Fluorinated Metal-Organic Frameworks (FMOFs): A Scientific Application Guide

How Do Fluorinated MOFs Differ from Conventional MOFs?

Fluorinated metal-organic frameworks (FMOFs) are crystalline porous materials where fluorine atoms are introduced into the organic linkers or inorganic nodes. The introduction of highly electronegative fluorine atoms significantly alters the physicochemical environment on the internal pore surfaces of the MOF compared to their traditional analogues. The inherent fluorination effect promotes hydrophobicity, enhanced chemical stability, particularly in aqueous media, and tunable electrostatic fields within the pore system. Collectively, these enhancements afford FMOFs with superior performance in areas such as gas separation, chemical sensing, and adsorption in extreme environments.

Fig.1 FMOFs have many superior properties compared to non-fluorinated MOFs^[1].

Fluorine atoms embedded in the linkers induce electropositive characteristics on adjacent carbon atoms, thus enhancing interactions with electron-rich guest molecules. Simultaneously, the fluorine-lined pores establish unique adsorption sites and improve host–guest compatibility via hydrogen bonding, van der Waals forces, and π–π stacking.

How Does Fluorination Enhance Hydrophobicity and Stability?

FMOFs often have exceptional hydrophobicity, which depends greatly on the presence and distribution of fluorine atoms within the pores. Contact angle measurements often exceed 150°, classifying many FMOFs as superhydrophobic. For instance, The contact angle of water for FMOFs built with perfluorinated biphenyltetrazole ligands is greater than 150°, showing high efficiency in oil–water separation.

Hydrophobicity of FMOFs can be tuned by presynthetic ligand design or postsynthetic modification. In the latter approach, some frameworks, such as DUT-67 (a MOF built with zirconium clusters and thiophenedicarboxylate ligands), are robust enough for ionic exchange with fluorinated carboxylates to render them more hydrophobic without losing crystallinity. The framework remains stable in water, allowing it to be used in the remediation of oil spills and adsorptive refrigeration.

Fig.2 (a) Comparison of the water contact angles of different fluorinated and non-fluorinated DUT-67-type MOFs and (b) the corresponding water–adsorption isotherms at room temperature^[2].

What Are the Key Applications of FMOFs in Environmental Remediation?

FMOFs are promising in the field of environmental remediation for oil spill cleanups and gas capture/separation. FMOFs can selectively adsorb non-polar hydrocarbons such as hexane, toluene, and xylene from aqueous solutions due to their inherent hydrophobic properties. FMOFs have an advantage over traditional absorbents because they do not have water intrusion and can be reused in the presence of moisture.

Gas adsorption and separation is another application area for FMOFs. They have high selectivity for CO₂, CH₄, and light hydrocarbons due to their tunable pore size and the high polarizability of the fluorine atoms. The chemical stability and the ability to selectively adsorb gases even in the presence of humidity make FMOFs suitable for applications such as flue gas purification and air quality monitoring.

Fig.3 Single crystal structural differences (a), hydrogen uptake comparison (b), and H2 adsorption isotherms (c) between fluorinated and non-fluorinated MOFs^[3].

How Are FMOFs Utilized in Sensing and Detection?

FMOFs are emerging as multifunctional platforms in chemical sensing, particularly fluorescence-based detection. By incorporating fluorinated ligands, lanthanide- or zinc-based MOFs have been demonstrated to sense small molecules, metal ions, or gases with excellent selectivity.

For example, a metal-organic framework known as LIFM-100, which was synthesized with the trifluorinated triphosphonic acid ligand and La³⁺, can serve as a crystalline sponge. The material immobilizes VOCs in the channels by strong host–guest interactions. It was confirmed that the guest molecules were orderly dispersed by single-crystal X-ray diffraction.

Fig.4 Synthesis of LIFM-100 and its corresponding fluorinated ligands, crystal structures, and guest molecule-backbone interactions using Cu(II)^[4].

