Design And Preparation Of Metal Nanoparticles-Metal Organic Framework Composite And Its Application In Catalysis

In recent years,the controllable integration of metal-organic framework(MOF)and functional components has promoted the development of new multifunctional composites,which are superior to the individual component of MOF because of the collective behavior of functional units.Among multifunctional composites,the composite material formed by metal nanoparticles(NPs)and MOF possesses advantages of controllable structure and host-guest synergistic effect,which has attracted the increasing attention with a good application prospect in the field of catalysis,especially in the chemical utilization of C1 resources.Among them,the[2+3]cycloaddition reaction between CO2 and epoxides is the classic C1 resource utilization process,and the product carbonates can be used as favorable organic solvents and high value-added chemicals.The multicomponent multifunctional composite structure of NPs-MOF has certain advantages in the catalytic transformation system,but there are still some problems to be solved,such as the complex preparation method of the composite material,the difficulty of embedding metal nanoparticles,the easy agglomeration of nanoparticles,and the requirement of stabilizers,which would affect its application in the field of catalysis.In order to solve the above problems,a new method of pseudocrystalline transformation-thermal decomposition has been developed in this paper through the exploration of crystal transformation and the comparison of nanoparticle reduction methods,achieving the accurate,rapid and controllable preparation of NPs-MOF composites.First of all,MOF-74 precursors were prepared by interfacial synthesis method.MOF-74 precursors can be transformed into classical MOF-74 crystal structure through pseudocrystalline transformation process.Secondly,thermal decomposition reaction or nanoparticle doping reaction were investigated to explore and compare different ways of nanoparticle introduction before pseudocrystalline transformation.The original copper metal nodes of MOF have been reduced in the thermal decomposition process to introduce nanoparticles,and realize the in-situ nanocrystallization of MOF metal center.The doping process of nanoparticles requires the recombination of aqueous gold nanoparticles with MOF precursors.Then,the pseudocrystalline transformation was used to convert the intermediate embedded with the nanoparticles into the classical MOF-74 crystal,and finally the composite Cu NP@MOF-74 was accurately synthesized.The catalytic function of NPs-MOF composites in the[2+3]cycloaddition reaction of CO2 and epoxy compound was further investigated.Cu NP@MOF-74 showed the good catalytic performance.Specific research results are as follows:(1)MOF crystal(Cu(H2dhtp)·3DMF)synthesized from copper salt and 2,5-dihydroxy terephthalic acid(H4dhtp)served as the precursor of Cu-MOF-74,which was partially decomposed by vacuum heating to synthesize MOF-74 intermediate embedded with copper nanoparticles.Methanol can promote the crystal transformation from intermediate to MOF-74,through the ligand and guest molecule isomerization,and generate multilevel nanostructure:Cu NP@MOF-74.In addition,Cu-MOF-74 precursors were also dispersed into gold nanoparticles solution,facilitating gold doped into the crystal by oscillation method,and the crystalline transformation can also be carried out.(2)In the further work,the composite material was used as a catalyst for the cycloaddition reaction of CO2,in which Cu NP@MOF-74 not only maintained the integrity and porosity of MOF skeleton,but also achieved the purpose of controllable recombination of copper nanoparticles,thus forming a good multistage composite catalytic structure,and the conversion rate reached 91.6%.Compared with the traditional Cu-MOF-74,the catalytic efficiency has been improved.This work provides a new idea for the preparation and application of NPs-MOF composites.