Investigation of the effects of ion and water interaction on structure and chemistry of silicalite MFI type zeolite for its potential use as a seawater desalination membrane

Abstract

In this work we study the structural response of silicalite based MFI type zeolite powder to the presence of water and cations commonly found in sea salts including Na+, K+, Ca2+, and Mg2+. MFI zeolite powder was exposed to different solutions (3.8 wt% TDS (total dissolved solids) seawater (SW), or 1 M NaCl, or 0.05 M KCl), and their effects on the structure of the zeolite powder were characterized. The results obtained from ion adsorption testing and synchrotron X-ray powder diffraction showed that the zeolite powder interacted with water, monovalent and divalent cations, and these interactions altered the crystal dimensions of the zeolite. Most exposures to ions and water led to increases in crystal volumes of <0.1%, but exposure to KCl solution was observed to make the biggest contribution to crystal expansion, at 0.59%, particularly in the a- and c-directions as determined by Rietveld refinement. Upon closer inspection of SW exposed silicalite, a rapid exchange of Na+ and K+ was observed within 1 h, but after 8 h, these monovalent cations were gradually exchanged with divalent ions. Ca2+ was especially observed to adsorb into the zeolite at 8 h. N2 adsorption and positron annihilation lifetime spectroscopy (PALS) measurements were made on SW exposed zeolites. PALS showed that the silicalite powder has mesopores (2.6 nm) and micropores (1.0 nm), which are both attributed to intercrystalline spaces. The intrinsic intracrystalline zeolite pore size was measured as 0.33 nm. Both N2 adsorption and PALS showed that the relative amount of micropores to total pores decreased with seawater exposure which was attributed to ion adsorption and ion exchange. The mesopore size and number increase due to exposure, and although mesopores represent defects in crystalline zeolites, their expansion on exposure to seawater is important for membrane stability and diffusion properties and warrants further investigation. The intrinsic zeolite pore size was smaller in zeolites exposed to seawater (∼0.29 nm), and the changes observed by PALS lead us to conclude that most of the ion activity observed is attributed to the intercrystalline micropores and mesopores. No other changes were observable by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) due to SW exposure. Results indicate that the interaction between silicalite and the water and ions in SW causes a change in structure and porosity expected to affect diffusion properties of these materials when used as membranes for desalination.