(19c) Rates of Levoglucosanol Hydrogenolysis over Brønsted and Lewis Acid Sites on Platinum Silica-Alumina Catalysts Synthesized By Atomic Layer Deposition
Bifunctional Pt/AlOx/SiO2 catalysts were synthesized using ALD to deposit sub-nanometer overcoats of AlOx onto SiO2, and kinetic measurements were made in the acid-limited regime. The ratio of Brønsted to Lewis acid sites, measured by NH3-TPD and pyridine-FTIR, systematically decreases with Al2O3 loading, while the Al coordination, measured by solid state 27Al NMR increases. Lewis acid sites on ALD-AlOx/SiO2 catalysts (potentially related to Al(V) species) have a 4-times higher turnover frequency (TOF) compared to Lewis acid sites on bulk Î³-Al2O3. A simple model is used to describe the reactivity trends for ALD-AlOx catalysts, assuming that is overall rate is the linear sum of independent contributions from Lewis and BrÃ¸nsted acid sites. Based on this model, BrÃ¸nsted acid sites on ALD-AlOx/SiO2 catalysts have a 6-times higher TOF than Lewis acid sites on these catalysts. With decreasing Al2O3 loading, the reactivity of these catalysts transitions from being entirely governed by Lewis acid sites, to largely governed by BrÃ¸nsted acid sites. This work provides a new approach to synthesize amorphous SiAls with tunable BrÃ¸nsted/Lewis acid site ratios, and reveals differences in the reactivity of BrÃ¸nsted and Lewis acid sites on these materials.
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