Abstract
The behavior of fluids under nano-confinement varies from that in bulk due to an interplay of several factors including pore connectivity. In this work, we use molecular dynamics simulations to study the behavior of two fluids—ethane and CO2 confined in ZSM-22, a zeolite with channel-like pores of diameter 0.55 nm isolated from each other. By comparing the behavior of the two fluids in ZSM-22 with that reported earlier in ZSM-5, a zeolite with pores of similar shape and size connected to each other via sinusoidal pores running perpendicular to them, we reveal the important role of pore connectivity. Further, by artificially imposing pore connectivity in ZSM-22 via inserting a 2-dimensional slab-like inter-crystalline space of thickness 0.5 nm, we also studied the effect of the dimensionality and geometry of pore connectivity. While the translational motion of both ethane and CO2 in ZSM-22 is suppressed as a result of connecting the pores by perpendicular quasi-one-dimensional pores of similar dimensions, the effect of connecting the pores by inserting the inter-crystalline space is different on the translational motion of the two fluids. For ethane, pores connected via inter-crystalline space facilitate translational motion but suppress rotational motion, whereas in the case of CO2, both types of motion are suppressed by pore connection due to the strong interaction of CO2 with the surface of the substrate.
Highlights
To understand the role of electrostatic interactions and pore connectivity on the structure and dynamics of guest molecules confined in sub-nanometer pores of zeolitic materials, we report here MD simulation studies on ethane or CO2 confined in ZSM-22 pores
The distribution for CO2 is shown on the left, while that for ethane is shown on the right as the loadings increase from bottom to top
MD simulations of ethane and CO2 reported here, in combination with a previous study involving ZSM-5, help us understand the role of pore connectivity in determining the structural and dynamical behavior of the confined fluids
Summary
Zeolites are microporous materials which are commercially used as adsorbents in size and shape selective separation of molecular species and as catalysts in the petrochemical industry [1]. Due to their well-ordered pore structure, guest molecules adsorbed in zeolites exhibit behavior that differs considerably from the behavior of these molecules in their bulk systems [2]. This deviation of the behavior of guest molecules from the bulk is significantly enhanced for pores with smaller dimensions.
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