1H MAS NMR studies of molecules adsorbed on activated zeolites

The aim of this paper is to study the interaction of organic molecules with zeolites of different pores sizes (NaY, dealuminated ZSM-5, and H-ZSM-5) pretreated under conditions which led to various catalytically active sites. The behavior of the adsorbed molecules is investigated by means of high-resolution H-1 MAS NMR spectroscopy. The measurements demonstrate that complex adsorption interactions occur between adsorbed 2-butene or acetone molecules and the OH groups in the zeolites which also have a strong influence on the activity of the catalytic sites. During the adsorption of the molecules on differently activated zeolites the H-1 MAS NMR spectra show changes which can be attributed to catalytic conversions, where Bronsted and Lewis acidic sites of the zeolite catalysts are involved. A proton transfer from the bridging AlOHSi groups of the zeolite framework to the adsorbed 2-butene molecules can be clearly detected. The results for the system acetone/dealuminated ZSM-5 and acetone/H-ZSM-5 point, in particular, to the role of Lewis acid centers which may be formed in the zeolites during the applied pretreatment procedure. (C) 2001 Academic Press.