Modifying the hydrophobic nature of MAF-6

Using a combination of molecular simulations techniques, we evaluate the structural tunability of the metal azolate framework with zeolitic RHO topology, MAF-6. Two mechanisms are explored to induce hydrophilicity to this hydrophobic material. The study at a molecular level of water adsorption takes place under a variety of conditions. On a first step, we consider water mixtures containing benzene or alcohols, paying special attention to the effect of the size of the alcohol molecules. On a second approach, we analyse the effect of small weight percentages of salt into the MAF-6 on the water adsorption. We first validate the accuracy of the host–guest interactions by reproducing experimental data. A new set of Lennard-Jones parameters for the interaction water- MAF-6 is also provided. The water adsorption behaviour of MAF-6 is studied in terms of adsorption isotherms, heats of adsorption, radial distribution functions, hydrogen bonds formation, and water distribution inside the material. We found that the presence of long molecules of alcohols favours the water adsorption at low values of pressure by smoothing the phase transition of water withing the MAF-6. On the other hand the addition of salt to the structure creates additional adsorption sites for water enhancing its adsorption, while reducing the saturation capacity of the material since the presence of salt reduces the accessible pore volume

Evaluation of ZIF-8 flexible force fields for structural and mechanical properties

Metal–Organic Frameworks (MOFs) offer considerable potential for applications in adsorption due to their large pore volumes and surface areas. Studies on mechanical stability of MOFs are scarce. Seminal experimental work has shed a new light on the role that elastic constants play in establishing the structural stability of the prototypical ZIF-8 MOF, with its elastic deformation mechanism being linked to the pliant ZnN4 tetrahedra (Tan et al., 2012). Over the past decade several classical flexible force fields have been proposed to study the physical properties of the system using simulations (Zheng et al., 2012; Hu et al., 2012; Zhang et al., 2013, Wu et al., 2014; Krokidas et al., 2015; Weng and Schmidt, 2019; Dürholt et al. 2019). In this work, we evaluated the majority of them for reproducing structural and mechanical properties (unit cell sizes as a function of temperature and pressure, and elastic constants as a function of pressure), compared them to existing DFT calculations (Tan et al., 2012; Maul et al., 2019) and found that they provide different results under the same testing conditions. The obtained results provide insight into the relationship between fundamental elastic properties and the chosen force field parametrization, allowing us to characterize the applicability of each of the force fields. Furthermore, the employed two-code approach allowed us to find significant discrepancies in elastic constant values for the same force field between methodologies that employ different energy minimization algorithms, suggesting that eigenmode-following approaches might be needed to guarantee true minimum energy configurations for ZIFs