Insight into the solvent, temperature and time effects on the hydrogenolysis of hydrolyzed lignin

- An efficient hydrogenolysis process of hydrolyzed lignin was proposed.
- Solvent effect was investigated and the supercritical methanol performed the best.
- Temperature showed a great effect on depolymerization, repolymerization and coking.
- High temperature promoted the repolymerization and β-β groups formed.
- More than 26.3 wt% monomers yield could be obtained at 300 °C, 2 MPa H2 for 4 h.

The aim of this study is to explore the reaction mediums and conditions for producing high yield of valuable monomers from concentrated sulfuric acid hydrolyzed lignin. The solvent, temperature and time effects on the hydrogenolysis of hydrolyzed lignin were investigated under the catalysis of Pd/C and CrCl3. Supercritical methanol exhibits the best depolymerization performance, because of its unique diffusion, dissolution and acid-base properties. Afterwards, the influence of reaction temperature and time on depolymerization, repolymerization and coking during hydrogenolysis was examined in methanol. The high temperature is found to favor the depolymerization, with the β-O-4 linkages cleaved significantly. However, the repolymerization is promoted simultaneously, and a high amount of β-β groups form. These reactions are in constant competition with each other and the repolymerization is preferred at excessive high temperature, producing bulk char residues, that is coking. This study will provide a beneficial reference for the maximization of lignin waste valorization.

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