Insights into reaction mechanisms of catalytic hydrogen production from alcohols : a density functional theory study

Abstract

Securing the world's clean energy future in the form of sustainable H₂ generation is a key challenge. Alcohols (and eventually carbohydrates from bio-waste) are potential carriers for H₂ storage, from which H₂ needs to be liberated catalytically. Morton and Cole-Hamilton have presented a classic catalytic system for dehydrogenation and decarbonylation of primary alcohols including methanol (Morton and Cole-Hamilton et al. 1988). A water gas shift reaction (WGSR) would allow the liberation of another equivalent of H₂. We now present a Density Functional Theory (DFT) investigation, using the B97-D dispersion-corrected functional, to probe the viability of such a pathway and to help design a system that can catalyse all processes, including dehydrogenation, decarbonylation and WGSR, equally well. Two different catalytic cycles that depart from those in molecular hydrogen production under basic conditions catalysed by [RuH₂(CO)(PPh₃)₃] (22) have been studied, the first involving a WGSR and the second covers the formation of gem-diol-(ate), formic acid and finally the CO₂ elimination. The overall computed energy barrier for the attack of water (in the form of OH⁻) on the CO ligand of complex 22 is prohibitively high because this attack is predicted to be highly endergonic; therefore, a WGSR is not plausible with this species. An alternative catalytic cycle for this process has been characterised computationally, starting from the formaldehyde complex (35) through the formation of a gem-diolate species, subsequently forming formic acid and, finally, CO₂. The gem-diolate pathway has insurmountable barriers, too. Another catalytic cycle involving complex [RuH₂(CO)₂(PPh₃)₂] (58) is characterised as a result of a second decarbonylation reaction where the attack of OH⁻ on the CO ligand is reasonable. Complete dehydrogenation of alcohols could thus be possible under basic conditions.