Kinetics of the gas-phase oxidation of methanol catalysed by hydrogen bromide

Abstract

The homogeneous, gas-phase oxidation of methanol, catalysed by small amounts of hydrogen bromide, has been studied in a boric acid coated vessel at 310°. Under these conditions no reaction takes place in the absence of hydrogen bromide. The kinetics of the reaction and the rate of accumulation of formaldehyde in the products are compared with previously published data on the nitric oxide catalysed reaction at 310° and the uncatalysed reaction at 390°, i.e. at comparable rates of oxidation. The kinetics of the reaction were studied by means of pressure-time curves, and these were found to be of a similar shape to those of the uncatalysed reaction at 390°, and the nitric oxide catalysed reaction at 310°. The maximum rate was increased by the addition of "inert" gas. This rate varied as the methanol and hydrogen bromide pressures raised to the powers 0.7 and 1.3 respectively. On the other hand, increase in the oxygen pressure inhibited the maximum rate. The overall activation energy was 27 kcal mole^-1. These kinetic data are similar to those of the nitric oxide catalysed reaction but differ markedly from those of the uncatalysed process at 390°. Under similar conditions, 15 mmHg hydrogen bromide were required to give a rate approximately equal to that obtained when using 2 mmHg nitric oxide. The maximum pressure of formaldehyde in the products was only about one-tenth of that obtained under similar conditions in the other two oxidations.