A Fecl3-Based Ionic Liquid for the Oxidation of Anthracene To

Fuel Processing Technology 135 (2015) 157–161

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Fuel Processing Technology

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AFeCl3-based ionic liquid for the oxidation of anthracene to anthraquinone

Yu-Gao Wang, Xian-Yong Wei ⁎, Sheng-Kang Wang, Rui-Lun Xie, Peng Li, Fang-Jing Liu, Zhi-Min Zong

Key Laboratory of Coal Processing and Efﬁcient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China article info abstract

Article history: N-butylpyridinium bromide ferric trichloride (NBPBFTC), a FeCl3-based ionic liquid, was prepared by a two-step Received 28 June 2014 method. NBPBFTC signiﬁcantly catalyzed the oxidation of anthracene to anthraquinone (AQ) using aqueous hy- Received in revised form 27 November 2014 drogen peroxide (AHPO) as the oxidant. The optimal conditions were determined to be 50 °C, 45 min, 100 mg Accepted 14 December 2014 NBPBFTC, and 1 mL AHPO for the oxidation of 50 mg anthracene. Under the conditions, AQ was obtained in a Available online 5 January 2015 yield of 99.5%. NBPBFTC can be reused at least 3 times without substantial decrease in activity. With the intense π–π Keywords: interaction between anthracene and pyridine-based cation in NBPBFTC, anthracene is soluble in the reaction system. Meanwhile, the iron-based anion and AHPO form a Fenton-like reagent to produce HOO· and HO·, which FeCl3-based ionic liquid Oxidation attack 9-position in anthracene to induce anthracene oxidation to AQ. Anthraquinone © 2014 Elsevier B.V. All rights reserved. Hydrogen peroxide

1. Introduction In the present investigation, N-butylpyridinium bromide ferric

trichloride (NBPBFTC) was prepared as a FeCl3-based IL and used in Anthraquinone (AQ) is one of important fine chemicals and chem- the oxidation of anthracene to AQ using AHPO as the oxidant. ical intermediates. It is often used to produce the anthraquinone dye [1] and hydrogen peroxide [2] and to enhance the Kraft process for 2. Experimental paper production [3]. Anthracene, as a main component in coal tar, is usually subject to oxidation to yield AQ in industry [3–5]. In general, 2.1. NBPBFTC synthesis gas-phase oxidation method involves high temperatures and heavy metal-based catalysts, while liquid-phase one requires strong acids, NBPBFTC was synthesized by a two-step method according to e.g., concentrated sulfuric acid and dichromic acid [4,5]. Scheme 1 [25]. The intermediate IL N-butylpyridinium bromide Ionic liquids (ILs) have been paid increasing attention due to their (NBPB) was prepared by the quaternization reaction of pyridine with negligible vapor pressure, thermal stability, excellent solubility, and de- 1-butyl bromide at 70 °C in cyclohexane in a round flask with a reflux sign ability by appropriate modifications of cationic or anionic condenser and magnetic stirrer for 48 h. The resulting crude product structures [6–9]. The FeCl3-based ILs not only have the above unique was purified by repetitious recrystallization and dried in a vacuum properties, but also exhibit a strong response to an additional magnetic oven. Then NBPBFTC was synthesized by mixing equimolar amounts

field [10–13]. These properties contribute a lot to the potential applica- of the purified NBPB and FeCl3·6H2O at 50 °C in a dry flask with a me- tion of the iron-based ILs in catalytic reactions [14], solvent effects [15], chanical stirrer for 24 h. The coarse product was washed several times and separation processes [16]. Three kinds of FeCl3-based ILs were used with diethyl ether and deionized water, and then dried in a vacuum to dissolve coal direct liquefaction residues to obtain asphaltene frac- oven to obtain a dark red solid, i.e., NBPBFTC, at room temperature. tions [17]. Among them, an IL with pyridine-based cation is the most effective, implying that the IL can effectively dissolve aromatics. Besides, 2.2. Characterizations of NBPB and NBPBFTC

FeCl3-based ILs have been employed as an effective medium and catalyst in benzene alkylation [18], isobutene oligomerization [19],and Fourier transformed infrared (FTIR) spectra of the two ILs were re- Friedel–Crafts sulfonylation of some aromatics [20]. More interestingly, corded on a Nicolet Magna IR-560 FTIR spectrometer by collecting 50 −1 FeCl3-based ILs along with aqueous hydrogen peroxide (AHPO) could scans at a resolution of 4 cm in reﬂectance mode with a measuring re- greatly facilitate oxidative desulfurization of liquid fuels [21–24] due gion of 4000–400 cm−1. Raman analysis was carried out using a Bruker to their good dissolubility and high catalytic activity for oxidation. Senterra Raman spectrometer with a 532 nm laser source at a resolution of 1.5 cm−1 with a measuring region of 200–450 cm−1. 1H-nuclear ⁎ Corresponding author. Tel.: +86 516 83885951. magnetic resonance (NMR) spectrum was recorded on a Bruker AV- E-mail address: [email protected] (X.-Y. Wei). 400 NMR spectrometer with tetramethylsilane as the external standard.

