Bryce A. Williams

Monomolecular cracking of n-hexane on Y, MOR, and ZSM-5 zeolites

The activity and activation energy for cracking of n-hexane were measured over three zeolites: HZSM-5, H-MOR, H-USY (ultrastable Y zeolite), and CDHY (AHFS dealuminated Y zeolite), under conditions where the product selectivities were nearly identical and there was no deactivation of the catalysts. The identical selectivities implied that the same reaction mechanism for cracking was occurring over all three catalysts. Within experimental error, the differences in apparent activation energies could be entirely attributed to differences in heats of n-hexane adsorption, such that the intrinsic activation energies were identical. These results suggest that the kinetics of cracking is insensitive to any possible differences in acid strength among these catalysts, and the large enhancement in activity at lower temperatures observed upon steam dealumination of Y zeolite is due to effects other than changes in acid strength.

The roles of acid strength and pore diffusion in the enhanced cracking activity of steamed Y zeolites

Steamed HY or ultrastable Y (H-USY) zeolites are active hydrocarbon cracking catalysts. The high activity of H-USY compared to HY zeolite has been previously explained by the generation of unusually strong and active Bronsted acid sites, or an increase in the number of accessible sites in a micropore diffusion-controlled reaction. However, neither model explains the accumulated literature observations. A model is proposed that incorporates a change in the predominant cracking reaction mechanism as a function of alkane conversion and the very different rates of these mechanisms. Additionally, an oligomeric cracking mechanism is introduced to explicitly account for coking and deactivation of the catalyst. The model is capable of accounting for most literature results. It concludes that the large enhancement in cracking activity by steaming is due to a proportionally smaller increase in external surface area of the zeolite crystals and possibly a small increase in the specific initiation activity of each site. These small changes lead to a much larger overall effect because of the sensitive dependence of oligomeric cracking and, to a lesser extent, bimolecular cracking on the alkene partial pressure.

Towards understanding the enhanced cracking activity of steamed Y zeolites

Abstract Ultrastable Y (H-USY) zeolite, prepared by steam treatment of Y zeolite, is a very active hydrocarbon cracking catalyst. However, the extent of enhancement in activity compared to a non-steamed sample depends on the reaction condition. A model that has been proposed to explain this behavior is summarized. The model incorporates the three different mechanisms for hydrocarbon cracking, and the dependence of their rates on the partial pressures of reactants and products and temperature. Depending on the reaction condition, such as hydrocarbon pressure, temperature, and conversion, the predominant cracking reaction mechanism may differ. The change in the predominant mechanism may also be a result of the proportionally small increase in external surface area caused by the steaming-induced structural destruction of the zeolite particles. However, these relatively small changes can lead to a much larger overall effect on the cracking rate because of the sensitive dependence of oligomeric cracking, and to a lesser extent, bimolecular cracking on the alkene partial pressure.

Enhanced hydrocarbon cracking activity of Y zeolites

It has been known for a long time that faujasite zeolites that have been steam‐dealuminated to form USY zeolite are much more active than the unsteamed parent material for hydrocarbon cracking. A popular model to explain this phenomenon is the generation of active sites of unusually high activity. In this paper, a brief review of recent advances towards understanding this phenomenon is presented. Evidence suggests that steaming does not generate sites of new chemical or catalytic properties for cracking. An alternate model is summarized that incorporates a change in the dominant cracking reaction mechanism as a function of conversion, and raises the possibility that the rates of bimolecular and oligomeric cracking are influenced by pore diffusion.

Surface equilibration in adsorption microcalorimetry of bases on H-USY

Abstract It is commonly assumed that adsorbed basic probe molecules freely equilibrate with surface-acid sites in microcalorimetric experiments to determine the strength and distribution of the acid sites. The validity of this assumption was tested by comparing the differential heat of adsorption on H-USY as a function of surface coverage of CD 3 NH 2 , NH 3 and CD 3 CN, which have widely different proton affinities, and by monitoring with FTIR the distribution of the adsorbed molecules between Bronsted- and Lewis-acid sites during adsorption and desorption. The results showed that full equilibration was achieved with the weakest base, CD 3 CN, but not with the stronger bases, CD 3 NH 2 or NH 3 .

Evidence for different reaction mechanisms as the cause of the enhanced hydrocarbon cracking activity of steamed Y zeolites

Experimental evidence is presented to support two key aspects of a conceptual model for explaining enhanced hydrocarbon cracking activity of steamed vs. unsteamed Y zeolites. First, it is shown how the cracking mechanism shifts with changing concentrations of adsorbed olefins. Second, evidence is presented to support the suggestion that diffusion of olefins inside the zeolite may influence the observed reaction rate at low temperatures.