(731g) Vacuum-Assisted Ion Exchange of Zeolite Membranes
Zeolite membranes with a variety of crystal structures and pore sizes can effectively perform molecular separations due to preferential adsorption and differences in diffusion rates. There have been significant efforts to modify the zeolite membranes in order to improve separation/catalytic performance using methods such as chemical vapor deposition, coking treatments, and catalytic hydrothermal cracking. One way to modify the membrane properties is through ion exchange, which is known to control the adsorption, diffusion, and catalytic properties of zeolite materials.However, the conventional ion-exchange methods (in which one immerses the zeolite membrane into the ion exchange solution) often lead to slow and limited exchange of ions. This is because of the large hydrated form of the ions in solutions, small zeolitic pore size, and the thick zeolite membrane layer, thereby causing a low driving force for ion exchange.
We will discuss a new vacuum-assisted ‘flow-through’ technique to effectively ion exchange the zeolite membranes. Na-containing ZSM-5 (MFI) zeolite membranes with different Si/Al ratios were synthesized on a porous α-alumina disk by in situ crystallization method and by secondary growth. In this vacuum-driven pressure differential method, the membrane surface is gently immersed into the ion exchange solution bath. We show how the large increases in the exchange of Ga3+, Zn2+, and Pt2+ ions can be achieved with this method. We find that, in contrast to conventional ion exchange involving counter-diffusion of the ions being exchanged in and out, the vacuum-driven method creates a unidirectional displacement of one type of ion with the other. The Na+ ions are transported into the vapor phase on the vacuum side in the form of hydrated clusters, and replaced with e.g., Ga3+ ions from the liquid side. Finally, we find that the creation of a low-pressure vapor phase is critical for this process, above and beyond the existence of a pressure differential. A similar liquid-phase process with the same pressure differential as the vacuum-driven method, but using a pressurized liquid feed and an atmospheric liquid permeate, leads to a very low degree of ion exchange. The talk will further discuss the mechanisms responsible for the large enhancement in ion exchange by the vacuum-driven process over conventional methods.