Using Postsynthetic X‐Type Ligand Exchange to Enhance CO₂ Adsorption in Metal–Organic Frameworks with Kuratowski-Type Building Units

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Summary:
Authors from The Ohio State University developed aniline-exchanged CFA-1 MOFs (1-NHPhx) that chemisorb CO₂ via Zn–NHPh sites, achieving 3.3 mmol mmol⁻¹ low-pressure uptake (1 mbar, 300 K) and 83 kJ mol⁻¹ binding energy—30 % higher than parent Zn–OH analogues.

Background:
1. Trace CO₂ capture demands strong yet reversible chemisorption; prior Zn–OH MOFs show moderate capacity and require cooperative H-bonding.
2. The team introduced postsynthetic X-type ligand exchange (PXLE) to replace Zn–OH with Zn–NHPh, creating stronger nucleophilic sites while preserving framework integrity.

Research Content:
1. Synthesis: CFA-1 (1-OH) was treated with aniline (0.25–5 eq) in 1,4-dioxane, 25 °C, 24–72 h → 1-NHPh0.9 & 1-NHPh2.5; MFU-4l analogue 2-NHPh prepared similarly.
2. Characterizations:
   – BET: 2005–2816 m² g⁻¹ (microporous, 0.8 nm pores).
   – PXRD/SEM: crystallinity & triangular particles retained; no aggregation.
   – DRIFTS: ν(N–H) 3391→3462 cm⁻¹ shift confirms Zn–O₂CNHPh carbamate; two distinct species (isolated & H-bonded) detected.
3. Application: 1-NHPh2.5 gives 3.31 mmol mmol⁻¹ CO₂ at 1 mbar vs 2.58 for 1-OH; 2-NHPh shows 0.95 mmol g⁻¹ and 65 kJ mol⁻¹ Qst—stable over 5 cycles.
4. Mechanism: DFT shows Zn–O₂CNHPh formation ΔH = 82.9 kJ mol⁻¹ vs 47.5 kJ mol⁻¹ for Zn–O₂COH; inter-cluster H-bonding in 1-NHPh2.5 further stabilizes CO₂, enabling reversible 150 °C regeneration.

Outlook:
PXLE on Kuratowski nodes offers a general, mild route to install tunable X-ligands for next-gen MOF sorbents that surpass traditional hydroxide chemistries in both capacity and stability.

Using Postsynthetic X‐Type Ligand Exchange to Enhance CO₂ Adsorption in Metal–Organic Frameworks with Kuratowski-Type Building Units
Authors: Caitlin E. Bien, Zhongzheng Cai, Casey R. Wade
DOI: 10.1021/acs.inorgchem.1c01077
Link: https://doi.org/10.1021/acs.inorgchem.1c01077

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