Toward Selective Lignin Liquefaction: Synergistic Effect of Hetero- and Homogeneous Catalysis in Sub- and Supercritical Fluids

Abstract

Selective liquefaction of lignin is important for synthesis of value-added phenolic monomers contributing to green chemistry and sustainable energy applications. In the present study, a synergistic effect of a supercritical carbon dioxide (scCO 2 ) acidic catalyst in combination with a heterogeneous metal oxide catalyst, specifically nickel oxide (NiO) or ceria-doped scandia-stabilized zirconia (CeScSZ), in sub- or supercritical water (sbcr/scH 2 O) for selective liquefaction of alkali lignin is demonstrated for the first time. The scCO 2 -assisted hydrothermal process in the temperature range of 100–400 °C resulted in highly selective synthesis of the phenolic monomers. On the basis of the total organic carbon (TOC) and gas chromatography–mass spectroscopy (GC–MS) analysis, it is evident that the scCO 2 catalyst is essential for enhancing the reaction selectivity in the presence of a heterogeneous catalyst. A combination of homogeneous scCO 2 and heterogeneous NiO catalysts resulted in the highest total relative yield of the phenolic monomers, reaching ∼97% at 200 °C. It is confirmed that the scCO 2 -assisted hydrothermal depolymerization of the alkali lignin in the presence of both scCO 2 and a heterogeneous catalyst is superior to that of the each of these catalysts alone and in comparison of the systems where scCO 2 is replaced by inert nitrogen.