Effects of SiO 2 Support on the Formation of Active Sites in HfO 2 /SiO 2 Catalysts for the Conversion of Ethanol and Acetaldehyde to 1,3-butadiene.

Efforts to improve the conversion efficiency of 1,3-butadiene (BD) synthesis from biomass feedstocks, such as bioethanol, have the potential to drive the development of sustainable chemical manufacturing processes. In this study, BD synthesis from ethanol and acetaldehyde was investigated over HfO2/SiO2 catalysts prepared using different SiO2 supports. Although BD yield strongly depended on the type of SiO2 support, a consistent relationship between conversion and BD yield was observed for all catalysts. The determined activation energies of the HfO2/SiO2 catalysts were nearly identical, regardless of the type of SiO2 support, indicating that the nature of the active sites for BD synthesis is consistent across the catalysts. The amounts of Lewis acid sites in the catalysts, determined via pyridine adsorption, exhibited a linear relationship with both the BD synthesis reaction activity and the quantity of silanol (Si-OH) groups present in the SiO2 supports. Dynamic nuclear polarization NMR further showed that BD yield increased with the concentration of Si-O-Hf species. These results demonstrate that the active sites for BD synthesis over HfO2/SiO2 catalysts are Lewis acid sites, specifically Si-O-Hf species formed from interactions between Si-OH groups and Hf ions on the SiO2 surface.