Hierarchical Sn-MFI zeolites prepared by facile top-down methods for sugar isomerisation

Alkaline treatment of hydrothermally synthesised Sn-MFI and alkaline-assisted stannation of silicalite-1 were investigated as industrially amenable post-synthetic strategies to generate hierarchical Lewis-acidic MFI zeolites for the isomerisation of mono- and disaccharides. The mesoporosity introduced upon both treatments produced materials that are able to convert glucose, xylose, and lactose, which suffer from access or diffusion limitations to the active sites of the purely microporous Sn-MFI. The stannation of silicalite-1 rendered more active catalysts than the base leaching of Sn-MFI. This is due to a better utilisation of Sn, which is mainly incorporated at the crystals' external surface in the former case but mostly remains in the inaccessible micropores in the latter. The amount of Sn introduced via stannation could also be controlled by varying the strength of the alkaline solution and the molarity of the tin salt added. A Sn loading of 0.25 wt.% already ensures remarkable activity and selectivity.