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 diffusion of benzene in NaX and NaY zeolites. The self-diffusion coefficient is found to decrease with increasing loading in both zeolites. In NaX, the diffusivities obtained from QENS are in reasonable agreement with those obtained by pulsed-field gradient NMR or frequency-response methods, but a large discrepancy is observed with results from the tracer zero-length column technique. In NaY, where the experimental results are scarce, the diffusion coefficients derived from QENS compare well with those from different simulation techniques. At low benzene concentrations, extrapolation of the QENS values 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 distribution 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.