Propylene oligomerization over SiO2-overcoated oxides

Catalytic oligomerization of light olefins is an important CC coupling strategy that can be used to produce high-value transportation fuels and chemicals from shale gas and biomass feedstocks. In this work, vapor-phase propylene oligomerization was studied over a series of overcoated SiO₂-MOx materials, whose acid site densities and strengths are tunable by the amount of SiO₂ deposition and the nature of the core oxide. These materials contain acid sites only on particle external surfaces, limiting pore diffusion artifacts. SiO₂-MOx materials were prepared by depositing stoichiometric amounts of tetraethyl orthosilicate (TEOS) onto Al₂O₃, anatase TiO₂, Nb₂O₅, and ZrO₂. Compared to the unmodified oxides, which are poor propylene oligomerization catalysts, the SiO₂-MOx materials exhibit moderate activity with Al₂O₃, TiO₂, and Nb₂O₅ core materials performing better than commercial amorphous silica alumina (ASA) on a surface area basis. The activity of the materials appears to be primarily driven by their Brønsted acid strength as measured by ³¹P TMPO MAS NMR with the rates ranked in order SiO₂/Al₂O₃ > SiO₂/TiO₂ ≈ SiO₂/Nb₂O₅ > SiO₂/ZrO₂. This study shows that SiO₂-overcoated materials are a tunable class of materials that are active for vapor-phase propylene oligomerization.