Controllable Preparation Of Metal Phosphates And Their Application In Biomass Catalytic Conversion

Producing vast chemicals and fine chemicals from biomass(instead of fossil resources)serves as one of the most efficient methods to realize the continuous development of mankind society.Traditionally,inorganic acids are used as catalysts during the conversion of biomass,and it surely contradict the concept of green chemistry.Metal phosphates solid acids are promising catalysts to replace ordinary liquid acids.However,general metal phosphates suffer from low acidity,low total acid sites,hydrothermal instability et al.,limiting their potential industrial application.Therefore,preparation of novel metal phosphates with strong acidity,high specific surface areas,ordered pore structure and hydrothermal stability represents one of the most critical points in the field of producing value-added chemicals from the catalytic conversion of biomass.The current thesis has reported the preparation of mesoporous zirconium phosphates and niobium phosphates through controllable methods.Then,their catalytic activity for the dehydration of cellulose and sorbitol was investigated.Finally,the reaction pathways and mechanism was further proposed.Firstly,controllable preparation of metal phosphates was realized by sol-gel methods.The formation of gelation was optimized through thermal dynamic and kinetic aspects,followed by controllable preparation by changing substrates amount,templates,and metal sources and so on.The dehydration of cellulose to glucose was used as probe reactions to reveal the intrinsic regulation between catalyst structure and catalytic activity.In this way,we have prepared metal phosphates with mesoporous structures and good catalytic activity.Moreover,the catalysts remained steady during consecutive runs with high thermal stability.Secondly,the relationship between catalysts structure and dehydration ability of zirconium and niobium phosphates during the dehydration of cellulose to glucose was studied.The effect of mass transfer,catalysts to substrates ratio,reaction temperature,and reaction time was investigated.It was found that the catalytic activity could be improved by changing the kinetic characteristic parameters,thus promoting the conversion of cellulose and the yield of glucose.The optimized reaction conditions are 160 ?,4 h,15 mL deionized water with catalyst to cellulose ratio being 1:3.In addition,the thermal stability of catalysts and accumulative conversion of cellulose was investigated by consecutive runs.It was observed that the catalyst possessed good thermal stability,and the accumulative conversion reached 80%,indicating promising prospect in industrial application.Thirdly,a solvent-free liquid-solid reaction system was developed for the dehydration of sorbitol to isosorbide.The effect between catalyst structure and dehydration ability of zirconium and niobium phosphates was then investigated.Appropriate catalyst to substrate ratio,reaction temperature,reaction time was beneficial for the performance of metal phosphates,and the optimized reaction conditions are 210 ?,1-2 h with catalyst to sorbitol ratio being 1:10.In addition,a practical purification process of the raw isosorbide was developed.In the present study,there was no solvent during the reaction,thus greatly decreased the moisture content of the dehydration product,indicating higher purity of the formed isosorbide.Therefore,the purification process was simple,reducing the separation costs of isosorbide.When compared the catalytic activity of metal phosphates with traditional catalysts,better catalytic performance of the current catalysts was obtained with complete sorbitol conversion and an isosorbide selectivity of 70%,indicating promising commercial application prospect.Finally,the dehydration path way and mechanism of cellulose and sorbitol was studied.A double-layer structure model of solid acids under aqueous conditions was proposed and demonstrated by the linear relationship between total H+ concentration and catalytic activity.Moreover,space structure simulation of sorbitol revealed that the oxygen of C4 hydroxy had the most potent electronegativity and was the most active atom regarding to Br(?)nsted acid sites.Consequently,a dehydration pathway of one-step dehydration of sorbitol to 1,4-sorbitan was proposed and the corresponding solid acid catalyzed dehydration mechanism of sorbitol was then deduced.