Catalytic liquefaction of Kraft lignin in water at subcritical and supercritical conditions.

Abstract

Concerns about the negative environmental impacts caused by the production and consumption of petroleum-based chemicals have made lignocellulosic biomass a promising source of valuable industrial chemicals. Kraft lignin (KL) is a by-product from cellulose production typically treated as a waste or used as a low value fuel in heat and power generation. However, a large amount of this biomass is still not utilized. In principle, a thermo-catalytic process working in subcritical (subCW) or supercritical water (SCW) could be used instead to produce a variety of valuable chemical products from KL. However, the development of such a process remains a challenge. This work aims to obtain oxygenated monoaromatic compounds from KL using SCW as reaction medium. Initially, the effect of temperature, pressure and reaction time on the conversion of KL was investigated. Catalysts were also tested in order to further improve the conversion of KL to bio-oil. These catalysts were prepared by a wet impregnation method and reduced Ni was observed in Ni-CeO2/carbon based catalysts, without using H2. Among these catalysts prepared via wet impregnation, Ni 15MgO/CNF promoted the highest yield of bio-oil (around 62 wt.%), followed by Ni 15CeO2/CNF (around 55 wt.%). In order to increase the stability and activity of Ni-CeO2/CNF, this catalyst was also synthesized via one-pot catalyst synthesis method in subCW and SCW in presence of formic acid as reducing agent. Highly stable and active Ni-CeO2-x/CNF catalysts were successfully obtained in subcritical water and without addition of H2. Ni CeO2-x/CNF promoted the conversion of KL to an extremely high yield of oxygenated monoaromatic-based bio oil (around 79 wt.%), without adding co-solvents and H2. The catalytic liquefaction process developed in this work brings a ground-breaking approach to the synthesis of stable and active catalysts for greener processes in the context of biomass valorisation in water at SCW.