Lignin Depolymerisation under Supercritical Process Conditions for Aromatic Chemicals Production

Valorisation of lignin plays a key role in the further development of biorefinery processes for biofuels and the production of biobased materials from lignocellulosic biomass. Today’s increased demand for alternatives to fossil carbon based products, such as the production of transportation biofuels and bulk “green” chemicals, expands the interest and the need to create added value to the unconverted lignin fraction. As a consequence of its poly-aromatic structure, lignin is the obvious candidate to serve as a source for aromatic chemicals. In this work organosolv hardwood lignin was converted in a mixture of super critical carbon dioxide, acetone and water (300 – 370°C, 100 bar) to aromatic monomeric compounds with a total yield of 10% based on dry lignin. Addition of formic acid increases the yield of aromatics from 7% to 10%. Main monomeric lignin derived products were 2,6-dimethoxyphenol, 4-hydroxy-3,5dimethoxybenzoic acid, 2-methoxy-4-methylphenol and 2methoxyphenol. During this process a strong competition occurs between depolymerisation of lignin and recondensation of fragments. This leads to a residual lignin fraction representing 40% of the starting material consisting of a substantially reduced oxygen/carbon ratio and a similar structure to carbon black.