In situ IR and catalytic studies of the effect of coke on acid properties of steamed zeolite Y

The effect of coke deposits in steamed zeolite Y on acid properties of zeolite was followed by IR spectroscopy with the use of ammonia. The concentration of protonic sites was determined in quantitative IR studies of ammonia sorption. We followed also the changes caused by coke in the spectra of OH groups by recording the spectra at low temperature (ca. 170 K). The entire treatment: zeolite activation at vacuum, m-xylene reaction in the flow conditions, sorption and desorption of ammonia were done “in situ” inside IR cell without exposing the sample to the air. IR studies of fresh zeolite evidenced that the acid strength of hydroxyl groups increased in the order: OH (3675 cm−1) < Si–OH–Al (3630 and 3525 cm−1) < Si–OH–Al…Al (3610 and 3525 cm−1). The deposition of coke eliminated some acidic hydroxyls, the most acidic ones were eliminated as the first and the less acidic were affected in the last order. Coke affected also hydroxyls inside hexagonal prisms which we relate to the formation of many structural defects during the dealumination by steaming. We performed also catalytic studies in which the zeolite was coked in situ in the IR cell (the acidity of such zeolite was controlled by IR spectroscopy) and catalytic test with such coked zeolite was realized in a microreactor. Catalytic test evidenced the decrease of activity by coke deposition.