Effects of Temperature and Salt Catalysts on Depolymerization of Kraft Lignin to Aromatic Phenolic Compounds

Hydrothermal liquefaction (HTL) of Kraft lignin was investigated with different salt catalysts (namely, the basic salts K2CO3, CaCO3; acidic salts ZrOCl(2 center dot)8H(2)O and AIK(SO4)(2)center dot 12H(2)O, and neutral salt KCl) in the temperature range of 200-350 degrees C for 40 min in a batch reactor. With an increase in reaction temperature from 200 to 350 degrees C, the bio-oil yield first increased up to 300 degrees C and then decreased at 350 degrees C. The highest total bio-oil yield of similar to 48.5 wt %, which involved similar to 7.5 wt % water-soluble organics (WSO) and lowest char formation (similar to 19.1 wt %), was obtained at 300 degrees C over K2CO3 catalyst. The GC MS results showed that WSO fractions were comprised of mainly guaiacol and catechol-type monomers, whereas the heavy bio-oil fractions contained mostly long-chain acids, aldehydes, and ketone-type compounds. In comparison with the other salt catalysts, the insertion of K2CO3 resulted in significant reduction in oxygen, nitrogen, and sulfur contents and consequently increased the high heating value (HHV) of 18.9 MJ/kg (in the original Kraft lignin) to 29.3 MJ/kg (in the bio-oil obtained with K2CO3 catalyst). The overall results suggested that K2CO3 in a subcritical water system was more efficient for the production of high-quality bio-oil in comparison to other tested acidic and neutral salt catalysts. The solid and liquid products obtained in different experiments were analyzed by FT-IR, SEM, GC-MS, and elemental analysis methods.