A Base-Resistant Zn^II -Based Metal-Organic Framework: Synthesis, Structure, Postsynthetic Modification, and Gas Adsorption

A Zn^II -based metal-organic framework (MOF), [Zn₂ (bdp-CHO)₂ ]⋅(DMF)(CH₃ CN)(H₂ O)₂ (BUT-31) is reported that was synthesized by the reaction between a newly designed aldehyde-tagged polypyrazole ligand 2,5-di(1H-pyrazol-4-yl)benzaldehyde (H₂ bdp-CHO) and a zinc salt. BUT-31 has a unique pillared layered framework structure with 3D intersecting channels approximately 3.4-5.4 Å in size. Powder X-ray diffraction and N₂ adsorption experiments revealed that BUT-31 is rigid and permanently porous with the Brunauer-Emmett-Teller surface area of 926 m² g^-1 . Notably, this MOF tolerates boiling water and even highly basic aqueous solution (4 m sodium hydroxide), although dilute acid gradually decomposes its framework. Owing the permanent porosity and chemical stability of BUT-31, covalent post-modification of the free aldehyde group exposed on the pore surface was accomplished by treating the MOF in a concentrated ammonia solution (25 %) at near room temperature, giving rise to an imine-functionalized analogue of BUT-31. Gas adsorption results show that the aldehyde- and imine-functionalized MOFs have high CO₂ adsorption capacities, as well as CO₂ /N₂ and CO₂ /CH₄ adsorption selectivities.