Application of Hydroxylamine Ionic Liquid Salts in

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5 Mar 2017 - An eco-friendly catalytic process for hydroxylation of benzene to phenol with (NH2OH)2¢ILs in ammonium molybdatecopper chlorideionic.
Application of Hydroxylamine Ionic Liquid Salts in Hydroxylation of Benzene to Phenol with Ammonium Molybdate­Copper Chloride­Ionic Liquid System Zhihui Li,1,2 Xudong Qi,2 Liya Gao,1 Yuanyuan Xu,1 Dongsheng Zhang,*1 Shufang Wang,1 Xinqiang Zhao,1 and Yanji Wang*1 1 School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, P. R. China 2 School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300130, P. R. China (E-mail: [email protected], [email protected])

An eco-friendly catalytic process for hydroxylation of benzene to phenol with (NH2OH)2¢ILs in ammonium molybdate­copper chloride­ionic liquid system was proposed. The new system demonstrated an enhanced phenol selectivity (100%) and yield (11.2%) due to the addition of Cl¹ and copper ions. That is, Cl¹ ions could increase the phenol selectivity and copper ions helped in improving the phenol yield.

REPRINTED FROM

Vol.46 No.3

2017 p.289–292 CMLTAG March 5, 2017

The Chemical Society of Japan

Received: October 19, 2016 | Accepted: November 27, 2016 | Web Released: December 10, 2016

CL-160936

Application of Hydroxylamine Ionic Liquid Salts in Hydroxylation of Benzene to Phenol with Ammonium Molybdate­Copper Chloride­Ionic Liquid System Zhihui Li,1,2 Xudong Qi,2 Liya Gao,1 Yuanyuan Xu,1 Dongsheng Zhang,*1 Shufang Wang,1 Xinqiang Zhao,1 and Yanji Wang*1 1 School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, P. R. China 2 School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300130, P. R. China (E-mail: [email protected], [email protected])

Scheme 1. Hydroxylation of benzene to phenol with (NH2OH)2¢ILs in ILs-Mo-Cu catalytic system.

Keywords: Hydroxylamine ionic liquid salts | Hydroxylation of benzene | Phenol

Chem. Lett. 2017, 46, 289–292 | doi:10.1246/cl.160936

Y phenol S phenol

2.5

2.6

2.4

31.2

30 25

22.6

2.0

Y phenol /%

Phenol is a valuable intermediate for the production of phenolic resin, pharmaceuticals, and agrochemicals.1 More than 90% of the world’s phenol is produced through the cumene process, which suffers from some drawbacks, such as multistep procedures, low atomic efficiency, and an equimolar amount of acetone by-product.2 Direct hydroxylation of benzene to phenol using clean oxidants like N2O,3 O2,4 and H2O25 has been realized and attracted worldwide attention. Generally, using O2 as an oxidant requires the use of reducing agents like H2,6 CO,7 NH3,8 ascorbic acid,9 etc., while N2O and H2O2 are costly and difficult to store. Recently we discovered that phenol could be synthesized directly from hydroxylamine sulfate ((NH2OH)2¢H2SO4) catalyzed by (NH4)6Mo7O24¢4H2O in H2O­HAc­H2SO4 medium.10 Although it offers a new route for hydroxylation of benzene to phenol, there are still several problems that must be resolved. For example, H2SO4 was used as the cosolvent, which caused severe problems such as equipment corrosion and environmental pollution. (NH2OH)2¢H2SO4 was used as the hydroxylation agent, which may also lead to corrosion and pollution problems because the salt released strong acid. The phenol selectivity was only 55%, which needed to be improved. Furthermore, the (NH4)6Mo7O24¢4H2O catalyst, together with H2SO4, was hard to recover from the reaction system. Therefore, it is desirable to develop an eco-friendly catalytic process beneficial for the hydroxylation of benzene to phenol with high yield and selectivity. Ionic liquids (ILs) have been widely researched as green reaction media due to their favorable physicochemical properties.11 Among them, SO3H-functionalized ILs are highly attractive because they are ideal media or/and catalysts for many classical acid-promoted organic reactions.12 More recently, a series of SO3H-functionalized ILs were employed as alternatives to conventional inorganic acids in hydroxylamine stabilization, and several eco-friendly hydroxylamine ionic liquid salts

