Acid-functionalized UiO-66(Zr) MOFs and their evolution after intra-framework cross-linking: structural features and sorption properties

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Summary:
The authors from Institut Lavoisier, ENSICAEN, and partner institutions developed acid-functionalized UiO-66(Zr) MOFs with free –COOH groups, achieving reversible intra-framework anhydride cross-linking and enhanced CO₂/CH₄ selectivity for gas separation and proton-conduction applications.

Background:
1. Prior work functionalized MOFs with –SO₃H or –COOH, but direct incorporation of free carboxylic acids remains scarce because they can coordinate metal ions and disrupt framework formation; post-synthetic routes often give incomplete or inhomogeneous modification.
2. The team now proposes a green, water-based one-pot synthesis using 1,2,4-BTC or 1,2,4,5-BTeC linkers with ZrCl₄, yielding crystalline UiO-66(Zr)-(COOH)ₓ that can be thermally cross-linked into anhydride-bridged networks without losing porosity.

Research Content:
1. Synthesis:
   Reflux of ZrCl₄ and polycarboxylic acid (H₃BTC or H₄BTeC) in pure water at 100 °C for 24 h; scale-up to 40 g demonstrated.
2. Characterizations:
   1) BET: 661 → 443 m² g⁻¹ (UiO-66-(COOH)) and 527 → 332 m² g⁻¹ ((COOH)₂) upon 30–190 °C activation; micropore volume 0.26 → 0.16 cm³ g⁻¹.
   2) In-situ EDXRD & PXRD show <30 min crystallization at 150 °C; frameworks stable to 200–250 °C.
   3) FTIR & ¹³C CP-MAS NMR confirm free –COOH (1712 cm⁻¹) converts to anhydride (1787 & 1852 cm⁻¹) at 100–200 °C; DFT models agree.
3. Application:
   CO₂ uptake at 30 °C/10 bar rises from 3.0 (30 °C outgas) to 3.5 mmol g⁻¹ (150 °C outgas) despite N₂-BET drop; CO₂/CH₄ selectivity enhanced. Proton conductivity of hydrated samples reaches 5 × 10⁻⁶ S cm⁻¹ at 25 °C, ten-fold higher than pristine UiO-66.
4. Mechanism:
   Adjacent –COOH groups H-bond (2640 cm⁻¹), dehydrate upon heating to form –CO–O–CO– bridges that rigidify triangular windows, reducing low-T N₂ accessibility but leaving room-temperature gas diffusion and Brønsted acidity intact; process reversibly hydrolyses back to –COOH under moist air.

Outlook:
The study delivers a scalable, eco-friendly route to robust carboxylic-acid Zr-MOFs whose reversible anhydride cross-linking modulates acidity, proton transport and gas-separation performance, offering a versatile platform for stimuli-responsive porous materials.

Acid-functionalized UiO-66(Zr) MOFs and their evolution after intra-framework cross-linking: structural features and sorption properties
Authors: Florence Ragon, Betiana Campo, Qingyuan Yang, Charlotte Martineau, Andrew D. Wiersum, Ana Lago, Vincent Guillerm, Callum Hemsley, Jarrod F. Eubank, Muthusamy Vishnuvarthan, Francis Taulelle, Patricia Horcajada, Alexandre Vimont, Philip L. Llewellyn, Marco Daturi, Sabine Devautour-Vinot, Guillaume Maurin, Christian Serre, Thomas Devic, Guillaume Clet
DOI: 10.1039/C4TA03992K
Link: https://pubs.rsc.org/en/content/articlelanding/2015/ta/c4ta03992k

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