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

Using density functional theory based molecular dynamics with the tight-binding approximation and experimental characterization, the H2O 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 H2O adsorption, and they even allow for the adsorption of multiple H2O molecules on the same site, making it possible to form stable H2O cluster at low temperatures. The adsorption heats of different adsorption processes were predicted, and the possibility of H2O cluster formation under vapor exposure at low and room temperatures was explored. The first adsorbed H2O can only be dissociated at elevated temperatures, therefore reducing the cyclic uptake near the room temperature. The vibrational spectra of the lattice and adsorbed H2O 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 H2O adsorption isotherm measurements have been conducted to verify the theoretical predictions, and good agreements are found.