Diffusion of benzene in NaX and NaY zeolites studied by quasi-elastic neutron scattering

Quasi-elastic neutron scattering (QENS) has been used to study the diffusio n of benzene in NaX and NaY zeolites. The self-diffusion coefficient is fou nd to decrease with increasing loading in both zeolites. In NaX, the diffus ivities obtained from QENS are in reasonable agreement with those obtained by pulsed-field gradient NMR or frequency-response methods, but a large dis crepancy is observed with results from the tracer zero-length column techni que. In NaY, where the experimental results are scarce, the diffusion coeff icients derived from QENS compare well with those from different simulation techniques. At low benzene concentrations, extrapolation of the QENS value s to 300 K yields diffusivities that are almost 2 orders of magnitude lower in NaY than in NaX. This reflects the strong influence of the cation distr ibution at this temperature. The activation energy for diffusion in NaY, 33 .7 kJ/mol, is about twice that in NaX, 17 kJ/mol, at low loading.