A new catalyst that performs the direct and selective hydrogenolysis of the C–OH bond of substituted phenols and naphthols — two of the major components of lignin — has been developed by professor Kyoko Nozaki and colleagues at the Dept. of Chemistry and Biotechnology, University of Tokyo (Japan; www.park.itc.u-tokyo.ac.jp/nozakilab). This is said to be the first report on the selective cleavage of sp2-hybrid C–O bonds without also cleaving the aromatic rings of lignin-derived bio-oils. The researchers say the catalyst shows promise as a way to produce basic aromatic hydrocarbons, such as benzene, toluene and xylenes (BTX) amd phenols from lignin instead of petroleum.
The catalyst, which is based on hydroxycyclopentadienyl iridium complexes, has achieved a 99% yield of the corresponding arenes, when reacting a phenol with H2 (1 atm pressure) at 200°C. Maximum turnover numbers of 255 are also observed. Furthermore, the same catalysts were applied to the “unprecedented” selective hydrogenolysis of the sp3-hybrid C–O bonds in aryl methyl ethers. Thus, the hydrodeoxygenation of vanillylacetone, a lignin model compound, produces alkylbenzenes as the major products via triple deoxygenation.