Stepwise dehydration of thomsonite (THO) with disordered Si/Al distribution: a new partially hydrated phase

Giordani, Matteo; Skrzynska, Katarzyna; Cametti, Georgia (2022). Stepwise dehydration of thomsonite (THO) with disordered Si/Al distribution: a new partially hydrated phase. Microporous and mesoporous materials, 346, p. 112308. Elsevier 10.1016/j.micromeso.2022.112308

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The structural transformations occurring as a function of increasing temperature in zeolites are of interest because the porous structure, and therefore the physical properties, can significantly change.
Zeolites with THO framework type are small-pore materials, which received attention because of their applications in catalytic processes. The majority of THO zeolites (synthetic and natural) are characterized by an ordered distribution of the cations at the tetrahedral sites. To date, few cases of disordered thomsonite have been reported. In this study, we investigated the dehydration behavior of a natural thomsonite with disordered Si/Al distribution and chemical composition Ca3.34Na2.66Si11Al9O40∙12H2O. The structure was determined from room temperature (RT) to 698 K in order to compare the thermal behavior with that reported for the ordered variety. Accurate structural analysis was performed by in situ single crystal X-ray diffraction. The dehydration starts at 348 K. Up to 498 K, thomsonite gradually releases four H2O. From 498 to 573 K, additional four H2O are lost and the space group changes from orthorhombic (Pbmn) to monoclinic (P21/n). This partially hydrated phase is characterized by a unit-cell volume contraction of -3% with respect to the RT phase and by a rearrangement of the extraframework cations in the zeolitic pores. The thermally treated thomsonite is able to reabsorb 50% of the lost H2O and transform to the orthorhombic phase, equivalent to that observed at lower temperatures. However, the diffraction pattern analysis indicated a high degree of mosaicity, most probably due to the residual stress accumulated during the phase transformation.

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