air catalyzed by sulfonic acid-functionalized ionic liquids

air catalyzed by sulfonic acid-functionalized ionic liquids

Catalysis Communications 10 (2008) 201–204 Contents lists available at ScienceDirect Catalysis Communications journal homepage:

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Catalysis Communications 10 (2008) 201–204

Contents lists available at ScienceDirect

Catalysis Communications journal homepage:

Nitration of aromatic compounds with NO2/air catalyzed by sulfonic acid-functionalized ionic liquids Guangbin Cheng *, Xuelei Duan, Xiufang Qi, Chunxu Lu Department of Chemistry, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

a r t i c l e

i n f o

Article history: Received 20 May 2008 Received in revised form 13 August 2008 Accepted 22 August 2008 Available online 30 August 2008 Keywords: Nitration NO2/air Ionic liquid

a b s t r a c t Nitration of aromatic compounds with NO2/air catalyzed by sulfonic acid-functionalized ionic liquid under solvent-free conditions is reported. A variety of ionic liquids [BSPy][HSO4], [BSPy][TfO], and [BSPy][pTSA] ([BSPy] = N-(4-hydroxsulfonylbutyl) pyridinium) were prepared. Their acidities were further investigated by Hammett method and the acidity-catalytic relationship was discussed. It was found that satisfactory results were obtained with [BSPy][HSO4]. Ionic liquids could be conveniently separated with the products and reused for five times with excellent yield of mono-nitration products and paraselectivity. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Nitration of aromatic compounds is one of the most fundamental therefore important reaction in organic synthesis, which provides key organic intermediates or energetic materials. Traditionally, nitration of aromatic compounds is performed with mixture of nitric and sulfuric acids [1,2]. However, this method is often deteriorated with poor selectivity for desired products and high environmental cost. Various clean nitration approaches have been explored which involve nitrogen oxides such as NO2 [3,4], N2O4 [5], and N2O5 [6] under ‘‘neutral” condition, or recyclable catalysts such as lanthanide triflates [7,8], perfluorinated resin immobilized sulfonic acid [9], claycop or zeolites [10–13]. Although much success has been achieved, some problems still remain, for example, the use of lanthanide triflate or polymeric sulfonic acid resins as catalysts did not improve the selectivity. Furthermore, volatile chlorinated solvents were still required in some processes. Over the past few years, ionic liquids received great attention because of their high polarity, negligible vapor pressure, and good solubility in a broad range of organic and inorganic compounds, which endorse them as useful ‘‘clean and green” solvents or catalysts for a number of reactions [14–18]. There were several reports on the nitration of aromatic compounds using HNO3 (67%) [19,20] or HNO3/Ac2O [21,22] in the media of ionic liquid. We have also reported the nitration of aromatic compounds with NO2 in N-protonized carprolactam based acidic ionic liquids, in which the ionic liquid plays dual roles: solvent and catalyst [23]. Our previous re-

* Corresponding author. Tel.: +86 25 8431 5948/8205; fax: +86 25 8431 5030. E-mail address: [email protected] (G. Cheng). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.08.019

port showed that acidic ionic liquid had great potential to be developed into a practical catalyst for a greener nitration method: ‘‘solvent-free”, good catalytic activity, recyclable. Here reported is our continuous effort in systematical investigation of this novel nitration method (Scheme 1). Instead of the N-protonized carprolactam based acidic ionic liquids used in our previous report, the easy accessible, ‘‘formula stable” pyridinium based ionic liquids (Fig. 1) were used in this study. As nitration is an electrophilic reaction and the acidity of catalyst is an important parameter related to its catalytic activity, compared to the ionic liquid previously used, pyridinium based sulfonic acid modified ionic liquids were expected to have much higher acidity. The acidities of the ionic liquids were determined using the Hammett method with UV–visible spectroscopy and the relationship of acidity and catalytic activity was discussed.

2. Experimental 2.1. General information All reagents were commercially available. Toluene, chlorobenzene, benzene and dichloromethane were dried over 4 Å molecular sieve prior to use. Product mixtures were analyzed on Agilent Technologies 6820 Gas Chromatography with OV-101 capillary column (30 m  0.32 mm) and FID detector. The 1H NMR spectra were recorded on a Bruker Advanced Digital 300 MHz spectrometer in D2O and calibrated with tetramethylsilane (TMS) as internal reference. The UV–visible spectra were recorded on Beijing Ruili UV-1100 spectrophotometer.


