Insights into Oxidative Conversion of Lignin to High-Added-Value Phenolic Aldehydes

In this paper, the influence of some lignin features on its oxidative conversion to high-added-value phenolic aldehydes is discussed. Four softwood and three hardwood lignins are considered due to their different origins regarding the wood species, pulping process, and lignin isolation process. The lignins were characterized based on nitrobenzene oxidation (NO), frequency of phenolic hydroxyl (OHph) groups, and molecular weight determination, attempting to evaluate their potential to produce vanillin (V) and/or syringaldehyde (Sy) through the alkaline oxidation with oxygen. The raw-materials, Pinus spp. kraft lignin (LWest), Lignoboost kraft lignin (LBoostS), and Organosolv beech lignin (LOrgs) were selected to perform bench-scale oxidations with O2. These lignins contain lower content of nonlignin material and show higher yields of NO and high OHph groups content. In the oxidation with O2, LWest provide the highest production of V, 4.4% w/w (corresponding to 36% of NO yield). Considering the characteristics of LOrgs, an unexpected low Sy yield (2.5% w/w) was obtained (19% of the NO yield). The high reactivity of syringyl nuclei may be pointed out as the main reason for the faster production and degradation of Sy in oxidation. The concentration profile of products, the variation of pH in the reactive medium, and the variation of temperature with the reaction time of oxidation with O2 are shown and discussed with reference to the investigated lignins features.