Effects of CrCl₃ pretreatment and surfactant on improving enzymatic hydrolysis of poplar

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℃, 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 ℃, 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.