Saccharification And Characterization Of Lespedeza Stalks With Low Severity Steam Explosion And Peroxide Treatment

Lespedeza crytobotrya is a perennial shrub species of the leguminous genus Lespedeza. The most important advantages of L. crytobotrya with respect to its use as a substrate for bioconversion to ethanol are that it has superior growth characteristics relative to other species and that its stalks contain a relatively large amount of carbohydrate. A combination of low severity steam explosion (1.25MPa,4min) and alkaline peroxide post-treatment of Lespedeza stalks was investigated in order to improve enzymatic digestibility of cellulose, and the removed lignin and hemicellulose were isolated from the filtration after this process.Steam explosion did not significantly change the composition of Lespedeza materials except for hemicellulose. The samples obtained by steam explosion were found to be slightly smoother and looser than the raw material through scanning electron microscopy images.In the post-treatment by alkaline peroxide, the experimental sets included variations in substrate concentration, peroxide content, time, temperature and ethanol content for investigating the delignification. The optimal reaction condition got from single factor experiments was substrate concentration3.3%, peroxide content0.6g H2O2/g substrate,24h, and60℃, resulting in a remove of73.20%of lignin from steam-pretreated material. The maximum delignification reached88.41%as the lignin content of sample was only5%.Samples with lignin content of15%,10%, and5%were produced by alkaline peroxide treatment for analyzing the influence of delignification on cellulose digestibility and the structure of lignin and hemicellulose. The cellulose-rich fractions were characterized by scanning electron microscopy and X-ray diffraction, illustrating a smoother surface, a looser structure, an rising crystalline index (from50.4%to53.1%), and a declining degree of polymerization (from333.9to229.3) with the increase of delignification severity. The glucose yield of the3cellulose-rich fractions were431.8、491.2和432.6mg/g raw material, respectively. The isolated lignin preparations were characterized by gel permeation chromatography, Fourier transform infrared spectroscopy, and thermogravimetric analysis/differential thermal analysis. Depolymerization and condensation occurred during the alkaline peroxide treatment. Condensation increased the thermal stability of lignin (mass of pyrolysis residue went up from26to 31%), and the average molecular weight rose to18,919g/mol with the increase of delignification severity. The isolated hemicellulos preparations were characterized by gel permeation chromatography, Fourier transform infrared spectroscopy,’H NMR spectroscopy. Almost all of the hemicellulose was removed from the material, and was further degraded as the average molecular weight was decreased from14,090to9,062g/mol.The maximum value of solid recovery in those three procedures reached88%, indicating that the consecutive pretreatment conducted in this research can not only enhance the enzymatic hydrolysis of lespedeza stalks, but also achieve the isolation and utilization of full composition with minimal loss.