Boron Lewis\nAcid Extraction of Wood Generates High\nQuality Lignin

The separation of lignocellulose into lignin, cellulose,\nand hemicellulose\nwithout significantly altering the chemical structures of these component\nbiopolymers remains a modern chemical challenge. Lignin, in particular,\nhas potential as a highly valuable feedstock material but remains\nunderutilized due to the difficulty of generating lignin with low\nmodification and condensation. This work investigates the lignin-rich\nsolids (“boron lignin”) generated from a previously\nreported boron Lewis acid-mediated lignocellulose separation and concludes\nthat (1) boron Lewis acid extraction removes 80–85% of carbohydrates\nfrom the original lignocellulose sample, and (2) the resulting lignin\npossesses a low condensation level and high similarity to native lignin\nstructure. Residual carbohydrate assessment, depolymerization efficiency\nanalyses, heteronuclear single quantum coherence (HSQC) and solid-state\nnuclear magnetic resonance (NMR) analyses are discussed, including\nbenchmarking results with alternate lignin sources known to possess\nlow and high condensation levels. Further, two different wood sources\n(white pine, a softwood, and beechwood, a hardwood) were employed\nto generate lignin samples. Depolymerization of a white pine-derived\nboron-lignin produced 47% (±9.5) of extractable monomers, which\ncompares well to a state-of-the-art method to generate low condensed\nlignin (56 ± 7.8%). An unexpected instability of the oil sample\nwas observed following hydrogenolysis of boron lignin generated from\nbeechwood. Dramatic color changes coupled with precipitation and lowered\nmonomer yields were observed when samples were aged (11% decrease)\nor concentrated (30% decrease). Based on NMR spectroscopic analyses,\nthis instability is postulated to arise due to boron-mediated demethylation\nof methoxy sites on the lignin scaffold.