Reaction duration-dependent formation of two Cu(ii)-MOFs with selective adsorption properties of C3H4 over C3H6

Abstract

The formation of metal–organic frameworks (MOFs) under given reaction conditions depends on various factors including reaction duration, temperature, used solvent, system pH, and others. Among them, the reaction duration is relatively less investigated. In this work, based on a Cu(II)-MOF system, the reaction duration was found to play an important role in directing the formation of two different products, (NH₂(CH₃)₂)[Cu₁₂(DDPN)₆(H₂O)₁₀Cl] (BUT-301) on shorter reaction time and (NH₂(CH₃)₂)₂[Cu(DDPN)] (BUT-302) on longer reaction time, when CuCl₂ reacted with 3,5-di(3,5-dicarboxylphenyl)nitrobenzene (H₄DDPN) in a DMA/MeOH mixed solvent at 120 °C. With increasing the reaction duration, BUT-301 can transform into BUT-302. Both MOFs have three-dimensional (3D) framework structures. BUT-301 is constructed from paddle-wheel Cu₂(COO)₄ units and DDPN^4– ligands as four-connected linkers, while BUT-302 is assembled from mononuclear Cu(II) centers connecting the ligands as also four-connected linkers but in a monodentate coordination fashion. N₂ adsorption confirmed that both MOFs are porous materials with the Brunauer–Emmett–Teller (BET) surface areas of 1953 and 561 m² g⁻¹, respectively. Interestingly, both BUT-301 and -302 show selective adsorption properties of C₃H₄ over C₃H₆. The C₃H₄/C₃H₆ adsorption selectivities were calculated to be 1.9 and 4.4 at 0.1 bar and 298 K by the ideal adsorbed solution theory (IAST) for a 1 : 99 C₃H₄/C₃H₆ mixture, respectively.