IN-SITU SN-119-MOSSBAUER SPECTROSCOPIC STUDY ON MFI, MEL, AND MTW TINSILICALITES

MFI, MEL, and MTW analogue tin silicalites were studied by in situ Mos sbauer spectroscopy to complement previous studies. Series of 77 and 3 00 K spectra were obtained from synthesized, reduced, and oxidized sam ples. Intensity changes of components as a function of temperature (d InA/dT values) were also considered in addition to the usual spectral parameters (isomer shift and quadrupole splitting) to evaluate the spe ctra. The data obtained for Sn(IV) support neither the octahedral nor the extraframework emplacement; instead, dominance of tetrahedral coor dination is supported. Upon reduction in hydrogen a Sn(ll) component i s formed and can be stabilized. A significant portion of Sn(ll) can be reoxidized to the Sn(IV) state by a subsequent medium oxidation. in v iew of the respective d InA/dT values and the stabilization of Sn(ll), the structural change of edge-sharing [SnO4/2] tetrahedra is suggeste d to form Si-O-Sn(ll)-O-Si bridges. The existence of these bridges als o can be correlated with the catalytic activity of tin silicalites obs erved in mild oxidation processes. (C) Elsevier Science Inc. 1997.