Salete S. Balula

An efficient oxidative desulfurization process using terbium-polyoxometalate@MIL-101(Cr)

An efficient and recyclable oxidative desulfurization process (ODS) to remove the most refractory sulfur-compounds (dibenzothiophene, 1-benzothiophene and 4,6-dimethyldibenzothiophene) from fuel is reported. The ODS process was catalyzed by terbium-polyoxometalate [Tb(PW11O39)2]11− (Tb(PW11)2) and its composite Tb(PW11)2@MIL-101. The tetrabutylammonium (TBA) salt of Tb(PW11)2 was prepared and further incorporated in the porous metal–organic framework MIL-101(Cr). The TBA compound and its composite were characterized by various techniques (powder X-ray diffraction, FT-IR, FT-Raman, SEM and elemental analysis), and their electrochemical behavior was investigated, indicating that the structure of the polyoxometalate anion must be retained after immobilization. The studied ODS process was based on a biphasic system formed by a model oil with various refractor sulfur-compounds and an extracting solvent using H2O2 as the oxidant. Two main steps in the process were carefully investigated: the initial extraction and the oxidative catalytic stage. The optimization of the ODS process was performed by the analysis of the most suitable extracting solvent and also comparing the desulfurization performance of the homogeneous Tb(PW11)2 and the heterogeneous Tb(PW11)2@MIL-101 catalysts. Acetonitrile was selected as the best solvent because it allowed the highest desulfurization rate, conciliating good initial extraction and high catalytic performance. The presence of the porous catalyst Tb(PW11)2@MIL-101 seemed not to influence the initial extraction step; however, with this porous hybrid catalyst were obtained higher desulfurization rates during the catalytic stage. Remarkably, using Tb(PW11)2@MIL-101 and the oil–acetonitrile system complete desulfurization of oil was achieved only after 5 h. The recyclability of the solid catalyst was investigated for three consecutive ODS cycles and its stability was confirmed by several techniques.

Novel Composite Material Polyoxovanadate@MIL-101(Cr): A Highly Efficient Electrocatalyst for Ascorbic Acid Oxidation

A novel hybrid composite material, PMo10V2@MIL-101 was prepared by the encapsulation of the tetra-butylammonium (TBA) salt of the vanadium-substituted phosphomolybdate [PMo10V2O40](5-) (PMo10V2) into the porous metal-organic framework (MOF) MIL-101(Cr). The materials characterization by powder X-ray diffraction, Fourier transform infrared spectra and scanning electron microscopy confirmed the preparation of the composite material without disruption of the MOF porous structure. Pyrolytic graphite electrodes modified with the original components (MIL-101(Cr), PMo10V2), and the composite material PMo10V2@MIL-101 were prepared and their electrochemical responses were studied by cyclic voltammetry. Surface confined redox processes were observed for all the immobilized materials. MIL-101(Cr) showed one-electron reduction process due to chromium centers (Cr(III) → Cr(II)), while PMo10V2 presented five reduction processes: the peak at more positive potentials is attributed to two superimposed 1-electron vanadium reduction processes (V(V) → V(IV)) and the other four peaks to Mo-centred two-electron reduction processes (Mo(VI) → Mo(V)). The electrochemical behavior of the composite material PMo10V2@MIL-101 showed both MIL-101(Cr) and PMo10V2 redox features, although with the splitting of the two vanadium processes and the shift of the Mo- and Cr- centered processes to more negative potentials. Finally, PMo10V2@MIL-101 modified electrode showed outstanding enhanced vanadium-based electrocatalytic properties towards ascorbic acid oxidation, in comparison with the free PMo10V2, as a result of its immobilization into the porous structure of the MOF. Furthermore, PMo10V2@MIL-101 modified electrode showed successful simultaneous detection of ascorbic acid and dopamine.

