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

✍ Scribed by Michele Sacerdoti; Giovanna Vezzalini; Simona Quartieri

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 293 KB
• Volume: 41
• Category: Article
• ISSN: 1387-1811
• DOI: 10.1016/s1387-1811(00)00278-x

✦ Synopsis

The crystal structure of the zeolite brewsterite (space group P2 1 am, ideal formula (Sr,Ba) 2 Al 4 Si 12 O 32 Á 10H 2 O) was studied at dierent dehydration levels. Six single crystals were kept in evacuated capillaries at dierent temperatures and for dierent treatment times, and the X-ray diraction data were collected at room temperature. The following results were obtained.

1. The structure re®nements of the partially dehydrated brewsterite indicate that, as dehydration advances, the cell volume decreases, the channels are 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 diraction study.

2. The T±O±T bridge breaking and the consequent formation of new 4-and 5-coordinated (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 complete. This phenomenon was not observed in the in situ powder diraction experiment, probably owing to the presence of residual water.

3. The eect 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 Sr site, with a wider spreading of the extra-framework cations.