G. Cheng et al. / Catalysis Communications 10 (2008) 201–204

a basic indicator was used to trap the acidic proton. The Hammett function (H0) can be expressed as






H0 ¼ pKðIÞaq þ logð½I=½IHþ Þ -


X = HSO4 , -



X = CF3SO3 ,


X- = H3C


Fig. 1. Structures of three sulfonic acid-functionalized ionic liquids.

2.2. Preparation of sulfonic acid-functionalized ionic liquids Three sulfonic acid-functionalized ionic liquids were synthesized following the literature reported method [24]. They are all colorless liquid and entirely miscible with water. The data of 1H NMR spectra of the three ionic liquids were shown as follows: [BSPy][HSO4]: 1H NMR (300 MHz, D2O): d 1.41 (m, 2H), 1.79 (m, 2H), 2.58 (t, 2H, J = 7.4 Hz), 4.28 (t, 2H, J = 7.3 Hz), 7.69 (t, 2H, J = 7.0 Hz), 8.17 (t, 1H, J = 8.0 Hz), 8.48 (d, 2H, J = 6.7 Hz). [BSPy][OTf]: 1H NMR (300 MHz, D2O): d 1.42 (t, 2H, J = 10.4 Hz), 1.78 (t, 2H, J = 6.4 Hz), 2.55 (m, 2H), 4.24 (m, 2H), 7.69 (t, 2H, J = 5.5 Hz), 8.16 (q, 1H, J = 7.0 Hz), 8.45 (t, 2H, J = 5.3 Hz). [BSPy][pTSA]: 1H NMR (300 MHz, D2O): d 1.87 (m, 2H), 2.01 (s, 3H), 2.28 (m, 2H), 3.19 (t, 2H, J = 6.2 Hz), 4.58 (t, 2H, J = 5.3 Hz), 6.92 (s, 2H), 7.47 (s, 2H), 7.70 (s, 2H), 8.14 (s, 1H), 8.53 (d, 2H, J = 4.9 Hz).

where pK(I)aq is the pKa value of the indicator, [I] and [IH+] are, respectively, the molar concentrations of the unprotonated and protonated forms of the indicator, which can usually be determined by UV–visible spectroscopy. The acidities of the three ionic liquids were examined using 4nitroaniline (Hammett content is 0.99, concentration is 10 m mol/ L) as indicator in dichloromethane and the results are shown in Fig. 2. Dichloromethane was chosen as test solvent, because it is an aprotic polar solvent, stable under acidic conditions and has considerable solubility for all tested ionic liquids. The maximal absorbance of the unprotonated form of 4-nitroaniline was observed at 349 nm in CH2Cl2 (Fig. 2. spectra a). As the acidic ionic liquid was added, the absorbance of the indicator decreased. We could determine the [I]/[IH+] ratio by measuring the absorbance when each ionic liquids was added (spectra b–d), and then the Hammett function (H0) is calculated (see Table 1). The experiments showed that the acidity order was: [BSPy][OTf] > [BSPy][HSO4] > [BSPy][pTSA]. It is clear that acidities of the ionic liquid depend on the anions. Xing et al. [26] determined the minimum energy geometries of these ionic liquids, the results manifested that anions had strong interaction with the sulfonic acid proton. It is considered that in addition to alkyl sulfonic acid group, the conjugate acid of the corresponding anion is likely to serve as potential catalytic acid. Therefore, the acidities and catalytic activities of the ionic liquids depend on the nature of anion as well.

To a cooled (15 °C, ice–salt bath) vigorously stirred mixture of the ionic liquid (15 mol% of substrate) and the substrate (10.0 m mol) under air atmosphere (provided by air balloon) was added liquid nitrogen dioxide (0.5 mL, ca. 14 m mol). The reaction was kept at 15 °C for 30 min, then at 0 °C for 5 h, and then warmed up to a higher temperature as indicated in the table. After a certain time as indicated the reaction was quenched with deionized water (ca. 5 mL), extracted with hexane (5 mL  4). The organic phase was isolated and organic solutions were combined and washed with saturated solution of sodium bicarbonate (5 mL), water (5 mL  3) and brine (5 mL  2), dried over Na2SO4 and analyzed by GC (different internal standard was used for toluene, and chlorobenzene, nitrobenzene; and for benzene, hexadecane 0.2 mL) [21]. The ionic liquid was recovered from the aqueous solution by removing the water at 70 °C under vacuum for 5 h and was reused without significantly lost of its reactivity.


