Substitution (CH3, Cl, or Br) effects of the imidazolate linker on benzene adsorption kinetics for the zeolitic imidazolate framework (ZIF)-8

Abstract

Herein, the time dependence of benzene adsorption uptake was examined for ZIF-8, Cl-ZIF-8, and Br-ZIF-8 and analysed using an intra-crystalline (Fick's) diffusion model, yielding the diffusion coefficient and saturated adsorption amount of benzene. The saturated adsorption amount of benzene decreased in the order of ZIF-8, Cl-ZIF-8, and Br-ZIF-8. Notably, ZIF-8, with an intermediate pore volume among the three specimens, accommodated the greatest number of molecules (5.5 molecules per micropore). The activation energy, E_a, and the pre-exponential factor, D₀, for benzene diffusion increased in the order of ZIF-8, Cl-ZIF-8, and Br-ZIF-8. These findings suggest that the 2-methylimidazolate moiety forms an effective attraction interaction with benzene molecules. The D₀ values also yielded the activation entropy, ΔS^‡, in the transition state when a benzene molecule passed through a six-membered ring aperture. The ΔS^‡ values at 303 K were negative, and their absolute values increased in the order of Br-ZIF-8, Cl-ZIF-8, and ZIF-8. Considering the degree of freedom of translation and rotation of the benzene molecule and the vibration and disorder of the linker, we found that the differences in ΔS^‡ were caused by the dynamic local structure of the six-membered ring aperture among the ZIF-8 analogues. Furthermore, infrared spectroscopy revealed a low-wavenumber shift of the C–H stretching band in both the imidazolate moiety and adsorbed benzene molecules. A solid-state ¹³C-nuclear magnetic resonance spectrum presented a downfield shift of ¹³C resonance peaks in the imidazolate moiety, suggesting that CH/π interactions reasonably explain the intermolecular interaction between the imidazolate moiety (including the methyl group) and π-electrons of benzene.