catalysts Article Chlorination of Toluene to o-Chlorotoluene Catalyzed by Ionic Liquids Aili Wang, Xiaoyan Zhu, Hengbo Yin *, Yujun Fu and Xiangxiang Hou Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China; [email protected] (A.W.); [email protected] (X.Z.); [email protected] (Y.F.); [email protected] (X.H.) * Correspondence: [email protected]; Tel.: +86-(0)511-8878-7591 Received: 13 October 2018; Accepted: 5 November 2018; Published: 9 November 2018 − − Abstract: Ionic liquids with [BMIM], [Et3NH], and [BPy] cations and AlnCl 3n+1, ZnnCl 2n+1, − and CunCl n+1 anions were used as the catalysts for the chlorination of toluene with Cl2. − The ZnnCl 2n+1 containing ionic liquids with high Lewis acid strength had high catalytic activity for the selective chlorination of toluene to o-chlorotoluene via the electrophilic substitution reaction. − Dichlorotoluenes were favorably formed when the AlnCl 3n+1 containing ionic liquids with both − Lewis and Brönsted acid sites were used as the catalysts. When the CunCl n+1 containing ionic liquids with weak Lewis acid strength were used as the catalysts, more benzyl chloride was formed via the radical chlorination of methyl group. When the [BMIM]Cl-2ZnCl2 ionic liquid was used as the catalyst, after reacting at 80 ◦C for 8 h, the conversion of toluene was 99.7% and the selectivities of o-chlorotoluene, p-chlorotoluene, m-chlorotoluene, benzyl chloride, and dichlorotoluenes were 65.4%, 26.0%, 4.0%, 0.4%, and 4.2%, respectively. The [BMIM]Cl-2ZnCl2 ionic liquid catalyst had good recycling performance. Keywords: ionic liquids; toluene; chlorination; o-chlorotoluene 1. Introduction o-Chlorotoluene as an important raw material has been widely used in the production of pharmaceuticals, pesticides, spices, and dyes [1–5]. o-Chlorotoluene has been commercially produced by the chlorination of toluene with gaseous chlorine over conventional Lewis acid catalysts, such as FeCl3 and AlCl3, accompanied by the formation of m-chlorotoluene, p-chlorotoluene, benzyl chloride, and dichlorotoluenes with high yields [6–8]. While using FeCl3 catalyst, only one half of the product is o-chlorotoluene. However, the conventional Lewis acid catalysts, FeCl3 and AlCl3, cannot be recycled, causing serious equipment corrosion and environmental pollution [7]. Ionic liquids have attracted considerable attention in recent years because of their negligible vapor pressure, high thermal stability, high ionic conductivity, and composition tunability [9–11]. Ionic liquids have been widely used in the catalysis, electrochemistry, and separation fields [12–19]. When the ionic liquids were used as the catalysts for the polymerization of α-pinene and the redistribution reaction of methyltrichlorosilane with low-boiling residue to dimethyldichlorosilane, the Lewis acid ionic liquids had excellent catalytic activity and good reusability [16,20,21]. The ionic liquid acidities could be adjusted by using different types of cations and anions and changing their mole ratios. Their acidities are crucial for the acid-catalyzed reactions. However, to the best of our knowledge, catalytic chlorination of toluene with gaseous Cl2 to chlorotoluene over ionic liquid catalyst has not been reported until now. This paper reports the catalytic