Synthesis, Characterization, and Application of Mesoporous Silica Functionalized with Alkyl-Hydroperoxides



A variety of alkyl hydroperoxides such as tert-butyl-, tert-octyl-, 1-cyclopentyl-, 1-cyclohexyl-, 3,4-disubstituted-1-cyclohexyl-, n-propyl, and n-undecyl-hydroperoxides have been functionalized onto ordered mesoporous silica, SBA-15, from the corresponding covalently anchored synthons. All the tert-hydroperoxides are prepared by autoxidation using molecular O2 and an initiator, whereas other hydroperoxides are obtained by reaction with H2O2. For autoxidation, the use of a combination of an azoinitiator (AIBN) and N-hydroxyphthalimide increased the hydroperoxide yield compared with using the azoinitiator alone. Synthons containing two or more tert- and sec-hydrogens lead to higher peroxide yield compared to synthons with a single reactive site. Oxidation of Si–OH (silanol groups) with acidic H2O2 at low temperature produces Si–OOH. Reusability of these alkyl hydroperoxides is carried out by oxidation of alcohols obtained from the corresponding alkyl hydroperoxides using H2O2. Both the covalently anchored synthons and the resulting hydroperoxides are thoroughly characterized by powder X-ray diffraction, 13C cross-polarized magic angle spinning NMR, TG/DTA, Fourier transform IR spectroscopy, sorption, and surface area measurements. The quantification of the amount of alkyl hydroperoxide was carried out by iodometric titration using a thio solution. The hydroperoxides exhibit high activity for the epoxidation of styrene to styrene oxide and exhibit reasonably high efficiency for oxygen transfer.