Synthesis Of Several Bis-M-Phenylene Diacid MOFs,Fluorescence Detection Of Cr₂O₇^2- And Adsorption Separation Of C₃H₄/C₃H₆

Metal-organic frameworks?MOFs?are nanoporous materials,also known as porous coordination polymers?PCPs?,which are constructed by metal centers?metal ions or clusters?and organic ligands through coordination bonds.MOFs have been applied in the fields of fluorescence sensing and gas adsorption and separation due to their various structures,adjustable pore size,high specific surface area and high porosity.In this study,highly stable Zr?IV?-MOF and Cu?II?-MOFs were synthesized and applied to fluorescence sensing of heavy metal chromium in water and adsorption and separation of C₃H₄/C₃H₆,respectively.As Cr₂O₇^2-anion is highly toxic,new sensors have been developing for its effective detection from water,among which metal-organic frameworks?MOFs?show distinct superiority over many other materials.Herein,a new fluorescent Zr?IV?-based MOF,[Zr₆O₄?OH?₈?H₂O?₄?sbtc?₂]?referred as to BUT-28?,based on the di-isophthalate ligand with a central CH=CH moiety,trans-stilbene-3,3',5,5'-tetracarboxylate(sbtc^4-),has been prepared and structurally determined.The MOF shows excellent stability in neutral,highly acidic and weakly basic aqueous solutions.Moreover,no essential uptake loss in three cycles of water vapor adsorption-desorption measurements was observed for BUT-28,suggesting the robustness of the porous framework and its great potential for long-term use.Fluorescent measurements were carried out for BUT-28 and an isostructural MOF,Zr-abtc,which is constructed from the di-isophthalate ligand with a central N=N moiety,azobenzene-3,3?,5,5?-tetracarboxylate(abtc^4-).Interestingly,Zr-abtc shows very weak fluorescent emission.In contrast,BUT-28 exhibits relatively strong fluorescence,and serves as a promising sensory material for the detection of trace Cr₂O₇^2-?limit of detection:36 ppb?in aqueous solutions by selective and sensitive fluorescence quenching effect.The formation of MOFs under given reactants depends on various factors including reaction duration,temperature,used solvent,system pH,and others.Among them,reaction duration is relatively less investigated.In this work,based on a Cu?II?-MOF system,the reaction duration was found to play a very important role in directing the formation of two different products,?NH₂?CH₃?₂?[Cu₁₂?DDPN?₆?H₂O?₁₀Cl]?BUT-301?on shorter reaction time and?NH₂?CH₃?₂?₂[Cu?DDPN?]?BUT-302?on longer reaction time,when CuCl₂ reacted with 3,5-di?3,5-dicarboxylphenyl?nitrobenzene?H₄DDPN?in DMA/MeOH mixed solvent at 120 ^oC.With increasing the reaction duration,BUT-301 can transform to BUT-302.Both MOFs have three-dimensional?3D?framework structures.BUT-301 is constructed by paddle-wheel Cu₂?COO?₄ units and 2 ligands as four-connected linkers,while BUT-302 is assembled by mononuclear Cu?II?centers connecting the ligands as also four-connected linkers but in a monodentate coordination fashion.N₂ adsorption confirmed that both MOFs are porous with the Brunauer-Emmett-Teller?BET?surface areas of 1953 and 561 m² g^-1,respectively.Interestingly,both BUT-301 and-302 show selective adsorption property of C₃H₄ over C₃H₆.The C₃H₄/C₃H₆ adsorption selectivities were calculated to be 1.9 and 4.4 at 0.1 bar and 298K by the ideal adsorbed solution theory?IAST?for a 1:99 C₃H₄/C₃H₆ mixture,respectively.