Designing hierarchical porous features of ZSM-5 zeolites via Si/Al ratio and their dynamic behavior in seawater ion complexes

Abstract

MFI-type zeolites were characterized using powder X-ray diffraction (XRD), and positron annihilation lifetime spectroscopy (PALS) to uncover the hierarchical porous properties with Si/Al ratios varying from 30 to >1000 (high silica MFI). Structural dynamics were observed in response to exposure of the zeolites to seawater solutions, mimicking service conditions for use in membrane desalination or other ionic solution applications (e.g. sensors). PALS results indicated MFI channels between 0.27 and 0.36nm. All materials had microporous grain boundaries of 1.1nm. Mesopores were also detected at approximately 8nm in size for all materials. However the volume of microporous grain boundaries reduced with increasing alumina as explained by increasing presence of cations. Seawater exposure had a strong and complex effect on all materials. Initially widely variable crystal dimensions were stabilized by seawater exposure. The highest alumina content ZSM-5 (Si/Al=30) demonstrated net exchange, replacing K+ from synthesis impurities with Na+ leading to little overall change in porous features. Higher silica materials, including high silica MFI, demonstrated an overall release of ions, principally Na+ and K+, which were liberated mainly from the zeolite channels, yet these materials adsorbed Ca2+ in the microporous grain boundaries and Mg2+ in the mesoporous grain boundaries. Si/Al=100 appeared to be a unique ratio, allowing the material to possess the same Na+ adsorption features (and hydrophilicity) of high alumina materials, but ion interactions resembling high silica materials. MFI-type zeolite is therefore shown to be highly configurable using Si/Al ratio for a wide variety of applications in the presence of ions.