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.
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