Chemistry and Catalytic Activity of Molybdenum(VI)-Pyrazolylpyridine Complexes in Olefin Epoxidation. Crystal Structures of Monomeric Dioxo, Dioxo-μ-oxo, and Oxodiperoxo Derivatives

The dioxomolybdenum(VI) complexes [MoO2Cl2(PzPy)] (1) and [MoO2(OSiPh3)2(PzPy)] (5) (PzPy = 2-[3(5)-pyrazolyl]pyridine) were synthesized and characterized by vibrational spectroscopy, with assignments being supported by DFT calculations. Complex 5 was additionally characterized by single crystal X-ray diffraction. Recrystallization of 1 under different conditions originated crystal structures containing either the mononuclear [MoO2Cl2(PzPy)] complex co-crystallized with 2-[3(5)-pyrazolyl]pyridinium chloride, binuclear [Mo2O4(μ2-O)Cl2(PzPy)2] complexes, or the oxodiperoxomolybdenum(VI) complex [MoO(O2)2Cl(PzPyH)], in which a 2-[3(5)-pyrazolyl]pyridinium cation weakly interacts with the Mo(VI) center via a pyrazolyl N-atom. The crystal packing in the different structures is mediated by a variety of supramolecular interactions: hydrogen bonding involving the pyridinium and/or pyrazolyl N-H groups, weak CH · · · O and CH · · · π contacts, and strong π-π stacking. Complexes 1 and 5 are moderately active catalysts for the epoxidation of cis-cyclooctene at 55 °C using tert-butylhydroperoxide as oxidant, giving 1,2-epoxycyclooctane as the only reaction product. Insoluble materials were recovered at the end of the first catalytic runs and characterized by IR spectroscopy, elemental analysis, scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS), and powder X-ray diffraction. For complex 5 the loss of the triphenylsiloxy ligands during the catalytic run resulted in the formation of a tetranuclear complex, [Mo4O8(μ2-O)4(PzPy)4]. The recovered solids could be used as efficient heterogeneous catalysts for the epoxidation of cyclooctene, showing no loss of catalytic performance between successive catalytic runs.

Synthesis and catalytic properties of molybdenum(VI) complexes with tris(3,5-dimethyl-1-pyrazolyl)methane.

The complex [MoO(2)Cl{HC(3,5-Me(2)pz)(3)}]BF(4) (1) (HC(3,5-Me(2)pz)(3) = tris(3,5-dimethyl-1-pyrazolyl)methane) has been prepared and examined as a catalyst for epoxidation of olefins at 55 °C using tert-butyl hydroperoxide (TBHP) as the oxidant. For reaction of cis-cyclooctene, epoxycyclooctane is obtained quantitatively within 5 h when water is rigorously excluded from the reaction mixture. Increasing amounts of water in the reaction mixture lead to lower activities (without affecting product selectivity) and transformation of 1 into the trioxidomolybdenum(VI) complex [{HC(3,5-Me(2)pz)(3)}MoO(3)] (4). Complex 4 was isolated as a microcrystalline solid by refluxing a suspension of 1 in water. The powder X-ray diffraction pattern of 4 can be indexed in the orthorhombic Pnma system, with a = 16.7349(5) Å, b = 13.6380(4) Å, and c = 7.8513(3) Å. Treatment of 1 in dichloromethane with excess TBHP led to isolation of the symmetrical [Mo(2)O(4)(μ(2)-O){HC(3,5-Me(2)pz)(3)}(2)](BF(4))(2) (2) and unsymmetrical [Mo(2)O(3)(O(2))(2)(μ(2)-O)(H(2)O){HC(3,5-Me(2)pz)(3)}] (3) oxido-bridged dimers, which were characterized by single-crystal X-ray diffraction. Complex 2 displays the well-known (Mo(2)O(5))(2+) bridging structure where each dioxidomolybdenum(VI) center is coordinated to three N atoms of the organic ligand and one μ(2)-bridging O atom. The unusual complex 3 comprises dioxido and oxidodiperoxo molybdenum(VI) centers linked by a μ(2)-bridging O atom, with the former center being coordinated to the tridentate N-ligand. The dinuclear complexes exhibit a similar catalytic performance to that found for mononuclear 1. For complexes 1 and 2 use of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate and N-butyl-3-methylpyridinium tetrafluoroborate as solvents allowed the complexes to be completely dissolved, and in each case the catalyst and IL could be recycled and reused without loss of activity.

