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, Ea, 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.