5 years ago

Free Energy Analysis for Adsorption-Induced Structural Transition of Colloidal Zeolitic Imidazolate Framework-8 Particles

Free Energy Analysis for Adsorption-Induced Structural Transition of Colloidal Zeolitic Imidazolate Framework-8 Particles
Shuji Ohsaki, Hideki Tanaka, Minoru T. Miyahara, Satoshi Watanabe
Particle size and shape of flexible metal–organic frameworks have been reported to change the gate adsorption phenomenon which is characterized by an abrupt increase in the adsorbed amount induced by structural transitions of a host framework, although a detailed mechanism has not yet been clarified. Here, we focus on zeolitic imidazolate framework-8 (ZIF-8) whose structural flexibility stems from the rotation of 2-methylimidazole linkers. We perform free energy analyses with the aid of adsorption simulations to understand the dependence of the gate adsorption phenomenon on the particle size and shape of ZIF-8. For the adsorption simulations, we construct a simulation cell referred to as the nanoparticle model, which has a ZIF-8 slab structure and gas phase regions. Our simulations demonstrate that a decrease in the slab width results in a reduction of the average adsorbed amount. This can be explained by the smaller amount adsorbed in the region close to the surface of the ZIF-8 structure, which reaches a thickness of 1.0 nm from the surface and is not altered by slab widths or linker rotational angles. In the free energy analysis, the gate adsorption of smaller sized ZIF-8 particle models occurs at higher pressures because of less stabilization by the gas adsorption. On the other hand, the crystal planes have little impact on the gate opening and closing pressures because of the similar free energy stabilization by adsorption, although variations in the crystal planes significantly alter the adsorption isotherms. Furthermore, we propose a model to predict the gate opening and closing pressures of ZIF-8 particles with arbitrary sizes. We find quantitative agreement between the proposed model and experiments, which suggests that the near-surface region with less adsorbed amounts is the origin of the particle size dependence of the gate adsorption phenomena.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b06836

DOI: 10.1021/acs.jpcc.7b06836

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