Kinetics and Mechanism of Ethane Oxidation to Acetic Acid on Catalysts Based on Mo-V-Nb Oxides

摘要

Kinetic and isotopic studies showed that C—H bond activation in ethane by surfaces essentially saturated with lattice oxygens is the sole kinetically relevant step in ethane oxidation on Mo—V—NbO_x mixed oxides. These conclusions are consistent with the dependence of oxidation rates on O2 and C2H6 pressures, with H/D exchange and kinetic isotope effects, and with the preferential initial incorporation of ~(16)O atoms from the oxide lattice into products formed from ~(18)O2—C2H6 mixtures. The precipitation of active components (Mo_(0.61)V_(0.31)Nb_(0.08)O_x) in the presence of colloidal TiO2 led to 10-fold increases in all rate constants (per active component), consistent with higher dispersion of active components resembling in structure and surface reactivity those prevalent in bulk powders. The concurrent presence of PdO_x cocatalyst, even as a separate solid, markedly increased all rate constants for oxidation of ethene intermediates and specifically that for ethene oxidation to acetaldehyde molecules, which are rapidly converted to acetic acid on active Mo—V—NbO_x sites. Water, whether formed as a byproduct or added with C2H6—O2 reactants, increases acetic acid selectivities by promoting the desorption of adsorbed acetate species as acetic acid. Ethene molecules, formed as reactive intermediates, inhibit ethane oxidation rates by depleting surface lattice oxygen atoms in fast oxidation reactions, thus decreasing the number of sites available at steady state for the kinetically relevant C—H bond activation step required for ethane conversion.