Dehydration mechanism in brewsterite: single-crystal X-ray diffraction study

The crystal structure of the zeolite brewsterite (space group P2(1)/m, idea l formula (Sr,Ba)(2)Al4Si12O32. 10H(2)O) was studied at different dehydrati on levels. Six single crystals were kept in evacuated capillaries at differ ent temperatures and for different treatment times, and the X-ray diffracti on data were collected at room temperature. The following results were obta ined. 1, The structure refinements of the partially dehydrated brewsterite indica te that, as dehydration advances, the cell volume decreases, the channels a re contracted and the exchangeable cations spread over several sites, which were occupied in the original structure by water molecules. These results are in agreement with a previous in situ powder diffraction study. 2. The T-O-T bridge breaking and the consequent formation of new 4- and 5-c oordinated (Si,Al) polyhedra, already observed in a previous single-crystal dehydration study, are caused by the necessity of the exchangeable cations to be coordinated by the framework oxygens when the water loss is almost c omplete. This phenomenon was not observed in the in situ powder diffraction experiment, probably owing to the presence of residual water. 3. The effect of the heating time was also evaluated: a longer exposure of the crystal at high temperature in vacuum favors a higher percentage of T-O -T bridge breaking, and induces the complete depopulation of the original S r site, with a wider spreading of the extra-framework cations. (C) 2000 Els evier Science B.V. All rights reserved.