PdO Supported on TiO₂for the Oxidative Condensation of Furfural with Ethanol: Insights on Reactivity and Product Selectivity

Pd nanoparticles were dispersed on TiO2 (P-25) to obtain catalysts for the oxidative condensation of furfural (FUR), aiming at the synthesis of furan-2-acrolein (F2A) as the main product. Synthesized catalysts were characterized by X-ray diffraction, transmission electron microscopy, N2 sorption at -196 °C, and X-ray photoelectron spectroscopy. Characterization studies pointed to the presence of well-dispersed and uniform PdO nanoparticles with very small particle sizes. Catalytic data showed that the incorporation of low Pd content (1-2 wt %) caused an increase in the conversion at shorter reaction times, with FUR conversion close to 95% and a F2A yield of 60%, after 3 h of reaction at 150 °C. However, at longer reaction times and higher temperatures, the high reactivity of F2A reduced product yields, accompanied by the formation of 2,2′-difuryl methane (DFM) via C-C coupling. Other parameters including the nature of oxidant and the strength of the base, play an important role in this catalytic process. The use of strong bases (NaOH or KOH) ameliorated activity, although they favored the formation of nondetected products, so it was necessary to use a softer base such as Na2CO3. The presence of an oxidant (H2O2) and a noble metal are also necessary to oxidize the alcohol and for use as solvents, which is necessary for the oxidative condensation.