<p>Greenhouse gas emissions remain a critical global concern. The adsorption and separation of biogas, primarily comprising carbon dioxide (CO₂) and methane (CH₄), present significant challenges. In this study, a modified chabazite zeolite containing different fractions of potassium and sodium as extraframework cations was used as an adsorbent. For the adsorption/separation of a mixture of CO₂ and CH₄ at 33:67 vol% at 298&#xa0;K and 250&#xa0;kPa, the 94K/Na-chabazite adsorbent (molar ratio of extraframework cations potassium (K) and sodium (Na) is 94:6) displays superior performance of effective separating this mixture. It exhibits the highest CO₂ adsorption capacity of 2.68&#xa0;mmol/g and dynamic selectivity for CO₂/CH₄ of 9.26 compared to chabazite adsorbents with K:Na ratios of 25:75 and 100:0. With larger cation of K, 100K-chabazite can create a pore constraint that selectively hindered the diffusion of larger CH₄ molecules while allowing CO₂ diffusion to a greater extent. In addition, the extraframework cation of Na, which has a higher electronegativity and cationic density than the K cation, contributes to a stronger electrical field gradient within the chabazite structure, thereby enhancing CO₂ adsorption. The balance between these properties contributes to a greater performance. However, the fastest CO₂ adsorption kinetics depend on the presence of a large amount of Na cations in the structure. Compared to benchmark commercial Na-X and Na-A zeolites, 94K/Na-chabazite is outperform for CO₂ adsorption capacity, dynamic selectivity, and CO₂ adsorption kinetics. The excellent architectural structure, high density of adsorption sites, and appropriate composition of 94K/Na-chabazite in dynamic separation experiments highlight its potential as a promising and competitive adsorbent for the industrial separation of CO₂ and CH₄.</p>