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Dehydration and rehydration process in boggsite: An in situ X-ray single-crystal study

¹ Dipartimento di Scienze della Terra, Università di Ferrara, Corso Ercole I° d’Este, 32, I-44100 Ferrara, Italy
² Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Largo S. Eufemia, 19, I-41100 Modena, Italy

Correspondence: ^* E-mail: zrs{at}unife.it

The dehydration-rehydration process in boggsite |K_0.06Na_0.36Sr_0.01Ca_7.00Mg_1.20Fe_0.05(H₂O)₇₀| [Al_17.52Si_78.62O₁₉₂]-BOG, a rare natural pentasil zeolite characterized by a 3D channel system of 10- and 12-rings, occurs in space group Imma with cell parameters a = 20.291(1), b = 23.840(1), c = 12.807(1) Å, and V = 6195.2 ų at 25 °C. Single-crystal X-ray data collections were carried out at room temperature, at 150, 350, and 500 °C in a hot nitrogen stream, and after the crystal was cooled down to 150 °C and then again at room temperature. During these processes, the variation in the unit-cell volume was always less than 1.4%. At room conditions boggsite is characterized by strong disorder in cation and H₂O molecule distribution, with many partially occupied sites weakly interacting with the framework. At 150 °C, where most of the water is lost, three extraframework cation sites were located. At 350 °C boggsite is fully dehydrated and five cation sites are present; three of these are fourfold coordinated and the others are sixfold coordinated. As a consequence of the migration of the cations, at 500 °C only four cation sites are present. The rehydration process causes distortion of the boggsite channels; the 10-ring reduces its free area by about 1 Ų, whereas the area of the 12-ring channel grows from 40 to 42 Ų. The dehydration process is rapid and completely reversible.