Formation And Fermentative Inhibition Mechanism Of The Degradation Products In Corn Stover During Dilute Sulfuric Acid Pretreatment

In order to solve the key technical bottleneck of fuel ethanol production in respect of pentose fermentation inhibitors formation and controling, four main non- lignin constituents of corn stover, such as cellulose, hemicellulose, hot water extractives and ethanol extractives, were prepared respectively in this thesis. The degradation profiles of non-lignin constituents of corn stover from dilute sulfuric acid pretreatment were established using gas chromatography- mass spectrometry(GC-MS), high performance anion-exchange chromatography(HPAEC), high performance liquid chromatography(HPLC) and other analytical technologies. And on this basis their water soluble degradation profiles and formation characterization during the dilute sulfuric acid pretreatment were investigated, whereas the inhibitory kinetics of their acidic prehydrolyzates on ethanol fermentation by Candida shehatae was evaluated.Formic acid, levulinic acid and 5-hydromethylfurfural(HMF) originated mainly from cellulose degradation during the dilute acid pretreatment at the yield of 1.4%, 2.7% and 2.2%, while acetic acid and furfural came from hemicellulose degradation at the yield of 3.1% and 7.8% on the basis of corn stover weight, respectively. Orthogonal tests showed that the acetic acid production was significantly affected by the concentration of sulfuric acid, while the formic acid, levulinic acid, HMF and furfural formation was seriously affected by the pretreatment temperature.The non- lignin constituents were treated with 0.75% dilute sulfuric acid at 180 ℃ for 40 min respectively, then the inhibitory kinetics of their acidic prehydrolyzates on ethanol fermentation by C. shehatae were evaluated. Under the concentration of 100 g/L prehydrolyzates(on the basis of corn stover pretreated with sulfuric acid), the products released from the cellulose degradation could completely inhibit the xylose fermentatio n; the utilization of the glucose and xylose could be completely inhibited by the hemicellulose degradation products, and these products even perform lethal toxicity to C. shehatae; the products generated from hot water extractives and ethanol extractives exert inhibition on cell growth and sugars utilization. The key inhibitors were mainly from cellulose and hemicellulose degradation during the dilute sulfuric acid pretreatment of corn stover. Besides formic acid, acetic acid, levulinic acid, HMF and furfural in corn stover prehydrolyzates, some unknown degradation products also presented toxicity or synergistic inhibition on ethanolic co- fermentation of hexose and pentose.Over 100 degradation chemicals derived from four non- lignin constituents of corn sto ver after dilute sulfuric acid pretreatment were separated and identified by a combination of GC-MS, HPLC, RP-HPLC, and HPAEC. These chemicals were primarily sugars, aromatics, carboxylic acids, furans, alcohols, aldehydes, and cyclenes. The degradation profiles were analyzed and screened carefully, then nine degradation chemicals which have the potential inhibitory on the ethanol co-fermentation by Candida shehatae and Pichia stipites, such as aromatics(1,2-benzenedicarboxylic acid diisooctyl ester), carboxylic acids(2- methylpropioric acid, 4-pentenoic acid, 2-hydroxypropionate, 2-hydroxycaproic acid, malonic acid, palmitic acid and stearic acid) and furans(3-methyl-2-furoic acid).2.0 g/L 3- methyl-2- furoic acid solely, carboxylic acids and nine degradation chemicals in the mixture styles(each chemical added was 0.5 g/L) were added into the fermentative mediums to test the pentose fermentation inhibitory effects on C. shehatae and P. stipites. The results have shown that the capacity of ethanol co-fermentation by two strains was inhibited to varying degrees. Especially, the utilization of xylose and its ethanol fermentation was intensively inhibited that leads to the decline of ethanol yield. Potential synergistic effects of the carboxylic acids or nine degradation chemicals have been studied in this experiment with C. shehatae and P. stipites. The results indicate synergistic effects of combinations of acids, as well as of combinations of aromatics and furans markedly.