35

3.0

20 1.5 15 1.0 0.5 0.0

S phenol /%

Several novel hydroxylamine ionic liquid salts were applied in direct hydroxylation of benzene to phenol in ammonium molybdate­copper chloride­ionic liquid system. Hydroxylamine ionic liquid salt was used as the eco-friendly hydroxylation agent, ammonium molybdate and copper chloride were used as the catalyst and additive, respectively, while the ionic liquid was used as the green cosolvent. The new system demonstrated an enhanced phenol selectivity (100%) and phenol yield (11.2%) due to the addition of Cl¹ and copper ions. Therefore, Cl¹ ions could increase the phenol selectivity and copper ions helped in improving the phenol yield. Furthermore, the ILs-Mo-Cu catalytic system could be recycled at least three times.

10 0.3

2.7

NaVO3

0.3

5

4.0

NH4VO3

Na2MoO4

(NH4)6Mo7O24

0

Catalysts Figure 1. Effect of catalyst on direct synthesis of phenol. Reaction conditions: 5.63 mmol benzene, n(benzene):n((NH2OH)2¢[HSO3-bmim]¢HSO4) = 1:1, 0.1 mmol catalyst, solvent: 6 mL HAc + 1.5 mL [HSO3-b-mim]¢HSO4, 80 °C, 3 h. Yphenol: The yield of phenol; Sphenol: The selectivity of phenol.

((NH2OH)2¢ILs) were obtained and applied in one-step synthesis of caprolactam.13 Considering this, we wondered whether these (NH2OH)2¢ILs could be used in phenol synthesis. The aim of this study was to explore the feasibility of using these (NH2OH)2¢ILs as the hydroxylation agents to replace the (NH2OH)2¢H2SO4, and as alternative medium to H2SO4 for the hydroxylation reaction. Therefore, direct hydroxylation of benzene to phenol with (NH2OH)2¢ILs in an eco-friendly and reusable catalytic system, i.e., ILs-Mo-Cu catalytic system, was proposed, as shown in Scheme 1. Firstly, the reaction was performed with hydroxylamine 1-sulfobutyl-3-methylimidazole hydrosulfate salt ((NH2OH)2¢ [HSO3-b-mim]¢HSO4) as the hydroxylation agent. The catalytic performance of several catalysts was evaluated. As shown in Figure 1, molybdenum species (especially (NH4)6Mo7O24) was favorable for the hydroxylation reaction probably because it had good activity in the activation of C­H bond of benzene.14

© 2017 The Chemical Society of Japan | 289

Entry 1b 2 3 4 5 6 7 8 9 10 11 12

ILs [HSO3-b-N(CH3)3]¢HSO4 [HSO3-b-N(CH3)3]¢HSO4 [HSO3-b-Py]¢HSO4 [HSO3-b-mim]¢HSO4 [HSO3-b-N(CH3)3]¢CF3SO3 [HSO3-b-Py]¢CF3SO3 [HSO3-b-mim]¢CF3SO3 [HSO3-b-N(CH3)3]¢p-TSA [HSO3-b-Py]¢p-TSA [HSO3-b-mim]¢p-TSA [HSO3-b-Py]¢Cl [HSO3-b-mim]¢Cl

Xbenzene/%

Yphenol/%

Sphenol/%

0 16.8 12.4 8.3 16.1 13.2 10.5 7.5 5.8 5.6 4.0 2.8

0 6.9 5.2 2.6 6.8 6.0 5.0 3.6 3.0 2.2 4.0 2.8

0 41.1 42.0 31.2 42.2 45.5 47.5 47.8 51.6 39.2 100 100

a Reaction conditions: 5.63 mmol benzene, n(benzene):n((NH2OH)2¢ [HSO3-b-mim]¢HSO4) = 1:1, 0.1 mmol (NH4)6Mo7O24, solvent: 6 mL HAc + 1.5 mL ionic liquid, 80 °C, 3 h. bNo (NH4)6Mo7O24 was added. Xbenzene: The conversion of benzene.

