The Synthesis Of Functionalized Zn-based MOFs For Catalytic CO₂ Cycloaddition With Epoxide

Carbon dioxide（CO₂）has gained great attention not only as main greenhouse gas leading to climate problems,but also as the endless C1 feed stock for the synthesis of chemical products.Carbon capture and utilization technologies could achieve the goal of reducing the accumulation of atmospheric CO₂and synthesizing high-value chemicals,which is of great strategic significance for environmental conservation and CO₂utilization.One of the most effective and promising strategy is the CO₂cycloaddition with epoxides to generate cyclic carbonate,which is a highly atom-economical reaction,the products of cyclic carbonates are widely applied as electrolytes,aprotic solvents and valuable precursors for medicine synthesis.Due to the chemical inertness of CO₂molecule,it’s necessary to develop efficient catalysts for catalytic CO₂conversion.Metal organic frameworks（MOFs）are promising heterogeneous catalysts for CO₂capture and fixation due to its high surface area,ordered pore structure and tunable functionalities.Based on the reported catalytic mechanisms for CO₂conversion,the ring-opening process of epoxides acted as the key step for the CO₂cycloaddition reaction,and the existence of HBD groups（including free-OH,-NH₂,-COOH）could form hydrogen bonds with O atoms of epoxides to achieve the activation of substrates.A novel Zn-based MOF Zn（dobdc）（datz）with micropores and HBD groups was designed and successfully synthesized by solvothermal method from mixed ligands of 2,5-dihydroxyterephthalic acid（H₄dobdc）and3,5-diamino-1,2,4-triazole（H₂datz）.The Zn（dobdc）（datz）was thoroughly characterized and applied for the synthesis of cyclic carbonates.The influence of co-catalyst species,reaction temperature and time,were further investigated to ascertain the optimum reaction conditions.With the assistance of Bu₄NBr,further catalytic experiments on CO₂cycloaddition conversion verified Zn（dobdc）（datz）efficient catalyst with 98%propylene carbonate yield under mild conditions of 80^oC,1.5 MPa and 7 h.Besides,the excellent versatility for various epoxides was also affirmed.The existence of plentiful oxygen-containing groups made most MOFs catalysts poor water stability.Herein,forming more Zn-N bonds and Zn centers with higher ligancy by the introduction of azolate-containing ligands,a novel nitrogen-containing Zn-based MOF Zn（Py）（Atz）with micropores was facilely prepared by employing mixed ligands of 3,5-Pyridinedicarboxylic acid（H₂Py）and3-Amino-1,2,4-triazole（HAtz）.Zn（Py）（Atz）showed superior water stability（>1week）and maintained stable structure over 72 h in acidic and basic aqueous solutions（p H=2-14）,and the existence of little water showed no obvious influence on its structure and catalytic performance.Nitrogen-containing units and abundant micropores led to good adsorption performance to CO₂（52.3 cm³/g at 273 K）.Zn（Py）（Atz）could efficiently catalyze CO₂cycloaddition reaction with 86%propylene carbonate yield under 60°C,1.5 MPa,24 h and solvent-free conditions in the presence of Bu₄NBr co-catalysts.Zn（Py）（Atz）also presented good versatility and the product yield was little decrease when it was recycled up to 7 times.The Zn（Py）（Atz）catalyst reported herein with excellent water stability and catalytic performance would be appealing in the CO₂capture and its chemical fixation.Besides,the microporous Zn（Py）（Atz）was further performed to prepare Zn-Py-370by simple pyrolysis process under N₂atmosphere.The acid strength of Zn-Py-370was obviously improved with partial oxygen-containing linkers removal,and forming mesopore structure.The catalytic activity was further enhanced,and PC yield up to 94%by prolonging reaction time to 36 h at room temperature.The existence of mesopores in MOFs was benefited for substrate diffusion and transfer in its framework,which was anticipated to improve catalytic activity.Zn-based MOF with hierarchical pores（Hie-Zn-MOF）was facilely synthesized by room-temperature stirring,and the synthesis process was remarkably reduced to 2 h by employing organic amines of TEA to facilitate the ligands deprotonation.The present room-temperature stirring synthesis for Zn-MOF was high-efficient and energy-saving compared with traditional solvothermal method.Besides,mesopores ranging from 5-30 nm were generated and hierarchical Zn-MOF was obtained.Due to the hierarchical pores and high surface area,Hie-Zn-MOF showed high CO₂adsorption capacity of 84.9 cm³/g at 273 K and catalytic activity for the CO₂cycloaddition with epoxide,97%PC yield was achieved under the conditions of room temperature,1 MPa and 16 h,which was superior to most microporous MOFs.Especially for the bulky epoxides,such as butyl glycidyl ether（BGE）,the corresponding product yield was obviously improved from 64%to 88%compared with microporous Mic-Zn-MOF-Sol.The time-and energy-saving synthesis strategy of efficient Zn-based MOF catalyst with hierarchical structure is of great potential in CO₂capture and utilization.