Utilization of Lignin in Biomass

Lignocellulosic biomass – the non-edible part of plants – consists mainly of three components: cellulose , hemi-cellulose (both carbohydrate polymers), and lignin (aromatic polymers). While several commercial processes have been developed for cellulose and hemi-cellulose, the lignin fraction has received less attention.

Lignin is the second most abundant natural polymer, representing 30 % of the weight and 40 % of the energy content of lignocellulosic biomass. It is essential for the economic feasibility of future bio-refineries that all fractions, including lignin, are utilized.

Lignin is an amorphous polymer with many chemical functionalities and a structure that varies from plant to plant. While this complexity makes utilization challenging, the phenyl propane units contained in lignin are a potentially rich source for chemical production. Among the most abundant structural unit of lignin, is the β–O-4 which represents approximately 60 % of the bonds in hardwood and 45-50 % of those in softwood. Birch and beech sawdust were subjected to the organosolv treatment for extraction of lignin, which was further processed.

Lignin depolymerisation with selective bond cleavage can be achieved by several alternative processes. One of these is heterogeneously catalysed oxidation, which results in the production of a platform of aromatic compounds.

In the project, the lignin model products veratryl alcohol and guaiacyl glycerol-β- guaiacyl ether (GGGE) were tested in oxidation reactions. An 89 % yield of veratraldehyde was obtained from the aerobic oxidation of veratryl alcohol in water at elevated temperature and pressure with Ru/Al2O3 catalyst. When the same catalyst was used in the oxidative transformation of GGGE, 34 % yield of guaiacol, 13 % of vanillin, and 11 % of vanillic acid were obtained. The catalyst was easy to regenerate and recycle. Importantly, it was found to be stable over five consecutive runs.

Further, a novel one-pot, two-step conversion route from lignin to aromatics was developed. The first step consisted of the oxidative depolymerisation of lignin at elevated pressure with pressurized oxygen. A second step followed, wherein the oxidation products were subjected to catalytic hydrogenolysis at elevated temperature with a pressure of hydrogen. Different catalysts were tested. The tandem Ru/SiO2 – Ni/H-mordenite catalyst system gave the highest degree of degradation of the organosolv lignin binding motifs, with 57 % of the β-O-4 binding motifs, and 46 % of the phenylcoumaran interunits cleaved. Prolonged reaction times for the oxidation step resulted in increased degradation of the organosolv lignin linkages, resulting in the cleavage of 76 % of the β-O-4 binding motifs, and 64 % of the phenylcoumaran interunits.

In conclusion, heterogeneous catalysis provides great advantages, not only in terms of separation and recyclability, but also providing the ability to use molecular oxygen as oxidant. While oxidation is not the only option for the depolymerisation of lignin, it is a relevant route as an eco-efficient process.