AbstractRealization of ideal molecular sieves, in which the larger gas molecules are completely blocked without sacrificing high adsorption capacities of the preferred smaller gas molecules, can significantly reduce energy costs for gas separation and purification and thus facilitate a possible technological transformation from the traditional energy‐intensive cryogenic distillation to the energy‐efficient, adsorbent‐based separation and purification in the future. Although extensive research endeavors are pursued to target ideal molecular sieves among diverse porous materials, over the past several decades, ideal molecular sieves for the separation and purification of light hydrocarbons are rarely realized. Herein, an ideal porous material, SIFSIX‐14‐Cu‐i (also termed as UTSA‐200), is reported with ultrafine tuning of pore size (3.4 Å) to effectively block ethylene (C₂H₄) molecules but to take up a record‐high amount of acetylene (C₂H₂, 58 cm³ cm⁻³ under 0.01 bar and 298 K). The material therefore sets up new benchmarks for both the adsorption capacity and selectivity, and thus provides a record purification capacity for the removal of trace C₂H₂ from C₂H₄ with 1.18 mmol g⁻¹ C₂H₂ uptake capacity from a 1/99 C₂H₂/C₂H₄ mixture to produce 99.9999% pure C₂H₄ (much higher than the acceptable purity of 99.996% for polymer‐grade C₂H₄), as demonstrated by experimental breakthrough curves.