Elsevier

Molecular Catalysis

Volume 498, December 2020, 111244
Molecular Catalysis

Highly efficient transformation of ethylbenzene into acetophenone catalyzed by NHPI/Co(II) using molecular oxygen in hexafluoropropan-2-ol

https://doi.org/10.1016/j.mcat.2020.111244Get rights and content

Highlights

  • Ethylbenzene was efficiently converted into acetophenone catalyzed by NHPI/Co(II).

  • Hexafuoropropan-2-ol significantly promoted the formation of acetophenone from ethylbenzene.

  • PINO and PhCHCH3 radicals were detected by in situ EPR and HRMS, respectively.

Abstract

Acetophenone is an important industrial intermediate and generally produced by the Friedel-Crafts acylation reaction, suffering from a low reactivity and serious equipment corrosion. Direct oxidation of ethylbenzene to acetophenone by molecular oxygen will be benign and cost-effective. The catalytic performance of NHPI/Co(II) herein was investigated by selective oxidation of ethylbenzene to acetophenone in different solvents at room temperature. The solvent hexafluoropropan-2-ol (HFIP) was found to markedly enhance the transformation efficiency from ethylbenzene to acetophenone in comparison with acetic acid, pyridine and ethanol, and the ethylbenzene conversion and the selectivity to acetophenone was high up to 87.9 % and 61.2 %, respectively. A higher concentration of phthalimide-N-oxyl (PINO) radicals was observed by an in situ electron paramagnetic resonance spectrometer (EPR) in HFIP with respect to other solvents, suggesting that HFIP may facilitate the generation of the N-oxyl radical and thus promote the selective oxidation of ethylbenzene to acetophenone. Furthermore, the benzylic carbon radical (PhCHCH3) from ethylbenzene was trapped by tetramethylpiperidine N-oxyl radical (TEMPO) and observed by a high resolution mass spectrometer. The findings of both PINO and PhCHCH3 under reaction conditions indicated that the selective oxidation of ethylbenzene to acetophenone catalyzed by NHPI/Co(II) should proceed via a radical mode. The selective oxidation of ethylbenzene to acetophenone using molecular oxygen by NHPI/Co(II) in HFIP exhibited an important industrial application prospect.

Graphical abstract

Ethylbenzene was efficiently transformed into acetophenone in the presence of 1,1,1,3,3,3-hexafluoropropan-2-ol at room temperature via phthalimide-N-oxyl and PhCHCH3 radicals.

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Introduction

The activation and selective oxidation of CHsingle bond bonds are of great importance in industrial transformation of inexpensive hydrocarbons into oxygenated compounds with a higher value. The selective oxidation of ethylbenzene to acetophenone is a typical reaction for selective oxidation reaction of benzyl CHsingle bond bonds [1,2]. As an important chemical intermediate, acetophenone is widely used in industrial production fields, such as perfumes [3], soaps [4], resins [5] and drugs [6]. Industrially, acetophenone is synthesized by Friedel-Crafts acylation reaction of benzene. However, the process suffers from a low reactivity, serious equipment corrosion and a high cost of wastewater treatment [7]. Therefore, a cost-efficient and benign technology for selective oxidation of ethylbenzene to acetophenone will be crucial.

Different strategies were developed to convert ethylbenzene into acetophenone via vapor phase or liquid phase oxidation in the past years. There are no solvents needed in vapor phase oxidation, in which the air was used as the oxidant directly. And the reactions via vapor oxidation can be carried out using a fixed-bed flowing reactor without fear of the separating and recovering of catalysts. However, the high temperature reaction in vapor phase oxidation often leads to the production a lot of products, such as styrene, benzaldehyde, benzoic acid, CO and CO2 [8,9,10]. Compared to the vapor phase oxidation, the liquid phase oxidation of ethylbenzene to acetophenone can be performed in milder reaction conditions, and exhibited a higher ethylbenzene conversion and a lower selectivity to overoxidized products like CO2, showing a promising industrial application prospect [11,12]. Heterogeneously catalytic transformation of ethylbenzene into acetophenone facilitates the separation of the catalyst, but the conversion of ethylbenzene and the selectivity to acetophenone are generally low [13,14]. The homogeneous catalysts can be molecularly dispersed in solvent, permitting a good contact with ethylbenzene molecules and thus exhibiting higher reactivity and selectivity [15].

Molecular oxygen is an environmentally friendly and cheap oxidant, and widely used in catalytic reactions in the oxidation of amines, alcohols and alkane [16,17,18]. However, it is necessary to activate the triplet state of molecular oxygen and/or ground CHsingle bond bond to realize the oxygen functionalization of hydrocarbon due to the spin − flip restriction between them [11]. Ishii’s group [19] developed a NHPI/Co(II) catalytic system to oxidize toluene and other substrates into value-added products in liquid phase under mild conditions for the first time, indicating the aerobic oxidation of CHsingle bond bonds under mild conditions was possible by NHPI/Co(II) catalysts.

