Molybdate- and tungstate-exchanged layered double hydroxides as catalysts for O-1(2) formation: Characterization of reactive oxygen species and a critical evaluation of O-1(2) detection methods

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 heter ogeneous catalysts for the decomposition of H2O2 into singlet molecular oxy gen (O-1(2)). The oxometalate anions are present on the LDH as monomeric, t etrahedral MO42- anions. With LDH-MoO42- and H2O2, O-1(2) is by far the pre vailing reactive oxygen species. EPR trapping detected a minor amount of fr ee OH. radicals, but these do not appreciably contribute to product formati on in olefin oxygenation. LDH-WO42- transforms H2O2 into O-1(2) about 4 tim es slower than the Mo catalyst, and additionally effects mono-oxygen transf er to electron-rich substrates such as amines and olefins. Only for alkenes with very low beta values, the Schenck hydroperoxidation dominates over th e epoxidation in the W-catalyzed reactions. While most tests for O-1(2) det ection were designed for photosensitized reactions, the applicability of th ese methods to dark catalytic reactions is not always clear. Therefore, a s eries of common O-1(2) detection methods was critically evaluated in the LD H-WO42- or LDH-MoO42- + H2O2 reactions. Isomer distributions in the olefin hydroperoxidation and detection of NIR luminescence are the most reliable a nd sensitive methods. Quenching or trapping methods that involve amines sho uld not be used when the catalyst also effects mono-oxygenation.