Characterization of metal-organic frameworks by water adsorption

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Summary：
The authors fromDresden University of Technology and Mahle Filtersysteme GmbH developed a series of metal-organic frameworks (MOFs) including HKUST-1, ZIF-8, MIL-101, MIL-100(Fe), and DUT-4, characterized by distinct water adsorption properties and stability. The study achieved comprehensive insights into the hydrophobicity/hydrophilicity and moisture stability of these materials, providing critical data for applications in trace water removal and molecular sensing.

Background：
1.Previous Work & Limitations: While MOFs have been widely studied for gas storage and separation, their water stability and adsorption behavior remain underexplored. Existing nitrogen adsorption data fail to reveal hydrophobicity or moisture-induced degradation, limiting industrial deployment.
2.Innovation: This work pioneers a comparative analysis of water vs. nitrogen adsorption isotherms, introducing the "bridging effect" to explain pore volume underestimation in hydrophilic MOFs and correlating framework chemistry with water stability.

Research Content：
1.Synthesis:
   - HKUST-1: Cu(NO₃)₂ + H₃BTC via solvothermal synthesis.
   - MIL-101/100(Fe): Cr/Fe salts + dicarboxylic acids with HF-assisted hydrothermal synthesis.
   - DUT-4: Al(NO₃)₃ + naphthalene dicarboxylate in DMF.
   - ZIF-8: Commercially sourced.
2.Characterizations:
   1)BET & Pore Analysis: HKUST-1 (1340 m²/g, 0.72 cm³/g), ZIF-8 (1255 m²/g, 0.64 cm³/g), MIL-101 (3017 m²/g, 1.61 cm³/g).
   2)Water Adsorption: HKUST-1 showed highest affinity (0.55 cm³/g) but degraded in water; ZIF-8 was hydrophobic (0.02 cm³/g); MIL-series exhibited balanced hydrophilicity (0.81–1.28 cm³/g) with excellent stability.
   3)XRD Stability Tests: Confirmed MIL-series and ZIF-8 retained crystallinity after 24h in 323 K water, while HKUST-1 and DUT-4 decomposed.
3.Application Testing:
   - HKUST-1: Suitable for trace water sensing despite instability.
   - MIL-100(Fe)/101: Promising for high-humidity adsorption due to mesopores and stability.
   - ZIF-8: Ideal for moisture-resistant coatings.
4.Mechanism:
   -Bridging Effect: Water clusters form on hydrophilic metal sites, leaving voids in hydrophobic regions, reducing apparent pore volume.
   -Stability Correlation: Fe/Cr-O bonds in MIL-series resist hydrolysis; open Cu sites in HKUST-1 promote irreversible water coordination.

Outlook：
This study establishes water adsorption as a critical complement to nitrogen physisorption for MOF characterization, guiding the design of moisture-stable frameworks for industrial gas drying, humidity sensing, and protective coatings.

Characterization of metal-organic frameworks by water adsorption
Authors: Pia Küsgens, Marcus Rose, Irena Senkovska, Heidrun Fröde, Antje Henschel, Sven Siegle, Stefan Kaskel
DOI: 10.1016/j.micromeso.2008.11.020
Link: https://www.sciencedirect.com/science/article/pii/S1387181108006100

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