05-P-07 - Microporous gallosilicate TNU materials and their implications for the synthesis of low-silica molecular sieves

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

“…2 above shows that the positions and relative intensities of all the X-ray peaks from TNU-6 are essentially the same as those reported in our preliminary study. 6 The observed hexagonal prismatic crystal morphology is also consistent with the symmetry determined crystallographically (see below). No noticeable weight loss at temperatures up to 800 ºC was observed from its TGA/DTA curves (ESI † Fig.…”

“…For comparison, the orthorhombic (Fdd2) member (Si/Ga = 1.57, with a high 25 degree of ordering) of the NAT family of sodium gallosilicate zeolites was prepared at 200 ºC under rotation (60 rpm) for 14 days. 6 Here we refer to this material as our previous laboratory code TNU-4 for the sake of clarity and distinction from PST-1. As-made PST-1 and TNU-4 in the K + and Na + forms were refluxed four times in 1.0 M NaNO 3 and 1.0 M KNO 3 solutions (2.0 g solid per 100 mL solution) for 6 h, respectively, in order to maximize the concentration of the exchanging cations in these two materials.…”

“…4 Nevertheless, PST-1 is thermally stable up to at least 800 o C. Also, it can withstand hydrothermal treatments up to at least 600 o C in the presence of 10% water vapor. These results are in sharp contrast with the behavior of its more siliceous sodium 60 gallosilicate counterparts with Si/Ga ~ 1.6 that lose crystallinity upon exposure to ambient air after heating at temperatures below 400 o C. [5][6][7] The most striking feature of PST-1 is its ability to selectively adsorb the smallest gases, He and especially H 2 , with Lennard-Jones (L-J) diameters of 2.60 and 2.89 Å, respectively. PST-1 can discriminate them from slightly larger molecules such as Ar and CO 2 with L-J diameters of 3.40 and 3.30 Å, respectively, making this Ga-rich material a potential candidate for fast, selective H 2 or He separation processes based on pressure swing adsorption or 70 membrane technology.…”

“…With prolonged heating in the K + -containing crystallisation medium, PST-1 was found to transform rapidly into TNU-6 (Taejon National University number 6), one of a series of gallosilicate materials that we have previously prepared without the use of any organic additive. 6 The crystal structure of TNU-6 has been solved by combining synchrotron powder diffraction and transmission electron microscopy, and its physical properties have been characterized by using elemental and thermal analyses, high-temperature powder X-ray diffraction, scanning electron microscopy, and 29 Si and 71 Ga MAS NMR.…”

“…S4) of TNU-6 exhibits only one broad asymmetric line around 177 ppm, typical of framework tetrahedral Ga species. [5][6][7][18][19][20][21] The 29 Si 30 MAS NMR spectrum of this dense gallosilicate phase, together with the simulated spectrum and its deconvoluted components, is shown in Fig. 8.…”