Deep oxidative desulfurization of gas oil by iron(III)-substituted polyoxometalate immobilized on nickel(II) oxide, ((n-C4H9)4N)4H[PW11FeO39]@NiO, as an efficient nanocatalyst

Sulfur compounds are among the most unfavorable constituents of petroleum derivatives, so stringent regulations have been established to curb their atmospheric emissions. In this regard, a new nanocomposite ((n-C4H9)4N)4H[PW11FeO39]@NiO) was synthesized composed of quaternary ammonium bromide salt of ironIII-substituted Keggin-type polyoxometalate immobilized on nickel(II) oxide nanoceramics via sol-gel method. The assembled (n-C4H9)4N)4H[PW11FeO39]@NiO nanocomposite was identified by FT-IR, UV-Vis, XRD, SEM, EDX, and TGA-DTG methods. The characterization results exhibited that ((n-C4H9)4N)4H[PW11FeO39] dispersed uniformly over the surface of the NiO nanoceramics. The ((n-C4H9)4N)4H[PW11FeO39]@NiO nanocomposite was employed as a heterogeneous nanocatalyst in the extractive coupled oxidation desulfurization (ECOD) of real gas oil and dibenzothiophene (DBT) as a model compound. Under relatively moderate conditions, the catalytic performance of the ((n-C4H9)4N)4H[PW11FeO39]@NiO in the ECOD procedure was studied by incorporating acetic acid/hydrogen peroxide as an oxidant system at a volume ratio of 1:2. According to the ECOD results, the ((n-C4H9)4N)4H[PW11FeO39]@NiO demonstrated the effectiveness of up to 95% with 0.1 g at 60 °C under optimal operating conditions. Moreover, the ((n-C4H9)4N)4H[PW11FeO39]@NiO nanocatalyst could be separated and reused for five runs without a noticeable decrease in the ECOD process. This study provides a promising way to meet the target of ultra-low sulfur as an essential process in oil refineries.

MOF/POM hybrids as catalysts for organic transformations

Insertion of molecular metal oxides, e.g. polyoxometalates (POMs), into metal-organic frameworks (MOFs) opens up new research opportunities in various fields, particularly in catalysis. POM/MOF composites have strong acidity, oxygen-rich surface, and redox capacity due to typical characteristics of POMs and the large surface area, highly organized structures, tunable pore size, and shape are due to MOFs. Such hybrid materials have gained a lot of attention due to astonishing structural features, and hence have potential applications in organic catalysis, sorption and separation, proton conduction, magnetism, lithium-ion batteries, supercapacitors, electrochemistry, medicine, bio-fuel, and so on. The exceptional chemical and physical characteristics of POMOFs make them useful as catalysts in simple organic transformations with high capacity and selectivity. Here, the thorough catalytic study starts with a brief introduction related to POMs and MOFs, and is followed by the synthetic strategies and applications of these materials in several catalytic organic transformations. Furthermore, catalytic conversions like oxidation, condensation, esterification, and some other types of catalytic reactions including photocatalytic reactions are discussed in length with their plausible catalytic mechanisms. The disadvantages of the POMOFs and difficulties faced in the field have also been explored briefly from our perspectives.

Cobalt complexes tuned by Anderson-type polyoxometalates and bis-amide derivative ligands featuring a 'V'-like connector for efficient ampere sensing and the visible-light catalytic reduction of Cr(VI)

To exploit polyoxometalate (POM)-based metal-organic complexes (MOCs) with outstanding electro- and photo-chemical performances, two new bis-amide derivative N-donor ligands featuring a 'V'-like connector, 4,4'-bis(3-pyridinecarboxamide)phenylmethane (L¹) and 4,4'-bis(3-pyridinecarboxamide)phenylketone (L²), were designed and reacted with Anderson-type POMs in the presence of Co(II) ions under solvothermal conditions, which generated four MOCs: [Co₂(L¹)₂(AlMo₆H₅O₂₄)]·4H₂O (1), [Co₂(HL¹)₂(H₂O)₆(TeMo₆O₂₄)]·2H₂O (2), [Co₂(HL²)₂(H₂O)₄[AlMo₆H₆O₂₄]₂]·5H₂O (3), and [Co₂(HL²)₂(H₂O)₆(TeMo₆O₂₄)]·2H₂O (4). All the complexes showed supramolecular structures via hydrogen bond interaction, which resulted from the 2D layers for 1, the satellite-like structural units for 2 and 4, but the 1D chains for 3. In these structures, the POMs and organic ligands exhibited different coordination modes. Both 2 and 4 showed efficient ampere sensing activities for Cr(VI) with lower limits of detection of 0.029 and 0.038 μM, respectively. Complexes 1 and 2 showed good visible-light catalytic behavior toward the reduction of Cr(VI), which offers more chances for developing electrochemical sensors and photocatalysts for Cr(VI).

