Catalytic Hydropyrolysis of Lignin for the Preparation of Cyclic Hydrocarbon-Based Biofuels

The demand for biomass utilization is increasing because of the depletion of fossil resources that are non-renewable in nature. Lignin is the second most renewable organic carbon source, but currently it has limited scope for application in the chemical and fuel industries. Lignin is a side product of the paper and pulp, sugar, and 2G bioethanol industries. Many research groups are working on the value-addition of lignin. Among the lignin depolymerization methods, catalytic hydropyrolysis is gaining attention and is playing a crucial role in developing biorefinery. The hydropyrolysis of lignin was conducted at a higher temperature in the presence of H2. The hydropyrolysis of lignin results in the selective formation of non-oxygenated cyclic hydrocarbons in a shorter reaction time. It is possible to use the cyclic hydrocarbons directly as a fuel or they can be blended with conventional gasoline. This review focuses on the prior art of pyrolysis and hydropyrolysis of lignin. Possible products of lignin hydropyrolysis and suitable synthetic routes to obtain non-oxygenated cyclic hydrocarbons are also discussed. The influence of various process parameters, such as type of reactor, metal catalyst, nature of catalytic supports, reaction temperature, and H2 pressure are discussed with regard to the hydropyrolysis of lignin to achieve good selectivity of cyclic hydrocarbons.