(684b) Synthesis and Characterization of Tin(IV) MFI: Sodium Inhibits the Synthesis of Phase Pure Materials

The synthesis and characterization of tin(IV) containing MFI zeolites is reported.  Using a variety of characterization methods, it is shown that via conventional hydroxide-mediated preparations that include sodium hydroxide it is difficult to make phase-pure Sn-MFI materials with more than approximately 3 wt% tin in the framework.  While at low tin contents and high pH values it is possible to make materials that appear more uniform, it is generally observed that the presence of sodium hydroxide in the synthesis has a deleterious effect on the quality of the material obtained.  This is manifested in several ways including the presence of a residual surface phase observed via scanning electron microscopy, a discrepancy between the bulk and surface Si/Sn ratios as determined by X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS) respectively, and a systematic decrease in the micropore volume as determined by nitrogen physisorption.  Moreover, it is observed that the tin/sodium rich overlayer formed is recalcitrant towards methods to dissolve it, including ion-exchange processes in acidic or basic media.  In contrast, Sn-MFI samples made in the absence of sodium hydroxide do not display these trends.  Rather, they appear to be phase pure and more homogeneous based on the methods outlined above.   The current work lays out a different approach to the synthesis of tin zeolites that appear to be highly uniform and should be of interest for the catalysis community.  Preliminary catalytic studies of these materials will also be described.