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.