Design And Performance Of Novel Hybrid Materials Based On Keggin-type Polyoxometalates

Polyoxometalates(POMs)have attracted much attentions owing to their excellent acidity and reversible redox property.However,some drawbacks hindered the application of POMs,such as agglomeration in non-polar solvent and poor machinability.We prepare a series of POMs@MOFs and POMs@GO hybrid materials to overcome these problems.The main research achievements are attached below:1.For the first time,we propose a new strategy that using organic ligands of MOFs acting as hydrophobic groups to introduce POMs into nonpolar reaction systems.Reducing the level of sulfur content in fuel oils has long been desired for environmental reasons.Polyoxometalates(POMs)can act as catalysts to remove sulfur-containing heterocyclic compounds by the process of oxidative desulfurization under mild conditions.However,one key obstacle to the development of POM-based catalysts is the poor solubility of POMs in the overall nonpolar environment.We report a novel strategy for the introduction of catalytically active POMs into nonpolar reaction systems by encapsulating the inorganic catalyst within the pores of a metal–organic framework structure in which the organic ligands act as hydrophobic groups.The nanocrystalline catalysts,obtained rapidly and conveniently by both solution and mechanochemical synthesis,showed remarkable activities in catalytic oxidative desulfurization reactions in both a model diesel environment and in real diesel wherein dibenzothiophene was converted rapidly and quantitatively into dibenzothiophene sulfone2.An inherent challenge in using metal-organic frameworks(MOFs)for catalysis is how to access the catalytic sites generally confined inside the porous structure,in particular for substrates larger than the pores.We present here a promising solution to bypass this roadblock by modulating the facets of a crystalline MOF NENU-3a to enhance the facet exposure of the catalytic sites and the adsorption of substrates.Specifically,by transforming it with encapsulated catalysisresponsible polyoxometalate from octahedron characterized entirely by {111} facets to cube with only {100} facets,much enhanced catalytic activities were observed,especially for sterically demanding substrates that are otherwise hard to diffuse into the pores.Crystallographic analysis and adsorption/desorption experiments collectively established the critical effects of morphological control on the enhanced catalysis.The cubic crystals were then applied for biodiesel production,reaching more than 90% conversion of fatty acids(C12-C22)in comparison to <22% using octahedral crystals.3.Polyoxometalate-functionalized metal-organic frameworks(MOFs)featuring uniform proton-conducting pathways in three orthogonal directions,good water retention and stability were prepared by a convenient method.Three dimensional hydrogen bonding networks were constructed by incorporating phosphotungstic acid(HPW)and isonicotinic acid(Ina),which serve as both proton donors and carriers,into the pores of HKUST-1.The proton conductivity of the hybrid material was observed to increase by 5 orders of magnitude comparing to the parent material.4.Polyoxometalate-modified sponge-like GO monolith(PEGO)with 3D crosslinking inner structure,which exhibits unique “shrink-expand” effect to polar solvent,are synthesized.Owing to the introduction of polyoxometalates and the replacement of unstable epoxy groups by ethylenediamine,PEGO exhibits hitherto the highest proton conductivity under low relative humidity(1.02×10-2 S cm-1 at 60% relative humidity)and excellent long-term stability(more than 1 month).The outstanding conductivity originates from 3D transporting pathways,high-density hopping sites,and eliminated grain boundary resistance.This study provides a practical way to design GO-based protonconducting material dominated by in-plane diffusion.5.Created nanocomposites featuring porous diamine-functionalized graphene oxide(FGO)impregnated with photocatalytically active polyoxometalates(POMs).Cross-linking of GO via diamination not only generated porous structures with positively charged interior that attracts and stabilizes the anionic POM guests,but also modulated the GO bandgap.The introduction of POMs improved loading capacity of FGO for cationic dye pollutants and promoted effective separation of electron-hole pair by trapping and transferring photo-generated electron.The two components act in synergy to result in much improved adsorption of certain common organic dyes as well as enhanced oxidative degradation by both the GO host and the POMs that led to complete regeneration of the adsorbents without compromising their performance in multiple rounds of reuse.