Ion exchange of zeolite membranes by a vacuum ‘flow-through’ technique

Abstract

Ion exchange of nanoporous (e.g., zeolite) membranes is of increasing importance in their applications as separation devices and catalytic reactors. Ion exchange processes in zeolite membranes are significantly limited by slow hydrated-ion transport rates and the low liquid–solid interfacial area available in comparison to ion exchange of zeolites in powdered form, thereby leading to long membrane processing and regeneration times. Here, we consider ion exchange processes in zeolite membranes in more detail, and show the much higher efficacy of a vacuum-assisted liquid–vapor ‘flow-through’ method in comparison to both the conventional ‘immersion/counter-diffusion’ method as well as a liquid–liquid flow-through method. Na-MFI zeolite disk membranes, made by both in situ and seeded growth, were ion-exchanged with Ga3+, Zn2+, and Pt2+ ions in the temperature range of 23–70°C and exchange times of 5–24h. The penetration of these ions into the zeolite membranes was investigated in detail by energy-dispersive X-ray (EDX) spectroscopy. Surprisingly, the quantity of exchanged ions in the membranes via the vacuum-assisted ‘flow-through’ technique is found to exceed that achieved by the other two methods by up to a factor of ten, with the liquid–liquid technique being the least efficient. Higher temperatures and longer ion exchange times increased the ion exchange efficiency in the vacuum-assisted method. Chemical analysis of the condensed permeate solution by inductively-coupled plasma (ICP) mass spectrometry revealed that both the original Na+ and replacement metal cations moved through the membrane in a co-current manner, unlike the conventional counter-current movement of ions in the immersion process. The Na+ ions in the membrane experience pressure-driven transport (along with water molecules) to the permeate side, and leave the membrane surface as hydrated vapor-phase cations, thereby allowing the maintenance of a high driving force for ion exchange. In contrast, the conventional counter-current flow technique leads to a decreasing concentration driving force with time. The liquid–liquid method, even though having the same ion concentration and applied pressure driving force as the liquid–vapor method, is very slow because of the large osmotic gradient opposing the permeation of ions and water from the feed to the permeate side. The liquid–vapor ion exchanged MFI membranes showed excellent integrity (as determined by H2 and CO2 permeation measurements).