Unavoidable but minimizable microdefects in a polycrystalline zeolite membrane: its remarkable performance for wet CO 2 /CH 4 separation

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2021-06, Vol.9 (21), p.12593-12605
• Main Authors: Jeong, Yanghwan, Lee, Minseong, Lee, Gihoon, Hong, Sungwon, Jang, Eunhee, Choi, Nakwon, Choi, Jungkyu
• Format: Article
• Language: English
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/D1TA01286J

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 nm 2 ) is ideal for molecular-sieve-based CO 2 (0.33 nm) separation from CH 4 (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 O 3 eliminated defect formation and, thus, achieved a very high performance for dry CO 2 permselectivity over CH 4 (CO 2 /CH 4 separation factor (SF) of ca. 523 ± 96 at ca. 50 °C, which is a representative temperature of biogas streams). Surprisingly, high separation performances (CO 2 /CH 4 SF of 422) for water-vapor-containing CO 2 /CH 4 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 O 2 . The defect structures were quantitatively analyzed by combining fluorescence confocal optical microscopy with gas-permeation modeling. Furthermore, the inhibition of water-molecule-adsorption on CO 2 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 CO 2 permselectivities for wet CO 2 /CH 4 mixtures. In contrast, the elimination of defects by calcination in O 3 was the key to achieving a very high dry CO 2 /CH 4 separation performance.