Synthesis,structure,selective CO₂ Adsorption And Photocatalytic CO₂ Reduction Properties Of Metal-organic Frameworks Based Upon Nitrogen-containing Aromatic Carboxylic Acid

Since the industrial revolution,intensified human activities have led to excessive carbon dioxide(CO₂)emissions,resulting in climate change or global warming.Thus,how to alleviate this problem has become one main focus of global attention.At present,CO₂ capture and conversion may provide an effective solution.In particular,the photoreduction of CO₂ into value-added chemicals(such as CO,HCOOH,CH₄,etc.)has attracted great attention,since it can be considered as a promising approach for solar-to-chemical energy conversion by mimicking the natural photosynthetic process to achieve a carbon neutral economic.Metal-organic frameworks(MOFs),due to their high surface area,inorganic-organic hybrid nature,structural and functional diversity and tunability,are regarded as the promising materials for CO₂capture.Since 2011,they may combine advantages of the traditional homogeneous/heterogeneous catalysts and are emerging as the promising platform for the photocatalytic CO₂ reduction.In this dissertation,based upon nitrogen-containing aromatic carboxylic acid ligands,5-(pyridin-4-yl)isophthalic acid(H₂L_A),5-(pyridin-3-yl)isophthalic acid(H₂L_B),nicotinic acid(H₂L_C)and pyrimidine-5-carboxylic acid(H₂L_D),twelve MOFs have been synthesized,and their structures,CO₂ selective adsorption and photocatalytic CO₂ reduction have been further investigated.1.Based on the T-type pyridylcarboxylate ligands,MOFs 1,2 and 3 were successfully constructed by pillaring strategy,and herein,we present an unprecedented approach that the lower symmetric cluster meets the lower symmetric ligand for sharply boosting MOFs'thermal stability by additional inter-linker interactions,and 3 was afforded.In comparison with similar MOFs 1 and 2,this method was further revealed clearly.Notably,3 exhibit exceptional thermal stability up to 450?,which is comparable to those of the famous thermostable MOFs based on polynuclear clusters(nuclearity?3)or rod-shaped building blocks,and is the highest among MOFs based on binuclear clusters.Meanwhile,3 with the high CO₂adsorption capacity and CO₂ selectivity may be a very promising candidate for CO₂capture.This work may open up a new avenue for constructing robust MOFs from the abundant,unstable,and lower symmetric binuclear clusters which have usually been ignored by most MOFs'chemists.2.Based upon the simple linear nitrogen-containing carboxylate ligands H₂L_C and H₂L_D,Cu₄I₄ and Cu₂(CO₂)₄ clusters based MOFs 4 and 5 were synthesized.4 is a 2D layered network with sql topology.Based upon 4,a porous 3D 5 was successfully synthesized by adding the N coordination sites of ligands to regulate the structure.5 is one of the fewest reported Cu_xI_y-MOFs with high CO₂ adsorption selectivity so far.Based upon 5,four novel Cu(I)/Cu(II)-MOFs 6-9 were obtained by adjusting metal clusters without changing the ligand.3.Based on the simple ligand H₂L_D,three semiconductive MOFs 10,11 and 12with excellent chemical stability were synthesized by constructing binary Cu(I)/Cu(II)SBUs,which successfully applied to the photocatalytic CO₂ reduction without additional photosensitizers and cocatalysts.10 is a 2D layered 4-connected network with sql topology based on Cu₄I₄ clusters,which can be applied into photocatalytic CO₂ reduction to CO.11 and 12 are 3D porous frameworks based on Cu₄I₄ and Cu₃OI(CO₂)₄ clusters,Cu₂I₂ and Cu₃OI(CO₂)₄ clusters,respectively,which show the widest adsorption band among the reported MOFs with the edge up to 1400 nm.They could be applied into photocatalytic CO₂ reduction to CH₄ with good photocatalytic performance.Notably,11 exhibits the CH₄selectivity of 72.8%in aqueous solution,which is the highest among the reported MOFs.11 was afforded by the positive crystal-to-crystal transformation from 10.Experimental and theoretical investigations reveal that the Cu₄I₄ and Cu₃OI(CO₂)₃ clusters in 11 play the role of photoelectron generators and collectors in driving the 8e^-/8H⁺process.It is the first time to extend the old Cu(I)_xX_yL_zcoordination polymers'application into the photoreduction of CO₂to CH4 and open up a new platform for the effective photoreduction of CO₂ to CH₄.