- Br- [FeBrCl3] N . Br + FeCl3 6H2O + 70 oC, 48 h N 50 oC, 24 h N in cyclohexane NBPBFTC

Scheme 1. Procedure for NBPBFTC preparation.

Ultimate analysis was performed using a Leco CHN-2000 elemental de- NBPBFTC exhibits the reversible melting and solidifying behaviors. terminator. The magnetic measurements were conducted on a Quan- At room temperature, NBPBFTC is a dark red crystal, while it begins to tum Design MPMS-XL-7 magnetometer at 27 °C. The melting point melt around 44 °C and become a fluid with a high viscosity at 50 °C, as was measured with a Beijing Keyi XT5 melting point determinator. displayed in Fig. 1. It turns into a solid when cooling the reaction system to room temperature. Such a temperature-responsive behavior may facilitate NBPBFTC recycle after the reaction by spontaneous phase 2.3. Anthracene oxidation to AQ separation. As depicted in Fig. 2, the magnetization of NBPBFTC shows a nearly About 50 mg anthracene, 5 mL CH CN, and prescribed amounts 3 linear field dependence over the applied magnetic field range of of NBPBFTC and AHPO were put into a glass tube reactor. Then the mix- −20–20kOe, which is in accordance with a typical paramagnetic be- ture was stirred at a designed temperature in a KEM PRS Start-up Kit havior. From the slopes of the fitted line, the magnetic susceptibility of parallel synthesizer. After reaction for an indicated period of time (15– NBPBFTC is determined to be 41.7 memu· g–1, which is similar to a re- 75 min), the reaction mixture was extracted with dichloromethane ported value for similar Fe(III)-based ILs [26]. Despite the fact that the (DCM). The extract was analyzed with an Agilent 7890/5975 gas chro- strong response to an additional magnetic field would be helpful for matograph/mass spectrometer (GC/MS) equipped with a capillary col- the recycle of the Fe(III)-based ILs, how to conveniently separate umn coated with HP-5 (cross-link 5% PH ME siloxane, 60 m length, Fe(III)-based ILs from the reaction system needs further investigation 0.25 mm inner diameter, 0.25 μm film thickness) and a quadrupole an- [27]. alyzer and operated in electron impact (70 eV) mode. Pure anthracene and AQ were adopted as external standards for quantitative analysis. The yield of AQ was calculated by dividing the molar mass of AQ by 3.2. Effects of reaction conditions onanthracene oxidation to AQ the total molar mass of anthracene (including converted and residual one), while the selectivity of AQ was determined by dividing the In the presence of NBPBFTC, the decomposition of H O in AHPO is molar mass of AQ by the converted molar mass of anthracene. 2 2 too rapid to control the oxidation. So, CH3CN was added to the reaction system as a dispersant to control the oxidation. As shown in Table 1, 3. Results and discussion under the same conditions except catalyst, AQ selectivity is 100% over

either NBPBFTC or FeCl3·6H2O or without catalyst, but AQ yields 3.1. Characterizations of NBPB and NBPBFTC are quite different over NBPBFTC (99.3%), FeCl3·6H2O (21.3%), and without catalyst (1.8%). Both the yield and selectivity of AQ from

The contents of carbon, hydrogen, and nitrogen in NBPB are 50.84%, anthracene oxidation over FeBr3 decrease compared to those 8.90%, and 6.61%, respectively, which are approximately equal to the over NBPBFTC due to the formation of the by-product 9,10- theoretical elemental analysis of the IL. The high purity of NBPB synthe- dibromoanthracene over FeBr3.The best result was obtained over 1 sized was further evidenced by FTIR (Fig. S1) and H NMR (Fig. S2) anal- [(CH3CH2CH2CH2)4NH4]6[BW11Mn(H2O)O39] in previous publications yses. FTIR spectrum of NBPBFTC is similar to that of NBPB except for [28–31], but compared to the expensive catalyst, higher temperature lacking the strong absorbance of –OH caused by the strong hygroscopic- (80 °C), and much longer time (24 h), NBPBFTC should be more appro- ity of NBPB, as shown in Fig. S1. The fact indicates that the cation of priate for catalyzing anthracene oxidation to AQ using AHPO as the NBPB does not participate in the complexation reaction during the for- oxidant. mation of NBPBFTC. In the Raman spectrum of NBPBFTC (Fig. S3), the −1 bands around 225, 246, 268, 333, and 351 cm correspond to the 0.9 − − − − stretch vibration of [FeClBr3] , [FeBrCl3] , [FeBr2Cl2] , [FeCl4] ,and − 3+ − [Fe2Br2Cl5] , respectively [25]. The result suggests that free Fe ,Br , 0.6 and Cl− could be combined to generate different anions in the production of NBPBFTC. 0.3 ) -1 g . 0.0 (emu g

