Enhancement of Ru on furoic acid synthesis from furfural oxidation over Rux/8-MnO2 catalysts

8-MnO2 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 8-MnO2 (Rux/8-MnO2) have been made to enhance its C-H bond breaking ability. The result showed that the furfural conversion was greatly enhanced on Rux/8-MnO2 due to the enhanced capability for breaking the C-H bond. Especially for Ru0.25/8-MnO2, whose conversion rate per unit mass was 18 times higher than 8-MnO2. The characterization results showed that the Ru species mainly existed as single atoms or clusters on Ru0.25/8-MnO2 and Ru0.5/8-MnO2, while started to aggregate to RuO2 nanoparticles on Ru1/8-MnO2 and Ru2/8-MnO2. With the increase of Ru particles, the Ru's average oxidation state (AOS) decreased from +6 on Ru0.25/8-MnO2 to +4.74 on Ru2/8-MnO2. The Ru's TOF (intrinsic activity per Ru) was positively linearly related to the Ru's AOS. The electron-deficient Ru in Ru0.25/8-MnO2 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 Ru0.25/8-MnO2 exhibited the highest TOF, which can even reach 29.85 mmolFF/mmolRu/h.