Direct synthesis of n-butanol from ethanol over nonstoichiometric hydroxyapatite

n-Butanol is an important industrial chemical usually produced by the oxo process, an expensive, energy-consuming set of reactions over metal catalysts, using petrochemical raw materials at high pressure. We developed nonstoichiometric hydroxyapatite (HAP), a highly active calcium phosphate compound and found it catalyzed selective conversion of ethanol to n-butanol in a single reaction at atmospheric pressure and low temperature, with maximum selectivity of 76%. Higher alcohols were also formed. We postulate that ethanol is adsorbed and activated on HAP as CH3CH2OH(a) and that a C-C bond was formed between beta-C in the CH3CH2OH(a) and alpha-C in n-CnH2n+1OH to produce n-CnH2n+1CH2CH2OH. We further postulate that, by successive propagation, part of this n-CnH2n+1CH2CH2OH is then adsorbed and activated on HAP as n-CnH2n+1CH2CH2OH(a) and that C-C bond was formed between beta-C in the n-CnH2n+1CHCH2OH(a) and alpha-C in n-alcohol to produce branched alcohols. Reaction simulation supported this hypothesis, suggesting that efficient, environmentally friendly production of n-butanol might be possible in future using bioethanol as raw material.