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

Ahmad Ziaee, Drahomir Chovan, Matteo Lusi, John J. Perry, Michael J. Zaworotko, Syed A.M. Tofail

Research output: Contribution to journalArticlepeer-review

Abstract

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 (Qst) 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 SiF62- 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 Qst 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.

Original languageEnglish
Pages (from-to)3890-3897
Number of pages8
JournalCrystal Growth and Design
Volume16
Issue number7
DOIs
Publication statusPublished - 6 Jul 2016

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