title: Atomistic insight into adsorption, mobility, and vibration of water in ion-exchanged zeolite-like metal-organic frameworks.
abstract: The adsorption, mobility, and vibration of water in ion-exchanged rho-zeolite-like metal-organic frameworks (ZMOFs) are investigated using atomistic simulations. Because of the high affinity for the ionic framework and nonframework ions, water is strongly adsorbed in rho-ZMOFs with a three-step adsorption mechanism. At low pressures, water is preferentially adsorbed onto Na(+) ions, particularly at site II; with increasing pressure, adsorption occurs near the framework and finally in the large cage. Upon water adsorption, Na(+) ions are observed to redistribute from site I to site II and gradually hydrated with increasing pressure. In Li-, Na-, and Cs-exchanged rho-ZMOFs, the adsorption capacity and isosteric heat decrease with increasing ionic radius attributed to the reduced electrostatic interaction and free volume. The mobility of water in Na-rho-ZMOF increases at low pressures but decreases upon approaching saturation. With sufficient amount of water present, the mobility of Na(+) ions is promoted. The vibrational spectra of water in Na-rho-ZMOF exhibit distinct bands for librational motion, bending, and stretching. The librational motion has a frequency higher than bulk water due to confinement. With increasing loading and hence stronger coordinative attraction, the bending frequency shows a blue shift. Symmetric and asymmetric modes are observed in the stretching as a consequence of the strong water-ion interaction. This study provides a fundamental microscopic insight into the static and dynamic properties of water in charged ZMOFs and reveals the subtle interplay between water and nonframework ions.
authors.affiliation: Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576.
authors.forename: A
authors.initials: A
authors.lastname: Nalaparaju
authors.forename: R
authors.initials: R
authors.lastname: Babarao
authors.forename: X S
authors.initials: XS
authors.lastname: Zhao
authors.forename: J W
authors.initials: JW
authors.lastname: Jiang
mesh_terms: D000327:Adsorption; D008670:Metals / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009010:Monte Carlo Method; D009038:Motion; D009930:Organic Chemicals / Q000737:chemistry*; D012964:Sodium / Q000737:chemistry; D014732:Vibration; D014867:Water / Q000737:chemistry*; D017641:Zeolites / Q000737:chemistry*
chemical_list: D008670:Metals; D009930:Organic Chemicals; D014867:Water; D017641:Zeolites; D012964:Sodium