Study On The One-step Synthesis Of Aniline By The Direct Catalytic Oxy-amination Of Benzene

For a long time, more attentions have been paid on the high selectivity and high yield of chemical reactions than atomic efficiency of the reactants. Thus the wastes of resource and environment pollution were simultaneously encountered. With the development of modern industry, especially the development of green chemistry, people care more about the environment and the energy consumption, and tend to apply the basic ideas of green chemistry to every fields of chemistry and chemical industry. Therefore, increasing attentions is paid to better atomic efficiency, and the"greening"trends of global chemical manufacturing requires new processes that economize resource and reduce or even eliminate environmental pollutions. The selective activation and directional functionalization of C–H bonds, especially those of benzene and other aromatics under mild conditions, remains one of the most significant challenges in both synthetic and green chemistry. The introduction of amino-group into benzene ring to produce aniline is involved in this research. By using suitable aminating reagent and oxidant, the direct amination of benzene to aniline under certain conditions is a green synthetic process because it reduces the formation of intermediate products and by-products, and increases the atomic efficiency of the reactants. The exploitation of new method for the one-step synthesis of aniline will lead the development of aniline industry to a simple, clean, and green energy-conservative direction. The present thesis deals with the study of the direct oxy-amination of benzene to aniline with aqueous ammonia using hydrogen peroxide as the oxidant under mild reaction conditions (low temperature and atmospheric pressure). Because of the peculiar stability of benzene ring, the selection and preparation of the catalyst are the key to the co-activation of C–H bond of benzene, N–H bond of ammonia, and O–O bond of H2O2, or one or two of them under mild conditions to initiate the reaction. Based on the previous research results, Ni, Mo and/or Mn were used as the active components and a series of supported catalysts (i.e. Ni/Al2O3, Mo/Al2O3, Mn/Al2O3, Mo-Ni/Al2O3, and Mn-Ni/Al2O3) were prepared and used in the direct oxy-amination of benzene to aniline under mild reaction conditions.It was found that the one-component catalysts (Ni/Al2O3, Mo/Al2O3, and Mn/Al2O3) are active with lower selectivity to aniline. Nickel species on Ni/Al2O3 are active for the co-activation of C–H bond of benzene and N-H bond of ammonia, and the activation of N–H bond of ammonia is enhanced by the reduced forms of nickel species. Both of MoO3 and the reduced forms of Mo species are active for the activation of C–H bond and N–H bond of the reactants and the reduced forms of Mo species on Mo/Al2O3 makes a more remarkable increase of C–H bond activation than N–H bond activation. The reduced forms of Mn species are responsible for the amination, but its oxides are inactive. The addition of Mo or Mn into Ni/Al2O3 results in an increase of both the yield and selectivity to aniline comparing to the one-component catalysts. Different Mo/Ni or Mn/Ni atomic ratios of the catalysts result in different activities and selectivities to aniline. For the Mn-Ni/Al2O3 catalysts, the highest aniline yield was obtained on the catalyst with a Mn/Ni atomic ratio of 0.16. The reduced Mo-Ni/Al2O3 catalyst with a Mo/Ni atomic ratio of 1.7 showed the best catalytic amination activity under the reaction conditions investigated.The catalytic performance of the catalyst is mainly determined by its structure, thus X-ray diffraction (XRD) and temperature-programmed reduction (TPR) were employed to characterize the catalysts prepared. The results showed that, the reduced forms of supported Mo species on Mo-Ni/Al2O3 makes a more remarkable increase of C–H bond activation than N–H bond activation, and the reduction of nickel species favors the activation of N–H bond of ammonia. The formation of NiMoO4 on Mo-Ni/Al2O3 decreases the amination activity. The interaction of Mn and Ni species or the interaction between the metals and the support leads to the co-activation of the C–H and N–H bonds of the reactants. The oxyamination was proved to be a heterogeneously catalyzed reaction. Phenol is the main by-product in the experiment. The amination activity and selectivity of the catalysts can be improved by properly optimizing the compositions of the catalysts and corresponding preparation method.The synthesis of aniline from benzene and hydroxylamine hydrochloride in acetic acid–water media under mild conditions was also studied. Using NaVO3 as the catalyst, its catalytic performance in the amination was investigated in detail. The influence of the acidity of the reaction media, the presence of air (oxygen), the amount of NaVO3 used, the feed ratio (nNH2OH/nbenzene), the reaction temperature, the reaction time, the feed order, and the presence of Na+ cation on the amination were investigated. It was found that, a relatively acidic reaction medium, especially an organic acid such as acetic acid-water medium, is advantageous for the present homogenously catalyzed amination. Acetic acid, used in the present study, actes not only as a good solvent for mixing the reactants into one phase, it also supplies an acidic surrounding, perhaps even coordinates with the vanadium center and affects the mechanism of the titled amination reaction. It was proved that the amination reaction takes place more efficiently in open air than in a closed system. The presence of air (oxygen) is favorable for the enhancement of the aniline yield and selectivity. Without the introduction of other anions, the Na+ cation in the catalyst favors the amination. The optimized reaction conditions investigated in