A set of acid ILs (Figure S1) were prepared and used in the hydroxylation reaction. Table 1 demonstrated that the ILs had no catalytic activity on the present reaction because no phenol was detected without the addition of (NH4)6Mo7O24. However, both the cations and anions of the ILs had great influence on the yield of phenol and conversion of benzene. When the anion was the same, the phenol yield and benzene conversion decreased in the order: [HSO3-b-N(CH3)3]+ > [HSO3-b-Py]+ > [HSO3-b-mim]+. With the same cation, they decreased in the order: CF3SO3¹ > HSO4¹ > Cl¹ > p-TSA¹. The [HSO3-b-N(CH3)3]¢CF3SO3, with [HSO3-b-N(CH3)3]+ as the cation and CF3SO3¹ as the anion, seems to be the best candidate. Indeed, when [HSO3-bN(CH3)3]¢CF3SO3 was used, the phenol yield (6.8%) was the second-highest, after [HSO3-b-N(CH3)3]¢HSO4 (6.9%). The possible reasons were as follows. The acidity of the HSO3functionalized ILs depended on the characteristics of both the cations and anions. We previously found that15 the H0 of each type of the ILs with the same cation and different anions increased in the order CF3SO3¹ < p-TSA¹ < HSO4¹, while the H0 of the ILs with the same anion and different cations increased in the order [HSO3-b-Py]+ < [HSO3-b-mim]+ < [HSO3-bN(CH3)3]+. The Hammett acidity for the cation and anion of the [HSO3-b-N(CH3)3]¢HSO4 was found to be the minimum, whereas the phenol yield was the highest when [HSO3-bN(CH3)3]¢HSO4 was used. Thus it could be concluded that slightly weaker acidity was suitable for the hydroxylation reaction. Furthermore, the ILs were used as the cosolvent with acetic acid to improve the solubility of hydroxylamine IL salt and (NH4)6Mo7O24. The better cooperative effect of the cation and anion of [HSO3-b-N(CH3)3]¢HSO4 may provide the highest solubility for these materials. Results in Table 1 also showed that when the anion of the ILs was HSO4¹, CF3SO3¹, or p-TSA¹, there was no significant difference in the phenol selectivity. While the anion was Cl¹, the phenol selectivity was always 100%, regardless of the type of cation used. Hence, the presence of Cl¹ ions was crucial for the phenol selectivity. When [HSO3-b-N(CH3)3]¢HSO4 was used, the phenol yield was highest with a low selectivity of 41.1%. On the other hand, when the ILs with Cl¹ was used, the phenol selectivity was

290 | Chem. Lett. 2017, 46, 289–292 | doi:10.1246/cl.160936

100

100

Y phenol, S pehnol/%

Table 1. Effect of ionic liquids as cosolvent on direct synthesis of phenola

Yphenol

93.0

Sphenol

80

75.9

75.5

71.7 60.4

60

58.0

57.6

41.1

40 20 8.9

0

9.3

CuCl2 CuCl

6.9

6.6

5.3

4.7

6.2

None NaCl FeCl3 NiCl2 FeCl2 -

5.5

KCl

5.1

ZnCl2

Cl promotors Figure 2. Effect of promoter on direct synthesis of phenol. Reaction conditions: 5.63 mmol benzene, n(benzene):n((NH2OH)2¢[HSO3bmim]¢HSO4) = 1:1, (NH4)6Mo7O24 0.1 mmol, catalyst promoter 0.1 mmol, solvent: 6 mL HAc + 1.5 mL [HSO3-b-N(CH3)3]¢HSO4, 80 °C, 3 h.

