Catalytic depolymerization of organosolv lignin to phenolic monomers and low molecular weight oligomers

Abstract

The lignin-first strategy provides significant potential for producing value-added phenolics and high-quality biofuels. In this contribution, stepwise extraction and depolymerization of lignin in corncob residue towards the production of phenolic monomers, lignin-derived dimers and trimers was investigated. H2O/n-butanol (4:6, v/v) co-solvent could extract 87.1% of lignin in corncob residue, retaining a high-purity (86.0%) cellulose pulp. Ni/HZSM-5 catalysts with different Ni loadings were prepared and applied for further degradation of organosolv lignin. Over 5% Ni/HZSM-5, the highest yield of phenolic monomer was 19.5% with the selectivity of 66.7% towards 4-ethyl phenols (4-ethyl phenol and 4-ethyl guaiacol) at 300 °C, and liquid products with higher heating value (HHV) of 29.2 MJ kg−1 were obtained. The 5% Ni/HZSM-5 catalyst promoted the cleavage of intramolecular β-O-4 and Cβ-Cγ linkages, resulting in substantially decreased weight-average molecular weight (Mw)/number-average molecular weight (Mn) of the organosolv lignin from 2541/1064 Da to 562/391 Da, which implied the degradation of high-Mw oligomers to dimers and trimers. The catalyst also contributed to the hydrogenation of CαCβ bonds in unsaturated intermediates (coniferyl alcohol and coumaryl alcohol). The possible cleavage mechanism for the degradation of lignin-derived oligomers was also proposed.