Theoretical Optimization of Pore Size and Chemistry in SIFSIX-3-M Hybrid Ultramicroporous Materials

详细信息    查看全文

• 作者：Ahmad Ziaee ; Drahomir Chovan ; Matteo Lusi ; John J. Perry IV ; Michael J. Zaworotko ; Syed A. M. Tofail
• 刊名：Crystal Growth & Design
• 出版年：2016
• 出版时间：July 6, 2016
• 年：2016
• 卷：16
• 期：7
• 页码：3890-3897
• 全文大小：661K
• 年卷期：0
• ISSN：1528-7505

文摘

Classical Grand Canonical Monte Carlo (GCMC), classical molecular dynamics (MD), and density functional theory (DFT) have been employed to study the effect of pore-size and pore-chemistry on the adsorption of carbon dioxide in the SIFSIX-3-M family of hybrid ultramicroporous materials (HUMs), where M = Zn²⁺, Ni²⁺, or Cu²⁺. These HUMs have been shown to exhibit exceptional affinity toward small polarizable molecules such as CO₂, and our simulated isotherms are in good agreement with those obtained experimentally. Isosteric heats of adsorption (Q_st) calculated using these theoretical methods also follow the experimentally observed trend, decreasing as M varies from Cu to Ni to Zn in SIFSIX-3-M. More specifically, the interaction energy between a CO₂ molecule and HUMs with varying pore-size was calculated using a DFT-D2 level of theory. The strongest interaction energy was calculated for SIFSIX-3-Cu (56.89 kJ mol^–1) where the pore-size is the smallest. The interaction energy decreased in SIFSIX-3-Ni (52.21 kJ mol^–1) and SIFSIX-3-Zn (48.46 kJ mol^–1), each of which exhibiting larger pore dimensions. The increase in the strength of the interaction in SIFSIX-3-Cu is attributed to the shorter distance between the negatively charged equatorial fluorine atoms of the SiF₆^2– pillar and the positively charged carbon atom of CO₂. Finally, DFT-D2 calculations were used to reconcile the uncertainty in locating the exact crystallographic position of equatorial fluorine atoms in the SIFSIX-3-M family. The calculated interaction energies agree best with experimental Q_st values when equatorial fluorine atoms are at a 45° angle with respect to the crystallographic a-axis. Our theoretical calculations thus create a foundation for first-principles based rational design of SIFSIX-3-M and other HUMs for selective adsorption of carbon dioxide and other relevant sorbates such as acetylene.