Catalytic Activities Of Graphene-based Nanocomposites In Alcohol/Phenol Oxidations

The oxidation of alcohol to aldehyde or ketone is a key step in certain organic synthesis. As a widely uesd catalyst for alcohol oxidations, Au nanoparticles are well known for its easy preparation, excellent physical/chemical stability and high selectivity for primary alcohols. However, the Au catalysts are almost inert for the oxidation procedure in the absence of base additives or basic supports, which restricts severely their practical applications. In this work we first prepared composites of Au nanoparticles(NPs) and graphene quantum dots(GQDs), abbreviated as GQDs/Au, and studied their catalytic activities to the oxidation of alcohol in both acidic and alkaline conditions.Phenolic compounds are important chemical raw materials and intermediates, also the byproducts of many industrial processes. The degradation of phenolic-containing waste water through catalytic oxidation has been used to reduce phenolic contamination. The degradation procedure catalyzed by enzyme is of high efficiency, but the poor stability and high price of enzyme greatly limits its practical application. Here, to overcome the disadvantages of enzyme, by using nano-catalysts and immobilized enzyme, we established an efficient and stable catalytic system.The main contents and results of the work are as follows:1. Preparation of graphene quantum dots/Au composite(GQDs/Au) and the catalytic activity towards the selective oxidation of veratryl alcohol(VA). With GQDs/Au composites as catalysts, VA can be oxidized into veratryl aldehyde or veratric acid by H2O2 with excellent selectivity and conversion efficiency, which can be further tuned by varying the pH and reaction time. Inhibition reactions and EPR trapping experiments show that the reaction is controlled by free radicals. Under acidic condition, singlet oxygen species that generated by GQDs with H2O2 play a critical role; while under basic condition, superoxide anion and singlet oxygen are involved. The excellent chemical stability, the periphery carboxyl groups and the large aromatic structure of GQDs all contribute to the excellent catalytic activity of GQDs/Au composites.2. Preparation of the magnetic composites of Fe3O4/GQDs/Au and its catalytic activity used for solvent-free oxidation of VA. Based on the excellent catalytic performance of GQDs/Au composites in aqueous phase, we extend it to the solvent-free system. Meanwhile, in order to simplify the recover procedure of GQDs/Au, we immobilize GQDs/Au on a magnetic substrate to get Fe3O4/GQDs/Au composites. Fe3O4/GQDs/Au shows good catalytic activity and high selectivity to solvent-free oxidation of VA. This enables also rapid separation of the catalyst, prevents the waste of solvent and secondary pollution and should be practically applicable.3. Synthesis of magnetic graphene quantum dots/Fe3O4 composites(GQDs/Fe3O4) and its catalytic activity for degradation of phenolic compounds. A facile approach for the synthesis of GQDs/Fe3O4 composites through co-precipitation is developed. It is demonstrated that the peroxidase-like activities of the GQDs/Fe3O4 are much higher than those of the individual GQDs, Fe3O4 NPs, and graphene oxide/Fe3O4 composites(GO/Fe3O4). We find that the high peroxidase-like activity of GQDs/Fe3O4 can be attributed to the unique properties of GQDs and the synergistic effect of GQDs and Fe3O4. Moreover, GQDs/Fe3O4 composite exhibits better or comparable catalytic efficiencies for some phenolic compounds compared to horseradish peroxidase(HRP). Having high catalytic activity, good stability and reusability, the GQDs/Fe3O4 composite should be a potent peroxidase mimic that holds great promise for industrial phenolic wastewater treatment.4. HRP immobilized on the composites of chemically reduced graphene oxide and Fe3O4(CRGO/Fe3O4) for the catalytic degradation of phenolic compounds. A CRGO/Fe3O4 supported HRP(CRGO/Fe3O4/HRP) is prepared to catalyze the degradation of phenolic compounds. The CRGO/Fe3O4/HRP shows improved stability, reusability and low cost in comparison with HRP. The degradation efficiency of several phenolic compounds with CRGO/Fe3O4/HRP are studied and compared with free HRP, revealing the potential in achieving low-cost, efficient degradation of phenolic compounds.