Low temperature selective hydrodeoxygenation of model lignin monomers from a homogeneous palladium catalyst

The molecular, homogeneous catalysts [Pd(tpy)Cl]Cl and [Ni(tpy)](PF6)(2), where tpy is 2,2':6',2 ''-terpyridine, have been utilized to perform selective hydrodeoxygenation of benzyl alcohol, benzaldehyde, and benzophenone under very mild conditions. The [Pd(tpy)Cl]Cl catalyst exhibits excellent catalytic activity, with the complete selectivity towards hydrodeoxygenation, even at room temperature. Results indicate that the single-site nature of the molecular catalysts is what leads to the complete selectivity and the absence of aromatic ring hydrogenation products. A two-step mechanism consisting of H-2 activation by the catalyst to form a metal hydride complex, followed by selective hydrodeoxygenation is proposed. These results illustrate the possible advantages for the use of homogeneous catalysts in the conversion of lignin biomass to fuel or chemical feedstocks.