Moreover, ESIPT (Excited-State Intramolecular Proton Transfer) active fluorinated ligands have been employed to fabricate FMOFs with dual emission behavior. These materials can respond to external stimuli such as water vapor, pH, or temperature, making them ideal for reversible sensors. A Zn-based FMOF using such a ligand shows reversible photoluminescent color change upon hydration/dehydration within seconds, allowing for rapid humidity sensing.

What Role Do FMOFs Play in Toxic Gas Detection?

FMOFs are uniquely suited to detect low concentrations of hazardous gases such as H₂S and SO₂, even under humid conditions. In the case of H₂S detection, an Fe(III)-based FMOF incorporating fluorinated dicarboxylic acid ligands was shown to interact with H₂S via redox mechanisms. ¹⁹F NMR revealed changes in fluorine's magnetic environment following Fe(III) reduction to Fe(II), confirming gas interaction at the molecular level.

Similarly, KAUST-7 and KAUST-8—nickel-based FMOFs—exhibit high selectivity for SO₂ due to optimal F···F distances and tailored pore environments^[5]. Single-crystal XRD revealed specific SO₂ binding sites within the fluorinated channels, where electrostatic interactions between F atoms and sulfur centers drive selectivity. These frameworks outperform traditional SO₂ sorbents like zeolites by enabling reversible capture and regeneration.

Table: Comparison of Selected FMOFs for Key Applications

FMOF Name	Ligand Type	Metal Node	Key Application	Notable Feature
LIFM-100	Trifluorinated triphosphonate	La3+	VOC detection (crystalline sponge)	Host–guest order validated by SCXRD
DUT-67 (modified)	Thiophene-2,5-dicarboxylate (fluorinated)	Zr4+	Hydrophobic adsorbent, heat exchange	Post-synthetic functionalization
KAUST-8	Perfluorinated inorganic pillar	Al3+/Ni2+	SO2 detection under humid conditions	Optimal F···F spacing, reversible capture
Zn–FMOF	ESIPT-active fluorinated ligand	Zn2+	Humidity-responsive fluorescence sensor	Dual emission switching within seconds
Ln–FMOF	Fluorinated imidazole-dicarboxylate (H2hpi2cf)	Eu3+, Tb3+	Cu2+ and small molecule sensing	Tunable f–f emission, enhanced sensitivity

Frequently Asked Questions (FAQs)

Q1: Can FMOFs be used for oil–water separation in marine environments?

Yes, due to their superhydrophobic pore surfaces and high structural stability in aqueous environments, FMOFs are highly efficient at adsorbing oil components while repelling water.

Q2: What makes FMOFs better than conventional MOFs in toxic gas sensing?

The fluorinated framework enhances specific interactions with polarizable gases like SO₂ and H₂S, improving selectivity and sensitivity, particularly under humid conditions.

Q3: Are FMOFs stable in acidic or basic environments?

Many FMOFs, especially those with zirconium or rare-earth metal nodes, exhibit excellent stability in a wide pH range, although this should be evaluated case-by-case.

Q4: How fast do FMOF-based sensors respond to humidity changes?

Some FMOFs, such as Zn-based ESIPT-responsive materials, show response times as fast as a few seconds, making them suitable for real-time environmental monitoring.

Q5: Do FMOFs have potential in biomedical applications?

Although not the primary focus, the selective adsorption and biocompatibility of certain FMOFs are under investigation for drug delivery and bioimaging applications.

References

1. Kumar S., et al. (2023). "Decoration and utilization of a special class of metal–organic frameworks containing the fluorine moiety." Coordination Chemistry Reviews. 476, 214876.
2. Drache F., et al. (2016). "Postsynthetic inner-surface functionalization of the highly stable zirconium-based metal-organic framework DUT-67." Inorg. Chem. 55, 7206-7213.
3. Pachfule P., et al. (2012). "Fluorinated metal-organic frameworks: advantageous for higher H2 and CO2 adsorption or not?" Inorg. Chem. 18, 688-694.
4. Qiu Q., et al. (2019). "A flexible Cu-MOF as crystalline sponge for guests determination." Inorg. Chem. 58, 61-64.
5. Tchalala M.R., et al. (2019). "Fluorinated MOF platform for selective removal and sensing of SO2 from flue gas and air." INat. Commun. 10, 1328.