Hydrolysis Of Cellulose To Glucose Catalyzed By Composite Zeolites

Cellulose is the most abundant biomass energy in nature.Glucose,the main product of cellulose hydrolysis,can be further converted into biofuels and high value-added chemicals,which is of great significance to the sustainable development of energy.Acid catalysis is one of the most effective methods for cellulose hydrolysis,while an ideal reaction process would be the utilization of zeolite catalyst owing to its advantages in easy separation and recyclability.Nevertheless,it is very difficult to use solid acid catalysts directly for degrading lignocellulose,since it has a dense structure and difficult to depolymerize.In addition,the more acidic sites on the surface of zeolites often lead to the decomposition of glucose into other by-products,resulting in low glucose yield and poor selectivity.In view of these above mentioned problems in zeolite catalyzed cellulose hydrolysis,the composite zeolite-ionic liquid cellulose catalytic hydrolysis reaction system was designed in this study based on the cellulose hydrolysis mechanism.The effect of acid and structure of zeolite on the hydrolysis of cellulose has been considered as well.In this work,three kinds of composite zeolites were synthesized and used to promoted the hydrolysis cellulose.The effect of composite zeolites synthetic conditions and hydrolysis conditions on the reaction performance were discussed.High yields of total reductive sugars(TRS)and glucose were obtained in 1-ethyl-3-methylimidazolium chloride([Emim]Cl)ionic liquid as solvent.The mechanism on cellulose hydrolysis over composite zeolites was also probed.The structure and properties of composite zeolites with different Si/Al ratios(HY(n(Si O₂))/n(Al₂O₃)=5.1,HUSY₁(n(Si O₂))/n(Al₂O₃)=11.0 and HUSY₂(n(Si O₂))/n(Al₂O₃)=17.9)were characterized and analyzed by XRD,N₂adsorption-desorption and Py-FTIR,and their catalytic performance for cellulose hydrolysis was investigated.It can be found that the acid amount of zeolite mainly affected the reaction rate.The larger the amount of acid,the faster the reaction rate,and the higher the TRS yield.While lowering down the total acid amount and Lewis acid amount,a higher glucose yield could be achieved.Among the three Y-type zeolites,cellulose hydrolysis catalyzed by HY zeolite was the fastest associated with the highest TRS yield.HUSY₂,representing the lowest total acid amount and Lewis acid amount,afforded the highest glucose yield(32.7%)under the optimal conditions.Based on the above studies,HY and HUSY were modified by HZSM-5(exhibits less Lewis acid sites)or Silicalite-1(without acidity),resulting in composite zeolites HY/ZSM-5,HUSY₁/Silicalite-1 and HY/Silicalite-1 respectively.The XRD,SEM,TEM and N₂adsorption-desorption data indicated that the composite zeolites have core-shell structure.Due to the introduction of ZSM-5 or inert Silicalite-1,the total acid,Br?nsted acid and Lewis acid amount of the three composite zeolites were all decreased as compared to the pure HY or HUSY.Cellulose hydrolysis results indicated that the composite zeolites with reduced acid amount could effectively inhibit the glucose degradation side reaction,thereby the selectivity and yield of glucose could be improved.Comparing to pure HY catalyzed cellulose hydrolysis,the optimal glucose yield obtained for HY/ZSM-5-catalyzed cellulose hydrolysis can be increased from 28.0%to 38.8%,and 5-HMF yield reduced from 13.3%to 5.0%.Comparing to HUSY₁,the optimal glucose yield obtained for HUSY₁/Silicalite-1-catalyzed cellulose hydrolysis increased from 30.9%to 41.3%,and 5-HMF yield decreased from7.8%to 4.0%.The yield of glucose can reach 50.8%when cellulose hydrolysis was catalyzed by HY/Silicalite-1 under the optimum conditions.In addition,the hydrolysis of cellulose promoted by HY/Silicalite-1 not only improved the glucose yield and selectivity,but also reduced the reaction temperature from 130 ^oC to 110 ^oC.The mechanism with respect to the hydrolysis of cellulose over composite zeolite was fully discussed.First,the ionic exchange experiments showed that a proton exchange reaction could be facilitated between the organic cations of ionic liquid and the Br?nsted acid sites of zeolites.The exchanging rate order of different organic cations with H⁺was[Emim]⁺>[Amim]⁺>[Bmim]⁺.Secondly,calculation results for the diffusion coefficient of cations in different ionic liquids indicated that the shell Silicalite-1 had a certain limitation effect on molecules of different sizes.Based on the above results,the process of cellulose hydrolysis catalyzed by composite zeolites in ionic liquids was speculated:the[Emim]⁺cation might enter the core of the zeolite through the pores of the shell Silicalite-1,then H⁺is released from HY zeolite to the outer surface layer of Silicalite-1 through ionic exchange reaction,which further catalyze cellulose hydrolysis.In the ionic liquids system,the composite shell Silicalite-1 has the effect to prolong the diffusion path of cations in the zeolite,thus preventing the direct contact of glucose with the Lewis acid sites on the surface of HY zeolite.Accordingly,the release rate of H⁺could be controlled,which is beneficial for achieving good yield and selectivity of glucose.