Integrated Melamine Molecules on Microspherical Boehmite Particles via Spray Drying for Highly Efficient CO₂/N₂ Adsorption Separation

A newly developed spherical boehmite and melamine composite with a mesoporous structure was successfully fabricated through a spray drying system utilizing a mixture of boehmite sol and melamine. EDX–SEM, FTIR, and TGA analyses confirmed the integration of melamine into the boehmite network within the resulting composite. With an increase in melamine content, the composites exhibited a gradual reduction in porosity compared to their pristine boehmite counterpart. However, the CO₂ uptake of the composites continued to demonstrate improvement. The boehmite sample modified with 5 mol% of melamine (IMB#5) demonstrated the highest CO₂ adsorption capacity at 19.2 cm³ g⁻¹. This value surpassed the original boehmite sample by 46.1% under conditions of 25 °C and 1 bar. The enhanced adsorption can be attributed to the development of adsorptive affinity facilitated by N-derived functional groups (–NH₂ and  –CN) within the melamine structure and their interaction with CO₂. As a result, the CO₂/N₂ separation factor and CO₂/N₂ adsorptive selectivity using the ideal adsorbed solution theory (IAST) over the IMB#5 sample were 113.3 and 3182, respectively, approximately 3 times and 9.2 times higher than those for the boehmite sample. Density functional theory (DFT) calculations were conducted to investigate the interaction of melamine on the boehmite surface, as well as the selective adsorption of CO₂ and N₂ gaseous molecules on the boehmite/melamine composite. It is shown that the melamine mainly interacts with the boehmite via a strong binding of the N atom of the triazine ring with the Al atom of the boehmite. The adsorption of CO₂ has lower binding enthalpies and free energies than that of N₂. These findings indicate that utilizing continuous spray drying holds promise as an effective pathway for scaling up the production of mesoporous boehmite/melamine composite spheres as CO₂ selective adsorbents.