Study Of Fenton-like Catalytic Oxidation Of Guaiacol As A Lignin Model Compound

Efficient utilization of biomass is of great significance for the establishment of a sustainable new chemical process,because of the depletion of fossil resources.Lignin and guaiacol?a derivative of lignin?are both important components of biomass feedstocks.Guaiacol is widely used as a model reactant of lignin in the studies of hydrodeoxygenation reactions,while the conversions of guaiacol to organic acids via oxidative cleavage have been seldom reported.A solution of H₂O₂ and Fe²⁺ ions,well-known as Fenton's reagent,can produce hydroxyl radicals?HO?that show strong oxidative ability in degradation of organic compounds,and are potentially applicable for selective oxidation of lignin to value-added chemicals.In this study,Fe/ZSM-5,Cu/ZSM-5,and FeCu/ZSM-5 catalysts were prepared via solid-state ion-exchange methods,and were successfully used to catalyze the Fenton-like process of guaiacol oxidation to formic acid with ultra-high selectivity,in which H₂O₂ was used as the oxidizing agent.At optimized reaction conditions?40 ?,3 h,0.01 mmol/mL guaiacol,and 0.25 mmol/mL H₂O₂?,the selectivity of formic acid among the products of the guaiacol oxidative cleavage reached up to 77%,whereas the corresponding selectivity of CO2 was merely 18%.We found that tert-Butanol used here as a radical quencher was able to completely restrain the oxidative cleavage of guaiacol,conforming that hydroxyl radicals formed from decomposition of H₂O₂ in situ were the true oxidants in these Fenton-like processes.The assessment on the reactivity and selectivity of C1-C4 carboxylic acids and their mixtures in such Fenton-like processes further unveiled that maleic acid and glycolic acid are plausible reactive intermediates involved in the reaction pathway of guaiacol oxidation to formic acid,and that propanedioic acid and oxalic acid are kinetically irrelevant to the formation of formic acid.In particular,it was found that formic acid was more reactive than longer carboxylic acids in the oxidation by hydroxyl radicals,but these hydroxyl radicals preferred to react with the longer carboxylic acids over formic acid when formic acid and the longer carboxylic acids coexisted in the solution,rendering a high selectivity of formic acid for the Fenton-like process of guaiacol oxidation.The comparison among Fe/ZSM-5,Cu/ZSM-5,FeCu/ZSM-5,FeCu/SiO2 and homogeneous Fe3+/Cu²⁺ catalysts on their activity and selectivity in oxidative cleavage of guaiacol showed that supported Fe/Cu-based solid catalysts were not only easier that the Fe3+/Cu²⁺ ions in catalytic recycling,but also more controllable on the oxidation degree of the products,which allows an achievement of high yields of formic acid.The formation rates of formic acid on FeCu/ZSM-5 were much higher than those on Fe/ZSM-5 and Cu/ZSM-5,indicative of synergy between Fe3+ and Cu²⁺ ions during the catalysis of the Fenton-like processes.Moreover,we found that the microporous framework of FeCu/ZSM-5 promoted the conversion of C²⁺ intermediates into formic acid with respect to those amorphous FeCu/SiO2 catalysts,reflecting the presence of confinement effects when such oxidative cleavage reactions occur in micropores.The above understanding on the reaction mechanism and the catalytic structure-activity relationship for the oxidative cleavage of guaiacol on Fenton-like solid catalysts are applicable to structurally more complicated diphenols and diphenyl ethers,and high yields of formic acid from these reactants can be obtained at optimized reaction conditions.These studies on the oxidative cleavage of aromatic compounds would promote the development of green and efficient processes for converting lignin into useful oxygenate chemicals.