Influence of coke on the aromatization of 3-carene in the vapour phase over zeolites

Abstract

Conversions of 3-carene to cymenes over Na13X, NaY and NaZSM-5 are less than over NaH13X (29% proton exchange), NaHY (33% proton exchange) and NaHZSM-5 (35% proton exchange). In the presence of oxygen, at its different

partial pressures, the formation of cymenes increases significantly with increase in time on stream, indicating the formations of some species in the coke during the reaction which enhances the activity of the catalysts. To identify the species that cause enhancement of the activity, zeolite NaHY (catalyst B) has been doped with different amounts of NaOH and the reactions have been carried out over them at 350°C, at a contact time of 1.0 h (contact time is expressed as the reciprocal of W_hsv, the weight hourly space velocity representing the weight of 3-carene passed over 1 g of catalyst per hour). The coke deposited has been examined by IR, ESR and XRD. Formation of quinone type of carbonyl group, responsible for the enhanced activity of the zeolites is observed. The participation of the quinone group in the dehydrogenation process is confirmed

by extracting the coke after the reaction and the same reaction is carried out over the extracted coke both in the presence and in the absence of oxygen under the same experimental conditions. The quinone group is present in an atmosphere of oxygen but is absent if there is no oxygen.