Catalytic transfer hydrogenation on a hydrogen-absorbing alloy (CaNi₅) using hydriding–dehydriding properties

Abstract

By using the characteristics of a hydrogen-absorbing alloy, the hydrogen produced by catalytic dehydrogenation of saturated compounds can be absorbed to form metal hydrides, and, vice versa, the resulting metal hydrides are able to hydrogenate efficiently unsaturated compounds upon dehydriding. Gas-phase reactions between 2-butene and 2-propanol on a hydrogen-absorbing alloy CaNi₅ have been studied in the temperature range of 393–473 K. CaNi₅ showed interesting characteristics as an active catalyst for the catalytic transfer hydrogenation of butene from propanol as a hydrogen donor. 2-propanol was effectively dehydrogenated at 423 K to yield acetone in which the dissociated hydrogen was completely absorbed by CaNi₅ to form the metal hydride. When the alloy was hydrided to some extent, butene was hydrogenated by the absorbed hydrogen in the metal hydride to produce butane. The overall reaction on CaNi₅ was expressed as catalytic transfer hydrogenation of 2-butene from 2-propanol through intermediate formation of metal hydrides, rather than the direct reaction between butene and propanol on the alloy. Thus, CaNi₅ effectively repeated hydriding–dehydriding cycles: hydriding of CaNi₅ by 2-propanol dehydrogenation with subsequent dehydriding for the hydrogenation of 2-butene. The use of hydrogen-absorbing CaNi₅ provides a novel reaction system for the catalytic transfer hydrogenation.