Heterogeneously Catalysed Mild Hydrogenolytic Depolymerisation of Lignin Under Microwave Irradiation with Hydrogen-Donating Solvents

A series of hydrogen-donating solvents have been investigated in lignin depolymerisation practices by using a mild, microwave-assisted, hydrogen-free, hydrogenolytic approach promoted by the use of insitu hydrogen generating reagents with Ni10%Al-SBA-15 as a heterogeneous catalyst. Selected solvents were tetralin, isopropanol, glycerol and formic acid. Final identified products, namely bio-oil, biochar and residual lignin were characterised by using GCMS, matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry and high-performance size-exclusion chromatography to evaluate the extension of the hydrogenolytic process. Interestingly, the obtained phenolic monomeric products were found to be remarkably affected by the type of hydrogen-donating solvent, with the best results obtained with formic acid, a potentially renewable derived solvent, which unexpectedly provided no biochar as compared to a maximum of 38% obtained for tetralin. The reported protocol constitutes another step towards the development of fully sustainable and green methodologies of low environmental impact for lignin depolymerisation.