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Strontium Ions Function as Both an Accelerant and Structure-Directing Agent of Chabazite Crystallization

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journal contribution
posted on 31.12.2020, 20:08 by Yu Liang, Allan J. Jacobson, Jeffrey D. Rimer
Zeolite crystallization often requires the presence of an organic structure-directing agent (SDA) to facilitate the formation of nanoporous cages and channels. Among the limited number of zeolites that can be synthesized in organic-free media, alkali metals are most commonly employed as inorganic SDAs. Many alkali metal zeolites are usually prepared under moderate conditions using temperatures less than 100 °C and synthesis times on the order of hours. One notable exception is zeolite chabazite (CHA), which is prepared from potassium ions and requires atypically long crystallization times on the order of weeks. Attaining practical synthesis times on the order of days necessitates an alternative approach, such as the use of zeolite crystal seeds. In this study, we show that growth solutions prepared via the partial substitution of potassium with strontium generate purely-crystalline chabazite within hours without the need for either organics or crystal seeds. Studies of strontium inclusion in seed-assisted syntheses also reveal shorter synthesis times. Notably, we show that strontium has a pronounced impact on the kinetics of chabazite formation, leading to 14- and 3-fold reductions in crystallization time compared with pure potassium syntheses in the absence and presence of crystal seeds, respectively. Using a combination of 29Si and 27Al MAS NMR spectroscopy, we also show that strontium functions as a SDA based on its ability to alter Q4(nAl)29Si speciation, thereby redistributing tetrahedral Al sites in the framework. Given the widespread application of chabazite in adsorption and separation processes, designing facile and efficient synthesis approaches with concomitant control of physicochemical properties is commercially relevant.