A forgotten participant in pore deblocking of zeolites: dicarbonate in NaMeA zeolites, Me = Na, K, Rb, Cs

Abstract

The dynamics of carbon dioxide, carbonate anion (CO₃^2-), and dicarbonate anion (C₂O₅^2-) in NaKA zeolite is studied at the DFT GGA level using ab initio molecular dynamics (AIMD). We show the easy formation of C₂O₅^2- dicarbonate from the reaction between CO₃^2- and CO₂ at high CO₂ loading and their equilibrium at low CO₂ loading. We have found that the dicarbonate anion can contact up to six cations (Me⁺ and Na⁺, Me = Na, K, Rb, Cs), which could reduce the separation properties of NaMeA zeolites relative to CO₂ mixtures. The K⁺ interaction with dicarbonate C₂O₅^2- species pushes the cation from 8R site in full analogy with the carbonate's deblocking studied earlier. The easy C₂O₅^2- formation in NaMeA is confirmed by modeling reaction of C₂O₅^2- formation at the DFT GGA (PBE-D3) and hybrid levels (B3LYP, HISS, HSE06) with cNEB. The calculated intensities for high and low frequency branches of valence vibrations in C₂O₅^2- are compared with calculated ones for Me₂C₂O₅ molecules and known IR spectroscopic data in the NaMeA zeolites. This new mechanism of deblocking could be important for a wide family of narrow pore zeolites (CHA, RHO, KFI, etc.) at room temperature where the carbonates are observed in the IR spectra. The possibility of tricarbonate formation is discussed.