Catalytic ramifications of steam deactivation of Y zeolites: An analysis using 2-methylhexane cracking

Kinetic analysis of experimental data for 2-methylhexane cracking demonstra tes that trends in activity and selectivity are well simulated by adjusting a single parameter that represents the acid strength of a Y-based FCC cata lyst. This acid strength may be modified via steam deactivation, and we hav e experimentally corroborated acidity changes using ammonia microcalorimetr y and infrared spectroscopy. Increased severity of steam treatment reduces the number and strength of catalyst acid sites, and it leads to a reduction in the turnover frequency of all surface processes and a decrease in overa ll site time yield. Steaming of the catalyst does not change the fundamenta l chemistry involved in catalytic cracking. However, change in acidity caus ed by steaming alters product selectivity by changing relative rates of var ious catalytic cycles in the cracking process. For example, steam treatment increases olefin selectivity by favoring catalytic cycles that produce ole fins. (C) 1999 Academic Press.