Dynamics of Propylene adsorbed in Na-Y and Na-ZSM5 Zeolites: A QENS and MD Simulation Study

Abstract

Abstract Here we report dynamics of propylene (smallest asymmetric hydrocarbon) adsorbed in two different host systems – Na-Y and Na-ZSM5 zeolites as studied by quasielastic neutron scattering (QENS) techniques and molecular dynamics (MD) simulation techniques. MD simulation studies suggested that the rotational motion of propylene is much faster than the translation. Therefore, two different spectrometers were used to determine the translational and rotational contributions. Analysis of the QENS data (ΔE~200 μeV) revealed that the translational motion of propylene in Na-ZSM5 zeolite is a factor of 3 slower than that in Na-Y zeolite. This is consistent with the smaller void volume available in Na-ZSM5 zeolite as also the fact that the adsorption of propylene is stronger in Na-ZSM5 as compared to that in Na-Y zeolite as indicated in the Fourier Transformed Infra Red (FTIR) studies. This result is further corroborated by the MD simulation studies. It was found that the translation motion occurs at three time scales in both the systems, only one of which corresponds to that observed through the QENS measurements. To probe the faster rotational motion experiments were carried out on a Triple axis spectrometer with wider energy window (ΔE~3meV). The variation of the elastic incoherent structure factor (EISF) suggests that the observed QE broadening corresponds to isotropic rotational motion of propylene in both the hosts. Rotational diffusion coefficients of propylene molecules are found to be of the same order in both zeolites. Trajectories obtained from MD simulation corresponding to the rotational motion revealed that although on a large time scale, the rotation appears isotropic, but at short time scales the channel structure of the Na-ZSM5 restrict the rotation of propylene molecules. This is in contrast with propylene adsorbed in Na-Y zeolite where rotation remains isotropic even at short time scale. A comparison with other hydrocarbon-zeolite systems as studied by us is also presented here.