Adsorptive Separation of C1–C2 Hydrocarbons in COFs by Microenvironment Modulation

The separation of light hydrocarbons (C1–C2) from different bicomponent mixed gases is considered one of the most critically important and complex processes in the chemical industry. Covalent organic frameworks (COFs) with a tunable structural microenvironment have emerged as a class of crystalline porous organic materials with huge potential for energy-efficient and environmentally friendly adsorptive-based gas separation. In this work, NDA-TAPA, an imide-based COF, and TFPA-TAPA, an imine-based COF, have been synthesized from a common node: tris(4-aminophenyl)amine (TAPA). Next, their performance in the adsorptive separation of various industrially important gas mixtures, including C2H2/C2H4, C2H2/CO2, C2H6/CH4, and CO2/CH4, has been examined. Despite having comparable BET surface areas, the imide-based NDA-TAPA COF demonstrated a superior separation performance compared to the imine-based TFPA-TAPA COF. This can be attributed to the differences in the pore width, polarity, and microenvironment around the pores in the two COFs. After calculating the isosteric heat of adsorption (Qst) and IAST selectivity, real-time dynamic breakthrough separation experiments were conducted on all binary gas mixtures. Finally, the adsorption binding sites and different adsorbent–adsorbate interactions have been investigated by density functional theory (DFT) calculations, which support the experimental findings.