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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- 作者：Alberto Alberti ; Giuseppe Cruciani ; Ermanno Galli ; Roberto Millini ; Stefano Zanardi
- 刊名：Journal of Physical Chemistry C
- 出版年：2007
- 出版时间：March 29, 2007
- 年：2007
- 卷：111
- 期：12
- 页码：4503 - 4511
- 全文大小：323K
- 年卷期：v.111,no.12(March 29, 2007)
- ISSN：1932-7455
文摘
Tschernichite, a very rare pentasil zeolite, is the natural aluminum-rich analog of zeolite beta, a large porealuminosilicate, 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 monoclinicand tetragonal symmetry, respectively. The monoclinic polytype crystallizes in the C2/c space group, withcell parameters a = 17.982(1), b = 17.985(1), c = 14.619(1) Å, 
= 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 wasstudied by single-crystal X-ray data diffraction collected at room temperature, at 80, 150, and 250 C in a hotnitrogen stream. During the dehydration process, the variation of the unit-cell volume was always less than1.3%. In room conditions, monoclinic tschernichite is characterized by disorder in cation sites and watermolecule distribution. At 80 C almost 65% of H2O is lost, and this involves a reorganization of extraframeworkcations. At 250 C, all H2O is lost and six extraframework sites were localized. Only one of these displaysa coordination number greater than four, and two are only coordinated to three framework oxygens. As aresult, about 90% of Ca cations are four- or three-coordinated. The structural collapse of the monoclinicpolytype of tschernichite occurs at a temperature below 350 C. The combination of a large frequency ofsilanols, associated with the stacking faults, together with the high Ca content, probably explains the relativelylow temperature of the structural collapse.
= 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 wasstudied by single-crystal X-ray data diffraction collected at room temperature, at 80, 150, and 250 C in a hotnitrogen stream. During the dehydration process, the variation of the unit-cell volume was always less than1.3%. In room conditions, monoclinic tschernichite is characterized by disorder in cation sites and watermolecule distribution. At 80 C almost 65% of H2O is lost, and this involves a reorganization of extraframeworkcations. At 250 C, all H2O is lost and six extraframework sites were localized. Only one of these displaysa coordination number greater than four, and two are only coordinated to three framework oxygens. As aresult, about 90% of Ca cations are four- or three-coordinated. The structural collapse of the monoclinicpolytype of tschernichite occurs at a temperature below 350 C. The combination of a large frequency ofsilanols, associated with the stacking faults, together with the high Ca content, probably explains the relativelylow temperature of the structural collapse.