Density Functional Theory Computational Study of Alkali Cation-Exchanged Sodalite-like Zeolitelike Metal鈥揙rganic Framework for CO2, N2, and CH4 Adsorption

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• 作者：Jin Shang ; Gang Li ; Jiaye Li ; Liangchun Li ; Paul A. Webley ; Jefferson Z. Liu
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：December 10, 2015
• 年：2015
• 卷：119
• 期：49
• 页码：27449-27456
• 全文大小：487K
• ISSN：1932-7455

文摘

Porous adsorbents are promising for carbon capture and other industrially important gas separations, for example, CO₂/N₂/CH₄ separations. Zeolitelike metal鈥搊rganic frameworks (ZMOFs), a new subclass of MOFs, have charged frameworks, similar to conventional zeolites, which endow them with promising potential such as a high adsorption capacity and different selective molecular admission schemes from those observed in zeolites. This paper presents a density functional theory computational study of alkali cation-exchanged sodalite-like ZMOF (sod-ZMOF) for CO₂, N₂, and CH₄ adsorption. We found that large Cs⁺ cations favor sites close to the pore aperture so that three Cs⁺ cations form a positively charged gate, controlling the admission of gas molecules. These gases have an expected sequence of binding energy values: 螖E_ads(CO₂) > 螖E_ads(CH₄) > 螖E_ads(N₂). Interestingly, the energy barrier of gases passing through the gates shows an unusual sequence: 螖E_a(CO₂) > 螖E_a(N₂) > 螖E_a(CH₄). This sequence can be largely attributed to their energy levels at the centers of the gates formed by Cs cations. The electrostatic interaction between the positively charged gate and CO₂ leads to a much higher energy level at the gate center. This is in contrast to the corresponding zeolite structures, where the apertures are enclosed by negatively charged oxygen atoms. In light of similar molecular structures at the apertures of all reported ZMOFs, our study suggests a new design route in which, by appropriate selection of extraframework cations, a unique positively charged gate can be designed that can lead to different gas admission behavior from conventional zeolite materials.