Selective ring Csingle bondH bonds activation of toluene over Fe/activated carbon catalyst

https://doi.org/10.1016/j.molcata.2013.05.005Get rights and content

Highlights

  • Iron interacted with carboxylic and lactonic groups forming single bondCOOsingle bond(Fe) like species.

  • Activated carbon activated benzene ring resulting in ring Csingle bondH bonds activation.

  • The ortho-selectivity might be due to the orientating role of activated carbon.

  • Fe/activated carbon catalysts were stable enough for the hydroxylation of toluene.

Abstract

Direct hydroxylation of toluene and related aromatics over Fe/activated carbon catalyst using hydrogen peroxide as oxidant in acetonitrile was studied. The catalysts were characterized by ICP-AES, N2 adsorption, FTIR, XPS and ion exchange experiment. It was found that iron was anchored on the surface of activated carbon forming iron carboxylate like species acting as active phase for toluene hydroxylation. 13.4% yield (TOF, 34.4 × 10−2 h−1) and 74.3% selectivity to cresols were obtained under the optimal conditions, and o-cresol was dominant with 56% selectivity. The interaction between benzene ring and activated carbon might be responsible for the selective ring Csingle bondH bonds activation. The ortho-selectivity might be due to the synergetic effect by activated carbon and high-valent iron-oxo species. The structure of the key intermediate for the titled reaction was proposed and confirmed by theoretical modeling using Gaussian 09 program package.

Graphical abstract

Interaction between the benzene ring and activated carbon was responsible for the selective ring Csingle bondH bonds activation. The carboxylic and lactonic groups on activated carbon played the role of ortho-positioning.

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Introduction

For alkyl substituted benzene, side-chain Csingle bondH bonds usually take the priority over those on the aromatic ring to be activated with the formation of benzaldehyde and benzoic acid [1], [2]. The selective activation of aromatic ring Csingle bondH bonds is presumed to occur where electrophilic catalytic species is generated in the oxidation of aromatic hydrocarbons [3], [4], [5], [6]. The hydroxylation of aromatics is of considerable importance, for it involves the control of selectivity for the oxidation of aromatic ring Csingle bondH bonds at the correct position.

Cresols are important intermediates for fine chemicals, which are widely employed for preparing phenolic resins, antioxidants, insecticides, agrochemicals, pharmaceuticals, herbicides, and dyes [7], [8]. Generally, worldwide industrial production of cresols are done from toluene by multistep reaction processes such as sulfonated alkali fusion process [8], chlorinated hydrolysis process [8], cymene process [9], or by vapor phase methylation of phenol [10], [11], [12], [13], and the obtained isomers are isolated by traditional distillation or crystallization [7], [14]. Nevertheless, these processes bear some drawbacks like low cresols yield, low atom utilization with the production of byproducts, and apparatus corrosion. The vapor phase methylation of phenol requires expensive original material phenol with the high cost for separating cresols from the mixtures [7].

With enhanced atom efficiency, the one step hydroxylation of toluene to cresols under mild condition has attracted worldwide attention for it meets the green and sustainable chemistry demands, and it is one of the most challenging issues in catalysis research at present [7]. The search of heterogeneous catalysts is important, for the drawbacks involving low selectivity to target products and hard separation of products from the reaction mixtures containing catalysts in homogeneous processes could be overcome. The catalysts investigated in the oxidation of toluene to cresols are mostly transition metals supported on porous materials, such as TS-1 [15], [16], vanadium based catalysts [7], Fe3+–Al2O3 [17], Fe-ZSM-5 [4], [18], H[Al]-ZSM-5 [19], and so on. However, because of their high cost, multi-step preparation, relatively low catalytic activity, low selectivity or low reusability, they are used restrictedly in industrial applications. Fe species have been studied by many researchers due to their low cost. Compared to other supports like silica, alumina or magnesia, activated carbon has some advantages as support. The source of activated carbon is wide and cheap. The large surface area and high porosity of activated carbon contribute to the high dispersion of active phases. The nature and amount of surface oxygen groups can be controlled, and they can act as anchorage sites that interact with metallic phase improving its dispersion. Pilot study on the interaction between iron and the surface oxygen groups revealed the formation of iron carboxylate like species [20]. Previously, we studied primarily the selective oxidation of aromatics, and found the predominant ring oxidation and predominant selectivity to ortho-hydroxylated products, where a schematic coordination mode between the substrate and the iron species was proposed [21].

In the present work, Fe/activated carbon catalyst was studied in more detail in the hydroxylation of aromatics with emphasis on the selective activation of aromatic ring Csingle bondH bonds and the prior ortho-regioselectivity.

Section snippets

Catalyst preparation

Commercially available coal-based activated carbon from Jiangsu Nantong Activated Carbon Cooperation (China) with a particle size of 40–80 mesh was washed by distilled water and then dried at 383 K. The obtained sample (100 g) was pretreated with 250 ml 12 M nitric acid solution at 363 K for 10 h in a 500 ml round bottom flask, and then washed by boiled distilled water to reach a constant pH of about 7 and finally dried at 383 K for 24 h. The resultant sample was denoted as ACN. The Fe/activated carbon

Nitrogen adsorption/desorption

As shown in Table 1, the iron content on the catalyst increased from 0.03 to 0.33 mmol/g with the concentration of Fe(NO3)3 increased from 0 to 0.698 mol/L, and then decreased to 0.30 mmol/g when the concentration of Fe(NO3)3 increased to 0.825 mol/L. It might be due to the limited surface oxygen groups on ACN which were the anchorage sites for iron. The N2 adsorption isotherms shown in Fig. 1 were corresponded to typical type I in the BDDT classification [27], which indicated that all the samples

Conclusions

Fe/activated carbon catalysts showed good catalytic performance for the direct hydroxylation of toluene to cresols by hydrogen peroxide under moderate conditions. Fe/activated carbon catalysts were reusable without any appreciable loss in activity and selectivity for 200 h reaction time in the continuous flow reaction. The ferric species anchored at carboxylic groups and lactonic groups on activated carbon with the formation of iron carboxylate like species. High-valent iron-oxo complex was

Acknowledgements

The financial support from the National Natural Science Foundation of China (Nos. 20872102, 21021001 and 20502017), and characterization of the catalyst from Analytic and Testing Center of Sichuan University are greatly appreciated.

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