Single-Molecule Traps in Covalent Organic Frameworks for Selective Capture of C2H2 from C2H4-Rich Gas Mixtures.

Abstract

Removing trace amounts of acetylene (C₂H₂) from ethylene (C₂H₄)-rich gas mixtures is vital for the supply of high-purity C₂H₄ to the chemical industry and plastics sector. However, selective removal of C₂H₂ is challenging due to the similar physical and chemical properties of C₂H₂ and C₂H₄. Here, we report a "single-molecule trap" strategy that utilizes electrostatic interactions between the one-dimensional (1D) channel of a covalent organic framework (denoted as COF-1) and C₂H₂ molecules to massively enhance the adsorption selectivity toward C₂H₂ over C₂H₄. C₂H₂ molecules are immobilized via interactions with the O atom of C=O groups, the N atom of C≡N groups, and the H atom of phenyl groups in 1D channels of COF-1. Due to its exceptionally high affinity for C₂H₂, COF-1 delivered a remarkable C₂H₂ uptake of 7.97 cm³/g at 298 K and 0.01 bar, surpassing all reported COFs and many other state-of-the-art adsorbents under similar conditions. Further, COF-1 demonstrated outstanding performance for the separation of C₂H₂ and C₂H₄ in breakthrough experiments under dynamic conditions. COF-1 adsorbed C₂H₂ at a capacity of 0.17 cm³/g at 2,000 s/g when exposed to 0.5 ml/min C₂H₄-rich gas mixture (99% C₂H₄) at 298 K, directly producing high-purity C₂H₄ gas at a rate of 3.95 cm³/g. Computational simulations showed that the strong affinity between C₂H₂ and the single-molecule traps of COF-1 were responsible for the excellent separation performance. COF-1 is also robust, providing a promising new strategy for the efficient removal of trace amounts of C₂H₂ in practical C₂H₄ purification.