A solid-state NMR spectroscopic study of the adsorption of toluene in zeolite LiK-L

The adsorption of toluene in partially Li+-exchanged zeolite K-L (LiK-L) was examined by solid-state NMR spectroscopy. The environment of the Li+ ions at different sites in the zeolite before and after adsorption was characterized by Li-7 magic-angle spinning (MAS) NMR. H-1 -> Li-7 cross polarization (CP) and Li-7{H-1} rotational-echo double-resonance (REDOR) experiments were performed to probe the cation-sorbate interactions. The data were also used for spectral assignment. The Li-7 NMR results indicate that most of the Li+ ions occupy the sites A (39%) and B (43%) and that there are fewer Li+ ions at sites C (17%) and D (1%). The four Li-7 NMR signals corresponding to the Li+ ions at four sites are not resolved in the spectrum of the dehydrated zeolite, but are well-separated after the adsorption of toluene. The NMR results show that a toluene molecule is directly coordinated to the Li+ ion at site D near the wall of the main channel via formation of a pi-complex, resulting in a significant change in Li-7 chemical shift toward a more shielded direction. Although not directly accessible by the toluene molecules, the Li+ ions at sites A and C also exhibit an observable shift upon adsorption. Only the Li+ ions at site B do not experience a significant change in chemical shift. The effect of the adsorption on Li-7 chemical shift is discussed. Dynamic behavior of toluene molecules inside the channels was also investigated by wide-line H-2 NMR.