TY - JOUR

T1 - Toward understanding the unusual reactivity of mesoporous niobium silicates in epoxidation of C = C bonds with hydrogen peroxide

AU - Ivanchikova, Irina D.

AU - Skobelev, Igor Y.

AU - Maksimchuk, Nataliya V.

AU - Paukshtis, Eugenii A.

AU - Shashkov, Mikhail V.

AU - Kholdeeva, Oxana A.

N1 - Publisher Copyright: © 2017 Elsevier Inc.

PY - 2017/12/1

Y1 - 2017/12/1

N2 - Niobium-containing mesoporous silicates reveal superior activity and selectivity in epoxidation of alkenes using hydrogen peroxide as a green oxidant and, in contrast to mesoporous titanium silicates, catalyze epoxidation of both electron-rich and electron-deficient C=C bonds. This report describes a kinetic and mechanistic investigation of epoxidation of two representative substrates, cyclooctene (Cy0) and 2-methyl-1,4-naphthoquinone (MNQ), over two mesoporous niobium silicates with predominantly di(oligo)meric or isolated Nb(V) sites. The observed kinetic regularities did not depend on the state of Nb but were strictly determined by the nature of the organic substrate. The rate law established for Cy0 is consistent with a mechanism that involves interaction of H2O2 with Nb(V) sites to give a hydroperoxo species NbOOH and water, followed by oxygen transfer to a nucleophilic C=C bond, producing an epoxide and regenerating the initial state of the catalyst. This mechanism is strongly supported by stereospecificity in epoxidation of cis-alkenes and high heterolytic pathway selectivity in the oxidation of cyclohexene. The NbOOH species is manifested by an absorption feature at 307 nm in diffuse reflectance UV-vis spectra. The addition of a base (NaOAc) causes a shift of the absorption band to 293 nm and produces a rate-retarding effect on the epoxidation reaction. Several lines of evidence, including zero reaction order in MNQ rate-accelerating effect of base, detection of acetamide in the reaction mixture, negligible reaction rate in ethyl acetate, and recognition of weak basic sites in the niobium silicates using infrared spectroscopy of adsorbed CDCl3, all indicate that MNQ epoxidation proceeds by another mechanism that involves rate-limiting oxidation of the solvent molecule (MeCN) to generate peroxycarboximidic acid, which reacts with electron-deficient C=C bonds, producing an epoxy derivative and acetamide. (C) 2017 Elsevier Inc. All rights reserved.

AB - Niobium-containing mesoporous silicates reveal superior activity and selectivity in epoxidation of alkenes using hydrogen peroxide as a green oxidant and, in contrast to mesoporous titanium silicates, catalyze epoxidation of both electron-rich and electron-deficient C=C bonds. This report describes a kinetic and mechanistic investigation of epoxidation of two representative substrates, cyclooctene (Cy0) and 2-methyl-1,4-naphthoquinone (MNQ), over two mesoporous niobium silicates with predominantly di(oligo)meric or isolated Nb(V) sites. The observed kinetic regularities did not depend on the state of Nb but were strictly determined by the nature of the organic substrate. The rate law established for Cy0 is consistent with a mechanism that involves interaction of H2O2 with Nb(V) sites to give a hydroperoxo species NbOOH and water, followed by oxygen transfer to a nucleophilic C=C bond, producing an epoxide and regenerating the initial state of the catalyst. This mechanism is strongly supported by stereospecificity in epoxidation of cis-alkenes and high heterolytic pathway selectivity in the oxidation of cyclohexene. The NbOOH species is manifested by an absorption feature at 307 nm in diffuse reflectance UV-vis spectra. The addition of a base (NaOAc) causes a shift of the absorption band to 293 nm and produces a rate-retarding effect on the epoxidation reaction. Several lines of evidence, including zero reaction order in MNQ rate-accelerating effect of base, detection of acetamide in the reaction mixture, negligible reaction rate in ethyl acetate, and recognition of weak basic sites in the niobium silicates using infrared spectroscopy of adsorbed CDCl3, all indicate that MNQ epoxidation proceeds by another mechanism that involves rate-limiting oxidation of the solvent molecule (MeCN) to generate peroxycarboximidic acid, which reacts with electron-deficient C=C bonds, producing an epoxy derivative and acetamide. (C) 2017 Elsevier Inc. All rights reserved.

KW - Epoxidation

KW - Heterogeneous catalysis

KW - Hydrogen peroxide

KW - Kinetics

KW - Mechanism

KW - Mesoporous niobium silicate

KW - MIXED OXIDES

KW - SELECTIVE OXIDATION

KW - OLEFIN EPOXIDATION

KW - MECHANISM

KW - ACTIVE-SITE

KW - MOLECULAR-SIEVES

KW - OXIDE CATALYSTS

KW - TI-SILICALITE

KW - CATALYTIC-OXIDATION

KW - ALKENE EPOXIDATION

UR - http://www.scopus.com/inward/record.url?scp=85032336253&partnerID=8YFLogxK

U2 - 10.1016/j.jcat.2017.09.011

DO - 10.1016/j.jcat.2017.09.011

M3 - Article

AN - SCOPUS:85032336253

VL - 356

SP - 85

EP - 99

JO - Journal of Catalysis

JF - Journal of Catalysis

SN - 0021-9517

ER -