Effect of steam dealumination on H-Y acidity and 2-methylpentane cracking activity

The relationship between 2-methylpentane cracking activity and the acid properties of H-Y (acidic Y zeolite), H-USY (acidic ultrastable Y zeolite), steamed H-USY, and (H,NH₄)-USY ((H,NH₄)-ultrastable Y zeolite) was investigated. The acid strength distributions of these samples were determined by microcalorimetry of NH₃ adsorption, and the types of acid sites by FTIR spectroscopy. It was found that even for an H-Y sample of a high degree of crystallinity its cracking activity per unit catalyst weight was 35 times lower than that of H-USY With further steaming of H-USY, the cracking activity decreased, although the activity per strong Brønsted site remained essentially constant. Interestingly, although the strongly acidic Lewis acid sites were covered by NH₃ in (H,NH₄)-USY, the catalyst had the same activity as H-USY Also, the heat of NH₃ adsorption on (H,NH₄)-USY did not exceed 130 kJ/mol. Thus, it was concluded that strong Lewis acid sites were not active for hydrocarbon cracking, and that 2-methylpentane cracking did not require Brønsted sites with a high heat of NH₃ adsorption. H-USY, with both Brønsted and Lewis sites, had a heterogeneous acid strength distribution, whereas zeolites containing only Brønsted sites had a homogeneous acid strength.