Study On Heterogeneous Catalysts For Benzene Hydroxylation And Styrene Epoxidation

As an important intermediate in chemical industry, phenol is heavily demanded as it can be used to synthesize bi-phenol A, nylon-66, phenolic resin, etc. Over95%of commercial phenol synthesis is made by the cumene route. The process contains three steps, i.e. Friedel-Crafts alkylation of benzene with propene to form cumene, cumene oxidation with oxygen to cumyl hydroperoxide and cleavage of cumyl hydroperoxide in acidic medium to obtain phenol and acetone with equimolar amount. However, at present, the demand for phenol is outpacing the demand for acetone. Furthermore, this multi-step reaction needs many organic reagents which cause much waste and environmental pollution. Based on the ecological and economical point of view, the direct hydroxylation of benzene to phenol has been attracted much attention.In this thesis, we studied the liquid phase direct hydroxylation of benzene to phenol with H2O2and O2as environmental friendly oxidant. A series of heterogeneous catalysts were synthesized and the correlation between the catalytic activity and the microstructure of the catalyst was investigated. Also, the possible reaction mechanism is proposed. It was found that the tetrahedrally coordinated vanadium species in the V-HMS catalyst may be the active species for the benzene hydroxylation reaction. Among the studied second metals added to the V-HMS catalyst, Fe was found to be effective in improving the catalytic performance. Also we successfully synthesized a series of cesium salts of molybdovanadophosphoric acid doped with metal (Mym+CsxH3+n-x-myPMo12-nVn04o, M=Ga3+, Fe3+, Cu2+, x=0.5-3.0, y=0.1-0.3, m=2,3, n=0,1,2) via a solid synthesis method. The compounds show remarkable catalytic activity and insolubility in the reaction. The yield of phenol achieves26.3%on the Ga0.2Cs2H2.4PMo10V2040using H2O2as oxidant. In addition, the catalytic activity and reaction kinetics of polyoxometalate coordinated compound was studied in the styrene epoxidation with tert-butylhydroperoxide (TBHP) as oxidant. All the heterogeneous catalysts we synthesized show high catalytic activity and recyclability in the studied reaction, thus have great potential in industrial application. The specific research work and results include several following parts.In the study of direct hydroxylation of benzene to phenol we prepared different vanadium containing mesoporous silica materials (V-HMS) through co-synthesis method. The characterization results show the presence of worm-like mesopores in the catalysts. The isolated VO4species are incorporated into the framework of HMS under low vanadium content. The polymeric and crystalline vanadium species appear over the catalyst with high vanadium content. The catalytic results of V-HMS catalysts in the benzene hydroxylation reaction with H2O2as oxidant show that the isolated VO4species are active species. A high phenol yield of11.1%is achieved under the optimized condition. The catalyst shows good reusability.We further studied the doping effect of Fe(III), Al(III), Cu(II), Ni(II), Co(II), Mo(VI), and Cr(III) introduced by impregnation method on the V-HMS catalyst. Among the studied elements Fe was found to be the most effective one for the improvement of the catalytic performance. Thus, we synthesized a series of FexVy-HMS catalysts containing different contents of Fe and V by the co-synthesis method. The characterization results show that the mesoporous structure of HMS maintains and the addition of Fe creates the new acid sites and stronger redox ability. Catalytic tests show that vanadium species are the active species and iron species play the role as a promoter for the reaction. Under the optimal reaction conditions the Feo.o4Vo.o6-HMS catalyst exhibits the best catalytic performance with the highest phenol yield of18.1%in comparison with12.7%over the Fe-free Feo.ooVo.o6-HMS catalyst.Heteropoly acids (HPAs) show good catalytic performance in many selective oxidation reactions. HPAs have several advantages as catalysts, e.g. they exhibit fast reversible multielectron transfer under rather mild conditions and their catalytic properties can be tuned by changing the identity of charge-compensating counter cations, heteroatoms, and addenda atoms. However, the application of HPA catalysts is hampered by their high solubility in polar media. Thus, a series of cesium salts of Keggin-type heteropoly acids, CsxH3+n-xPMoi2-nVn04o (n=0,1,2, x=0.5-3.0), were prepared using solid synthesis method. The characterization results show that, doping with Cs+, the crystal phase changes from the triclinic phase to cubic phase with Keggin structure remains unchanged. The redox ability of the catalysts decreases gradually with the hydrophobicity of the catalysts increases with increasing Cs+substitution number. These cesium salts were employed in the hydroxylation of benzene using H2O2as oxidant in the aqueous acetic acid solvent. Catalytic results show that the substitution of molybdenum addenda atoms by vanadium in the primary Keggin structure can remarkably enhance the catalytic activity, indicating that vanadium play an important role in the reaction. The volcanic trend in catalytic activity with increasing Cs+content may arise from the combined effect of the redox ability and hydrophobicity of the catalysts. Also the amount of released heteropoly species into the solution decreases significantly with increasing the cesium content. According to the catalytic and leaching results, Cs2H3PMo10V2O40exhibits the optimal catalytic activity among the CsxH3+n-xPMo12-nVnO4o catalysts with high phenol yield of19.2%and good reusability in the benzene hydroxylation reaction.Ga3+, Fe3+, Cu2+were doped into the polyoxometalate in order to further improve the catalytic activity. The characterization results show that the Keggin structure and crystalline phase of polyoxometalates retain. However, the redox ability improves with doping of metal cations. Catalytic results in the hydroxylation of benzene with H2O2as oxidant show that, doping with Ga3+, the phenol yield achieves26.3%on Gao.2Cs2H2.4PMo10V204o with TOF value of19.9h"1. The order of catalytic activity is Ga3+> Fe3+> Cu2+. Also, a series of reaction conditions and reusability of the catalyst were investigated, the used catalyst during the fifth recycle show high catalytic activity in the reaction. Besides H2O2, O2were used as oxidant in the hydroxylation of benzene. The ascorbic acid plays an important role in the reaction and the order of catalytic activity for the polyoxometalate doping with metal cations is Cu2+> Ga3+> Fe3+. Over the Cu0.2Cs2H2.6PMo10V2O40catalyst the phenol yield and phenol selectivity achieve17.3%and100%, respectively, with TOF value of3.6h-1under the optimized condition.Polyoxometalate can coordinate with transition metal cations and organic ligands to form polyoxometalate coordinated compounds with unique catalytic performance. The reaction conditions and kinetics over the [Co(H20)4Mo10O31(OH)2-2NH4-7H20]n were investigated in the epoxidation of styrene. Catalytic results show that styrene can be oxidized into benzaldehyde and styrene oxide by two different routes, and partial styrene oxide is converted to other by-product via further ring-open reactions. The solvent affects the distribution of products where the main product is benzaldehyde and styrene oxide with CH3CN and DMF as solvent, respectively. The highest styrene oxide selectivity is66.9%under the optimized condition. The catalytic activity and structure of the catalyst maintain after the third recycle.