Hydrogen transfer reactions relevant to Guerbet coupling of alcohols over hydroxyapatite and magnesium oxide catalysts

Abstract

Hydrogenation and dehydrogenation reactions were performed over hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAP) and magnesia (MgO) to explore their role in the reaction network for the Guerbet coupling of ethanol to butanol. In particular, the dehydrogenation of benzyl alcohol at 633 K and the hydrogenation of ethene and acetone at 473 K using both H₂ and ethanol as a hydrogen source were studied. The H₂–D₂ exchange reaction at room temperature and the Guerbet coupling of ethanol at 613–673 K in the presence of D₂ were also performed. Although there was no consequence of adding D₂ to the Guerbet coupling of ethanol in terms of rate or selectivity, incorporation of deuterium into product butanol was only observed over MgO. This was attributed to the rapid exchange of H₂–D₂ that can occur over MgO but not over HAP. Hydrogenation of acetone occurred with ethanol as a sacrificial hydrogen donor via an MPV-like reaction whereas hydrogenation with H₂ was not observed. Hydrogenation of ethene with H₂ or ethanol was not observed above background. Comparing the rate of benzyl alcohol dehydrogenation to the rate of ethanol coupling over HAP and MgO suggests that the MPV-like hydrogen transfer reaction over HAP is mostly responsible for generating intermediate acetaldehyde during the Guerbet reaction instead of direct dehydrogenation.