(329c) Revealing Mechanistic Insights into Lignin Decomposition through Pygc-MS Analysis

100 million tons of lignin waste is generated from the pulp and paper industries annually.¹ Lignin is the aromatic macromolecule of biomass composing between 15 – 30% of plant matter.^2,3 Due to its complex aromatic structure, lignin has historically been difficult to characterizend degrade without severe reaction conditions. If we are able to apply precision molecular scissors to the macrostructure of lignin, then we can transform lignin from a million ton waste problem to a valuable resource capable of producing synthetic wood materials with enhanced strength and flexibility and aromatic specialty chemicals traditionally derived from fossil fuels.

Lignin serves as an important renewable biomolecule, yet differences in its fractionation method and native structure can result in varying degrees of depolymerization and product yields. This work aims to understand the effect that fractionation can have on decomposition pathways through an analysis of Kraft and CELF lignins. Hydrothermal liquefaction (HTL) is a promising technique to produce a carbon-rich biocrude, however, it is not suitable for understanding the complex degradation pathways of lignin due to long heat-up and cool-down timescales. HTL of lignin has shown increased biocrude yields from hardwood lignin, yet monomer yields remain below 20 mg/g lignin. For this reason, fast pyrolysis coupled with a GC-MS was utilized to understand the effects of wood type and fractionation method of lignin.

In this way, CELF lignin has been shown to result in increased monomer production compared to Kraft- highlighting decreased β-O-4 linkages in CELF lignin, as identified with NMR. By using step-wise pyGC, the temperature-dependent decomposition products were determined from Kraft and CELF fractionated soft and hardwoods. The total liquid carbon yield from CELF softwood lignin increases steadily with increasing pyrolysis temperature, whereas Kraft softwood experiences a maximum liquid carbon yield at 400 °C before a 10% decrease at 500 °C. In addition to pyrolysis carbon yields, the initial lignin as well as the resultant char were both analyzed with ATR FT-IR to determine the changes in bulk functional groups after degradation. This work will reveal compositional-dependent pathways for the thermal degradation of pre-fractionated lignin.