Investigation of the Chemocatalytic and Biocatalytic Valorization of a Range of Different Lignin Preparations: The Importance of β-O-4 Content

A set of seven different lignin preparations was generated from a range of organosolv (acidic, alkaline, ammonia treated, and dioxane-based), ionic liquid, autohydrolysis, and Kraft pretreatments of lignocelluloses. Each lignin was characterized by 2D HSQC NMR spectroscopy, showing significant variability in the beta-O-4 content of the different lignin samples. Each lignin was then valorised using three biocatalytic methods (microbial biotransformation with Rhodococcus jostii RHA045, treatment with Pseudomonas fluorescens Dyp1B or Sphingobatterium sp. T2 manganese superoxide dismutase) and two chemocatalytic methods (catalytic hydrogenation using Pt/alumina catalyst, DDQ benzylic oxidation/Zn reduction). Highest product yields for DDQ/Zn valorization were observed from poplar ammonia percolation-organosolv lignin, which had the highest beta-O-4 content of the investigated lignins and also gave the highest yield of syringaldehyde (243 mg L-1) when using R. jostii RHAO4S and the most enzymatic products using P. fluorescens Dyp1B. The highest product yield from the Pt/ alumina hydrogenation was observed using oak dioxasolv lignin, which also had a high beta-O-4 content. In general, highest product yields for both chemocatalytic and biocatalytic valorization methods were obtained from preparations that showed highest beta-O-4 content, while variable yields were obtained with preparations containing intermediate beta-O-4 content, and little or no product was obtained with preparations containing low beta-O-4 content.