Irina D. Ivanchikova

Magnetically separable titanium-silicate mesoporous materials with core–shell morphology: synthesis, characterization and catalytic properties

The preparation and characterization (XRD, N2 adsorption, TEM, EDX, DRS-UV) of a novel catalytic material Ti-SMCMS (solid magnetic core–mesoporous shell) is reported. The material has quasi spherical particles with an ordered mesoporous silicate shell containing isolated Ti atoms and a silica core which, in turn, comprises superparamagnetic iron oxide nanoparticles. The magnetic kernels are embedded in the nonporous silica core and, thus, are completely protected from ambient medium. The material was found to combine the advantages of high activity and selectivity in H2O2-based selective oxidations with the merits of an easy magnetic separation from the reaction mixture.

Kinetics and mechanism of the oxidation of alkyl substituted phenols and naphthols with tBuOOH in the presence of supported iron phthalocyanine

2,3,5-Trimethylbenzoquinone (precursor of vitamin E) and 2-methylnaphthoquinone (vitamin K3) were obtained in good yields by oxidation of 2,3,6-trimethylphenol and 2-methyl-1-naphthol, respectively, with tBuOOH catalyzed by supported iron tetrasulfophthalocyanine. The mechanism of this heterogeneous oxidation was studied using 18O2labeling experiments, EPR spectroscopy with spin traps, kinetic studies, and complete analysis of reaction products including minor ones. 18O2labeling experiments did not indicate the involvement of O2 in the oxidative process. EPR study of reaction mixtures of 2,3,6-trimethylphenol and 2-methyl-1-naphthol oxidations in the presence of 3,5-dibromo-4-nitrosobenzenesulfonic acid spin trap showed no formation of any radical intermediates. Besides the target quinones, epoxyquinones and formyldimethyl-1,4-benzoquinones, as over-oxidation minor products have been found. C–C and C–O coupling products relevant to one-electron oxidation pathways were detected in trace amounts. Based on the experimental results, a mechanism of oxidation of alkyl-substituted phenols and naphthols mediated by the supported iron phthalocyanine catalyst has been proposed which involves two successive electron transfers without escape of radical species in solution.

Highly efficient production of 2,3,5-trimethyl-1,4-benzoquinone using aqueous H2O2 and grafted Ti(iv)/SiO2 catalyst

The oxidation of 2,3,6-trimethylphenol with aqueous H2O2 over titanium(IV) grafted on commercial mesoporous silica produces 2,3,5-trimethyl-1,4-benzoquinone, with nearly quantitative yield.

Alkene oxidation by Ti-containing polyoxometalates. Unambiguous characterization of the role of the protonation state

Kinetic and DFT studies revealed that protonation of Ti-containing polyoxometalates (Ti-POM) lowers significantly the energy barrier for the heterolytic oxygen transfer from the Ti hydroperoxo intermediate to the alkene, increasing the activity and selectivity of alkene oxidation.

User-friendly synthesis of highly selective and recyclable mesoporous titanium-silicate catalysts for the clean production of substituted p-benzoquinones

Mesoporous titanium-silicates have been prepared following the evaporation-induced self-assembly (EISA) methodology and characterized by elemental analysis, XRD, N2 adsorption, SEM, DRS UV–Vis and Raman techniques. The use of acetylacetone during synthesis allowed the formation of highly dispersed dimeric and/or small oligomeric Ti species, within the mesostructured silica network, to be realized. The materials catalyse oxidation of alkylsubstituted phenols to corresponding p-benzoquinones with 100% selectivity using the green oxidant – 30% aqueous hydrogen peroxide. The titanium-silicates prepared by the convenient and versatile EISA-based procedure reveal the true heterogeneous nature of the catalysis and do not suffer from titanium leaching. They show advantages over other types of mesoporous Ti,Si-catalysts, such as TiO2–SiO2 mixed oxides and grafted Ti/SiO2, in terms of the catalyst stability and reusability.

Mechanistic Insights into Oxidation of 2-Methyl-1-naphthol with Dioxygen: Autoxidation or a Spin-Forbidden Reaction?

Oxidation of 2-methyl-1-naphthol (MNL) with molecular oxygen proceeds efficiently under mild reaction conditions (3 atm O(2), 60-80 °C) in the absence of any catalyst or sensitizer and produces 2-methyl-1,4-naphthoquinone (MNQ, menadione, or vitamin K(3)) with selectivity up to 80% in nonpolar solvents. (1)H NMR and (1)H,(1)H-COSY studies revealed the formation of 2-methyl-4-hydroperoxynaphthalene-1(4H)-one (HP) during the reaction course. Several mechanistic hypotheses, including conventional radical autoxidation, electron transfer mechanisms, photooxygenation, and thermal intersystem crossing (ISC), have been evaluated using spectroscopic, mass-spectrometric, spin-trapping, (18)O(2) labeling, kinetic, and computational techniques. Several facts collectively implicate that ISC contributes significantly into MNL oxidation with O(2) at elevated pressure: (i) the reaction rate is unaffected by light; (ii) C-C-coupling dimers are practically absent; (iii) the reaction is first order in both MNL and O(2); (iv) the observed activation parameters (ΔH(‡) = 8.1 kcal mol(-1) and ΔS(‡) = -50 eu) are similar to those found for the spin-forbidden oxidation of helianthrene with (3)O(2) (Seip, M.; Brauer, H.-D. J. Am. Chem. Soc.1992, 114, 4486); and (v) the external heavy atom effect (2-fold increase of the reaction rate in iodobenzene) points to spin inversion in the rate-limiting step.