Tuning acidity in silica-overcoated oxides for hydroalkoxylation

Abstract

Mixed oxides are ubiquitious solid acid catalysts for a number of transformations. Here, the chemistry of mixed oxide interfaces was experimentally probed by constructing model SiO2 on MOx interfaces using a liquid-phase, stoichiometrically-controlled deposition of tetraethyl orthosilicate (TEOS). Catalytically active Brønsted sites were generated in four series of overcoated SiO2 on metal oxide materials prepared by depositing TEOS onto Al2O3, anatase TiO2, Nb2O5, and ZrO2. Pyridine DRIFTS studies indicate that the acidity of deposited silanol groups was enhanced by interaction with the metal oxide surface, suggesting that pseudo-bridging silanol type structures were formed. Trimethylphosphine oxide (TMPO) 31P NMR experiments showed that Brønsted acid strength varied in the overcoated materials, with increasing strength in the following order: SiO2/ZrO2 < SiO2/TiO2 < SiO2/Nb2O5 < SiO2/Al2O3. This order correlated with the Lewis acidity and chemical hardness of the underlying metal oxides. The catalytic performance of the materials was evaluated in the liquid-phase hydroalkoxylation of dihydropyran with n-octanol, revealing that activity was highly dependent on SiO2 loading, which impacted the density of acid sites, and scaled exponentially with Brønsted acid strength, determined by the core metal oxide identity. Brønsted acid strength appears to be the key driver of the materials’ activity, with the highest areal rate of 2.2 mmol / m2 – h being measured over SiO2/Al2O3. This study shows that SiO2-overcoated materials are a systematically tunable class of catalysts that can be applied to acid catalyzed reactions.