In situ X-ray single-crystal study on the dehydration mechanism in the monoclinic polytype of tschernichite, the mineral analog of zeolite beta

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) A, beta = 114.33 (1)degrees, and V = 4308 A(3) at 25 degrees 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(H(2)0)(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 degrees 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 degrees 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.