Lewis Acid Sites and Surface Aluminum in Aluminas and Zeolites: A High-Resolution NMR Study

Abstract

It was the main purpose of this work to study the nature of Lewis acidity in various catalysts (aluminas and zeolites) pretreated under controlled atmosphere at different temperatures. Isotopically enriched ammonia [sup 15]NH[sub 3] chemisorption was employed to probe the surface active sites and to allow for magnetization transfer to surface and close-to-surface Al nuclei. A special deconvolution procedure was applied to obtain [sup 27]Al spectral parameters for different Al sites in partially resolved spectra. The procedure accounted for the normal distribution of electric field gradient tensor components at a particular site. It is shown that there are two kinds of Lewis sites on aluminas: namely, a tetrahedral site with isotropic shift of about 58 ppm and quadrupolar coupling constant QCC [approx equal] 6 MHz and a pentagonal site with a isotropic shift of about 40 ppm and a slightly smaller QCC. In zeolites the same kinds of sites exist in the nonframework aluminum debris, and in addition, the Broensted sites are associated with framework aluminum. 32 refs., 8 figs., 5 tabs.