Synthesis Of Supported Metal Complexes And Metal Oxide Catalysts For Catalytic Epoxidation Of Alkenes

The selective epoxidation of alkenes plays an important role in the production ofsurfactants, drugs, food additives and perfumes etc. Up to now, there are many waysto produce epoxides, but they also have some problems, such as pollutionenvironment and low economic benefits etc. Therefore, the subject of high efficiencyand no pollution in the epoxidation of alkenes is still attractive and challenging.Many Salen and8-hydroxyquinoline transition metal complexes exhibit excellentcatalytic activity and selectivity for the homogeneous oxidation of olefins, but thesecatalysts are difficult to be recycled. To heterogenize homogeneous catalysts byanchoring them onto insoluble supports is a possible strategy to overcome thisdifficult. Meanwhile this method can also isolate the active site to prevent fromforming polymer so that it can improve catalytic activity and stability of the catalyst.Besides, some metal oxides have been used in the epoxidation of olefins, but theycan’t achieve ideal results. Consequently, it is still a meaningful study to develop newand efficient catalysts for the epoxidation of alkenes.Based on the above, this thesis mainly focuses on the synthesis of supportedmetal complexes of Salen,8-hydroxyquinoline and metal oxides catalysts and theircatalytic properties in epoxidation of olefins were studied. A series of characterizationtechniques are used to study the constitutes and structures of the prepared catalysts.Moreover, the effect of different oxidants, solvents, catalyst dosage and reactiontemperature on the catalytic activities, selectivities and stability was investigated. Inaddition, the impact of the distribution of active sites and structures on the catalyticactivities was also studied. The main contents and results are as follows:1. Oxovanadium(IV) and iron(III) Salen complexes tethered ontoamino-functionalized graphene oxide for the aerobic epoxidation of styreneVO-Salen-GO and Fe-Salen-GO were prepared by covalent immobilization of oxovanadium(IV) and iron(III) Salen complexes onto amino-modified graphene oxide(NH2-GO) and the synthesized materials were used as catalysts in the aerobicoxidation of styrene. The catalytic results showed that these prepared catalysts wereactive and VO-Salen-GO was more active than its homogeneous analogue in theaerobic epoxidation of styrene due to better site-isolation. Besides that, it could bereused three times without significant loss of activity and selectivity. However, theselectivity of the styrene oxide significantly decreased for Fe-Salen-GO. It may bedue to the variation of coordination environment around Fe during the reaction.2. Dioxomolybdenum(VI) complex covalently attached to amino-modifiedgraphene oxide for the epoxidation of alkenesMoO2-Salen-GO was synthesized by immobilization of transition metal Salencomplex MoO2-Salen onto NH2-GO and it was applied in the epoxidation of alkenes(styrene, cyclooctene,1-octene etc.) using TBHP or H2O2as oxidant. The catalyticresults exhibited that MoO2-Salen-GO performed higher conversion in theepoxidation of alkenes (styrene, cyclooctene etc.) using TBHP as oxidant under thesimilar reaction conditions as compared with the catalytic results using H2O2asoxidant, which might be attributed to the decomposing of H2O2into molecular oxygenand water. Furthermore, the effect of different solvents on the catalytic activities wasdiscussed. In addition, the MoO2-Salen-GO catalyst was more active than itshomogeneous analogue MoO2-Salen in the epoxidation of cyclooctene because ofbetter site isolation. MoO2-Salen-GO could be reused three times without significantloss of activity. Combining with the ICP result, it confirmed the stability ofMoO2-Salen-GO and no loss of active specie.3. Immobilization of Cu(II), Co(II), Fe(III) or VO(IV) Salen complexes ontohybridization of graphene sheets and carbon-coated Fe3O4nanoparticles for theaerobic epoxidation of styreneNovel core@shell structural magnetic nanocatalysts RGO/Fe3O4@C-Salen-Mand Fe3O4@C-Salen-M [M=Cu(II), Co(II), Fe(III) or V(IV)] involving carbon-coatedmagnetic Fe3O4nanoparticles supported on graphene oxide sheets (RGO/Fe3O4@C)and carbonaceous coatings on magnetic Fe3O4nanoparticles (Fe3O4@C) for immobilizing metal (Cu, Co, Fe or VO) Salen complexes were prepared by a facile,green and efficient chemical approach and examined as catalysts in the aerobicepoxidation of styrene. Compared with the magtic materials Fe3O4@C-Salen-M (M=Cu, Co, Fe or V), RGO/Fe3O4@C-Salen-M (M=Cu, Co, Fe or V) showed highercatalytic reactivity. This is possibly due to better site-isolation, good dispersion ofmagnetic Fe3O4nanoparticles and efficiently synergistic effect between the metalcomplexes and the supports. Beyond that, RGO/Fe3O4@C-Salen-Co showed highconversion and selectivity for the aerobic epoxidation of styrene. These magneticcatalysts were stable because they could be reused three times without significant lossof activity. Even more importantly, these novel catalysts can be easily recovered bysimple magnetic separation.4. Enhanced alkenes epoxidation reactivity of discretebis(8-quinolinol)oxovanadium(IV) or bis(8-quinolinol)dioxomolybdenum(VI)complex attached onto graphene oxide by a metal-template/metal-exchangemethodA series of imprinted catalysts were prepared by covalent attachment of discretebis(8-quinolinol)oxovanadium(IV) or bis(8-quinolinol)dioxomolybdenum(VI)complex onto graphene oxide (GO) through the metal-template/metal-exchangemethod to control the distribution of covalently attached independent ligands (thetemplated catalysts has uniform composition and structure). The results of EPR andelement analysis exhibited the ligands of the templated catalysts are bis(8-quinolinol).Meanwhile, they were applied in the epoxidation of olefins using TBHP or H2O2asoxidant in comparison with their homogeneous counterparts and materials preparedby random grafting of the ligand.Catalytic results exhibited that these imprinted oxovanadium(IV) ordioxomolybdenum(VI) complex catalysts showed higher activity and higherselectivity to the target product than either their homogeneous ones or randomlygrafted analogues due to better site-isolation and uniform composition and structure.The influence of different solvents and oxidants on the catalytic activities was alsoinvestigated. The Mo and V catalysts perform well with CHCl3and CH3CN as the solvent, respectively. The catalysts showed higher conversions in epoxidation ofcyclooctene using TBHP as oxidant than those achieved using H2O2as oxidant, whichmight be attributed to the decomposing of H2O2into molecular oxygen and water. Inaddition, these imprinted catalysts could be reused three times without significant lossof activity. But the conversion of cyclooctene and the selectivity to the cycloocteneoxide significantly decreased over the VO(IV) catalyst prepared by random graftingof the ligand. Repeated and leaching tests indicated that these synthesed imprintedcatalysts were stable and recyclable.5. Co3O4nanoparticles supported on N-doped carbon-coatedFe3O4/graphene for styrene epoxidation with molecular oxygenMagnetic core@shell nanocomposite catalysts involving Co3O4immobilized onN-doped carbon-coated Fe3O4/graphene (RGO/Fe3O4@C-N) were prepared by a facile,green and efficient chemical approach and were examined as catalysts in theepoxidation of styrene and cyclooctene with air as the oxidant. N-dopants in theRGO/Fe3O4@C-N-Co3O4can strengthen the interaction between Co3O4and thesupport and enable generation of smaller Co3O4nanoparticles and more active speciesthan Co3O4supported on N-free RGO/Fe3O4@C (RGO/Fe3O4@C-Co3O4).Compared with bulk Co3O4and RGO/Fe3O4@C-Co3O4,RGO/Fe3O4@C-N-Co3O4displayed higher catalytic reactivity (i.e.93.2%conversionof styrene and88.4%selectivity to styrene oxide). It may be due to the increase ofactive site, good dispersion of Co3O4nanoparticles and efficiently synergistic effectbetween Co3O4and the magnetic support. In addition, the reaction conditions werealso optimized by changing the catalyst dosage and reaction temperature. Furthermore,the higher conversion of cyclooctene than styrene is due to electron density effectover RGO/Fe3O4@C-N-Co3O4. In addition, RGO/Fe3O4@C-N-Co3O4exhibited goodrecoverability by simple magnetic separation with negligible loss in activity andselectivity within several successive runs due to superparamagnetism. Repeated testsand leaching experiment results suggested that RGO/Fe3O4@C-N-Co3O4was a trulyheterogeneous catalyst.