(764c) Nanobowls: A Platform for Selective Acid Catalysis on External Oxide Surfaces
In the Brønsted acidic case, spherical Al2O3 particles were overcoated with SiO2 in the liquid phase using TEOS as the Si precursor. These overcoated materials were calcined at 650 oC with a high ramp rate, 20 oC/min, that lead to the formation of strong Brønsted acid sites contained within a thin, accessible microporous overcoat. Their performance in a Brønsted acidity probe reaction, the vapor phase catalytic cracking of Triisopropylbenzene (TIPB) was on par with industry standard amorphous SiO2-Al2O3 and slightly less active than zeolite H-Y on a surface area basis. In the Lewis acidic case, shape selective Ti sites were synthesized on SiO2 spheres by grafting Cp*TiCl3, calcining off the ligands at 550 oC, overcoating with SiO2 using the same liquid phase deposition technique, and recalcining the material at 550 oC with a high ramp rate of 10 oC/min. This lead to the formation of strong Lewis acid sites contained within a thin, accessible microporous overcoat. Their shape selectivity performance is being tested in several Lewis acidity probe reactions including competitive Tetrahydropyranylation of Benzyl alcohol and 2,4,6-Trimethylbenzyl alcohol, competitive MPV reduction of Acetophenone and 2,4,6-Trimethylacetophenone, and Limonene Epoxidation.
Overall, we have demonstrated this approach as a highly tunable synthetic pathway to alternative selective Brønsted and Lewis acidic oxide materials. Future materials development will include application to bifunctional catalysis such as biomass conversion reactions and the synthesis of shape selective Brønsted sites via sulfation of Al2O3 or TiO2 within 1-2 nm SiO2 cavities.