Synergistic and competitive effect of H2O on CO2 adsorption capture: Mechanism explanations based on molecular dynamic simulation

The presence of water vapor in post-combustion gas streams is a challenging technical issue that limits the utilization of CO2 adsorbents. Understanding the physical and chemical interaction mechanisms between adsorbents and adsorbates is the key to effectively deal with the competitive adsorption of CO2 and H2O. In this paper, a thermodynamic molecular pump (TMP) is proposed to resolve the contradictions between the promotion and impedance of CO2 adsorption by H2O. Specifically, the TMP model was proposed for quantitative analyses on a molecular scale for the first time. The equilibrium adsorption isotherms and adsorption enthalpies for CO2 and H2O at 298–388 K were obtained by grand canonical Monte Carlo (GCMC) simulations, while the adsorption entropies were obtained by density functional theory (DFT). Furthermore, the Gibbs free adsorption energy was then obtained by considering the effects of temperature and adsorption sites. The TMP-based analysis showed that the Gibbs free adsorption energy of H2O was a significant factor that influenced the CO2 adsorption result due to its contribution to the total driving energy. For CuBTC (BTC: benzene-1,3,5-tricarboxylate), the CO2 adsorption entropy provided a greater driving force to adsorb CO2 preferentially at 348–388 K. For zeolite AFI, the adsorption enthalpy was always the largest driving force. The results show that the adsorption temperature had a strong influence on zeolites. At 320–360 K, the zeolites showed better CO2/H2O adsorption selectivity than metal–organic frameworks (MOFs) due to their sensitivity to the thermal driving forces, as predicted by the TMP model. This is especially significant for the potential application of zeolites in temperature swing cycles driven by low- and medium-grade energy. The TMP model can provide guidance for screening CO2 adsorbents in the presence of H2O.