(318g) Gas Transport at the Polymer-Zeolite Interface Using Atomistic Simulations

Membrane
based separation is considered as efficient, productive, and environmentally
friendly processes that operate in a continuous fashion. The
development of mixed matrix membranes (MMM) for gas separation is an active and
rapidly growing area of research, due to the challenges with the current
spectrum of polymeric and inorganic membranes. MMMs have been conventionally
prepared by incorporating inorganic fillers such as zeolites, metal organic
framework (MOFs) and carbon nanotubes (CNTs) in a continuous polymer matrix.
However, the ultimate success of these advanced membranes depends on the
material selection and interface defect elimination. Hence, nanoscale
understanding of polymer structure near a surface and gas transport at the
interface is critical to the design of these advanced gas separation
technologies. Here we report a comparison of the adsorption and transport
characteristics of CO₂ and CH₄ as single components in
polyimide (PI), MFI zeolite and PI-MFI zeolite composite membrane as a function
of temperature in the range of 300-500 K using equilibrium Molecular Dynamics
(EMD) simulations. It is seen that incorporation of MFI zeolite into PI results
in the formation of densified polymer layers near the surface, having thickness
around 1.2 nm, before bulk-like behavior of the polymer is attained. This
region offers an extra resistance to the gas diffusion especially for the gas
with larger kinetic diameter, CH₄ in this case, thus improving the
CO₂/CH₄ kinetic selectivity in the PI-MFI composite
membrane. Further, we find that the kinetic selectivity of CO₂ over
CH₄ in the rigidified region increases with temperature.
Furthermore, the temperature dependence of the collective diffusivity of CO₂
and CH₄ follows Arrhenius behavior in PI, MFI zeolite and PI+MFI
hybrid membranes. Also, it is seen that crystal size has little effect on the
polymer structure and gas transport at the polymer-filler interface. In
addition, to extract the isotherms for gas adsorption, we implemented a
two-step methodology by performing Grand Canonical Monte Carlo (GCMC)
simulations coupled with EMD simulations by considering the dynamics and
structural transitions in the polymer matrix upon gas adsorption, and
investigated the resulting adsorption isotherms of CO₂, CH₄
in pure and composite polymer membranes. The sorption results indicate that
incorporation of MFI zeolite into PI improves the adsorption selectivity of CO₂
over CH₄. A significant increase in CO₂/CH₄
selectivity as well as the gas permeability is observed in the PI-MFI composite
membrane compared to the pure PI polymer membrane, which is correlated with the
high selectivity of the rigidified interfacial layer in the polymer.