activities of the ionic liquid catalysts with the cation counterparts, triethylammonium ([Et3NH]), 1-butyl-3-methylimidazolium ([BMIM]), and 1-butylpyridinium ([BPy]) − − − and the anion counterparts, ZnnCl 2n+1, AlnCl 3n+1, and CunCl n+1 for the chlorination reaction Catalysts 2018, 8, 532; doi:10.3390/catal8110532 www.mdpi.com/journal/catalysts Catalysts 2018, 8, x FOR PEER REVIEW 2 of 15 This paper reports the catalytic activities of the ionic liquid catalysts with the cation counterparts, triethylammonium ([Et3NH]), 1-butyl-3-methylimidazolium ([BMIM]), and 1-butylpyridinium ([BPy]) and the anion counterparts, ZnnCl−2n+1, AlnCl−3n+1, and CunCl−n+1 for the Catalysts 2018, 8, 532 2 of 15 chlorination reaction between toluene and gaseous Cl2. The Fourier transform infrared spectroscopy (FT‐IR) technique with the use of pyridine as the molecular probe was used to determine the Lewis and betweenBrönsted toluene acid strengths and gaseous of these Cl2. The ionic Fourier liquid transform catalysts. infrared The catalytic spectroscopy chlorination (FT-IR) technique reaction was with the use of pyridine as the molecular probe was used to determine the Lewis and Brönsted acid investigated at different reaction temperatures, reaction time periods, and catalyst loadings. The strengths of these ionic liquid catalysts. The catalytic chlorination reaction was investigated at different reaction routes were also briefly discussed. reaction temperatures, reaction time periods, and catalyst loadings. The reaction routes were also briefly discussed. 2. Results and Discussion 2. Results and Discussion 2.1. Acidity of Ionic Liquid 2.1. Acidity of Ionic Liquid Pyridine is commonly used as a molecular probe to determine the Lewis and Brönsted acidities Pyridine is commonly used as a molecular probe to determine the Lewis and Brönsted acidities of of ionic liquids by the IR technique [22]. The IR peak appearing at ca. 1450 cm−1 indicates the pyridine ionic liquids by the IR technique [22]. The IR peak appearing at ca. 1450 cm−1 indicates the pyridine −1 moleculemolecule coordinated coordinated to tothe the Lewis Lewis acid acid site.site. TheThe IRIR peak peak appearing appearing at ca.at ca. 1540 1540 cm− 1cmmeans means the the formationformation of pyridinium of pyridinium ions ions resulting resulting fromfrom the the Brönsted Brönsted acid acid site. site. The The blue blue shift shift extent extent of the peakof the at peak at ca.ca. 1450 1450 cm cm−1− in1 indicatesdicates the the increase increase inin LewisLewis acid acid strength. strength. PurePure pyridine pyridine shows shows a well a well resolved resolved band band at at1437 1437 cm cm−1 −(Figure1 (Figure 1a)1a).. When When pyridine pyridine was was mixed withmixed the [BMIM]Cl with the- [BMIM]Cl-nAlCl3, [Etn3AlClNH3]Cl, [Et-n3AlNH]Cl-Cl3, andnAlCl [B3Py, and]Cl- [BPy]Cl-nAlCl3 (nnAlCl = 1, 32,( nand= 1, 2.5) 2, and ionic 2.5) liquids, ionic the absorptionliquids, thepeaks absorption appearing peaks at appearing 1448, 144 at9 1448,, 144 1449,9; 14 1449;45, 144 1445,5, 1445,1447; 1447;1449, 1449, 1449, 1449, 1449 1449 cm cm−1− 1were observed,were observed,respectively, respectively, indicating indicating the thepresence presence of ofLewis Lewis acid sites.sites. InIn addition, addition