Hydrogenation of acetylene over supported metal catalysts. Part 4.—[14C]tracer studies of the reaction catalysed by nickel–silica

Abstract

The hydrogenation of acetylene, at low pressure, has been studied over a 25.4% w/w nickel-on-silica catalyst at 293 K. Under the conditions used, the acetylene was hydrogenated to ethylene and ethane, with the selectivity for ethylene formation being 76%. Some n-butane was also formed during the later stages of the reaction.

Adsorption of [¹⁴C]acetylene on the catalyst, in the absence of hydrogen, showed two regions in the adsorption ‘isotherm’. The species adsorbed in the primary region, identified from [¹⁴C]carbon monoxide adsorption studies as being hydrocarbon adsorbed directly on the metal, was not directly involved in the catalysis. The extent of this adsorption diminished as the catalyst was brought to its steady-state activity, when the rate of reaction was approximately one-sixth its initial value, by successive hydrogenation reactions. Hydrogenation of [¹⁴C]acetylene showed that during the course of the reaction, the acetylene surface coverage passed through a maximum, which was attained at the point corresponding to the acceleration point in the pressure-fall–time curve for the reaction.

Hydrogenation of [¹²C]acetylene +[¹⁴C]ethylene mixtures showed that the predominant reaction pathway to ethane formation was a direct route from acetylene, rather than via ethylene as an intermediate [graphic omitted] values of k₁= 0.089, k₂= 0.0042 and k₃= 0.0163 being evaluated.

The results are interpreted in terms of four types of sites on the catalyst surface which are responsible for acetylene self-hydrogenation, conversion of acetylene to ethylene, ethane production from acetylene and the hydrogenation of ethylene to ethane. These sites also account for the selectivity of the catalyst in acetylene hydrogenation.