A STUDY OF DEALUMINATED FAUJASITES

We have examined a series of steamed and chemically dealuminated faujasites, with lattice parameters varying from 2.4488 to 2.4242 nm, using a range of techniques in order to obtain a better understanding of the acid sites in these materials. Temperature programmed desorption (TPD) and thermogravimetric analysis (TGA) measurements of isopropylamine showed that only a fraction of the sites associated with framework Al give rise to Bronsted-acid sites, as determined by decomposition of the amine between 575 and 650 K. Infrared spectroscopy indicated that these acidic sites are associated with the hydroxyls observed in a band at 3640 cm-1. No significant differences were observed for TPD-TGA of isopropylamine or for propene oligomerization between a steamed and a chemically dealuminated H-Y having nearly the same lattice parameter; however, TPD-TGA measurements of isopropanol suggest that additional reaction sites are present in the steamed sample for this probe molecule. n-Hexane cracking measurements at 748 and 798 K indicate that turnover frequencies (molecules reacted/site sec) are significantly enhanced by steaming, but that acid leaching reduces the activity back to that observed for a chemically dealuminated material. Microcalorimetry measurements for pyridine and isopropylamine provided no evidence for the presence of special sites in steamed materials. It is suggested that the enhanced cracking activities observed in steamed faujasites are not due to the enhanced acidity of the hydroxyl sites. (C) 1994 Academic Press, Inc.