Preparation And Application In Tandem Catalysis Of Zirconium-Based Metal-organic Frameworks Composites

As a kind of organic-inorganic hybrid porous solid phase carriers,metal-organic framework(MOFs)materials have many advantages,such as tunable pore size,designed structure,easy functionalization and high guest loading efficiency,which make them show broad application prospects in the field of industrial biocatalysis.Among different metal-organic frame materials,zirconium-based metal-organic framework(Zr-MOF)has attracted much attention because of its excellent structural stability,and it can also be used as an ideal biological enzyme loading material.In this paper,UiO-66 zirconium-based MOFs was used as immobilized carrier,lipase and glucose oxidase were selected as model enzyme molecules,and the in-situ encapsulation process of zirconium-based MOF to biological enzyme molecules was studied by post-synthesis exchange method under room temperature and aqueous phase.The morphology and structure of the composite catalysts were characterized and analyzed by SEM,TEM,NMR,FTIR,XRD,XPS and other testing techniques,and the catalytic performance of the same/different types of catalysts encapsulated by UiO-66 MOF was further studied.The main conclusions are as follows:(1)Lipase was immobilized in zirconium-based metal organic framework in the form of in-situ by ligand exchange process at room temperature using water as solvent to prepare UiO-66-NH2@Lps immobilized enzyme catalyst,and the encapsulation efficiency of lipase was more than 95%.The results of SEM and TEM showed that the MOF crystal maintained the original irregular granular morphology after in-situ encapsulation of biological enzymes compared with the pure MOF crystal morphology.The crystal structure and ligand exchange process of UiO-66-NH2 were characterized by XRD,FTIR,NMR and XPS before and after loading biomolecules.Then,using 4-nitrophenyl acetate as the model substrate,the specific enzyme activity of the immobilized enzyme catalyst was tested under different reaction conditions.The results showed that the optimum reaction temperature of the immobilized enzyme was increased by about 10℃,and the pH tolerance range of the reaction environment and resistance to protease degradation were significantly improved compared with the free enzyme catalyst.In addition,the immobilized enzyme catalyst also exhibited good cycling stability and still retained 86.1%catalytic activity after 6 times of reused.(2)Based the biological enzyme encapsulation procedure of the mild conditions,a double enzyme co-immobilization catalysis system was further designed and constructed,and the UiO-66-NH2@glucose&oxidase-catalase(UiO-66NH2@GOx&CAT)double enzyme composite catalyst was prepared,the average entrapment efficiency of the enzyme was more than 85%.Similarly,the morphology of the composites before and after co-immobilization was characterized by SEM and TEM,and the process of dual enzyme encapsulation and ligand exchange was monitored and analyzed by XRD,FTIR,XPS and other testing techniques.The series catalytic performance of the double enzyme composite catalyst was further tested using glucose as the model substrate.The results showed that when GOx and CAT were encapsulated at the equal mass ratio,the composite catalytic system has the highest catalytic activity,which was 2.8 times of the immobilized GOx single enzyme system.At the same time,the optimum reaction temperature of the double enzyme composite catalyst is 30℃,the optimum pH is 6,it also has good tolerance to protease,and can still retain 87.6%catalytic activity after after 6 times of reused.(3)Due to the abundant amino functional groups on the surface of UiO-66-NH2,it can serve as an effective site for coordination binding of metal ions.Therefore,based on the coordination between metal ions and primary amine groups in the MOF structure,metal nanoparticles were in-situ synthesized,and metal-organic framework@lipase@metal nanoparticles(UiO-66-NH2@Lps@MNP)composite catalysts were further prepared.The results of SEM,TEM,XPS and XRD showed that different metal nanoparticles can be reduced successfully in UiO-66-NH2@Lps structure,the changes of the reaction products in the process of enzyme-metal tandem catalysis were monitored using 4-nitrophenyl acetate as the model substrate by UV-vis spectroscopy,and the one-pot tandem catalytic capability of the biological/chemical composite catalyst was verified.The results showed that the optimum reduction efficiency of 4-nitrophenol(0.4482 min-1)was obtained when silver/palladium alloy nanoparticles were prepared by the same molar ratio silver/palladium precursor system compared with single metal silver nanoparticles(0.1871 min-1).At the same time,the enzyme-metal composite catalyst can successfully complete the two-step tandem reaction,but the reaction time is longer than the step-by-step reaction process.