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

The dynamics of carbon dioxide, carbonate anion (CO32-), and dicarbonate anion (C2O52-) in NaKA zeolite is studied at the DFT GGA level using ab initio molecular dynamics (AIMD). We show the easy formation of C2O52- dicarbonate from the reaction between CO32- 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 C2O52- species pushes the cation from 8R site in full analogy with the carbonate's deblocking studied earlier. The easy C2O52- formation in NaMeA is confirmed by modeling reaction of C2O52- 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 C2O52- 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.

Carbonate-Promoted Drift of Alkali Cations in Small Pore Zeolites: Ab Initio Molecular Dynamics Study of CO2 in NaKA Zeolite.

An effect of deblocking of small size (8R, D8R) pores in zeolites due to cation drift is analyzed by using ab initio molecular dynamics (AIMD) at the PBE-D2/PAW level. The effect of carbonate and hydrocarbonate species on the carbon dioxide uptake in NaKA zeolite is demonstrated. It is shown that a hydrocarbonate or carbonate anion can form strong complexes with K+ cation and withdraw it from the 8R window, so that the probability of CO2 diffusion through 8R increases. For the first time, correlations between cationic and HCO3-/CO32- positions are demonstrated in favor of their significant interaction leading to the cationic drift from 8R windows. This phenomenon explains a nonzero CO2 adsorption in narrow pore zeolites upon high Na/K exchange. In a gas mixture, such deblocking effect reduces the separation factor because of the possible passage of both components through the plane of partly open 8R windows.

The role of water in the elastic properties of aluminosilicate zeolites: DFT investigation.

The bulk and Young moduli and heats of hydration have been calculated at the DFT level for fully optimized models of all-siliceous and cationic zeolites with and without water, and then compared to the corresponding experimental data. Upon the addition of water, the monovalent alkali ion and divalent alkaline earth ion exchanged zeolites presented opposite trends in the elastic modulus. The main contribution to the decrease in the elastic modulus of the alkali ion exchanged zeolites appeared to be a shift of cations from the framework oxygen atoms upon water addition, with the coordination number often remaining the same. The contrasting increase in elastic modulus observed for the divalent (alkaline earth) ion exchanged zeolites was explained by cation stabilization resulting from increased coordination, which cannot be achieved within a rigid zeolite framework without water.

Computational differentiation of Brønsted acidity induced by alkaline earth or rare earth cations in zeolites.

For bi- and trivalent Me(q+) (Me = metal) cations of alkaline earth (AE) and rare earth (RE) metals, respectively, the formation of the nonacid MeOH((q-1)+) species and acid H-Ozeo group, where Ozeo is the framework atom, from water adsorbed at the multivalent Me(q+)(H2O) cation in cationic form zeolites was checked at both isolated cluster (8R or 6R + 4R) and periodic (the mordenite framework) levels. Both approaches demonstrate qualitative differences for the stability of the dissociated water between the two classes of industrial cationic forms if two Al atoms are closely located. The RE forms split water while the AE ones do not, that can be a basis of different proton transfer in the RE zeolites (thermodynamic control) than in the AE forms (kinetic control). The cluster models allow quantitatively explaining nearly equal intensities IHF ∼ ILF of the high frequency (HF) and low frequency (LF) OH vibrations in the RE forms and lowered IHF ≪ ILF in the AE forms, where HF bands are assigned to the Me-OH groups in the RE and AE forms, respectively, while LF bands are assigned to the Si-O(H)-Al groups. The role of electrostatic terms for water dissociation in the RE and AE forms is discussed.