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%.

Biomass energy has the advantages of recyclability and environmental friendliness. Poplar grows rapidly and has a relatively high cellulose content, making it an excellent raw material for the production of biomass energy. However, the cellulose, hemicellulose and lignin of poplar are intertwined to form a dense structure, making it difficult to be hydrolyzed in normal cases. In this work, poplar was pretreated by CrCl₃ as catalyst, the effect of different temperatures was compared, and the optimal pretreatment temperature was determined. Results showed that under the conditions of 0.05 mol/L CrCl₃, solid-liquid ratio of 1/10, pretreatment time of 20min, stirring speed of 300 r/min and pretreatment temperature of 160°C, the components of poplar were effectively separated after pretreatment with CrCl₃, while 10.4% lignin and 90.0% hemicellulose removal, and cellulose content increased by 35.6%, hemicellulose and lignin content decreased by 85.5% and 29.9%, respectively. The yield of glucose increased by 568.1% after 72h. In addition, the structure and physical properties of the raw and pretreated poplar were characterized by scanning electron microscope (SEM)、diffraction of X-rays (XRD) and Fourier transform infrared spectrometer (FT-IR). Experimental results indicated that the structure of poplar became loose after CrCl₃ pretreatment, improving the contact surface of cellulose. The crystallinity (CrI) of poplar raw material was 63.84%, then increased to 71.07% after CrCl₃ pretreatment, which was due to the removal of amorphous hemicellulose during pretreatment. In the FT-IR test, the characteristic absorption peak of acetyl groups in hemicellulose was significantly weakened after CrCl₃ pretreatment compared with the raw material, a strong benzene ring characteristic absorption peak was retained at 1510 cm⁻¹, and the characteristic peak of β-glycosidic bonds still appeared around 898 cm⁻¹. It is further demonstrated that CrCl₃ pretreatment removed most of hemicellulose and retained the majority of cellulose, which promoted the subsequent enzymatic hydrolysis process, but had less effect on lignin. The enzymatic hydrolysis of pretreated poplar was conducted under the conditions of pH value is 4.8, enzymatic hydrolysis temperature of 50 °C, shaker speed of 150 r/min, enzyme addition of 20 FPU/g substrate, surfactant dosage of 150mg/g substrate, exploring the promotion effect of surfactant type, dosage and enzyme addition on enzymatic hydrolysis. It was found that the glucose yield of 69.7% was obtained after 24 h with calcium lignosulfonate, which was higher than that after 72 h without additive. Further extending the hydrolysis time to 72 h, the glucose yield reached 85.5%. When the cellulase loading was 5 FPU/g substrate, the glucose yield with calcium lignosulfonate (68.0%) was constant with that when the enzyme dosage was 20 FPU/g substrate without additive. That the addition of calcium lignosulfonate could reduce the hydrolysis time, decrease the dosage of cellulase, and enhance the efficiency of enzymatic hydrolysis. Consequently, the CrCl₃ pretreatment effectively removed the lignin and hemicellulose, while remained cellulose to promote hydrolysis efficiency. Furthermore, the addition of calcium lignosulfonate as a surfactant in the enzymatic hydrolysis also improve hydrolysis efficiency. This study provides some theoretical support for the improvement of pretreatment and the reduction of enzymatic hydrolysis cost.

As a green and renewable energy, bio-ethanol produced from lignocellulosic biomass is great important for China to achieve the "double carbon" strategic goal. However, the dense and complex structure composed of cellulose, hemicellulose and lignin makes it difficult for cellulase to degrade lignocellulose directly. Therefore, appropriate pretreatment was needed to disrupt the intact structure, enhance the accessibility of enzyme to cellulose, thus enhancing the ethanol production. In this study, poplar was regarded as the raw poplar, then the influence of acid-alkali combination pretreatment on hemicellulose and lignin degradation, the retention of cellulose, and subsequent simultaneous saccharification and fermentation were investigated. Subsequently, scanning electron microscopy (SEM), X-ray diffraction (XRD), and fourier transform infrared spectroscopy (FT-IR) were used to determine the effect of pretreatment on the alteration of surface morphology, composition and thermal stability. The two-step combination pretreatment included the HAc and NaOH pretreatment. The first step was conducted at (160-200 ℃) with 1% HAc catalyst. The pretreated solid obtained from HAc pretreatment was regarded as the material for the second NaOH pretreatment with different NaOH concentrations (0.3%-1.2%). The fermentation efficiency of two-step HAc-NaOH combination pretreated samples were significantly higher than that obtained from one-step HAc pretreatment. The highest ethanol concentration of 18.72 g/L was obtained from 200 ℃ HAc-0.8% NaOH pretreated substrate. The analysis of SEM, XRD and FT-IR revealed that the removal of hemicellulose and lignin during pretreatment significantly increased the cellulose content in pretreated poplar, resulting in the disruption of dense structure of poplar, which improved the accessibility of the enzyme to the cellulose and enhanced the subsequent fermentation efficiency. SEM showed that with the increase of the severity of the HAc-NaOH combination pretreatment, the structure of the poplar fiber bacame looser, the surface of the fiber was seriously broken, various cracks appeared, and the degree of fiber fracture increased. After 200 ℃ HAc-0.8% NaOH combination pretreatment, the crystallinity (CrI) was increased from 66.73% of raw poplar to 77.31%, ascribing to the removal of amorphous hemicellulose and lignin. The FT-IR analysis found that the characteristic absorption peaks of β-(1,4) -glycosidic bond (898 cm⁻¹) and-OH stretching (3340 cm⁻¹) of cellulose were observed in all samples, indicating that cellulose degraded less in the two-step HAc-NaOH combination pretreatment stage. These results were consist with the fermentation efficiency. In addition, the correlation between glucan, xylan, acid-insoluble lignin content, the removal of xylan and acid-insoluble lignin (AIL), the reserved cellulose and structural properties and ethanol yield were analyzed, suggesting that pretreatment samples with lower hemicellulose and lignin content and higher cellulose crystallinity had a higher ethanol yield.

