Study On Enzymatic Hydrolysis And Saccharification Of Cotton Straw Pretreated With Ethylene Glycol-chloride Salts

China is the world’s largest country in cotton cultivation and production with cotton straw of 54 million tons produced per year.Cotton straw is rich in carbohydrates that can be hydrolyzed and saccharified to produce biofuels and biochemicals.However,it is difficult to be hydrolyzed because of its dense and complex network structure.In this work,cotton straw was pretreated by ethylene glycol-chloride salts,and subsequently enzymatic hydrolyzed to fermentable sugars for producing value-added products.Cotton straw was from Xinjiang Uygur Autonomous Region,mainly consisted of 32.9%cellulose,16.4%hemicellulose and 27.8%lignin.The pretreatment of cotton straw with organic solvents-chloride salts was conducted in two different reactors:autoclave and three-necked flask equipped with heating pot.The pretreatment conditions of ethylene glycol-ferric chloride were optimized according to an orthogonal design.The pretreated cotton straw was hydrolyzed with cellulase,and cellulase loading and reaction time of enzymatic hydrolysis were optimized.Detailed results are listed below:First,cotton straw was pretreated in the autoclave.Effects of different types of chloride salts,organic solvents and concentrations on the compositions of cotton straw were preliminarily studied by a single factor method.After pretreated at 121℃ for 60 min(solid/liquid ratio of 1:20 w/w)with ethylene glycol(90 wt.%)-ferric chloride(0.15 mol/L),cellulose content(53.0%)increased by 61.1%but hemicellulose(6.9%)and lignin content(22.0%)decreased by 57.9%and 20.9%,respectively as compared to original cotton straw.Secondly,cotton straw was pretreated in the three-necked flask.The effects of the type and concentration of chloride salts,reaction temperature and solid/liquid ratio on the compositions of cotton straw were studied by a single-factor design.The pretreatment conditions of ethylene glycol-ferric chloride were subsequently optimized with orthogonal experiments.The results showed that the order of influencing factors on the hemicellulose and lignin removal of cotton straw was ferric chloride concentration>solid/liquid ratio>reaction temperature>reaction time.The optimal conditions were 0.1 mol/L ferric chloride,90 wt.%ethylene glycol,solid/liquid ratio of 1:15,reaction temperature 160℃,reaction time 20 min with 79.7%cellulose remaining,but 85.7%lignin and 88.9%hemicellulose removal,or cellulose content increased by 133.0%,hemicellulose and lignin content decreased by 67.2%and 58.0%,respectively.In addition,the structure and physical properties of original and pretreated cotton straw were characterized by Fourier infrared spectrum,N2-adsorption desorption and X-ray diffraction.The results indicated that the crystallinity,specific surface area and pore volume of pretreated cotton straw were increased by 9.5%,420.8%and 450%,respectively as compared to the original cotton straw.The functional groups(O-H,C=O,-C=C-,-CH and-CH2)in the pretreated cotton straw were changed that benefited the access of enzyme to cellulose for hydrolysis.Finally,enzymatic hydrolysis of cotton straws pretreated with ethylene glycolferric chloride was conducted(cellulase loading of 8.3 FPU/g-dry substrate,substrate concentration of 5%(w/v),pH of 4.8,temperature of 50℃,and the rotary speed of 150 rpm)in a shaker.Compared with solid/liquid ratio,temperature and time,the concentration of ferric chloride had a significant effect on enzymatic hydrolysis.After 48 h,glucose yield reached the highest(88.5%)from the sample pretreated under the optimal conditions(0.1 mol/L ferric chloride,solid/liquid ratio of 1:15,reaction temperature 160℃,reaction time 20 min).Cellulose in pretreated cotton straw was completely converted to glucose at 50 ℃,hydrolysis time of 72 h,substrate concentration of 5%(w/v),cellulase loading of 8.3 FPU/g-dry substrate and pH of 4.8.It can be concluded that ethylene glycol-chloride salt can effectively remove lignin and remain cellulose to promote hydrolysis,especially for the system of ethylene glycolferric chloride.The enzymatic hydrolysis efficiency can be further improved by increasing hydrolysis time and enzyme loading,which provided a new method for cotton straw pretreatment.