Electrostatic force-driven lattice water bridging to stabilize a partially charged indium MOF for efficient separation of C2H2/CO2 mixtures

Abstract

Highly stable metal–organic frameworks (MOFs) with accessible sites have gradually been valued because of their excellent performance. Here, we proposed a steric hindrance effect to construct a partially charged MOF with electrostatic forces for efficient C₂H₂/CO₂ separation. Two MOFs, [Me₂NH₂]·[In(BDTA)] 5.5DMF·4H₂O (FJU-117, BDTA is [4,4′,4′′,4′′′-([1,1′-biphenyl]-4,4′-diylbis(azanetriyl)) tetrabenzoic acid]) and [In(4Me-BDTA)·0.5H₂O]·6DMF·2H₂O (FJU-118) were synthesized using a similar procedure except that BDTA in FJU-117 was replaced with 4Me-BDTA in FJU-118. The four methyl groups in 4Me-BDTA in FJU-118 obviously restrict the rotation of the two C–N bonds of ligands. The steric hindrance effect of the 4Me-BDTA results in 7-coordinated In³⁺ ions in FJU-118, compared to 8-coordinated ones in FJU-117. Accessible sites in FJU-118 with partially remaining charge can provide electrostatic force to anchor lattice water. Electrostatic force-driven lattice water bridging stabilizes the FJU-118 framework. The activated sample FJU-118a exhibits the third highest Brunauer–Emmett–Teller (BET)/Langmuir surface area (1860/2106 m² g⁻¹) among the reported indium MOFs. The absorption capacities for C₂H₂ and CO₂ at 296/273 K are 88.6/155.1 cm³ g⁻¹ and 35.6/66.9 cm³ g⁻¹, respectively. The single crystal results show that the partially remaining charge on indium sites has an electrostatic force on the –CC– bond of C₂H₂ molecules for the selective separation of C₂H₂/CO₂ mixtures.