Pretreatment has been one of the most important procedures to convert the lignocellulosic biomass into the sugar-based chemicals. The dense structure of lignocellulosic biomass can be destroyed to reduce the biological resistance. This study aims to investigate the influence of pretreatment on the enzymatic hydrolysis of sugarcane bagasse, the compositions of solid residue, and pretreatment liquid. The 0.025 mol/L metal salts (FeCl₃, CrCl₃, AlCl₃, CuCl₂, FeCl₂, ZnCl₂, MnCl₂, MgCl₂, CaCl₂, NaCl, LiCl, and Na₂CO₃) were selected to enhance the ethanol/H2O. The pretreatment was conducted at 160 °C for 10 min. Compared with the raw sugarcane bagasse, the glucan content in the metal salt-enhanced ethanol pretreated samples increased from 45.5% to 77.2%. The sugars in the pretreatment solution were mainly xylose. There was the low content of glucose, indicating that the metal salts was effectively removed the hemicellulose and lignin during ethanol/H₂O pretreatment. In the enzymatic hydrolysis of pretreated samples, the efficiency of metal salts on enzymatic hydrolysis was ranked in the descending order of: trivalent metal salt (FeCl₃, CrCl₃ and AlCl₃), divalent metal salt (CuCl₂, FeCl₂, ZnCl₂, MnCl₂, MgCl₂ and CaCl₂), and monovalent metal salt (NaCl, LiCl and Na₂CO₃). Furthermore, the surface morphology and structure in the native sugarcane bagasse and pretreated solids were characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric. SEM images showed that the morphological structure of bagasse changed more violently with the increase of metal hydrochloric acid degree, ranging from the original smooth surface to the rough, where many irregularly shaped fine particles appeared, even a large number of cracks and pores. XRD analysis showed that the crystallinity of bagasse raw materials was 51.9%. Furthermore, the crystallinity was improved differently in the various metal salts after pretreatment. Among them, the most crystallinity was found after trivalent metal chloride-enhanced ethanol pretreatment, which were FeCl₃ (64.5%), CrCl₃ (65.1%) and AlCl₃ (64.4%). While the crystallinity of MgCl₂ and NaCl only increased to 57.5% and 55.8%, respectively. FT-IR analysis showed that there was the weak or even disappeared characteristic absorption peaks at 1 730 cm^-1 of the carbonyl group in hemicellulose, and the characteristic absorption peaks at 1 600 and 1 510 cm^-1 of the benzene ring in lignin after ethanol pretreatment enhanced by trivalent metal salts (AlCl₃, CrCl₃, FeCl₃), compared with the raw materials. Therefore, the trivalent metal salts (AlCl₃, CrCl₃, FeCl₃) during ethanol pretreatment were greatly removed the lignin and hemicellulose in bagasse. TG analysis showed that the maximum weight loss peaks of the metal salt CrCl₃, AlCl₃, FeCl₃, MgCl₂ and NaCl enhanced ethanol/H2O pretreatment samples appeared at 334, 346, 360, 360 and 365 °C, respectively, and the maximum weight loss rates were -1.86, -1.72, -1.59, -1.17 and -1.16% /°C, respectively. There was the decrease in the peak temperature of maximum weight loss decreased, whereas, the maximum weight loss rate increased with the increase of metal hydrochloric acid. As such, the acidic metal salt enhanced ethanol/H2O pretreatment can effectively destroy the bagasse structure suitable for the degradation of biomass. These characterization datasets were in better agreement with the previous enzymatic hydrolysis of trivalent metal salt-enhanced ethanol/H2O pretreatment. These findings can provide the valuable insights to utilize the lignocellulosic biomass.