Shuang Yao

Incorporating Polyoxometalates into a Porous MOF Greatly Improves Its Selective Adsorption of Cationic Dyes

Various polyoxometalates (POMs) were successfully immobilized to the mesoporous coordination polymer MIL-101 resulting in a series of POM-MOF composite materials POM@MIL-101 (POM = K4PW11VO40, H3PW12O40, K4SiW12O40). These materials were synthesized by a simple one-pot reaction of Keggin POMs, tetramethylammonium hydroxide (TMAH), terephthalic acid (H2bdc), and Cr(3+) ions. XRD, FTIR, thermogravimetric analyses (TG), inductively coupled plasma (ICP) spectrometry, and energy-dispersive X-ray spectroscopy (EDX) collectively confirmed the successful combination of POMs and the porous framework. Further, these composites POM@MIL-101 with different loading of POMs were achieved by variation of the POM dosage. Notably, the uptake capacity of MIL-101 towards organic pollutants in aqueous solution was significantly improved by immobilization of hydrophilic POMs into cages of MIL-101. An uptake capacity of 371 mg g(-1), comparable to that of the graphene oxide sponges, and much higher than that of the commercial activated carbon, was achieved at room temperature in 5 min when dipping 20 mg PW11V@MIL-101 in the methylene blue (MB) solution (100 mL of 100 mg L(-1) MB solution). Further study revealed that the POM@MIL-101 composite materials not only exhibited a fast adsorption rate towards dye molecules, but also possessed of selective adsorption ability of the cationic dyes in wastewater. For example, the adsorption efficiency of PW11V@MIL-101 (10 mg) towards MB (100 mL of 10 mg L(-1)) could reach 98 % in the initial 5 min, and it could capture MB dye molecules from the binary mixture of the MB and MO with similar size. Also, the POM@MIL-101 materials could be readily recycled and reused, and no POM leached in the dye adsorption process.

A polyoxometalate-based ionic crystal assembly from a heterometallic cluster and polyoxoanions with visible-light catalytic activity

A polyoxometalate-based inorganic–organic hybrid [WO4{Ni(en)2(H2O)}3] [Ni(en)3]{P2W18O62}·[Ni(en)3]CO3·H2O (1) (en = ethanediamine), was synthesized under hydrothermal conditions, and characterized by IR, elemental analyses, XPRD, UV diffuse reflectance spectroscopy and single crystal X-ray diffraction analyses. Compound 1 consists of a heterometallic cluster [WO4{Ni(en)2(H2O)}3]4+, two [Ni(en)3]2+ groups, one [CO3]2− group, one water molecule and a Wells–Dawson anion [P2W18O62]6− ({P2W18}), which represents the first POM-based ionic crystal with the heterometallic macrocation cluster. Photocatalytic studies indicate that compound 1 is not only actively photocatalytic for degradation of methylene blue (MB) under visible light irradiation, but very stable and easily separated from the reaction system for reuse as well.

Polyoxometalate-based supramolecular architecture constructed from a purely inorganic 1D chain and a metal–organic layer with efficient catalytic activity

A new polyrotaxane structure, {Ag8O(Htrz)4(4,4′-bpy)2}{AgPMo12O40} (Htrz = 1,2,4-triazole, 4,4′-bpy = 4,4′-bipyridine) (1), was obtained by the reaction of Ag+, H5PMo10V2O40 and mixed organic linking ligands under hydrothermal conditions. The structure has been characterized by IR, TG analysis, elemental analysis, X-ray powder diffraction and single-crystal X-ray diffraction. In 1, a purely inorganic 1D chain composed of Keggin-type polyoxoanions and Ag+ linkers, interluded into the 2D Ag–organic layers, resulting in the polyrotaxane structure. This is the first polyrotaxane compound constructed from a POM-based purely inorganic 1D chain and Ag-Htrz-4,4′-bpy 2D networks. The ratio of the mixed ligands played a crucial role in the synthesis of the title compound. Moreover, compound 1 exhibited efficient catalytic activity for the oxidation of desulfurization, and possessed electrocatalytic activity for the reduction of H2O2 and BrO3−.

