264-269Conversions
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 Whsv, 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