Sn-MFI molecular sieves: synthesis methods, 29Si liquid and solid MAS-NMR, 119Sn static and MAS NMR studies

Four different methods of the hydrothermal synthesis of Sn-sil-1 (MFI structure) (Si/Sn > 15) molecular sieves are described. Three of the methods are in a basic medium (pH = 12.4) and the last is in a fluoride medium (pH = 6.4). The procedure in which SnCl₄.5H₂O is dissolved first in Si(OC₂H₅)₄ (TEOS) before hydrolysis by tetrapropyl ammonium hydroxide (method A) seems to be the most suitable in terms of the crystallinity of the resulting material and its activity in the hydroxylation of phenol by aqueous H₂O₂ (72% efficiency for H₂O₂). The course of hydrolysis of TEOS and SnCl₄ is studied systematically by ²⁹Si liquid NMR spectroscopy. It is inferred that as soon as SnCl₄ is partially hydrolysed, it combines with monomeric Si(OH)₄ (Q⁰) species. This combination leads to the formation of Q¹, Q² and Q³ species and a clear solution. The liquid NMR data provide direct evidence for the interaction of Sn species with silicate species in the synthesis mixture. Further, ²⁹Si MAS NMR and ¹¹⁹Sn static and MAS NMR spectral studies of the crystalline product indicate that the Sn⁴⁺ ions are probably attached to the defect silanol groups in a tetrahedral coordination, but tend to assume five- or six-fold coordination very easily due to their large size.