Abstract
The selectivity of platinum and rhodium in the hydrogenation of 1-butene, 1,3-butadiene and 1-butyne have been determined and compared with that of palladium. The experimental results are rationalized using a single reaction scheme accounting for primary parallel formation of 1-butene, 2-butenes and butane and consecutive formation of 2-butenes and butane from the 1-butene initially produced. Platinum and rhodium behave very similarly and are not very different from palladium for hydrogenations of 1-butyne and 1,3-butadiene except for the initial formation of butane: platinum and rhodium produce a lot of “initial” butane ( S BA pO) unlike palladium which is characterized by a selectivity index S BA p of zero. This particular behaviour of palladium is also apparent in the trans/cis ratio for 2-butenes produced from butadiene hydrogenation, this ratio being 3.5 for platinum and rhodium and 12 for palladium. Results from of our laboratory and from the literature on Group VIII metals, either as heterogeneous or homogeneous catalysts, show that the initial formation of butane increases while the trans/cis ratio decreases. It seems that this correlation is not by chance and we propose that butane is formed via a semi-hydrogenated species, originating from a syn-diadsorbed 1,3-butadiene, from which cis-2-butenes are formed.
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