(764c) Nanobowls: A Platform for Selective Acid Catalysis on External Oxide Surfaces

Metal oxide catalysts are workhorse materials in the production of petroleum products and fine chemicals. Crystalline, microporous materials (zeolites) have long dominated catalysis in these industries since being introduced in 1960. Zeolites act as shape selective solid acid catalysts and often possess both strong Brønsted and Lewis acidity. As the chemical industry looks to switch over to biomass and tar sands feedstocks, reactant diffusion and catalyst stability issues have arisen. These problems could be addressed by running these same reactions over (somehow) selective, non-porous metal oxides. In addition, using bulk oxides for zeolite chemistry would open the door to tunable materials development covering a wide array of elemental compositions. Therefore, we propose the Nanobowls approach as an alternative, highly tunable synthesis method to create selective acid catalysts. In this approach, we construct 1-2 nm diameter cavities on the surface of nonporous oxides. The Nanobowls approach has been applied to both major types of acid catalysts: those with Brønsted sites and those with Lewis sites.

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.