Manipulating the organic counter cations, which serve as pore gatekeepers to selectively obstruct the channels in anionic metal-organic frameworks (MOFs), offers a highly effective strategy for optimizing the separation performance. Here, we report an yttrium-based MOF, Y-ebdc, featuring cage-type structures that accommodate protonated dimethylamine (DMA) as both counter cations and molecular sieving gates. Subsequent optimization of the adsorption separation performance for propylene/propane (C₃H₆/C₃H₈) was achieved through regulation of DMA’s thermal decomposition. The temperature dependence of DMA decomposition was elucidated using temperature-resolved in situ infrared spectroscopy and breakthrough studies. With approximately 70% of DMA removed, the expanded aperture window and increased pore volume remarkably enhance dynamic C₃H₆ uptake while simultaneously facilitating the direct production of polymer-grade (> 99.5%) C₃H₆ in a single adsorption–desorption cycle. This study exemplifies how engineering the pore environment via co-existing counter cations within MOFs can effectively boost gas adsorption and separation performance.