Carbon monoxide hydrogenation selectivity of catalysts derived from ruthenium clusters on acidic pillared clay and basic layered double-hydroxide supports

Abstract

Acidic pillared clays, e.g. alumina pillared montmorillonite (APM), and basic layered double hydroxides, e.g. hydrotalcite (HT), provide well defined surface environments for dispersing metal-cluster carbonyl complexes. In the present work, FTIR spectroscopic studies have been used to elucidate the surface organometallic chemistry of Ru₃(CO)₁₂ on APM and HT. For APM as the support, cluster binding occurs initially by protonation to form H Ru₃(CO)₁₂⁺ cations on the intracrystalline gallery surfaces of the clay. Further reaction results in the grafting of mononuclear sites of the type [Ru(CO)_x(OAl)₂]_n(x= 2, 3) to the pillar surfaces. The reaction of Ru₃(CO)₁₂ with HT affords chemisorbed H Ru₃(CO)₁₁^– anions which can be tranformed to surface-bound [Ru(CO)_x(OM)₂]_n(M = Al, Mg) complexes analogous to the grafted species on APM. The reduction of the grafted complex on both supports results in active ruthenium catalysts for CO hydrogenation. Ru-APM exhibits very high selectivity for isomerized hydrocarbons (branched alkanes and internal alkenes). The isomerized products arise from the unique texture and bifunctional nature of Ru-APM; the clay-embedded ruthenium catalyses Fischer–Tropsch chain propagation, and the intracrystalline Brønsted acidity of the clay host catalyses alkene rearrangements through carbenium-ion mechanisms. In contrast, the Ru-HT system gives very different product distributions containing a high fraction of oxygenates, specifically methanol and lesser amounts of C₂–C₄ alcohols. The high alcohol selectivity, which is atypical for CO hydrogenation over Ru, is ascribed in part to the inhibition of CO dissociation on the metal particles by decoraments provided by the highly basic support.