(411b) Acidity Trends in SiO2 Overcoated Oxides

Solid acid catalysts are critical components in both fine chemical synthesis and bulk commodity production. In addition to being intrinsically safer than liquid acids like hydrofluoric acid, solid acids can be operated neatly, decreasing chemical waste and often enhancing reaction kinetics. However additional synthetic control over their often heterogeneous distribution (Lewis vs. Brønsted) of acid sites is needed to take full advantage of these materials.

This challenge is particularly difficult in the mixed oxide space since the acidic properties of materials like amorphous silica-alumina are heavily synthesis dependent. We have attempted to simplify the structural complexity of these materials by depositing SiO₂ domains onto various oxides to create analogous SiO₂-MO_x interfaces. We then study the distribution, strength, and quantity of acid sites as a function of the Si surface density.

In this work we have prepared 4 series of materials by depositing SiO₂ onto Nb₂O₅, TiO₂, Al₂O₃, and ZrO₂ using a self-limiting sol-gel process followed by thermal treatment in static air. This process successfully deposits sub-monolayer to multilayer overcoats of amorphous SiO₂ without significantly changing the underlying oxide.

The Brønsted acidity of the SiO₂/MO_x catalysts was probed at mild conditions with liquid-phase hydroalkoxylation of n-octanol with 3,4-dihydro-2H-pyran and at moderate conditions with gas-phase propylene oligomerization. These reactions show that activity increases in each series of materials as a function of the Si surface density at sub-monolayer Si loadings. Interestingly, the extent of this activity enhancement is dependent on the core metal oxide and is ranked in the order SiO₂/Al₂O₃ > SiO₂/Nb₂O₅ > SiO₂/TiO₂ > SiO₂/ZrO₂. Pyridine DRIFTS and NH₃ TPD experiments indicate that the distribution of acid sites also evolves as a function of the Si surface density, providing a handle for engineering the acidity of these materials for individual applications.