Similarities in Recalcitrant Structures of Industrial Non‐kraft and Kraft Lignin

This work compares the structure of industrially isolated lignin samples from kraft pulping and three alternative processes: butanol organosolv, super-critical water hydrolysis, and sulfur dioxide/ethanol/water fractionation. It is known that kraft processes produce highly condensed lignin, with reduced potential for catalytic depolymerization, while the alternative processes have been hypothesized to impact the lignin less. Structural properties most relevant to catalytic depolymerization are characterized by elemental analysis, quantitative 13 C NMR, 2D HQSC NMR, GPC, and TGA. Quantification of the bO4 ether bond content shows partial depolymerization, with all samples having less than 12 bonds per 100 aromatics. This results in theoretical monomer yields of less than 5%, strongly suggesting the alternative fractionation processes generate highly condensed lignin structures that are no more suitable for catalytic depolymerization than kraft lignin. However, the different thermal degradation profiles suggest there are physicochemical differences that could be leveraged in other valorization strategies.