Oxidation sites in dealuminated mordenite. A combined EPR, NMR and product distribution study

Abstract

Dealuminated mordenite zeolites have been prepared by different modification procedures and characterised by a variety of techniques such as ¹H, ²⁷Al and ²⁹Si magic-angle spinning (MAS) NMR, nitrogen adsorption and elemental analysis with special emphasis on detecting irreversible changes of the framework and the presence of non-framework species. Radical cations were generated on the dealuminated H-mordenite zeolites following adsorption of alkenes, and were detected by EPR spectroscopy. The organic species present after 30 min reaction on the zeolites were distilled off and analysed using gas chromatography (GC). The generation of radical cations was found to be dependent on the pretreatment of the zeolite. The intensity of the EPR signal, and the reaction products could be correlated with the presence of extra-framework alumina in the zeolite pores. It is postulated that the formation of radical cations occurs on Lewis acid sites associated with the extra-framework alumina formed in the presence of oxygen during the activation process.