Adsorption and Diffusion of CO2 and CH4 in Zeolitic Imidazolate Framework-8: Effect of Structural Flexibility
详细信息 查看全文
- 作者：Liling Zhang ; Gang Wu ; Jianwen Jiang
- 刊名：Journal of Physical Chemistry C
- 出版年：2014
- 出版时间：May 1, 2014
- 年：2014
- 卷：118
- 期：17
- 页码：8788-8794
- 全文大小：514K
- 年卷期：v.118,no.17(May 1, 2014)
- ISSN：1932-7455
文摘
A hybrid molecular simulation study is reported to examine adsorption and diffusion of CO2 and CH4 in zeolitic imidazolate framework-8 (ZIF-8). The structure flexibility of ZIF-8 is described using a recently developed force field (Zhang, L. J. Am. Chem. Soc. 2013, 135, 3722). The simulated adsorption isotherms in rigid and flexible ZIF-8 are nearly identical and agree well with experimental data; thus, the effect of structure flexibility on adsorption is indiscernible. In remarkable contrast, the effect on diffusion is significant. No diffusive motion is observed for CO2 and CH4 in rigid ZIF-8; by incorporating structure flexibility, however, the predicted diffusivities are close to experimental and simulated data reported in the literature. From the analysis of free energy, CO2 has a lower barrier for diffusion than CH4 and hence a higher diffusivity. With increasing loading, CO2 and CH4 exhibit different trends. CO2 diffusivity slightly decreases due to enhanced steric hindrance; however, CH4 diffusivity substantially increases because CH4 is preferentially located near the aperture and thus the free energy barrier for diffusion is reduced. For a CO2/CH4 mixture, CO2 is more strongly adsorbed than CH4 and blocks the diffusion pathway of CH4; therefore, CH4 diffusivity in the mixture decreases upon comparison with pure CH4. This simulation study provides microscopic insight into adsorption and diffusion in ZIF-8 and highlights the indispensable effect of structure flexibility on diffusion behavior.