Horizontal Restacking Control of Covalent Organic Framework Nanosheets
for Ultrathin Gas Separation Membranes
Abstract
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, the traditional methods mainly
rely on increasing the thickness (vertical restacking control) to
achieve high separation selectivity but with limited permeance. Herein,
the fabrication of ultrathin COF membranes through a horizontal
restacking control with the aid of introducing thermal perturbation is
reported, i.e., a heating vacuum filtration method to optimize the
relative displacement of nanosheets in horizontal direction without
sacrificing thickness control. The obtained membrane exhibits high H2
permeance (655.6 GPU) and H2/CO2 selectivity (43.2) for CO2
pre-combustion capture. Furthermore, this method also displayed its
universality to control the restacking microstructures of other COF
nanosheets and the resulted properties of membranes.