2.3. General nitration procedure







a : blank b : [BSPy][pTSA] c : [BSPy][HSO4] d : [BSPy][OTf]


0.6 0.4 0.2 0.0 250

2.4. UV–visible acidity determination






Wavelength (nm) The ionic liquids are all colorless liquid with good fluidity at room temperature; the experiments were carried out at room temperature. Dichloromethane solutions of the ionic liquids (10 m mol/L) were prepared. 4-Nitroaniline was used as indicator (10 m mol/L), and the UV–visible determination was carried out in the scale of 220–500 nm.

Fig 2. Absorption spectra of 4-nitroaniline in various ionic liquids solution in dichloromethane.

Table 1 H0 values of ionic liquids in dichloromethane at room temperaturea Ionic liquids

Absorbance (AU)b

[I] (%)

[IH+] (%)


3.1. Acidities of the sulfonic acid-functionalized ionic liquids

Blank [BSPy][HSO4] [BSPy][OTf] [BSPy][pTSA]

1.134 0.709 0.615 1.007

100 62.5 54.2 88.8

0 37.5 45.8 11.2

– 1.21 1.06 3.98

A common and effective way to evaluate the acidity of Brønsted acid is Hammett method, which was first proposed by Gilbert et al. in 2003 on the basis of the Hammett acidity function [25], wherein

Concentration: 10 m mol/L; indicator: 4-nitraniline. The maximal absorbance of the unprotonated form of 4-nitroaniline was observed at 349 nm in CH2Cl2.

3. Results and discussion




G. Cheng et al. / Catalysis Communications 10 (2008) 201–204

3.2. Nitration under different conditions using [BSPy][HSO4] as catalyst

Table 3 Nitration of simple aromatic compounds catalyzed by different ionic liquids

Toluene was chosen as a test substrate and [BSPy][HSO4] was used as catalyst for the investigation on nitration reaction with NO2/air, the results are summarized in Table 2. The reaction proceeded well and mono-nitrotoluene was gained in moderate yield and dramatically improved regioselectivities (Table 2, entry 3). Compared to the 1.67 ratio of ortho/para of the nitration of toluene in HNO3/H2SO4 [1], the nitration in acidic ionic liquid with NO2/air has regioselectivity as high as 1.2–1.3. The enhanced regioselectivity is attributed to much bigger size of the catalyst (ionic liquid) [27]. When the equivalents of [BSPy][HSO4] increased (Table 2, entries 1–4), the yield was also increased (Table 2, entry 3). When it reached up to 20%, the conversion and yield decreased slightly. It may be due to the solvating effect. It seems that concentration of [BSPy][HSO4] has little effect on the para-selectivity. The equivalents of NO2 used in the reaction had a significant effect on conversion and yield, but little effect on the distribution of products (Table 2, entries 5–7). When 0.5 mL (ca.14 m mol) NO2 was used, which was a little excess, the best yield was gained. We also investigated the effect of reaction temperature. At the temperature below 0 °C the reaction preceded very slow (Table 2, entry 9). It may need much longer time to gain reasonable conversion. Therefore, after a certain time at lower temperature, the reaction was heated at higher temperature for 20 h and good yield and selectivity were obtained (Table 2, entries 3 and 8). An even better ortho/para ratio (Table 2, entry 8, ortho/ para = 1.13) was obtained at 45 °C, however, both the conversion and yield decreased. This is probably due to the fact that higher temperature accelerate the evaporation of nitrogen oxides. Due to the fact that nitrogen dioxide has low boiling point (21.2 °C), to avoid the evaporation of the nitrogen dioxide it is necessary to remain a low temperature before a higher temperature can be adapted. To our surprise, there was no significant difference in terms of the regioselectivity of the reaction under different temperatures. From the result shown above, the best reaction condition is: toluene 10 m mol, NO2 0.5 mL, ionic liquid 15 mol%, 15 °C/30 min, then 0 °C/5 h, and rt/20 h.




