Thermal behavior of stilbite and stellerite revisited and dehydration of their Na-exchanged forms: Considerations on the memory effect of the STI framework type

Abstract

The thermal behavior of Na-exchanged stellerite and stilbite was investigated by in-situ single crystal X-ray diffraction. For comparison with the exchanged forms new data were collected on natural stellerite and stilbite under the same experimental conditions. With the increase of temperature, strong disorder at T and O sites of the tetrahedra of the four-membered ring developed in natural forms. Such disorder was associated with the rupture of TO -T connections and transition from the A to the B phase. Differently from previous studies, stellerite B at T >300°C was found to be monoclinic (space group A2/m). In addition, at 400°C, a new TO -T connection occurred, analogous to that in the B phase of barrerite. Na-stellerite and Na-stilbite were at RT monoclinic, space group F2/m. Upon heating, they also displayed the same structural modifications as observed in natural barrerite and Na-barrerite and adopted space group A2/m. Compared to natural stellerite and stilbite different TO -T connections ruptured leading to a different topology of the B phase. The total volume contraction was-16% at 350°C compared to-8% of pristine materials. The highly-condensed D phase, which does not form in natural stellerite and stilbite, was obtained by heating a Na-stellerite crystal ex-situ at 525°C. The structure corresponded to the D phase of natural barrerite and Na-barrerite. All investigated STI members, after being exchanged with Na, have identical symmetry and demonstrate corresponding behavior upon heating and associated dehydration. Thus, a previously assumed memory effect of the symmetry of the natural parent structure, is not confirmed.

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

“…Ag similarly to the Na-exchanged form 34 , the Ag uptake provoked a distortion of the orthorhombic symmetry. Such distortion was even more pronounced in Ag-STI as indicated by the higher value of the beta angle (90.55 and 90.06° for Ag and Na-STI 34 , respectively) and by the dimension of the ten membered-ring channels (Fig. 1).…”

“…1987 39 . From 50 to 275°C the symmetry lowered from F to A centering and the structures were solved and refined in space group A2/m 34 . Structure refinements were carried out using SHELXL-2014 40 and neutral atomic scattering factors.…”

“…For Extended X-ray Absorption Fine Structure (EXAFS) analyses, pseudo radial distribution functions were obtained by Fourier transforming k 3 -weighted χ(k) functions between 2.0 and 11 Å -1 using a Kaiser-Bessel window function. Structural information were extracted by a multishell fit approach using FEFF8.2 49,50 calculated scattering-paths based on the structure of Na-STI 34 exchanged with Ag + . By fitting common Debye-Waller factors to the first two Ag-O and the following two Ag-Ag and Ag-Si paths, respectively, we could reduce the total number of independent fitting parameters to 11, hence remained below the Nyquist limit of 12.9 calculated for the used k-range of 2.0 to 9.5 Å -1 and the used R-range of 0.9 to 3.6 Å.…”

“…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 .…”