Development of MOF-5-like ultra-microporous metal-squarate frameworks for efficient acetylene storage and separation

Abstract

Although MOF-5 and its related family members are landmark metal–organic frameworks (MOFs), they have large specific surface area and high porosity, which have long been a subject regarding their instability. The introduction of new inorganic building blocks, in addition to classical [Zn₄O(COO)₆], is a promising way to maintain the highly attractive architecture of MOF-5. Herein, a family of new MOF-5-like frameworks were synthesized by using squaric acid (SQ) and transition metal salts, namely, {(NH(CH₃)₂)₂[M₄X₄(SQ)₃]}_n (SNNU-505-M, M = Zn, Co and Ni, X = F⁻ and OH⁻). In this SNNU-505-M series of compounds, except for the μ₃-bridging atom of SNNU-505-Zn being an F⁻ ion, all the others are OH⁻. Compared with MOF-5, SNNU-505-M exhibits good stability in organic solvent and aqueous solutions (pH = 3–13). Furthermore, benefiting from its ultra-micropore size and multiple polarity F⁻/OH⁻ adsorption sites, SNNU-505-M shows high C₂H₂ adsorption capacity and excellent separation performance. In particular, SNNU-505-Zn exhibits the best C₂H₂/C₂H₄ (1 : 1) separation performance and its breakthrough interval time is 51 min g⁻¹ at 298 K at a flow rate of 2 mL min⁻¹. The GCMC calculations further demonstrate that the excellent C₂H₂ uptake and C₂H₂/C₂H₄ separation performance of SNNU-505-Zn can be mainly attributed to the multiple C–H⋯F hydrogen bonds between the C₂H₂ molecules and MOF skeleton.