Molecular Insight into the Slow Dynamics of C₄ Hydrocarbons in the Zeolitic-Imidazole Framework (ZIF-8)

The family of zeolitic-imidazole framework (ZIF) materials is currently considered for the challenging separation of C4 hydrocarbons. However, yet, the microscopic diffusion mechanism for these hydrocarbons in these narrow gate porous materials remains elusive by conventional methods due to its very slow nature. Experimental (solid-state ²H nuclear magnetic resonance-NMR) and computational (molecular dynamics-MD) approaches were applied together to derive slow diffusional dynamics of n-butane and 1-butene in ZIF-8. By means of the ²H NMR technique, we evidenced the presence of two adsorption sites for the guests localized inside the cages of ZIF-8 and in the vicinity of the gates. We characterized the molecular mobility at each site and revealed that the translational intercage diffusion is realized by a slow directional motion associated with the gate-crossing. MD simulations provide an in-depth analysis of the diffusion and fully support the proposed dynamics picture for both n-butane and 1-butene. These calculations enable the derivation of the diffusivity and barriers for the long-range diffusion of both hydrocarbons in ZIF-8 and unraveled the microscopic diffusion mechanism implying intracage and intercage motions. We show that this NMR approach combined with modeling is a valuable tool to probe the molecular mobility for slow diffusing species in ordered cagelike porous frameworks.