Kinetics of the partial oxidation of methanol over a Fe-Mo catalyst

Abstract

The intrinsic steady-state kinetics of the partial oxidation of methanol to formaldehyde over a commercial Fe-Mo catalyst has been studied experimentally in a differentially operated reactor at temperatures of 230–260 °C, over a wide range of methanol and oxygen concentrations. The principal products found were formaldehyde, water, dimethyl ether (DME) and dimethoxymethane (DMM). The kinetics of the formaldehyde formation from methanol could be well described with Langmuir–Hinshelwood kinetics, assuming two different metal oxide sites, one containing adsorbed oxygenates and the other one containing lattice oxygen. The presence of water vapor lowered the formaldehyde formation rate significantly, especially at lower water partial pressures. These results could be well explained in terms of competitive adsorption of water with methanol on the free active catalyst sites. For the most important side reactions, i.e. dimethyl ether formation as well as dimethoxymethane formation the forward reaction rates were determined from the selectivity data. The conversion rate of dimethyl ether to formaldehyde was also measured with separate experiments in the differential reactor. Carbon monoxide was not formed during the differential kinetic measurements of formaldehyde formation from methanol. Therefore, the rate of formaldehyde oxidation to CO was studied separately in a dual bed catalyst, where formaldehyde was formed in the first integral reactor at low temperatures and subsequently converted to CO in a differential reactor. The rate of CO formation was first order in formaldehyde and the oxygen dependency was the same as that for the formaldehyde formation from methanol. Rate expressions for all reactions were formulated based on the above assumptions and a multivariate Levenberg–Marquardt method was used to fit all the model constants to all the experiments for all reaction rates simultaneously, while additionally accounting for axial concentration profiles in the reactor. The observed influences of composition and temperature on the reaction rates could be well described.