Encapsulation Of Polyoxometalates Research Articles

Biaxially‐Strained Phthalocyanine at Polyoxometalate@Carbon Nanotube Heterostructure Boosts Oxygen Reduction Catalysis

AbstractIron phthalocyanine (FePc) with unique FeN4 site has attracted increasing interests as a promising non‐precious catalyst. However, the plane symmetric structure endows FePc with undesired catalytic performance toward the oxygen reduction reaction (ORR). Here, we report a novel one‐dimensional heterostructured ORR catalyst by coupling FePc at polyoxometalate‐encapsulated carbon nanotubes (FePc‐{PW12}@NTs) using host‐guest chemistry. The encapsulation of polyoxometalates can induce a local tensile strain of single‐walled NTs to strengthen the interactions with FePc. Both the strain and curvature effects of {PW12}@NT scaffold tune the geometric structure and electronic localization of FeN4 centers to enhance the ORR catalytic performance. As expected, such a heterostructured FePc‐{PW12}@NT electrocatalyst exhibits prominent durability, methanol tolerance, and ORR activity with a high half‐wave potential of 0.90 V and a low Tafel slope of 30.9 mV dec−1 in alkaline medium. Besides, the assembled zinc‐air battery demonstrates an ultrahigh power density of 280 mW cm−2, excellent charge/discharge ability and long‐term stability over 500 h, outperforming that of the commercial Pt/C+IrO2 cathode. This study offers a new strategy to design novel heterostructured catalysts and opens a new avenue to regulate the electrocatalytic performance of phthalocyanine molecules.

Biaxially-Strained Phthalocyanine at Polyoxometalate@Carbon Nanotube Heterostructure Boosts Oxygen Reduction Catalysis.

Iron phthalocyanine (FePc) with unique FeN4 site has attracted increasing interests as a promising non-precious catalyst. However, the plane symmetric structure endows FePc with undesired catalytic performance toward the oxygen reduction reaction (ORR). Here, we report a novel one-dimensional heterostructured ORR catalyst by coupling FePc at polyoxometalate-encapsulated carbon nanotubes (FePc-{PW12}@NTs) using host-guest chemistry. The encapsulation of polyoxometalates can induce a local tensile strain of single-walled NTs to strengthen the interactions with FePc. Both the strain and curvature effects of {PW12}@NT scaffold tune the geometric structure and electronic localization of FeN4 centers to enhance the ORR catalytic performance. As expected, such a heterostructured FePc-{PW12}@NT electrocatalyst exhibits prominent durability, methanol tolerance, and ORR activity with a high half-wave potential of 0.90 V and a low Tafel slope of 30.9 mV dec-1 in alkaline medium. Besides, the assembled zinc-air battery demonstrates an ultrahigh power density of 280 mW cm-2, excellent charge/discharge ability and long-term stability over 500 h, outperforming that of the commercial Pt/C+IrO2 cathode. This study offers a new strategy to design novel heterostructured catalysts and opens a new avenue to regulate the electrocatalytic performance of phthalocyanine molecules.

Host-Guest Chemistry in Boron Nitride Nanotubes: Interactions with Polyoxometalates and Mechanism of Encapsulation.

Boron nitride nanotubes (BNNTs) are an emerging class of molecular container offering new functionalities and possibilities for studying molecules at the nanoscale. Herein, BNNTs are demonstrated as highly effective nanocontainers for polyoxometalate (POM) molecules. The encapsulation of POMs within BNNTs occurs spontaneously at room temperature from an aqueous solution, leading to the self-assembly of a POM@BNNT host-guest system. Analysis of the interactions between the host-nanotube and guest-molecule indicate that Lewis acid-base interactions between W═O groups of the POM (base) and B-atoms of the BNNT lattice (acid) likely play a major role in driving POM encapsulation, with photoactivated electron transfer from BNNTs to POMs in solution also contributing to the process. The transparent nature of the BNNT nanocontainer allows extensive investigation of the guest-molecules by photoluminescence, Raman, UV-vis absorption, and EPR spectroscopies. These studies revealed considerable energy and electron transfer processes between BNNTs and POMs, likely mediated via defect energy states of the BNNTs and resulting in the quenching of BNNT photoluminescence at room temperature, the emergence of new photoluminescence emissions at cryogenic temperatures (<100 K), a photochromic response, and paramagnetic signals from guest-POMs. These phenomena offer a fresh perspective on host-guest interactions at the nanoscale and open pathways for harvesting the functional properties of these hybrid systems.

