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

Abstract

Hydrothermal liquefaction (HTL) of Kraft lignin was investigated with different salt catalysts (namely, the basic salts K₂CO₃, CaCO₃; acidic salts ZrOCl₂·8H₂O and AlK(SO₄)₂·12H₂O, and neutral salt KCl) in the temperature range of 200–350 °C for 40 min in a batch reactor. With an increase in reaction temperature from 200 to 350 °C, the bio-oil yield first increased up to 300 °C and then decreased at 350 °C. The highest total bio-oil yield of ∼48.5 wt %, which involved ∼7.5 wt % water-soluble organics (WSO) and lowest char formation (∼19.1 wt %), was obtained at 300 °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.