In order to clarify the adsorption mechanism of the zeolitic imidazolate framework, ZIF-8, for bulky molecules larger than its aperture diameter, the local structure of ZIF-8 and its host–guest interactions were studied by adsorption kinetics, potential energy analysis, and solid-state 1H nuclear magnetic resonance (1H-NMR) spectroscopy. Adsorption kinetics measurements lead to apparent diffusion coefficients for CS2, CDCl3, and CCl4 in ZIF-8 that provide residence times for each molecule in the ZIF-8 micropores. Potential energy analyses using van der Waals pairwise interactions reveal two kinds of feasible sites for bulky molecules in the ZIF-8 micropores, where there are minimal intermolecular interactions between an adsorbate and the pore wall. Each site lies on one of the four diagonal axes of the cubic lattice of the ZIF-8 unit cell. One set of sites corresponds to the side of the micropore where methyl groups form a bottleneck, and the other to the side where CH moieties form the bottleneck. Second-moment analysis of the 1H-NMR resonance line reveals that the 2-methylimidazolate rings undergo flip motions, with flip angles of ~55°, in addition to the rapid axial rotations of their methyl groups. Temperature-dependent spin–lattice relaxation time analyses indicate that adsorptions of these molecules reduce the motions of the 2-methylimidazolate rings. In particular, steric hindrance from bulky molecules such as CDCl3 and CCl4 significantly lowers the flipping frequency of the 2-methylimidazolate ring. The flipping of the 2-methylimidazolate ring results in instantaneous aperture expansion that enables bulky molecules to pass through the ZIF-8 aperture.
Adsorption – Springer Journals
Published: Aug 2, 2017
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