Two polymolybdate-directed Zn(ii) complexes tuned by a new bis-pyridine-bis-amide ligand with a diphenylketone spacer for efficient ampere sensing and dye adsorption

Two polymolybdate-based Zn(ii) complexes were constructed from a new bis-pyridine-bis-amide ligand, which can be used as electrocatalysts and electrochemical sensors for Cr(vi), KBrO3, H2O2 and AA, and exhibit selective adsorption of CV and MB.

Various amide derivatives induced Keggin-type SiW12O404−-based cobalt complexes: assembly, structure, electrochemical sensing and dye adsorption properties

Three new [SiW12O40]4−-based complexes assembled from three amide derivative ligands displayed excellent electrochemical sensing activities for BrO3−, NO2− and Cr(vi) ions and showed good adsorption capacities for methylene blue and neutral red dyes.

Supramolecular network of a framework material supported by the anion-π linkage of Keggin-type heteropolyoxotungstates: experimental and theoretical insights

A novel inorganic-organic hybrid, {Na[Tb(L)(H2O)4]3(SiW12O40)}·4H2O, in which L = 4-hydroxypyridine-2,6-dicarboxylic acid (chelidamic acid), was prepared and characterized by elemental analyses, Fourier transform infrared spectroscopy, and single-crystal X-ray diffraction. It consists of a trinuclear terbium cluster bonded to a Keggin-type polyoxotungstate. In the terbium cluster three crystallographically equivalent Tb3+ ions are bridged by carboxylate oxygen atoms belonging to three chelidamic acid units to form an equilateral triangle. These triangle units connect to each other through hydrogen-bonding and anion-π interactions. A CSD survey was carried out focusing on the cation and the ligand roles in the structure formation of this type of compounds. Finally, DFT calculations were used to analyse the role of the anion-π interactions that are important in the formation of 1D supramolecular assemblies in the solid state. The noncovalent interaction plot (NCIplot) computational tool has been used to characterize the anion-π interaction.

Polyoxometalate-based metal–organic frameworks for heterogeneous catalysis

POM-based MOFs simultaneously possessing the virtues of POMs and MOFs exhibit excellent heterogeneous catalytic properties.

CuII Complexes and Coordination Polymers with Pyridine or Pyrazine Amides and Amino Benzamides—Structures and EPR Patterns

Isonicotine amide, picoline amide, pyrazine 2-amide, 2- and 4-amino benzamides and various CuII salts were used to target CuII complexes of these ligands alongside with 1D and 2D coordination polymers. Under the criterion of obtaining crystalline and single phased materials a number of new compounds were reliably reproduced. Remarkably, for some of these compounds the ideal Cu:ligand ratio of the starting materials turned out to be very different from Cu:ligand ratio in the products. Crystal and molecular structures from single-crystal XRD were obtained for all new compounds; phase purity was checked using powder XRD. We observed exclusively the Oamide and not the NH2amide function binding to CuII. In most of the cases; this occurred in chelates with the second pyridine, pyrazine or aminophenyl N function. µ-O,N ditopic bridging was frequently observed for the N = pyridine, pyrazine or aminophenyl functions, but not exclusively. The geometry around CuII in these compounds was very often axially elongated octahedral or square pyramidal. X-band EPR spectra of powder samples revealed various spectral symmetry patterns ranging from axial over rhombic to inverse axial. Although the EPR spectra cannot be unequivocally correlated to the observed geometry of CuII in the solid state structures, the EPR patterns can help to support assumed structures as shown for the compound [Cu(Ina)2Br2] (Ina = isonicotine amide). As UV-vis absorption spectroscopy and magnetic measurement in the solid can also be roughly correlated to the surrounding of CuII, we suggest the combination of EPR, UV-vis spectroscopy and magnetic measurements to elucidate possible structures of CuII compounds with such ligands.

Syntheses, structures and catalytic properties of Evans–Showell-type polyoxometalate-based 3D metal–organic complexes constructed from the semi-rigid bis(pyridylformyl)piperazine ligand and transition metals

Two 3D Evans–Showell-type POM-based different transition metals–bis-pyrazine–bis-amide complexes act as heterogeneous catalysts for the oxidation of benzyl alcohol.