| The rapid development of human society has caused the fast depletion of fossil fuels and serious environmental problems. Concerted efforts have been made to develop effective and environmental friendly renewable energy in order to meet the requirement of sustainable development. China has the largest population and agricultural production. The annual straw production is estimated to be over 700 million tons, most of which is currently unused. The burning of straw could even become a source of pollution. Therefore,lignocellulosic biomass has a huge potential to be used as a feedstock for bioenergy production. It is difficult to use in the area of hydrolysis and fermentation due to the complex structure of the lignocellulosic material. The pretreatment process is considered as one of the most important steps in terms of the efficient use of biomass. Ionic liquid (IL), which is a type of novel and environment-friendly organic solvent, can be used to disrupt the highly ordered structure of plant cell wall and remove the lignin due to its unique physicochemical properties. Ionic liquid has a potential in the pretreatment of lignocellulosic biomass which could improve the biological digestibility.However, the whole mechanism of the ILs pretreatment on the biomass is not clear yet, which limited both the application of pretreatment technology and the development of the efficient ILs using in biomass pretreatment. The impact of ILs pretreatment on the change of crystalline structure of cellulose in biomass was studied by analyzing XRD data to reveal the process of swelling and solubilization. The influence of IL pretreatment on the physicochemical characteristics and internal structure was studied. After that,the impacts of ILs and alkaline pretreatments on the enzymatic hydrolysis and thermal degradation of corn stover, respectively, were investigated in order to better understand the mechanism of biomass pretreatment. Finally, the anaerobic digestibility of biomass after IL pretreatment was invesgated. This research aims to further understand the mechanism of ILs pretreatment of the lignocellulosic biomass. The results are as follows.X-ray diffraction (XRD) was used to investigate the swelling and solubilization process of biomass crystalline cellulose in the ionic liquid. The different biomass materials (switchgrass, corn stover and rice husk) was pretreated in 1 -butyl-3-methylimidazolium acetate ([C4mim][OAc]) at temperature of 50-130℃ for 6 hours. At a biomass loading of 5 wt.%, the crystallinity index (CrI) decreased with increasing of pretreatment temperature,which was due to swelling of crystalline cellulose in biomass. The low-order structure was formed, after most of crystalline cellulose swollen in IL. With further increasing the temperature, cellulose II structure was formed due to solubilization and subsequent regeneration of cellulose in [C4mim][OAc].Thermogravimetry (TG) was used to study the interaction between the biomass components and ILs. The thermal degradation data of biomass samples pretreated in [C4mim][OAc] at different temperature were provided by TG analysis of lignocellulosic biomass. Samples of Avicel (PH101), xylan(beechwood) and alkaline lignin as well as corn stover and switchgrass were pretreated by [C4mim][OAc] at temperatures of 50-130℃ for 6 h. Analysis of TG profiles supplied insight into the patterns of degradation of xylan and lignin in [C4mim][OAc]. After pretreatment, Avicel samples showed higher decomposition temperatures than that of untreated Avicel because of transformation from cellulose I into cellulose II. Switchgrass and com stover samples exhibited a different thermal stability due to removal of minerals by[C4mim][OAc]. Increasing pretreatment temperature led to a decrease of thermal resistance as a result of degradation of biomass components and decrystallization.The impacts of IL pretreatment, as well as NaOH pretreatments, on the enzymatic hydrolysis and thermal degradationand of corm stover were studied to know the mechanism of biomass pretreatment better. Com stover samples,at a 5 wt.% biomass loading, were pretreated in [C4mim][OAc] at different temperatures range of 50-110℃ for 6 h. Three different concentrations of NaOH solutions (0.5, 1.0 and 4.0 wt.%) were used to pretreat the corn stover at temperatures of 35 and 50℃ for 6 h. The results showed that different pretreatment conditions affected structures of biomass in varying degree.Structural changes were compared and discussed, which led to different enzymatic digestibility and variations in thermal degradation behavior. After pretreatment, enzymatic digestibility of corn stover was enhance with the decrease in thermal degradation temperature and more materials degrading at lower temperatures. Activation energy of the thermal degradation of biomass samples did not always decrease b.ecause the activation energy was mainly affected by cellulose crystallinity. Analysis of XRD data also showed that[C4mim][OAc] molecules interacted with (101) and (101) lattice planes of cellulose in biomass during the initial dissolution process.Samples of corn stover, at biomas loading of 5 wt.%, were pretreated in[C4mim][OAc] at temperature range of 50-150℃ for 1.5-4.5 h. The impact of IL pretreatment on the components, crystalline structure and thermal stability of corn stover at different conditions was investigated. More hemicellulose and lignin were lost to the liquid phase of mixed solvent of IL and water than cellulose during the pretreatment. The maximum lignin removal rate was 84.5%. With increasing the IL pretreatment severity, the cumulative biogas and methane yields of corn stover increased. The cumulative biogas and methane yields of untreated corn stover were 258.3 mL/g vs and 50.9 mL/g vs,respectively. The highest biogas and methane yields of 463.6 mL/g vs and 155.8 mL/g vs were obtained at 130℃ for 4.5 h. The maximum biogas and methane yields of pretreated corn stover were 79.5% and 206.1% higher than those of untreated samples. Accounting for losses during IL pretreatment, the pretreatment condition of 110 0C for 4.5 h resulted in maximum overall biogas and biomethane yields, which were 347.9 mL/g vs and 107.9 mL/g vs,respectively.This study revealed the crystalline structure transformation of biomass in IL pretreatment process. This would deepen the understanding of the swelling and dissolving process of bimass in ionic liquid. Different methods were used to study the influence of the IL pretreatment on physical and chemical properties changes of biomass to better understand the mechanism of IL pretreatment. Combined with the advanced energy conversion technologies,this study would provide new idea for the development and application of ionic liquids and the utilization of lignocellulosic biomass. |
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