Application Of Graphene-based Catalysts In Selective Hydrogenation Of Aromatic Compounds

Selective hydrogenation of aromatic compounds has attracted much attention of chemists, because of the target products tend to have a high commercial value. For example, hydrogenation of phenol to prepare cyclohexanone, which is the precursor of caprolactam and commodity acid. While 1,3-cyclohexanedione (1,3-CHD) was the significant intermediate in making maize, In recent years, graphene was ususally used as an ideal support in preparation of catalysts due to its great theoretical specific surface area and stability. Graphene-based catalyst was widely used in catalytic oxidation, catalyzed hydrolysis, esterification, catalytic decomposition, electric catalysis and photocatalysis field. However, it is still preliminary in the application of hydrogenation.In this paper, the noble metal supported graphene catalyst was prepared by a series of methods and used to catalysic the hydrogenation of resorcinol, phenol and benzene. Furthermore,the reaction mechanism was studied based on the density functional theory calculation.Firstly, graphene oxide was prepared by Hummers method and both the palladium and ruthenium graphene catalysts (Pd/rGO and Ru/rGO) were prepared by co-precipitation method. Catalysts were characterized by Tranmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Raman and Brunauer-Emmett-Teller (BET). The TEM graphic demonstrates that Pd and Ru metal particle was distributed uniformly and the average particle size of Pd was 4.61 nm while the mean particle diameter of Ru was 1.96 nm. By Raman spectrum analysis, it is indicated that Pd/rGO and Ru/rGO catalysts have much edge and defect sites, which may be related to the destruction of the graphite layer caused by chemical oxidation reduction method. The specific surface area of Pd/rGO and Ru/rGO was not large, which was 80 m2·g-1 and 74 m2·g-1, respectively.In the process of hydrogenation of aromatic phenolic compounds, e.g., Resorcinol (RES), Phenol. Compared with ordinary commercial activated carbon based palladium catalyst (Pd/AC), Pd/rGO shows better catalytic activity and selectivity. Take the hydrogenation of resorcinol for example, even at room temperature,99% conversion and 94% selectivity of 1,3-cyclohexanedione (1,3-CHD) can be obtained. While during the catalytic hydrogenation of phenol, under the room temperature and 1 MPa hydrogen pressure, the conversion rate could be achieved 100% after 5 h. As for Pd/AC, under the same conditions, only 70% conversion could be obtained 9 hour’s later. In this thesis, the effect of solvents was investigated, which shows that polar solvents can inhibit the activity of graphene catalysts.A theoretical investigation has also been carried out on the reaction mechanism of hydrogenation and the interaction between different compounds and graphene model by using DFT calculation with the density functionals B3LYP. In this present study, we performed all calculations using the computational software package Gaussian09, and all molecular visualizations used the Gaussview 5.08 program. We used a triple-ζ basis set, which includes polarization functions and diffuse functions,6-311++G(d,p), to obtain accurate energy values, but the geometry optimization, transition state (TS) search, vibrational frequency were performed by using a slightly smaller basis set,6-31+G(d), because the difference in the geometries optimized with these two basis sets would be negligible and the double-ζ basis set is much less time-demanding. Generally in the IRC calculations, thirty points were computed from the transition state in each direction towards the reactants and products with a step size of 0.1 Bohr. The solvent effect towards interaction between graphene and substrates was calculated by force field model CPCM.Our calculation showed that graphite structures exhibit stronger adsorption toward RES due to the π-π interactions between them, which means that RES is superior to 1,3-CHD in occupying active sites. After addition of 1 mol of hydrogen to RES,1,3-CHD (enol isomer) was generated, and its less interaction force with graphene surface allows it to leave away easily. Therefore, excessive hydrogenation is prevented and the selectivity toward 1,3-CHD is enhanced. When the hydrogenation occurred in polar solvents, the solvents molecule could form hydrogen bond with RES, which interfering with the π interactions and reducing conversion rate and selectivity of 1,3-CHD.