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

X. Peng, L.-C. Lin, W. Sun, and B. Smit, Water Adsorption in Metal-Organic Frameworks with Open-Metal Sites AIChE J.  61 (2), 677 (2015) http://dx.doi.org/10.1002/aic.14707

Abstract In this study, we investigated H2O adsorptions inside porous materials, including silica zeolites, zeolite imidazolate frameworks (ZIFs), and metal-organic frameworks (MOFs) by using molecular simulations with different water models. Due to the existence of coordinately unsaturated metal sites, the predicted adsorption properties in M-MOF-74 (M=Mg, Ni, Co, and 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-O4 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, we used a simple approach to adjust metal-O(H2O) sigma parameters to reproduce the DFT-calculated binding energies. 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 we observed that there is an interesting two-layer water network formed by using three- and four-site models. This article is protected by copyright. All rights reserved.

© Berend Smit 2019