Diffusion of hydrocarbon in zeolite and effect due to pore topology: Neutron scattering and MD simulation studies

- Dynamical landscape of propylene adsorbed in two different zeolites is obtained.
- MD simulation indicated three different time scales for the translational motion.
- Neutron scattering data correspond to one of the three different time scales.
- Faster rotational motion observed in a wider energy window neutron spectrometer.
- Propylene found to be in preferred orientation in ZSM5 but not in Na-Y zeolite.

Here we report detailed dynamical landscape of propylene adsorbed in ZSM5 and Na-Y zeolites as studied by neutron scattering and molecular dynamics (MD) simulation. Separation of propylene molecule from propane in the petrochemical industry is an important issue because it is one of the most demanding energetic separation processes due to their very close relative volatilities and molecular sizes. Aim here is to investigate the correlation of the host topology towards the dynamics of guest molecules. ZSM5 zeolite is typified by a network of intersecting channels while Na-Y has a network of spherical supercages interconnected by windows. Both neutron scattering and MD simulation studies indicated that translational diffusion of propylene is more restricted in ZSM5 compared to Na-Y zeolite. Fully atomistic MD simulation studies showed that the translation involves three different time scales and rotational motion of the propylene is much faster than translation. The observed dynamics in QENS spectrometer (ΔE ∼ 200 μeV) corresponds to one of the three translational components indicated in the MD simulation. The faster rotational motion is observed in a wider energy window spectrometer (ΔE ∼ 3 meV). MD simulation results also show some interesting features like, non-isotropic rotation in ZSM5 while it has been isotropic in Na-Y zeolite. It is also found that propylene molecules prefer to orient along channels of ZSM5 zeolite.