Oxidative catalytic versatility of a trivacant polyoxotungstate incorporated into MIL-101(Cr)

The first immobilization of the trivacant Keggin-type polyoxometalate ([A-PW9O34]9−, PW9) to prepare a novel heterogeneous oxidative catalyst is here reported. PW9 was incorporated into the cavities of the chromium terephthalate metal–organic framework MIL-101(Cr). Characterization of the composite PW9@MIL-101 by powder X-ray diffraction, SEM-EDX, FT-IR, FT-Raman spectroscopy, N2 adsorption–desorption isotherms and 31P solid-state NMR confirmed that the structures of MIL-101 and the polyoxometalate anion were retained after immobilization. The composite PW9@MIL-101 revealed versatility as a heterogeneous catalyst to oxidize efficiently monoterpenes as well as to reach a complete desulfurization of a model oil containing the most refractory sulfur compounds in fuel, using in both systems acetonitrile as the solvent and H2O2 as the oxidant. Complete conversion of geraniol to 2,3-epoxygeraniol was achieved after the first 30 min at room temperature, while the total desulfurization of the model oil containing 1707 ppm of sulfur was attained after 2 h. In both systems the catalyst was recyclable for various cycles without a significant loss of activity. The stability and heterogeneity of the catalyst were confirmed by several techniques and by leaching tests.

Influence of a porous MOF support on the catalytic performance of Eu-polyoxometalate based materials: desulfurization of a model diesel

An aluminum 2-aminoterephthalate based metal–organic framework (MOF) material was applied for the first time to prepare highly efficient heterogeneous catalysts in desulfurization processes. Sandwich-type [Eu(PW11O39)2]11− polyoxometalate (POM) was supported on Al(III) and Cr(III) MIL-type MOFs, NH2-MIL-53(Al) and MIL-101(Cr), and extensive characterization confirmed the incorporation of the POM on the two supports. The catalytic performance of the two composite materials, POM/MIL(Al) and POM/MIL(Cr), was evaluated in the oxidative desulfurization (ODS) of a model fuel containing some of the most common refractory sulfur compounds in diesel. Both composite materials have shown to be active and robust heterogeneous catalysts for the efficient removal of the sulfur-containing compounds from the model diesel, and the influence of the solid support on the catalytic performance of the active species was further assessed. The POM/MIL(Al) revealed notable catalytic performance, since complete desulfurization was obtained after 2 h of reaction. Furthermore, this remarkable heterogeneous catalyst revealed to be stable and recyclable for various catalytic cycles.

Mono-substituted silicotungstates as active catalysts for sustainable oxidations: homo- and heterogeneous performance

A series of tetrabutylammonium (TBA) salts of the transition metal mono-substituted silicotungstates [SiW11M(H2O)O39]n−, M = CoII, FeIII, MnIII, (SiW11M) were explored as homogeneous catalysts for the oxidation of geraniol and styrene with H2O2. The most active homogeneous catalysts (SiW11Co and SiW11Fe) were immobilized onto an amine-functionalized SBA-15 (aptesSBA-15) and the resulting composites were characterized using several techniques (FT-IR, FT-Raman, UV-Vis/DRS, elemental analysis, powder XRD, SEM and N2 adsorption–desorption isotherms). The catalytic performance of the new composites SiW11Co@aptesSBA-15 and SiW11Fe@aptesSBA-15 was investigated under similar experimental conditions to those used for homogeneous counterparts. 2,3-Epoxygeraniol and benzaldehyde were the main products obtained from geraniol and styrene oxidation, respectively, for all the catalysts. SiW11Co and SiW11Co@aptesSBA-15 showed to be the most active catalysts for the oxidation of geraniol and styrene. The recyclability of the composite SiW11Co@aptesSBA-15 was investigated for three reaction cycles. The stability of the composites was confirmed using several techniques after catalytic cycles.

