Tailoring of MFI membranes for enhanced selectivity

Sammanfattning: Zeolite membranes can potentially be used for the 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 membranes in order to enhance the selectivity for a given separation. Traditional methods to tailor zeolite membranes include; choice of zeolite framework, adjustment of Si/Al ratio and choice of counterion. In zeolite catalysts the properties are often also tailored by incorporating metal or metal oxide clusters in the zeolite pores by impregnation. In this work the traditional methods for membrane tailoring by adjusting the Si/Al-ratio and exchanging the counterions have been used. In addition, a new method where the impregnation concept often used in catalyst preparation is adapted to tailor the properties of zeolite membranes, was used. The polarity of a zeolite can be tailored by changing the Si/Al ratio, and to facilitate the separation of polar and non polar molecure e.g. H2O and H2, the Si/Al ratio should be relatively low. In the present work, separation of mixtures of H2O, H2 and n-hexane was investigated for membranes with two different Si/Al ratios (silicalite-1 and ZSM-5), in the temperature range 25 to 350 C. The highest separation factors H2O/H2 were observed at 25 C and were 14.3 and 19.7 for silicalite-1 and ZSM-5, respectively. The membranes were selective also at 100 C and the separation factors were about 3.2 and 6 for silicalite-1 and ZSM-5, respectively. These results confirm that the selectivity for this separation can be controlled by changing the polarity the zeolite. The aim of the new and innovative modification procedure was to prepare CO2 selective membranes with high flux. The pores of a silicalite-1 membrane were impregnated with calcium compounds to aid the chemisorption of CO2, which is essential to achieve a membrane which is CO2 selective even at high temperatures. The result of the impregnation was evaluated by separation of CO2 and H2. Calcined membranes were impregnated with a solution of Ca(NO3)2 in methanol and heated to 600 to thermally decompose the Ca(NO3)2 Calcium compounds were evenly distributed in the pores of the silicalite-1 film and there were also some relatively large CaCO3 crystals on the surface. The separation experiments with of mixtures of CO2 and H2 showed that the separation factor CO2/H2 at 25C was drastically changed from 0.7 (H2 selective) to 3.7 (CO2 selective) by this modification. These results show that the properties of the H2 selective silicalite-1 membrane could be tailored by impregnation to prepare a CO2 selective membrane.