Acidity and cracking activity changes during coke deactivation of ultrastable Y zeolite

Changes in acidic ultrastable Y zeolite (H-USY) due to coke deactivation during hexane cracking were determined by ammonia TPD, microcalorimetry, IR spectroscopy of adsorbed pyridine, C-13 NMR, powder XRD, TGA of hexane adsorption, and nitrogen and argon adsorption. Small amounts of coke produced a large decrease in activity; however, few acid sites were poisoned. Compared to a fresh USY zeolite, coke deactivated H-USY did not show significant changes in the acid strength distribution, number of acid sites, fraction of Bronsted and Lewis acid sites, pore size distribution, or hexane diffusivity that could account for the large decrease in the cracking activity. It is proposed that the rapid loss in activity resulting from a small decrease in the number of acid sites is consistent with a site poisoning deactivation model for a diffusion-limited reaction, i.e., the rate of hexane reaction is greater than the rate of diffusion. For a diffusion limited reaction, the initial hexane cracking and coke deposition occurs primarily at active sites near the external surface of the crystal with little reaction at the particle interior. As deactivation proceeds, the thickness of coked region grows and hexane diffuses further into the zeolite particle before reaction.