Synthesis Of Pyridine And Imidazole Cationic Metalloporphyrin Polymers And Their Catalytic Properties In CO₂ Cycloaddition

The global environment and climate problems caused by excessive accumulation of carbon dioxide（CO₂）have attracted widespread attention in the world.At the Central Economic Work Conference in 2020,General Secretary Xi Jinping proposed the strategic goal of"dual carbon",requiring China to strive to peak CO₂emissions by 2030 and achieve carbon neutrality by 2060.The development of CO₂storage and transformation technology has become an important way to achieve the goal of"dual carbon".Cyclocarbonate,an important industrial raw material,is prepared by the cycloaddition reaction of CO₂and epoxides,which has a broad industrial application prospect.However,the development of this technology is limited by low catalytic activity,harsh use conditions,difficult separation of catalyst and product.Therefore,the optimum design of catalyst has become a key issue in the field of preparation of cyclocarbonate from CO₂.The easy modification of the peripheral substituents of metalloporphyrins is very beneficial to the design of bifunctional heterogeneous catalysts and has natural advantages for catalytic CO₂cycloaddition reaction.In this paper,by modifying the peripheral substituents of metalloporphyrins and introducing rich catalytic active sites,a series of cationic metalloporphyrins polymers bifunction catalysts with novel structure,good stability,mild use conditions and easy separation were designed and synthesized,which improved the yield of CO₂cycloaddition to generate cyclocarbonate.Specific research contents include:1.Three A₂B₂free ester porphyrins 5,15-bis（pyridyl）-10,20-bis（4-carboxymethylphenyl）porphyrin（H₂Pp₁（COOMe））,5,15-bis（4-（N-imidazolyl）phenyl）-10,20-bis（4-carboxymethylphenyl）porphyrin（H₂Pp₂（COOMe））,5,15-bis（phenyl）-10,20-bis（4-carboxymethylphenyl）porphyrin（H₂Pp₃（COOMe））and zinc porphyrin complex ZnPp₃（COOMe）were synthesized.Two cationic zinc porphyrin complexes ZnPp₁⁺（COOMe）and ZnPp₂⁺（COOMe）were obtained by two-step post-modification reaction.Their molecular structures were characterized.The catalytic properties of three zinc porphyrins for the cycloaddition reaction of CO₂and phenyl glycidyl ether（PGE）were studied.The catalytic activity of ZnPp₃（COOMe）for cycloaddition reaction of CO₂and PGE is low.ZnPp₁⁺（COOMe）and ZnPp₂⁺（COOMe）show the high catalytic activity for cycloaddition reaction of CO₂and PGE.At 0.5 MPa CO₂initial pressure,the catalytic yields of ZnPp₁⁺（COOMe）and ZnPp₂⁺（COOMe）for PGE reached 99%and 96%,respectively.The results show that the two cationic zinc porphyrins exhibit high catalytic activity for the cycloaddition reaction of CO₂and PGE under low CO₂initiation pressure through the synergistic effect of nucleophile I^-introduced by alkylation of two basic groups of A₂B₂zinc porphyrins and Zn²⁺as the central metal of Lewis acid.2.Three A₂B₂free carboxyl porphyrins 5,15-bis（pyridyl）-10,20-bis（4-carboxylphenyl）porphyrin（H₂Pp₁（COOH））,5,15-bis（4-（N-imidazolyl）phenyl）-10,20-bis（4-carboxylphenyl）porphyrin（H₂Pp₂（COOH））and 5,15-bis（phenyl）-10,20-bis（4-carboxylphenyl）porphyrin（H₂Pp₃（COOH））were obtained by alkaline hydrolysis of the above three A₂B₂type ester free porphyrins.Take three free carboxyl porphyrins as monomers,six free porphyrins polymers were obtained by condensation with phenyltethylamine tetrachloride（BTA）and triphenylhexamine hexachloride（HATP）,respectively.The six cationic zinc porphyrin polymers were obtained,named ZnPp₁⁺-BTA、ZnPp₂⁺-BTA、ZnPp₃-BTA and ZnPp₁⁺-HATP、ZnPp₂⁺-HATP、ZnPp₃-HATP,after six free porphyrins polymers alkylated and metallized successively.Their structures were characterized and it was proved that they were polymers with benzimidazole as linker.Their catalytic performances for CO₂cycloaddition reaction were tested.ZnPp₁⁺-BTA and ZnPp₁⁺-HATP have excellent catalytic performance.At 0.5 MPa CO₂initial pressure,ZnPp₁⁺-BTA and ZnPp₁⁺-HATP could turn most epoxides into products.The catalytic performance of ZnPp₁⁺-BTA was not significantly decreased after four cycles.Under the same reaction conditions,the catalytic capacity of ZnPp₁⁺-HATP decreased with the increase of recycling times,and the recyclability needed to be further improved.The results show that the A₂B₂metalloporphyrin polymer with benzimidazole as the linker can effectively combine the structural functional groups with the catalytic functional groups,and show good catalytic performance for the cycloaddition reaction of CO₂and epoxides under low CO₂initial pressure.3.Two free porphyrin polymers were obtained by condensation of free porphyrin 5,10,15,20-tetra(4-（3-carboxyl）phenoxy phenyl porphyrin（TCPP）with BTA and HATP,respectively.After two free porphyrin polymers modified by alkylation and metallization,two cationic zinc porphyrin polymers were obtained,named Zn TCPP-BTA and Zn TCPP-HATP.Their structures were characterized and it was proved that they were polymers withbenzimidazole as linker.The catalytic performance of Zn TCPP-BTA and Zn TCPP-HATP for CO₂cycloaddition reaction was tested.Both of them had good catalytic performance for CO₂cycloaddition reaction.Under 0.5 MPa CO₂initial pressure,both of them could convert most epoxides into corresponding cyclocarbonates.Among them,Zn TCPP-HATP has higher specific surface area and thermal stability,but its recyclability needs to be further improved.The results show that the benzimidazole as the linker of A₄metalloporphyrin polymer,thebenzimidazole at the same time undertake structure functional groups and catalytic functional groups,flexible substituent of the metalloporphyrin effectively increased the porosity ofpolymer.The catalytic activity for the cycloaddition reaction of CO₂and epoxides have been further improved under the lower the initial pressure of CO₂.4.Two cationic zinc porphyrin complexes ZnPp₁⁺（COOH）and ZnPp₁⁺’（COOH）were synthesized,and their molecular structures were characterized.After combined with amino functionalized carbon nanotubes（CNT）,two supported catalysts were obtained,named ZnPp₁⁺/CNT and ZnPp₁⁺’/CNT,respectively.The structures of these two supported catalysts were characterized and their CO₂cycloaddition catalytic performance was tested.Their catalytic performance for CO₂cycloaddition reaction proved to be poor.The results show that CNT as the support is not conducive to the effective dispersion of cationic zinc porphyrin homogeneous catalyst.As the active center of the reaction,the loading of zinc porphyrin is an important factor affecting the catalytic activity.