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Study On The Direct Catalytic Hydroxylation Of Benzene To Phenol

Posted on:2006-01-20Degree:DoctorType:Dissertation
Country:ChinaCandidate:J ZhangFull Text:PDF
GTID:1101360155963713Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Traditionally, more attention is paid to the yield and selectivity than to the atomic efficiency. Thus, it is hard to avoid the embarrassed situation, that is, quite a few synthetic processes not only waste resource but also cause serious pollution. Since 20 century, people have cared more about the environment and resource. Especially, with the development of Green Chemistry, people concern more and more about how to improve the atomic efficiency, and how to use catalyst to replace the traditional reagent in chemical synthesis, in an attempt to eliminate environmental pollution and to save resource. The direct catalytic hydroxylation of benzene to phenol using benzene as the raw material, needs only one step from benzene to the final product. This accords with the view point of Green Chemistry. This reaction is directly connected to the activation of C-H bond on aromatic ring. This is one of the most important subjects that deserve detailed investigation. In addition, it is also one of the most difficult problems in synthetic chemistry. In order to realize this goal, the design and preparation of effective catalysts are fatally important. In this thesis, the direct catalytic hydroxylation of benzene to phenol was studied. Four kinds of catalysts were prepared and characterized. The catalytic activity was investigated and the active phase for the reaction was studied. The possible mechanism of catalytic reaction was also discussed and the reaction conditions were optimized. Sixteen kinds of heteropolyacids including heteropolymolybdates and vanadium-substituted heteropolymolybdates, heteropolytungstates and vanadium-substituted heteropolytungstates, arsenic-molybdenic and vanadium-substituted arsenic-molybdenic, arsenic-tungstic and vanadium-substituted arsenic-tungstic, silicomolybdenic and vanadium-substituted silicomolybdenic, silicotungstic and vanadium-substituted silicotungstic heteropolyacids were prepared using multi-step method. The products were characterized by elementary analysis, potential titration, thermal gravimetric analysis, infrared spectroscopy, UV-vis spectroscopy, X-ray powder diffraction, and NMR techniques. The results showed that the as-prepared compounds were HPAs with Keggin structure. The as prepared HPAs were used as catalysts for the direct hydroxylation of benzene in glacial acetic acid solvent with hydrogen peroxide as oxidant. The experiment showed that HPAs containing no vanadium species had no catalytic activity for the target reaction. In contrast, all the vanadium-containing HPAs had catalytic activity. This indicates that vanadium species in HPAs are active phase for the direct hydroxylation. Mo-containing HPAs have higher catalytic activity as compared with the W-containing HPAs (H4P(As, Si)Mo11VO40>H4P(As, Si)W11VO40). Keeping the coordinating atoms and substituted atoms the same, the catalytic activity of HPAs varies with different centeral atom, the following order is obtained: H4PMo11VO40>H4AsMo11VO40>H5SiMo11VO40;H4AsW11VO40>H4PW11VO40 ≈H5SiW11VO40. The catalytic activity and stability of mono-vanadium substituted and multi-vanadium substituted HPAs are different. The mono-vanadium substituted heteropolymolybdates(H4PMo11VO40) shows the highest turnover based on vanadium atom. In the present work, the turnover number of mono-vanadium substituted HPA catalyst was 50 mol phenol/mol catalyst with a yield of 22.1 % and a selectivity of 90.7 % to phenol. It is also showed that the mono-vanadium substituted heteropolymolybdates (H4PMo11VO40) was more stable than the multi-vanadium substitutedheteropolymolybdates (H4PMo12-nVnO40; n=2 or 3). The H4PMo11VO40 was supported onγ-Al2O3, NaX-zeolite, TiO2 and SiO2 carriers using impregnation method. It is showed that all the supported heteropolyacid catalysts are catalytically active in the hydroxylation of benzene to phenol. The catalytic activity is closely related to the final contents of supported HPA. The more HPAs are supported, the higher the catalytic activity. With the same amount of heteropolyacid supported, the catalytic activity varies with the surface area of the carrier, that is, the bigger the surface area of carrier, the higher the catalytic activity. By optimizing the experimental conditions, the supported heteropolyacid catalyst of HPA (8.7 %)/ NaX-Zeolite was found to possess the best catalytic activity with a yield of 9.5 % and a selectivity of 93.1 % to phenol. When mono-vanadium substituted heteropolymolybdates (H4PMo11VO40)   was used as the catalyst, glacial acetic acid as the solvent, and H2O2 as the oxidant, the kinetics research of the hydroxylation of benzene to phenol exhibited that the hydroxylation rate of benzene to phenol shows first-order dependence with respect to the substrate, the catalyst and the oxidant, respectively, with an activation energy of 57.73 kJ.mol-1. The macro-kinetics rate equation was estimated to be dc/dt=k [HPA] [H2O2 ] [C6H6]. Twenty two Ni (or Fe, Ca, Ce, Pt)-based catalysts were prepared by impregnation and co-impregnation method. The supports and catalysts were characterized by BET and XRD. The as-prepared catalysts were used directly to catalyze the conversion of benzene to phenol. These experiments showed that the catalytic activity of the supported catalysts varies with the main component supported on the same carrier. For alumina-supported ones (Ni6.1/?