SITE-HOPPING DYNAMICS OF BENZENE ADSORBED ON CA-LSX ZEOLITE STUDIED BY SOLID-STATE EXCHANGE C-13 NMR

First-time applications of solid-state exchange C-13 NMR techniques to the study of the reorientation dynamics of hydrocarbon molecules adso rbed on zeolites have enabled the geometry and time scales of molecula r hopping processes between adjacent adsorption sites to be characteri zed directly and model free. Two-dimensional exchange C-13 MMR on stat ic samples establishes the geometry of the site-hopping dynamics, whil e one-dimensional magic-angle spinning (MAS) exchange-induced-sideband s (EIS) C-13 NMR permits motional correlation times on the order of mi lliseconds to seconds to be extracted directly from the experimental d ata. Variable-temperature experiments performed on Ca-LSX zeolite samp les with average bulk loadings of 0.5, 1, and 2 benzene molecules per supercage yield apparent Arrhenius activation energies of about 66 +/- 6 kJ mol(-1) for the discrete, localized reorientation dynamics of be nzene molecules among different Ca2+ cation adsorption sites (similar to 0.5 nm apart). Arrhenius preexponential factors were established to be on the order of 1 x 10(12) s(-1), consistent with elementary hoppi ng processes. Motional correlation times exhibit only minor variations upon changes in benzene loading over the range studied.