Fe-MOF-74: 1D Magnetic MOF for Adsorption

• Fe-MOF-74 is a metal–organic framework characterized by quasi-linear chains of Fe centers, providing open Lewis-acid sites for gas adsorption and distinct 1D magnetic properties.
• Density functional theory reveals strong ferromagnetic coupling along the Fe chains (J_NN ~28.1 cm⁻¹) with weaker inter-chain antiferromagnetic interactions, underpinning its 1D Ising model behavior.
• Its open Fe centers exhibit selective adsorption energies—water binds strongly, impacting CO₂ capture performance and highlighting challenges in water-induced structural stability.

Fe-MOF-74 is a member of the MOF-74 family—a class of metal–organic frameworks characterized by quasi-linear arrays of open-shell transition metal ions coordinated by organic linkers, yielding highly periodic, one-dimensional channels. When Fe is incorporated as the metal center, unique properties arise, impacting magnetism, adsorption phenomena, redox behavior, water stability, thermal expansion, and potential applications in catalysis, gas separation, and low-dimensional quantum materials. The versatility of Fe-MOF-74 stems from its predictable crystalline topology, tunable chemistry via metal and linker selection, and the interplay of local electronic states with guest molecules.

1. Crystalline Structure and One-Dimensionality

Fe-MOF-74 is constructed via Fe(II) ions coordinated by the linker 2,5-dihydroxyterephthalic acid (DOBDC), generating quasi-linear chains of Fe centers. These chains are aligned along the framework’s channel axis and separated by lengthy organic linkers, which spatially isolate magnetic and electronic interactions within the chains. The periodicity and orientation of the channels produce highly anisotropic physical properties (Canepa et al., 2013). The local environment at each Fe site is square-pyramidal, with coordination unsaturation providing accessible Lewis-acid centers for guest adsorption. The architecture is retained across metal substitutions (Co, Ni, Mg, Zn, etc.), enabling systematic comparative studies.

2. Magnetism and Model Hamiltonians

First-principles density functional theory analysis demonstrates that Fe-MOF-74 is an exemplary quasi-one-dimensional magnet. Magnetic coupling is strong and ferromagnetic along Fe chains (nearest-neighbor $J_{\rm NN} \approx 28.1$ cm$^{-1}$), while next-nearest-neighbor intra-chain coupling ($J_{\rm NNN}$) is weaker (6.0 cm$^{-1}$), and inter-chain coupling ($J_{\rm I-I}$) is antiferromagnetic and much smaller ($\approx -1.2$ cm$^{-1}$) (Canepa et al., 2013). The magnetism is well described using an Ising-model Hamiltonian: $$\hat{H} = -2 \sum_{i,j} J_{ij}\, \hat{S}^z_i \cdot \hat{S}^z_j$$ where $\hat{S}^z_i$ represents the z-component spin operator at Fe site $i$. This formalism, supported by DFT-calculated coupling constants, allows exact mapping onto 1D theoretical treatments and experimental susceptibility ($\chi_M$) analysis. The ratio $|J_{\rm NN}|/|J_{\rm I-I}| \gg 1$ confirms ideal 1D magnetic character. Fe-MOF-74 thus provides a simple, experimentally accessible realization of the 1D Ising model, with transitions between ferromagnetic (intra-chain) and antiferromagnetic (inter-chain) order occurring at low but accessible temperatures.

3. Gas Adsorption: Energetics and Selectivity

The open Fe(II) centers act as strong adsorption sites, exhibiting well-defined energetics for small molecules (Canepa et al., 2013). The calculated adsorption energies (ΔE, kJ mol$^{-1}$) for representative gases are:

Adsorbate	ΔE (kJ mol$^{-1}$)	ΔEᶜ (kJ mol$^{-1}$)	δE$_{\rm MOF}$ (kJ mol$^{-1}$)	δE$_{\rm M}$ (kJ mol$^{-1}$)
H$_2$	–19.8	–19.1	–0.2	–0.4
CO$_2$	–51.2	–47.2	1.4	–5.4
CH$_4$	–39.8	–35.4	1.5	–5.9
H$_2$O	–129.7	–163.8	30.1	–4.0

Water binds most strongly, followed by CO$_2$, CH$_4$, and H$_2$: H$_2$O $\gg$ CO$_2$ > CH$_4$ > H$_2$. Water’s strong adsorption often results in competitive “site poisoning,” reducing available sites for CO$_2$ and H$_2$ and compromising gas uptake, especially in humid environments. The deformation energies ($\delta E_{\rm MOF}$, $\delta E_{\rm M}$) reveal that water adsorption induces significant rearrangement of the framework, which can be a precursor to structural instability.

Fe-MOF-74 is competitive with other MOF-74 variants for hydrogen storage from a binding-energy perspective. For carbon capture, the moderate CO$_2$ adsorption energy is favorable, but compromised by water’s stronger competitive adsorption. High-throughput computational screening further distinguishes Fe-MOF-74 from noble-metal MOFs (Rh, Pd, Ir, Os, Pt), which exhibit greater CO$_2$ selectivity.

