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Comparing the mechanism and energetics of CO₂ sorption in the SIFSIX series
Summary:
The authors from the Department of Chemistry, University of South Florida developed five SIFSIX MOMs (SIFSIX-1-Cu, SIFSIX-2-Cu, SIFSIX-2-Cu-i, SIFSIX-3-Zn, SIFSIX-3-Cu) with tunable ultramicroporous structures and SiF₆²⁻ pillars, achieving benchmark CO₂ capture capacity and selectivity for flue-gas separations.
Background:
1. To address the challenge of efficient CO₂ capture, earlier studies focused on open-metal-site MOFs such as M-MOF-74; these showed high uptake but suffered from water instability and difficult regeneration.
2. The authors now propose the SIFSIX platform—pillared MOMs that exploit strong physisorption on SiF₆²⁻ ions while possessing saturated metal centers for enhanced hydrothermal stability.
Research Content:
1. Synthesis: The materials were crystallized hydrothermally by combining Cu²⁺ or Zn²⁺, linear N-donor ligands (4,4′-bipyridine, 4,4′-dipyridylacetylene, pyrazine) and Na₂SiF₆.
2. Characterizations:
1) BET and He-porosimetry gave pore volumes of 0.178–1.083 cm³ g⁻¹ and pore sizes tunable from 3.54 Å to 13.05 Å.
2) PXRD confirmed pcu topology; SEM revealed micrometer-sized crystals.
3) Low-pressure CO₂ isotherms at 273–298 K and Qst measurements (22–55 kJ mol⁻¹) agreed with GCMC simulations including explicit polarization.
3. Application: SIFSIX-3-Cu and SIFSIX-3-Zn reached saturation uptakes of ~2.7 mmol g⁻¹ below 0.2 atm with record CO₂/N₂ and CO₂/CH₄ selectivities.
4. Mechanism: Smaller pores increase Qst via simultaneous interaction with four equatorial F atoms; interpenetration in SIFSIX-2-Cu-i promotes cooperative CO₂–CO₂ T-shape packing, while larger pores in SIFSIX-2-Cu favor high-pressure storage.
Outlook:
The SIFSIX series offers a scalable, water-stable platform whose pore chemistry can be further tuned by metal/ligand substitution or pillar exchange (e.g., TiF₆²⁻), promising next-generation sorbents and membranes for industrial carbon capture.
Comparing the mechanism and energetics of CO₂ sorption in the SIFSIX series
Authors: Katherine A. Forrest, Tony Pham, Brian Space
DOI: 10.1039/c7ce00594f
Link: https://pubs.rsc.org/en/content/articlelanding/2017/ce/c7ce00594f
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