Effect of amine modification on the properties of zirconium–carboxylic acid based materials and their applications as NO2 adsorbents at ambient conditions

• Melamine and urea were used to modify zirconium–carboxylic acid based materials.
• Chemical interactions of urea and melamine with zirconium and ligands differ.
• Introduction of urea results in crystalline microporous materials.
• Introduction of melamine leads to amorphous solids of low porosity.
• Relatively high amounts of NO2 are retained on the adsorbents in moist conditions.

Zirconium–carboxylic ligand-based porous materials modified with –NH2 groups originated either from melamine or urea were synthesized. The samples were used as adsorbents of NO2 from either dry or moist air. The surface features were analyzed using SEM, XRD, nitrogen sorption, thermal analysis and FT-IR. The incorporation of urea, which is a planar molecule, results in a highly porous crystalline materials. Melamine, a triazine compound, on the other hand obstructs the nucleation and growth of the crystals due to its interaction with metallic centers and carboxylic ligands and leads to amorphous materials with low porosity. Water enhances the NO2 adsorption process. Introducing Lewis basic sites by the incorporation of –NH2 groups promotes chemical reactions on the surface. The amine (NH2) or the carbonyl (CO) groups in urea directly interact with NO2 molecules in both moist and dry conditions, which leads to the formation of surface bound nitrates. In the case of melamine modified materials the hydrolysis of the terminal secondary –NH2 creates oxygen rich functional groups that cause the formation of surface bound nitrate species.

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