4. CO$_2$ Capture: Thermodynamics and Metal–Adsorbate Coupling

Thermodynamic screening with van der Waals density functional theory establishes that the room-temperature CO$_2$ adsorption enthalpy for Fe-MOF-74 (Fe-DOBDC) is –32.4 kJ mol$^{-1}$ (Koh et al., 2016). This is weaker than ideal MOFs (Mg-, Ca-, Sc-DOBDC, etc.) that fall within the target window (–40 to –75 kJ mol$^{-1}$) for practical carbon capture and regeneration. The diminished CO$_2$ binding arises from a lower partial charge on Fe centers ($+0.004$ e/Å$^3$) and corresponding weak polarization of CO$_2$—as compared to Mg-DOBDC ($+0.011$ e/Å$^3$). A direct correlation exists between partial charge and enthalpy, making the former a reliable descriptor for screening MOF performance. Consequently, Fe-MOF-74 is less optimal than its more electropositive analogues for flue gas CO$_2$ removal at low pressures.

5. Redox Chemistry and Metal–Insulator Transitions

Fe-MOF-74 exhibits redox activity at its iron centers. Fe(II) can be oxidized to Fe(III), mediating charge transfer to guest molecules—most notably O$_2$—and forming superoxo (O$_2^{-}$) species via: $$\mathrm{Fe}^{2+} \rightarrow \mathrm{Fe}^{3+} + e^{-}$$

$\mathrm{O}_2 + e^- \rightarrow \mathrm{O}_2^-$

This process can induce or drive quasi-1D metal–insulator transitions (MIT), where charge localization/delocalization alters electronic conductivity (Maximoff et al., 2013). The framework acts as an electron donor semiconductor, with band gap narrowing or closure upon electron transfer to O$_2$ or through doping. Such mechanisms are phenomenologically and computationally validated, and have implications for gas-selective adsorption, catalytic activity, and the development of nano-porous materials that intertwine fundamental chemistry with MIT physics.

6. Water Dissociation, Structural Stability, and Passivation

Water dissociation in Fe-MOF-74 follows: $$\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{OH} + \mathrm{H}$$ Adsorbed water at open Fe centers undergoes thermally activated splitting ($T>150^{\circ}$C). The resulting OH group binds to Fe, while H migrates to an adjacent linker oxygen, confirmed via the appearance of a sharp IR peak at 970 cm$^{-1}$ with D$_2$O (Tan et al., 2014). This mechanism blocks active adsorption sites—leading to passivation—and reduces capacity for further uptake, e.g., CO$_2$ ($\sim$60% decrease). The products also weaken Fe–O (linker) bonds, potentially causing crystal lattice breakdown in humid conditions (Zuluaga et al., 2016).

Cluster-assisted pathways further lower the water dissociation barrier (by $\sim$37%), accelerating degradation. Introduction of inert gases (He) disrupts water cluster formation, thereby suppressing dissociation and providing a route to stabilize MOF-74 frameworks (Zuluaga et al., 2016). Enhanced stability through linker modification (e.g., O$\rightarrow$S substitution) or mixed-metal strategies can increase the activation barrier.

7. Thermal Expansion and Phonon Anisotropy

The thermal expansion of MOF-74 is anomalously small and anisotropic—a consequence of compensatory effects between negative tensor coefficients (contraction in the xy plane) and positive coefficients (expansion along channel axes) (Kamencek et al., 2022). This macroscopic behavior arises from summing individual mode contributions via Grüneisen theory: $$Y^{(\lambda)}_{ij} = -\frac{1}{\omega^2_\lambda} \frac{\partial \omega^2_\lambda}{\partial \varepsilon_{ij}}$$

$$\alpha_{ij}(T) = \frac{1}{V} \sum_\lambda c_{v,\lambda}(T) [S_{ijkl} Y^{(\lambda)}_{kl}]$$

Only the lowest-frequency phonons ($<$3 THz) matter; higher modes contribute negligibly at typical operating temperatures. Accurate computation requires high-order finite-difference schemes for dynamical matrix derivatives and rigorous convergence with respect to supercell and $k$-point mesh. These features yield the observed compensation and ensure mechanical integrity of Fe-MOF-74 over a range of conditions.

8. Two-Dimensional Fe-Based MOF Realizations

Recent theoretical studies of Fe$_3$(C$_6$X$_6$)$_2$ (X = O, S, Se) MOF monolayers have shown them to be thermally stable (up to 1500 K), linearly elastic, and “magnetic semiconductors” (Mortazavi et al., 2019). For Fe-based systems, spin polarization is zero at the Fermi level, yet the magnetic moment per Fe atom remains high ($\approx$2 μ$_B$). The interplay of Fe 3d orbitals with chalcogens and carbon underpins this behavior. These monolayers are promising for robust integration into energy storage, nanoelectronic, and spintronic devices where both charge and spin can be manipulated.

9. Significance and Prospects

Fe-MOF-74 exemplifies how a single framework chemistry can produce coupled functionalities: 1D quantum magnetism, selective adsorption, tunable redox properties, and resilience/downfall in the face of water. Its ease of synthesis, structural uniformity, and amenability to theoretical modeling position it as a prototypical material for fundamental paper and applied research. Persistent challenges in water stability and competitive adsorption motivate ongoing design efforts—both at the metal center and linker—toward frameworks with maintained performance under realistic operating conditions. The synergy between computational screening, advanced spectroscopy, and materials engineering continues to drive refinement of Fe-MOF-74 for diverse technological applications.