Hydrogen production from ethanol catalysed by Group 8 metal complexes

Different strategies for the catalytic thermal production of hydrogen from ethanol are discussed and demonstrated using various Group 8 metal catalysts, in the presence of added base. Where the metal has a low affinity for carbon monoxide, e.g. in [PtH(PEt3)3]+, simple dehydrogenation of ethanol to ethanal and its aldol condensation products is observed. When the metal has a high affinity for CO, CO abstraction from the formed ethanol occurs and, as in reactions catalysed by [RhCl(PPh3)3] or [RhH(PPri3)3], can poison the reaction. In some cases, the CO abstraction reaction can be used to promote the thermodynamically favourable reaction of formation of hydrogen, methane, and carbon monoxide; although irradiation with visible light is often required to release the carbon monoxide from the metal centre {e.g. [RhH(CO)(PPri3)2] in the absence of base}. Finally, in catalytic reactions carried out in the presence of base, water–gas shift type chemistry is observed in reactions catalysed by [Rh(bipy)2]Cl, so that ethanol can be converted into 2H2, CH4, and CO2. In the cases of [Rh(bipy)2]Cl and [RuH2(N2)(PPh3)3], rates of hydrogen production of > 100 catalyst turnovers h–1, corresponding to > 1 l per litre of catalyst solution per hour can readily be sustained over long periods. The role of base in, and the mechanisms of, these interesting reactions are discussed; as are synergistic effects and reasons for the success of [Rh( bipy)2]Cl and [RuH2(N2)(PPh3)3] as catalysts for hydrogen production.