The influence of spatial limits on the modeling chemical reactivity: The example of CO₂ hydration in MeX zeolites (Me = K, Rb, Cs)

Abstract

Zeolite cell size variations upon cationic exchange are frequently disregarded while calculating chemical reactions with rather bulky reagents. An example of the reaction between the small reagents (H2O and CO2) illustrates a necessity to check this assumption for faujasite (FAU) zeolites with large porous space. The interplay of the space for the reaction center and the mobility of alkali cations forces lattice parameters to play a crucial role for the accurate computation of activation barrier of CO2 hydration. While the CO2 hydration is modeled in the MeX forms (Me = Rb, or Cs) with a fixed volume of NaX one, barrierless reactions were predicted that is not confirmed by experimental data. When the RbX and CsX cell parameters were optimized with Vienna Ab‐initio Simulation Package (VASP), activation energies were obtained in agreement with the experimental data that CO2 hydration in CsX occurs at room temperature.

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

“…Due to a large band splitting Δ ν values calculated 32,33,49,51–53 in HCO 3 − can be formally considered as the candidates to interpret the peaks related herein to dicarbonates. However, one cannot find a mechanism in the literature which allows the complete conversion of CO 3 2− to HCO 3 − under CO 2 loading.…”

“…If the first CO 2 → HCO 3 − step (1) in the reaction CO 2 → HCO 3 − → CO 3 2− → C 2 O 5 2− sequence (1–3) was studied earlier in detail, 12,13,33 the second HCO 3 − → CO 3 2− step (2) was analyzed herein. The barrier of 0.8 eV is rather large, thus requiring a different site for H transfer in the NaKA to achieve better agreement with the experimental data, which confirms easy CO 3 2− formation from HCO 3 − at RT.…”

“…3.2.2 NaA zeolite. Admitting that the stronger attraction of Na + cation to 8R window could hinder the stabilization of transition state such as for CO 2 hydrolysis, 33 we repeated the same reaction with all Na cations per the LTA pseudocell (Table 2). A small activation energy of 0.082 eV was also calculated for the exothermic process (DU = À0.293 eV, the reaction profile can be seen in the ''Naa_c2o5.avi'' file in ESI †).…”

“…The exothermic step (1) has been already studied at the cNEB level earlier (heat of reaction is À0.3 eV 13,32,33 ), while the endothermic reaction (2) was considered with cNEB herein (Part 3.1.2). The reaction (3) was investigated using cNEB (Part 3.2) and AIMD (Part 3.1.2).…”