Carbonate “door” in the NaKA zeolite as the reason of higher CO2 uptake relative to N2

Theoretical identification of carbonate geometry in zeolites from IR spectra

“…S4 where from which a negligible CH 4 adsorption can be suggested at 293 K as well. As illustrated in our work [19], the carbonate motion together with a K cation, i.e., KCO À 3 , only partially opens the 8R window ( Fig. 4a in Ref.…”

“…This CO/N 2 separation cannot for example be explained by the trapdoor model [18]. We predicted that the measurements from binary CO 2 /N 2 mixtures would lead to lower separation coefficients [19]. Such prediction is not confirmed by the recent results [18], where similar coefficients were measured from either isotherms, or from binary CO 2 /CH 4 mixture at 293 K in r1CsCHA and r2KCHA zeolites.…”

“…Because of the large UC size of NaKA we reduced the cell model up to two a-cages instead of eight, conserving however the correct Si/Al alternations in the 8R window between the cages where the interaction between potassium cation KII and carbonate should be studied (more details is presented in [19]). We observed three NaKA models (A, B, C) which correspond to the carbonate migration starting from different initial positions in the pseudo-UC.…”

“…We addressed to this ''artificial model'' of Me 2 CO 3 alkali molecule in all-siliceous MOR (open triangles in Fig. 1) regarding the CO 2À 3 formation to evaluate the possible concurrence between CO 2À 3 and zeolite (in the absence of Al substitution) for the cation coordination [19]. For all three A-C models of NaKA, a bond is formed between the O of CO 2À 3 and Si of MOR with nearly the same length, i.e., |Si-O carb | = 1.858, 1.857, Table 1 Band splitting (BS, in cm À1 ), d = R i = 1,3 (||C-O| i -|C-O| aver |) values and |C-O| distances (in Å) calculated at the B3LYP/6-31G ⁄ , GGA-PW91, LDA-CA, and GGA-PBE levels for the carbonate models which obey (upper part) and do not obey (lower part) to the linear ''BS-d'' dependence.…”

“…The effect of separation was assigned to groups of CO 2 molecules which can temporally displace an alkali cation from its position in the center of the 8R window [18]. An alternative assignment connects such effect with carbonates whose form and binding to the framework becomes important [19]. Details regarding the various approaches for the calculations of carbonates are compared in the ''Discussion'' part.…”