Direct Measurement of Intercage Hopping in Strongly Adsorbing Guest-Zeolite Systems

Solid-state exchange ${}^{13}\mathrm{C}$ NMR has been used to measure directly discrete intercage hopping of strongly adsorbed molecules on a nanoporous zeolite. Such intercage motions represent the molecular transport events that are central to macroscopic diffusion of guest molecules through zeolite pore spaces. Apparent activation energies of 73 and $76\mathrm{kJ}/\mathrm{mol}$ have been measured for intra- and intercage hopping, respectively, of benzene molecules on Ca-Y zeolite. Exchange NMR measurements at 338 to 368 K yield benzene self-diffusion coefficients from ${10}^{-19\mathrm{}}–{10}^{-18\mathrm{}}\mathrm{m}{}^2/\mathrm{s}$, values which are well below the range of diffusivities accessible from other equilibrium techniques.