Intermolecular interactions governing adsorption of bulky molecules on ZIF-8: Insight from adsorption kinetics of benzene and 6-membered ring alicyclic hydrocarbons

Abstract

This study examined the adsorption kinetics of benzene and six-membered ring alicyclic hydrocarbons on Zeolitic imidazolate framework-8 (ZIF-8) to elucidate the interactions between the adsorbed molecules and linkers on the adsorption behavior of bulky molecules. The temperature dependence of the apparent Fickian diffusion coefficient was analyzed to determine the activation entropy and energy of diffusion. The significant negative values observed for the activation entropy, from − 170 to − 220 J K^− 1 mol^− 1, indicate a restriction on the degree of freedom of the adsorbate molecules within the diffusion transition state, thereby reducing the apparent diffusion coefficients. The molecular orientations of the adsorbate molecules traversing the ZIF-8 aperture were investigated using Monte Carlo simulations. Order parameter analysis revealed that the molecular orientation was constrained to minimize the energy barrier with passage through the aperture. Fourier-transform infrared (FT-IR) spectroscopy and ¹³C cross-polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) measurements supported the CH-π interactions between the adsorbate molecules and 2-methylimidazolate linkers. This interaction maintained the orientation of the adsorbate molecules, facilitating their passage through the ZIF-8 apertures. CH-π interactions influence molecular orientation control in the adsorption and diffusion of large molecules containing π electrons in porous materials with organic ligands.