N-hydroxyphthalimide (NHPI) was employed to catalyze the activation and functionalization of CHsingle bond bonds in many reaction systems. Wang et al. [20] achieved a selective aerobic oxidation of cyclohexane to ε-caprolactone under mild conditions in the presence of NHPI and aldehyde. Carboxylic functionalized β-carbolines were successfully synthesized by aerobic oxidation in the presence of NHPI and transition metal salts using molecular oxygen at room temperature [21]. Li et al. [22] successfully achieved the chlorination of benzylic CHsingle bond bond of toluene by NHPI and 2,3-dichloro-5,6-dicyano-benzoquinone (DDQ).

NHPI is also used in liquid phase oxidation of ethylbenzene to acetophenone [11,12,23,24]. Miao et al. [11] successfully achieved highly selective oxidation of ethylbenzene to acetophenone (yield 70 %) in the presence of Fe(NO3)3 and NHPI. However, the reaction was completed in a longer period of time, probably due to the relatively low reactivity of the catalyst. Zhang et al. [12] realized controllable activation of CHsingle bond bond in the presence of α-Fe2O3 and NHPI, the conversion of ethylbenzene and the selectivity to acetophenone in 4 h were 55 % and 96 %, respectively. In addition, di-dodecyl-dimethyl ammonium bromide (DDDAB) [23] and ion liquids [24] were found significant promoting role on solvent-free oxidation of ethylbenzene to acetophenone catalyzed by NHPI/Co(II). All of the above researches implied that NHPI is active for oxidation of ethylbenzene to acetophenone in liquid phase, but the efficiency of producing acetophenone currently is relatively low. Therefore, an enhanced efficiency of oxidation of ethylbenzene to acetophenone in liquid phase is raised here to meet increasing needs of industrial application.

1,1,1,3,3,3-Hexafluoropropan-2-ol (HFIP) is a non-nucleophilic polar solvent with weak acidity, high dielectric constants and ionization power [25]. In addition, HFIP is a strong hydrogen-bond donor that pairs with hydrogen-bond acceptor groups, thereby can interfere with the catalytic reactions cycle, promote the kinetics of polar reactions and significantly increase the substrate conversion and selectivity to the desired product [26,27,28]. Pappo et al. [29] developed a simple and efficient method for selective oxidation of toluene to benzaldehyde using HFIP as the solvent. Based on Ishii and his co-author's picture [19], Pappo’s group increased significantly the yield of benzaldehyde (> 90 %).

In the present work, we employed HFIP as solvent in liquid phase oxidation of ethylbenzene in the presence of NHPI and Co(II) in molecular oxygen at room temperature. The concentrations of PINO radicals in different solvents were investigated by electron paramagnetic resonance spectrometer (EPR). Further, the benzylic carbon radical was trapped by tetramethylpiperidine N-oxyl (TEMPO) radicals and detected by high resolution mass spectrometry (HRMS). Accordingly, the possible mechanism of ethylbenzene oxidation was proposed.

Section snippets

Chemicals

Ethylbenzene, acetic acid, absolute ethanol, cobalt acetate tetrahydrate and N-hydroxyphthalimide were purchased from China Pharmaceutical Group Chemical Reagents Co., Ltd. 1,1,1,3,3,3-Hexafluoropropan-2-ol was purchased from Aladdin-Reagent. All reagents were not purified before use.

Catalytic reactions and product analysis

Liquid phase oxidation of ethylbenzene was performed in a Schlenk tube. Ethylbenzene (2 mmol), Co(OAc)24H2O (0.01 mmol), NHPI (0.05 mmol) and solvent (1 mL) were added to the Schlenk tube. Nitrobenzene of 0.5 mmol

Results and discussion

Table 1 compares the effects of different solvents on the liquid phase oxidation of ethylbenzene. There were no products from ethylbenzene oxidation detected when ethanol was used as the solvent (entry 1), and very low ethylbenzene conversion was observed in pyridine (entry 2). Under the same reaction conditions, an ethylbenzene conversion of 21.2 %, a selectivity to acetophenone of 73.5 %, a selectivity to 1-phenylethanol of 22.7 % and a selectivity to benzaldehyde of 2.4 % were observed when

Conclusions

In summary, the solvent HFIP markedly improved the transformation of ethylbenzene into acetophenone in liquid phase using molecular oxygen. Under the experimental conditions, the conversion of ethylbenzene was as high as 87.8 % in 4 h, and the selectivity to acetophenone was up to 61.2 %. The analysis via EPR indicated that the solvent HFIP is beneficial to the rapid generation of PINO radical. The PINO radical may be the actual catalyst for the reaction, which activates the ethylbenzene

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Sihao Xu: Conceptualization, Methodology, Funding acquisition, Writing - review & editing, Conceptualization, Methodology, Funding acquisition, Writing - review & editing. Guojun Shi: Investigation, Writing - original draft. Ya Feng: Investigation. Chong Chen: Resources, Investigation. Lijun Ji: Methodology.

Acknowledgements

The authors gratefully acknowledge the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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