Abstract
In this study, molecular dynamics simulation is used to investigate the effects of water-based substitutional defects in zeolitic imidazolate frameworks (ZIF)-8 membranes on their reverse osmosis (RO) desalination performance. ZIF-8 unit cells containing up to three defect sites are used to construct the membranes. These substitutional defects can either be Zn defects or linker defects. The RO desalination performance of the membranes is assessed in terms of the water flux and ion rejection rate. The effects of defects on the interactions between the ZIF-8 membranes and NaCl are investigated and explained with respect to the radial distribution function (RDF) and ion density distribution. The results show that ion adsorption on the membranes occurs at either the nitrogen atoms or the defect sites. Complete NaCl rejection can be achieved by introducing defects to change the size of the pores. It has also been discovered that the presence of linker defects increases membrane hydrophilicity. Overall, molecular dynamics simulations have been used in this study to show that water-based substitutional defects in a ZIF-8 structure reduce the water flux and influence its hydrophilicity and ion adsorption performance, which is useful in predicting the type and number of defect sites per unit cell required for RO applications. Of the seven ZIF-8 structures tested, pristine ZIF-8 exhibits the best RO desalination performance.
Highlights
Zeolitic imidazolate frameworks (ZIFs) are a subset of metal-organic frameworks (MOFs) that are topologically isomorphic with zeolites due to their metal-imidazole-metal angles resembling the 145◦ Si-O-Si angle in zeolites
The pristine ZIF-8 unit cell was selected as a point of reference to assess the reliability of the halving method applied during geometric optimization, and the computed values of the unit cell length (17.05 Å), included sphere (11.47 Å), and free sphere (3.40 Å) diameters were found to be consistent with those obtained from experimental studies (16.99 Å [1], 11.6 Å [1,5], and 3.40 Å [1,5], respectively)
The results of this study suggest that the reverse osmosis (RO) seawater desalination performance of the pristine ZIF-8 membrane is superior to that of the defective membranes, which is in good agreement with the study of Zhang et al [10], in which even a small number of defects in a ZIF-8 unit cell significantly affected the performance of the corresponding RO desalination membrane
Summary
Zeolitic imidazolate frameworks (ZIFs) are a subset of metal-organic frameworks (MOFs) that are topologically isomorphic with zeolites due to their metal-imidazole-metal angles resembling the 145◦ Si-O-Si angle in zeolites. Due to the hydrophobic pores of ZIF-8 as a result of stable tetrahedral clusters formed between the Zn atoms and the inhibitive structure of the imidazolate linkers, ZIF-8 is highly resistant to water degradation [5]. In RO seawater desalination, ZIF-8 is mainly employed as an additive for improving the desalination performance of thin-film-composite RO polymer membranes by increasing the water flux through them while maintaining a high NaCl rejection rate. In an molecular dynamics (MD) simulation study on ZIF-8 RO membranes, Hu et al explored a pristine ZIF-8 membrane whose pores were orientated in a direction parallel to the general water flow direction to allow for maximum water flow through the membrane [5] Their results indicate a NaCl rejection rate of 100% [5]. The effects of pressure and temperature on water transport within these membranes were investigated
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