Dramatic simplification of lignin HSQC spectra from ring-puck milling followed by oxidation

Conversion of biomass into usable fuels and chemicals, especially under very low-cost processing, remains a major challenge. Mechanochemical treatment of lignin can greatly improve the yield of monomer products that can be obtained by oxidative deconstruction. In this study, we applied a mechanochemical approach using ring-and-puck milling with KOH in a solvent-free system to promote lignin degradation. We investigated how milling with this larger scale method would affect the success of a two-step oxidative lignin deconstruction strategy, in which we previously employed small-scale ball milling. We monitored changes in the lignin structure and functionality using heteronuclear single quantum coherence (HSQC) and P-31 nuclear magnetic resonance (NMR) spectroscopy. P-31 NMR indicated that 2 h of ring-and-puck milling can generate significant amounts of aliphatic hydroxyls and phenols. Dramatic simplification of the HSQC spectra and a pronounced downfield shift in NMR resonances (to delta(H) 8.5 and 8.8 ppm) after porphyrin oxidation suggests that ring-and-puck milling produced a significantly different set of reactions and products than is produced by the ball-milling technique that we used in a previous study. Also, in contrast to our findings with ball-milled material, porphyrin oxidation of the ring-and-puck milled material produced new products that could not be reduced with NaBH4 to reform the starting material. In addition, Baeyer Villiger oxidation produced a remarkable increase in the amount of the ethyl-acetate-soluble portion as well as a decrease in the insoluble solid compared to the amounts obtained from the ball-milling process. A short 2 h period of ring-and-puck milling produced nearly half of the amount of vanillic acid (isolated as the methyl ester) as was previously isolated after an extended (48 h) period of ball milling. The significant changes in the HSQC spectrum, the irreversibility of the oxidation step, and the dramatic increase in ethyl-acetate-soluble products suggest that previously unobserved chemistry is taking place.