CO2 promoted hydrogenolysis of benzylic compounds in methanol and water

Abstract

This study successfully demonstrated the hydrogenolysis of benzylic alcohols and their derivatives over a Pd/C catalyst by using CO₂-expanded methanol (CX-methanol) and compressed CO₂/water as two green reaction media. It was found that with the addition of low-pressure CO₂ (1 MPa), the reaction conversions of benzylic alcohols and their derivatives could all be increased in the CO₂ promoted systems. For example, the conversion of 1-phenylethanol could be elevated from 23% to 63% in CX-methanol and the conversion of benzyl alcohol could be elevated from 75% to 92% in compressed CO₂/water. The positive effects of CO₂ could be attributed to the decrease of mass transfer resistance and the increase of hydrogen solubility. CO₂ could also form methylcarbonic acid and carbonic acid in methanol and water, respectively. Therefore, it could enhance the departing ability of the protonated hydroxyl leaving groups by providing a more acidic environment. In addition, the hydrogenolysis (or deoxygenation) of aromatic aldehyde and ketone by using compressed CO₂/water as the solvent were also studied in this work. The results suggested that both CX-methanol and compressed CO₂/water could be used as two efficient solvents for the hydrogenolysis reactions.