Thermocatalytic depolymerization of lignin and hydrodeoxygenation of lignin-derived monomers

In recent years, lignin, the third fraction of lignocellulosic biomass, has evolved as one of the most prominent renewable resources for value-added aromatic chemicals. Lignin constitutes around 10-30% of the total lignocellulosic biomass. It is made up of aromatic monomers (coniferyl, sinapyl, and p-coumaryl), which brands it as the most abundant renewable source of aromatic chemicals. The removal of aromatic entities from lignin is a big challenge to date, mainly due to its heterogeneous nature and tendency of the polymer to repolymerize a more condensed structure. While working in a similar area in the last few years, we have narrowed down the specific needs and targets in this area of interest. The early and recent advancement has always given a strong focus on the development and screening of efficient catalysts for lignin depolymerization. Although the catalyst is a crucial component for the lignin depolymerization to monomeric products, significant attention is also required on the catalyst-to-lignin interactions and collision possibility of inter-unit linkages with the catalyst. An appropriate solvent might regulate the collision possibility of inter-unit linkages with active site of the catalyst. The use of a suitable solvent can also reduce the formation of char and restrain the repolymerization reactions. Therefore, Hansen solubility parameters (HSP) were utilized for selecting a suitable solvent for the lignin polymer. A mixture of water and THF was observed to have higher thermodynamic compatibility for lignin and guaiacol with a RED ~ 1. The beneficial effect of water and THF mixture was also observed in the depolymerization reactions of Kraft lignin using HZSM as a catalyst and NaOH as co-catalyst. A higher lignin conversion of 77% and 79% selectivity towards guaiacols with negligle char could be attained at the optimal reaction conditions from statistical analysis - Response Surface Methodology (RSM). The water-THF ratio was also found as the major i