Functional group tuning of CAU-10(Al) for efficient C2H2 storage and C2H2/CO2 separation

Abstract

Adsorptive separation of acetylene (C₂H₂) from a C₂H₂/CO₂ mixture has emerged as an alternative to replace cryogenic distillation to obtain high purity C₂H₂ in a safer manner. Al-based metal–organic frameworks, such as MIL-160 and CAU-10–NH₂, have been reported as good acetylene adsorbents with high selectivity and high acetylene packing density. According to this, incorporation of hetero atoms in the MOF pore wall clearly has a positive impact on enhancing C₂H₂ capture performance. Nonetheless, a systematic evaluation of the role played by functional groups within the same MOF structure in their C₂H₂ sorption performance is still required. In this study, CAU-10 samples were prepared according to the functional group using Al metal ions and various functionalized organic linkers, including pristine terephthalate (–H) and its modified forms (–OH, –F, –NO₂, –NH₂, and –CH₃) to investigate their C₂H₂ and CO₂ adsorption properties. The C₂H₂/CO₂ separation performance of the adsorbents was investigated using single gas adsorption isotherms and by application of the ideal adsorbed solution theory as well as dynamic breakthrough experiments for the gas mixture under dry and humid conditions. Interestingly, CAU-10–CH₃ was demonstrated to exhibit a record high packing density of 543 g L⁻¹ at 25 °C for acetylene, 2.8 times higher than the CO₂ uptake at 1 bar. A breakthrough mixture experiment using CAU-10–CH₃, and CAU-10–NH₂ as comparative samples confirmed CAU-10–CH₃ as a prominent candidate for acetylene selective adsorption under both dry and humid conditions. The microscopic origin of these attractive adsorption/separation performances was understood from density functional theory calculations and single component/binary mixture grand canonical Monte Carlo simulations.