Enhanced Selectivity of CO₂ over CH₄ in Sulphonate-, Carboxylate- and Iodo-Functionalized UiO-66 Frameworks

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Summary:
The authors from Ghent University, Dalian Institute of Chemical Physics (Chinese Academy of Sciences), and Augsburg University developed three functionalized UiO-66-X (X=-SO₃H, -CO₂H, -I) frameworks with high thermal/chemical stability and enhanced CO₂/CH₄ adsorption selectivity, achieving promising results in biogas purification.

Background:
1. To address the issues of insufficient CO₂ capture performance of traditional adsorbents and poor physicochemical stability of some metal-organic frameworks (MOFs), previous researchers studied UiO-66 (high stability) but faced instability of UiO-66-SO₃H after guest molecule removal.
2. The authors proposed an improved solvothermal synthesis method, obtained stable UiO-66-SO₃H and other functionalized UiO-66, with higher CO₂/CH₄ selectivity than unfunctionalized UiO-66.

Research Content:
1.Synthesis: The authors synthesized the three UiO-66-X materials via solvothermal method, using different Zr salts (ZrCl₄, ZrOCl₂·8H₂O, ZrO(NO₃)₂·xH₂O), BDC-X linkers, and modulators in polar amide solvents, followed by activation via solvent exchange and vacuum heating.
2.Characterizations:
   1) BET: Specific Langmuir surface areas were 769, 842, 799 m²/g for X=-SO₃H, -CO₂H, -I respectively; micropore volumes 0.26, 0.29, 0.27 cm³/g.
   2) SEM/TEM: Not mentioned in the documents.
   3) Other tests: XRPD confirmed isostructure with UiO-66; TGA showed stability up to 260, 340, 360℃; DRIFT verified functional groups.
3.Application: Tested in CO₂/CH₄ adsorption; at 25 bar, 33℃, CO₂ uptakes were 5.6, 6.4, 5.1 mmol/g, with ideal selectivities 4.5-5.2 (higher than UiO-66’s 2.6).
4.Mechanism: CO₂ adsorption is governed by functional group polarity, CH₄ by pore size; polar groups enhance CO₂ affinity, improving selectivity.

Outlook:
This research provides stable functionalized UiO-66 materials for CO₂ separation, verifies theoretical predictions, and lays a foundation for biogas/landfill gas purification applications.

Enhanced Selectivity of CO₂ over CH₄ in Sulphonate-, Carboxylate- and Iodo-Functionalized UiO-66 Frameworks
Authors: Shyam Biswas, Jian Zhang, Zhibao Li, Ying-Ya Liu, Maciej Grzywa, Lixian Sun, Dirk Volkmer, Pascal Van Der Voort
DOI: 10.1039/c3dt32288b
Link: https://pubs.rsc.org/en/content/articlelanding/2013/dt/c3dt32288b

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