DYNAMICS OF ETHANE AND PROPANE IN ZEOLITE ZSM-5 STUDIED BY QUASI-ELASTIC NEUTRON-SCATTERING

Quasi-elastic neutron scattering has been used to study the dynamics of ethane and propane in zeolite ZSM-5. The experiments were performed at different loadings for the two alkanes and at different temperatures for propane. The long-range translational motion of the molecules has been observed and has been interpreted with a jump diffusion model, the mean jump lengths being of the order of 10 angstrom, slightly decreasing with increasing loading. The self-diffusion coefficients of ethane are about 2 x 10(-9) m2 s-1 at 300 K, those of propane at the same temperature being smaller by a factor of two. An activation energy of 5 kJ mol-1 is obtained for the self-diffusion of propane. These results are in good agreement with the pulsed-field gradient n.m.r. measurements. In addition to the translational motion, a rotational motion of the two molecules is also observed; it is described by an uniaxial rotational diffusion model and rotational diffusion constants are derived.