| dc.description.abstract | HY zeolites are an essential component as catalysts in fluidized catalytic cracking (FCC) process. The Steamed HY or Ultra Stable Y (USY) zeolites have proved to be active hydrocarbon cracking catalysts. Therefore, in this research we have conducted reactions for cracking of n-hexane on various HY zeolites (commercial and lab synthesized) which were prepared by steaming and/or acid leaching of the conventional HY zeolite. Catalytic cracking of n-hexane has been widely accepted as a probe for strong acidity in zeolites, and henceforth, it shall be a good measure of comparing activity differences in various HY zeolites.
The high activity of H-USY compared to HY has been previously explained by isolation of Brønsted Acid Sites (BAS) to create highly active BAS, or increased accessibility in a microporous diffusion-limited HY, or generation of Extra-framework Aluminum (EFAl) which greatly helps in the activity enhancement of HY zeolites. However, neither of the reasons have been proven to be fully correct and agreed upon by majority of the observations seen in literature. Therefore, properties of HY zeolites were studied using characterization methods like BET adsorption, 27Al MAS NMR, 29Si MAS NMR and IPA TPD. Calculated activation energies, reaction rates and turnover frequencies (TOF) were helpful in comparing the zeolites for any diffusion limitations or activity enhancements. After comparing activity of HY zeolites with HZSM-5, it was clear that the cracking of n-hexane over HY zeolites is not governed by diffusion of the reactant in terms of entering the pores of the HY zeolites. Cracking activity on various commercial HY zeolites was compared with quantified values of Framework Al (FAl) and EFAl. These comparisons helped in knowing that isolated FAl might be a necessary, but insufficient condition for activity enhancement. Presence of EFAl might play a positive or a negative role in enhancing the activity. Furthermore, a comparison study of product distribution over a range of conversions between steamed HY and acid leached HY showed that longer diffusion paths inside the zeolite can lead to a large selectivity towards secondary reaction products formed from oligomerization and isomerization. | en_US |
