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In Situ X-ray Single-Crystal Study on the Dehydration Mechanism in the Monoclinic Polytype of Tschernichite, the Mineral Analog of Zeolite Beta

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posted on 29.03.2007, 00:00 by Alberto Alberti, Giuseppe Cruciani, Ermanno Galli, Roberto Millini, Stefano Zanardi
Tschernichite, a very rare pentasil zeolite, is the natural aluminum-rich analog of zeolite beta, a large pore aluminosilicate, the peculiar structure and acidity of which makes it one of the most important acid catalysts. Tschernichite, like zeolite beta, is a disordered structure consisting of two distinct polytypes with monoclinic and tetragonal symmetry, respectively. The monoclinic polytype crystallizes in the C2/c space group, with cell parameters a = 17.982(1), b = 17.985(1), c = 14.619(1) Å, β = 114.33 (1)°, and V = 4308 Å3 at 25 °C, and its structure is characterized by a three-dimensional channel system of 12-membered rings of tetrahedra. The dehydration process of the monoclinic polytype |Na0.8K0.3Mg0.4Ca8.0(H20)67| [Al18.0Si46.0O128]-BEA was studied by single-crystal X-ray data diffraction collected at room temperature, at 80, 150, and 250 °C in a hot nitrogen stream. During the dehydration process, the variation of the unit-cell volume was always less than 1.3%. In room conditions, monoclinic tschernichite is characterized by disorder in cation sites and water molecule distribution. At 80 °C almost 65% of H2O is lost, and this involves a reorganization of extraframework cations. At 250 °C, all H2O is lost and six extraframework sites were localized. Only one of these displays a coordination number greater than four, and two are only coordinated to three framework oxygens. As a result, about 90% of Ca cations are four- or three-coordinated. The structural collapse of the monoclinic polytype of tschernichite occurs at a temperature below 350 °C. The combination of a large frequency of silanols, associated with the stacking faults, together with the high Ca content, probably explains the relatively low temperature of the structural collapse.