No 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 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.

Lignin is considered as the most abundant renewable carbon source after cellulose and noncommercialized waste product with constantly growing annual production exceeding 50 million tons per year. This article is focused on a newly developed selective synthesis of high-value phenolic products from lignin in presence of supercritical carbon dioxide (scCO 2) known as sustainable, non-flammable, naturally abundant, and catalytically active solvent. Depending on the synthesis conditions, such as temperature (250, 300, and 350°C) and water-to-scCO 2 ratio (1:5, 1:2, 1:1, and 2:1), high yield of the specific phenolic compounds such as phenol, guaiacols, and vanillin has been achieved. The GCMS analysis reveales the trend of the increased total phenolic yield with temperature and strong dependence of the selectivity on the water-to-scCO 2 ratio. The maximum selectivity toward formation of the specific phenolic products, such as guaiacol and vanillin was observed at the highest H 2 O:scCO 2 =1:5 ratio. At 350 °C the relative yield of guaiacol was ~39% whereas at 250 °C vanillin was a dominant phenolic monomer with a relative yield of ~33%. Moreover, at the intermediate temperature of 300 °C both guiacol and

No 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 (scCO2) 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/scH2O) for selective liquefaction of alkali lignin is demonstrated for the first time. The scCO2-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 scCO2 catalyst is essential for enhancing the reaction selectivity in the presence of a heterogeneous catalyst. A combination of homogeneous scCO2 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 scCO2-assisted hydrothermal depolymerization of the alkali lignin in the presence of both scCO2 and a heterogeneous catalyst is superior to that of the each of these catalysts alone and in comparison of the systems where scCO2 is replaced by inert nitrogen.