KINETICS AND MECHANISMS OF THE PYROLYSIS OF DIMETHYL ETHER: III. THE REACTION ACCELERATED BY HYDROGEN SULPHIDE

Abstract

The thermal decomposition of dimethyl ether in the presence of hydrogen sulphide was studied in the temperature range of 480 to 530 °C, and over the pressure range of 5 to 500 mm of Hg. For a given pressure of dimethyl ether the rate of decomposition increased with the addition of increasing quantities of hydrogen sulphide, reaching a plateau after approximately 30% H2S had been added. The relative rate of decomposition then remained constant until more than 50% H2S had been added. Further increase in the hydrogen sulphide concentration produced a linear increase in the relative rate. In the region between 30 and 50% added H2S, the decomposition of dimethyl ether was three-halves order with respect to ether pressure, and zero order with respect to hydrogen sulphide pressure. In this region the three-halves-order rate constant can be expressed by k = 1.06 × 1014e−53,200/RT cc1/2 mole−1/2 sec−1. In the region beyond 50% added H2S, the reaction was first order with respect to dimethyl ether, and one-half order with respect to hydrogen sulphide. In this region the three-halves-order rate constant is given by k = 4.98 × 1014e−52,500/RT cc1/2 mole−1/2 sec−1. The experimental facts are shown to be consistent with a mechanism involving hydrosulphide radicals as the principal chain carriers. These HS radicals are produced mainly from the reaction of methyl radicals with hydrogen sulphide. The work leads to a value of 85.6 kcal for the dissociation energy of H2S into H + SH.