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Molybdate and tungstate exchanged layered double hydroxides as catalysts for 1O2 formation: characterization of reactive oxygen species, and a critical evaluation of 1O2 detection methods

par Sels, B.;De Vos, Dirk E.;Grobet, Piet J.;Pierard, Frédéric ;Kirsch-De Mesmaeker, Andrée ;Jacobs, P. A.
Référence The Journal of Physical Chemistry. B, 103, 50, page (11114-11123)
Publication Publié, 1999

Article révisé par les pairs

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Résumé :

Layered double hydroxides (LDHs) with the formula [Mg0.7Al0.3(OH)2](NO3)0.3 were partially exchanged with MoO42- and WO42- and were used as heterogeneous catalysts for the decomposition of H2O2 into singlet molecular oxygen (1O2). The oxometalate anions are present on the LDH as monomeric, tetrahedral MO42- anions. With LDH−MoO42- and H2O2, 1O2 is by far the prevailing reactive oxygen species. EPR trapping detected a minor amount of free OH• radicals, but these do not appreciably contribute to product formation in olefin oxygenation. LDH−WO42- transforms H2O2 into 1O2 about 4 times slower than the Mo catalyst, and additionally effects mono-oxygen transfer to electron-rich substrates such as amines and olefins. Only for alkenes with very low β values, the Schenck hydroperoxidation dominates over the epoxidation in the W-catalyzed reactions. While most tests for 1O2 detection were designed for photosensitized reactions, the applicability of these methods to dark catalytic reactions is not always clear. Therefore, a series of common 1O2 detection methods was critically evaluated in the LDH−WO42- or LDH−MoO42- + H2O2 reactions. Isomer distributions in the olefin hydroperoxidation and detection of NIR luminescence are the most reliable and sensitive methods. Quenching or trapping methods that involve amines should not be used when the catalyst also effects mono-oxygenation.