Abstract
CO 2 and H 2 O compete for the same adsorption site. If Δ G CO2 is more negative than Δ G H2O , then the TMP will drive CO 2 to be adsorbed. • The CO 2 /H 2 O adsorption selectivity is highly related to the difference between the ΔG H2O and ΔG CO2 . • The driving force needed for preferential adsorption of CO2 and H2O can be predicted by the thermodynamic molecular pump model (TMP). • For CuBTC, the adsorption entropy was the main thermodynamic driving force to promote CO 2 adsorption at low temperature. The presence of water vapor in post-combustion gas streams is a challenging technical issue that limits the utilization of CO 2 adsorbents. Understanding the physical and chemical interaction mechanisms between adsorbents and adsorbates is the key to effectively deal with the competitive adsorption of CO 2 and H 2 O. In this paper, a thermodynamic molecular pump (TMP) is proposed to resolve the contradictions between the promotion and impedance of CO 2 adsorption by H 2 O. 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 CO 2 and H 2 O 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 H 2 O was a significant factor that influenced the CO 2 adsorption result due to its contribution to the total driving energy. For CuBTC (BTC: benzene-1,3,5-tricarboxylate), the CO 2 adsorption entropy provided a greater driving force to adsorb CO 2 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 CO 2 /H 2 O 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 CO 2 adsorbents in the presence of H 2 O.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call