Synthesis of FAU-type zeolite membrane: An original in situ process focusing on the rheological control of gel-like precursor species

This study reports the preparation of a FAU-type membrane for the separation of CO2/N2. The membrane is prepared by in situ seeding followed by a two-step secondary growth to plug the 0.2 μm macropores of a multilayer α-Al2O3 tube. The key factor for successful in situ seeding is the infiltration of the synthesis mixture in the pores of the α-Al2O3 tube. Infiltration is achieved by cooling down of the initial mixture in order to slow down its gelation. The infiltrated support is hydrothermally treated to provide a uniform layer of zeolite crystals in pores of the α-Al2O3 support. The hydrothermal treatment is repeated with a gel mixture of low viscosity. SEM analysis shows that the FAU crystals close the interparticle porosity after two such hydrothermal treatments. However, there is also some zeolite formation in the larger pores (1.5 and 12 μm) of the multilayer α-Al2O3 tube, which can negatively affect the membrane performance. Nevertheless, the membrane exhibits a CO2 permeance of 3.5 × 10−7 mol s−1 m−2 Pa−1 and a separation selectivity of 12 in the separation of equimolar CO2 and N2 gas mixtures at 323 K.