Key features of thermochemical pretreatment induced lignin structure to saccharification and fermentation

Abstract

This dissertation introduced and investigated an innovative biorefinery platform to convert urban waste derived lignocellulosic biomass into bioethanol and reactive lignin. Woody biomass derived from single or mixed species were investigated separately for softwood, hardwood, and then universally identical woods, respectively. Softwood contains large amount of easily fermentable hexoses (mannose plus glucose) and G-lignin, while hardwood is with more xylose and S-lignin; thereby the two types of woody biomass may be more feasible in different biorefinery approaches for valorization based on a comprehensive review on current processes (Chapter 2). Softwoods were subjected to sulfite pretreatment followed by the "whole slurry" saccharification and fermentation (SSF) process, which aims to utilize all the dissolved sugars (in pretreatment liquor) plus enzyme hydrolyzed sugars in fermentation. New insights on the duel effects of inhibiting/improving mechanisms of pretreatment derived lignosulfonates were studied by surface tension analysis, enzyme activities, and fitting of linearized kinetic models (Chapter 3). It was discovered that pretreatment liquor could increase the inhibiting effects to fermentation at high temperature, and an extremely high ethanol titer (82.1 g/L) were yielded by simply optimizing the operation temperature. Without the need of detoxification, the resulting ethanol titer is approaching the theoretical yield and is currently among the highest in softwood conversion (Chapter 4). Energy balance calculation was conducted to evaluate the trade between energy lose from temperature control and benefited higher energy yield from product titer. The net energy yield of the new process was 2,410 MJ per ton oven-dried wood, which is approximately 730-1,690 MJ higher than that of the other biorefinery processes. The water input before reclamation was 3.65 tons per ton dried wood, which is 25.8-51.2% lower than most of the other processes (Chapter 5). Finally for hardwood, a new reactive lignin with great solvent solubility and preserved β-O-4 linkages was obtained from eucalypts after a modified organosolv pretreatment using 1,4-butanndiol (1,4-BDO). 2D HSQC NMR analysis showed that this 1,4-BDO lignin contain relative higher amount of β-O-4 linkages, indicating a higher integrity than ethanol pretreated lignin. This result agreed with ¹³P NMR analysis and suggested that alcohols can quench the benzyl carbocation intermediate and formed ether linkage at the α position of the lignin. Although dioxane structure was not observed in the NMR spectra, solubility tests revealed that grafting aliphatic hydroxyl groups on 1,4-BDO lignin can increase its dissolution. This phenomenon was demonstrated for four other diols with similar structures; and more than 90% cellulose conversion was obtained from all the diol pretreated eucalyptus coupling with enzymatic hydrolysis at only 7.5 FPU/g glucan. Diol pretreatment offers an attractive reaction pathway to coincide with the trading among lignin fractionation, lignin structural integrity, and cellulose hydrolysis.