| Fundamental separation mechanisms by zeolite membranes were investigated, which include adsorption and diffusion. A density-bottle method was developed to directly measure absolute adsorbed amounts of liquid mixtures on zeolites. This technique measures both ideal and non-ideal liquid mixture adsorption under the same condition as in pervaporation by avoiding the introduction of the non-adsorbing solvent. Adsorption of benzene mixtures on silicalite-1 and NaX zeolite showed that both heat of adsorption and entropy (size and configurational entropy) control the mixture adsorption selectivity. IAST and RAST predicted or correlated measured mixture adsorption data, depending on the ideality of mixtures.; Diffusion of polar organic mixtures through MFI zeolite membranes was studied by isotopic transient pervaporation. Labeled molecules were added to the feed during steady-state pervaporation, and their transient responses in the permeate were measured. Methanol/ethanol, methanol/2-propanol mixtures showed correlated diffusion behavior, which means that the more mobile species was slowed down in the mixture and the tardy species was speeded up, as predicted by Maxwell-Stefan (M-S) model, although much stronger correlation effect was found, which may result from strong interaction between these polar molecules. Acetone/methanol mixtures showed abnormal diffusion behavior, and both components in the mixture diffused slower than slower diffusing pure component, acetone. Adjusting the value of the self-exchange coefficient D ii in the M-S equations did not provide an explanation of the observed experimental results; it appeared that the component diffusivities, D1 and TH2, in the mixture were both lower than the values of the pure components, an effect that has not earlier been reported in the literature.; Microstructure of MFI zeolite membrane was found to be flexible and changed upon adsorption of some molecules in zeolite pores and upon temperature change. At 303 K, the pervaporation flux of n-hexane was only 5-20% of the fluxes of several larger molecules (2,2-dimethylbutane (DMB), isooctane, and 1,3,5-trimethylbenzene (TMB)), because n-hexane expanded zeolite crystals and shrank non-zeolitic pores. Benzene that can also adsorb in MFI zeolite pores did not show this effect. Because of this expansion effect by n-hexane, dramatically different MFI membrane quality was indicated by n-hexane and benzene porosimetry. Benzene porosimetry is suggested to characterize MFI zeolite membranes. Single-component vapor permeation of large molecules, such as DMB and isooctane, showed capillary condensation in non-zeolitic pores, and thus can be used to characterize relative non-zeolitic pore sizes and absolute pore size if combined with appropriate pore size calculation model. Non-zeolitic pore sizes decreased with temperature from 300 to 348 K, as indicated by isooctane vapor permeation, apparently because of the thermal expansion of zeolite crystals.; A new hydrogen storage concept, storing hydrogen in SAPO coated pellets, was demonstrated by high pressure hydrogen permeation through a SAPO-34 zeolite membrane. Methanol adsorbed in SAPO layer efficiently blocked hydrogen permeation at ∼7MPa, and heating recovered the hydrogen permeance. Therefore, high pressure hydrogen can be stored in coated pellets by blocking SAPO layer with methanol and released by heating. |