Yieldc (%)

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

[BSPy][HSO4] [BSPy][OTf] [BSPy][pTSA] [hexPy][PTSA]d [BSPy][HSO4] [BSPy][OTf] [BSPy][pTSA] [BSPy][HSO4] [BSPy][OTf] [BSPy][pTSA] – – –

CH3 CH3 CH3 CH3 Cl Cl Cl H H H CH3 Cl H

77.0 63.6 30.8 13.4 34.7 32.6 16.4 38.9 28.6 15.4 11.4 4.5 8.6

3.3. Nitration of simple aromatic compounds catalyzed by different ionic liquid

Product distribution (%)





54.2 53.0 53.2 54.5 22.5 19.8 24.3 – – – 57.9 39.8 –

3.3 3.3 6.5 8.0 2.2 0.5 1.8

42.5 43.7 40.3 37.5 75.3 79.7 73.8

1.27 1.21 1.32 1.45 0.30 0.25 0.33

5.7 0

36.4 60.1

1.59 0.66

Substrate 10 m mol, NO2 0.5 mL; reaction time and temperature: a 15 °C/30 min, then 0 °C/5 h, and rt/20 h (when the air balloon was removed). b 15 °C/30 min, then 0 °C/5 h, and rt/40 h (when the air balloon was removed). c Calculated by quantitative GC. d [hexPy][PTSA]: N-hexyl pyridinium p-toluenesulfonic acid salt.

All of the ionic liquids used were found to be able to promote the NO2/air nitration of benzene and chlorobenzene. Nitration of toluene gave the best result in terms of yield and conversion. However, the yields of nitration of chlorobenzene and benzene are significantly lower, which reflect the fact that benzene and chlorobenzene is less electron rich compared to toluene, and the nitration in these ionic liquid is a mild nitration method. An ionic liquid with stronger acidity might be required to get those type aromatic compounds nitrated. The significant higher para-selectivity in the case of chlorobenzene is most likely due to the fact that its ortho position is deactivated by electron induction. The anions also have great effect on the yield or selectivity of the reaction. Reactions conducted in [BSPy][HSO4] and [BSPy][OTf] were much better in terms of yield than in [BSPy][pTSA]. This can be explained by their acidities order: [BSPy][OTf] > [BSPy][HSO4] > [BSPy][pTSA]. However, [BSPy][HSO4] has the best catalytic activity, due to that [BSPy][HSO4] has two protons, which provide more acid source for nitration reaction. [BSPy][OTf] showed best para-selectivity, probably due to the higher polarity of [BSPy][OTf] which give it good affinity to aromatic compounds. 3.4. Reusability of [BSPy][HSO4]

In view of the success of the new reaction, the reaction scope was explored with various ionic liquids and variety of aromatic compounds. All reactions were conducted under the identical conditions indicated above. The results are summarized in Table 3.

The reusability of [BSPy][HSO4] in nitration of toluene was examined and the results were listed in Table 4. [BSPy][HSO4] was used for five cycles and the yield was lost about 9.4% in the

Table 2 Nitration of toluene in [BSPy][HSO4] under different conditionsa Entry

1 2 3 4 5 6 7 8b 9c

[BSPy][HSO4]d (mol%)

5 10 15 20 15 15 15 15 15

NO2 (mL)

0.5 0.5 0.5 0.5 0.2 0.8 1 0.5 0.5

Conversione (%)

49.9 49.0 79.4 61.7 43.9 41.9 31.8 46.8 15.9

Substrate 10 m mol; reaction time and temperature: a 15 °C/30 min, then 0 °C/5 h, and rt/20 h (when the air balloon was removed). b 15 °C/30 min, then 0 °C/5 h, and 45 °C/20 h (when the air balloon was removed). c 15 °C/30 min, then 0 °C/5 h. d Mole ratio to toluene. e Calculated by quantitative GC.

Yielde (%)

33.0 33.3 77.0 52.8 16.7 35.9 28.7 35.7 4.6

Product distribution (%)





55.4 53.7 54.2 54.1 54.2 54.3 54.5 51.0 56.0

3.9 3.7 3.3 3.5 4.3 3.7 3.5 3.9 0

40.7 42.6 42.5 42.4 41.5 42.0 42.0 45.1 44.0

1.36 1.26 1.27 1.27 1.31 1.29 1.30 1.13 1.27


G. Cheng et al. / Catalysis Communications 10 (2008) 201–204

Table 4 Reusability of [BSPy][HSO4] for nitration of toluenea Entry


Yieldb (%)

1 2 3 4 5

1 2 3 4 5

77.0 76.3 74.9 70.1 67.6

Product distribution (%) Ortho



54.2 53.3 54.0 53.3 53.5

3.3 3.9 4.4 4.8 4.2

42.5 42.8 41.6 41.9 42.3


1.27 1.25 1.30 1.27 1.26

a Substrate 10 m mol, 15 °C/30 min, then 0 °C/5 h, and rt/20 h (when the air balloon was removed). b Calculated by quantitative GC.



