ZnCl2 induced catalytic conversion of softwood lignin to aromatics and hydrocarbons

Abstract

Selective cleavage of C–O–C bonds in lignin without disrupting the C–C linkages can result in releasing aromatic monomers and dimers that can be subsequently converted into chemicals and fuels. Results from this study showed that both biomass-derived lignin and lignin model compounds were depolymerized in a highly concentrated ZnCl₂ solution under relatively mild conditions (120 °C–200 °C, 4–6 h). Zn²⁺ ions in highly concentrated ZnCl₂ aqueous solutions appeared to selectively coordinate with C–O–C bonds to cause the key linkages of lignin to be much easier to cleave under mild conditions. In a 63 wt% ZnCl₂ solution at 200 °C for 6 h, nearly half of the softwood technical lignin was converted to oil products, of which the majority were alkylphenols. Results indicated that most of the β-O-4 and C_methyl–OAr bonds of the lignin model compounds were cleaved under the above reaction conditions, providing a foundation towards understanding lignin depolymerization in a concentrated ZnCl₂ solution. Furthermore, by adding Ru/C as a co-catalyst, the phenolic products were further converted into more stable cyclic hydrocarbons via hydrodeoxygenation and coupling reactions.