Modification of MFI membranes for enhanced selectivity
Sammanfattning: Zeolite membranes can potentially be used for separation of many types of mixtures. The membranes can be tailored by a number of methods to suit a specific separation application. In this work both traditional and new innovative methods were used to tailor the properties of zeolite membranes in order to enhance the selectivity for a given separation. In this work the traditional methods for tailoring of zeolite membranes by adjusting the Si/Al-ratio of, and exchanging the counterions in the zeolite have been used. In addition, two new methods have been developed: One where the impregnation concept often used in the catalysis field is adapted to tailor the properties of zeolite membranes for the first time, and another, where methylamine is used to modify the zeolite and form more and stronger basic sites. The polarity of a zeolite can be tailored by changing the Si/Al ratio in order to facilitate the separation of polar and non polar molecules. A low Si/Al ratio gives a more polar zeolite and vice versa. In the present work, separation of mixtures of water, hydrogen and n-hexane was investigated for membranes with two different Si/Al ratios (silicalite-1 and ZSM-5). The highest separation factors α-water/hydrogen were observed at 25 °C and were 14.3 and 19.7 for silicalite-1 and ZSM-5, respectively. Mixtures of methanol/ethanol, hydrogen, carbon dioxide and water were also investigated, and the highest measured methanol/hydrogen separation factor, 32, was achieved for a ZSM-5 membrane, while a silicalite-1 membrane was found to give the highest ethanol/hydrogen separation factor of 46. The polar ZSM-5 favours the separation of the polar methanol, whereas the less polar silicalite-1 is more favourable for separation of the less polar ethanol. These results confirm that the selectivity for these separations can be controlled by tailoring the polarity the zeolite. The effect of the counter ions in ZSM-5 was studied by preparing, silicalite-1 and ZSM-5 membranes with three different counter ions (Na+, Li+ and Ba2+) and evaluating these for separation of quadrupolar carbon dioxide from binary and ternary mixtures of carbon dioxide, hydrogen and water. The aim was to develop a membrane suitable for separation of carbon dioxide from synthesis gas. A separation factor α-carbon dioxide/hydrogen of 6.2 was achieved for a BaZSM-5 membrane at room temperature. In the BaZSM-5 membranes, the permeances of both carbon dioxide and hydrogen were decreased by the presence of the Ba2+counter ion, but due to enhanced adsorption of the more quadrupolar carbon dioxide the carbon dioxide permeance was decreased much less than the hydrogen permeance. By development of a new and innovative impregnation procedure, carbon dioxide selective membranes with high flux were prepared by impregnating the pores of a silicalite-1 membrane with calcium compounds to aid the adsorption of carbon dioxide. The separation experiments with mixtures of carbon dioxide and hydrogen showed that the separation factor α-carbon dioxide/hydrogen at 25 °C was drastically changed from 0.7 (hydrogen selective) to 3.7 (carbon dioxide selective) by this modification. A second new modification procedure was also developed, where MFI membranes with high Si/Al ratio were modified with methylamine to increase the carbon dioxide affinity and thus increase the carbon dioxide selectivity. These membranes were then evaluated for separation of carbon dioxide from various mixtures of carbon dioxide, hydrogen, methane and water. The modification had significant effects on both permeances and separation factors and the selectivity towards carbon dioxide was increased considerably for all the feed mixtures tested. The results of the different modifications were evaluated by techniques such as SEM, TEM, XRD, DRIFT spectroscopy and by single component permeation and mixture separation experiments. High quality membranes with few defects are critical to study the effects of membrane modification and throughout this thesis adsorption-branch permporometry is used as a standard tool to evaluate membrane quality. The permporometry technique was also studied more in detail. It is shown how the distribution of flow-through defects can be estimated from the permporometry pattern. The estimated defect distribution is supported by SEM observations. In addition the permporometry data can be used to predict the permeance of molecules diffusing through defects in the membrane in mixture separation experiments and also indicate the separation factor.
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