(252d) Influence of Industrial Fractionation Methods on Lignin Structure

As the only large-scale source of natural aromatics, the potential value of lignin as an alternative feedstock for petroleum-derived chemicals has sparked interest in recent years. A major hurdle for all pathways for chemically valorizing lignin is the recalcitrance of the lignin structure due to chemical transformations during fractionation. The process of fractionating lignin from cellulose and hemicellulose creates reactive sites in the lignin structure that condense to form very stable C-C bonds. The prevailing strategy to isolate lignin from biomass is Kraft pulping that generates black liquor, containing lignin polymers that show diminished promise for upgrading to chemicals. As alternatives to Kraft lignin, many new biorefineries isolate lignin using milder solvents and reaction conditions, but many of these lignin structures have yet to be thoroughly characterized. Broad characterization of lignin isolated using industrially feasible methods will aid in developing tailored valorization strategies.

Herein, eight widely used analytical techniques were used to characterize the structures of eight industrial lignin samples from wood to determine the effects different industrial fractionation and extraction methods have on the purified lignin stream. The scope of characterization covers hardwood and softwood lignin extracted using Kraft, super-critical water, and organosolv fractionation processes. Molecular weights were determined through gel permeation chromatography (GPC). Nuclear magnetic resonance (NMR) spectroscopy and infrared spectroscopy were used to identify common linkages, prevalence of various monolignol units, and functional moieties. Thermogravimetric analysis (TGA) provided insight to the thermal-physio properties of the lignin polymer. Extensive characterization of the lignin allows for correlations between the fractionation and extraction methods and final lignin structure, as well as, a comparison between the utility of the many characterization techniques. The non-Kraft lignin samples, in general, show lower levels of recalcitrance compared to the Kraft lignin, but still exhibit significant alteration to the original structures.