H₂O Adsorption/Desorption in MOF-74:Ab InitioMolecular Dynamics and Experiments

Abstract

Using density functional theory based molecular dynamics with the tight-binding approximation and experimental characterization, the H 2 O vapor adsorption/desorption processes in Zn-MOF-74 and the corresponding influence on the lattice dynamics were investigated. It was found that the Zn sites are preferred for H 2 O adsorption and they even allow for the adsorption of multiple H 2 O molecules on the same site, making it possible to form stable H 2 O cluster at low temperatures. The adsorption heats of different adsorption processes were predicted and the possibility of H 2 O cluster formation under vapor exposure at low and room temperatures were explored. The first adsorbed H 2 O can only be dissociated at elevated temperatures, therefore reducing the cyclic uptake near the room temperature. The vibrational spectra of the lattice and adsorbed H 2 O were also calculated using molecular dynamics simulation to illuminate the variation of lattice dynamics during the adsorption. The framework was found to be stable after water vapor exposure near room temperature but may start to collapse at around 570 K. X-ray diffraction and H 2 O adsorption isotherm measurements have been conducted to verify the theoretical predictions and good agreements are found.

A Tunable Bimetallic MOF‐74 for Adsorption Chiller Applications

“…Unlike the CO 2 capacity, H 2 O adsorption did not decrease significantly as the content of Ni decreased (Zn increased). Crystalline defects such as intergrowth have previously been thought to limit access to open metal sites, especially in the Zn‐MOF‐74 analog , . Thus, from the results shown in Figure a and 2b, it is speculated that the presence of even a small fraction of Ni atoms in a primarily Zn‐containing MOF‐74 structure may suppress this intergrowth within the crystals.…”

“…The adsorption of water in pure Ni‐MOF‐74 (Ni 100 Zn 0 ) and Zn‐MOF‐74 (Ni 0 Zn 100 ) samples was also consistent with prior literature. [9a], Interestingly, apart from Ni 0 Zn 100 , the isotherms were much more closely packed. Unlike the CO 2 capacity, H 2 O adsorption did not decrease significantly as the content of Ni decreased (Zn increased).…”

Experimental and Theoretical Evaluation of the Stability of True MOF Polymorphs Explains Their Mechanochemical Interconversions

“…It is known that commonly used DFT functionals underestimate the energies of van der Waals interactions, 65 potentially causing errors in calculated lattice energies and geometrical parameters of MOFs. 50,[66][67][68][69] Correct description of dispersion forces was found to be crucial for modelling guest absorption in MOFs [70][71][72][73][74][75] and for deriving their mechanical properties. 76,77 While functionals tailored to reproduce van der Waals forces have been developed 78,79 and applied to ZIF systems, 80 a more widely adopted approach for modelling them remains the semi-empirical dispersion correction (SEDC) in combination with one of the available general gradient approximation (GGA) or hybrid functionals.…”

Controlling Chemical Reactions in Confined Environments: Water Dissociation in MOF-74

“…It is thus the presence of additional water molecules that stabilizes the product state and makes the reaction observable. Note that experimentally, it is very difficult to determine the exact amount of water molecules inside the MOF pore and nearby the reaction site, but it is estimated based on isotherm measurements corrected to the higher temperature (180 • C), that 8 Torr of water result in at least 6 to 8 water molecules in the MOF unit cell (calculations based on the ideal gas law) [42].…”

“…Comparing the two pathways that obstruct the reaction, a schematic of the reaction mechanism of IPA/water is presented in Figure 3b. The IPA blocks the reaction by its interaction with the metal center, requiring a lower pressure to occupy all metal centers in the unit cell [42]. On the contrary He/water reaction mechanism presented in Figure 3d illustrates how adding guest molecules physically prevent the formation of organized water clusters, hence reducing water dissociation reaction at higher pressures.…”