Effect And Mechanism Of NADES Pretreatment On Enzymatic Hydrolysis Of Rice Straw

With social and economic development,more and more countries are beginning to realize the urgency of seeking clean energy and renewable energy.China is the world's third largest bioethanol producer and consumer after Brazil and the United States.Lignocellulose is the most abundant and easily available renewable resource on earth.Through pretreatment,enzymatic hydrolysis,saccharification and fermentation,the cellulose can be converted into bioethanol.China is a large agricultural country,which produces a large number of crop straw and other natural lignocellulose every year.In natural lignocellulose,cellulose is wrapped by hemicellulose and lignin,and it also has strong crystallinity.Pretreatment can break the bond between cellulose and other components,remove lignin and hemicellulose,greatly promote the enzymatic hydrolysis and saccharification of cellulose,and improve the yield of bioethanol.Therefore,pretreatment is the key step of lignocellulosic conversion into fuel ethanol.In recent years,natural deep eutectic solvent(NADES)has been widely concerned by researchers and applied in the pretreatment of lignocellulose due to its green environmental protection,low cost and easy availability.However,current research results show that NADES pretreatment is generally inefficient.In view of this deficiency,this paper develops a NADES combined with alkali/urea pretreatment technology,which greatly improves the efficiency of enzymatic hydrolysis and glycosylation.On this basis,a new NADES was synthesized and applied to the pretreatment of lignocellulose.Through this paper,the main results and conclusions are as follows:(1)Four kinds of NADES,including choline/glycerol(CC/GL),choline/proline(CC/PL),choline/malic acid(CC/MA)and choline/citric acid(CC/CA),were selected for the pretreatment of rice straw.Among them,CC/CA had the best pretreatment effect and the saccharification rate reached 43.9%.Further CC/CA(temperature130 ?,the mole ratio of 1:5,pretreatment time 4 h)with alkali/urea(concentration4% / 6%,temperature 20 ?,pretreatment time 4 h)combined with joint was carried out on the rice straw pretreatment,saccharification rate increased significantly,86%,is 1.95 times and 2.49 times of the sole CC/CA and alkali/urea preprocessing,respectively.In the combined pretreatment of CC/CA and alkali/urea,the NADES CC/CA pretreatment in the first step reduced the conditions for the subsequentalkali/urea pretreatment,indicating that the combined pretreatment of CC/CA and alkali/urea produced synergistic effects.(2)The solubility parameter theory of Hildebrand was used to calculate the performance of NADES in dissolved lignin.Four novel NADES were synthesized based on the calculation results.Among them,choline chloride/citric acid/glycerol(CC/CA/GL)showed the highest lignin removal rate and the best pretreatment effect of rice straw.Experimental results show that the molar ratio of 1:0.5 : 1.5,4 h under the condition of temperature 130 ?,after the CC/CA/GL pretreatment,the saccharification rate as high as 91.6%.Compared with the combined pretreatment of CC/CA and alkali/urea,CC/CA/GL pretreatment has the advantages of simple operation,high efficiency and green environmental protection.(3)The mechanism of CC/CA/GL pretreatment of rice straw was systematically studied.SEM,AFM and BET the determination results show that the new NADES severely damaged the ultrastructure of rice straw,in addition to the large amounts of hemicellulose and lignin removel of rice straw,increase the pore volume and specific surface area of rice straw,improved the accessibility of cellulose and cellulose enzyme on the surface of rice straw and cellulose enzyme adsorption rate to promote saccharifying enzyme solution.The results of FTIR,XRD and TG showed that the novel NADES CC/CA/GL pretreatment could destroy the bond between cellulose,hemicellulose and lignin in rice straw,including the destruction of C=O in the structure of lignin benzene ring,and change the crystallinity of rice straw,finally making the enzymatic hydrolysis reaction easier.