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All-silica DDR (decadodecasil 3R, Si-DDR) zeolites with a pore size of 0.36 × 0.44 nm 2 are highly desirable for separating CO 2 (0.33 nm) from N 2 (0.364 nm) on the basis of the size difference. Despite their potential as CO 2 separators, the synthetic protocols that allow for the mono-dispersed DDR zeolite particles have not been systematically investigated. Here, we found that a seeded growth of irregular Si-DDR particles, obtained by non-seeded growth, resulted in mono-dispersed, diamond-like Si-DDR particles. Regardless of the origin of seeds, the size of the Si-DDR particles was decreased with increasing seed amount. Adsorption isotherms of CO 2 , N 2 , and H 2 O, the three main components in the flue gas from coal-fired power plants, in Si-DDR particles and the corresponding heats of adsorption (∼25–27, ∼15–22, and ∼32–40 kJ·mol −1 , respectively) were comparable and in good agreement with the literature data. The resulting CO 2 /N 2 ideal sorption and permeation selectivities were estimated to be ∼15.3–18.0 and ∼7.7–9.0 at 303 K, respectively, indicating that Si-DDR zeolites can serve as adsorbents and membranes for CO 2 /N 2 separations. In addition, we demonstrated that Al atoms could be incorporated into the DDR framework via seeded growth. The adsorption of H 2 O depends considerably on the concentration of defects (mainly OH groups) present in the Si-DDR framework and more strongly on Al species incorporated into the DDR framework. [ABSTRACT FROM AUTHOR]