Enhancement of Ru on furoic acid synthesis from furfural oxidation over Rux/δ-MnO2 catalysts

δ-MnO 2 exhibits excellent activity for furfural oxidation to furoic acid under base-free conditions. However, its poor C H bond breaking capacity leads to lower conversion efficiency per unit mass catalyst than noble metal. Thus, a series of Ru-modified δ-MnO 2 (Ru x /δ-MnO 2 ) have been made to enhance its C H bond breaking ability. The result showed that the furfural conversion was greatly enhanced on Ru x /δ-MnO 2 due to the enhanced capability for breaking the C H bond. Especially for Ru0.25/δ-MnO 2 , whose conversion rate per unit mass was 18 times higher than δ-MnO 2 . The characterization results showed that the Ru species mainly existed as single atoms or clusters on Ru0.25/δ-MnO 2 and Ru0.5/δ-MnO 2 , while started to aggregate to RuO 2 nanoparticles on Ru1/δ-MnO 2 and Ru2/δ-MnO 2 . With the increase of Ru particles, the Ru's average oxidation state (AOS) decreased from +6 on Ru0.25/δ-MnO 2 to +4.74 on Ru2/δ-MnO 2. The Ru's TOF (intrinsic activity per Ru) was positively linearly related to the Ru's AOS. The electron-deficient Ru in Ru0.25/δ-MnO 2 can strongly adsorb the H atom of the C H of geminal diol intermediate and accelerate the breakage of its C H bond. As a result, Ru in the Ru0.25/δ-MnO 2 exhibited the highest TOF, which can even reach 29.85 mmol FF /mmol Ru /h.