A STUDY OF DEALUMINATED FAUJASITES

We have examined a series of steamed and chemically dealuminated fauja sites, with lattice parameters varying from 2.4488 to 2.4242 nm, using a range of techniques in order to obtain a better understanding of th e acid sites in these materials. Temperature programmed desorption (TP D) 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 t he amine between 575 and 650 K. Infrared spectroscopy indicated that t hese 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 molecu le. n-Hexane cracking measurements at 748 and 798 K indicate that turn over frequencies (molecules reacted/site sec) are significantly enhanc ed by steaming, but that acid leaching reduces the activity back to th at observed for a chemically dealuminated material. Microcalorimetry m easurements for pyridine and isopropylamine provided no evidence for t he presence of special sites in steamed materials. It is suggested tha t the enhanced cracking activities observed in steamed faujasites are not due to the enhanced acidity of the hydroxyl sites. (C) 1994 Academ ic Press, Inc.