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

Abstract

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 SnCl 4.5H 2O is dissolved first in Si(OC 2H 5) 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 H 2O 2 (72% efficiency for H 2O 2). The course of hydrolysis of TEOS and SnCl 4 is studied systematically by 29Si liquid NMR spectroscopy. It is inferred that as soon as SnCl 4 is partially hydrolysed, it combines with monomeric Si(OH) 4 (Q 0) species. This combination leads to the formation of Q 1, Q 2 and Q 3 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 Sn 4+ 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.