Synthesis Of Heteropolytungstate@ZIF Composite And Its Adsorption/Detection Performance For Cations

In this work,a series of heteropoly tungstate coated by the Zeolitic imidazole-framework(ZIF)materials(POM@ZIF)by the one-pot method were successfully prepared and characterized by FTIR,TEM,XRD,UV-Vis and Fluorescence Spectra and other characterization.A series of materials were synthesized by encapsulating hydrophilic and electronic-negative heteropolytungstate capsules in the framework of hydrophobic ZIF materials to form a series of host-guest composites,which can be applied to the efficient adsorption or detection of cations in water(1)Using the self-assembly method,the Keggin type unsaturated silicotungstic acid Na₁₀[A-?-SiW₉O₃₄]·5H₂O(SiW₉),K₈[?₂-SiW₁₁O₃₉]·14H₂O(SiW₁₁)and saturated K₄[?-SiW₁₂O₄₀]·11H₂O(SiW₁₂)were encapsulated into ZIF-8 and ZIF-67,respectively,to prepare three kinds of POM@ZIF-8 complexes and three kinds of POM@ZIF-67 complexes.This kind of composite combines the stability and electronegative property of polyoxometalate with the hydrophobicity and limiting effect of ZIF material to prepare POM@ZIF host and guest composites,and synergistically exploit the advantages of both.The compound was used as an dye adsorbents to adsorb methylene blue(MB)dye in aqueous solution,and the effects of different conditions on the adsorption properties of dyes were investigated.Through comparison,it is found that SiW₁₁@ZIF has the highest adsorption efficiency,which is attributed to the higher solubility and negative charge of SiW₁₁compared with SiW₉ and SiW₁₂.The experimental results show that the composite can quickly adsorb MB in the solution,while the adsorption of the anionic dye methyl orange(MO)is very small,indicating that the composite has a good selectivity for the adsorption of cations.Moreover,the composite can be reused.After 5 times of cyclic adsorption-elution experiments,it was found that the adsorbent still maintained its high adsorption capacity.(2)In the process of self-assembly reaction,the rare earth class polyoxometalate Na₉Eu W₁₀O₃₆·32H₂O(Eu W₁₀)with fluorescence property was successfully encapsulated into the ZIF-8 material by introducing lithium salt,and the starting light type composite material(Eu W₁₀@ZIF-8)was prepared.The introduction of lithium salt can not only form ion pairs with Eu W₁₀to solve the problem of the very low solubility in organic alcohols,but also greatly enhance the luminescence intensity.Compared to the aqueous solution of Eu W₁₀,the fluorescence intensity of Eu W₁₀@ZIF-8 increased by about 13 times,which was mainly due to the introduction of Li⁺and the hydrophobicity of ZIF-8.Further study showed that the fluorescence quenching effect of Eu W₁₀@ZIF-8 composite material can be used for the detection of Pb²⁺and S^2-in the aqueous phase,and it shows good selectivity and high sensitivity.The minimum detection limits for Pb²⁺and S^2-can reach 0.66?M and 1.15?M,respectively.We investigated the specific detection mechanism of Pb²⁺,and the results showed that it could be attributed to fluorescence quenching induced by FRET.the fluorescence quenching caused by S^2-is due to the distortion of some ZIF-8 lattice caused by the introduction of Eu W₁₀and the exposed unsaturated zinc ions which combine with S^2-to generate Zn S,and the strong absorption of UV light by Zn S will induce the fluorescence quenching behavior of Eu W₁₀@ZIF-8.The research work in this paper not only provides a new technical route for the design and synthesis of POM@ZIF composites,but also expands the application of the materials in cationic adsorption or ion detection and other fields.