Ion Exchange Synthesis And Properties Study Of The Crystalline Porous Materials Based On Polyoxometalates

The construction and characterization of porous materials with versatilities have been thefocus of intense research interest. Metal-organic frameworks （MOFs） are a new class ofcrystalline porous materials consisting of metal ions and organic ligands that have highporosities and specific functionalities: catalysis, magnetism, separation, and gas adsorption.They not only offer high surface areas but also provide tunable pore shapes/sizes. As aconsequence, they are allowed greater chemical alteration on a periodic scale and can be usedas hosts for encapsulating other guest molecules to construct new multifunctional materials.Polyoxometalates （POMs） have been incorporated into MOFs as guests or templates toconstruct novel hybrid materials. These hybrid materials based on porous MOFs and POMs,which present intriguing structures, excellent properties, and corresponding applications, havereceived increasing attention. In this paper, a series of crystalline porous materials have beensynthesized by incorporating POMs into MOFs. The relationships between structures andproperties, including stability and porosity, were also carried out.Five new crystalline porous compounds based on POMs have been synthesized by thehydrothermal reaction and ion exchange. In addition to elemental analysis, thermalgravimetric analysis, IR, single-crystal X-ray diffraction, and powder X-ray diffraction, theadsorption properties were studied by adsorption measurements, including N2, H₂, NO,volatile organic compounds （VOCs）, and nerve gas stimulant. Furthermore, some compoundsdisplayed their potential application in the removal of toxic gas. The main achievements aresummarized as follows:1. A sodalite-type porous MOFs with Keggin-type polyoxoanion [PW₁₂O₄₀]3-template,H3[（Cu4Cl）3（BTC）8]2[（C4H12N）6PW₁₂O₄₀]·3H₂O （1）（BTC=1,3,5-benzenetricarboxylate）,was obtained by the hydrothermal reaction of simple reagents. As a reasonable candidate foreliminating nerve gas, it displayed good adsorption behavior for dimethyl methylphosphonate（DMMP）. The adsorbed amount of DMMP reached23.82%（1.92mmol g-1）, which isequivalent to the adsorption of15.5DMMP molecules per formula unit. In virtue of thecatalytic activity of POMs guests, this nerve gas mimic could be facilely decomposed by ahydrolysis reaction. It was found that the conversion increased gradually with temperature,reaching93%at50℃.2. Another isotypic compound, H₄[（Cu₄Cl）₃（BTC）₈]₂[(C4H₁₂N)₆SiW₁₂O₄₀]·3H₂O （2） wassynthesized under similar condition by changing Keggin-type polyoxoanion [PW₁₂O₄₀]3-to[SiW₁₂O₄₀]4-and employed in NO removal for the first time. The open structure with exposedCu2+sites and POMs guests increased the adsorption of NO. Furthermore, the introduction ofPOMs improved the thermal stability and brought a positive possibility for NO decomposition. About64%of adsorbed NO decomposed, yielding N2by rapid heating at573K.3. Three isomorphic POMs-based porous compounds have been prepared by alkali metals ion （Li+、K+） exchange.Li2[Cu12（BTC）8·12H2O][HPW12O40]·27H2O （3）Li2[Cu12（BTC）8·12H2O][H2SiMo12O40]·5H2O （4）K2[Cu12（BTC）8·12H2O][HPW12O40]·28H2O （5）By initiating a systematic evaluation, we have demonstrated that POMs and alkali metal cations played important roles for increasing the adsorption capacity of these compounds. The adsorption enthalpies estimated by the modified Clausius-Clapeyron equation provided insight into the impact of POMs and alkali metal cations on the adsorption of H2and volatile organic compounds （VOCs）. The introduction of POMs not only improved the stability, but also brought the increase of adsorption capacity by strengthening the interaction with gas molecules. Furthermore, the exchanged alkali metal cations acted as active sites to interact with adsorbates and enhanced the adsorption.