100%, while the phenol yield was no more than 4.0%. Therefore, additives containing Cl¹ ions were added to the [HSO3-b-N(CH3)3]¢HSO4 medium to explore whether these promoters had a beneficial effect on the phenol yield or/and selectivity. Figure 2 indicated that all the additives containing Cl¹ ions could dramatically improve the phenol selectivity. Nevertheless, only CuCl2 and CuCl helped in increasing the phenol yield. To gain information on the role of copper ions, CuSO4 and CuNO3 were used as the catalyst promoters (Table S1). Both of them demonstrated a noticeable improvement in phenol yield, but there was no obvious change in phenol selectivity. Based on these observations, the following conclusion could be drawn: Cl¹ ions could effectively increase the phenol selectivity and copper ions helped in improving the phenol yield. Surprisingly, when CuCl2 was used, the phenol selectivity was 100%. The possible reason was as follows. No further phenol was observed when ethanol as a free radical scavenger was added into the reaction system (Table S1),16 indicating that hydroxyl radical (•OH) was the active species for the present reaction. Copper ions could stabilize the free radicals.17 Cl¹ ions could scavenge the •OH,18 resulting in a decrease in the amount of •OH. Hence, further oxidation of phenol with excess •OH was retarded. Moreover, the existence of CuCl2 greatly decreased the electron cloud density of +NH3OH due to the metal cation coordination to + NH3OH via N and O.19 Therefore, the electrophilicity of + NH3OH was increased, making it easier for +NH3OH to gain electrons to form •OH. Hence, CuCl2 was selected as the catalyst promoter. The effects of various reaction variables, including IL amount, reaction temperature, time, and catalyst amount, on the yield and selectivity of phenol were investigated (Figures S2 and S3). The optimum reaction condition was at 80 °C for 4 h, benzene and (NH2OH)2¢[HSO3-b-mim]¢HSO4 molar ratio was 1:1 (5.63 mmol), (NH4)6Mo7O24 and CuCl2 were both 0.1 mmol, 6 mL HAc, and 1.5 mL [HSO3-b-N(CH3)3]¢HSO4 as the solvent. Under the optimal reaction condition, the phenol yield and selectivity were 11.3% and 100%, respectively. A comparison between the (NH2OH)2¢[HSO3-b-mim]¢ HSO4 and other (NH2OH)2¢ILs, as well as the traditional

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Table 3. Recycle of the ILs-Mo-Cu catalytic systema

Table 2. Effect of different hydroxylamine salts on synthesis of phenola Entry Hydroxylamine slats 1 2 3 4 5 6

None (NH2OH)2¢[HSO3-b-Py]¢HSO4 (NH2OH)2¢[HSO3-b-N(CH3)3]¢HSO4 (NH2OH)2¢[HSO3-b-mim]¢HSO4 NH2OH¢HCl (NH2OH)2¢H2SO4

Xbenzene /%

Yphenol /%

Sphenol /%

0 9.8 11.2 11.3 12.5 21.5

0 9.1 11.2 11.3 12.5 15.9

0 92.7 100 100 100 74.0

Reaction conditions: 5.63 mmol benzene, n(benzene):n(NH2OH) = 1:2, (NH4)6Mo7O24 0.1 mmol, CuCl2 0.1 mmol, solvent: 6 mL HAc + 1.5 mL [HSO3-b-N(CH3)3]¢HSO4, 80 °C, 4 h.

Run 1 2 3

Xbenzene/% 11.2 10.8 10.1

Yphenol/% 11.2 10.8 10.1

Sphenol/% 100 100 100

Reaction conditions: 5.63 mmol benzene, n(benzene):n((NH2OH)2¢ [HSO3-b-N(CH3)3]¢HSO4) = 1:1, (NH4)6Mo7O24 0.1 mmol, CuCl2 0.1 mmol, solvent: 6 mL HAc + 1.5 mL [HSO3-b-N(CH3)3]¢HSO4, 80 °C, 4 h. a

a

OH NH2OH

N 2O

(1)

(4) Mo(VI)

H

OH

Mo(V) (2)

(3)

OH

NH3OH+

CuCl2

Cu +

NH3OH

Cl Cl

Scheme 2. Plausible mechanism of benzene hydroxylation reaction.

