Two-Stage Thermal and Heterogeneous Catalytic Conversion of Hydrolysis Lignin in Supercritical n-Hexane

Hydrolysis lignin conversion in supercritical n-hexane medium is conducted in two stages: (1) heat treatment of lignin in an autoclave in argon atmosphere at 250°C and 15 MPa and (2) the hydrogenation of the products in solution formed at the first stage at 250°C and 9.0 MPa in the presence of Ru/C catalyst. The products obtained at the first and second stages are analyzed by high-performance liquid chromatography, size exclusive chromatography, gas chromatography, gas chromatography–mass spectrometry, and elemental analysis. At the first stage, the degree of depolymerization is 17%; the mono- and oligomeric fragments of lignin that pass into solution due to depolymerization have molecular mass distribution (MMD) in the range of 65 Da to 270 kDa. It is found that the composition of monomers in the first-stage depolymerization products is as follows: mono-, di-, tri-, and tetraalkyl benzene derivatives (2.8 wt %), guaiacol, and 4-alkyl guaiacol derivatives (0.6 wt %), including coniferyl alcohol (0.06 wt %). The following processes occur at the second stage: (i) the depolymerization of oligomers to monomers, (ii) the hydrogenation of monomeric phenols to oxo- and alkyl cyclohexane derivatives, and (iii) the formation of gaseous products (mostly methane, 95 vol %) due to the catalytic hydrocracking of the solvent.