Structural and Hydrolytic Stability of Coordinatively Unsaturated Metal–Organic Frameworks M₃(BTC)₂ (M = Cu, Co, Mn, Ni, and Zn): A Combined DFT and Experimental Study

Abstract

Metal–organic framework (MOF) materials have shown great potential in numerous practical applications such as storage, gas separation, and heterogeneous catalysis, largely due to their versatile porous structures and functional tunability. However, most MOFs still suffer from low structural stability, particularly low hydrolytic stability in a humid environment. Herein, the structural and hydrolytic stabilities of coordinatively unsaturated, paddlewheel M3(BTC)2 (M = Cu, Co, Mn, Ni, and Zn) have been studied using density functional theory (DFT)-based simulations combined with experimental measurements. Ab initio molecular dynamics (AIMD) simulations of isostructural metal-substitution analogues of M3(BTC)2 show that the structural oscillation intensity relies heavily on the metal type, where the skeleton of Ni3(BTC)2 and Zn3(BTC)2 dramatically vibrates. The presence of water molecules further induces oscillations of all coordinatively unsaturated M–O bonds at the metal node to some extent, leading to the precursor state for the initial M–O bond breaking. For the hydrolytic breakdown of M3(BTC)2 that involves adsorption, substitution, and dissociation steps, DFT calculations indicate that water substitution is more facile than water dissociation. The hydrolytic stability trend is predicted as follows Cu3(BTC)2 > Co3(BTC)2 > Mn3(BTC)2 > Ni3(BTC)2 > Zn3(BTC)2, which is consistent with our experimental observation. Finally, it is also found that the spin state of metal plays an important role in the hydrolytic stability of Co3(BTC)2, whose higher spin states cause more facile hydrolytic breakdown.

Enhanced water-resistant performance of Cu-BTC through polyvinylpyrrolidone protection and its capture ability evaluation of methylene blue

“…Some similar reports about the instability of Cu-BTC in water can be found in literature. 27,28 Herein, two factors might be responsible for the enhanced water resistance performance of Cu-BTC. Firstly, during the synthesis process of Cu-BTC, Cu(II) and H 3 BTC were accordingly used as unsaturated coordination metal ions and ligand in the absence of PVP, which combined with each other and extended infinitely to form Cu-BTC with uncontrolled growth.…”

“…One of the most studied and representative cases, Cu-BTC [Cu 3 (BTC) 2 , BTC = 1,3,5-benzenetricarboxylate] also denoted as HKUST-1 or MOF-199, is a widely focused and developed metal-organic frameworks (MOFs). 1,2 Thanks to the advantages of tunable options, organic functionality, high thermal and mechanical stability, open metal sites in the skeleton, large pore sizes, high surface areas, and easy surface modification, Cu-BTC exhibited a very promising candidate for adsorptive removal of toxic compounds (e.g. methylene blue (MB)) from effluent wastewaters.…”

CO Oxidation over HKUST-1 Catalysts: The Role of Defective Sites

“…The pristine HKUST-1 catalyst could be degraded into the defective HKUST-DE1 model catalyst in the presence of water molecules. Water molecules attack the Cu–Cu metal node of HKUST-1, causing one of the organic linkers to detach and a hydroxyl group to dangle on one of the Cu 2+ sites (Figure c). , The calculated Gibbs free energy profile for CO oxidation over HKUST-DE1 is shown Figures and S4 with the singlet and triplet spin states. Because of the detachment of one organic linker, the Cu 2+ site at the Cu–Cu node is more open.…”

Metal Exchange Boosts the CO2 Selectivity of Metal Organic Frameworks Having Zn-Oxide Nodes

“…Cu–Cu paddlewheel MOFs are one of the most commonly examined M-MOFs in the literature. 17 H 2 uptake of M-BTC, where M = Co, Ni, Fe, and Zn, was experimentally compared with that of Cu-BTC and gravimetric uptakes were found to increase by 10, 58, 59, and 60% for Co, Ni, Fe, and Zn-BTC, respectively. 18 By postsynthetic partial metal substitution, O 2 /N 2 selectivities of Mn-, Fe-, and Co-exchanged Cu-BTC were experimentally found to be higher than the O 2 /N 2 selectivity of Cu-BTC in the order of Mn-BTC > Fe-BTC ≈ Co-BTC ≫ Cu-BTC at 77 K, which was supported with the difference in binding energies for O 2 and N 2 in M-MOFs calculated with density functional theory.…”

“…Cu–Cu paddlewheel MOFs are one of the most commonly examined M-MOFs in the literature . H 2 uptake of M-BTC, where M = Co, Ni, Fe, and Zn, was experimentally compared with that of Cu-BTC and gravimetric uptakes were found to increase by 10, 58, 59, and 60% for Co, Ni, Fe, and Zn-BTC, respectively .…”