Quasi-elastic neutron scattering and molecular dynamics simulation as complementary techniques for studying diffusion in zeolites

Quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations have enabled the quantitative study of molecular motion in pure and mixed fluids sorbed in zeolites over length scales of 0.1–100 nm and time scales of 10 fs–100 ns. After an introduction to the statistical mechanics of diffusion, this review describes the principles and practice of time-of-flight (TOF), backscattering (BS), and neutron spin-echo (NSE) measurements, as well as methods to analyze the results. A brief overview of MD simulations is provided, with emphasis on how to calculate QENS observables and how to detect signatures of local anisotropic translational dynamics within the inhomogeneous periodic force field of zeolite crystals. Illustrations of what can be learned from combined application of QENS and molecular simulation are provided from recent work on specific systems and problems: self-diffusion of n-alkanes up to C16 in MFI zeolites, self-diffusion of methane co-adsorbed with n-butane in silicalite-1, and transport diffusion of N2 and CO2 in silicalite-1.