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) 2Cl, so that ethanol can be converted into 2H2, CH4, and CO2. In the cases of Rh (bipy) 2Cl 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) 2Cl and RuH2 (N2) (PPh3) 3 as catalysts for hydrogen production.