Thermal stability of barrerite and Na-exchanged barrerite: An in situ single crystal X-ray diffraction study under dry conditions

Abstract

Elpidite Na2ZrSi6O15•3H2O is a natural zirconosilicate with space group Pbcm. A sample from Mt. Saint Hilaire (Canada) was investigated between 25 and 350°C under different humidity conditions (RH = 0% and at RH = 100%). The largest structural channels are eight-and ten-rings delimited by corner-sharing SiO4 tetrahedra and ZrO6 octahedra. Under dry conditions, the structure started liberating water at 75°C. At 100°C, when it lost one H2O molecule, elpidite underwent a structural change from space group Pbcm to Cmce. The structure continuously released water up to 225 °C until it became anhydrous. From 225°C to 350°C no structural changes were observed. Associated with the transition from Pbcm to Cmce symmetry, the unit-cell volume reduced by 2.7%. An additional decrease is observed up to 225°C when the contraction reached 3.4% of the starting volume. After release of all H2O molecules, increase of temperature led to gradual expansion of the unit cell from 2940.79(7) Å 3 at 225°C to 2947.4(5) Å 3 at 350°C. Supplementary ex-situ heating experiments showed that the structure was retained up to 950°C but after heating at 1200°C the sample melted. Under humid conditions a similar dehydration-trend similar to that found under a dry N2atmosphere was observed but the structural changes occurred at slightly higher temperatures. Immersion of a dehydrated sample in water for 30 days led to only marginal rehydration and the Cmce symmetry was preserved.

Framework Modifications and Dehydration Path of a Ag+-Modified Zeolite with STI Framework Type

“…The EF occupants also influence the thermal stability of the STI framework, as demonstrated by the different behavior upon heating of the three minerals 29,30,31,32 . Once exchanged with the same extraframework cation, the three minerals undergo the same structural transformations driven by dehydration 33,34 . The completely Na-exchanged forms (Na16Al16Si56O144 •58H2O) are monoclinic (space group F2/m) at room temperature (RT) and they are referred to as phase A 33,34 .…”

“…Once exchanged with the same extraframework cation, the three minerals undergo the same structural transformations driven by dehydration 33,34 . The completely Na-exchanged forms (Na16Al16Si56O144 •58H2O) are monoclinic (space group F2/m) at room temperature (RT) and they are referred to as phase A 33,34 . Upon heating T-O-T connections of the framework break leading to a different topology accompanied by unitcell volume contraction with respect to the natural forms 33,34 .…”

“…Stellerite is the member with the highest symmetry Fmmm , stilbite is monoclinic F 2/ m , and barrerite is orthorhombic with space group Amma . The EF occupants also influence the thermal stability of the STI framework, as demonstrated by the different behavior upon heating of the three minerals. − Once exchanged with the same EF cation, the three minerals undergo the same structural transformations driven by dehydration. , The completely Na-exchanged forms (Na 16 Al 16 Si 56 O 144 ·58H 2 O) are monoclinic (space group F 2/ m ) at room temperature (RT) and they are referred to as phase A. , Upon heating, the T–O–T connections of the framework break, leading to a different topology accompanied by unit-cell volume contraction with respect to the natural forms. , The high-temperature (HT) modification is known as phase B. Its structural topology is characterized by the statistical breaking of the T–O–T connections and the formation of a system of partially occupied face-sharing tetrahedra, mutually exclusive .…”

“…33,34 Upon heating, the T−O−T connections of the framework break, leading to a different topology accompanied by unit-cell volume contraction with respect to the natural forms. 33,34 The high-temperature (HT) modification is known as phase B. Its structural topology is characterized by the statistical breaking of the T−O−T connections and the formation of a system of partially occupied face-sharing tetrahedra, mutually exclusive.…”

“…Its structural topology is characterized by the statistical breaking of the T–O–T connections and the formation of a system of partially occupied face-sharing tetrahedra, mutually exclusive . Moreover, when heated ex situ at 525 °C, the Na forms transform to a highly contracted (a volume reduction of 19%) structure , known as the D phase . The D phase is described in the space group A 2 1 ma .…”