Study On The Pretreatment Of Cornstalk And Dissolution Mechanism In Ionic Liquids

As a sustainable and renewable energy resource with large reserves and wide distribution, biomass has been widely applied in the preparation of liquid fuel and high value-added chemicals. However, the complex network structures among the compositions of biomass make the biomass direct utilization rate very low. Biomass pretreatment could effectively remove lignin, lower cellulose crystallinity, and greatly increase the enzymolysis rate and improve biomass utilization significantly. Therefore, pretreatment procedure has become the key step in biomass efficient utilization. As a new medium, Ionic liquids (ILs) possess good solubility and stability, and has been gradually applied in biomass pretreatment in recent years. Improving biomass pretreatment efficiency and understanding the pretreatment mechanism clearly are two challenges in the biomass pretreatment with ILs. In this work, cornstalk and rice straw were used as raw material; cornstalk was pretreated by different composite ILs systems to obtain cellulose rich materials (CRM), which would be used to prepare high value-added chemicals through enzymolysis or catalytic conversion; meanwhile, laser scanning confocal microscope (CLSM), atomic force microscope (AFM) and nuclear magnetic resonance (NMR) were used to observe or characterize the interaction bewteen rice straw and ILs in different scales, and the mechanism of rice straw dissolution in ILs was obtained. The main contents and results are as follows:(1) Study on cornstalk pretreatment in binary compound system of IL+additives. The delignification of cornstalk was efficiently accomplished by using 1,5,7-triazabicyclo [4.4.0]dec-5-ene (TBD) as an additive in 1-allyl-3-methylimidazolium acetate ([Amim][OAc]), and good CRM was obtained. The influences of ILs types, additives kinds, pretreatment time, and TBD loading on cornstalk pretreatment were also investigated. When 1.0 wt% TBD was added to [Amim][OAc], the cellulose and lignin contents of CRM were achieved to be 39.12% and 6.74%, respectively. With the addition of 0.1 wt% TBD to [Amim][OAc], the lignin content of CRM could be reduced to 2.06%. Simultaneously, the CRM regenerated from the system of [Amim][OAc]+TBD was effectively hydrolyzed by cellulase with 98% enzymatic hydrolysis yield. The density functional theory (DFT) calculations indicated that the interaction energy of [Amim]+and [O Ac]- decreased from 99.1 kcal·mol-1 to 89.2 kcal·mol-1 with the addition of TBD, which made the [Amim]+and [OAc]- easier to interact with the cornstalk components. Besides, the alkalinity and exposed nitrogen atoms made TBD efficient in cleaving the β-0-4 ether bond of lignin, thus the structures of lignin were disrupted and lignin was degraded into small molecular substances, eventually the enhanced delignification was achieved.(2) Study on cornstalk pretreatment with two-step ILs method. A two-step ILs method combining choline hydroxide (ChOH) and 1-ethyl-3-methylimidazolium dimethyl phosphate ([Emim][DMP]) for cornstalk delignification was developed. The influences of ILs types, temperature and time on cornstalk pretreatment were investigated. In the two step ILs method, cornstalk sample was first soaked in ChOH aqueous solution for 3 h, and the residue was dissolved completely by [Emim][DMP] in 10 min at 130 ℃. The results showed that 77.28% lignin was removed and cellulose content was increased to 52.14% in the CRM. Simultaneously, the CRM could be catalyzed and converted into 5-Hydroxymethylfurfural (HMF) with a high yield of 87.21%. The DFT calculations indicated that the synergy of Ch+and OH- is helpful for delignification, meanwhile, the cellulose crystalline structures were disrupted by [Emim][DMP]. In this two step method, the ChOH is biodegradable, and the [Emim][DMP] can be easily synthesized and recycled; simultaneously, the dissolution time at high temperature was reduced and the energy consumption was lowered. Therefore, this two-step method is promising for biomass pretreatment and provides a new way of thinking for biomass industrial utilization.(3) The dissolution mechanism of rice straw in ILs was studied from micron scale to nanoscale. In the micron scale, the three major cell types in the cell walls, i. e. the sclerenchyma cells, tracheids, and parenchyma cells were all first swelled and then dissolved in 1-ethyl-3-methyl-imidazolium acetate ([Emim][OAc]), but the rates of swelling and dissolving were variously, sclerenchyma cells were first swelled, tacheids cells took second place, and the parenchyma cells were swelled and distorted at the same time. Swelling is the key step in the cell walls dissolution, when the thickness change of cell wall is close to the original cell wall thickness, the cell walls start to be dissolved. Lignin and hemicellulose can be quickly dissolved in traditional acid and alkali solution, thus the cellular structures were destroyed; 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) could not swell the cell walls of rice straw. In the nanoscale, the microfibres were first swelled and then dissolved in [Emim][OAc]. When the change of microfibre diameter is close to the original microfibre diameter, the microfibers begin to be dissolved. Acid/alkali aqueous solution and [Bmim][PF6] could not swell the microfibres, microfibers could be swelled to a little degree but can’t be dissolved in pure glacial acetic acid. Therefore, swelling is the key process for microfibers dissolution, microfibers would not be dissolved in the system that could not effectively swelling microfibers. In the molecular scale, the NMR characterizations and simulation results indicated that ILs hydrogen bonds (ILs-HBs), which were different from conventional hydrogen bonds, were formed among the anions, cations of ILs and glucose chains of cellulose. Simulation results show that ILs anions play a leading role, NMR chemical shift changes suggest that ILs cations could interact with glucose chains. The changes of NMR chemical shifts of ILs have close relationship with the chain length of the oligosaccharides. Cellulose dissolution in ILs is great influenced by the size of cations and anions of ILs and the position of hydrogen bonds that formed with glucose chains.According to the results above mentioned, the possible mechanism can be summarized as followed:1) the anions and cations of ILs firstly formed strong ILs-HBs with the exposed hydroxyl of glucose chains, the hydrogen bonds networks among the glucose chains were weakened; 2) the loose hydrogen bonds networks increased the gaps among the original glucose chains, which would induce more anions and cations of ILs to be inserted into the spaces to form more ILs-HBs with the hydroxyl of glucose chains, the "space effect" of ILs-HBs enhanced the interaction between ILs and glucose chains; the "size effect" of anions and cations of ILs further expanded the gaps among the original glucose chains, and gradually disrupted the hydrogen bonds networks among the glucose chains, then the microfibrils were swelled; 3) the "time effect"and the consistency of the ILs-HBs, which was attributed to the long lifetime of the ILs-HBs, would disrupt the hydrogen bonds among glucose chains completely, and the microfibers were dissolved in ILs gradually; 4) the microfibers swelling and dissolution induced the cell walls swelling and dissolution; meanwhile, the cellulose crystalline was destructed and ultimately increased the biomass enzymolysis.