Hybrid Assembly toward Enhanced Thermal Stability of Virus-like Particles and Antibacterial Activity of Polyoxometalates

In an effort to improve both the stability of virus-like particles (VLPs) and the medical activity of polyoxometalates (POMs), a new hybrid assembly system between human papillomavirus (HPV) capsid protein L1 and a europium-containing POM (EuW10) has been constructed, for the first time, via the electrostatic interactions between them. The co-assembly of EuW10 and HPV 16 L1-pentamer (L1-p) in buffer solution resulted in the encapsulation of POMs in the cavity of VLPs, which was further confirmed by cesium chloride (CsCl) gradient ultracentrifugation, SDS-PAGE, dynamic light scattering, and transmission electron microscopy, whereas the post-assembly of EuW10 with the as-prepared VLPs leads to the adsorption of POMs only on the external surface of particles, and both cases improved the thermal and storage stabilities of VLPs obviously. Particularly, the encapsulation of POMs in VLPs largely improved the antibacterial activity of EuW10, and thereby, the present study will be significant for both the stability improvement of protein vaccines and the development of POM medicine.

Trichloroacetic acid-modulated synthesis of polyoxometalate@UiO-66 for selective adsorption of cationic dyes

In this study, POM@UiO-66 nanoparticles were fabricated by encapsulation of POM (K6P2W18O62 polyoxometalate) into mesoporous UiO-66 metal organic framework through a solvothermal method with trichloroacetic acid as a modulator to promote the defects formation of UiO-66. The as-prepared samples were characterized by TEM, SEM, PXRD, TG, XPS, and EDX elemental mapping, and the successful combination of POM and UiO-66 was confirmed. Two cationic dyes, rhodamine B and malachite green, and one anionic dye orange G were employed to investigate the adsorption performance of POM@UiO-66. The adsorption data showed that the removal process of cationic dyes by POM@UiO-66 matched well with the pseudo-second-order model and Langmuir isothermal model. The resulting POM@UiO-66 nanoparticles exhibited high adsorption to cationic dyes but low adsorption to anionic dyes, and the adsorption capacities of rhodamine B, malachite green, and anionic dye orange G were 222.6, 190.6 and 40 mg g−1, respectively. Furthermore, the cationic dyes could be selectively removed from a cationic-anionic dye binary system. These results suggested that the novel polyoxometalate-based UiO-66 material is a promising candidate for the adsorption of cationic dyes from effluent.

Highly selective oxidation of cyclohexene to 2-cyclohexene-1-one over polyoxometalate/metal–organic framework hybrids with greatly improved performances

H3+xPMo12−xVxO40@MIL-100 (Fe) (x = 0, 1, 2) hybrids were prepared by encapsulation of polyoxometalates (POMs) within the metal–organic framework using a direct hydrothermal method.

Encapsulation of polyoxometalates into supramolecular cages based on flexible ligands: Synthesis, structure and properties

Three Keggin-type polyoxometalates as noncoordinating guests wrapped into supramolecular cages based on flexible ligands, named, [H2L][HL][L]2[PMo12O40]·4H2O (1), [HL]4[SiW12O40] (2) and [HL]2[L]2[PWVWVI11O38(O2)]·4H2O (3) (L=4-(3-pyridinyloxy)benzoic acid) have been synthesized via hydrothermal conditions and have been characterized by elemental analysis, IR spectroscopy, TG analysis, and single-crystal X-ray diffraction. Single-crystal X-ray diffraction analyses revealed that the Keggin polyanions are wrapped as guests in the supramolecular cages that are formed by the ligands with hydrogen bonds (O–H⋯O and N–H⋯O). Moreover, the fluorescence of the title complexes and the electrochemical properties of 1 have been investigated. In addition, the catalytical properties of compounds have also been investigated.

Immobilization of polyoxometalates in the Zr-based metal organic framework UiO-67.

The encapsulation of polyoxometalates within the large pores of the Zr(iv) biphenyldicarboxylate UiO-67 metal-organic framework has been achieved, for the first time, by direct solvothermal synthesis. The resulting POM@UiO-67 composite materials were fully characterized by XRPD, IR, MAS NMR, N2 porosimetry measurements and cyclic voltammetry.

Induced circular dichroism of polyoxometalates via electrostatic encapsulation with chiral organic cations.

To explore the principle of chiral induction in inorganic clusters, chiral organic cations with two stereocenters, R- and S-BPEA, are used to encapsulate a series of polyoxometalates (POMs) bearing different structures and transition absorption bands in aqueous solution, constructing a series of chiral supramolecular complexes. Due to the induction of chiral organic cations, POMs possessing both chiral and achiral structures show an induced circular dichroism (ICD) effect. ICD signals in the absorption bands corresponding to ligand to metal charge transfer (LMCT) transitions, d-d transitions and intervalence charge transfer (IVCT) transitions are observed for different complexes. Moreover, the ICD of the POMs exhibits a direct correlation with the degree of POM distortion and the distance between the chiral center and the POM surface. The encapsulation of POMs with chiral organic cations via electrostatic interactions provides a facile and effective method for constructing optically pure POM-based materials.

Encapsulation of polyoxometalates within layered metal–organic frameworks with topological and pore control

Organic links of varied structure metrics and functionalities connect with Cu2(COO)4 clusters to afford layered metal–organic frameworks. Through topological and pore control, polyoxometalates could be synthetically encapsulated within the cavities. The templating effects of polyoxometalates and specific link functionalizations provide two strategies for controllably incorporating these active species into frameworks.