Pretreatment Of Corn Stover By Ionic Liquids And Its Application In Butanol Fermentation

Increasing awareness on environmental pollution and energy depletion has prompted the search for clean and renewable fuels as alternatives for fossil energy. Production of biofuels from abundant and cheap lignocellulosic biomass such as crop straw is a promising method. However, the natural structure of crop straw is complicated and the inner reducing sugars are hard to be accessed and utilized. As a result, an efficient pretreatment method can not only lower the fermentation cost of biofuels but also beneficial to the environment. In recent years, ionic liquids(ILs) have emerged as promising solvents in biomass pretreatment owing to their excellent physico-chemical properties, especially the high dissolving capacity of various compounds. In this study, two ILs, a traditional IL(1-butyl-3-methylimidazolium chloride, [Bmim][Cl]) and a novel IL(deep eutectic solvent), were used in the pretreatment of corn stover(CS), and the pretreatment conditions as well as the enzymatic hydrolysis conditions were optimized. Biobutanol fermentation of these CS hydrolysates by Clostridium saccharobutylicum DSM 13864 was also investigated.Firstly, pretreatment of CS with fresh [Bmim][Cl] and recycled [Bmim][Cl] combining Na OH were performed, respectively. After enzymatic hydrolysis, a glucose concentration of 18.7 g·L-1 using ten-time recycled [Bmim][Cl] combining Na OH was obtained, representing about 77% of that produced with fresh [Bmim][Cl] combining Na OH(24.2 g·L-1). Hydrolysate I and II were obtained by hydrolyzing the fresh and ten-time reused [Bmim][Cl] combining Na OH pretreated CS. Fermentation of hydrolysate I using C. Saccharobutylicum DSM 13864 resulted in 7.4 g·L-1 butanol, with a specific yield of 0.21 g·g-1total-sugar and productivity of 0.11 g·L-1·h-1, while 7.9 g·L-1 butanol was achieved in fermentation using concentrated hydrolysate II with a specific yield of 0.23 g·g-1total-sugar and productivity of 0.12 g·L-1·h-1. Meanwhile, similar levels of by-products(acetone and ethanol) were obtained from above two fermentations. The results suggest that [Bmim][Cl] could be reused and the CS hydrolysate pretreated by reused [Bmim][Cl] combining Na OH can be applied in butanol fermentation without obvious inhibitory effect.Deep eutectic solvent(DES) is widely recognized as a new ILs, which is cheaper, lower toxic and easier biodegradable than traditional ILs. In this study, seven DESs were successfully synthesized with choline chloride(Ch Cl) as hydrogen-bond acceptor and five carboxylic acid as well as two neutral reagents as hydrogen-bond donors. Based on the pretreatment of CS with these seven DESs, Ch Cl:formic acid showed excellent performance in CS pretreatment, displaying the highest glucose yield in enzymatic hydrolysis. By scanning electron microscope(SEM), fourier transform infrared spectroscopy(FTIR) and X-Ray diffraction(XRD) analysis, the effect of Ch Cl:formic acid was confirmed and the mechanism of Ch Cl:formic acid in CS pretreatment was elucidated. Then, the pretreatment and enzymatic hydrolysis conditions of Ch Cl:formic acid were optimized, and the best conditions were as following: pretreatment temperature of 130°C, pretreatment time of 2 h, CS/Ch Cl:formic acid ratio of 1:20(w/w), solid/liquid ratio in enzymatic CS hydrolysis of 1:10. Hydrolysate III was obtained by hydrolyzing the CS pretreated by Ch Cl:formic acid. Fermentation in concentrated hydrolysate III resulted in 5.6 g·L-1 butanol(49 h) with a specific yield of 0.17 g·g-1total-sugar and productivity of 0.12 g·L-1·h-1. The results demonstrate that DES could be used as a promising and biocompatible pretreatment method, and provide theoretical guidance for the conversion of lignocellulosic biomass into biofuel.