Dinuclear Lanthanide(III) Complexes by Metal-Ion-Assisted Hydration of Di-2-pyridyl Ketone Azine

The initial employment of di-2-pyridyl ketone azine in 4f metal chemistry has led to a unique ligand transformation; the resulting anionic ligand is able to bridge two Ln(III) ions, affording neutral and cationic dinuclear complexes with interesting properties.

Redox behaviour, electrochromic properties and photoluminescence of potassium lanthano phosphomolybdate sandwich-type compounds

Sandwich-type phosphomolybdates with general formula Kn[LnIII(PMo11O39)2], where LnIII = Sm, Eu, Gd, Tb and Dy, were prepared and characterized by several techniques. The crystal structure of Sm(PMo11)2 and Gd(PMo11)2 were studied and showed that they crystallise in a P21/c space group. All Ln(PMo11)2 revealed four Mo-based electrochemical reduction processes with very similar E1/2 values (≈0.5, ≈0.3, ≈0.02 and ≈−0.1 V) for all the LnIII atoms. The two more positive reduction processes correspond to pH independent one-electron reduction processes, whereas the two more negative processes correspond to pH dependent two-electron reduction processes. Electrolysis at two different potentials, 0.1 V – related to the two one-electron reduction processes – and −0.3 V – related to the two more negative two-electron reduction processes, confirmed the electrochromic properties of the Ln(PMo11)2 species: their original yellow coloured solutions turned blue, corresponding to the appearance of four new electronic bands in the near UV-Vis-near IR region. These electronic bands were tentatively assigned based on their molar absorption coefficients (e) and absorbance (Abs) variation as a function of electrolysis time: bands A (λmax ≈ 855–870 nm) and B (λmax ≈ 670–695 nm) were assigned to MoV → MoVI intervalence charge transfer transitions, band C (λmax ≈ 525 nm) to d–d transition due to d1 configuration of the reduced addenda atom (MoV) and band D (λ ≈ 310–315 nm) to an O → MoV charge transfer transition (CTT). The emission features of the Eu(PMo11)2 and Tb(PMo11)2 samples reveal broad emission in the UV/vis spectral region resulting from d–d transition transitions. The Eu(PMo11)2 also display the typical Eu3+ 5D0 → 7F0–4 transitions, when excited through the O → EuIII and O → MoVI CCTs.

Cobalt aluminate nanoparticles supported on MIL-101 structure: catalytic performance investigation

The first catalytic active composites based on CoAl2O4 nanoparticles with different size (5.5 and 2.5 nm) were successfully prepared using a simple methodology of incorporation into MIL-101(Cr) framework, CoAl-x@MIL(Cr). Characterization of CoAl-x@MIL(Cr) composites by elemental analysis, vibrational spectroscopy (FT-IR and FT-Raman), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) confirmed the successful preparation and stability of the support structure after nanoparticle immobilization. A remarkable catalytic performance was found for thioanisole oxidation under sustainable conditions (95% of conversion after 30 min of reaction) and the catalytic application of the most active composite was extended to styrene oxidation. Higher catalytic performance was achieved for the composite prepared with larger CoAl2O4 nanoparticles. The recyclability and the stability of composites after catalytic use were investigated. For the CoAl-x@MIL(Cr) catalytic systems, the loading parameter instead of the nanoparticle size seemed to have a pronounced influence in the heterogeneous catalytic performance. The confinement effect promoted by MIL-101(Cr) cavities associated to the higher number of catalytic active centers (CoAl2O4) is clearly more important than the size of the catalytic nanoparticles used.