A comparative study between Keggin-type tungstophosphates and tungstosilicates in the oxidation of cyclooctane with hydrogen peroxide

The tetrabutyl ammonium (TBA) salts of the Keggin-type polyoxotungstates [XW12O40](n-), [XW11O39]((n+4)-), [XW11VO40](m-) and [XW11MIII (H2O)O-39](p-), X = P or Si and M = Fe or Mn, proved to be effective catalysts for the oxidation of cyclooctane with hydrogen peroxide, in acetonitrile, to the corresponding alkyl hydroperoxide, ketone, and alcohol. High turnover numbers and selectivity for cyclooctyl hydroperoxide were obtained, with values of cyclooctane conversion, after 9 h, between 13 and 96%, depending on the catalyst and reaction conditions. In general tungstosilicates were less active than tungstophosphates but presented higher selectivity for cyclooctyl hydroperoxide. Results obtained with H2O, near the stoichiometric ratio and in excess are compared. Excess of hydrogen peroxide afforded higher selectivity for cyclooctyl hydroperoxide. In the presence of (TBA)(4) [PW11Fe(H2O)O-39], 74% of cyclooctane conversion and 80% of selectivity for cyclooctyl hydroperoxide were obtained after 2 h of reaction, using an excess of H2O2, In the same conditions, 100% of cyclooctyl hydroperoxide was obtained with (TBA)(4)H4SiW11O39, after 9 h (55% conversion). These results indicate that these are very promising systems for the oxidation of alkanes. (C) 2004 Elsevier B.V. All rights reserved.