A hierarchical porous N-doped carbon material as catalyst derived from the folic acid-Zn/KBr hydrogel: Reaction network and kinetics and mechanism for the oxidation of furfural to maleic acid

A novel hydrogel (named FA-Zn/KBr) was prepared, by taking advantage of the hydrogen-bonds and π-π stacking interactions between the pterin heterocycles of the folic acid (FA) molecules. The hydrogel is composed of highly dispersed KBr, coordinated zinc ions, and stacked arrays of folate tetramers. Due to the presence of KBr, a hierarchical porous nitrogen-doped carbon (PNC-800) material with a specific surface area of PNC-800 is 2807.9 m2/g can be formed through the calcination of the FA-Zn/KBr xerogel under N2. PNC-800 is used to catalyze the oxidation of furfural to maleic acid in the presence of H2O2. Representative nine reactions constitute the reaction network. The reaction rate constants and apparent activation energy were obtained for each reaction. Based on the kinetics parameters, the simulated results are in good agreement with the experimental data. The mechanism of the reaction catalyzed by PNC-800 is proposed based on ample experimental evidence. The graphitic nitrogen species in PNC-800 are the active sites. PNC-800 catalyzes the decomposition of hydrogen peroxide to generate hydroxyl radical (HȮ), which oxidizes furfural to furfural radical, and the furfural radical is subsequently converted to 5-hydroxy-furan-2(5H)-one (HFONE). The conversion of furfural to maleic acid via the formation of HFONE is the main reaction pathway. In addition, the hierarchical porous structure with a high specific surface area facilitates the interaction of reactant with the active sites. PNC-800 has been shown an excellent catalytic activity for the production of maleic acid from furfural. For the oxidation of 1000 mM furfural using 50 mg PNC-800 at 80 °C within 9 h, the conversion of furfural is 100 % with a maleic acid yield 67.5 %.