Optimized Separation of Acetylene from Carbon Dioxide and Ethylene in a Microporous Material

Abstract

Selective separation of acetylene (C₂H₂) from carbon dioxide (CO₂) or ethylene (C₂H₄) needs specific porous materials whose pores can realize sieving effects while pore surfaces can differentiate their recognitions for these molecules of similar molecular sizes and physical properties. We report a microporous material [Zn(dps)₂(SiF₆)] (UTSA-300, dps = 4,4'-dipyridylsulfide) with two-dimensional channels of about 3.3 Å, well-matched for the molecular sizes of C₂H₂. After activation, the network was transformed to its closed-pore phase, UTSA-300a, with dispersed 0D cavities, accompanied by conformation change of the pyridyl ligand and rotation of SiF₆^2- pillars. Strong C-H···F and π-π stacking interactions are found in closed-pore UTSA-300a, resulting in shrinkage of the structure. Interestingly, UTSA-300a takes up quite a large amounts of acetylene (76.4 cm³ g^-1), while showing complete C₂H₄ and CO₂ exclusion from C₂H₂ under ambient conditions. Neutron powder diffraction and molecular modeling studies clearly reveal that a C₂H₂ molecule primarily binds to two hexafluorosilicate F atoms in a head-on orientation, breaking the original intranetwork hydrogen bond and subsequently expanding to open-pore structure. Crystal structures, gas sorption isotherms, molecular modeling, experimental breakthrough experiment, and selectivity calculation comprehensively demonstrated this unique metal-organic framework material for highly selective C₂H₂/CO₂ and C₂H₂/C₂H₄ separation.