x -0.3

-0.6

-0.9 -20 -15 -10 -5 0 5 10 15 20 o room temperature 50 C1 H (kOe)

Fig. 1. Reversible melting and solidifying behaviors of NBPBFTC. Fig. 2. Magnetization of NBPBFTC as a function of applied magnetic ﬁeld at 27 °C. Y.-G. Wang et al. / Fuel Processing Technology 135 (2015) 157–161 159

Table 1 Anthracene oxidation to AQ under different conditions.

Entry Anthracene (mmol) AHPOa (mL) Catalyst (μmol) Organic solvent (mL) RT (°C) Time (h) Yield (%) Selectivity (%) Reference

1 0.28 1 NBPBFTC (260) CH3CN (5) 50 1 99.3 100.0 This work

2 0.28 1 FeBr3 (260) CH3CN (5) 50 1 94.7 95.1 This work

3 0.28 1 FeCl3·6H2O (260) CH3CN (5) 50 1 21.3 100.0 This work

4 0.28 1 None CH3CN (5) 50 1 1.8 100.0 This work b 5 1.5 1.2 H5BW12O40 (25) CH3CN (20) reﬂux 0.25 85.0 [28]

6 0.28 3.5 RuCl3 (0.196) CH3COOH (20) 100 0.5 98.0 N98.0 [29]

7 0.5 0.5 Complex A (0.75) CH3CN (3) 80 24 100.0 100.0 [30]

8 0.5 0.5 Complex B (0.75) CH3CN (3) 80 24 93.0 100.0 [30]

9 0.25 1.5 HTSB BrCu(NCCH3) (10) CH2Cl2 (3) & CH3CN (3) 80 2 N93.0 N98.0 [31]

RT denotes reaction temperature; complexes A and B are [(CH3CH2CH2CH2)4NH4]6[BW11Mn(H2O)O39] and [(CH3CH2CH2CH2)4NH4]6[BW11Fe(H2O)O39], respectively; HTSB denotes hydrotrispyrazolylborate. a 50% H2O2 for entry 6 and 30% H2O2 for other entries. b Operated under microwave irradiation.

resulting active radicals and thus resulting in the decrease in AQ yield. 97 Thereby, 50 °C is the appropriate temperature for anthracene oxidation to AQ over NBPBFTC. The amount of NBPBFTC plays an important role in anthracene oxidation to AQ, as depicted in Fig. 4. Under the conditions of 1 mL AHPO, 91 50 °C, and 1 h, AQ yield is only 46.9% over 5 mg NBPBFTC and increases to 86.5% over 25 mg NBPBFTC. With further increasing the amount up to 100 mg, AQ yield gradually increases up to 99.3%. However, further in- 85 creasing the amount over 100 mg does not further increase AQ yield. Therefore, 100 mg NBPBFTC appears to be suitable for the oxidation of

AQ yield (mol%) yield AQ 50 mg anthracene. 79 As demonstrated in Fig. 5, under the conditions of 100 mg NBPBFTC, 50 °C, and 1 h, AQ yield gradually increases with increasing AHPO 100 mg NBPBFTC, 1 mL AHPO, 1 h amount up to 1 mL, but decreases with further increasing the amount 73 due to the formation of 1-hydroxyanthracene-9,10-dione, suggesting 40 45 50 55 60 that the optimal amount of AHPO is 1 mL for the oxidation of 50 mg Temperature (oC) anthracene. With prolonging time from 15 to 45 min under the conditions of

Fig. 3. Effect of temperature on anthracene oxidation to AQ. 100 mg NBPBFTC, 1 mL AHPO, and 50 °C, AQ yield increases from 96.6% to 99.5%, but does not increase when the time is longer than 45 min (Fig. 6). Therefore, the appropriate reaction time is 45 min.

