Effects of different metal salts enhanced ethanol/H₂O pretreatment on enzymatic hydrolysis of sugarcane bagasse

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.