H-1 NMR studies of the mobility of olefins adsorbed in zeolites of type NaX

High-resolution H-1 magic angle spinning (MAS) NMR investigations of I-bute ne and I-pentene molecules sorbed in zeolite NaX are presented in which a c onsiderable decrease in the linewidths compared with common experiments is achieved by using the MAS technique. The highly resolved spectra were recor ded as a function of the loading (pore filling) in the range 0.05-4 molecul es per supercage. The measurements at these very low loadings give the poss ibility of studying the behaviour of quasi-isolated adsorbed molecules. Onl y very small changes of the H-1 NMR chemical shifts of most molecular group s of the adsorbed olefins can be detected with varying pore filling factors . At higher pore fillings the values are similar to those in the bulk liqui ds. At lower pore fillings the chemical shifts are close to the values meas ured in the gas phase. The interaction of the olefins with Naf cations as a dsorption sites can only be seen in the chemical shift of the =CH- group wh ich are nearly independent of the loading. This can be explained by the inf luence of exchange processes and the formation of weak adsorption complexes . The gain in resolution achieved also allows the application of H-1 NOESY NMR spectroscopy. H-1 NOESY NMR and H-1-C-13 cross-relaxation studies are p resented to investigate the mobility of I-butene and l-pentene molecules ad sorbed in NaX zeolite down to very low pore filling factors. The comparison of the different homonuclear and heteronuclear cross-relaxation time studi es permits a more detailed discussion of the motion of the guest molecules which cannot be described by a single correlation time. The mobility of the adsorbed species can be characterized by a fast libration-type motion of t he molecules and an overall reorientation-translation motion with a correla tion time which is comparable to the lifetime of the molecules in a cavity with respect to their jump into a neighbouring cavity, Copyright (C) 1999 J ohn Wiley & Sons, Ltd.