for, for the the [Et NH]Cl-nAlCl , [BMIM]Cl-nAlCl , and [BPy]Cl-nAlCl (n = 1, 2, and 2.5) ionic liquids, the IR [Et3NH3]Cl-nAlCl3, [BMIM]Cl3 -nAlCl3, and3 [BPy]Cl-nAlCl3 (3n = 1, 2, and 2.5) ionic liquids, the IR peaks peaks at 1538 cm−1 attributed to the Brönsted acid sites were also observed, due to the interaction at 1538 cm−1 attributed to the Brönsted acid sites were also observed, due to the interaction between between AlCl3 and water from air [22]. AlCl3 and water from air [22]. 1540 1450 a Pyridine [BPy]Cl-2.5AlCl3 [BPy]Cl-2AlCl3 [BPy]Cl-AlCl3 [Et3NH]Cl-2.5AlCl3 Absorbance [Et3NH]Cl-2AlCl3 [Et3NH]Cl-AlCl3 [BMIM]Cl-2.5AlCl3 [BMIM]Cl-2AlCl3 [BMIM]Cl-AlCl3 1600 1500 1400 1300 1200 1100 -1 Wavenumber (cm ) Figure 1. Cont. Catalysts 2018, 8, 532 3 of 15 Catalysts 2018, 8, x FOR PEER REVIEW 3 of 15 1540 1450 b [BPy]Cl-2.5ZnCl2 [BPy]Cl-2ZnCl2 [BPy]Cl-ZnCl2 [Et3NH]Cl-2.5ZnCl2 Absorbance [Et3NH]Cl-2ZnCl2 [Et3NH]Cl-ZnCl2 [BMIM]Cl-2.5ZnCl2 [BMIM]Cl-2ZnCl2 [BMIM]Cl-ZnCl2 1600 1500 1400 1300 1200 1100 -1 Wavenumber (cm ) 1540 1450 c [BPy]Cl-2.5CuCl [BPy]Cl-2CuCl [BPy]Cl-CuCl [Et3NH]Cl-2.5CuCl Absorbance [Et3NH]Cl-2CuCl [Et3NH]Cl-CuCl [BMIM]Cl-2.5CuCl [BMIM]Cl-2CuCl [BMIM]Cl-CuCl 1600 1500 1400 1300 1200 1100 -1 Wavenumber (cm ) − − FigureFigure 1. Fourier 1. Fourier transform transform infrared infrared spectroscopy spectroscopy (FT–IR)(FT–IR spectra) spectra of (aof) pyridine/Al(a) pyridine/AlnCl 3n+1nCl 3n+1 − anionanion-containing-containing ionic ionic liquids liquids;; (b (b) )pyridine/Zn pyridine/ZnnClCl 2n+12n+1 anionanion-containing-containing anionanion ionic ionic liquids; liquids and; and (c) (c) − pyridine/Cupyridine/CunCl−n+1nCl anionn+1 anion-containing-containing ionic ionic liquids. liquids. The The volume volume ratio ratio of of pyridine to to ionic ionic liquid liquid is 1:1.is 1:1. When the [BMIM]Cl-nZnCl2, [Et3NH]Cl-nZnCl2, and [BPy]Cl-nZnCl2 (n = 1, 2, and 2.5) ionic When the [BMIM]Cl-nZnCl2, [Et3NH]Cl-nZnCl2, and [BPy]Cl-nZnCl2 (n = 1, 2, and 2.5) ionic liquids were mixed with pyridine, IR peaks at 1448, 1449, 1449; 1447, 1448, 1449; 1449, 1449, 1449 cm−1 liquids were mixed with pyridine, IR peaks at 1448, 1449, 1449; 1447, 1448, 1449; 1449, 1449, 1449 were observed (Figure1b). When the [BMIM]Cl- nCuCl, [Et3NH]Cl-nCuCl, and [BPy]Cl-nCuCl (n = 1, −1 cm 2,were and observed 2.5) ionic liquids (Figure were 1b). mixed When with the pyridine, [BMIM]Cl IR peaks-nCuCl, at 1444,[Et3NH 1444,]Cl 1444;-nCu 1446,Cl, and 1446, [B 1446;Py]Cl 1445,-nCuCl (n = 1,1445, 2, and 1445 2.5) cm− ionic1 were liquid observeds were (Figure mixed1c). Thewith results pyridine, indicated IR peaks that only at Lewis1444, acid1444 sites, 144 are4; present1446, 1446, − −1 − 1446;in 144 the5, Zn 144nCl5, 1442n+15, cm and Cu werenCl observedn+1 anion-containing (Figure 1c). ionic The liquids.results indicated that only Lewis acid sites are present in the ZnnCl−2n+1, and CunCl−n+1 anion-containing ionic liquids. According to the blue shift extent of the IR absorption bands attributed to the Lewis acid sites, it could be found that the Lewis acid strengths of the as-synthesized ionic liquids were in an order of ZnnCl−2n+1 anion-containing ionic liquids > AlnCl−3n+1 anion-containing ionic liquids > CunCl−n+1 anion-containing ionic liquids.