the present work are: n benzene/n NH2OH 1:1, HOAc: H2O (v/v) 4:1, conducted at 353 K for 4 h under atmospheric pressure. Satisfactory aniline yield and turnover (64 mol%, 48 mol aniline per mol V), with a selectivity of 95.6% to aniline, were obtained under the optimized reaction conditions.The decomposition of hydroxylamine hydrochloride under the reaction conditions was studied to understand its function during the amination process. The effects of temperatures, the concentrations of acetic acid, the amount of NaVO3 used, the valences of vanadium, etc., on the decomposition of hydroxylamine hydrochloride were investigated in detail. It was found that an appropriate decomposition rate of hydroxylamine favors the amination, probably because the decomposition process is involved in the formation of active aminating agent.Several spectroscopic techiniques, including 51V NMR, EPR, and UV-VIS, were used to monitor the variation of the valence of vanadium in the amination process. A computational study on the UV-Vis spectra of some typical vanadium complexes was carried out using the time dependent DFT (TD-DFT) method to help explain the electronic spectra. A free radical mechanism is proposed based on both the results of spectroscopic and activity measurements as well as quantum chemical calculation. The proposed mechanism involves the interaction of a VO2+ cation with hydroxylamine to form the lower-valent vanadium species, mainly VIV. Gaseous N2O is evolved during this redox course. The probably formed VIII species is unstable in open air and is oxidized to the relatively stable VIV. The VIV species, however, would also be probably oxidized to VV in the presence of hydroxylamine, acetic acid, and atmospheric oxygen. On account of the relatively stronger reductive power of lower-valent vanadium species, the hydroxylamine present in the system then acts as oxidative agent and is reduced to a protonated amino-vanadium complex (HO-VV-?NH3+), then attacks the benzene ring to give a protonated aminocyclohexadienyl radical intermediate, which is subsequently oxidized by VV species to form protonated aniline accompanying the regeneration of lower-valent vanadium species, completing the catalytic cycle. The exhaustion of hydroxylamine stops the formation of HO-VV-?NH3+ which further results in the termination of the amination process. In addition, it is clear that the presence of the VV species is essential for the amination process, because they are not only involved in the production of lower-valent vanadium species, but also in the re-aromatization of the aminocyclohexadienyl radical intermediate to form aniline. The lower-valent vanadium species are further involved in the formation of the active protonated amino-vanadium radical species. Atmospheric air favors the co-existence of VV and VIV, which is crucial for the amination progress.Using vanadium as the main active component, various supported vanadium-based catalysts were designed and prepared. The structures of these catalysts were characterized by means of X-ray diffraction (XRD), X-photoelectron spectroscopy (XPS), and Diffuse reflectance UV-VIS (DRUV-VIS) spectroscopy. The as-prepared catalysts were used in the direct amination of benzene to aniline with hydroxylamine hydrochloride. The amination conditions were optimized. It was found that the activities of these catalysts with different supports are different. The activity of the supported catalysts with identical preparation conditions and reaction conditions are as follows:γ–Al2O3 > TiO2 > SiO2 > activated carbon > HASM-5 > CeO2. The actual vanadium loadings on the supports are related with the characters of the supports. The V/γ–Al2O3, with the highest vanadium loading, shows the best catalytic performance in the amination reaction. Using V/γ–Al2O3 as the catalyst, different vanadium loadings on the supports results in different catalytic activities, that is, the more of the vanadium loaded, the higher of the activity. The vanadium species are well dispersed onγ–Al2O3 when the vanadium content on the support is lower than 5%, and deposites on the support to form a small amount of V2O5 and VO2 when the vanadium content is higher than 5%. The variation of the impregnating solution has no obvious effect on the structure and performance of theγ–Al2O3 supported catalyst. The increase of the calcination temperature (above 700 oC) transforms theγ–Al2O3 phase toθ–Al2O3, and favors the formation of crystalline vanadium species on the support, and further results in the increment of the catalytic performance. The activity of V/γ–Al2O3 decreases to half after five consequent runs, this is attributed to the loss of active metals and the dissolution of the solid catalyst in the solution. In addition, an acidic reaction medium is favorable for the amination, but the addition of strong acid results in the dissolution of the catalyst and the decrease of the aniline yield. The amination takes place more efficiently in open air than in closed system. The presence of air (oxygen) favors the amination. Over the V/γ–Al2O3 catalyst, an aniline yield of 64%, with a selectivity of 95.8%, was obtained when using 2 mL of benzene in acetic acid-water solution (VHOAc: VH2O = 4:1) at 80 oC for 4 h. This value is comparable with that obtained in homogeneous NaVO3 catalyzed amination system. It was proved that the V/γ–Al2O3 catalyzed amination is mainly a heterogeneously catalyzed reaction, and the leaching vanadium ions are also responsible for the amination, although this contribution is not as high as the heterogeneous catalysis.The amination of other aromatics (i.e. ethylbenzene, dimethyl benzene, chlorobenzene, and nitrobenzene) was carried out over V/γ–Al2O3. It was found that the C–H bond in benzene ring could be activated but this activation is influenced by the nature of the substituted groups.