Depositing SiO₂ on Al₂O₃: A Route to Tunable Brønsted Acid Catalysts

Strongly Brønsted acidic silica-alumina materials are workhorse catalysts in petroleum processes, including cracking, isomerization, and hydrocarbon synthesis. Here thin, conformal overcoats of SiO₂, 2-5 nm by TEM, were synthesized on pre-existing Al₂O₃ supports by stepwise addition of tetraethyl orthosilicate under basic conditions, and the surfaces were interrogated by N₂ physisorption along with NH₃ and pyridine chemisorption. SiO₂ layers thicker than 5 nm give largely inert surfaces, but adding only 2 nm of SiO₂ is shown to quench the underlying Lewis acidity and unexpectedly form Brønsted acid sites strong enough to protonate gas-phase pyridine. Alternately, the use of a molecular template grafted to the alumina surface before SiO₂ deposition selectively preserves the most reactive AlOH. In the catalytic cracking of 1,3,5-triisopropylbenzene at 450 °C, Al₂O₃ overcoated by 2 nm of SiO₂ proved to be a highly active catalyst with 10 times higher conversion in comparison to that of Al₂O₃. Finally, the silica overlayer was deliberately cracked to expose strong interfacial sites, likely between tetrahedral Al and SiO₂. In comparison to a material with an intact overlayer, this catalyst had 1.5 times higher conversion and 3 times higher selectivity to deep dealkylation products, including cumene and benzene. This core-shell SiO₂@Al₂O₃ catalyst gives total dealkylation yields, per surface area, similar to those of conventionally prepared SiO₂-Al₂O₃ or zeolite Y catalysts while providing for new synthetic handles for catalyst optimization.