Diffusion of Benzene in ZSM-5 Measured by the Neutron Spin−Echo Technique

The diffusion of benzene in ZSM-5 has been measured by neutron spin−echo. This technique allows probing for much longer times (20 ns in this work) than with conventional quasi-elastic spectrometers. In this zeolite, the diffusion coefficient of benzene extrapolated to 300 K is 1.3 × 10 -14 m 2 s -1 at 300 K, the activation energy being 30 kJ/mol. This is one of the few systems in which agreement can be observed between a microscopic technique and macroscopic methods, that is, gravimetry, frequency-response, infrared, and zero-length column (ZLC) technique. Comparison with molecular simulation studies shows that inclusion of the lattice flexibility is crucial when the molecule is similar in size to the zeolite pore, as for benzene in MFI.