Enhancing CO2 adsorption selectivity of MOF-199: Investigating the synergistic effect of Mg metal doping and polyethyleneimine impregnation

[Display omitted] •A highly efficient bi-metallic MOF adsorbent with a large surface area was synthesized.•Only up to 2 mol % of Cu metal can be successfully substituted with Mg metal in MOF-199.•Polyethyleneimine (PEI) was incorporated into bi-metallic MOF to alter its pore size and environment.•The bi-metallic MOF composite loaded with 40 wt% PEI showed highest CO2/N2 adsorption selectivity. The fine-tuning of metal–organic frameworks (MOFs) can enable the development of effective adsorbents for the selective capture of less polarizable carbon dioxide (CO2) from flue gases. In this work, a series of bi-metallic MOFs with different Mg metal concentrations (1–10 mol%) were synthesized by in situ metal substitution followed by post-synthetic functionalization with 10–50 wt% of polyethyleneimine (PEI). Detailed characterization confirmed the successful incorporation of the secondary metal and amine group into the MOF framework. Results showed that only up to 2 mol% of copper (Cu) can be substituted by Mg metal in the framework of MOF-199; further increase in Mg content disrupts the electrostatic equilibrium, leading to the formation of carbon matter-based copper nanoparticles instead. The sorption results indicated that 2 mol% Mg-doped MOF (MOF@Mg2) demonstrate a CO2 uptake of 8.61 (mmol/g) at 1 bar and 273.15 K. This reflects a 17.78 % increase in CO2 absorption capacity compared to the pristine MOF. Furthermore, 40 wt% PEI-loaded MOF exhibited a 37.90 % enhancement in CO2 adsorption selectivity. This study explores in detail the intricate process of optimizing the metal ratio and PEI loading in the bimetallic MOF-199(Cu, Mg), aiming to improve its CO2 adsorption efficiency.