A typical phosphonic acid metal–organic framework, Ni-STA-12, is synthesized and its unique C₂H₂/CO₂ separation mechanism is explored. The widely distributed adsorption sites in the activated framework effectively adsorbed C₂H₂ with good selectivity.

Due to their comparable molecular dimensions and volatility, distinguishing C₂H₂ from CO₂ during purification remains a significant challenge in industrial applications. Achieving effective isolation of C₂H₂ from binary mixtures of C₂H₂/CO₂ is therefore a critical objective in petrochemical processes. Herein, an adsorption mechanism enabling selective C₂H₂/CO₂ separation has been elucidated in the phosphonate metal–organic framework (MOF) Ni-STA-12. The high C₂H₂ uptake and remarkable CO₂ selectivity of Ni-STA-12 arise from the synergistic effect of diverse adsorption sites distributed throughout its structure, including various oxygen atoms and open metal sites. The adsorbed C₂H₂ interacts strongly with the exposed adsorption sites in the framework and its binding capacity is much larger than that of CO₂. Dynamic breakthrough experiments demonstrated the practical potential for the separation of C₂H₂ in mixtures, and excellent separation potential (Δq) demonstrating high C₂H₂ recovery from C₂H₂/CO₂ mixtures. Theoretical calculations show the synergistic interaction of various oxygen atoms of the MOF with the open metal site Ni and the dominant role of uncoordinated oxygen atoms in the adsorption of C₂H₂.