Microwave-assisted depolymerisation of organosolv lignin via mild hydrogen-free hydrogenolysis: Catalyst screening

Abstract

Abstract Lignin depolymerisation to simple aromatics was investigated by using a mild microwave-assisted approach. The reaction was catalysed by different supported metal nanoparticles on mesoporous Al-SBA-15 including -nickel (2, 5 and 10 wt.%), palladium (2 wt.%), platinum (2 wt.%) and ruthenium (2 wt.%). Three main products were indentified in the proposed lignin valorisation protocol, namely a bio-oil, a bio-char and a fraction of residual lignin. These have been characterised by means of several techniques including GC–MS, MALDI-TOF, HPSEC and elemental analysis in order to evaluate the extension and mechanism of the selected depolymerisation approach. In this work, the objective has been the maximisation of bio-oil yield as well as its content in phenolic monomeric compounds. All tested catalysts exhibited improved results as compared to the blank depolymerisation (without catalyst), demonstrating a synergetic effect of metal nanoparticles in lignin hydrogenolysis. The most abundant phenolic products observed in most studied catalysts have been solvent derived products such as diethyl phthalate and butyl-octyl phthalate ester. Interestingly, phenolic compounds derived from lignin were also obtained including mesitol and syringaldehyde as well as desaspidinol and aspidinol, depending on the utilised solvent in the systems. The bio-oil obtained was primarily composed of monomers, dimers and trimers. Biochar yield was relatively high in most cases (>35% in some cases) due to existing oligomerisation reactions under the investigated conditions except for the case of the use of formic acid which gave no biochar in the process. Residual lignin content was often high and repolymerisation was observed in some cases. The catalyst containing 10 wt.% nickel was confirmed to achieve the highest lignin depolymerisation degree, with a maximum yield of 30% bio-oil after a short time of reaction (typically 30 min of microwave irradiation).