The steric influence of extra-framework cations on framework flexibility: an LTA case study

Abstract

The theoretical extent of framework flexibility of Zeolite A (LTA) in response to the steric and geometric effects of different Si/Al compositions and extra-framework cation content has been explored using GASP software. Flexibility windows and compression mechanisms for siliceous LTA and aluminosilicate Na-LTA, Ca-LTA and K-LTA have been modelled. As expected, relatively small cations in the zeolite pores have little effect on the range of flexibility observed. Aluminosilicate LTA, Na-LTA and Ca-LTA frameworks exhibit identical flexibility windows and these frameworks also follow the same compression mechanisms. The introduction of larger K+ ions, however, results in greater steric hindrance. This restricts the flexibility of the framework and alters the compression mechanism to accommodate these larger cations. It is shown that the limits of the flexibility window of Zeolite A are dependent on framework aluminium content and extra-framework cation size.

Selective Adsorption of CO2 on Zeolites NaK-ZK-4 with Si/Al of 1.8–2.8

“…There is an ongoing discussion on the structural details of 8-ring zeolites and how those relate to the trapdoor, swing door, potential gating of the CO 2 adsorption, and framework flexibility. 14 , 15 , 15 , 16 , 42 , 43 We recently deduced from in situ XRD experiments that the cations of zeolite NaK-A did not redistribute upon CO 2 adsorption but instead were slightly displaced toward the center of the α-cage. Here, we performed a similar in situ X-ray diffraction study to investigate the cation position in zeolite K-ZK-4 upon CO 2 adsorption and the positioning of the CO 2 molecules upon adsorption.…”

“…There is an ongoing discussion on the structural details of 8-ring zeolites and how those relate to the trapdoor, swing door, potential gating of the CO 2 adsorption, and framework flexibility. 14,15,15,16,42,43 We recently deduced from in situ XRD experiments that the cations of zeolite NaK-A did not redistribute upon CO 2 adsorption but instead were slightly displaced toward the center of the α-cage.…”

Effect of mono- and divalent extra-framework cations on the structure and accessibility of porosity in chabazite zeolites

“…In this study, SS MAS NMR ( 29 Si and 27 Al NMR) was employed to further investigate the effect of extra-framework cations on the structural properties of chabazite zeolites. In the chemical shifts of 29 Si NMR (Table S4 in SI), a very minor difference is seen between the different samples, indicating that the local environment of Si 4+ in the framework remains unchanged. The ratio of Si/Al can be calculated using Equation 1, and results in a Si/Al ratio of ~2.0 which is consistent with the EDX results discussed above.…”

“…Figure 5. 29 Si NMR spectrum of parent KNa-CHA (a) and Ca-CHA (b). Assignment of spectra shows five distinct local environments for 29 Si in a zeolite.…”

“…18 These are given a three-letter code, which identifies each framework. Some of the most commonly studied zeolites are made of following frameworks: MFI, [19][20][21][22] FAU, [23][24][25] LTA, [26][27][28][29] CHA [30][31][32][33][34][35] and EMT. [36][37][38][39] Because zeolites have found so many uses, there has been a considerable push to predict which zeolites could be synthesised, 40,41 how new zeolites could be produced, 36,42,43 and what new properties could be designed and developed among known zeolites.…”