Properties and modeling of MFI membranes

Abstract: The permeation properties of thin (<2µm) film MFI molecular sieve membranes have been studied in the present work and a model has been developed. The synthesis of such materials has been studied to a smaller extent. The films have been grown on graded a-alumina microfiltration filters using a seeding method. Scanning electron microscopy and x-ray diffraction were used in addition to permeation measurements for characterization of the materials. In particular, a simple and unique model describing single component permeation was developed. The model is a combination of simple and basic equations for permeation and adsorption. The important defect distribution of the membrane and the properties of the support are measured in separate experiments. The model is unique since it is accounting for the effect of defects and support on the permeation properties. The model can adequately describe the performance of various MFI membranes. The model indicates mass transfer limitations of the supports that strongly affect, for instance, permeance ratios. It was also found that these ratios are dependent on crystallographic orientation, film thickness and experimental conditions in addition to the amount of defects. Permeance ratios can thus only be used to compare membranes with similar morphology and tested under similar conditions. It was found that defects formed in thicker films. Membranes prepared on masked substrates were of higher quality than membranes prepared on unmasked substrates. MFI membranes with low and varying aluminum content with similar material properties, such as defect distribution and thickness, were evaluated with multi-component hydrocarbon isomers permeation. The silicalite-1 membrane showed a minimum in separation selectivity between two C6 isomers whereas the ZSM5 membrane showed an almost constant selectivity, independent of temperature, but with lower permeances. The effect of the calcination rate on the membrane quality was investigated for silicalite-1 membranes. Based on a number of permeation characterization techniques, the membrane quality was independent of the calcination rate. It was found that the permeation properties of membranes comprised of small crystals in several layers were different from membranes comprised of one layer of larger crystals, although the quality of the membranes was similar. ZSM-5 membranes with high aluminum content showed catalytic conversion of ethanol into diethylether and ethylene under simultaneous separation of the ethanol / water azeotrope