Alkali-based Pretreatment And Enzymatic Hydrolysis Of Lignocellulosic Biomass And Their Related Mechanisms

Converting lignocellulosic materials,such as sugarcane bagasse（SCB）,to fuel ethanol is of great significance to alleviate the increasingly serious energy crisis.However,it is difficult to achieve efficient conversion of lignocellulose for its natural recalcitrant structure.Therefore,an effective pretreatment method is necessary to improve the efficiency.In this study,to further improve the traditional alkaline pretreatment method,two pretreatment systems:（1）vacuum-assisted NaOH pretreatment（VANP）and（2）CH₃ONa with glycerol pretreatment（CWGP）,for ethanol production from SCB were established.Subsequent enzymatic hydrolysis and fermentation processes were also successively implemented.The mechanisms of VANP and CWGP to improve the hydrolysis efficiency of SCB were preliminarily deduced by studying the surface morphology,the crystallinity,the changes of chemical groups and the thermal cracking performance of the pretreated SCB.The main research contents and the results are shown as follows:（1）The pretreatment effect of NaOH on the SCB under vacuum conditions was firstly investigated.And the pretreatment parameters including NaOH concentration,pretreatment time and pretreatment temperature were optimized by a response surface methodology（RSM）.The results revealed that the concentration of the inhibitor at the supernatant of pretreatment hydrolysate under vacuum conditions was reduced by 50%-80%compared with the traditional pretreatment method under moderate pressure condition,while the total fermentable sugar yield（TFSY）increased by 8.54%-31.38%.Under the optimal pretreatment conditions,the delignification of SCB was 78.29%and the TFSY was 0.5146 g/g SCB.By conducting a separate hydrolysis and fermentation（SHF）using the SCB pretreated with the optimal VANP conditions,the highest concentration of ethanol was 4.5 g/L.The ethanol yield was134.6 g/kg raw SCB,which was 63.46%of the theoretical value and increased by 3.33 times compared with that of untreated SCB.The above results indicated that VANP could improve the ethanol concentration and ethanol yield of SCB and was an effective pretreatment method for SCB.（2）The pretreatment effect of various concentrations of CH₃ONa in 80%of glycerol solution on SCB was investigated,and the process parameters including CH₃ONa concentration,pretreatment time and pretreatment temperature were optimized by a RSM.The results showed that CWGP could significantly improve the enzymatic hydrolysis efficiency of SCB compared with pretreatment with glycerol or CH₃ONa alone.Under the optimal conditions,the delignification of SCB was 79.49%and the TFSY was 0.5666 g/g SCB.The highest concentration of ethanol was 4.58 g/L when the pretreated SCB was used as feedstock in a SHF.The ethanol yield was 147.2 g/kg raw SCB,equivalent to 66.42%of the theoretical value,and increased by 4.89 times compared with that of untreated SCB.The above results suggested that CWGP could improve the ethanol production from SCB and was an effective pretreatment method for SCB.（3）The synergistic effect between cellulase and xylanase was investigated.The enzymatic hydrolysis experiments of SCBs pretreated by VANP and CWGP were conducted with various proportions of cellulase and xylanase mixtures,respectively.The results showed that cellulase and xylanase had an obvious synergistic effect on enzymatic hydrolysis of pretreated SCBs and the synergism was more apparent at the initial stage of hydrolysis than the later stages.The ratio between cellulase and xylanase mainly influenced the speed of enzymatic hydrolysis rather than the final production of fermetabale sugar.In addition,the synergistic effect was dependent on the pretreatment method of SCB.（4）The structure and properties of SCBs pretreated with VANP and CWGP were characterized using field emission scanning electron microscope（SEM）,laser confocal scanning microscope（LCSM）,nuclear magnetic resonance（NMR）and so on.The mechanisms of two pretreatment methods to improve the enzymatic hydrolysis efficiency of SCBs were preliminarily clarify by the changes of the structure and properties of SCBs during the pretreatment process.The results showed that the surface structure of the pretreated SCBs was seriously damaged;the lignin contents were decreased;the crystallinities were increased;the sulphur content and the relative unsaturation degree were reduced;the thermal stability was increased.The above results indicated that VANP and CWGP would effectively destroy the integrity of SCB.