Application of Diffusion-Ordered NMR Spectroscopy to the Characterization of Sweet Sorghum Bagasse Lignin Isolated After Low Moisture Anhydrous Ammonia (LMAA) Pretreatment

Lignin isolated from sweet sorghum bagasse after the low moisture anhydrous ammonia (LMAA) pretreatment was characterized by using diffusion-ordered nuclear magnetic resonance (NMR) spectroscopy in combination with one-dimensional H-1, multinuclear two-dimensional NMR (HSQC, HMBC, HSQC-TOCSY), and elemental analysis to correlate structural observations with molecular weight. The LMAA lignin was compared with a commercial soda lignin product. The H-1 diffusion-ordered spectra indicate that the aromatic components (6.0-8.5 ppm) in the LMAA lignin cover a wider diffusivity range, and therefore broader molecular weight range, than did the soda lignin, and that its weight average molecular weight was also higher. Fractionation of the LMAA lignin based on solubility in methanol revealed that this property was largely dependent on the molecular weights of the components, as both the soluble and insoluble fractions contained similar structures. Also observed by H-1 NMR was a significant amount of residual ammonium present from the biomass pretreatment. This ammonium was largely fractionated into the methanol soluble fraction.