Rational regulation of acetylene adsorption and separation for ultra-microporous copper-1,2,4-triazolate frameworks...

It is well known that the introduction of exposed fluorine (F) sites into metal-organic frameworks (MOFs) can effectively promote acetylene (C₂H₂) adsorption via C-H⋯F hydrogen bonds. However, such super strong hydrogen bonding interactions usually lead to very high acetylene adsorption enthalpy and thus require more energy during the adsorbent regeneration process. As the same group elements, chlorine (Cl), bromine (Br) and iodine (I) also can act as hydrogen bond acceptors but with relatively weak forces. So, it is speculated that the decoration of Cl, Br or I sites on the pore surface of MOF adsorbents may enhance acetylene adsorption but with lower energy consumption. Herein, ultra-microporous MOFs constructed by Cu₄X₄ (X = Cl, Br, I) motifs and 1,2,4-triazolate linkers, namely, [Cu₈X₄(TRZ)₄] (TRZ = 3,5-diethyl-1,2,4-triazole or detrz for SNNU-313-X, and 3,5-dipropyl-1,2,4-triazole or dptrz for SNNU-314-X), provide an ideal platform to investigate the effect of C-H⋯X (X = Cl, Br, I) hydrogen bonding on C₂H₂ adsorption and purification performance. Benefiting from the small pore size and pore environment, the C₂H₂ uptake and separation properties of this series of MOFs are systematically regulated. Detailed gas adsorption results show that with the same organic linker, the C₂H₂ uptake and separation (C₂H₂/C₂H₄ and C₂H₂/CO₂) performance decrease clearly with the electronegativity of halogen ions (SNNU-313-Cl > SNNU-313-Br > SNNU-313-I). With the same halogen ion, the gas adsorption decreases with the bulk of decorated alkyl groups (SNNU-313-Cl > SNNU-314-Cl). Remarkably, SNNU-313/314 series MOF adsorbents exhibit moderate C₂H₂ uptake capacity and high separation ability, but the C₂H₂ adsorption enthalpies are much lower than those of MOF materials with exposed F sites. Dynamic fixed-bed column breakthrough experiments and Grand Canonical Monte Carlo (GCMC) simulations further indicate the critical effects of halogen hydrogen bonds on acetylene adsorption and separation. Overall, this work demonstrated an effective regulation of acetylene adsorption and separation by rational C-H⋯X hydrogen bonding, which may provide a new route for the exploration of energy-efficient acetylene adsorbent materials.