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

Hydrolysis lignin conversion in a 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, 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 a 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 %), in particular, 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.