Water Adsorption in Metal-Organic Frameworks with Open-Metal Sites

H2O adsorptions inside porous materials, including silica zeolites, zeolite imidazolate frameworks, and metal-organic frameworks (MOFs) using molecular simulations with different water models are investigated. Due to the existence of coordinately unsaturated metal sites, the predicted adsorption properties in M-MOF-74 (M=Mg, Ni, Co, Zn) and Cu-BTC are found to be greatly sensitive to the adopted H2O models. Surprisingly, the analysis of the orientations of H2O minimum energy configuration in these materials show that three-site H2O models predict an unusual perpendicular angle of H2O plane with respect to the Metal-O-4 plane, whereas those models with more than three sites give a more parallel angle that is in better agreement with the one obtained from density functional theory (DFT) calculations. In addition, the use of these commonly used models estimates the binding energies with the values lower than the ones computed by DFT ranging from 15 to 40%. To correct adsorption energies, simple approach to adjust metal-O(H2O) sigma parameters to reproduce the DFT-calculated binding energies is used. With the refined parameters, the computed water isotherms inside Mg-MOF-74 and Cu-BTC are in reasonable agreement with experimental data, and provide significant improvement compared to the predictions made by the original models. Further, a detailed inspection on the water configurations at higher-pressure region was also made, and observed that there is an interesting two-layer water network formed using three- and four-site models. (c) 2014 American Institute of Chemical Engineers AIChE J, 61: 677-687, 2015