Self-assembly and photocatalytic property of germanoniobate [H6Ge4Nb16O56]10−: encapsulating four {GeO4} tetrahedra within a {Nb16} cage

An unprecedented organic-inorganic hybrid germanoniobate compound Na4[Cu(en)2(H2O)2]5[Na6Ge8Nb32O108H8(OH)4]·41H2O (1) was synthesized under the hydrothermal condition. In compound 1, the {Nb16} cage containing four {GeO4} tetrahedra in its internal cavity results in a heteropolyniobate anion [H4Ge4Nb16O54(OH)2](10-) ({Ge4Nb16}), which is connected by a {Na6} cluster into the first germanoniobate-based sandwich-type structure. Further, the sandwich germanoniobates are connected by [Na2Cu(en)2O6H8] groups into a porous network with one dimensional channels along the a-axis. Photocatalytic study reveals that compound 1 exhibits good photocatalytic activity for the degradation of methylene blue.

Heterometallic 3d–4f cluster-containing polyoxotungstate obtained by partial disassembly of preformed large clusters

A new heterometallic 3d–4f cluster-containing polyoxotungstate KNa5{K2Dy2Cu2(H2O)8(SbW9O33)2}·17H2O (1) was designed and synthesized by reaction of the preformed large cluster [N(CH3)4]10Na12[Na2Sb8W36O132(H2O)4]·26H2O ({Sb8W36}), transition-metal (TM) and Ln3+ cations. Compound 1 is the first 3d–4f heterometallic cluster-containing sandwich-type tungstoantimonite obtained by partial decomposition of a preformed large cluster. Further, the sandwich-type polyoxoanion combines with two additional 4p cations resulting in a ring-like 3d–4f–4p cluster {K2Dy2Cu2(H2O)8}-containing polyoxotungstate. The sandwich polyoxoanion {K2Dy2Cu2(H2O)8(SbW9O33)2} was structurally similar to the well-known anion {K3Cu3(H2O)10(SbW9O33)2}, in which one alkali metal cation and one TM cation were both replaced by the rare earth metal cations. Furthermore, the 3d–4f–4p cluster-containing polyoxoanions were bridged by the K+ ion located at their central metal belt into a one-dimensional (1D) wavelike chain. An electrochemical study revealed that compound 1 exhibited electrocatalytic activity for reduction of the NO2− and O2, and a direct 4-electron electrochemical reduction process of O2 was achieved in 1-containing aqueous solution.

Mixed-valence manganese cluster containing a sandwich-type polyoxometalate

Reaction of Na12[α-P2W15O56] · 18H2O with Mn(CH3COO)2 · 4H2O in the presence of K2S2O8 in acidic aqueous solution leads to isolation of a { } cluster-containing sandwich-type compound: αββα-K3Na11{ (H2O)2(P2W15O56)2} · 40H2O (1). Single-crystal X-ray diffraction analysis revealed that 1 shows the well-known Weakley-type sandwich-type structure consisting of two trivacant [α-P2W15O56]12− Dawson moieties linked by a central symmetric rhomb-like mixed-valence manganese cluster via W–O–Mn and P–O–Mn connections. Mixed-valence Mn-cluster-substituted sandwich-type complexes based on trivacant Dawson units are rarely observed in polyoxometalate chemistry. Further, the sandwich-type polyoxoanions were connected by [K3Na2] clusters into a 44-layer. Magnetic study indicates that antiferromagnetic interactions exist in this compound.

Supermolecular assembly of polyoxoanion and metal–organic cationic units towards a model for core–shell nanostructures

Two novel core–shell-like molecular composites, [Ni(dap)3]4[HVIV12VV6O42(PO4)] (1) and [Ni(dap)2]4[HVIV12VV6O42(PO4)]·4H2O (2) (dap = 1,2-diaminopropane), were obtained from the supermolecular assembly of polyoxoanions and metal–organic cationic units. These two inorganic–organic hybrids were prepared under hydrothermal conditions and characterized by elemental analysis, IR spectroscopy, PXRD and single crystal X-ray diffraction analysis. In the well defined molecular core–shell structure of composite 1, the sphere-like anion [PV18O46]9− was surrounded by a shell composed of eight windmill-type [Ni(dap)3]2+ cationic units. In 2, ten sheet-like [Ni(dap)2]2+ metal–organic groups surrounded the sphere-like anion [PV18O46]9−, which was achieved by the easy alteration of the chemical composition of the core–shell structure by simply adjusting the feeding amount of dap. The resulting core–shell-like molecular composites represent a promising structural model toward core–shell nanostructures and exhibit electrocatalytic activity for the reduction of H2O2 and oxidation of NO2−.

