Hydrophobic pillared square grids for selective removal of CO₂ from simulated flue gas

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Summary:
The authors from Alexandria University, PNNL, USF, and University of Limerick developed hydrophobic ultramicroporous SIFSIX-3-M (M = Ni, Co) pillared square-grid MOFs with tuned pore size (~3.3–3.7 Å), achieving >50× CO₂/N₂ selectivity and stable CO₂ capture from wet simulated flue gas.

Background:
1. To mitigate CO₂ emissions, amine scrubbing and conventional MOFs have been tested, yet suffer from high energy penalty (25–40 % heat loss), corrosion, and poor moisture stability.
2. The authors proposed metal substitution within the inexpensive SIFSIX-3 platform to fine-tune pore size and electrostatics, obtaining robust Ni/Co analogues that retain performance under 50 % RH.

Research Content:
1. Synthesis: Solvothermal reaction of MSiF₆ (M = Ni, Co) with pyrazine in MeOH at 85 °C for 3 days.
2. Characterizations:
   1) BET (77 K N₂): 223 m² g⁻¹ (Co), 368 m² g⁻¹ (Ni); ultramicropores 3.3–3.7 Å.
   2) PXRD and Rietveld refinement confirm pcu topology; SEM reveals micron-sized crystals.
   3) In-situ PXRD under CO₂ shows slight lattice contraction and CO₂ centering along c-axis.
3. Application: Column breakthrough (15 % CO₂/85 % N₂, 1 bar, RT) shows capacities 2.54–2.65 mol kg⁻¹; breakthrough times and capacities unchanged under 50 % RH.
4. Mechanism: GCMC and DFT reveal electrostatic interaction between SiF₆²⁻ fluorines and CO₂ quadrupole; shorter a/b lattice parameters (Cu < Ni < Co < Zn) correlate with higher Qst (51 kJ mol⁻¹ for Ni).

Outlook:
This work delivers low-cost, water-tolerant MOFs that outperform amine-functionalized and open-metal-site sorbents for post-combustion CO₂ capture, guiding rational metal substitution for pore engineering.

Hydrophobic pillared square grids for selective removal of CO₂ from simulated flue gas
Authors: Sameh K. Elsaidi, Mona H. Mohamed, Herbert T. Schaef, Amrit Kumar, Matteo Lusi, Tony Pham, Katherine A. Forrest, Brian Space, Wenqian Xu, Gregory J. Halder, Jun Liu, Michael J. Zaworotko, Praveen K. Thallapally
DOI: 10.1039/c5cc06577a
Link: https://pubs.rsc.org/en/content/articlelanding/2015/cc/c5cc06577a

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