Unavoidable but minimizable microdefects in a polycrystalline zeolite membrane: its remarkable performance for wet CO2/CH4 separation

Abstract

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.