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

The zeolitic imidazolate framework ZIF-4 is a metal–organic framework made of tetrahedral Zn²⁺ 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.