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

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 increase of the readily volatile organic carbon in the liquid fractions resulting from the treatments at 300 and 400 °C at the expense of less volatile organic carbon and recalcitrant pyrolyzed carbon. Evaluated for the first time, a significant effect of CO₂ versus N₂ was demonstrated, providing both a higher yield of volatile products and more selective synthesis of guaiacols