NO2/O2 ionic liquid


R = CH3, H, Cl Scheme 1. Nitration of simple aromatic compounds.

fifth cycle. The yield of nitro products was 67.6% with an ortho/ para isomers ratio of 1.26 when the catalyst was used for the fifth time. This indicated that [BSPy][HSO4] has excellent reusability. A possible reason for the yield decreasing along the recycled times of the ionic liquid was that side-products were adsorbed in the ionic liquid phase. This decreased the amount of catalytically active species of ionic liquid, thus decreased the catalytic activity of the ionic liquid. 4. Conclusions Nitration of simple aromatic compounds in the presence of acidic ionic liquid with NO2/air has showed higher para-selectivity and moderate yield. Furthermore, the use of NO2/air is a clean nitration, and air is very cheap and facile. The acidic ionic liquid

could be easily separated from the products and easily recycled with the nitration yield lost about 9.4% during five cycles. This methodology offers significant improvements for nitration of aromatic compounds with regard to yield of products, simplicity in operation, mild reaction condition and ‘‘green” aspects by avoiding toxic catalysts and solvents. References [1] K. Schofield, Aromatic Nitration, Cambridge University Press, Cambridge, 1980. [2] G.A. Olah, R. Malhotea, S.C. Narang, Nitration: Methods and Mechanisms, VCH, New York, 1989. [3] H. Sato, K. Hirose, Appl. Catal. 174 (1998) 77. [4] H. Suzuki, S. Yonezawa, N. Nonoyama, T. Mori, J. Chem. Soc.: Perkin Trans 1 (1996) 2385. [5] N. Iranpoor, H. Firouzabadi, M.A. Zolfigol, Syn. Commun. 28 (1998) 2773. [6] R.R. Bak, A.J. Smallridge, Tetrahedron Lett. 42 (2001) 6767. [7] A.G.M. Barrett, D.C. Braddock, R. Ducray, R.M. Mckinnell, F.J. Waller, Syn. Lett. (2000) 57. [8] F.J. Waller, A.G.M. Barrett, D.C. Braddock, D. Ramprasad, Chem. Commun. (1997) 613. [9] G.A. Olah, R. Malhotra, S.C. Narang, J. Org. Chem. 43 (1978) 4628. [10] L. Delaude, P. Laszlo, K. Smith, Acc. Chem. Res. 26 (1993) 607. [11] B. Gigante, A.O. Prazeres, M.J. Marcelo-Curto, J. Org. Chem. 60 (1995) 3445. [12] S.P. Dagade, S.B. Waghmode, V.S. Kadam, M.K. Dongare, Appl. Catal. A 226 (2002) 49. [13] X. Peng, H. Suzuki, C.X. Lu, Tetrahedron Lett. 42 (2001) 4357. [14] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000) 3772. [15] R. Sheldon, Chem. Commun. (2001) 2399. [16] A.C. Cole, J.L. Jensen, I. Ntai, K.L.T. Tran, K.J. Weaver, J. Am. Chem. Soc. 124 (2002) 5962. [17] D.M. Li, F. Shi, J.J. Peng, S. Guo, Y.Q. Deng, J. Org. Chem. 69 (2004) 3582. [18] E.J. Angueira, M.G. White, J. Mol. Catal. A 227 (2005) 51. [19] M.J. Earle, S.P. Katdare, WO. Pat. WP0230865, 2002. [20] K. Qiao, C. Yokoyama, Chem. Lett. 33 (2004) 808. [21] K. Smith, S.F. Liu, G.A. El-Hiti, Ind. Eng. Chem. Res. 44 (2005) 8611. [22] N.L. Lancaster, L.M. Verónica, Chem. Commun. (2003) 2812. [23] X. Qi, G. Cheng, C. Lu, D. Qian, Syn. Commun. 38 (2008) 537. [24] J.Z. Gui, X.H. Cong, D. Liu, X.T. Zhang, Z. Hu, Z.L. Sun, Catal. Commun. 5 (2004) 473. [25] C. Thomazeau, H. Bourbigou, S. Luts, B. Gillber, J. Am. Chem. Soc. 125 (2003) 5264. [26] H.B. Xing, T. Wang, Z.H. Zhou, Y.Y. Dai, J. Mol. Catal. A 264 (2007) 53. [27] It was considered that N2O4 ionized to ½NOþ ½NO 3  in the presence of Brønsted acidic ionic liquid (see Ref. [23]). It is possible that the nitration of simple aromatics with NO2/air in the presence of sulfonic acid-functionalized ionic liquids at low temperature the active nitration species was activated  [NO+][X] (X = pTSA, HSO 4 , and CF3 SO3 ).