Acceptor-free dehydrogenation of secondary alcohols by heterogeneous cooperative catalysis between Ni nanoparticles and acid–base sites of alumina supports

Nickel-nanoparticle-loaded θ-Al2O3 (Ni/θ-Al2O3), prepared by H2-reduction of NiO/θ-Al2O3, acts as an effective and reusable heterogeneous catalyst for acceptor-free dehydrogenation of alcohols in a liquid phase. Among various supports, amphoteric supports (such as θ-Al2O3), having both acidic and basic sites, gave higher activity than acidic or basic supports. Among Ni/θ-Al2O3 catalysts with different Ni particle sizes, turnover frequency (TOF) per surface Ni increases with decreasing Ni particle size. These results suggest that low-coordinated Ni0 sites and metal/support interfaces play important roles in the catalytic cycle. The reaction mechanism is investigated by in situ IR study combined with kinetic analysis (kinetic isotope effect), and the following mechanism is proposed: (1) reaction of an alcohol with Lewis acid (Alδ+)–base (Alsingle bondOδ−) pair site of alumina yields an alkoxide on the Alδ+ site and a proton on the Alsingle bondOδ− site, (2) Csingle bondH dissociation of the alkoxide by Ni0 site to form Nisingle bondH and a ketone, and (3) protolysis of Nisingle bondH by a neighboring proton to release H2 gas. The proposed mechanism provides fundamental reasons for the higher activity of Ni on the acid–base bifunctional support (Al2O3) than on basic and acidic ones.