Rhodium-catalysed hydrogenation of allene as revealed by [14C]propylene and [14C]carbon monoxide tracer studies

Abstract

The low-pressure hydrogenation of allene has been studied over alumina-supported rhodium catalysts. During a series of hydrogenation reactions the activity of the catalyst progressively decreases to a steady-state value and thereafter remains constant. The reaction proceeds in two distinct stages. During the first stage the selectivity for the formation of propylene is ca. 95%.

Hydrogenation of allene+[¹⁴C]propylene mixtures shows that, in the first stage of the allene hydrogenation, the yield of propane from the hydrogenation of propylene is relatively small. Direct hydrogenation of adsorbed allene to propane is the major route to formation of the latter, the selectivity being a measure of the relative rates of hydrogenation of allene directly to propylene and propane.

Adsorption of [¹⁴C]propylene on freshly reduced catalysts occurs in two distinct stages: a non-linear primary region followed by a linear secondary region. No primary region is observed for propylene adsorption on steady-state catalysts or on freshly reduced catalysts in the presence of allene. However, [¹⁴C]propylene adsorption and hydrogenation occurs in the presence of allene on the secondary region with both freshly reduced and steady-state catalysts.

Adsorption of [¹⁴C]carbon monoxide shows that, whilst the decrease in activity of the catalyst to a steady-state constant value corresponds to the progressive build-up of a surface hydrocarbonaceous layer, the combined effects of allene and hydrogen on a carbon monoxide-precovered surface leads to an increase in the capacity of that surface for carbon monoxide adsorption. Treatment of the carbon-monoxide-precovered surface with hydrogen alone does not lead to such an increase. It is suggested that, under the influence of the allene hydrogenation reaction, the surface undergoes some reconstruction.

Evidence is presented to show the presence of separate surface sites for the hydrogenation of allene to propylene, the hydrogenation of allene to propane and for the hydrogenation of propylene to propane.