Towards the Design of Advanced MOF-Based Air Dehumidification Membranes: Molecular Simulation Study of Nitrogen, Oxygen, and Water in Rigid and Flexible UiO-66 MOF

Abstract

A significant source of energy consumption in hot and humid regions is the dehumidification of conditioned air delivered to the building areas. The conventional air dehumidification methods currently in practice are condensing moisture air dehumidification and desiccant absorption-based air dehumidification, which are energy-consuming and have high associated costs. Therefore, new methods such as membrane air dehumidification have been attracting much interest thanks to their lower energy consumption and higher efficiency. Metal-Organic Frameworks (MOFs) feature high porosity, uniform but controllable pore diameters, and the ability of surface functionalization. MOFs separate the molecules by their size and shape or their interaction with the membrane material. UiO-66 is a recently-developed MOF that exhibits high thermal, mechanical, acidic, and water stability and remains unaltered in water adsorption/desorption cycles. There are several studies on the applications of UiO-66, such as carbon dioxide capture, storage and separation of short-chain hydrocarbons, and removal of warfare agents, which are mostly adsorption based. However, studies on the diffusion of confined molecules within its pores are still scarce. In this work, the transport properties of humid air in UiO-66 and, more specifically, the diffusion of water, nitrogen, and oxygen are studied using Molecular Dynamics simulations. This was achieved by firstly validating the force fields through individual simulations of these gases at bulk, comparing the results against the available MD and experimental data, and then finally studying them in a mixture of all the gases in air composition confined in UiO-66. The MOF force field implementation was also validated against the literature by studying the diffusion of methane gas in UiO-66. Initially, a rigid force field was used to model the MOF, and then the work was extended by moving to a flexible force field and calculating the diffusion coefficients of the mentioned gases within UiO-66. This was followed by adsorption computations to obtain Henry’s law constant, permeability, and selectivity. All the final results show that water is the slower component compared to nitrogen and oxygen, and has a higher solubility in this MOF. Due to the disagreement in the water solubility obtained from the simulations and experimental values, the calculation of the permeability and selectivity was done using the experimental water solubility. The final permeability and selectivity show an excellent performance of water separation from humid air using this MOF.