Synthesis And Hydrostability Of Metal Organic Frameworks (MOF-5)

Abstract:Metal-Organic Frameworks (MOFs), the highly porous crystalline materials constructed by coordination bonds between metal ions and organic ligands, have been developed due to the specific pore size, intriguing structures and the potential applications, such as gas separation and storage, catalysis, biochemistry and drug delivery. MOF-5is one of the most prominent representatives of these porous materials. But being a hydrogen storage material, MOF-5is moisture sensitive because of the relative weak metal-oxygen coordination bond which is easy to be attacked by water molecules and that would result in the phase transformation and structure collapse. Hence, finding a method to enhance the hydrostability of MOF-5has become a challenge for chemists.The hydrostability of MOF-5was focused on in this paper, the main results were as followed:(1)MOF-5samples were synthesized using direct mixing of TEA and solvothermal synthesis methods. The properties of the products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), thermal gravimetric analysis (TG), and FT-IR respectively. The effect of the different synthesis methods on the morphologies and hydrostability of MOF-5was investigated. Yellow cube crystal with the size of100μm to200μm and high crystallinity was synthesized by solvothermal method, while the white and low crystallinity powder was prepared by direct mixing method. IR and DTG results revealed that exposure of MOF-5to air resulted in dramatic drop in crystallinity due to adsorbing water molecules. Compared with MOF-5_Z, MOF-5_R had better hydrostability. The serial interactional models of Zn4O(BDC)3(the secondary structural units of MOF-5) and H2O were built and the quantum calculations were performed. The results indicated that the weak bond between H2O and Zn4O(BDC)3exsited and induced to stabilize. And water molecules at side-position were easilier to be bonded with the frame structure than the water molecules at central-position.(2)In order to improve the hydrostability of MOF-5, Ni(Ⅱ) ions were doped into the MOF-5framework during the crystallization process. The chemical composition of Ni-MOF-5determined by energy-dispersive X-ray spectroscopy (EDS) suggested that the At%of Ni:Zn was about1:1. Though the IR results of the undoped-MOF-5and Ni-MOF-5were samilar, the TGA analysis showed that the thermo-decoposition temperature of Ni-MOF-5decreased apparently compared with that of pure MOF-5, which confirmed the exist of the iso-structure in Ni-MOF-5. After8days of being exposure on the air, TGA and IR curves of Ni-MOF-5were unchanged, which indicated that the hydrostability increased after Ni(II) ions doping.(3)The hydrostability of MOF-5_Z and MOF-5_R activated mechanically in planetary centrifugal mill for30minites was explored by XRD, SEM and FT-IR respectively. After2days of exposure to air, the structure of MOF-5_R occurred phase transformation, instead, MOF-5_Z was only partly decomposed. The hydrostability of MOF-5after ball-milling significantly decreased because of the larger contact area with H2O. The relationship between water stability and the degree of collapse of MOF-5was evaluated using quantum calculation results, which presented that the water stability energy increased with the enlargement of the framework decomposition, which demonstrated the low water-resistance of MOF-5materials. The position of water molecules in the framework were geometrically optimized theoretically, it could be found that water molecules situated at the side of the collapsed Zn-O bond could be attached with MOF-5in some ways, which would lead to lower energy state and more stable structure.