Abstract
Lignin is the only naturally occurring, renewable biopolymer and an alternative for the production of six-membered aromatic chemicals. The utilization of lignin can increase the additional revenue of biorefineries and reduce the dependence on crude oil for the production of aromatic chemicals. Therefore, the development of technologies for the production of valuable chemicals from lignin waste in biorefineries is of great importance. Catalytic hydrotreatment of lignin is considered one of the most promising technologies for the production of biobased aromatic chemicals and fuels. Among the various hydrotreatment routes, the solvent-free hydrotreatment approach is advantageous because this process reduces production costs and is similar to petroleum refinery processes such as cracking and heteroatom removal. This review addresses recent developments in solvent-free catalytic hydrotreatment of various lignins such as sulfur-containing, sulfur-free, and pyrolytic lignins to produce low oxygen-containing aromatics such as alkylphenolics in batch, semicontinuous, and continuous reactors. Special emphasis is given to the various noble and non-noble metal catalysts, the best route between single and two-stage processing, key factors in solvent-free depolymerization of lignin, techno-economic evaluation, crude oil vs lignin oil refining, challenges and future prospects, etc.