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

Abstract

The dehydration-rehydration process in boggsite |K 0.06 Na 0.36 Sr 0.01 Ca 7.00 Mg 1.20 Fe 0.05 (H 2 O)70| [Al 17.52 Si 78.62 O 192 ]-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 Å 3 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 H 2 O 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 Å 2 , whereas the area of the 12-ring channel grows from 40 to 42 Å 2 . The dehydration process is rapid and completely reversible.