Can COFs replace MOFs in flue gas separation? high-throughput computational screening of COFs for CO2/N2 separation

Abstract

Covalent organic frameworks (COFs) are under study as adsorbent and membrane candidates for gas separation applications. However, experimental testing of all synthesized COF materials as adsorbents and membranes under different operating conditions is not practical. Herein, we used a high-throughput computational screening approach to investigate adsorption- and membrane-based flue gas separation performances of 295 COFs. Adsorption selectivity, working capacity, percent regenerability and adsorbent performance score of COFs were calculated for separation of CO₂/N₂ mixture for three different cyclic adsorption processes, pressure swing adsorption (PSA), vacuum swing adsorption (VSA) and temperature swing adsorption (TSA). The top performing COFs were identified for each process based on the combination of several metrics. Selectivities of the top COFs were predicted to be greater than those of zeolites and activated carbons. Molecular simulations were performed considering the wet flue gas for the top COF adsorbents and results revealed that most COFs retained their high CO₂ selectivities in the presence of water. Using COFs with detailed geometry optimization and high-accuracy partial charges in molecular simulations led to lower selectivities and adsorbent performance scores compared to using experimentally reported COFs with approximate charges. Membrane-based flue gas separation performances of COFs were also studied and most COFs were found to have comparable CO₂ permeabilities with metal organic frameworks (MOFs), up to 3.96 × 10⁶ barrer, however their membrane selectivities were lower than MOFs, 0.38–21, due to their large pores and the lack of metal sites in their frameworks. Structure–performance relations revealed that among the COFs we studied, the ones with pore sizes <10 Å, accessible surface areas <4500 m² g⁻¹ and 0.6 < porosity <0.8 are not only highly selective adsorbents but also CO₂ selective membranes.