Design And Synthesis Of Hierarchical Porous Zeolite Imidazolate Framework And Its Catalytic Performance

Zeolitic imidazolate frameworks?ZIFs?are a new type of nanoporous crystal materials,which have received much attention in recent years.ZIFs have huge specific surface area and ultra-high porosity,high thermal stability and chemical stability,tunable properties and functional diversity.Therefore,ZIFs have shown broad application prospects in the fields of catalysis,gas adsorption,separation,and photoelectricity.Compared with the traditional ZIFs with only a single microporous structure,the hierarchical porous ZIFs have a larger pore volume,its hierarchical porous channel structure can effectively improve the mass transfer performance.The hierarchical porous ZIFs can widen its applications in adsorption and separation of complex gases,the macromolecular gas transfer and transportation and macromolecular catalytic reaction,etc.,has shown great advantages in practical applications.By introducing mesopores or macropores into the traditional microporous ZIFs,a hierarchical porous channel structure is formed,combining the advantages of the mesoporous material channel size with the characteristics of the high specific surface area and high stability of the microporous material.Hierarchical porous ZIFs with excellent characteristics of ZIFs and effective in reducing mass transfer resistance have gradually become research hotspots in the field of ZIFs material synthesis.Based on the previous work,here we designed and synthesized the ZIFs materials rationally through structural design and regulation and acid-base modification methods,and studied the synthesized materials in combination with experimental characterization techniques and computer simulation.Meanwhile,the catalytic performance of various hierarchical porous ZIFs catalysts in the cycloaddition reaction between CO₂ and epoxide was also studied.Based on the design and synthesis of deep eutectic solvent?DES?,a simple,efficient,and environmentally friendly ZIF-8 material preparation method was designed.The synthesis process does not require organic solvents and can be carried out in a stoichiometric molar ratio,which provides a feasible alternative method for the synthesis of ZIF materials.The BET specific surface area and micropore volume of ZIF-8 prepared by the eutectic method reached1557 m²/g and 0.58 cm³/g,respectively,which is close to that of ZIF-8 prepared by the methanol method,but much higher than that of the hydrothermal synthesis ZIF-8 material.ZIF-8 has superior thermal stability,and due to its high pore volume,the CO₂ absorption capacity of ZIF-8 reaches 8.9 mmol/g,which indicates that it shows application potential in CO₂ capture and chemical fixation.A new process combining the self-assembly method and the eutectic method was explored.Different sophisticated ZIF-8,such as solid spheres,hollow spheres,hierarchical particles,and stackable particles,were obtained by using different surfactants as templates.A series of characterization analysis results show that:?1?Compared with traditional dodecahedron ZIF-8materials,solid spheres,hollow spheres,and hierarchical porous structures of ZIF-8 have a larger specific surface area,higher porosity,and micro-mesoporous pore structure;?2?ZIF-8solid spheres have large total pore volume?0.74 cm³/g?and mesopore volume?0.25cm³/g?and rich acid-base sites.Therefore,it exhibits high catalytic activity in the cycloaddition reaction of CO₂ and styrene oxide,and the catalytic conversion rate reaches 98.2%,which is much higher than the traditional dodecahedron ZIF-8 and some ZIFs materials reported in the literature;?3?The conversion rate of ZIF-8 solid spheres remained above 95%after five cycles of use,indicating that it has good stability,which indicates its application prospects in other macromolecular reaction systems.ZIF-8 precursor and surfactant self-assembly method were used to prepare hierarchical ZIF-8 material,and the synthetic process was optimized.The as-synthesized ZIF-8 has rich 3D pores,large total pore volume?1.26 cm³/g?and mesopore volume?0.83cm³/g?,and good thermal stability.he formation process of hierarchical ZIF-8 was investigated via dynamic light scattering?DLS?measurement and dissipative particle dynamics?DPD?simulation.the anionic surfactant SDS self-aggregate spontaneously and form supramolecular aggregate akin to micelle,the crystallization of ZIF-8 occurs near the aggregates surface,abundant mesopores,macropores and interstices were obtained among the ZIF-8 after the SDS removed.the size of SDS aggregates can be regulated and controlled by changing the water content or the concerntration of SDS in solutions,sequentially regulate the porosity of ZIF-8.The catalytic activity of hierarchical ZIF-8 in the cycloaddition between CO₂ and small molecule epoxide was mainly affected by surface area and amount of acid-base.Whereas,for the cycloaddition between CO₂ and bulky molecule epoxide,The catalytic activity of HZIF-8 was mainly affected by porosity and amount of acid-base.The excellent properties and enhanced catalytic activity of the HZIF-8 make it into a promising material in varieties of bulky-molecule catalytic reactions that cannot be performed through the use of purely microporous ZIF-8.From the perspective of acid-base modification,ZIF-8 doped with different transition metal ions?Ni,Fe,Cu,and Co?were synthesized,the doped metal ions replace part of Zn and form framework with 2-Mim to obtain ZIF-8 derived hybrid material.A series of characterization analysis results show that:?1?The as-synthesized metal-doped ZIF-8 samples maintain rhombic dodecahedral morphology,high specific surface areas and high thermal stability of pure ZIF-8;?2?ZIF-8 doped with different metal ions have different acid-base characteristics,thus display different catalytic activities for the CO₂ cycloaddition with styrene oxide;?3?Fe/ZIF-8 catalyst shows the best catalytic activity among the metal-doped ZIF-8samples tested with a 96.6%yield of cyclic carbonate;?4?A suitable doping amount of Fe is beneficial to the activity of the ZIF-8 catalyst,while an excessive of Fe doping results in a decrease in activity;?5?Both weak and moderate acid-base sites are the active sites,but the weak acid-base sites contribute more to the increase in catalytic activity;?6?Fe/ZIF-8 catalyst can be separated by simple centrifugation and reused at least five times with a little loss of catalytic activity.Fe/ZIF-8 catalyst operated at relatively mild reaction conditions,and exhibit a great performance compared with other MOF-based heterogeneous catalysts.An efficient heterogeneous catalyst hierarchical mixed metal Co/ZIF-8 materials were developed for the cycloaddition reaction betweem CO₂ and styrene oxide.According to a series of characterization analysis,the synthesized Co/ZIF-8 possesses large surface areas,high pore volume,favorable thermal stability,and have hierarchical porous structure with micro-meso-macropores.The acid–base characteristics of hierarchical Co/ZIF-8 were investigated by NH₃and CO₂ adsorption.A maximum cyclic carbonate yield can be obtained over hierarchical 70%Co/ZIF-8?Co salt content in the substrates mixture is 70%?.The styrene oxide conversion reached 96.8%with a selectivity of 97.9%after 8 h of reaction at 0.05 g catalyst dosage,120?,0.70 MPa.The hierarchical Co/ZIF-8 catalyst can be separated by simple centrifugation and reused at least four times with a little loss of catalytic activity.The hierarchical Co/ZIF-8showed enhanced catalytic activity and excellent recyclability and are thus potential heterogeneous catalysts in various catalytic reactions.