Active <i>N</i> ‐Hydroxyphthalimide Species Confined in Silica Mesopores With Tunable Size and Architecture for Selective Aerobic Oxidation of Methyl‐Substituted Aromatics

ABSTRACT This study focused on the development of immobilized N ‐hydroxyphthalimide (NHPI) species anchored to mesoporous silicas as catalysts for the aerobic oxidation of hydrocarbons and their derivatives. Four types of SiO 2 with distinct textural properties were employed in the grafting process, that is, commercial silica gel, as well as MCM‐41, MCM‐48, and SBA‐15 synthesized by templating methods. 4‐(Chlorodimethylsilyl)phthalic anhydride was first anchored to their surfaces and subsequently converted into the catalytically active NHPI form. The formation of active centers was confirmed by scanning electron microscope‐energy dispersive x‐ray spectroscopy (SEM‐EDS), Fourier transform infrared (FT‐IR), NMR, and x‐ray photoelectron spectroscopy (XPS) measurements. Despite the highest concentration of surface silanol groups, silica gel yielded the lowest content of active sites compared to the other supports, which was attributed to differences in porosity that consequently determined the catalytic performance of the investigated materials. The catalysts showed high activity and selectivity in the aerobic conversion of various organic substrates, particularly aromatic hydrocarbons bearing two or three methyl substituents on the ring, which were predominantly transformed into the corresponding monocarboxylic derivatives. In the presence of the most active catalyst with NHPI species anchored on the MCM‐48 surface and Co 2+ acting as an efficient cocatalyst, a 59.4% conversion of p ‐xylene was achieved after 24 h at 80°C, with a selectivity toward 4‐toluic acid of 95.9%. The role of the cocatalyst was explained as enhancing NHPI performance by facilitating molecular oxygen activation and the generation of phthalimide N ‐oxyl (PINO) radicals. Combined experimental and DFT studies indicated that the initial hydrogen abstraction from the aromatic substrate is most likely the rate‐limiting step of the reaction.