Binder-free 3D printing of covalent organic framework (COF) monoliths for CO2 adsorption

Covalent organic frameworks (COFs) present a large group of crystalline porous polymeric materials, which are formed with organic building blocks by strong covalent bonds. They have great potential in energy, environment, and biotechnology areas due to their high surface areas, tunable pore size distribution, and versatile functionality. However, they are largely synthesized in powder form only. The recent 3D printing technology requires the addition of binders. For the targeted applications in gas separation and adsorption, the existence of any binders not only increases the cost, but also reduces the surface area of the active materials and adds on dummy weight, which is especially harmful to functional porous materials where the retention of surface functionality and surface area is paramount. Herein, we report a binder-free, 3D printing process for COF materials (SNW-1 and ATFG-COF), capable of producing free-standing, crack-free 3D COF monoliths by the control of the solvent choices and their diffusion and evaporation among the different parts of the monoliths. Compared to SNW-1 with F127 binder added, the binder-free 3D monolith shows a larger surface area of 794 m2/g, which is similar to SNW-1 powder. The higher CO2 adsorption and CO2/N2 sorption selectivity compared to monoliths made with F127 binder further demonstrate the advantage of binder-free, 3D printing technology applied to COFs