Synthesis and in situ transformation of PST-1: a potassium gallosilicate natrolite with a high Ga content

Abstract

The synthetic details of a novel potassium gallosilicate natrolite with Si/Ga = 1.28, denoted PST-1, are described. The presence of K(+) and Ga with well-defined levels of concentration in the synthesis mixture is essential for directing its crystallisation. PST-1 transforms rapidly into TNU-6 under hydrothermal conditions, behaviour that contrasts sharply with its very high thermal and hydrothermal stability, which is unusual for a material of such a high Ga content. These stability issues are discussed and rationalized based on chemical composition, likely violations of Loewenstein's rule and the temperature of dehydration of as-made K-PST-1. The crystal structure of TNU-6 has been resolved through the combined use of synchrotron X-ray and electron diffraction data; it has the BaFeGaO(4) structure type with an additional radical 3a x radical 3a "GeAlO(4)" superstructure that arises from tilting of some of the tetrahedral units in all of the 6-rings.

Tetrahedral Atom Ordering in a Zeolite Framework: A Key Factor Affecting Its Physicochemical Properties

“…As seen in Figure , dehydration occurs in the 300–400 °C range for Na-GaNAT materials, with the dehydration temperature increasing as the degree of T-atom ordering increases, while the K + -exchanged materials dehydrate below 150 °C. In our opinion, this low dehydration temperature affords the higher stability of K-GaNAT materials, in a similar way as we already proposed to explain the outstanding thermal stability of PST-1, a synthetic potassium gallosilicate natrolite with a high Ga content (Si/Ga = 1.28): when the temperature is high enough to afford structural modifications by reorganization of T–O–T bonds, there is no water that can mediate such a reorganization …”

“…This was substantiated by the observed large unit cell shrinkage (ca. 16%) of the material upon reversible dehydration, with no noticeable structural degradation . Natural aluminosilicate natrolites displaying full Si–Al ordering have been reported to shrink around 18 or 24% upon dehydration to the so-called metanatrolite phase, , although in this case substantial structural degradation does occur.…”

Violations of Löwenstein's rule in zeolites

“…At present there is no available method that can accurately distinguish framework aluminium from framework silicon with Ångström resolution. 4 A 2010 work by Shin et al 8 concerning possible non-Löwensteinian structures observed in gallosilicates, discusses the possibility of using of 17 O magic angle spinning (MAS) NMR to detect non-Löwensteinian ordering, a method which has been successful in identifying –Al–O–Al– linkages in aluminosilicate glasses. 47 We have examined the global minimum H-SSZ-13 structures (Si/Al = 17) and predicted 29 Si, 27 Al solid-state, MAS NMR shifts and vibrational frequencies, in an attempt to discern whether spectroscopic signatures exist that would be indicative of the presence of non-Löwensteinian ordering (see ESI † ).…”

Thermochemistry of gallosilicate zeolites with the NAT topology: An energetic view on their in situ disorder–order transformation and thermal stability

“…Their degrees of orthorhombic distortions, which were calculated using the equation given in our previous paper [10], can be found in Table 1. PST-1 was synthesized at higher alkali cation and Ga concentration levels (8.0K 2 OÁ2.0Ga 2 O 3 Á10SiO 2 OÁ150H 2 O) following the procedure recently reported by us [11,12]. K + or Na + ion exchange was done by stirring zeolite powder twice in 1.0 M KNO 3 or NaNO 3 solutions (2.0 g solid per 100 mL solution) at 60°C for 6 h.…”

“…We have also shown that at a given temperature the disordered phase is first formed and then transformed in situ into the ordered one in the crystallization medium. While PST-1 is a new gallosilicate analog of natrolites with an unusually high Ga content which we have recently synthesized in the presence of K + ions as a structure-directing agent [11,12], on the other hand, this material was found to be much more thermally stable than its Ga-poor gallosilicate counterparts.…”