Abstract: Imine-linked covalent organic frameworks (COFs) have been attracted considerable attention as stationary phases for chromatographic separation and as sample pretreatment materials. However, existing synthesis methods have the difficulty in controlling morphologies and shapes of COFs, limiting their practical applications. In this work, we report a new method for preparation of COF monoliths through the aldehyde-amine cryopolymerization reactions. Briefly, a homogenous prepolymerization mixture of 5% monomers and dimethyl sulfoxide was frozen in liquid nitrogen bath using a unidirectional freezing technique. The solvent of dimethyl sulfoxide formed crystals, in which the monomers were confined and then in-situ polymerized at -18 ^oC. The resulting amorphous polymeric monolith was then dried, immersed in suitable solvents for crystallization treatment, and finally transformed into crystal framework monoliths. The characterization using scanning electron microscopy, powder X-ray diffraction instrument and nitrogen adsorption measurement indicated that crystal framework monoliths possess hierarchical porosity including unidirectional macropores and ordered nanopores. The versatility of our two-step method of the freeze shaping followed by crystallization was illustrated by using different building blocks. It is anticipated that this method can produce COFs with desired shapes and morphologies to meet practical application requirements in the field of separations and adsorptions.

Key words: covalent organic frameworks, monolith, cryopolymerization, through-pore