The hydrogenation of alkadienes, Part II. The hydrogenation of buta-1,3-diene catalysed by rhodium, palladium, iridium, and platinum wires

Abstract

The hydrogenation of buta-1,3-diene has been studied by use of wires of rhodium (100–288°), palladium (90–224°), iridium (190–270°), and platinum (132–336°) as catalysts. Butene compositions were dependent upon temperature, but independent of activity, initial hydrogen pressure, and conversion over the range 0–50%. Butane yields were zero (Pd) or in the range 1–15%(Rh ⩽ Pt < Ir under comparable conditions). A less extensive study has been made with palladium films (20–206°).

But-1-ene was normally the major product. For both palladium and platinum the temperature-dependence of the but-1-ene yield exhibited a minimum, the position of which may be related to the concentration of special sites or defects in the metal. The trans: cis ratio in the but-2-ene was 1·0–1·3 (Ir, Pt), or 2·0–2·3 (Rh), and was independent of temperature. Over palladium the ratio decreased markedly with increasing temperature (14·4 at 38°, 2·6 at 190° for the thin film). This decreasing ratio, and a concomitant rise in the but-1-ene yield under certain conditions, indicate that chemisorption of buta-1,3-diene by the interaction of only one double bond with the surface becomes increasingly important as temperature is raised.

The hydrogenation activity of clean iridium wire was replaced by activity for hydrogenolysis when the metal surface became coated with carbon residues. Methane was the sole product, at 245°, of the hydrogenolysis of but-1-ene, pent-1-ene, buta-1,3-diene, penta-1,3-diene, pent-2-yne, and benzene.

Product distributions reported are compared with those reported earlier for alumina-supported metals. Reaction mechanisms appear to be characteristic of the metal and are not greatly modified by a change in the physical form of the catalyst from polycrystalline supported metal to bulk metal.