As Fig. 3 shows, over NBPBFTC (100 mg), AQ yield rapidly increases 3.3. Regeneration and recycling of NBPBFTC with raising temperature from 40 to 45 °C, then slowly reaches the peak at 50 °C, but obviously decreases at higher temperatures. The high tem- The regeneration and subsequent recycling of an IL are of great im- peratures up to 50 °C facilitate the rapid dispersion of NBPBFTC in the portance for practical use of the IL. So, we investigated the possibility solution, as exhibited in Fig. S4, which may be responsible for the best of recycling NBPBFTC in anthracene oxidation to AQ. After extraction yield of AQ at 50 °C. However, at temperatures higher than 50 °C, the de- with DCM, the DCM-insoluble portion was distillated under reduced composition of H2O2 in AHPO to H2O and O2 [32] could signiﬁcantly pro- pressure to remove CH3CN and water. Afterwards, fresh AHPO, ceed, leading to the decrease in the concentrations of H2O2 and its

98 95

85 90

75 82

65 AQ yield (mol%) yield AQ AQ yield (mol%) yield AQ 74 55

o o 1 mL AHPO, 50 C, 1 h 100 mg NBPBFTC, 50 C, 1 h 45 66 0255075100125 0.25 0.50 0.75 1.00 1.25 NBPBFTC (mg) AHPO (mL)

Fig. 4. Effect of NBPBFTC amount on anthracene oxidation to AQ. Fig. 5. Effect of AHPO amount on anthracene oxidation to AQ. 160 Y.-G. Wang et al. / Fuel Processing Technology 135 (2015) 157–161

[33–35], as displayed in Fig. S6. Fenton reagent was widely used to oxidize organic compounds [36–39].ThefreeFe3+ can be dissociated from the anion of NBPBFTC and form a Fenton-like reagent with AHPO. As strong oxidizing species, HOO· and HO· are effectively produced from

H2O2 [40] in the Fenton-like reagent. A carbon atom in an aromatic ring with a larger superdelocalizability (Sr) accepts active species, such as radicals, more readily [41,42]. As displayed in Table S1, C9 and C10 have the largest Sr in anthracene. Therefore, as Scheme 2 illustrates, either HOO· or HO· preferentially attacks C9 and C10 in the soluble anthracene molecules to initiate anthracene oxidation to generate AQ via subsequent water elimination from 9,10-dihydroperoxy-9,10- dihydroanthracene or via subsequent hydrogen abstraction by HO..

4. Conclusions

The characterizations demonstrate that the prepared NBPBFTC can reversibly melt and solidify with the temperature change and strongly respond to the additional magnetic ﬁeld, facilitating its convenient Fig. 6. Time proﬁle of anthracene oxidation to AQ. separation in the organic reaction system. With the good dissolubility for anthracene and high catalytic activity of the formed Fenton-like

agent, the NBPBFTC–AHPO–CH3CN system could effectively oxidize anthracene to AQ. Under the optimal conditions, AQ was obtained in a anthracene, and CH CN were added for the next oxidation. As displayed 3 yield of 99.5% from anthracene oxidation. Furthermore, the reuse of in Fig. S5, AQ yield did not obviously decrease for the 3 recycles, indicat- the oxidation system proved to be feasible and satisfactory. The investi- ing that NBPBFTC could be recycled at least 3 times without a substantial gation provides an effective approach for oxidizing anthracene to AQ. decrease in its activity.

3.4. Possible formation pathway of AQ in NBPBFTC–AHPO–CH3CN Acknowledgments oxidation system This work was supported by National Basic Research Program of Anthracene oxidation in the system may involve two steps, China (Grant 2011CB201302), National Natural Science Foundation of i.e., dissolution and subsequent catalytic oxidation, as exhibited in China (Grant 21276268), Fund from National Natural Science Founda- Scheme 2. With the intense π–π interaction between anthracene and tion of China for Innovative Research Group (Grant 51221462), and pyridine-based cation, anthracene is easily dissolved in the system the Research and Innovation Project for College Graduates of Jiangsu

Step 1. Dissolution of anthracene in the CH3CN-NBPBFTC system

- 3+ - - 3+ 2+ . + 2+ 3+ . - . . [FeBrCl3] Fe +3Cl +Br Fe +H2O2 Fe + HOO +H Fe +H2O2 Fe + HO +HO H2O2 + HO HOO +H2O H O O H O H O H O

. . + HOO + HOO -2H2O

O H O O H H O. H H H O H O. OH OH OH O O

. . . +2HO - H2O +2HO - H2O +2HO -2H2O

O H O. OH O O H H H O. H Step 2. Possible pathways for the formation of HOO. and HO. and anthracene oxidation to AQ over NBPBFTC

Scheme 2. Possible pathway for the formation of AQ from anthracene oxidation in the NBPBFTC–AHPO–CH3CN system. Y.-G. Wang et al. / Fuel Processing Technology 135 (2015) 157–161 161

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