Homogeneous catalysis in gas phase reactions.

Abstract

The gas-phase, thermal decomposition of two chlorinated compounds -1,1,2-trichloroethane and ethylene chlorohydrin - have been investigated in an all-glass static reactor system at temperatures between 590°C and 484°C. The decompositions were followed both by pressure change and by detailed product analysis. Both compounds exhibit two modes of breakdown. 1,1,2-trichloroethane decomposes into 1,1-dichloroethylene or 1,2-dichloroethylene with elimination of HC1, whilst ethylene chlorohydrin undergoes dehydration to vinyl chloride or dehydrochlorination to acetaldehyde. The effects of chain accelerators and chain inhibitors on these decompositions have been studied. The experimental evidence points to the existence of two separate modes of decomposition for each substrate - namely a radical chain mode and a molecular mode. The detailed nature of these mechanisms is discussed in the light of the experimental results. The homogeneous gas phase catalysis of these decompositions is also described. The choice of the gas phase catalysts was made on the basis of the currently accepted theories of radical chain and molecular reactions. The effect of these additions on the overall rate and, in particular, on the selectivity of these decompositions was investigated. Changes in the selectivity of the chain processes were observed with BBr and HgS addition. These are discussed with reference to the existing knowledge regarding selectivity. The effect of Lewis acid and base additives on the molecular modes of decomposition was shown to be small and is compared to the much greater effects observed in analogous liquid phase and heterogeneous systems. A mathematical model of the decomposition of ethylene chlorohydrin has been formulated. The decomposition of ethylene chlorohydrin alone was simplified to 29 elementary reactions. The solution of this scheme involved the use of an IBM 360-50 computer and gave a numerical rather than an analytical result for the formation of the various products which were observed in the decomposition.