Enhancement of Lignin Degradation and Cellulose Hydrolysis Using Peracetic Acid

Abstract

The objective of this research is to develop an integrated process based on peracetic acid (PAA) to enhance degradation of lignin and separate components of lignocellulosic biomass, thereby increasing hydrolysis of the cellulose in lignocellulosic biomass. The single process with enzymatically-generated PAA was inefficient in lignin oxidation from Kraft lignin and lignocellulosic biomass due to low concentration of PAA. High concentration of PAA or several cycles at low concentration of PAA are required for oxidizing lignin effectively. The strategy for degradation of Kraft lignin using a combined treatment with PAA followed by supercritical water was optimized. Optimization of combined treatment enhanced degradation of Kraft lignin and produced catechol. In addition, synergistic effect of the combined treatment led to reduction in the supercritical water reaction time from 20 to 3 min. PAA treatment limits its use due to only lignin removal from yellow poplar (Liriodendron tulipifera) as lignocellulosic feedstock. Therefore, sequential processes are developed for fractionation of cellulose, hemicellulose and lignin by addition hydrothermal process to enzymatically-generated PAA treatment. First, combined pretreatment using hot compressed water (HCW) and PAA was established for separation of components from yellow poplar. The combined pretreatment started with HCW (200 oC, 1.5 MPa 15 min), which selectively solubilized most of the xylan. And then, subsequent PAA treatment increased the amount of lignin removal to 80% while single PAA treatment removed 20% of original lignin. In addition, the combined pretreatment enhanced the purity of cellulose and the accessibility of cellulase to the cellulose resulting in more efficient cellulose hydrolysis. Next, a mild pretreatment was developed by addition of sulfuric acid to hydrothermal process for fractionation of components. The optimization of dilute acid pretreatment (140 oC, 5min) was achieved for 80% xylose recovery. And then, subsequent PAA enhanced separation of lignin. The mild pretreatment using sequential dilute acid and PAA efficiently separated the three major components of yellow poplar. This sequential pretreatment enhanced the purity of cellulose and the enzymatic digestibility of the cellulose. Sequential process has the advantage of efficient separation of cellulose, xylose and lignin fraction. It requires only single treatment with dilute peracetic acid to remove lignin. This indicate that requirement of peracetic acid can be decreased when preceded by hydrothermal process that modified the biomass structure. In other words, accessibility of peracetic acid to lignin can be more easily done by separation of xylan. Pretreatment of biomass with dilute acid requires high temperatures of >140 °C to remove xylan, but does not remove lignin. Finally, one-step process was developed for achieving cellulose with high purity. The addition of PAA to dilute acid pretreatment dramatically increased removal of lignin. The optimization of one-step pretreatment (120 oC, 5 min) improved both xylan and lignin removal, and resulted in more efficient enzymatic hydrolysis by increasing relative amount of cellulose. Thus, the addition of PAA dramatically increases the effectiveness of dilute acid pretreatment of biomass and reduces the requirement of temperature and time. This one-step process achieved similar hemicellulose removal, delignification and enzymatic digestibility compared to other organosolv pretreatment such as ethanol, glycerol, ethyl acetate or THF, but it has the advantage of requiring low temperature and short time. Overall, integrated process based on PAA could enhance degradation of lignin and purity of cellulose resulting in more efficient cellulose hydrolysis. This process could also reduce energy consumption and cellulase requirement. This research is expected to reduce dependence on petroleum-based fuels and chemicals, and provide research direction for economic success of biorefinery utilizing all the components of lignocellulosic biomass.