Preparation And Structural Characterization Of Metal-Organic Frameworks

Metal organic frameworks?MOFs?are porous network framework materials in which metal ions and organic ligands are linked by coordination bonds.As a new type of adsorbent,MOFs have the greatest advantage in that the reactive metal sites and various functional groups can be introduced in their structures according to the structure and physicochemical properties of the adsorbate,and their skeleton structure,pore size and pore surface physics.The chemical properties are regulated to adjust the affinity between the host and the guest,change the adsorption selectivity to the coexisting guest molecules,increase the adsorption capacity,and increase the adsorption rate.MOFs have great potential for adsorption removal of environmental pollutants,especially for the adsorption of low concentrations of heavy metal ions and organic contaminants in water.In this paper,five metal organic framework?MOFs?crystal materials were s ynthesized hydrothermally.They are MOF-Cd?II?-CA from citric acid?H₃CA?and cadmium nitrate,[Cd₃?AC?₃�?H₂O?₂]_n,MOF-Co?II?-FA from fumaric acid?H₂FA?and cobalt nitrate,[Co₃?FA?₂�?H₂O?₄]_n,MOF-Cd?II?-IcA from isonicotinic acid?HIcA?and cadmium nitrate,[Cd?IcA?2�?H2O?4]n,MOF-Zn?II?-IcA from isonicoti nic acid?HIcA?and cadmium nitrate,[Zn?IcA?₂�?H₂O?₄]n,and MOF-Cd?II?-FA sy nthesized by fumaric acid and cadmium nitrate,[Cd?FA?�H₂O�NH₂]n.The synthe sized crystals were characterized through infrared spectroscopy,single crystal X-r ay diffraction,thermogravimetry-differential thermal analysis and specific surface area measurement,respectively.The isothermal adsorption characteristics of MOF-Co?II?-FA for methane was studied in the range of pressure 0¹2MPa at 296K and 313 K respectively,with a high-pressure gas adsorption instrument,The beha viors of MOF-Co?II?-FA was explored in the isothermal and kinetic adsorption fo r divalent copper ions in water at 0?,30?,40?,and 50?,respectively,with a visible-ultraviolet spectrophotometer.The results from single crystal X-ray diffraction showed that MOF-Cd?II?-CA belongs to monoclinic system,P 21/c space group,with unit cell parameters of a???=6.1021?2?,b???=15.296?4?,c???=9.7816?3?,?=90,?=102.79?3?,?=90,a nd volume??³?=890.32?5?;MOF-Co?II?-FA belongs to monoclinic system,P 21/c space group,with unit cell parameters a???=6.6200?2?,b???=8.1642?3?,c???=15.577?6?,?=90,?=96.295?3?,?=90,and volume??³?=836.80?5?;MOF-Zn?II?-IcA belongs to monoclinic system,C 2/c space group,with unit cell parameter a???=7.0287?5?,b???=18.954?10?,c???=11.954?8?,?=90,?=102.56?7?,?=90,and volu me??³?=1554.4?18?;MOF-Cd?II?-IcA belongs to monoclinic system,C 2/c space group,with unit cell parameters a???=7.0313?3?,b???=18.969?8?,c???=11.939?6?,?=90,?=102.58?5?,?=90,and volume??³?=1554.3?13?;MOF-Cd?II?-FA b elongs to triclinic system,P-1 space group,with unit cell parameter a???=7.5186?4?,b???=7.5851?3?,c???=8.1908?4?,?=76.266?4?,?=63.141?5?,?=64.875?4?,and volume??³?=376.69?4?.Thermogravimetric and differential thermal analysis showed that for the five MOF crystals MOF-Cd?II?-CA,MOF-Co?II?-FA,MOF-Zn?II?-IcA,MOF-Cd?II?-IcA and MOF their decomposition temperatures are 250?,370?,395?,400?and 420?,respectively,indicating that these five crystal materials have good thermal stability.Among them,the MOFs from rigid ligands have better thermal stability than the MOF from the flexible ligand.The adsorption capacity of MOF-Co?II?-FA for methane is 3.50mol/kg at 296K and 5bar and 7.53mol/kg at 296K and 100bar,respectively,exhibiting a pote ntial for its application in methane adsorption storage.The data of the isothermal adsorption of MOF-Co?II?-FA for copper ions in water are fitted to the Langmu ir isothermal equation.The adsorption equilibrium constant,b,decreases with the increase of temperature,and the saturated adsorption amount,Qe,decreases with the increase of temperature.At 20?,b was 2.58 L/mmol and Qe was 86.4 mg/g.And the adsorption effieicncy of MOF-Co?II?-FA for Cu²⁺was 55.2%at 20?as the initial concentration of Cu²⁺was 1 mmol/L.The adsorption kinetics c an b e wel l fi t t ed t o t h e La g e rg r en s e co n d ar y a d s o rp t i o n eq u at i o n,tQt=1k₂Qe²+tQe,where the k₂ was found increasing with increasing temper ature first and then decreasing with increasing temperature.At 20?the k₂ is eq ual to 6.21�10^-4g.mg^-1.min^-1,and at 50?the k₂ value is equal to 7.87�10^-4g.mg^-1.min^-1.