Study On The Direct Catalytic Hydroxylation Of Several Typtical Aromatics Over Fe/Activated Carbon Catalyst

Phenols are one kind of fundamental compounds, and widely used as intermediates for the synthesis of drugs, polymers, pesticides, etc. However, most of phenols are currently manufactured via multi-step processes. In addition, some processes for phenol production are resource wasteful and environmentally pollutive. The directly selective oxidation of aromatics to phenols over catalyst is atom economically efficient and in good agreement with the view point of green chemistry, thus has been payed an increasing interest. Further more, the selective oxidation of aromatics to phenols is also one of the important subjects that deserve detailed investigation because of the following reasons. Firstly, it involves the activation of C-H bond on aromatic ring, which is much difficult among activation of C-H bond on all the organic compound. Secondly, the regio-selectivity of hydroxylation on substituted aromatics is not easy to be controlled, for there are three sites (o-,m-,p-) could be hydroxylated on ring of substituted aromatics. Thirdly, in the oxidation of aromatic hydrocarbons, the higher bond energy of aromatic C–H bonds (435 kJ mol?1 for benzene) compared to aliphatic ones (typically, 380–415 kJ mol?1) made the hydroxylation of aromatic ring even more difficult than the side chain substituted groups. In the present work, the direct catalytic hydroxylation of aromatic ring was studied. The interaction between the surface oxygen groups on activated carbon and the supported iron species was investigated. The hydroxylation of a series of typical aromatics was also investigated. The possible mechanism for the hydroxylation of aromatics was discussed and the reaction conditions for the hydroxylation of benzene, toluene and acetophone were optimized.A series of Fe-based catalysts was prepared by impregnating activated carbon in the aqueous solution of ferric sulfate. These catalysts were employed to the hydroxylation of benzene to phenol using hydrogen peroxide as the oxidant. The Fe/Activated Carbon catalyst were characterized by BET, adsorption of benzene, XRD and XPS. It was found that ferric species were mainly anchored on activated carbon via its interaction with surface carboxylic oxygen group. These supported ferric species was found catalytically active for the hydroxylation of benzene to phenol. The interaction between the surface carboxylic oxygen group and iron species was responsible for the synergistic effect of activated carbon for the hydroxylation of benzene to phenol.The adsorption capacity of benzene on activated carbons (ACs) born different textures and oxygen species was also studied in acetonitrile or acetic acid media in the presence or absence of water. The AC samples were pretreated by nitric acid or air to change the distribution of surface oxygen groups. It was found that the amount of benzene adsorbed on the pretreated samples was dependent on the distribution of surface oxygen groups and the media employed. Acetonitrile solvent favored benzene adsorption, in comparison to acetic acid solvent in both water and water–free solutions. The adsorption capacity of benzene decreased with the increase of the amount of carboxylic groups on ACs in both acetonitrile and acetic acid media.The Fe/Activated Carbon catalyst was further used for the hydroxylation of several typical substituted aromatics. The results showed that the Fe/Activated Carbon catalyst was effective for the hydroxylation of aromatics and the ring oxidation was predominant for all the substrates studied. Only trace amount of side chain oxidation was observed except xylene. The selectivity to ring oxidation was much greater and the reaction conditions were much milder than those reported previously. A comparison of the conversions revealed that electron-donating substituents (-CH3, -CH2CH3, -OCH3, -O-Ph ) increased the conversion of the substrates, while electron-withdrawing substituents (-Cl, -NO2, -CO-Ph, -COOH, -COCH3) decreased the conversion. The incease of yields from benzene to Ph-O-Ph could be attributed to the electron-donating ability. For electron-donating group substituted aromatics, except anisole, ortho- and para-substituent products were generally detected with almost equivalent selectivties, indicating an electrophilic character of the reaction. The faint difference in selectivity could be ascribed to steric hindrance. Thus it could be considered that the reaction goes through iron-oxo complexes mechanism (electrophilc mechanism). It is found that when electron-withdrawing group substituted aromatics were used as the substrates in this catalytic system, only a small amount of or even no meta- hydroxylated products were obtained while ortho- hydroxylated products were predominant with a small amount of para- hydroxylated products. For benzoic acid and anisole, only ortho- hydroxylated products were obtained in spite of the electron-donating or electron- withdrawing ability of the substituents. These data could not be explained by the reported electrophilic mechanism. An examination of the structure of these substrates indicated that the substituent contained at least one atom (O or Cl ) capable of coordinating to the FeIII. Thus a new mechanism involving the coordination of the substituent to the active site of the catalyst was tentatively proposed. The formation of 5 or 6 number ring with the catalyst allowed only the ortho- position to occur the hydroxylation. That is to say, the reaction bore also electrophilic character, which explained the decrease of substrate conversion. Thus the hydroxylation of substituted aromatics might be dominated by the competition between the two mechanisms mentioned above, while the electron-donating or electron-withdrawing character and the steric hindrance of the substituents played important roles for the activity and selectivity.The selective partial oxidation of toluene to cresols was also studied using Fe/Activated Carbon catalyst and the reaction conditions were optimized. BET and X-ray photoelectron spectroscopies (XPS) methods were used to characterize the catalysts. A toluene conversion of 28.1 % and a yield of 22.7 % with a selectivity of 80.8 % to cresols were obtained under optimized conditions: 303K, atmospheric pressure in acetonitrile medium. It was found that the pretreatment of activated carbon by nitric acid was favorable to the catalytic performance of the finished catalysts, and this effect was ascribed to the increase of surface carboxylic oxygen group on activated carbon with which ferric species might be anchored properly to provide active phases for the hydroxylation of toluene.Similar optimized conditions were obtained for the hydroxylation of acetophone and benzene. For the hydroxylation of acetophone, the yield of o-hydroxyl-acetophone was 7.7% with a acetophone conversion of 10.7% and a selectivity of 72% to o-hydroxyl-acetophone. For the hydroxylation of benzene, a benzene conversion of 19.6 %, a phenol yield of 17.5% with a selectivity of 89.3% was obtained under optimized conditions: 303K, atmospheric pressure, and using acetonitrile as the solvent.