Comparison of Acidic and Alkaline Oxidative Catalytic Fractionation Processes to Recover Aromatic Monomers from Native Poplar Wood and Straw Digestates: A Critical Lab-to-Lab Comparison

Abstract Lignin from native lignocellulose is a potentially attractive raw material for the production of bio-based aromatics due to its high abundance and its aromatic structure rich in ether bonds. Several oxidative catalytic fractionation processes have been presented in literature, through which oxygenated aromatics can be obtained from native lignocellulose-containing biomass in high yields. These processes can be divided into acidic processes using catalysts such as heteropolyacids in alcoholic solvents and alkaline processes using copper catalysts in aqueous NaOH solutions. Some of these processes have been tested only in one laboratory so far and the comparability among each other is very limited. Consequently, it is the aim and main novelty of this study to reproduce these processes with poplar wood and to compare them among each other and with the well-established reductive catalytic fractionation using the same reactor setup and biomass sample. The reaction conditions from selected reference processes were reproduced, with the main deviation that a smaller reactor size was used. Highest monomer yields on lignin basis around 40 wt% were obtained by reductive catalytic fractionation, leading to reduced instead of oxygenated monomers. The tested one-step alkaline oxidative catalytic fractionation process resulted in monomer yields up to around 30 wt%. Lowest yields around 10 wt% were obtained for one-step acidic catalytic processes using a methanol–water mixture as solvent and phosphomolybdic acid as catalyst, for which similar monomer yields could be achieved using sulfuric acid instead of phosphomolybdic acid. An acidic two-step process yielded very low monomer yields around 2 wt% largely deviating from literature values. For all oxidative catalytic processes, lower yields compared to literature values were observed, which is comprehensively discussed. It appears that, compared to the stable monomers obtained by reductive catalytic fractionation, the instability of the oxygenated monomers leads to a higher vulnerability to changes in the reaction process, probably shifting the optimal conditions. Straw digestate was applied as alternative substrate and yielded significantly less monomers than poplar wood. Graphical Abstract