Selective Synthesis of Phenolic Compounds from Alkali Lignin in a Mixture of Sub- and Supercritical Fluids: Catalysis by CO2

Abstract

A successful selective liquefaction of lignin has been demonstrated in the presence of a H2O–CO2 mixture at 300 °C, yielding 40–50 wt % organic phenolic phase. The effect of the temperature at a constant pressure and short residence time on the selectivity and yield of phenolic products from the hydrothermal reforming of alkali lignin in a mixture of sub- and supercritical fluids (H2O mixed with CO2 or N2) has been investigated. Dependent upon the processing conditions, the lignin samples produced a homologous series of phenols, such as guaiacol, homovanillic acid, quaiacyl carbonyls, guaiacyl dimers, phenol, and cresol. Gas chromatography–mass spectrometry (GC–MS), total organic carbon (TOC), and pyrolysis–GC–MS (Py–GC–MS) were used for chemical analysis of the organic liquid and solid phases. The results from GC–MS analysis of the liquid organic phases demonstrated the trend of increasing the amounts of major guaiacol products with the temperature. The thermal carbon analysis (TCA) showed a significant ...