Novel Cholinium Ionic Liquid-based Pretreatment Of Wheat Straw

Presently, fossil fuels such as coal and oil are still the main fuels for the world. With the depletion of these non-renewable resources and environmental problems caused by their use, green renewable fuels have attracted considerable interest. Lignocellulosic biomass is the most abundant, cheap and renewable resource. Production of biobased fuels and chemicals from lignocellulosic biomass is an effective way for the sustainable development of the society. Lignocellulosic biomass is highly recalcitrant to chemical and biological degradation due to its complicated structure. Therefore, pretreatment is essential to deconstruct the biomass prior to degradation. Ionic liquids(ILs), a new class of solvents, have many advantages, especially their powerful abilities to dissolve biopolymers such as cellulose and lignin. IL-based biomass pretreatment has become a hot research topic. In addition, large water consumption is an important factor affecting the sustainable development of biorefinery. Therefore, using abundant and cheap seawater to replace freshwater is of interest for the large-scale application of biorefinery. As a result, nine novel and renewable cholinium ILs were synthesized in this thesis, and these ILs were used to pretreat wheat straw for improving the subsequent enzymatic hydrolysis. Then, the feasibility of using seawater as an alternative to freshwater for IL pretreatment of wheat straw and enzymatic hydrolysis was explored.Firstly, 9 novel and renewable cholinium ILs were synthesized and characterized. A high solubility of lignin(up to 483 mg/g) and xylan(up to 721 mg/g) was observed in four ILs containing organic anions, while cellulose, chitosan, and keratin were scarcely soluble in all ILs. Among the nine ILs tested, cholinium taurate([Ch][Tau]) was the best solvent for wheat straw pretreatment. Upon [Ch][Tau] pretreatment, 68.8% of lignin was removed from wheat straw, and a reducing sugar yield of 74.2% was achieved in the subsequent enzymatic hydrolysis. The effects of pretreatment temperature, pretreatment time, biomass particle sizes, water contents, and biomass loadings on the IL pretreatment were studied. The optimal pretreatment temperature and time were 80℃ and 6 h, respectively. Readily digestible residues were obtained after wheat straw up to 2 mm size was pretreated by [Ch][Tau]. Additionally, this IL pretreatment process was highly tolerant toward moisture. A good reducing sugar yield(79.7%) was achieved in the enzymatic hydrolysis of wheat straw pretreated by [Ch][Tau] at a biomass loading of 10% under 80℃ for 6 h. [Ch][Tau] was a good pretreatment solvent for grass lignocellulosic biomass, while being ineffective for wood biomass pretreatment.The tested IL-seawater mixtures were found to be effective for wheat straw pretreatment, leading to the improved enzymatic hydrolysis, except cholinium levulinate([Ch][Lev]). Among the IL-seawater mixtures, cholinium argininate([Ch][Arg])-seawater mixture was the best, the mixture pretreatment resulted in removal of 71.3% lignin, and the reducing sugar yield of 70.7% was achieved in the subsequent enzymatic hydrolysis. In addition, delignification ability of [Ch][Arg]-seawater mixture decreased with the increase of seawater content, while the enzymatic hydrolysis efficiencies and reducing sugar yields(70.1-77.0%) were comparable in the subsequent enzymatic hydrolysis of all the pretreated biomass. The enzymatic hydrolysis of the pretreated wheat straw was also studied in different reaction media. The reducing sugar yield of 73.8% and initial saccharification rate of 0.78 mg m L-1 h-1 were achieved in pH 4.8 seawater adjusted with citric acid, which are similar to the results obtained in freshwater-based citrate buffer(50 m M, pH 4.8). However, increasing seawater concentration would result in an inhibitory effect to the enzymatic hydrolysis. In addition, [Ch][Arg]-seawater mixture is effective for pretreatment of grass lignocellulosic biomass, leading to the improved reducing sugar yield in the subsequent enzymatic hydrolysis, while it is a poor solvent of wood pretreatment.The structure-function relationship of cholinium ILs has been revealed preliminarily in the present work, which would lay the theoretical foundation for the rational design of novel ILs for highly efficient pretreatment of lignocellulosic biomass. In addition, a cholinium IL-based, simple, green, and highly efficient pretreatment process of lignocellulosic biomass has been established. Besides, the present study would provide a promising option for biorefinery in the regions suffering from freshwater-shortage.