ИСТИНА

Effective separation of gas mixtures containing carbon dioxide is one of the central problems of carbon capture and storage. While studying the separation of carbon dioxide from dry CO2/N2 gas mixtures, it was found that NaKA zeolites with low and moderate contents of K, have been observed to have an exceptionally high selectivity for CO2 [1]. Despite several mechanisms that explain how CO2 can penetrate through “closed” 8R windows were introduced, the specific feature of CO2 molecules to form carbonates remains out of discussion. The mechanism of CO2 uptake associated with the carbonate formation has been proposed in [2]. It assumes that negatively charged carbonate species can withdraw cations from the windows by strong Coulomb interaction. Here, we use ab initio molecular dynamics to show that even a small amount of carbonate anions concentrated near 8R windows or D8R prisms can significantly change the gas penetration by displacing K+. The pseudo-NaKA zeolite cell (chemical composition KNa23Al24Si24O96) contains two α-cages divided by the 8R window in which unique K cation is located. The ciNEB analysis was undertaken to confirm carbonate formation [3]. The AIMD dynamics of mixtures containing 7 CO2, CO3-2, HCO3-, and 7 CO2 plus HCO3- within one α-cage is simulated using PBE functional combined with a plane wave basis set, the PAW pseudopotentials, and D2 semiempirical dispersion corrections as implemented in the VASP package. These simulations were run with a time step of 1 fs for a 10-30 ps at 300 K in a Nose-Hoover thermostat with 40 fs thermal oscillations. In the pico-second scale the amplitude of K+ cation motion relative to its equilibrium position in the plane of the 8R window of NaKA zeolite is compared without and with hydrocarbonate or carbonate anion. Hydrocarbonate starts to displace/shift K+ farther away from 8R window at 12 ps [4]. Due to the softer interaction with cations, hydrocarbonate has higher mobility than carbonate. The carbonate anion does not reproduce the same scale of motions, as it is in more tight contact with the cations [4]. At the same time neither 3 CO2, nor 7 CO2 can withdraw K+ from the 8R window without hydrocarbonate or carbonate. The long K+ shift explains the non-zero CO2 adsorption in NaKA with the Na/K exchange being higher than 33 % when CO2 cannot pass all 8R windows because of its kinetic diameter is more than 3 Å. The results indicate that the microscopic model of carbonates should be taken into account when interpreting breakthrough experiments with CO2 mixtures. The authors thank the Russian Foundation of Basic Researches within the grant 17-53-18026-Bolg_а. The research is carried out using the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University [5]. References [1] Q. Liu, et al., Chem. Commun. 46 (2010) 4502. [2] A. V. Larin, et al., Microporous Mesoporous Mater. 162 (2012) 98. [3] Rybakov A.A., et al., Int. J. Quant. Chem., 119 (2019) e25820 [4] Bryukhanov I.A., et al., J. Phys. Chem. Lett., 10 (2019) 2191 [5] V. Sadovnichy, A. Tikhonravov, V. Voevodin, V. Opanasenko, «Lomonosov»: Supercomputing at Moscow State University, в: J.S. Vetter (Ред.), Contemp. High Perform. Comput. From Petascale Towar. Exascale, CRC Press, Boca Raton, USA, 2013: сс. 28.