Post-synthetic Modification Of UiO-68 For Photocatalytic Reduction Of Carbon Dioxide

In recent decades,because large amounts of fossil fuels have been consumed for industrial development,energy shortages and environmental problems have become increasingly serious.Therefore,a large number of researchers are committed to storing or converting CO2.Compared to carbon capture and storage(CCS),photocatalytically reduction of CO2 to the energy carrier products such as CH4,CH3OH,HCOOH and CO is a better choice.However,it is known that CO2 molecule with stable carbon-oxygen bond is difficult to be activated under the visible light.Therefore,the development of efficient photocatalysts is critical for the reduction of CO2.To date,semiconductors,homogeneous molecular complexes,metal-doped inorganic zeolite materials,and metal-organic frameworks(MOFs)have been investigated to induce photocatalytic conversion of CO2.However,semiconductor materials and homogeneous molecular complexes are not ideal materials for photocatalysts,in general homogeneous transition-metal systems are not reusable even though they show high efficiency in reduction of CO2 in the presence of photosensitizers,while metal-incorporated and inorganic semiconductors are robust but usually photocatalytically reduce CO2 in low efficiency.MOFs are generally considered to be an ideal photocatalytic material because of their diverse framework structures,relatively high stability and easy modification.Moreover,three-dimensional porous MOFs which are linked by metal ions or metal clusters and organic ligands have semiconductor-like properties,in which metal nodes can be used as semiconductor QDs,and organic ligands serve as light-harvesting antenna.In addition,PSM methods can be employed to introduce other functional groups or metal-organic complexes into MOFs to improve the absorption of visible light and photocatalytic activity of the parent MOFs.The UiO series is an important discovery in the history of MOFs.They are constructed by non-toxic zirconium ions and dicarboxylate ligands,which have superior chemical and thermal stability.Therefore,they can be applied in the field of photocatalysis.In this paper,it is aimed to improve the photocatalytic performance of UiO-68-NH2 which was selected as the research object for reduction of CO2 by using PSM.The final results will have a positive impact on the design of highly active photocatalysts based on amino-functionalized MOFs.The research in this paper consists of the following two parts:1.UiO-68-NH2 was obtained by solvothermal reaction of 2’-amino-[1,1’:4’,1"-terphenyl]-4,4"-dicarboxylic acid(amino-TPDC)and ZrCl4 in DMF in the presence of acetic acid(HOAc)as a modulator.The UiO-68-bpy was obtained from UiO-68-NH2 and 4’-methyl-[2,2’-bipyridine]-4-carbaldehyde(mebpy)through amine-aldehyde condensation reaction.The pure UiO-68-bpy and FeCl3 are mixed to create the dark red product UiO-68-Fe-bpy.Compared with the parent UiO-68-NH2,UiO-68-Fe-bpy not only broadens the absorption range of visible light,but also exhibits enhanced photocatalytic ability for reduction of CO2.2.UiO-68-NH2 is synthesized as above,then UiO-68-NH2 and three aldehyde-containing organic compounds(3-fluorobenzaldehyde,3-methylbenzaldehyde and 3-methoxybenzaldehyde)are mixed to give UiO-68-PSMs(UiO-68-F,UiO-68-CH3 and UiO-68-OCH3)through amine-aldehyde condensation reaction,which is used to investigate the effect of different functional groups on photocatalytic reduction of CO2.Electrochemical measurements show that the electron donating group can reduce the electron transfer resistance and promote the charge separation efficiency compared with electron-accepted group.The final photo catalytic results reveal that UiO-68-OCH3 has the better photocatalytic performance on reduction of CO2,while UiO-68-F is the worst.