Influence of coke on the acid properties of a USHY zeolite

Abstract

The composition of coke and its effect on the acid sites were determined with samples of a USHY zeolite (with total and framework Si/Al ratios of 3.1 and 5.4, respectively) used for m-xylene transformation at 520 and 720 K. At low coke contents, coke is mainly constituted by methyl substituted polyaromatic compounds with three (520 K) or four (720 K) aromatic rings trapped in the zeolite pores. Coke causes a decrease in the intensity of the IR hydroxyl band. The most acidic hydroxyls, i.e., the bridging hydroxyls in interaction with extra-framework aluminum species are most affected by coke, whereas no interaction is observed between the non-acidic hydroxyls and the coke molecules. Pyridine adsorption shows that, while the number of protonic sites able to retain pyridine adsorbed decreases by coking, this was not the case for the Lewis sites. Pyridine adsorption facilitates the desorption under vacuum of coke molecules from the coked zeolites, the effect being more significant for coking at 520 K. This indicates that the retention of coke molecules is not only due to their low volatility (at 520 K) or to their steric blockage (720 K), but also to their adsorption on the acid sites.