Synthesis Of Reduction-oxidation Metal-oxo Cluster-based Framework And Their Photocatalytic Performance Of Carbon Dioxide Conversion

Due to the increasing energy shortages and greenhouse effect,the use of solar energy to convert carbon dioxide（CO₂）into energy substances is an important project.Designing and synthesizing stable and efficient photocatalysts to decrease the activation energy of the reaction and accelerate the reaction process are the keys to achieve the above goal.Compared with traditional semiconductor photocatalysts,crystalline catalysts with precise structure information are conducive to exploring the structure-activity relationship of catalytic reactions and further guide the advantages of synthesizing efficient catalysts.Metal-oxo cluster-based coordination polymer is a kind of crystalline materials consisted with a metal-oxo cluster as a secondary building unit,and organic ligands connected by coordination bonds to form a definite periodic structure.Due to the rapid transport of multiple electrons and the strong redox capability,metal-oxo cluster-based coordination polymers have often been used as catalysts for photocatalytic CO₂ reduction reactions in recent years.However,there are still some problems and challenges:for example,the CO₂ reduction reaction is often carried out in organic solvents,which does not meet the requirements of green chemistry and is also not conducive to the separation of liquid phase products;the catalytic system in the aqueous phase is often accompanied by hydrogen evolution as a competition reaction,reducing the utilization efficiency of photogenerated charge;a single metal catalyst is often difficult to achieve the CO₂ reduction and water oxidation at the same time,often need to add additional sacrificial agents or photosensitizers;the conversion rate of using CO₂ directly as C₁ source to combine with organic catalysis is weak.Based on this,a series of stable heterometalltic oxygen cluster-based coordination polymers were constructed and synthesized as efficient CO₂ photocatalysts.（1）A novel polyoxometalate-based metal-organic framework（POMOF-CF₃）was synthesized in situ using a hydrophobic ligand 4,4’-（perfluoropropyl-2,2-diyl）bis（4,1-phenylene）bis（oxy）bis（methylene）dibenzoic acid and reductive polyoxometalate cluster{Zn₄PMo₈^VMo₄^VI}.Due to the strong hydrophobicity of the ligand and the suitable position of spatial arrangement,POMOF-CF₃ is firstly to be reported as a polyoxometalate-based complex with hydrophobic properties,which makes it exhibit outstanding chemical stability.POMOF-CF₃ was used as a catalyst in photocatalytic CO₂ reduction reaction,of which the reductive polyoxometalate clusters contribute to the reduction of CO₂,and hydrophobic groups restrain the hydrogen generation.In the photocatalytic CO₂ reduction system with POMOF-CF₃as the catalyst,the yield of HCOO^-reached 35.2μmol in the aqueous solution with a selectivity of 97.9%after 16hours.（2）A kind of crystalline heterometallic oxygen cluster-based catalysts were designed and synthesized by connecting reductive{M₃L₈（H₂O）₂}（M=Zn,Co and Ni for RO-1,2,3 respectively）cluster and oxidative{PMo₉V₇O₄₄}cluster through a single oxygen atom bridge.All of RO-1,2,3 have a broad range of light absorption in 250-2500 nm.RO cluster-based catalysts achieved artificial photosynthesis only with the participation of water vapor and CO₂ in a gas-solid system under light irradiation.When RO-1 as the photocatalyst,the catalytic rate of CO production was 13.8μmol g^-1 h^-1,and the selectivity was close to 100%.Moreover,under low concentrations of CO₂（30%CO₂+70%argon）and continuous light for 30 hours,RO cluster-based catalysts still have good activity,which is of great significance to practical application.The concomitantly reductive and oxidative active sites and the effective charge transfer between the units in these RO photocatalysts are the key factors to complete the overall photosynthesis.（3）It is found that selecting suitable oxidation and reducing clusters to construct RO cluster-based catalyst can complete the artificial photosynthesis.Based on this,the reductive{Cu₈^I}and oxidative{PMo₈V₆O₄₂}cluster were constructed to obtain a RO cluster-based coordination polymer（RO-4）with a well-defined single crystal structure and explored its photocatalytic performance.Under light irradiation,RO-4 can achieve efficient artificial photosynthesis with a high CO production activity of 20.06μmol g^-1h^-1（>99.5%selectivity）along with O₂ release.Through comparative experiments,we found that the catalytic performance of RO-4 is about 4 times that of the physical mixture of the reduction site and the oxidation site,indicating the necessity of direct bonding of the two components for rapid electron transport.Importantly,we captured the reaction intermediates by in situ infrared diffuse reflection spectroscopy and combined with the theoretical calculations to deeply explor the catalytic mechanism and structure-activity relationship at the molecular level.（4）Using N,N-bis（3,5-dicarboxy）-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic diimide（H₄BTTD）as an organic ligand,benzoic acid/1-naphthoic acid/9-anthracic acid as second auxiliary ligands and binuclear cerium-oxcn cluster to construct a series of cerium-based complexes CeMOF-B/CeMOF-N/CeMOF-9en.Through the analysis of the structural energy band,CeMOF have a good oxidation capacity.Because the cerium-oxo cluster can be used as the site for Lewis acid catalysis,we use CeMOF as photocatalysts,CO₂ as the C1 source in organic synthesis to achieve one-step styrene oxidation reaction and CO₂ cycloaddition to obtain carbonic acid 1-phenylethylene ester.We found that CeMOF-9en can catalyze the conversion rate of styrene to ester by 95%under the atmosphere of O₂/CO₂（1/1）at normal pressure and temperature with excellent substrate tolerance.Under oxygen atmosphere conditions,CeMOF-9en has superoxide radicals,9-anthroic acid positive radicals(9en^·+)and BTTD^·-radicals.Based on the capacity of oxygen activation and strong oxidizing property,we further tested the organic catalytic properties of benzylamine coupling.The results showed that CeMOF-9en can catalyze the conversion of benzylamine to N-benzylenebutylamine for 99.5%in two hours under an air atmosphere.