Single-phase covalent organic frameworks membranes make CO2-selective separation possible

A research group led by Prof. Yang Weishen and Dr. Peng Yuan from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) developed a novel strategy to fabricate single-phase CO₂-selective covalent organic frameworks (COFs) nanosheet membranes featured with both high CO₂/H₂ separation factors and high CO₂ permeance.

This study was published in Angewandte Chemie International Edition on July 20.

Two-dimensional (2D) COFs, a class of crystalline organic skeletons possessing permanent one-dimensional pores, are promising to serve as high-quality membrane materials in CO₂ capture field. However, their inherent pore sizes (>0.8 nm) are too large to provide desired molecular sieve capability for gas separations currently.

The researchers exfoliated three kinds of 2D COFs, that is, TpPa-1, TpPa-2 and TpHz, with an average particle size of 3 um and different pore sizes into 2 nm-thick nanosheets. Intact crystallinities and functionalities were well-retained after exfoliation treatment.

They found that even the TpHz with relatively smallest inherent pore size could not exhibit desired gas separation performance in a form of conventional bulk membrane. With ultrathin nanosheets served as membrane building units, they developed novel COF nanosheet membranes exhibiting distinctive staggered stacking micro-structures.

“Just like the fishing net, you can irregularly pile up several nets to catch tiny fish,” said Dr. Peng.

The intriguing membrane structures in collaboration with inherent CO₂-selective adsorption capacities of COF frameworks endowed the COF nanosheet membranes with permeation priority of large CO₂ molecules from mixed CO₂/H₂ gas based on a surface diffusion mechanism.

Among the membranes, TpPa-2 nanosheet membranes with medium pore size showed the highest CO₂/H₂ separation factor and CO₂ permeance, which reached the target with commercial feasibility for syngas separations.

The researchers identified that the TpPa-2 nanosheet membranes exhibited proper narrowed pores derived from the staggered stacking patterns, which allowed exactly two columns of CO₂ molecules to pass through and maximized the blocking effect for H2 passage.

This study provides a new pore engineering strategy for the design and manufacture of high-performance COF membranes in gas separation realm.

“Taking full advantage of this pore-engineering strategy, different categories of COFs with diversified pore structures can enable specified membrane-based gas separations,” said Prof. Yang.