Structural Lewis sites in zeolite beta-role on coking of the catalyst

Abstract

A new infrared band at 880-885 cm−1 was observed on spectra of some dealuminated beta zeolites after activation. Pyridine adsorption and deuterium exchange allowed us to define it as an infrared fingerprint of specific Lewis acid sites due to structural defects. Catalyst deactivation in working conditions (o-xylene conversion) or in static conditions (cyclohexene conversion) was monitored by FTIR spectroscopy. The decrease in catalytic activity in o-xylene conversion is correlated with the decrease of the band intensity of the acidic hydroxy groups at 3600 cm−1 due to their poisoning by coke deposition. For these two reactions, infrared spectra of deactivated catalysts show that the Lewis acid sites associated with the band at 880 cm−1 are unaffected by coke. Pyridine adsorption on deactivated catalyst confirms that (i) Bronsted acidity has strongly diminished, in agreement with the disappearing of acidic hydroxy group (ii) global Lewis acidity of catalyst is unaffected by coke deposition.