Reactions of ethene over silica, <img src='/image/spc_char/gamma2.gif' border=0>-alumina, YBa₂Cu₃O_6.9 perovskite and ZSM-5 zeolite: Thermal versus catalytic processes

Abstract

The reaction products formed during the flow of ethene at temperatures above 500°C over silica, YBa₂Cu₃O_6.9 perovskite and through an empty quartz reactor have been found to be similar to those obtained during its reaction over ZSM-5 zeolite and <img src='/image/spc_char/gamma2.gif' border=0>-alumina samples at lower temperatures. The prominent reaction products consisted of methane, propane, butane, propene, butene, pentene, methylpentane, cyclohexene, hexene, benzene and toluene. The product distribution, however, varies with the nature of the sample and the reaction temperature. The addition of oxygen in ethene leads to higher yields of almost all the reaction products, particularly at sample temperatures above 500°C. The enhancement increases progressively with the increase in oxygen content, the effect being more pronounced on the lower hydrocarbons containing odd number of carbon atoms, e.g., propene and pentene. It is suggested that at reaction temperatures above 500°C, the free radicals such as CH₃, CH₂ and CH, formed during thermal dissociation or self hydrogenation of C₂H₄ molecules, help in the hydrocarbon chain growth. The formation of the highly unstable epoxy species, viz. <img src='http://www.niscair.res.in/jinfo/equation41.gif' border=0> in the presence of oxygen facilitates this process. In the case of catalytic reactions over ZSM-5 or <img src='/image/spc_char/gamma2.gif' border=0>-alumina, the lewis acid sites rather than the Bronsted hydroxyl groups are proposed to be the reaction sites where ethene adsorbed in the form of a cyclic complex transforms to the similar transient surface species at lower temperatures.