Quality and performance of zeolite membranes

Abstract: Zeolite membranes displaying high flux and high selectivity are of major interest for potential industrial applications, such as gas and liquid separations. In order to achieve high flux and high selectivity, the zeolite membrane must be thin (< 1 µm) and free from flow-through defects. The development of thin defect-free zeolite membranes requires powerful tools for characterisation of the defects in the membranes. Permporometry is one of the most straightforward, non-destructive and powerful techniques for characterisation of flow-through pores in inorganic membranes. In permporometry, the flow of a non-adsorbing gas, e.g., helium, through the membrane is monitored as a function of the activity of a strongly adsorbing compound, e.g., hydrocarbon.The work showed how permporometry can be used to quantify defects in the mesopore range in MFI membranes. The results were in excellent agreement with SEM observations and separation experiments. For the first time, it was also shown that flow-through defects down to 0.7 nm in size in MFI membranes could be detected by permporometry and quantified using the permporometry data. The total amount of defects in high quality MFI membranes was found to be small accounting for less than 1% of the total membrane area. In turn, defects with a width below 1 nm constituted as much as 97% of the total area of defects for the best membranes. The permporometry results were consistent with SEM observations and separation experiments, demonstrating that permporometry data adequately reflect membrane quality and that the technique is a very powerful and reliable characterisation tool.This work also illustrated that single gas permeance ratios could not detect slight variations in the membrane quality. For membranes with similar however slightly different amount of defects, the ratios are mainly affected by the membrane thickness and support morphology.The MFI membranes were also evaluated for separation of dilute aqueous solutions of n-butanol and ethanol, and 1 µm zeolite X membranes were evaluated for separation of water from ethanol using pervaporation. The MFI membranes were selective to n-butanol and ethanol, whereas zeolite X membranes were selective to water. The flux observed for the MFI membranes was about 100 times higher than those previously reported for n-butanol/water and about 5 times higher than the highest reported for ethanol/water separation by pervaporation. The zeolite X membranes showed good pervaporation performance in terms of both flux and selectivity. However, both flux and selectivity were found to be reduced by a significant mass transfer resistance in the support in all the pervaporation experiments. At the same time, heat transfer limitations were found to be negligible.