Reverse Separation of Carbon Dioxide and Acetylene in Two Isostructural Copper Pyridine-Carboxylate Frameworks

Separating acetylene from carbon dioxide is important but highly challenging due to their similar molecular shapes and physical properties. Adsorptive separation of carbon dioxide from acetylene can directly produce pure acetylene but is hardly realized because of relatively polarizable acetylene binds more strongly. Here, we reverse the CO₂ and C₂H₂ separation by adjusting the pore structures in two isoreticular ultramicroporous metal-organic frameworks (MOFs). Under ambient conditions, copper isonicotinate (Cu(ina)₂), with relatively large pore channels shows C₂H₂-selective adsorption with a C₂H₂/CO₂ selectivity of 3.4, whereas its smaller-pore analogue, copper quinoline-5-carboxylate (Cu(Qc)₂) shows an inverse CO₂/C₂H₂ selectivity of 5.6. Cu(Qc)₂ shows compact pore space that well matches the optimal orientation of CO₂ but is not compatible for C₂H₂. Neutron powder diffraction experiments confirmed that CO₂ molecules adopt preferential orientation along the pore channels during adsorption binding, whereas C₂H₂ molecules bind in an opposite fashion with distorted configurations due to their opposite quadrupole moments. Dynamic breakthrough experiments have validated the separation performance of Cu(Qc)₂ for CO₂/C₂H₂ separation.