Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2021
Publication Date: 1/5/2022
Citation: Strahan, G.D., Mullen, C.A., Stoklosa, R.J. 2022. Application of diffusion ordered NMR spectroscopy to the characterization of sweet sorghum bagasse lignin isolated after low moisture anhydrous ammonia (LMAA) pretreatment. BioEnergy Research. https://doi.org/10.1007/s12155-021-10385-y.
Interpretive Summary: Biorefineries are operations that convert lignocellulosic biomass such as wood, grasses, and crop residues to fuels and chemical products. The feedstock biomass is comprised of cellulose, hemicellulose and lignin. Biorefineries are very good at efficiently converting the cellulose and hemicellulose to higher value products, but lignin is often left as a low value byproduct and is typically burned for energy. It is more difficult to develop a process to convert lignin efficiently because it is irregular and variable in its structure and the size of its molecules. Characterization is important to be able to develop processes to utilize lignin for production of higher value products and thereby make biorefineries more competitive with fossil refineries. In this paper we applied a technique called diffusion ordered nuclear magnetic resonance (NMR) spectroscopy in combination with other methods to lignin isolated from sweet sorghum bagasse after a biorefinery process called low moisture anhydrous ammonia pretreatment. This treatment is used to release sugars so they can be fermented to ethanol. Lignin from this type of process has not been previously well characterized, so the objective of the study was to both learn more about the chemical nature of this lignin and also demonstrate the utility of the diffusion NMR technique for lignin. This information will be of interest to those involved with lignocellulosic biorefineries.
Technical Abstract: Lignin isolated from sweet sorghum bagasse after the low moisture anhydrous ammonia (LMAA) pretreatment was characterized using diffusion order nuclear magnetic resonance (NMR) spectroscopy in combination with one dimensional 1H NMR, 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. Based on estimates from the weighted diffusion values of the aromatic region (6.0-8.5 ppm) of the 1H diffusion ordered spectra, the LMAA lignin had a broader molecular weight range and a higher average molecular weight than did the soda lignin. Fractionation of the LMAA lignin based on solubility in methanol revealed that solubility was largely dependent on molecular weight as both fractions contained similar structures. One lignin dimer, 4,4’-dihydroxystilbene, was identified in the methanol soluble fraction aided by its signal's separation from the rest of the mixture's signals via diffusion NMR.