Various strategies were used to graft N-hydroxyphthalimide groups onto the surface of SBA-15 mesoporous silica by the formation of O−Si-C bonds in order to activate them in the aerobic oxidation of p-xylene.

Abstract

The surface of SBA-15 mesoporous silica was modified by N-hydroxyphthalimide (NHPI) moieties acting as immobilized active species for aerobic oxidation of alkylaromatic hydrocarbons. The incorporation was carried out by four original approaches: the grafting-from and grafting-onto techniques, using the presence of surface silanols enabling the formation of particularly stable O−Si−C bonds between the silica support and the organic modifier. The strategies involving the Heck coupling led to the formation of NHPI groups separated from the SiO₂ surface by a vinyl linker, while one of the developed modification paths based on the grafting of an appropriate organosilane coupling agent resulted in the active phase devoid of this structural element. The successful course of the synthesis was verified by FTIR and ¹H NMR measurements. Furthermore, the formed materials were examined in terms of their chemical composition (elemental analysis, thermal analysis), structure of surface groups (¹³C NMR, XPS), porosity (low-temperature N₂ adsorption), and tested as catalysts in the aerobic oxidation of p-xylene at atmospheric pressure. The highest conversion and selectivity to p-toluic acid were achieved using the catalyst with enhanced availability of non-hydrolyzed NHPI groups in the pore system. The catalytic stability of the material was additionally confirmed in several subsequent reaction cycles.