Chemoselective alkane oxidation by superoxo-vanadium(V) in vanadosilicate molecular sieves

Electron paramagnetic resonance (EPR) spectroscopy of reactive superoxo-vanadium(V) species in vanadosilicate molecular sieves (microporous VS-1 and mesoporous V-MCM-41) generated on contact with H2O2, tert-butyl hydroperoxide (TBHP), or (H2 + O2) is reported for the first time. By suitable choice of the silicate structure, solvent, and oxidant, we could control the vanadium-(O2−•) bond (i.e., the V-O bond) covalency, the mode of O-O cleavage (in the superoxo species), and, therefore, chemoselectivity in the oxidation of n-hexane: Oxidation by TBHP over V-MCM-41, for example, yielded 27.2% of (n-hexanol + n-hexanal + n-hexanoic acid), among the highest chemoselectivities for oxidation of the terminal -CH3 in a linear paraffin reported to date. Over these vanadosilicates, oxidation of the primary C-H bond occurs only via a homolytic O-O bond cleavage; the secondary C-H bond oxidations may proceed via both the homo- and heterolytic O-O cleavage mechanisms

Rationalising the role of solid-acid sites in the design of versatile single-site heterogeneous catalysts for targeted acid-catalysed transformations

A versatile design strategy for rationalising the role of well-defined and isolated multifunctional solid-acid active centres, employing Mg(II)Si(IV)AlPO-5 nanoporous architectures has been demonstrated, with a view to affording structure–property correlations compared to its corresponding mono-substituted analogues (Mg(II)AlPO-5 and Si(IV)AlPO-5). The simultaneous incorporation of Mg(II) and Si(IV) ions, as isomorphous replacements for Al(III) and P(V) ions in the microporous architecture, plays an important role in modulating the nature and strength of the solid-acid active sites in the industrially-important, vapour-phase Beckmann rearrangement of cyclohexanone oxime to produce ?-caprolactam (the precursor for renewable nylon-6) and in the isopropylation of benzene to cumene. The structural integrity, coordination geometry and local environment of the active (Brønsted-acid) sites could be rationalised at the molecular level, using in situ spectroscopic techniques, for tailoring the catalytic synergy by adroit design of the framework architecture

Synergistic behavior of bimetallic Rhenium cluster catalysts: Spectroscopic investigation into the nature of the active site

Single-site Re nanoparticles were produced by anchoring dirhenium organometallic clusters on to the inner walls of mesoporous silica. The presence of oxophilic atoms (Sb or Bi) is essential to obtain well dispersed Re0 centers. The interaction between the organometallic cluster and the silica support is critical for the generation of well-defined and isolated Re0 single sites. FTIR spectroscopy was used to track the decomposition of the organometallic precursors and the adsorption of probe molecules such as CO on the metal sites sheds valuable information on the catalytic potential of this new class of bimetallic nanocatalysts.<br/

Toward Understanding the Catalytic Synergy in the Design of Bimetallic Molecular Sieves for Selective Aerobic Oxidations

Structure–property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and <i>in situ</i> FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO²⁺ ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems

Toward understanding the catalytic synergy in the design of bimetallic molecular sieves for selective aerobic oxidations

Structure–property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and in situ FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO2+ ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems