Lucie S. Nogueira

A supramolecular cucurbit[6]uril/Mo ^VI hydrogen-bonded assembly displays dual-function (oxidation/acid) catalytic behavior for the one-pot conversion of olefins to alkoxy products.

MoO₃/2,2′-bipyridine hybrids have been prepared (hydro)thermally from [MoO₃(2,2′-bipy)] and compared as catalysts in styrene oxide ethanolysis and olefin epoxidation.

The synthesis of molybdenum(0) tricarbonyl and tetracarbonyl complexes of the form [Mo(CO)₃(ptapzpy)Br] (1) and cis-[Mo(CO)₄(ptapzpy)]Br (2) is reported, where ptapzpy = 2-(1-propyltrimethylammonium-3-pyrazolyl)pyridine. Preparation of these derivatives was accomplished either through thermal replacement of CO in Mo(CO)₆ (for 1) or substitution under milder conditions of piperidine ligands in the precursor cis-[Mo(CO)₄(pip)₂] (for 2). The crystal structures of the ligand [ptapzpy]Br and complexes 1 and 2 were determined. Thermal treatment of 2 at 125–150 °C leads to mono decarbonylation and formation of 1. On the other hand, oxidative decarbonylation of 1 and 2 by reaction with tert-butylhydroperoxide (TBHP, 10 equiv.) gives a molybdenum oxide hybrid material formulated as [Mo₃O₉([ptapzpy]Br)₂]·nH₂O (3), which was characterised by FT-IR and Raman spectroscopy, thermogravimetric analysis, and ¹³C{¹H} CP MAS NMR spectroscopy. Compounds 1–3 were effective (pre)catalysts for the epoxidation of cis-cyclooctene at 55 °C with aqueous H₂O₂ or TBHP (slightly better results were obtained with the former). The characterisation of the Mo-containing solids isolated after the catalytic reaction showed that poorly soluble β-octamolybdate salts, (L)_x[Mo₈O₂₆], were formed from 1–3 with TBHP and from 1 with H₂O₂, while soluble oxoperoxo species were formed from 3 with H₂O₂. These findings helped to explain the different catalytic performances obtained.

The chemistry and catalytic performance of the dichlorido complex [MoO₂Cl₂(pbim)] (1) [pbim = 2‐(2‐pyridyl)‐benzimidazole] in the epoxidation of olefins is reported. Complex 1 acts as a precatalyst and is more effective with tert‐butylhydroperoxide (TBHP) as the oxidant than with aq. hydrogen peroxide: the cis‐cyclooctene (Cy) reaction with TBHP gave 98 % epoxide yield at 70 °C/24 h. Catalyst characterization showed that 1 is transformed in situ to the oxidodiperoxido complex [MoO(O₂)₂(pbim)] (2), with H₂O₂ and a hybrid molybdenum(VI) oxide solid formulated as [MoO₃(pbim)] (3) with TBHP. The hybrid material 3 was prepared on a larger scale and explored for the epoxidation of the biorenewable olefins methyl oleate, methyl linoleate, and (R)‐(+)‐limonene. With TBHP as the oxidant, 3 acts as a source of soluble active species of the type 2. A practical method for recycling oxidodiperoxidomolybdenum(VI) catalysts for the Cy/TBHP reaction is demonstrated by using an ionic liquid as the solvent for the molecular catalyst 2.

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.

A novel method to prepare silica nano-sized particles incorporating polyoxometalates was developed leading to a new efficient heterogeneous oxidative catalyst. Zinc-substituted polyoxotungstate [PW₁₁Zn(H₂O)O₃₉]⁵⁻ (PW₁₁Zn) was encapsulated into silica nanoparticles using a cross-linked organic–inorganic core, performed through successive spontaneous reactions in water. The potassium salt of PW₁₁Zn and the composite formed, PW₁₁Zn-APTES@SiO₂, were characterized by a myriad of solid-state methods such as FT-IR, FT-Raman, ³¹P and ¹³C CP/MAS solid-state NMR, elemental analysis and SEM-EDS, confirming the integrity of the PW₁₁Zn structure immobilized in the silica nanoparticles. The new composite has shown to be a versatile catalyst for the oxidation of olefins and also to catalyze the desulfurization of a model oil using H₂O₂ as the oxidant and acetonitrile as the solvent. The novel composite material was capable of being recycled without significant loss of activity and maintaining its structural stability for consecutive desulfurization and olefin oxidative cycles.