In situ 119 Sn-Mossbauer spectroscopic study on MFI, MEL, and MTW tin silicalites

Abstract

MFI, MEL, and MTW analogue tin silicalites were studied by in situ Mossbauer spectroscopy to complement previous studies. Series of 77 and 300 K spectra were obtained from synthesized, reduced, and oxidized samples. Intensity changes of components as a function of temperature (d InAldTvalues) were also considered in addition to the usual spectral parameters (isomer shift and quadrupole splitting) to evaluate the spectra. The data obtained for Sn(lV) support neither the octahedral nor the extraframework emplacement; instead, dominance of tetrahedral coordination is supported. Upon reduction in hydrogen a Sn(ll) component is formed and can be stabilized. A significant portion of Sn(ll) can be reoxidized to the Sn(lV) state by a subsequent medium oxidation. In view of the respective d InAldTvalues and the stabilization of SnW), the structural change of edge-sharing [SnO4/2] tetrahedra is suggested to form Si-0-Sn(lI)-0-S bridges. The existence of these bridges also can be correlated with the catalytic activity of tin silicalites observed in mild oxidation processes.