Effects of different metal-salts catalyzed ethanol pretreatment and additives on the enzymatic hydrolysis of poplar

The biochemical transformation of lignocellulosic biomass is one of the important methods for producing environmentally friendly bioenergy from agricultural and forestry wastes. Among them, lignocellulose is the most important form of biomass energy, and it is one of the important ways of biorefinery to convert lignocellulosic into fermentable sugar by means of biochemical transformation, and further conversion into fuel ethanol and chemicals, which has received extensive attention and research from various countries. In this study, Poplar wood was added to 60% ethanol aqueous solution with a solid-liquid ratio of 1:10 (w/v) with 0.025 mol/L of metal salts (FeCl₃, CrCl₃, CuCl_2, FeCl₂, ZnCl₂, MnCl₂, MgCl₂, CaCl₂, NaCl and LiCl) at a speed of 300 r/min, and the reaction continued for 10 min at 200 ℃ in a high-pressure reactor. The influence of different metal-salts catalyzed ethanol pretreatment of poplar on the chemical composition, sugar composition, microstructure and enzymatic glucose yield of poplar were studied. Furthermore, the additives (Tween 80, tea saponin, BSA, and xylanase) were added to the enzymatic hydrolysis, the effects of different metal-salts catalyzed ethanol pretreatment and additives on enzymatic hydrolysis of poplar were explored by comparing the glucose yield, resulting in the optimal combination of metal-salt and additive for poplar enzymatic hydrolysis. Through the pretreatment of poplar wood with different metal salts catalyzed ethanol pretreatment, it was found that the degradation of hemicellulose and lignin by trivalent and divalent metal salts was obvious. Through XRD, the crystallinity (CrI) of poplar raw material was 64.14%, and its CrI increased to 78.63%, 78.65%, 78.92%, 77.26%, 75.79% and 70.33% after pretreatment with FeCl₃, CrCl₃, FeCl₂, MgCl₂, NaCl and LiCl- catalyzed ethanol. This is because the relative crystallinity of crystalline cellulose in pretreated poplar is increased due to the removal of amorphous hemicellulose and lignin during the ethanol pretreatment process catalyzed by metal salts. It has been found that all metal salt-catalyzed ethanol pretreatment has the effect of promoting enzymatic hydrolysis efficiency. Among them, the trivalent metal salt is better than the trivalent metal salt and the monovalent metal salt as a whole. Through XRD and SEM, it could be seen that the dense structure formed by poplar fiber was destroyed, and the increase of cellulose content led to an increase in its relative crystallinity and exposed more accessible sites, thereby improving its enzymatic hydrolysis efficiency. By analyzing the effects of the addition of four additives on the yield and growth rate of enzymatic hydrolysis of poplar in the pretreatment of metal salt- catalyzed ethanol, it was found that the addition of additive could increase the enzymatic hydrolysis speed, shorten the enzymatic hydrolysis time and increase the enzymatic hydrolysis efficiency. Among them, Tween 80 has the widest application range and better effect on poplar pretreated with metal salt catalyzed ethanol. The addition of Tween 80 to the enzymatic hydrolysis of poplar after pretreatment with FeCl₃-catalyzed ethanol yielded the highest glucose yield of 93.44%.