Atomistic Insights into the Activation of Fe-Based Metal–Organic Framework Catalysts

To design efficient porous catalysts for selective light alkane oxidation¹ understanding H₂O and OH interactions at active sites in metal–organic frameworks (MOFs)² is essential. Spin-polarized first-principles DFT calculations were performed using the PBE exchange–correlation functional, with Grimme D3 dispersion corrections, as implemented in Quantum ESPRESSO to investigate dehydration and dehydroxylation in MIL100-(Fe), MIL88-(Fe), and Fe-substituted UiO-66. Single and multiple H₂O/OH configurations were studied to examine local Fe-O coordination environment and coverage dependent desorption processes related to the formation of open-metal-sites(OMSs)³. Stronger stabilization of the remaining coordinated water molecules causes sequential dehydration to become more endothermic at lower hydration levels. The thermodynamics of dehydration under activation conditions are influenced by entropy, as shown by the fact that ΔG values remain negative because favorable entropy contributions from H₂O release outweigh the increasing electronic energy cost. Weakly coordinated H₂O shows elongated Fe–O bonds compared to hydroxyl species, consistent with stronger OH coordination. Proton-assisted dehydroxylation via H₂O formation is exergonic compared to direct OH radical desorption, which is thermodynamically unfavorable. Charge-density difference and Bader Charge analyses show that electron redistribution toward Fe-O nodes makes Fe more positive while maintaining structural stability, offering fundamental insight into the creation of unsaturated Fe centers that are crucial in the catalytic oxidation of small hydrocarbons into high-value fuels.