LEWIS-ACID SITES AND SURFACE ALUMINUM IN ALUMINAS AND ZEOLITES - A HIGH-RESOLUTION NMR-STUDY

The distribution of surface aluminum atoms among different coordinatio n states is a key to the understanding of the acidity of aluminas and zeolites. Single-pulse Al-27 MAS NMR spectra do not distinguish the su rface from the bulk species. Proton-aluminum cross-polarization (CP-MA S) spectra, when hydroxyls are the source of protons, give a biased re presentation of the surface topology. Indeed, water chemisorption invo lves an unknown number of reconstructed layers because chemisorption i s dissociative. On the other hand, when CP is operated from chemisorbe d ammonia, the distribution of the surface Al is observed, and it repr esents the state of the active catalytic surface. It was the main purp ose of this work to study the nature of Lewis acidity in various catal ysts (aluminas and zeolites) pretreated under controlled atmosphere at different temperatures. Isotopically enriched ammonia (NH3)-N-15 chem isorption 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 Al-27 spectral parameters for different Al sites in partially resolved spectra. The procedure accounted for the normal distribution of electric field grad ient tenser components at a particular site. It is shown that there ar e two kinds of Lewis sites on aluminas: namely, a tetrahedral site wit h isotropic shift of about 58 ppm and quadrupolar coupling constant QC C similar or equal to 6 MHz and a pentagonal site with a isotropic shi ft of about 40 ppm and a slightly smaller QCC. In zeolites the same ki nds of sites exist in the nonframework aluminum debris, and in additio n, the Bronsted sites are associated with framework aluminum.