Alkali Metal–Organic Frameworks Stabilized by the Hydrogen Bond-Assisted Coordination Strategy for Efficient CO2 Separation from Flue Gas and Biogas Mixture

The inherent lightness of alkali metal–organic frameworks (AMOFs) confers gravimetric advantages for gas adsorption; however, their unpredictable coordination geometry limits their prevalence. Reinforcing the coordination of alkali metal centers would enhance stability, thereby mitigating application challenges. In this work, by using tris(4-carboxyphenyl)amine as a carboxylate linker and regulating the proportions of alkali metals, we successfully prepared two novel AMOFs (TCPA-K1 and TCPA-K2) through a hydrogen bond-assisted coordination strategy. Interestingly, the structure of TCPA-K1 is based on 24-connected K4 clusters with hydrogen bond-assisted coordination spheres, exhibiting an isolated cage-like framework, while the structure of TCPA-K2 is based on 16-connected K2 clusters and possesses two types of 1D permanent channels. Structural analysis reveals that the K2 clusters in TCPA-K2 exhibit additional 8-fold strong hydrogen bond interactions. Thermogravimetric analysis indicates that both TCPA-K1 and TCPA-K2 exhibit high thermal stability. More importantly, TCPA-K2 can capture CO2 from CO2/CH4 (50/50, V/V) and CO2/N2 (15/85, V/V) mixtures under different conditions and effectively separate CO2/N2 mixtures even at 100% humidity. Theoretical calculations further demonstrate that the pore surface in TCPA-K2 is beneficial for the preferential capture of CO2. The present work may provide insights into the rational design of robust and porous MOFs based on alkali metals.