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 are reported. It is shown that with 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 final solid. 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 tin/sodium 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 overlayer formed is recalcitrant towards methods to dissolve it, including ion-exchange processes in acidic or basic media. In contrast, Sn-MFI materials 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 an approach to the synthesis of tin zeolites that appear to be highly uniform and should be of interest to the catalysis community.

Tin MFI samples have very different properties when made with (left) and without sodium present in the preparation.Figure optionsDownload as PowerPoint slideHighlights
► A thorough synthesis and characterization study of tin(IV) MFI zeolites is reported.
► Syntheses in the presence of sodium lead to inhomogeneous materials with tin and surface sodium enrichment.
► Syntheses in the absence of sodium lead to high-quality phase pure materials.
► The reported work shows a route forward to high–quality tin–containing zeolites with high (>5 wt.%) tin loading.