Study On Enzymatic Hydrolysis Of Cellulose Under Supercritical Carbon Dioxide System

As an important part of biomass resources,cellulose is the most widely distributed and abundant polysaccharide in nature.It is a hot topic research about the efficient use of cellulose.In particular,the cellulose was hydrolyzed to sugar by cellulase.The sugar as a platform compound to synthesize various important chemicals is the key to alleviating the petrochemical crisis and developing a low-carbon economy.However,it is impossible to directly obtain a high concentration of reducing sugar after hydrolysis in traditional system,which limits the use of cellulose.As a special reaction medium,supercritical carbon dioxide?SC-CO₂?has more advantage than other reaction media,which makes it widely used in enzymatic reaction and cellulose pretreatment.If the cellulose can be hydrolyzed under SC-CO₂,after the reaction is completed,carbon dioxide is released,and a high concentration of reducing sugar solution can be obtained.The process has no complicated and cumbersome post-processing operations,reducing energy consumption and being environmentally friendly.It is an effective way to use cellulose efficiently.The specific findings are as follows:The effects of SC-CO₂ system on the structure and properties of microcrystalline cellulose,pulp,corn stover were studied.The results showed that the surface of these materials became rough and a large number of pits and wrinkles appeared on the surface after treatment by SC-CO₂ system.At the same time,the results of infrared fitting and other tests showed that SC-CO₂ reduced the crystallinity,hydrogen bond and hydroxyl content of the substrate,and increased the accessibility of the enzyme between the substrate resulting the increase of enzymatic reducing sugar.The pretreatment conditions of the three substrates were optimized,and the optimal conditions were obtained:microcrystalline cellulose as substrate,pretreatment time 10h,pretreatment pressure 10 MPa,pretreatment temperature 60?,pretreatment water content 2%;pulp as substrate,pretreatment time 10 h,pretreatment pressure 10 MPa,pretreatment temperature 60?and the pretreatment water content 2%;corn stalk as substrate,pretreatment time 10 h,pretreatment pressure 10 MPa,pretreatment temperature 60?and pretreatment water content 0.2%.The effects of SC-CO₂ on the structure and activity of cellulase were studied.The results showed that the SC-CO₂ system in some degree affected the stability of cellulase.However,the effect was less with the low frequent and short-time SC-CO₂treatment.The optimal reaction conditions for enzymatic hydrolysis of cellulose under SC-CO₂ system are as follows:temperature 50?,pressure 8.2 MPa?35 g carbon dioxide?,3 mL pH=4.8 buffer solution,300 mg cellulose,15 mg cellulase.Cellulase was chemically modified with monomethoxy polyethylene glycol propionaldehyde?mPEG-ALD?,activated dextran?mDEX-ALD?,monomethoxy polyethylene glycol maleimide?mPEG-MAL?,monomethoxy polyethylene glycol succinimide?mPEG-SPA?,monomethoxy polyethylene glycol hydrazide?mPEG-HZ?and phthalic anhydride?PA?as modifiers.Cell-ALD,Cell-DEX,Cell-MAL,Cell-SPA,Cell-HZ,Cell-PA modified enzymes were obtained,respectively.The results showed that the modified enzyme had higher activity and stability than the natural enzyme under both the conventional system and the SC-CO₂.Among the modified enzyme,Cell-ALD cellulase and Cell-DEX cellulase was of the best activity and the amount of reducing sugar is increased by about 35%.In addition,both immobilization and dialysis methods were used to recover natural and modified enzymes.The results showed that the activity of the enzyme recovered by the immobilization method was maintained at 80%or more,and the activity of the enzyme recovered by the dialysis method was maintained at about 60%.