<p>Low-concentration methane (CH₄) resources, such as coalbed methane (CBM) and coal mine methane (CMM), represent a vast but underutilized source of clean energy, primarily due to the difficulty in separating CH₄ from nitrogen (N₂). Adsorptive separation offers a promising pathway, yet conventional adsorbents suffer from limited selectivity. Here, we report a substantial improvement in the CH₄/N₂ selectivity of large-pore FAU zeolites by modulating their cations with smaller charge-to-size ratios. Combined analyses of adsorption isotherms, isosteric heats, and density functional theory (DFT) binding energies reveal that this strategy suppresses N₂ adsorption by weakening gas-cation electrostatic interactions, while concurrently enhancing CH₄ uptake through confinement effects that enable a CH₄ molecule to interact with multiple cations. By leveraging this strategy, Cs/TMA-Y, incorporating the cations featuring the smallest charge-to-size ratio in this study (cesium: Cs⁺ and tetramethylammonium: TMA⁺), exhibited the highest CH₄/N₂ separation factor under both static and dynamic conditions, along with excellent reusability. Notably, Cs/TMA-Y also delivered the highest CH₄/N₂ selectivity (7.5) reported to date under dynamic binary conditions (50/50, vomlue ratio). Process simulations further identified vacuum swing adsorption (VSA) as the most effective operational mode, highlighting the practical potential of this material. This study establishes a mechanistic framework for cation-controlled CH₄/N₂ separation and provides new design principles for zeolitic adsorbents targeting efficient methane upgrading. Furthermore, this strategy opens a promising pathway to enhance confinement effects in medium- and large-pore zeolites, extending their applicability to a broad range of adsorption- and catalysis-related applications.</p>