Ozonation pretreatment-assisted catalytic hydrogenation for efficient depolymerization of lignin

As one of the most important biomass resources, lignin is regarded as an ideal feedstock to produce high-value chemicals because of the abundance of its aromatic structures. However, the complex structure and large molecular weight (MW) of lignin prevent its efficient depolymerization into monomers, even under harsh processing conditions. In this work, an ozonation process was used to pretreat organosolv lignin followed by catalytic hydrogenation to promote lignin depolymerization. Upon ozonation pretreatment in different acid/alkaline environments, lignin was preliminarily depolymerized, and all the ozonated products could be divided into insoluble and water-soluble fractions, suggesting that the structure of lignin was changed to a certain extent. Moreover, the MW of the ozonated products markedly changed, and unexpectedly, some MWs obtained at pH = 6, 8, and 9 did not decrease but increased with possible recondensation. Afterwards, the ozonated product obtained at pH = 10 was used as the feedstock for the subsequent hydrogenation process because of the lowest MW (5574 g/mol). Due to the good hydrodeoxygenation activity, Ni-Nb composite material was used as hydrogenation catalyst. After hydrogenation at 280 °C and 4.0 MPa, the monomer yield of the untreated organosolv lignin was 19.5 wt%. By contrast, the water-soluble fraction had a higher monomer yield (83.3 wt%), while the insoluble fraction had a lower one (12.5 wt%) because of the greater number of C-C bonds. This work confirmed that ozonation pretreatment increased the relative content of C-C bonds, which may inhibit hydrodepolymerization to some extent, but the total depolymerization rate was still improved.