ZeoTIPS MEMBRANE FORMATION

            For many years it has been desired to produce membranes that can efficiently separate chemical species of similar size.  Typical applications include gas separations (such as O₂/N₂ and CH₄/CO₂), the removal of organics and salts from water, and the separation of ethanol and water.  Numerous polymer membranes have been used to varying degrees of success for these separations; however, more refined size separations remain the goal of considerable ongoing research.

Attempts have been made to achieve more refined size-based separations by using structured inorganic materials, such as zeolites, in the form of flat sheet membranes.  These membranes show great potential due to their narrow pore size distributions.  However, due to their fragile nature and difficult formation procedures, they have not yet been widely used.  Furthermore, since these membranes cannot be made in hollow fiber form, their low surface area to volume ratio also limits their use.  In applications that require superior chemical and thermal stability, ceramic membranes have proven to be better than polymer membranes; however, these membranes are expensive, difficult to produce, and fragile.

            Over the past decade, mixed matrix membranes have been proposed as an answer to the above drawbacks.  By suspending zeolite particles in a continuous matrix of low permeability polymer matrix, it has been possible to achieve separations not possible by polymer membranes.  Like zeolite films, mixed matrix membranes are not without drawbacks.  The most important being the limit to their separation capabilities.  Because the zeolite particles are by no means a continuous separation layer, only a small improvement over polymeric membranes can be achieved. 

Therefore, the remaining challenge is to produce a membrane with the effectiveness of a continuous zeolite sheet, but with the flexibility and durability of a mixed matrix membrane.  The proposed research presents one possible approach to achieving this goal.

            The objective of this work is to address the drawbacks of polymeric membranes, zeolite films, and mixed matrix membranes while maintaining most of their benefits.

          The membranes being developed in this research consist of nano-porous zeolite particles dispersed within a microporous membrane.  The formation of these membranes begins with a suspension of zeolite particles in a homogeneous polymer−diluent solution.  Through the process of liquid−liquid thermally induced phase separation (L−L TIPS), diluent droplets form within a polymer-rich phase.  As the droplets grow, they come in contact with the particles.  When this suspension is cooled to induce polymer solidification, it is these droplets in contact with the particles that connect the structure.  The diluent is then extracted and the extractant is evaporated to yield a microporous membrane containing nano-porous zeolite particles.  The membranes are referred to as ZeoTIPS membranes.

          Transport through a ZeoTIPS membrane is shown schematically in Figure 1, where the blue regions represent the impermeable polymer, the beige blocks represent the zeolite, and the white areas represent micrometer-size pores.  In Figure 4, smaller component (green line) can traverse the membrane, but the larger component (red line) is rejected once it reaches the first zeolite particle blocking it’s potential path.   

            Work on ZeoTIPS membranes to date has included the determination of feasible polymer−diluent−zeolite systems.  The interactions of the polymer and diluent with the zeolite surfaces is paramount to producing an effective membrane.  Without roughly equal affinity of the polymer and diluent for the zeolite surface, either a coating of polymer surrounding the zeolite will prevent all flux, or a coating of diluent, which is removed following formation, will result in a void around each particle, eliminating the separation capabilities of the membrane.  Work on surface modification of the zeolite particles has also been undertaken to allow the use of polymer systems that would be otherwise infeasible for ZeoTIPS membranes. 

            Figure 2 shows a polyethylene ZeoTIPS membrane.  Yellow boxes highlight the positions of some of the zeolite particles for clarity. 

            To better understand the formation of ZeoTIPS membranes, two other aspects of the project are currently been investigated.  The first of these studies includes the development of empirical models to predict the effects of zeolite particle fillers on L−L TIPS cell growth.  Different polymer−diluent systems will have different relative interactions with zeolite surfaces.  The extent to which a particular component is favored by the zeolite will have a direct impact on the cell growth in the solution.  Empirical models will be developed to predict the cell growth trends for three types of polymer−diluent−zeolite interactions.  Furthermore, this portion of the research will be used to predict all processing conditions needed to produce desired ZeoTIPS membrane structures.  Figure 3 shows a plot of experimental results relating cell size to the cooling rate used in the TIPS process and the polymer concentration for a zeolite loading of 35 wt-%. 

            Finally, the modeling of ZeoTIPS membrane formation is being performed using Monte Carlo methods.  When completed, the model will be able to predict the necessary zeolite loading to block all microporous paths through a membrane with a given particle size distribution, cell size distribution, and polymer concentration.