No abstract

The polycrystalline CHA-type zeolite layer with Si/Al = 18 was formed on the porous α-Al2O3 tube in this study, and the gas permeation properties were determined using single-component H2, CO2, N2, CH4, n-C4H10, and SF6 at 303–473 K. The membrane showed permeation behavior, wherein the permeance reduced with the molecular size, attributed to the effect of molecular sieving. The separation performances were also determined using the equimolar mixtures of N2–SF6, CO2–N2, and CO2–CH4. As a result, the N2/SF6 and CO2/CH4 selectivities were as high as 710 and 240, respectively. However, the CO2/N2 selectivity was only 25. These results propose that the high-silica CHA-type zeolite membrane is suitable for the separation of CO2 from CH4 by the effect of molecular sieving.

The preparation of a double-layer membrane by the combination of Pd metal and FAU zeolite was attempted to enhance the catalytic activity of the membrane reactor. The membrane was successfully synthesized on a porous ¡-Al 2 O 3 support tube by the hydrothermal synthesis of the FAU zeolite layer followed by chemical vapor deposition of the Pd layer. The thicknesses of the FAU and Pd layers were ca. 5 µm and 1 µm, respectively. The membrane functioned efficiently as a hydrogen separation membrane without deterioration, due to the rigid connection of each layer. During the model reaction, such as the reaction of benzene and permeated hydrogen species from Pd membrane, the inner surface of the Pd layer that was in contact with the FAU layer exhibited high hydrogenation activity, in contrast to the outer surface. Scanning electron microscope observations reveal that one of the reasons for this observed difference is the roughness of the inner surface. Loading of Pd particles in the micropores of FAU zeolite layer by ion-exchange treatment resulted in enhancement of the hydrogenation activity. The FAU zeolite layer is suitable for loading of the highly dispersed active metal particles. The combination of Pd metal membrane and fine metal particles has potential for acceleration of catalytic membrane reactions.

For use in a microwave heating system, a pore-filling type Pd membrane was prepared. This membrane was constructed by embedding Pd nanoparticles in micropores of an alumina support tube. Pd loading of 0.10 g/m 2 was sufficient for both microwave heating and stopping gas leak. The amount of Pd converted into an equivalent thickness gives a thin metal membrane of 0.07 m, suggesting a reduction in the amount of Pd in comparison with conventional thin membranes. The Pd membrane was easily heated to 700 K by microwave radiation. The temperature was precisely controlled by changing the microwave power, indicating effective control of the H 2 permeation rate. In addition, this Pd membrane can be used as a catalytic membrane. As the model reaction, hydrogenation of benzene and toluene were demonstrated under microwave radiation.