Preparation And Photocatalytic Properties Of Porphyrin-based Covalent Organic Polymers

The excessive use of fossil fuels has caused a series of environmental pollution problems,such as excessive emissions of carbon dioxide（CO₂）leading to the greenhouse effect,and the photosynthesis of plants is an effective way to counter the the greenhouse effect.Researchers have been inspired by plant photosynthesis to develop artificial photocatalysis.The molecular configuration of CO₂ is linear and extremely stable,requiring high energy to convert it,and is also a carbon resource that cannot be wasted.Therefore,artificial photocatalysis is an ideal strategy for converting CO₂ into a storable chemical energy source.Among them,finding a suitable and efficient catalyst is the crucial.Metalloporphyrins have a wide range of applications in the photocatalytic reduction of CO₂.Porphyrins are composed of four pyrrole substituents whoseα-carbon atoms are interconnected by hypomethyl bridges.π-conjugated electronic structures give porphyrins high molar absorption coefficients and fast charge transfer capabilities.The four nitrogen atoms on the porphyrin ring act as strong Lewis base sites and form stable MN₄ coordination sites with metal ions.The formed metalloporphyrin has a strong interaction force on CO₂molecules,which will contribute to their adsorption and activation in the active site of the metalloporphyrin.In addition,the excited state of the porphyrin ring can provide free electrons to the metal site through the ligand-metal charge transfer mechanism,thus reacting with the adsorbed CO₂ molecules and achieving the photoreduction of CO₂.Although the photocatalytic performance of porphyrins is relatively excellent,porphyrins still suffer from poor stability and easy recovery in catalysis.Meanwhile,conjugated organic polymers have been applied to photocatalytic reduction of CO₂ due to their high stability,easy recovery,and the possibility of modifying more active sites.based on the above research background,we constructed a series of metal porphyrin-based conjugated organic polymers with metal porphyrins as the basic unit,the main research content is as follows:1.5,10,15,20-Tetra（4-bromophenyl）metalloporphyrin（MTBPP）and 3,8-diethynyl-1,10-phenanthroline（DTPA）were constructed as porphyrin-based conjugated organic polymer materials（COPs）MTBPP-DTPA-COPs（M:Fe,Co,Ni,Cu,Zn）by Sonogashira coupling reaction.All prepared MTBPP-DTPA-COPs materials showed excellent photocatalytic performance for CO₂ reduction reaction under visible light irradiation,and the catalytic process used only H₂O as sacrificial reagent.The results showed that the reduction yields ofTBPP-DTPA-COP,FeTBPP-DTPA-COP,CoTBPP-DTPA-COP,NiTBPP-DTPA-COP,CuTBPP-DTPA-COP and ZnTBPP-DTPA-COP as photocatalysts after 4 h of light irradiation were 89.6μmol g^-1,76.0μmol g^-1,42.9μmol g^-1,31.9μmol g^-1,38.6μmol g^-1 and 152.1μmol g^-1,while ZnTBPP-DTPA-COP showed better photocatalytic activity than other MTBPP-DTPA-COPs.Moreover,the total amount of CO products of ZnTBPP-DTPA-COP was 274.56μmol g^-1 after 12 h of light exposure,indicating its good photocatalytic ability and light stability.2.5,10,15,20-Tetra（4-bromophenyl）porphyrin copper（CuTBPP）and 5,5’-bis（ethynyl）2,2’-bipyridine（BPY）were constructed as porphyrin-based COP（CuTBPP-BPY-COP）by Sonogashira coupling reaction.Co/CuTBPP-BPY-COP was prepared by ligating CuTBPP-BPY-COP with Co²⁺,and Co/CuTBPP-BPY-COP is a polymer with two metal sites,which are copper in the porphyrin ring and cobalt ligated with bipyridine.All the prepared CuTBPP-BPY-COP and Co/CuTBPP-BPY-COP showed excellent photocatalytic performance for the reduction reaction of CO₂ under visible light irradiation without any additional sacrificial reagent except H₂O.The results showed that Co/CuTBPP-BPY-COP with bimetallic sites performed better photocatalytic activity than CuTBPP-BPY-COP with monometallic sites.The total amount of CO products was 263.2μmol g^-1 after 10 h of Co/CuTBPP-BPY-COP light exposure.Co/CuTBPP-BPY-COP maintained a higher photocatalytic capacity after ten consecutive cycles.And after theoretical calculations,it is known that the HOMO-LUMO gap is greatly reduced due to the introduction of the metallic cobalt center and nitro,which is favorable to the transfer of electrons to the excited state;and because the Cu center becomes a more active region,it was easier to bind with CO₂,making the catalytic reaction easier,so we though there was a synergistic effect of the bimetallic sites.