Kinetic isotope effect in the selective oxidation of methanol to formaldehyde over some molybdate catalysts

Kinetic isotope effects were measured using fully deuterated methanol, CH3OD, and CD3OD. Four catalysts were tested: a commercial preparation containing a 3:1 molar ratio of MoO3 and Fe2(MoO4)3, pure MoO3, pure Fe2(MoO4)3, and FeMoO4. Surface areas varied from 2.3 m2/g for MoO3 to 6.1 m2/g for Fe(MoO4)3. 3 g of catalyst were used in a differential reactor with external recycle. Mass and heat transfer were rapid compared to the rate of the reaction. The feed gas contained 2 to 20% methanol, 5 to 40% oxygen, and the balance nitrogen. Differential rates were obtained under steady state conditions over a wide range of conversions. The rate constants were obtained by fitting the data to a rate equation of the power law. Results show that the rate-limiting step with all four catalysts is the abstraction of a hydrogen from the methyl group. This fact is confirmed by changes in product distribution. CH3OH and CH3OD yield identical product distributions, but with CD3OD the selectivity to dimethyl ether increases by a factor of 4 to 6