Two isostructural aluminum-based metal − organic frameworks with multiple polar sites for reversible NH3 capture

Abstract

Efficient ammonia (NH₃) capture is essential for both human health and environmental protection. Porous adsorbents play a crucial role in NH₃ removal from polluted air, as reversible physical adsorption eliminates the need for solvents or water, thereby minimizing waste generation. The structural tunability and diversity of metal–organic frameworks (MOFs) make them promising candidates for NH₃ uptake, however, developing stable and efficient MOFs for NH₃ adsorption remains a big challenge. In this study, we developed two isostructural Al-MOFs, hydroxyl-functionalized CAU-1 (CAU-1-OH) and amino-functionalized CAU-1 (CAU-1-NH₂), which featured specific polar sites and optimal porosity for efficient NH₃ removal from air. At 25 ℃ and 1 bar, CAU-1-OH and CAU-1-NH₂ achieved NH₃ uptakes of 16.3 and 16.5 mmol g⁻¹, respectively. Theoretical calculations reveal that the hydroxyl group in CAU-1-OH, the amino group in CAU-1-NH₂, and the dense electronegative μ₂-O group in both frameworks serve as high-density NH₃ binding sites, playing a critical role in NH₃ capture. Dynamic breakthrough experiments demonstrate the regeneration stability and excellent separation performance of these materials, highlighting their potential for cost-effective industrial applications in low-concentration NH₃ removal from air.