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

The dehydration-rehydration process in boggsite \K0.06Na0.36Sr0.01Ca7.00Mg1.20Fe0.05(H2O)(70)\ [Al17.52Si78.62O192]-BOG, a rare natural pentasil zeolite characterized by a 3 D 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) Angstrom, and V = 6195.2 Angstrom(3) at 25 degreesC. Single-crystal X-ray data collections were carried out at room temperature, at 150, 350, and 500 degreesC in a hot nitrogen stream, and after the crystal was cooled down to 150 degreesC 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 degreesC, where most of the water is lost, three extraframework cation sites were located. At 350 degreesC 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 degreesC 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 Angstrom(2), whereas the area of the 12-ring channel grows from 40 to 42 Angstrom(2). The dehydration process is rapid and completely reversible.