Interaction of Solvents and Mechanical Pretreatment with Enzymatic Lignocellulose Hydrolysis

  • Wechselwirkung von Lösungsmitteln und mechanischer Vorbehandlung mit enzymatischer Lignocellulose-Hydrolyse

Wang, Yumei; Spieß, Antje (Thesis advisor); Büchs, Jochen (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

Renewable plant biomass is considered as an alternative raw material for the production of fuels and chemicals instead of decreasing fossil resources. Enzymatic hydrolysis of pretreated lignocellulosic material to produce high sugar concentrations is one important step in a bio-refinery process, and can be operated under moderate conditions without by-products. However, the efficiency of the enzymatic hydrolysis is hindered by lignocellulose recalcitrance. Therefore, pretreatments of the biomass are applied to increase the accessibility of the biomass to the enzymes. However, the enzymatic hydrolysis of cellulosic substrates is significantly hampered by a strong slowdown of the reaction after its start. The initial rate and final biomass conversion strongly depend on the type of cellulose, ranging from commercial celluloses towards various pretreated lignocelluloses. In addition, the solvent in the enzymatic hydrolysis, either the remaining pretreatment solvent or the recycled solvent of the fermentation product, will affect the cellulase activity and stability. The objective of this work was to investigate the interaction of the solvents and mechanical pretreatment with enzymatic lignocellulose hydrolysis in depth.Mechanical pretreatment, i.e. in this work by a screw press, disrupts beech wood and wheat straw into smaller fibers. Combined with alkaline soaking, a synergetic enhancement of enzymatic hydrolysis was observed. Delignification by the chemical pretreatment played a major role, whereas enzymatic hydrolysis was nearly not affected using only the mechanical screw press. Alkaline assisted screw reactor pretreatment under moderate conditions enhanced sugar recovery by 96% and 126% for glucose and xylose, respectively. Hydrogen peroxide pretreatment after alkaline soaked screw press further enhanced the enzymatic hydrolysis.Various solvent pretreatments were investigated to evaluate the solvent effects onto enzymatic hydrolysis. Organosolv pretreatment with ethanol, pentanol, hexanol, heptanol and 2-MTHF could all delignify and increase the cellulose content, and thus enhance the enzymatic hydrolysis. All sugar yields were over 80% under 10% solid loading by Celluclast 1.5L. The ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EMIM Ac) could disintegrate lignocellulose, but the extent strongly depends on the water or acid content in the pretreatment liquid. To elucidate the enzymatic hydrolysis of the pretreated biomass, cellulase adsorption during hydrolysis was investigated. The initial reaction rate correlated to the adsorbed enzyme loading as predicted by the adsorption isotherm. However, reaction rate slows down during hydrolysis progress, and the adsorbed cellulase loading during the hydrolysis process drifts away from the enzyme adsorption isotherm. Particle size decrease, porosity increase and composition change partly contribute to the adsorption drift. The irreversible cellulose adsorption and activity loss during hydrolysis mainly cause the rate decrease. In addition, during hydrolysis, most solvents, such as IL, ethanol, or 2-MTHF had a negative effect on the hydrolysis yield. In contrast, itaconic acid, as the platform chemical produced from biomass, had no significant effect on enzymatic hydrolysis. Thus, itaconic acid could be recycled as a buffer component in the enzymatic hydrolysis. The solvent effect on the hydrolysis mostly depended on the effect to the cellulase activity. 2% ethanol and 1% 2- methyltetrahydrofuran (MTHF) was found to enhance the cellulase activity, whereas 10% ethanol caused 20% activity decrease. The cellulase stability was not affected by 10% ethanol compared to pure buffer. In contrast, 10% 2-MTHF caused more than 40% activity loss, and the stability decreased significantly while incubated in the 2-MTHF solution.To determine the enzyme activity quickly and easily, an online cellulase activity measurement method based on HPAEC-PAD was developed. Besides, the enzyme mixture was optimized to reach higher enzymatic hydrolysis yield for fermentation in next bio-refinery step.In summary, several pretreatment methods and their effects on the enzymatic hydrolysis were evaluated. Fundamental understanding of enzymatic hydrolysis, in particular, the relationship of the biomass properties and the hydrolysis rate, as well as solvent effects were identified. In the future, the online cellulase activity assay could be used for further enzyme mixture optimization for other substrates and for the control of enzymatic hydrolysis. All these findings can be useful for the economic conversion of lignocellulosic biomass in a further bio-refinery process.