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Microporous chabazite (CHA) zeolite is very promising for CO 2 capture because of its appropriate pores with molecular dimensions for the preferential adsorption of CO 2 molecules. Herein, CHA type zeolite particles and membranes were prepared by using a seeded growth method in the absence of an organic structure directing agent (OSDA) or template. After substantial effort to find appropriate and reliable conditions for obtaining continuous CHA type zeolite membranes, it was recognized that the formation of these membranes is a highly sensitive function of the Si/Al ratio in the synthetic precursor. Using the appropriate Si/Al ratio of ~50, OSDA-free CHA type zeolite membranes were manufactured with high reproducibility. The resulting OSDA-free CHA type zeolite membranes showed maximum CO 2 /N 2 and CO 2 /CH 4 separation factors of ~12.5 ± 3.8 and ~28.8 ± 6.9, respectively, with a moderate CO 2 permeance of ~1 × 10 −7 mol m −2 s −1 Pa −1 . Notably, under more realistic wet conditions (i.e., in the presence of H 2 O vapor), the separation performance at temperatures above 75 °C was comparable to that obtained under dry conditions, although permeation was hindered below 50 °C, apparently due to the strong adsorption of H 2 O vapor. [ABSTRACT FROM AUTHOR]