Preparation Of Pyridine-based Porous Organic Framework And Their Catalytic Properties For CO₂ Cycloaddition Reaction

Carbon dioxide（CO₂）is the main gas that causes environmental problems such as the greenhouse effect,but it is also a widely sourced and non-toxic renewable resource in nature.Among them,the reaction between epoxides and CO₂ to synthesize cyclic carbonates is one of the significant methods to realize CO₂ resource utilization.The reaction does not use toxic raw materials,does not generate any by-products and is environmental friendly.Cyclic carbonates have high solubility,high polarity,and low toxicity,and have been widely used in adhesives,battery electrolytes,pharmaceutical chemical intermediates,and other fields in recent years.However,the currently reported catalytic systems generally have problems such as cumbersome preparation processes,difficulty in separating from products,or harsh reaction conditions.Therefore,designing new catalytic materials which have high activity and easy to separate to achieve efficient conversion of CO₂ under mild conditions is of great significance.Based on the Friedel-Crafts alkylation reaction mechanism,a series of nitrogen-rich pyridinium-and aromatic-based hypercrosslinked organic framework（HCPs）materials were constructed and characterized by using different functionalized aromatic monomers.The effects of monomers types and composition on hypercrosslinked materials were studied.The prepared HCPs were applied to CO₂ adsorption and cyclic carbonates synthesis.The results showed that the adsorption capacity of HCPs for CO₂can reach 3102.2μmol/g at 273 K and 1.0 atm CO₂ pressure.It is found that under the synergistic catalysis of tetrabutylammonium iodide（TBAI）,T-HCP-Py had the best catalytic activity,which could efficiently and selectively convert CO₂ into cyclic carbonate through material structure regulation and the comparative study of catalytic activity.Excellent product yield and selectivity were achieved even at lower CO₂pressure（simulated flue gases）.The effects of reaction temperature,reaction time,T-HCP-Py loading,and CO₂ pressure on the cycloaddition reaction were systematically discussed.The yield of PC was 96%and selectivity was 99%at 80°C,2.0 MPa,6 h.The recyclability and universality of T-HCP-Py catalytic materials were studied under the optimized process conditions.There was no significant change in the yield and selectivity of PC after five catalytic reactions.In addition,they had good catalytic effects on epoxides with different substituents.Finally,based on the experimental results,the mechanism of T-HCP-Py/TBAI synergistic catalytic cycloaddition of CO₂ was proposed.In order to avoid the use of cocatalyst in the cycloaddition reaction,triazinyl-and pyridinium-based ionic organic framework（TPHCPs）materials were synthesized by“one-pot method”based on Friedel-Crafts alkylation and quaternization reaction by reasonably designing the structure of monomers.The structure of the prepared TPHCPs were verified using different characterization methods.The influence of the types of monomers and the molar ratio of different monomers on the TPHCPs catalysts were investigated.The adsorption performance of different TPHCPs on CO₂ was studied.At273 K,the adsorption capacity of TPHCP（?）3Cl^-for CO₂ could reach 1067μmol/g.The catalytic activity of TPHCPs with different structures was systematically investigated by using the cycloaddition reaction of PO and CO₂ as a model reaction.The comparative studies showed that TPHCP（?）3Br^-had the highest catalytic activity.Then,the reaction temperature,reaction time,TPHCP（?）3Br^-loading and CO₂ pressure were optimized.It was found that 96%PC yield and 99%selectivity could be obtained at 130 ^oC,2.0 MPa and 6 h.The cyclicity and universality of TPHCP（?）3Br^-catalyst were studied under the optimal reaction conditions.The catalytic activity of TPHCP（?）3Br^-did not change significantly after being reused for 5 times,and it could catalyze the efficient conversion of epoxides into the corresponding cyclic carbonates.Combined with experimental data,the mechanism of CO₂ cycloaddition catalyzed by multiple active sites was proposed.