[Preparation of imine-linked covalent organic framework monoliths through aldehyde-amine cryopolymerization]

Imine-linked covalent organic frameworks （COFs） have attracted considerable attention as stationary phases for chromatographic separation and as sample pretreatment materials，owing to their exceptional properties including high specific surface areas，well-defined nanopores，and excellent thermal and chemical stability. However，the practical deployment of COFs in separation science is still hampered by the limited control over their macroscopic morphologies and microscopic architectures，especially when they are constructed as monolithic materials that require continuous through-pores to facilitate rapid mass transfer and low backpressure. In our previous work，we discovered that the aldehyde‑amine polycondensation reaction could proceed under cryogenic conditions，and that using 1，4‑dioxane as the solvent led to the formation of pine needle‑like crystals upon freezing. These crystals acted as sacrificial templates to create unidirectional macroporous channels within the resulting COF monoliths. Nevertheless，the structural regularity of these channels was less than ideal. This motivated us to search for alternative solvents that could form more regular crystal templates. In this work，we propose dimethyl sulfoxide （DMSO） as a new template for the aldehyde‑amine polymerization system. We found that DMSO，when subjected to unidirectional freezing，forms highly uniform needle‑shaped crystals that are different from the pine‑needle morphology of 1，4‑dioxane crystals. These DMSO crystals provide a more orderly template，guiding the monomer assembly into a well‑aligned framework. Based on this finding，we have developed a novel two‑step approach for preparing imine‑linked COF monoliths. Briefly，a homogeneous prepolymerization solution containing 5% （mass fraction） total monomers （a combination of multi‑aldehyde and multi‑amine building blocks） in DMSO is prepared at room temperature. The solution is then cast into a mold and subjected to unidirectional freezing in liquid nitrogen，which forces the DMSO to crystallize in an oriented manner，forming needle‑like crystals. During this process，the monomers are excluded from the growing crystals and concentrated in the narrow gaps between them. The frozen system is then transferred to a -18 °C cryostat and allowed to react for a certain period，during which the monomers polymerize into an amorphous polymer. After washing to remove DMSO and drying under vacuum，the obtained amorphous monolith is subjected to a crystallization treatment in a suitable solvent mixture under solvothermal conditions，which triggers the structural rearrangement into crystalline COFs，while preserving the macroscopic channel architecture. The resulting COF monoliths were characterized by scanning electron microscopy （SEM），powder X‑ray diffraction （PXRD），and nitrogen physisorption measurements. SEM images show well‑oriented，unidirectional macroporous channels with diameters in the micrometer range. Compared with the materials prepared using 1，4‑dioxane templates，the channels obtained with DMSO are significantly more regular，with smoother inner walls. PXRD patterns confirm the crystallinity of COF monoliths. The Brunauer-Emmett-Teller （BET） surface areas are comparable with those of powdery materials prepared by conventional solvothermal methods，and those of monoliths obtained with 1，4-dioxane templates. We finally demonstrated the versatility of our "freeze-shaping followed by crystallization" strategy by using different aldehyde-amine monomer pairs. The two‑step strategy offers a general platform for shaping COFs into monolithic forms with desired architectures，which is expected to enhance mass transfer and separation efficiency. This work provides valuable insights into morphology engineering of COFs and is anticipated to promote their practical application in high‑performance liquid chromatography，solid‑phase extraction，and other separation technologies.