Promoting effect of unreducible metal doping on OMS-2 catalysts for gas-phase selective oxidation of ethanol

Abstract

Various unreducible-metal cations can be successfully doped into the tunnel of manganese octahedral molecular sieve (OMS-2) by using “one-pot” refluxing method to tune the acid-base and redox properties. The resulting M-OMS-2 materials were used as efficient catalysts for gas-phase aerobic oxidation of ethanol to acetaldehyde. The activity of the M-OMS-2 catalysts was demonstrated to be proportional to the surface basicity. Na-OMS-2 exhibited the highest catalytic activity at 200 °C, which is higher than and comparable to the previously reported preferred Fe-OMS-2 and Ag-OMS-2 catalysts, respectively. The beneficial effect of Na+-doping is related to the enhancement of surface oxygen vacancies, basicity and reducibility, which also result in better catalytic stability. Kinetic studies indicate that reaction rates depend weakly on the concentration of O2 and ethanol. Ethanol OH and α-CH bond cleavages are both kinetically-relevant, with the latter being the more difficult step. O2 is the most efficient oxidant and is thought to be activated at oxygen vacancies of Mn2+/Mn3+ species to form superoxide-type O2− and lattice-oxygen-type O2− species, which act as dual active sites for the rate-controlling ethanol activation steps.