-Al2O3, Fe6.2/?-Al2O3, Ca6.0/?-Al2O3, Ce6.0/?-Al2O3 and Pt1.1/?-Al2O3), the Ni-based catalyst (Ni6.1/?-Al2O3) exhibites the highest catalytic activity with a yield of 11.8 % and a selectivity of 94.0 % to phenol, while the catalytic activity of Pt-based catalysts (Pt1.1/?-Al2O3) is the lowest with a yield of only 0.5 % and a selectivity of 85.7 % to phenol. The dispersion of nickel species is different ondifferent carriers. Among the four supports (?-Al2O3, NaX-Zeolite, TiO2 and SiO2) used, nickel species are well dispersed on ?-Al2O3, NaX-Zeolite, and TiO2, while the dispersion on SiO2 support is the lowest. Ni-based catalysts exhibited different catalytic activity when different supports were used to prepare the catalyst, and Ni/?-Al2O3 catalyst showed the highest catalytic activity. The catalytic activity of supported Ni-based catalysts varies with the amount of nickel supported. With increasing amount of nickel species, the catalytic activity increases firstly, reaches the highest phenol yield of 11.8 % with a nickel content of 6.1 %, decreases with further increase of nickel content. To double-component Ni-based catalysts containing Ca or Ce (Ca0.5Ni6.1/?-Al2O3, Ce0.6Ni6.0/?-Al2O3), Ca and Ce have little effect on the dispersion of nickel species. To double-component Ni-based catalysts containing Fe or Pt (Fe0.6Ni5.9/?-Al2O3,Pt0.6Ni6.1/?-Al2O3), Fe and Pt has significant effect on the dispersion of nickel species. The activity measurement showed that the catalytic activity of all the additive-contained double-component Ni-based catalysts is lower than that of Ni/?-Al2O3 catalysts. For Ca-contained triple-component Ni-based catalysts(Ca0.6Ce0.5Ni6.2/?-Al2O3, Ca0.6Pt0.5Ni6.1/?-Al2O3 and Ca0.5Fe0.6Ni6.1 /?-Al2O3), the additive has little effect on the dispersion of nickel species. In triple-component Ni-based catalysts without Ca species (Ce0.6Pt0.6Ni6.2/?-Al2O3, Ce0.5Fe0.5Ni6.1/?-Al2O3 and Fe0.6Pt0.5Ni6.0/?-Al2O3), the additive has significant effect on the dispersion of nickel. The activity measurement showed that the catalytic activity of all the additive contained triple-component Ni-based catalysts is not only lower than that of mono-component Ni-based catalysts, but also lower than that of the double-component Ni-based catalysts. Five activated carbon samples were characterized by using EBT, Boehm titration and XPS. These activated carbons are used as catalyst for the direct hydroxylation of benzene to phenol. The experiment showed that the amount of oxygen containing groups on the surface of the five activated carbons is different. Among them, the amount of oxygen containing groups in date nucleonactivated carbon is the biggest. After treating the five activated carbons with dilute nitric acid, the amount of carbonyl group, carboxylic group and lactonic group on their surface increased, while the amount of phenolic group decreased. The amount of oxygen containing groups on the surface of activated carbon is found dependent on the concentration of nitric acid used for its treatment. After dealing with 2 M nitric acid, the amount of carbonyl group on the surface of activated carbon is the biggest, while the catalytic activity is also the highest with a phenol yield of 14.3 % and a selectivity of 92.6 %, respectively. When dealing with activated carbon with 2 M hydrochloric acid, there is little effect on the amount of oxygen containing groups on activated carbon surface, the catalytic activity changes also little. When dealing with activated carbon with 2 M sulfuric acid, the amount of carbonyl group on activated carbon surface increased and the catalytic reactivity increased slightly. In the direct hydroxylation reaction of benzene to phenol, the order of catalytic activity among the five activated carbons is: date nucleon>kichory nut nucleon>coconut shell>wood based carbon>coal based carbon. The experiment also showed that carbonyl group species on activated carbon surface is the active phase, while the catalytic activity of activated carbon increases with the amount of the surface carbonyl group. The mechanism of benzene direct catalytic hydroxylation to phenol by using activated carbons catalyst was also speculated.
Keywords/Search Tags:Benzene, Hydroxylation, Phenol, Yield of phenol, Selectivity of phenol, Heteropolyacid, Support, Supported heteropolyacid, Ni-based catalyst, Activated carbon, Active phase, hydrogen peroxide, Glacial acetic acid, Carbonyl group.
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