Amine-appended bimetallic MOF-74 for highly selective and energy-efficient CO₂ capture under vacuum swing adsorption

Rapid societal transformation and the widespread use of fossil fuels have increased atmospheric carbon dioxide levels, causing climate change that threatens human society. To improve the stability and number of open metal sites, we have developed a mixed bimetallic MOF-74 composed of Magnesium and Cadmium in this study. Furthermore, these open metal sites are grafted with guanidine to enhance the CO₂ capture performance of the developed amine appended bimetallic MOF-74 (Gu@CdMgMOF-74). The as-prepared adsorbent was fully characterized using various techniques, and the digestion ¹H NMR revealed a DHTA linker-to-guanidine ratio of 1:0.24, corresponding to approximately 25% amine loading. Gu@CdMgMOF-74 demonstrates a strong affinity for CO₂ with an uptake of 5.1 mmol g⁻¹ at 273 K, moderate heat of adsorption (Q_st) of 33.2, and exceptional IAST CO₂/N₂ selectivity of 1720 at 298 K. This enhancement is attributed to the presence of the amine moiety that offers selective interaction with CO₂ over N₂. Furthermore, the as-synthesized adsorbent showed cyclic stability over 10 cycles without any performance loss. Breakthrough experiments demonstrate that Gu@CdMgMOF-74 can efficiently capture CO₂ from the CO₂/N₂ (15/85, v/v) gas mixtures under ambient conditions. Moreover, the isotherm data were used to evaluate performance indicators for a cyclic vacuum swing adsorption (VSA) process and to estimate the practical suitability of the as-prepared adsorbent. The results from VSA confirmed the excellent working capacity (2.7 mmol g⁻¹) and regenerability (72%), demonstrating the practical role of the adsorbent for CO₂ capture with minimal regeneration energy cost. DFT calculations show that bimetallic CdMgMOF-74 offers optimal stability for guanidine grafting, whereas monometallic MOF-74 does not. Guanidine preferentially binds via –NH sites, significantly enhancing CO₂ adsorption (−0.59 eV) and CO₂/N₂ selectivity.