Efficient depolymerization of kraft lignin using zinc chloride based (molten) salts

Lignin is an attractive feed for biofuels and biobased chemicals. We here report the conversion of kraft lignin (Lignoboost) to liquids by hydropyrolysis using a zinc based (molten) salt in a semi-batch reactor. m-Cresol was used to dissolve and inject the lignin into the heated and pressurized reactor containing the molten salt. The liquid yield and fraction of organics in the liquid was optimized by varying the process parameters (hydropyrolysis temperature, pressure, hydrogen gas flow rate and salt to lignin ratio). The highest liquid yield of around 78 wt% was obtained using small amounts of salts (2.5 wt% of salts based on lignin input, 350 °C, 30 bar, 33 wt% lignin in m-cresol, hydrogen flow rate of 160 mL/min). A larger excess of salts gave a lower liquid yield, implying that catalytic amounts of salts are preferred. In addition, small amounts of salts are essential for depolymerization and the formation of lower molecular weight components, as was evident by comparing experiments with and without salts. The liquid product was shown to consist of low molecular weight phenolics like dimethyl- and ethyl-cresol, methyl guaiacol and oligomers (GC-MS, 2DGC-FID, NMR and GPC analysis). The product oil may have potential for biofuel production when coupled with a catalytic hydrotreatment step to obtain hydrocarbons or, after downstream processing and catalytic conversions, to important biobased chemicals like green phenol.