Hydrogen Species, Vacancies, and Alkadiene Hydrogenation Selectivity on Copper-Based Hydrogen Reservoirs

Abstract

Hydrogenation of 2-methylbuta-l,3-diene (isoprene) under helium flow in the absence of gaseous hydrogen is used to provide evidence for reactive hydrogen species (H*) present in copper-based oxides (CuCrO, CuAlO, CuZnO). These compounds are hydrogen reservoirs with marked diffusion properties for hydrogen species (H*, OH−). Moreover, analysis of the distribution of hydrogenated products obtained under H2 + isoprene in relation to the solid structure of the catalyst leads to the proposition of particular active sites, XM-YM′. These are constituted of two cations: XM is the 3-coordinatively unsaturated Cu+ ion and YM′ is a second cation (Cr3+, Al3+, Zn2+) with Y unsaturations of coordination. Under H2 a high degree of unsaturation can exist (3M-3M′), whereas under helium a change in the selectivity of hydrogenated products is observed that is attributed to the variation of Y (3 down to 0). This phenomenon corresponds to a progressive blocking of the surface unsaturations, probably due to the migration of OH− groups.