Calcination of Octadecasil:  Fluoride Removal and Symmetry of the Pure SiO2 Host

• Published in: Chemistry of materials 1998-12, Vol.10 (12), p.3966-3973
• Main Authors: Villaescusa, Luis A, Barrett, Philip A, Camblor, Miguel A
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Ion-exchange; Surface physical chemistry; Zeolites: preparations and properties
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm9804113

Pure silica octadecasil (AST) has been synthesized hydrothermally in a fluoride medium using a new structure-directing agent (tert-butyltrimethylammonium). F- is occluded in the material inside the small [46] cage and is removed completely upon calcination, in sharp contradiction with recent predictions based on computational studies. We propose chemically feasible pathways for the migration of F- out of the [46] cage that are based on the known chemical reactivity of F- and SiO2 and on the presence of small cations formed by thermal degradation of the organic species. After calcination to remove the guest species, the structural integrity of the host is completely preserved, yielding a pure SiO2 framework devoid of connectivity defects. Its structure has been solved by direct methods and refined using synchrotron X-ray powder diffraction data. Despite the removal of guest species, the framework is seen to adopt a tetragonal symmetry rather than the cubic maximum topological symmetry. It is proposed that symmetry lowering in AST is a consequence of the energetically unfavorable angles in the higher space group, in which 20% of the Si present four linear Si−O−Si angles. 29Si MAS NMR spectroscopy and energy minimization calculations support this notion and the refined structure proposed. At 500 °C calcined octadecasil undergoes a reversible phase transition to pseudocubic symmetry. Finally, the reported AlPO4-16 or octadecasil cubic forms are proposed to actually be disordered pseudocubic structures.