Abstract: Nb-containing catalysts have attracted growing attention of the selective oxidation community. Mesoporous niobium-silicates proved to be efficient catalysts for epoxidation of alkenes with the green oxidant – hydrogen peroxide [1-3]. In contrast to mesoporous titanium-silicates, they demonstrate high heterolytic pathway selectivity and unusual regioselectivity and catalyze epoxidation of both electron-rich and electron-deficient C=C bonds [2,3]. So far, few attempts have been made to rationalize the catalytic performance of Nb, and the conclusions were contradictory. Here we compare, in terms of activity, selectivity and stability, Nb and Ti catalysts prepared by the same methodology and present a kinetic and mechanistic investigation of H2O2-based epoxidation of two representative substrates, cyclooctene (CyO) and 2-methyl-1,4-naphthoquinone (MNQ). Niobium-silicates with different state of active centers (mostly oligomeric or site-isolated) were employed. Using kinetic, product and spectroscopic tools, we have inferred two different mechanisms for epoxidation of electron-rich and electron-deficient C=C bonds over Nb catalysts. The first mechanism involves adsorption of Н2О2 on coordinatively unsaturated Nb(V) sites, interaction with Н2О2 to give a hydroperoxo species NbOOH and water, followed by a concerted oxygen transfer from NbOOH to nucleophilic С=С bond, producing epoxide and regenerating the initial state of the catalyst. This mechanism is compatible with the previous investigations reported in the literature and all the data gained in our study, including high heterolytic pathway selectivity for oxidation of alkenes with highly reactive allylic hydrogen atoms (cyclohexene, limonene), stereospecificity of epoxidation of cis-alkenes, the rate law established for CyO epoxidation, and rate-retarding effect of base. The ‘NbOOH’ species is manifested by a DRS UV-vis absorption feature at 307 nm, which reveals a blue shift upon treatment with aqueous NaOAc. Product, spectroscopic, computational, and kinetic studies using tractable molecular models, Nb-containing polyoxometalates, allowed us to get further insights into the structure and reactivity of active Nb species. Several lines of evidence, including zero reaction order in substrate, rate-accelerating effect of base, formation of acetamide, lack of reaction in ethylacetate, and detection of weak basic sites in niobium-silicates, all indicate that epoxidation of α,β-unsaturated carbonyl compounds proceeds through another mechanism (Payne type oxidation) that involves rate-limiting oxidation of the solvent molecule (MeCN), most likely, with HOO– generated from H2O2 on the basic sites, formation of peroxycarboximidic acid Н3СC(=NH)OOH and its interaction with electron-deficient C=C bond, producing epoxy derivative and acetamide. This mechanism does not operate with Ti catalysts, which do not possess basic sites, as confirmed by IR spectroscopy of probe molecules.

Cite: Kholdeeva O.A.
H2O2-Based Selective Oxidations: Nb(V) versus Ti(IV)
8th World Congress on Oxidation Catalysis 03-08 Sep 2017