D), and 1°to 1°(F). Group contribution methods were implemented to assess adsorption enthalpies and entropies of C 4 −C 6 olefin isomers on H-ZSM-5 via the development of group correction terms for surface alkoxides; a linear dependence of enthalpy (or entropy) of formation difference between a surface alkoxide and a gas-phase alkane on carbon number was considered. Tertiary alkoxides have the smallest adsorption constants among surface adsorbates, and the resulting low coverage of highly substituted alkoxides restricts their contribution to alkene cracking pathways. Intrinsic β-scission rate constants (k E :k C :k D :k F ratio of 1094:21:8:1 at 783 K) and activation energies (E inE < E inC < E inD < E inF ) from experimentally observed effluent compositions of C 4 −C 6 olefin cracking consistent with computational studies were derived after rigorously accounting for adsorption constants and surface coverages of each C 4 −C 6 olefin isomer. These results demonstrate that shape selectivity constraints prevent equilibration of surface alkoxides on surfaces under reaction conditions relevant for alkene cracking and present a quantitative description of C−C bond cracking reactions of olefins catalyzed by solid acids.