Research On The Synthesis And Properties Of Composites Based On Metal-organic Framework And Dawson-type Polyoxometalate

With the rapid industrial development, the porous material with many excellent properties has been one of the hot areas for researchers. Metal-organic framework(MOF) as a new multifunctional material has attracted wide attention due to its unique application value. However, it still need to be modified in practical application because of their brittleness and lack of flexibility. Thus, metal-organic framework composite appeared in recent years. So far, designing and even tailor-making MOF-based composite by introducing polyoxometalate(POM) was mostly used in the field of catalysis, but was rarely used in the liquid-phase adsorption field. The aim of this thesis was to explore the catalytic activity and the selective adsorption performance of metal-organic framework composites based on polyoxometalate towards cationic dyes form dyeing wastewater. There were two different methods to be used successively:H6P2W18O62/Cu3(BTC)2(BTC=1,3,5-benzenetricarboxylate) was prepared by impregnation method. Three novel metal-organic framework composites based on polyoxometalate including H6P2W18O62/Cu3(BTC)2, H6P2W18O62/MOF-5 and H6P2Mo15W3O62/Cu3(BTC)2 were synthesized by a simple one-pot solvent-thermal method. The chemical structure, morphology, thermostability and surface charge of the composite were characterized by Fourier Transform Infrared Spectroscopy(FTIR),X-ray diffraction(XRD), Scanning Electron Microscope(SEM), Thermogravimetric Analysis(TGA), N2 adsorption-desorption isotherms, Zeta potential and so on.In Chapter 2, H6P2W18O62/Cu3(BTC)2(BTC=1,3,5-benzenetricarboxylate) as a new catalyst was prepared by impregnation method and used to synthesize cyclohexanone ethylene ketal as probe reaction, which has shown its favorable catalytic activity. FT-IR, XRD, SEM, N2 adsorption-desorption isotherms and TG/DTA were used to characterize its composition, structure, morphology, stability and toleration for thermal in order to further prepare the catalyst with effective performance. The catalyst was prepared under the optimum conditions: calcination temperature is at 150 oC, calcination time is 4 h, the content of H6P2W18O62 is 30 %.Moreover, we have also explored the optimized conditions of synthesizing probe reaction by orthogonal experiments. The yield of the acetals and ketones could reach54.9 %~83.5 % under the optimum conditions. H6P2W18O62/Cu3(BTC)2 exhibited good reusability and the approaches above may enable rational design of advanced and environmental-friendly MOF-based catalysts.In Chapter 3, one novel metal-organic framework composite H6P2W18O62/Cu3(BTC)2 based on polyoxometalate were synthesized by a simple one-pot solvent-thermal method and applied as an adsorbent to remove cationic dye methylene blue(MB) from aqueous solution. The effect factors containing the initial concentration, contact time, initial solution pH and temperature of MB adsorption onto the composite were systematically explored. The results showed that the lower pH and higher initial concentration benefited MB adsorption. The surface area of porous MOF increased through the modification of polyoxometalate, making that the removal rate of the composite was obviously greater than that of the pure MOF.Besides, the composite exhibited a fast adsorption rate and perfect adsorption ability towards the cationic dyes in aqueous solution. The adsorption process of the composite towards MB was described by a multilayer Freundlich model adsorption isotherm while kinetic data obeyed the pseudo-second-order model. The thermodynamic parameters ΔG<0 and ΔH<0 illustrated that the adsorption process was spontaneous and exothermic process.In Chapter 4, the composite H6P2W18O62/MOF-5 was synthesized by the same simple one-step reaction under solvothermal conditions. The industrial application of the composite and its selective separation ability in the mixed dye molecules were explored through simulating the real dye wastewater systems. The results showed that the removal rate was up to 97 % for cationic dyes methylene blue(MB) and 68 % for rhodamine B(RhB) within 10 min. However, anionicdye methyl orange(MO) can only reach to 10 %. The maximum uptake capacity of the composite at 20 oC was 51.81mg/g and considerably high compared to that of many other conventional materials towards MB reported in the literature. The experiment data could be well described by the Langmuir equations and pseudo-second-order kinetic model. Thermodynamicparameters ΔG<0 and ΔH>0 indicated that the MB adsorption onto H6P2W18O62/MOF-5 was spontaneous and endothermic.In order to study the adsorption property of the composite based on other Dawson-type polyoxometalate and metal-organic framework, the composite H6P2Mo15W3O62/Cu3(BTC)2 was prepared in Chapter 5 by a method similar to the the first two chapters. All the results illustrated that H6P2Mo15W3O62/Cu3(BTC)2composite can effectively and selectively remove cationic organic pollutants,represented by MB. The maximum uptake capacity of the composite at 30 oC was77.22 mg/g. The adsorption process was related to the electrostatic attraction between negatively charged composite surface and positively charged MB molecules. The adsorption process fitted the pseudo-second-order kinetic model and the Langmuir isotherm model well. The thermodynamic parameters indicated that the adsorption was spontaneous and exothermic process. These results showed that designing a metal-organic framework composite is a quite promising strategy to achieve extreme application for metal-organic framework.