Aerobic Oxidation of 5-Hydroxymethylfurfural to High-Yield 5-Hydroxymethyl-2-furancarboxylic Acid by Poly(vinylpyrrolidone)-Capped Ag Nanoparticle Catalysts

An efficient method for the catalytic aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) was developed using size-controlled Ag nanoparticles (NPs) in an aqueous solution. A series of AgNPs catalysts stabilized by poly(vinylpyrrolidone) (PVP) supported on ZrO2with different Ag loadings were prepared. XRD, TEM, XPS, and FT-IR techniques were used to characterize the nature of these catalysts. The impact of the Ag loading, the molar ratio of PVP to Ag, the reaction temperature and time, and the base type on the conversion of HMF and yield of HMFCA was investigated. The reaction results show that Ag–PVP/ZrO2 catalysts with a bigger particle size (>10 nm) exhibited superior catalytic activity and HMFCA selectivity compared to Ag/ZrO2 with a smaller particle size (ca. 8.5 nm), indicating that the addition of PVP has a promoting effect on the aerobic oxidation of HMF into HMFCA. The 2.5%Ag–PVP/ZrO2(1:1) catalyst presents an excellent activity with 100% conversion of HMF and 98.2% yield of HMFCA in oxygen flow at 20 °C for 2 h.The basicity is also very critical to obtain a high HMFCA yield, especially for the use of high concentration of HMF; i.e., Ca(OH)2 afforded a higher selectivity toward HMFCA compared to the use of NaOH or Na2CO3 under an identical reaction condition. It is revealed for the first time that the presence of a capping agent (PVP) can effectively modulate the metal–support interaction. Interestingly, it is found that the relatively weak interaction between Ag NPs and the ZrO2 support has a beneficial effect on the conversion of HMF. This work provides new insights into the relationship between the metal–support interaction mediated by PVP molecule and the activity and selectivity of the oxidation reaction.