FTIR and EPR characterisation of copper-exchanged mordenites and beta zeolites

Abstract

Sodium mordenites and beta zeolites, partially exchanged with Cu²⁺, were characterised by means of Fourier transform infrared (FTIR) and both continuous-wave and pulse electron paramagnetic resonance (EPR) spectroscopies, in order to shed light on the catalytic role of copper ions in the ammoxidation of 1-methylnaphthalene. FTIR analysis showed that ammonia interacts with both Brønsted and Lewis acid sites, while 1-methylnaphthalene is mainly coordinated through π-complexes. The acidity of the catalysts was also determined through FTIR analysis of the adsorbed pyridine. The main changes in the coordination sphere of Cu²⁺ ions were observed by EPR analysis after ammonia adsorption and after catalyst use, while adsorption of 1-methylnaphthalene had a very minor effect. Primary electron spin echo envelope modulation spectra after catalytic use showed the presence of organic radicals, having a relaxation time of longer than the Cu²⁺ ions. The latter are confirmed as the active reaction centres and should be located in the side pockets of the mordenites, and be unaccessible to the aromatic system of 1-methylnaphthalene. This explains the higher selectivity observed with mordenites. In the more acidic beta zeolites the presence of copper ions on the walls of large channels initially favours side reactions, but also makes adsorbed ammonia more available for ammoxidation.

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