André D. S. Barbosa

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.

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.

Metal-Organic Framework Materials MIL‑101(Cr) ([Cr₃X(H₂O)₂O(bdc)₃]∙n(H₂O), where X⁻ = F⁻ or OH⁻, n ≈ 25 and H₂bdc stands for 1,4-benzene-dicarboxylic acid] and MOF‑5(Zn) [Zn₄O(bdc)₃] were prepared by hydrothermal or solvothermal methods as well as Microwave‑Assisted Synthesis (MWAS), for which the detailed synthetic parameters were optimized. The crystal structures were confirmed by powder X-ray diffraction and the materials were further characterized by FT‑IR absorption spectroscopy. MIL‑101(Cr) and MOF‑5(Zn) showed weak catalytic activity in the oxidation of terpene, thiophene and cis-cyclooctene. Reasonable catalytic activity was observed for MOF-5(Zn) in the epoxidation of cis-cyclooctene and a 100 % of selectivity was observed for the epoxide. The structural stability of the materials was tested under the employed catalytic medium for oxidation reactions. MOF-5(Zn) revealed a remarkable structural stability at high temperature and also in the presence of high oxidant amounts.

The Keggin-type TBA₄H₂[BW₁₁Mn(H₂O)O₃₉].H₂O (BW₁₁Mn) compound was isolated and characterized. The first sphere of coordination for the manganese atom was determined by EXAFS analysis. Six oxygen atoms were found in the range of 1.80-2.64 Å from the manganese. BW₁₁Mn showed to be an active, efficient and selective catalyst for cis-cyclooctene (1a) epoxidation in the presence of stoichiometric quantities of H₂O₂. After 5 h of reaction almost complete conversion was achieved (86% of conversion). Selectivity was always 100 % for the epoxide.