| Summary: | Thin pure-silica chabazite (Si-CHA) membranes have been synthesized by using a secondary growth method on a porous silica substrate. A CO<sub>2</sub> permeance of 2.62 × 10<sup>−6</sup> mol m<sup>−2</sup> s<sup>−1</sup> Pa<sup>−1</sup> with a CO<sub>2</sub>/CH<sub>4</sub> permeance ratio of 62 was obtained through a Si-CHA membrane crystallized for 8 h using a parent gel of H<sub>2</sub>O/SiO<sub>2</sub> ratio of 4.6. The CO<sub>2</sub> permeance through the Si-CHA membrane on a porous silica substrate was twice as high as that through the membrane synthesized on a porous alumina substrate, which displayed a similar zeolite layer thickness.
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