hydroxylamine inorganic acid salts was summarized in Table 2. It can be seen that phenol was not provided without hydroxylamine, indicating mechanistically that hydroxylamine was the OH source of the reaction and essential. Furthermore, (NH2OH)2¢[HSO3-b-mim]¢HSO4 and (NH2OH)2¢[HSO3-bN(CH3)3]¢HSO4 demonstrated better reactivity than (NH2OH)2¢ [HSO3-b-Py]¢HSO4. When these two salts were used, the phenol yields were roughly the same and the phenol selectivities were both 100%. Considering that trimethylamine is much cheaper than N-methylimidazole, (NH2OH)2¢[HSO3-bN(CH3)3]¢HSO4 seems better than the other. In addition, the reactivity of (NH2OH)2¢[HSO3-b-N(CH3)3]¢HSO4 was better than (NH2OH)2¢H2SO4 and similar to NH2OH¢HCl. Hence, (NH2OH)2¢[HSO3-b-N(CH3)3]¢HSO4 was regarded as a better alternative to the traditional hydroxylamine salts. Based on these observations, a plausible radical mechanism was proposed for the hydroxylation reaction (Scheme 2). Mo(VI) was reduced to Mo(V) by NH2OH20 after the release of N2O (1), followed by the reductive cleavage of the N­O bond in +NH3OH in the benzene hydroxylamine system to generate the hydroxyl radicals (•OH)21 due to the relatively stronger reductive power of lower-valent Mo(V) species (2). Then the •OH species attacked the benzene ring to generate a hydroxycyclohexadienyl radical intermediate (3),21a,22 which was subsequently oxidized by Mo(VI) species to form the final product phenol, along with the regeneration of lower-valent Mo(V) to complete the catalytic cycle (4). The role of CuCl2 was to increase the electrophilicity of +NH3OH, which was favorable for +NH3OH to obtain electron to form •OH. Detailed study is still ongoing.

Chem. Lett. 2017, 46, 289–292 | doi:10.1246/cl.160936

To investigate the feasibility of the recycling of the ILs-MoCu catalytic system, the recycle experiments were conducted (Table 3). No obvious decrease in phenol yield and selectivity was detected as the system was reused three times. However, problems appeared after three runs. As we know, NH3 was produced when phenol was formed from benzene and (NH2OH)2¢ ILs (Scheme S2), which could react with the acid ILs in the catalytic system to form the solid white zwitterionic salt and the relevant ammonium salts (Scheme S3).23 Along with the recycling experiments, more NH3 was generated and reacted with the acid ILs. Therefore, more zwitterionic salt and ammonium salts were produced and remained in the recycled ILs-Mo-Cu catalytic system. The zwitterionic salt could be dissolved in the ILs. Since the amount of ILs was only 1.5 mL, when the third cycle was over, the system turned very sticky. For this reason, it was difficult to recycle the ILs-Mo-Cu catalytic system more than three runs. In summary, an eco-friendly catalytic process for hydroxylation of benzene to phenol with (NH2OH)2¢ILs in ammonium molybdate­copper chloride­ionic liquid system was proposed. (NH2OH)2¢[HSO3-b-N(CH3)3]¢HSO4 and [HSO3-b-N(CH3)3]¢ HSO4 were found to be the suitable hydroxylation agent and cosolvent, respectively, which effectively resolved the corrosion and pollution problems caused by (NH2OH)2¢H2SO4 and H2SO4. An enhanced phenol selectivity of 100% and phenol yield of 11.2% were obtained due to the adding of Cl¹ and copper ions. Furthermore, the ILs-Mo-Cu catalytic system was stable enough to be recycled. This work was financially support by the National Natural Science Foundation of China (Nos. 21236001, 21646015, and 21576069), National Natural Science Foundation of Hebei Province (Nos. B2016202335 and B2015202369) and Science and Technology Research Project of Hebei Province (No. Z2015049). Supporting Information is available on http://dx.doi.org/ 10.1246/cl.160936.

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