Synthesis Of Lanthanide Metal-Organic Frameworks Under Microwave-Assisted Solvothermal Condition

Metal-organic frameworks （MOFs）, which have unique frameworks, have intenseapplications in the areas of catalysis, adsorption, electrical, optical, and magnetic, so MOFsbecome a focus of chemical researches. Most of metal-organic frameworks are constructedthrough coordination of metal ions with multi-carboxylic acid ligands. This is due to theirintriguing topological structures and potential application as functional materials. In addition,oxalic acid has been widely used for the preparation of such complexes at the present time. Wechoose oxalic acid as a simple chain ligand, which can coordinate with lanthanide to form thecoordination polymers that have8-or12-membered rings or channels and may lead to two-orthree-dimensional network structures.At present many methods are used to synthesize metal-organic frameworks, which arehydrothermal （solvothermal） method, ultrasonic method, microwave method and so on. Amongthese, the most widely used method is hydrothermal （solvothermal） method, but it requires along reaction time, which extends the reaction cycle. While microwave method can overcomethis shortcoming, and it will help you get the products within a very short time, and also improvetheir yield and purity. Therefore we used oxalic acid and rare earth metal ions to firstlysynthesize the coordination compounds under microwave-assisted solvothermal condition.We got the products [Ln（C₂O₄）_1.5（H₂O）₃]·2H₂O （Ln=La、Pr、Sm、Gd、Dy） in20minutesunder microwave-assisted solvothermal condition, using rare earth nitrates and oxalic acid asstarting materials and DMF as solvent. The compound was characterized by X-ray Diffraction（XRD）, Thermogravimetry （TG）, Scanning Electron Microscopy （SEM）, IR spectroscopy, andFluorescent Spectrometry. The XRD result shows the compound has high crystallinity, and TGshows it has high thermal stability and can be stable until350-400°C in air. In addition, theproduct has good fluorescence properties. Besides, we studied the influences of different reaction time, temperature and the initialconcentration of the reactant on the complexes. It was found that the degree of crystallinity wasbad and the morphology became irregular when increasing the reaction temperature. When weprolonged the reaction time （less than30min）, the morphology was becoming more and moreregular, but when the reaction time reached30min the crystallinity started to deteriorate. Inaddition, higher initial concentration of the reactants leaded to worse morphology, and when thereaction concentration was0.25mol/L, the morphology was the best.In addition, there are few reports on the alkali metal ions together with rare earth ions tobuild MOFs. Framework decorated with unsaturated alkali metal ions will provide ideal scaffoldwith specific and strong binding sites for adsorbates, particularly H2. This paper studies the rareearth MOFs composed of potassium ion together with holmium ion, using holmium nitrate,oxalic acid, potassium nitrate as raw materials, and DMF as solvent, to synthesize[KHo（C₂O₄）₂（H₂O）₄]_n, which was characterized by XRD, SEM, TG, and IR. The test resultsshow that the product has high degree of crystallinity, and good thermal stability of up to350°Cin air. Also the nitrogen adsorption test shows Langmuir specific surface area is125.3m²/g,which is higher than the reported69.1m²/g in the literature.