日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

DDR-type zeolite membranes show good performance for the separation of carbon dioxide from methane in a mixed gas. Two DDR membranes with a thickness of about 2.5 µm were synthesized on alumina supports under two synthesis conditions. These membranes showed different carbon dioxide/methane separation factors (α): one membrane showed α=76 and the other showed α=1.2. In a dyeing test to find defects in the DDR membranes, all surface of the DDR membrane with the low separation factor was found to be uniformly dyed and we surmised that it had many defects. However, the reason for the formation of defects was not clarified after the dyeing test and the defects could not be observed by SEM. Therefore, the DDR membranes were observed by TEM. In particular, DDR/DDR grain boundaries in the DDR membrane and composite layers of the DDR membrane and the alumina support were carefully observed. The results of TEM observation revealed that typical DDR grains were adhered closely to each other and composite layers did not have amorphous layers such as amorphous silica. However, some gaps with a width of about 10～20 nm were observed at DDR/DDR grain boundaries in the DDR membrane with the low separation factor. It was investigated whether the formation of the gaps was caused by the difference in thermal expansion between the DDR membrane and the alumina support during the heating treatment to remove the structure-directing agents. A DDR membrane with a thick composite layer was synthesized to check whether the composite layer is effective in decreasing the formation of cracks in DDR membranes. This DDR membrane showed a good separation factor (α=57). It was concluded that the composite layer is effective in buffering the difference in thermal expansion between the DDR membrane and the alumina support.