The adsorptive separation of CH₄ from CO₂ is a promising process for upgrading natural gas. However, thermodynamically selective adsorbents exhibit a strong affinity for CO₂ and thus require a high energy compensation for regeneration. Instead, kinetic separation is preferred for a pressure swing adsorption process, although precise control of the aperture size to achieve a tremendous discrepancy in diffusion rates remains challenging. Here, we report a guest solvent-directed strategy for fine-tuning the pore size at a sub-angstrom precision to realize highly efficient kinetic separation. A series of metal–organic frameworks (MOFs) with isomeric pore surface chemistry were constructed from 4,4′-(hexafluoroisopropylidene)-bis(benzoic acid) and dicopper paddlewheel notes. The resultant CuFMOF·CH₃OH (CuFMOF-c) exhibits an excellent kinetic separation performance thanks to a periodically expanding and contracting aperture with the ideal bottleneck size, which enables the effective trapping of CO₂ and impedes the diffusion of CH₄, offering an ultrahigh kinetic selectivity (273.5) and equilibrium-kinetic combined selectivity (64.2). Molecular dynamics calculations elucidate the separation mechanism, and breakthrough experiments validate the separation performance.