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 degrees 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 N-2) 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 degrees C at the expense of less volatile organic carbon and recalcitrant pyrolyzed carbon. Evaluated for the first time, a significant effect of CO2 versus N-2 was demonstrated, providing both a higher yield of volatile products and more selective synthesis of guaiacols.