Adsorption Properties Of Porous MOFs Constructed By Hetero Nitrocarboxylate Ligands

Carbon dioxide is one of the main greenhouse gases and impurity gases of natural gas.Its effective and selective capture has become a hot topic for scientists.In the past two decades,metal-organic frameworks（MOFs）as a type of emerging porous functional material has been widely used in the fields of gas storage and separation,catalysis,drug sustained release,fluorescence sensing,etc.because of their rich and diverse structures,huge surface area and adjustable and functionalized pores.Compared with traditional porous materials,MOFs with expected structure and function can be prepared by reasonably selecting or designing organic ligands with specific functional groups.By introducing open metal sites,Lewis base sites and other polar groups into the structure of MOFs materials can effectively improve their CO₂adsorption and selective separation performance.Therefore,we mainly construct MOFs materials with carbon dioxide adsorption and separation properties by choosing appropriate organic ligands and carry out a series of research work.In this paper,by using the effective secondary building unit synthesis strategy,twelve novel metal-organic materials were constructed based on hetero N-donor carboxylate ligands（mainly azolate carboxylate acid and pyridine carboxylic acid organic ligands）with different metal sources.The single crystal structure analysis,basic characterization and performance test of these compounds were carried out.The properties of these compounds in CO₂ capture and separation,organic dyes adsorption and separation,iodine sorption and release were also explored.Our main research achievements are mainly including the following aspects:（1）Five novel MOF materials（compounds 1-5）were constructed by using Y-type ligands:5-（Tetrazol-5-yl）isophthalic acid（H₃TZI）and 5-（4-（Tetrazol-5-yl）phenyl）isophthalic acid（H₃TZPA）with different metal sources.Compound 1-3 based on double-crown like hexanuclear[Cu₆N₁₂X₆]（X=Cl,Br,I）cluster are isostructural and constructed by the ligand H₃TZI and different copper halides（CuCl₂,CuBr₂ and CuI）.Because the radius of coordinated halogen atom is Cl?Br?I,the surface area of the compound is 1?2?3,so the gas adsorption capacity of the compound is also 1?2?3.Compound 1 with the largest surface area of three compounds has high-density Open Metal Sites（OMSs）and Lewis Base Sites（LBSs）in its structure,so it has a good CO₂ capture and selective separation ability.Compound 4 based tetranuclear[Cu₄（μ₃-OH）₂N₄（CO₂）₄]cluster with rtl topology is constructed by ligand H₃TZI and copper chloride by modulating various factors,such as metal-ligand ratios,solvent polarity,pH values,temperature,etc.Compound 5 based on tetranuclear zinc clusters and mononuclear zinc SBUs is constructed by ligand H₃TZPA and zinc nitrate with irregular channels inside the structure.Because of its good pore characteristics and the existence of LBSs in the structure,we have tested the adsorption properties of compound 5 for small gases.The experimental results show that compound 5 also has a good CO₂ selective adsorption ability and exhibits good separation performance for CO₂/CH₄ and C₃H₈/CH₄ mixed gases.（2）Two novel MOFs（compounds 6-7）were constructed by using the same Y-type ligand:5-（4H-1,2,4-Triazol-4-yl）benzenedicarboxylic acid（H₂TIA）with CuI and CuCl₂?2H₂O as metal sources respectively.Compound 6 based on[Cu₂I]_n chain and classical[Cu₂（CO₂）₄]paddlewheel SBUs can be regarded as a new（3,4,4）-connected topological structure.Most of the reported copper（I）iodide-based MOFs are not stable enough to test their gas adsorption properties.Compound 6 has a good CO₂ selective adsorption property due to its stable structure and the presence of high-density OMSs in the structure.The selectivity of CO₂/CH₄ mixture is higher than most reported Cu_xI_y-based MOFs.In addition,compound 6 is also a good candidate for iodine capture and release.Compound 7 based on six-connected[Cu2（CO2）4N2]SBU also has a good CO₂ selective adsorption ability.（3）Three novel coordination polymers（compounds 8-10）were constructed by using the linear ligand:4-（1H-Imidazol-1-yl）benzoic acid（HIBA）and4（p-Carboxyphenyl）-1,2,4-triazole（HCPT）with different metal sources.Among them,compound 8 based on rod SBU with a novel（5,8）-connected topology was constructed by linear imidazole carboxylate acid ligand HIBA with copper nitrate.The compound has good chemical stability,especially water stability and acid-base stability.In addition,compound 8 also exhibits CO₂ selective adsorption ability.Compounds 9 and 10 based on trinuclear metal clusters were isostructural and constructed by the same linear triazole carboxylate acid ligand HCPT with nickel chloride and cobalt chloride,respectively.Both of the two frameworks are three-dimentional supramolecular networks.Compound 9 with high porousity is stable enough to adsorb small gases.Also,for its cationic skeleton,it can effectively adsorb and separate organic dyes with different charges,such as MO and GO II.（4）Two novel MOFs（compounds 11-12）were constructed by using the same organic ligand pyridine carboxylate acid ligand:5-（3,5-Dicarboxyphenyl）picolinic acid（H₃DCPPA）with indium nitrate and copper nitrate,respectively.Compound 11with a novel（3,4,4）-connected topology is made up of three kinds of mononuclear indium with different coordination modes and the organic ligand.As its anionic skeleton,specific pore characteristic and the presence of a large number of unsaturated coordination carboxylate groups in the pore channels,the adsorption properties of organic dyes were tested.The experimental results show that compound11 has good adsorption capacity for small cationic dyes:methylene blue and Rhodamine B.Compound 12 with a new（3,4,4）-connected topology is composed of the classical[Cu₂（CO₂）₄]paddlewheel and other rare binuclear copper cluster-based SBUs.The compound also has a high surface area.