Efficient Decarbonylation of Furfural to Furan Catalyzed by Zirconia-Supported Palladium Clusters with Low Atomicity

Decarbonylation of furfural to furan was efficiently catalyzed by ZrO₂-supported Pd clusters in the liquid phase under a N₂atmosphere without additives. Although Pd/C and Pd/Al₂O₃have frequently been used for decarbonylation, Pd/ZrO₂exhibited superior catalytic performance compared with these conventional catalysts. Transmission electron microscopy and X-ray absorption fine structure measurements revealed that the size of the Pd particles decreased with an increase in the specific surface area of ZrO₂. ZrO₂with a high surface area immobilized Pd as clusters consisting of several (three to five) Pd atoms, whereas Pd aggregated to form nanoparticles on other supports such as carbon and Al₂O₃despite their high surface areas. The catalytic activity of Pd/ZrO₂was enhanced with a decrease in particle size, and the smallest Pd/ZrO₂was the most active catalyst for decarbonylation. When CeO₂was used as the support, a decrease in Pd particle size with an increase in surface area was also observed. Single Pd atoms were deposited on CeO₂with a high surface area, with a strong interaction through the formation of a Pd−O−Ce bond, which led to a lower catalytic activity than that of Pd/ZrO₂. This result suggests that zero-valent small Pd clusters consisting of more than one Pd atom are the active species for the decarbonylation reaction. Recycling tests proved that Pd/ZrO₂maintained its catalytic activity until its sixth use.