Pressure-induced symmetry changes in body-centred cubic zeolites

Abstract

Previous work has shown a strong correlation between zeolite framework flexibility and the nature of structural symmetry and phase transitions. However, there is little experimental data regarding this relationship, in addition to how flexibility can be connected to the synthesis of these open-framework materials. This is of interest for the synthesis of novel zeolites, which require organic additives to permutate the resulting geometry and symmetry of the framework. Here, we have used high-pressure powder X-ray diffraction to study the three zeolites: Na-X, RHO and ZK-5, which can all be prepared using 18-crown-6 ether as an organic additive. We observe significant differences in how the occluded 18-crown-6 ether influences the framework flexibility—this being dependent on the geometry of the framework. We use these differences as an indicator to define the role of 18-crown-6 ether during zeolite crystallization. Furthermore, in conjunction with previous work, we predict that pressure-induced symmetry transitions are intrinsic to body-centred cubic zeolites. The high symmetry yields fewer degrees of freedom, meaning it is energetically favourable to lower the symmetry to facilitate further compression.

Understanding the Anion‐Templated, OSDA‐Free, Interzeolite Conversion Synthesis of High Silica Zeolite ZK‐5**

“…(A distortion of the ZK-5 structure to this tetragonal space group has been observed at high pressure. [31] ) A good fit to the data was achieved in this space group, with similar cation occupancies to those measured in the dehydrated form (Tables S1, S2 and Figures S13, S14). The large K + and Cs + cations (with additional NO 3 À anions occluded) are not very Chemistry-A European Journal mobile and do not move upon the adsorption of water, as recently noted by Yang et al [32] The symmetry change is attributed to the presence of water.…”

“…Working from the 2 : 1 intensity of the split peaks (well resolved in the 53–60 °2 θ range, Figure 1) and removing the cubic symmetry, the structure was refined in the tetragonal symmetry I 4/ mmm . (A distortion of the ZK‐5 structure to this tetragonal space group has been observed at high pressure [31] …”

Impact of Fly Ash Fractionation on the Zeolitization Process

“…In order to increase synthesis effectiveness and the quality and quantity of zeolite as final product, a series of works has been undertaken, including the use of microwaves [43,44], ultrasonic assistance [27,45,46], the merging of fusion-hydrothermal syntheses [26,37] or molten salt methods [47]. Attempts have also been made to understand the impact of various factors affecting zeolite synthesis (e.g., temperature [48][49][50], pressure [51][52][53], Si/Al ratio [54][55][56], reaction time [25,57,58] and alkalizing agent concentration [59][60][61]). Fly ash fractions can be counted as the next important factor affecting zeolite synthesis from this CCP waste.…”

Differentiating the role of organic additives to assemble open framework aluminosilicates using INS spectroscopy

“…This agrees with our previous findings from high-pressure X-ray diffraction. 56 In contrast, for zeolite EMC-2 the supercages are more voluminous than the t-fau cavity (Table 5), so the rationale of size fails to explain the softening of the vibrational modes. Instead, we suspect the softening is due to the elliptical geometry of the zeolite EMC-2 supercages, 13 providing a degree of geometric match-up and stronger organic-framework interactions to the 18C6 species.…”