All-silica DDR (decadodecasil 3R, Si-DDR) zeolites with a pore size of 0.36 x 0.44 nm(2) are highly desirable for separating CO2 (0.33 nm) from N-2 (0.364 nm) on the basis of the size difference. Despite their potential as CO2 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 CO2, N-2, and H2O, the three main components in the flue gas from coal-fired power plants, in Si-DDR particles and the corresponding heats of adsorption (similar to 25-27, 1522, and similar to 32-40 kJ.mol(-1), respectively) were comparable and in good agreement with the literature data. The resulting CO2/N-2 ideal sorption and permeation selectivities were estimated to be similar to 15.3-18.0 and similar to 7.7-9.0 at 303 K, respectively, indicating that Si-DDR zeolites can serve as adsorbents and membranes for CO2/N-2 separations. In addition, we demonstrated that Al atoms could be incorporated into the DDR framework via seeded growth. The adsorption of H2O 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. (C) 2016 Elsevier B.V. All rights reserved.