Partial oxidation of methane over halogen modified palladium catalyst.

Abstract

The vapor phase air oxidation of methane to formaldehyde was investigated in an isothermal integral flow reactor at atmospheric pressure in a temperature range of 390° to 510°C. The catalyst, 0.5% palladium supported on alumina, was modified by a constant and continuous supply of small amounts of chlorine, bromine and iodine compounds. The effect of various process variables, namely the weight ratio of halogen additive to methane, the feed ratio of air to methane, reaction temperature and the reciprocal of space velocity, on the conversion of methane and the product distribution was determined. The products and reactants were analysed by gas chromatography. It was observed that in the presence of halogen compounds, though the overall conversion of methane decreased, the selectivity of the catalyst for formaldehyde formation significantly increased. The effect of different halogen compounds are compared and discussed. Methylene chloride, which affected in producing relatively high yields of formaldehyde, was used as a modifier for the detailed kinetic study of methane oxidation reaction. The weight ratio of modifier additive to methane in the feed mixture at a given temperature was found to be an important variable in the selectivity of the catalyst. It is suggested that the charged methane and oxygen are selectively adsorbed on the catalyst surface and that the partial oxidation of methane is a p-type reaction and its further oxidation to carbon dioxide is an n-type reaction under the reaction conditions. The results indicate that the promotional effect of the modifier, in the sense of increasing the catalyst selectivity, is mainly due to its successfully suppressing further oxidation of aldehyde to undesired products. The active phase of the catalyst in selective oxidation of methane was found to be palladium oxide. Among the several mechanisms proposed for the oxidation of methane over chlorine modified palladium catalyst, the experimental data were found to be in best correlation with a mechanism involving the surface reaction between charged adsorbed methane and oxygen with surface reaction being the rate controlling step. The rate of reaction is given by the following expression: r=ksKMPM 1+KMPM+KFPF ˙KOPO1+KO PO where ks, KM, KO and K F are temperature dependent constants.