Capped Polyoxometalate Pillars between Metal–Organic Layers for Transferring a Supramolecular Structure into a Covalent 3D Framework

Two robust metal-organic frameworks (MOFs), {H4[Ni(π-H2O)2]2[Ni(rt-H2O)2]8Ni4(Tri)24}[VIVW12O40]2·24H2O (1) and {H[Ni(π-O)2]2[Ni(rt-H2O)2]8Ni4(Tri)24}[VIVW10VV2O40V2][VIVW9VV3O40VIV2]·24H2O (2) (Tri = 1,2,4-triazole), composed of polyoxometalates (POMs) and metal-organic units, were designed and synthesized by a hydrothermal method. Structure analysis indicates that there is a metal-organic crown [{Ni3(Tri)6(H2O)4}4] ({Ni12}) in these two compounds. In 1, the {Ni12} crown embraces four pendant Tri ligands that could capture a cationic [Ni(H2O)2]2+ group, resulting in the Ni13-Tri building unit [Ni(H2O)2{Ni3(Tri)6(H2O)4}4] ({Ni13}). The {Ni13} building unit was fused together by Tri bridges into the 2D metal-organic layers, which are pillared by a typical Keggin-type POM [VW12O40]4- to construct a 3D supramolecular framework via the hydrogen bonds. Interestingly, the 2D metal-organic layer in 1 was successfully transferred into a 3D covalent MOF via extension of the length of the pillars by capping a Keggin-type POM with V-O units. Moreover, electrochemical behaviors and electrocatalytic properties of these two compounds were both studied, which can act as bifunctional electrocatalysts toward the reduction of H2O2 and oxidation of nitrite in neutral aqueous solution.

Assembly of polyoxometalates and Ni-bpy cationic units into the molecular core–shell structures as bifunctional electrocatalysts

Four polyoxometalate-based supramolecular assemblies were designed and synthesized under hydrothermal conditions: [Ni(2,2-bpy)3]5[PW11NiO39(H2O)]2·1.08H2O (1), [Ni(2,2-bpy)3]1.5[Ni(2,2-bpy)2(H2O)BW12O40] (2), {[Ni(2,2-bpy)3]1.5[Ni(2,2-bpy)2(H2O)GeW12O40]}− (3), and {[Ni(2,2-bpy)3]1.5[Ni(2,2-bpy)2(H2O)PW12O40]}2− (4). These structures were characterized by IR, TG analysis, elemental analysis, X-ray powder diffraction and single-crystal X-ray diffraction. In compound 1, two mono-substituted [PW11NiO39(H2O)]5− anions were fused together via the H-bonding interactions forming a dimeric polyoxoanion [PW11NiO39(H2O)]210−, which was surrounded by Ni-bpy cationic units into the molecular core–shell structures. Compounds 2–4 were composed of saturated polyoxoanions [BW12O40]5−, [GeW12O40]4− and [PW12O40]3−, respectively. These saturated anions were surrounded by [Ni(2,2-bpy)3]2+ and [Ni(2,2-bpy)2(H2O)]2+ cationic units forming isostructural core–shell organic–inorganic hybrid materials. These isostructural polyoxometalates with different heteroatoms and charges all formed the isostructural materials, indicating that their charges do not have a significant influence on the structure of supramolecular assemblies. A detail study showed that substituted polyoxometalate with the terminal H2O molecule is essential for constructing the larger core–shell structure. An electrocatalytic study indicated that these four compounds are bifunctional electrocatalysts towards water oxidation and reduction of nitrite.