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 adsorbed on zeolites have enabled the geometry and time scales of molecular hopping processes between adjacent adsorption sites to be characterized directly and model free. Two-dimensional exchange C-13 MMR on static samples establishes the geometry of the site-hopping dynamics, while one-dimensional magic-angle spinning (MAS) exchange-induced-sidebands (EIS) C-13 NMR permits motional correlation times on the order of milliseconds to seconds to be extracted directly from the experimental data. Variable-temperature experiments performed on Ca-LSX zeolite samples 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 benzene 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 hopping processes. Motional correlation times exhibit only minor variations upon changes in benzene loading over the range studied.