(411b) Acidity Trends in SiO2 Overcoated Oxides
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-MOx 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/MOx 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.