THE EFFECTS OF EFAL EXTRACTION ON 2-METHYLPENTANE CRACKING OVER STEAMED HY

We have examined the cracking of 2-methylpentane at 400 degrees C on a steamed HY catalyst and on the same catalyst after extraction of its extra-framework aluminum (EFAl) using ammonium hexafluorosilicate. MAS-NMR data show that our procedure removes the EFAl, leaving the framework aluminum (FAl) intact. Kinetic runs on the various catalysts produced by steaming alone and by steaming followed by EFAl extraction have yielded quantitative reaction parameters, which we obtain by fitting the observed conversion and selectivity data with well-defined theoretical equations. The parameters thus obtained show that monomolecular and bimolecular reactions are equally active on both the extracted and unextracted catalysts; the feed is converted by monomolecular and bimolecular reactions in a ratio of similar to 1:2.3 on both catalysts. Closer examination of the kinetic parameters shows that the extraction of EFAl results in an similar to 30% increase in the total rate of first-order initiation and a much smaller increase in the second-order rate constant for decay, indicating that swaths of sites rather than isolated sites have been activated, or made accessible, by the removal of the EFAl. At the same time, initial product selectivities show that there is no significant change in the nature of the FAl sites after EFAl is removed. None of the individual elementary reactions of the cracking mechanism is affected by the removal of the EFAl. There is therefore no evidence that EFAl participates in a selective way in the cracking of 2-methylpentane under our experimental conditions. We conclude that the role of the EFAl in the cracking of 2-methylpentane is small, involving purely mechanical blocking of a random selection of the active sites, leaving the catalytically active centres at the FAl in an unchanged condition. There is little evidence in our data of any catalytically significant chemical interaction between the Bronsted acid sites residing on the crystal lattice framework and the EFAl. Such evidence as there is comes from the slight increase in the rate of catalyst decay and is discussed in detail. (C) 1995 Academic Press, Inc.