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

Abstract

High-resolution 1 H magic angle spinning (MAS) NMR investigations of 1-butene and 1-pentene molecules sorbed in zeolite NaX are presented in which a considerable decrease in the linewidths compared with common experiments is achieved by using the MAS technique. The highly resolved spectra were recorded as a function of the loading (pore filling) in the range 0.05-4 molecules per supercage. The measurements at these very low loadings give the possibility of studying the behaviour of quasi-isolated adsorbed molecules. Only very small changes of the 1 H NMR chemical shifts of most molecular groups 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 liquids. At lower pore fillings the chemical shifts are close to the values measured in the gas phase. The interaction of the olefins with Na C cations as adsorption sites can only be seen in the chemical shift of the CH— group which are nearly independent of the loading. This can be explained by the influence of exchange processes and the formation of weak adsorption complexes. The gain in resolution achieved also allows the application of 1 H NOESY NMR spectroscopy. 1 H NOESY NMR and 1 H- 13 C cross-relaxation studies are presented to investigate the mobility of 1-butene and 1-pentene molecules adsorbed in NaX zeolite down to very low pore filling factors. The comparison of the different homonuclear and heteronuclear cross-relaxation time studies 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 the molecules and an overall reorientation-translation motion with a correlation time which is comparable to the lifetime of the molecules in a cavity with respect to their jump into a neighbouring cavity. Copyright © 1999 John Wiley & Sons, Ltd. KEYWORDS: NMR; 1 H MAS NMR; 1 H NOESY NMR; 1 H- 13 C cross-relaxation; zeolites; adsorbed molecules