Heating-driven assembly of covalent organic framework nanosheets for gas separation

Covalent organic frameworks (COFs) are a family of promising membrane materials because of their intrinsic pores with uniform size, tunable functionalities, and high stability. Especially, two-dimensional (2D) COFs can be exfoliated into single- or few-layered nanosheets and restacked into molecular sieving membranes through controlling restacking modes and thickness. However, traditional methods mainly rely on increasing the thickness (vertical restacking control) to achieve high separation selectivity but with compromised permeance. Herein, the fabrication of ultrathin COF (CTF-BTD) membranes through the restacking of the nanosheets with the aid of introducing thermal perturbation is reported, i.e., a heating vacuum filtration method to optimize the relative displacement of nanosheets without sacrificing thickness control. As a result, a membrane exhibits high H2 permeance (655.6 GPU) and H2/CO2 selectivity (43.1) for CO2 pre-combustion capture, which are superior than those of the membranes prepared at room temperature. This work realizes an enhanced separation performance through the control of restacking modes, which will inspire the optimization of other membrane architectures.