SN-MFI MOLECULAR-SIEVES - SYNTHESIS METHODS, SI-29 LIQUID AND SOLID MAS-NMR, SN-119 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 met hods are in a basic medium (pH = 12.4) and the last is in a fluoride m edium (pH = 6.4). The procedure in which SnCl4 . 5H(2)O is dissolved f irst in Si(OC2H5)(4) (TEOS) before hydrolysis by tetrapropyl ammonium hydroxide (method A) seems to be the most suitable in terms of the cry stallinity of the resulting material and its activity in the hydroxyla tion of phenol by aqueous H2O2 (72% efficiency for H2O2). The course o f hydrolysis of TEOS and SnCl4 is studied systematically by Si-29 liqu id NMR spectroscopy. It is inferred that as soon as SnCl4 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 mixt ure. Further, Si-29 MAS NMR and Sn-119 static and MAS NMR spectral stu dies of the crystalline product indicate that the Sn4+ ions are probab ly 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. (C) 1997 Elsevier Science B.V.