Unavoidable but minimizable microdefects in a polycrystalline zeolite membrane: Its remarkable performance for wet CO₂/CH₄separation

We prepared a hydrophobic deca-dodecasil 3 rhombohedral@chabazite (DDR@CHA) zeolite hybrid film comprised mainly of DDR zeolite. Specifically, the pore size of the DDR zeolite (0.36 × 0.44 nm2) is ideal for molecular-sieve-based CO2 (0.33 nm) separation from CH4 (0.38 nm), which is critical for upgrading biogas. We demonstrated that an appropriate choice of calcination conditions was the key factor controlling the formation of defects and, consequently, determining the final membrane performance. Simply put, low-temperature calcination in O3 eliminated defect formation and, thus, achieved a very high performance for dry CO2 permselectivity over CH4 (CO2/CH4 separation factor (SF) of ca. 523 ± 96 at ca. 50 °C, which is a representative temperature of biogas streams). Surprisingly, high separation performances (CO2/CH4 SF of 422) for water-vapor-containing CO2/CH4 mixtures (at 100% humidity and 50 °C) required the formation of a few defects, which in turn necessitated optimal calcination at ca. 450 °C in O2. The defect structures were quantitatively analyzed by combining fluorescence confocal optical microscopy with gas-permeation modeling. Furthermore, the inhibition of water-molecule-adsorption on CO2 permeation rates was estimated. This clearly revealed that fully opening the all-silica hydrophobic DDR zeolite micropores, while minimizing the formation of concomitant defects, helped to achieve the highest ever CO2 permselectivities for wet CO2/CH4 mixtures. In contrast, the elimination of defects by calcination in O3 was the key to achieving a very high dry CO2/CH4 separation performance.