Fractal Characterization Of Acacia During Acetic Acid Pretreatment And Enzymatic Hydrolysis

With the development of economic globalization, the progress of science and technology and the improvement of living standard, the worldwide energy shortage and environmental pollution problems have been increasingly intensified. So it is urgent to develop renewable biomass energy, more and more attention has been paid to lignocellulosic bio-ethanol conversion. Pretreatment and enzymatic hydrolysis are two key steps in the conversion process, so the thorough investigation into the mechanism is quite necessary.The acacia were enzymatic hydrolyzed after acetic acid pretreatment, then the UV-Vis (Ultraviolet-Visible spectrophotometry), XRD (X-ray diffraction spectrum), SEM (scanning electron microscopy), TG-IR,31P-NMR (nuclear magnetic resonance spectroscopy) and nitrogen adsorption were used to analyze the composition, functional structure and microstructure of liquid and solid samples in the conversion process. In order to study the effect of different pretreatment conditions on the fractal structure of acacia, the method for the calculation of fractal dimension were discussed, and then the relationship between fractal dimension and chemical composition, functional structure and microstructure were revealed. Based on the above, the fractal structure model was established. The effect of pretreatment and enzymatic hydrolysis conditions including pretreatment temperature, acetic acid dosage, substrate concentration, glucose inhibition, cellulase and Tween-80 dosage on the reducing sugars yield were also discussed. Finally, the variation of reaction rate constant (k) and fractal exponent (h) were discussed by using the improved fractal kinetic model.Before enzymatic hydrolysis, the acacia chips were pretreated by acetic acid and refined in three stages by the high consistency refiner. Results indicate that both the pretreated temperature and acetic acid dosage have certain effect on enzymatic hydrolysis, and the higher temperature or the larger amount of acetic acid leads to higher hydrolysis efficiency. After PFI refining, the crystallinity index of cellulose decreases, part of the fiber become the fragments and the phenomenon of fibrillation become more obvious, which result in significantly improved enzymatic hydrolysis efficiency.Chemical composition and structural characterization results show that partial hemicelluloses and low molecular weight of lignin hydrolyze after pretreatment, the crystallinity index of cellulose decreases by 11.31%. The wood surface becomes rough and generates a large number of pores, the fiber length of enzymatic hydrolysis residues is shorten and partially broken. A shoulder peak lacks around 280℃ on the curve of DTG due to the degradation of hemicelluloses. Aliphatic hydroxyl content of lignin decreases in different degree, while the phenolic hydroxyl and the ratio of S/G increases, and the ratio of phenolic/non-phenolic hydroxyl groups decreases with increased pretreatment temperature.The porous structure of wood and pretreated pulps has fractal characteristics, and its fractal dimension ranges from 2 to 3, which shows obvious fractal characteristics. The acetic acid pretreatment results in higher fractal dimension, which indicates increased irregularity of pore structure. As to the temperature effect, the higher temperature leads to lower fractal dimension. Compared to the FHH dimension, the fitting range of YBM dimension is wider with lower correlation coefficient. In addition, certain correlation is shown between fractal dimension and chemical composition, crystallinity and the ratio of phenolic/ non-phenolic hydroxyl groups. The fractal dimension decreased with the increase of cellulose or lignin content, but increased with the increase of the hemicellulose content, cellulose crystallinity or the phenolic-non phenolic hydroxyl group ratio. Fractal structure model shows that lignin plays a major role in the fractal structure of samples, while the influence of other factors is not significant.The results of enzymatic hydrolysis kinetics show that larger rate constant of k is shown for higher pretreatment temperature or acetic acid dosage, while there is no clear regularity with the fractal exponent of h. Moreover, the value of h increases for higher cellulase loading as well as substrate concentration, and the rate constant k increases with the former while decreases with the latter. The glucose inhibition also leads to the reduction of k and h. The effect of Tween-80 on the enzymatic saccharification of lignocellulose should be mainly contributed to the elimination of lignin inhibition by decreasing the fractal exponent h and increasing the rate constant k.