The main goals of this project are: (1) to better understand how zeolite membranes work and how non-zeolitic pores affect separations, (2) to understand how adsorbed-induced crystal expansion and shrinkage of non-zeolitic pores change separation selectivities, (3) improve separation selectivities by adding a third component to a binary (or multi-component) mixture to take advantage of these adsorbed-induced changes, and (4) to develop better characterization methods for zeolite membranes. The project will build on the recent discovery that adsorption of certain molecules (e.g., n-hexane, n-octane) in MFI zeolite membranes swells the MFI crystals slightly (~1% linear expansion), and this expansion shrinks the size of the non-zeolitic pores (defects) sufficiently to dramatically decrease permeation through non-zeolitic pores. The successful completion of this research will result in new understanding of the micro-structure of zeolite membranes, will indicate directions for improved zeolite membrane preparation, and will potentially result in new methods of zeolite membrane separations by addition of certain molecules to mixtures. It will identify molecules that can cause improved selectivity by shrinking the size of non-zeolitic pores. Comparing membranes prepared in different laboratories is difficult, and characterization methods that measure flux through non-zeolitic pores and measure their pore sizes will be developed. MFI membranes with different fractions of their flow through non-zeolitic pores will be used. Methods to measure changes due to adsorption, for crystals or membranes, include XRD, TPD, and confocal microscopy. Permeation methods include pervaporation to obtain high adsorbed loadings, which are more representative of industrial applications, vapor permeation as a function of pressure (and capillary condensation in non-zeolitic pores), permporosimetry, and transient permeation of mixtures with mass spectrometer detection. Separations will be carried out by pervaporation for binary and ternary mixtures.
Separations use a significant fraction of the energy in the US, and membrane separations have the potential to significantly reduce that usage. Understanding adsorbed-induced microstructural changes in zeolite membranes will impact how zeolite membranes are prepared and utilized. Zeolite membranes have the potential to separate organic isomers (structural and stereo isomers), and azeotropes, and these abilities combined with their high temperature stability give them significant advantages over polymer membranes. Zeolite layers have potential applications for hydrogen storage and in micro devices. Continuation of membrane technology patents having the potential to rapidly and directly benefiting society through commercialization is anticipated. It is anticipated that at least one student will be supported by a GAANN fellowship, which has K-12 and community outreach as one of the required aspects. Undergraduate students will be supported, and one high school student will be involved in this research each summer.
Zeolite membranes, which have the potential to significantly reduce the energy required to separate gas mixtures, were studied to understand the influence of flux through defects on the overall separations. The size of the defects were discovered to change as molecules adsorbed in the zeolite pores, and the amount of change depended on the molecule adsorbed, its loading, and the temperature. X-ray diffraction was used to show that the percentage volume change of zeolite crystals correlated with the increase or decrease in the flux through defects. When the crystals contracted due to expansion, the flux through defects increased, and when the crystals expanded the flux decreased. Small percent changes in the volume (less than 1%) had relatively large changes in the size of the defects. Carbon nanotube membranes with high densities of carbon nanotubes were prepared by growing aligned carbon nanotubes and collapsing the nanotubes together. These membranes had much higher fluxes than expected for Knudsen diffusion and exhibited large changes in ion fluxes in water for small changes in temperature or in the presence of ultrasound. Composite zeolite membranes were modified by depositing a thin layer (10 nanometers) of a porous oxide onto a zeolite membrane using molecular layer deposition. These membranes were highly selective for hydrogen at high pressures and elevated temperatures and have potential to significantly improve hydrogen separations.
