Catalysis and chemistry of lignin depolymerization in alcohol solvents - A review

The second-generation biofuel strategy aims to fully utilize lignocellulose, which is the major component of the plant cell wall and the most abundant form of renewable organic resources. Among three major components of lignocellulose, i.e. cellulose, hemicellulose and lignin, lignin has been the least utilized one up to now. Nevertheless, lignin depolymerization (LD) to produce aromatic chemicals and fuels has been intensely explored in the recent decade. Alcohols have been the mostly employed solvents in LD reaction, and are also involved into the LD reaction. This review provides an overview of the catalysis and chemistry of LD reaction in alcohols, especially in methanol, ethanol and isopropanol. The recent advances are firstly summarized, and then the roles of alcohol in LD reaction are outlined. The alcohol self-conversions are firstly discussed, and then the roles of alcohol are discussed in four subtopics: supplying hydrogen, depolymerizing lignin, hindering repolymerization and affecting monomer structure. Alcohol and alcohol-derived intermediates provide active hydrogen for reductive catalytic LD reaction carried out without hydrogen input, effectively break ether linkages but not C-C linkages in lignin, and also react with active intermediates and monomers, suppressing the repolymerization side reactions. In addition, alcohol also inhibits the hydrogenation of benzene rings and involves in the formation of products, affecting the structure of monomers. With these understandings, the challenges and opportunities of LD are proposed.