Multi-objective optimization of alkali/alkaline earth metals doped graphyne for ultrahigh-performance CO₂ capture and separation over N₂/CH₄

Adsorbents with excellent CO₂ capture and separation performances are critical in reducing the excessive CO₂ concentration in the atmosphere. Herein, alkali/alkaline earth metals doped graphynes (AM_n-GYs, AM = Li, Na, K, Mg and Ca; GY = graphyne; n = 0.5, 1, and 2) were evaluated by employing grand canonical Monte Carlo and density functional theory approaches. The results showed that AMs were strongly bonded on GY with binding energies of −0.09 to −2.52 eV and diffusion barriers of 5.20–16.32 eV. Electronic structure analyses proved that there was a strong covalent bond, large charge transfer, and distinct orbital overlap characters between the AMs and GY, thus constructing a stable and feasible gas adsorption environment. Among all structures, AM₁-GYs with one AM doping in a GY unit cell displayed the best CO₂ adsorption behavior. Thereafter, Li₁/Na₁/K₁-GY with a pore size of 6.60 Å and Mg₁/Ca₁-GY with a pore size of 6.50 Å were screened as the potential adsorbents. In comparison, the best performing Ca₁-GY had an ultra-high CO₂ adsorption capacity of 9.01 mmol/g at 298 K and 1 bar, which was superior to most of the previously reported adsorbents. At 298 K and 1 bar, the CO₂ selectivity over N₂/CH₄ for Ca₁-GY reached 2246 and 5602, respectively. Interaction and gas distribution analysis confirmed that comparing to other AM₁-GYs, Ca₁-GY has a stronger affinity with CO₂ and larger isosteric heat differences between CO₂ and other gases, rendering Ca₁-GY to possess excellent adsorption capacity and selectivity.