Rates of levoglucosanol hydrogenolysis over Brønsted and Lewis acid sites on platinum silica-alumina catalysts synthesized by atomic layer deposition

Abstract

Nanoscale coatings of AlOx were deposited onto SiO2 using atomic layer deposition (ALD) to synthesize amorphous silica-alumina (SiAl) catalysts, and these catalysts were investigated for levoglucosanol (Lgol) hydrogenolysis. With decreasing Al2O3 loading, the ratio of Brønsted to Lewis acid sites, measured by NH3-TPD and pyridine-FTIR, systematically increases, while the Al coordination, measured by solid state 27Al NMR, decreases. These structural changes correspond to an increasing mass-normalized rate of Lgol hydrogenolysis. We model the mass-normalized reaction rate as the sum of independent contributions from Brønsted and Lewis sites, showing that Brønsted acid sites on ALD-AlOx/SiO2 catalysts have a 6-times higher turnover frequency (TOF) than Lewis acid sites on these catalysts. Additionally, Lewis acid sites on ALD-AlOx/SiO2 catalysts (potentially related to Al(V) species) have a 4-times higher TOF than Lewis acid sites on bulk γ-Al2O3. The overall mass-normalized reactivity of ALD-AlOx/SiO2 catalysts is due to Lewis acid sites at the highest Al2O3 loading, while it is predominantly due to Brønsted acid sites at the lowest Al2O3 loadings. This work provides a new approach to synthesize amorphous SiAls with tunable Brønsted/Lewis acid site ratio and reveals differences in the reactivity of Brønsted and Lewis acid sites on these materials.