Molecular Siting of C1–C6 n-Alkanes in ZIF-4: A Hybrid Monte Carlo Study

Abstract

The zeolitic imidazolate framework ZIF-4 is a metal− organic framework made of tetrahedral Zn 2+ nodes connected by imidazolate ligands to form zeolite-like pores. ZIF-4 exhibits many interesting phenomena such as framework flexibility and phase transitions. Recently, the "window effect" has also been demonstrated experimentally in ZIF-4. The window effect refers to an unusual, nonmonotonic trend in the diffusivity of chain molecules in nanoporous materials. In ZIF-4, the diffusivities decrease with chain length from methane to n-butane but with n-pentane having a higher diffusivity than n-butane and n-hexane. This is hypothesized to be due to the difficulty of the longer chains (n-pentane, n-hexane) to coil up inside the cages, causing them to extend out of the cage through the flexible cage windows. As a result, the diffusion mechanisms could change as the chain length increases from n-butane to n-pentane. We investigate this possibility by simulating the adsorption of C1−C6 n-alkanes in ZIF-4 using hybrid Monte Carlo simulations and analyzing the adsorbed configurations for molecular siting differences among the n-alkanes from methane to n-hexane.

Structural Chemistry of Zeolitic Imidazolate Frameworks

“…Guest molecules having larger kinetic diameters than the D f value of a ZIF may still be able to enter the pores considering the factors mentioned above. For a more rigorous analysis on the mechanism, it is essential to conduct simulation studies or in situ powder X-ray diffraction on prospective candidates that are near to the boundaries of our classification …”

Molecular Simulations of the Chain Length Dependent Adsorption of C7‐C14 n‐Alkanes in ZIF‐8

“…S3. In ZIFs, the chain length dependent effects are often closely connected to the presence and flexibility of the windows (e.g., the window effect 7, 55). However, for C7‐C14 n ‐alkanes in ZIF‐8, the adsorption happens mostly away from the windows and the chains do not extend from one cage to another through the windows.…”