Chlorination of Toluene to O-Chlorotoluene Catalyzed by Ionic Liquids

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 to the the Lewis Lewis acid acid site. site. The The IR IR peak peak appearing appearing at ca. at ca. 1540 1540 cm− 1 cmmeans 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, the peaks 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, forthe 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.

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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 (a of) 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. The intensities of those absorption peaks attributed to the Lewis acid sites increased with the increase in anion content, meaning that the Lewis acid amount increased upon increasing the anion content. It was interesting to find that the intensities of the IR peaks at

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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. The intensities of those absorption peaks attributed to the Lewis acid sites increased with the increase in anion content, meaning that the Lewis acid amount increased Catalysts 2018, 8, x FOR PEER REVIEW 4 of 15 upon increasing the anion content. It was interesting to ﬁnd that the intensities of the IR peaks −1 at 1537 cm attributed to the Brönsted acid sites present in [BMIM]Cl-nAlCl3, [Et3NH]Cl-nAlCl3, −1 3 3 3 1537 cm attributed to the Brönsted acid sites present in [BMIM]Cl-nAlCl −, [Et NH]Cl-nAlCl , and and [BPy]Cl-nAlCl3 ionic liquids also increased with the increase in− AlnCl 3n+1 content, indicating [BPy]Cl-nAlCl3 ionic liquids also increased with the increase in AlnCl 3n+1− content, indicating that the that the Brönsted acid amount also increased upon increasing the AlnCl 3n+1 content. Brönsted acid amount also increased upon increasing the AlnCl−3n+1 content. 2.2. Chlorination Reaction Catalyzed by Ionic Liquids 2.2. Chlorination Reaction Catalyzed by Ionic Liquids Figure2 shows the conversions of toluene and the selectivities of o-chlorotoluene, m-chlorotoluene, Figure 2 shows the conversions of toluene and the selectivities of o-chlorotoluene, p-chlorotoluene, benzyl chloride, and dichlorotoluenes in the chlorination reaction between toluene m-chlorotoluene, p-chlorotoluene, benzyl chloride, and dichlorotoluenes in the chlorination reaction and gaseous Cl2 catalyzed by the ionic liquids. between toluene and gaseous Cl2 catalyzed by the ionic liquids.

100 [BMIM]Cl/AlCl3 a 80

0 [Et NH]Cl/AlCl 100 3 3

Conversions and selectivities (%) [BPy]Cl/AlCl 100 3

0 1:1 2:1 2.5:1 Mole ratios of AlCl to [BMIM]Cl, [Et NH]Cl and [BPy]Cl 3 3 Figure 2. Cont.

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100 [BMIM]Cl/ZnCl2 b 80

0 [Et NH]Cl/ZnCl 100 3 2

0 [BPy]Cl/ZnCl Conversions and selectivities (%) 100 2

0 1:1 2:1 2.5:1

Mole ratios of ZnCl to [BMIM]Cl, [Et NH]Cl and [BPy]Cl 2 3

Figure 2. Cont.

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[BMIM]Cl/CuCl 100 c 80

0 100 [Et3NH]Cl/CuCl

Conversions and selectivities (%) 100 [BPy]Cl/CuCl

0 1:1 2:1 2.5:1 Mole ratios of CuCl to [BMIM]Cl, [Et NH]Cl and [BPy]Cl 3

Figure 2. 2. CChlorinationhlorination of oftoluene toluene with with gaseous gaseous Cl2 Clover2 over the ioni thec ionic liquid liquid catalysts catalysts, , with different with different mole ratiosmole ratios of (a) of AlCl (a)3 AlCl, (b) 3 ZnCl,(b) ZnCl2, and2, ( andc) CuCl (c) CuCl to [BMIM]Cl to [BMIM]Cl,, [Et3NH [Et3]ClNH]Cl,, and and[BPy [BPy]Cl,]Cl, respectively. respectively. ■, conversion, conversion of toluene; of toluene; ●, selectivity, selectivity of o- ofchlorotoluene;o-chlorotoluene; ▲, selectivityN, selectivity of m of-chlorotoluene;m-chlorotoluene; ▼, selectivityH, selectivity of p of-chlorotoluene;p-chlorotoluene; ◄,J selectivity, selectivity of of benzyl benzyl chloride; chloride; ►I, ,selectivity selectivity of of dichlorotoluenes. dichlorotoluenes. Reaction conditions: toluene, 0.5 mol;mol; chlorine,chlorine, 2525 mLmL minmin−−1;; catalyst catalyst loading loading,, 3 mol mol/100/100 mol mol toluene; toluene; reaction t temperature,emperature, 80 °◦CC;; reaction time, 8 h.

When the [BMIM]Cl-[BMIM]Cl-nAlCl3,, [ [EtEt3NHNH]Cl-]Cl-nnAlClAlCl33, ,and and [B [BPy]Cl-Py]Cl-nnAlClAlCl33 ionicionic liquids liquids w wereere used used as ◦ the catalyst catalysts,s, after after reacting reacting at 80 °CC for for 8 8 h, h, t thehe conversion conversionss of of toluene toluene increased increased from from 93.7% 93.7% to to 98.9%, 98.9%, 91.9% to 95.2%, andand 87.0%87.0% toto 94.3%,94.3%, respectively, respectively, with with increasing increasing the then valuesn values from from 1 to 1 2.5to 2.5 (Figure (Figure2a). 2a).The T selectivitieshe selectivities of o-chlorotoluene of o-chlorotoluene decreased decreased from from 52.3% 52.3% to 42.5%, to 42.5%, 52.1% 52.1% to 41.6%, to 41.6%, and 45.0% and 45.0% to 42.4%. to 42.4%.The selectivities The selectivities of p-chlorotoluene of p-chlorotoluene decreased decreased from 41.6% from to 41.6% 18.8%, to 39.0% 18.8%, to 39.0% 17.3%, to and 17.3%, 42.5% and to 42.5% 17.3%. toHowever, 17.3%. However, the selectivities the selectivities of dichlorotoluenes of dichlorotoluenes rapidly increased rapidly increased from 1.9% from to 34.1%, 1.9% to 8.1% 34.1%, to 36.1%, 8.1% toand 36.1%, 3.2% and to 35.7%. 3.2% to The 35.7%. selectivities The selectivities of m-chlorotoluene of m-chlorotoluene and benzyl and chloride benzyl werechloride less were than 4.7%less than and 4.7%6.8%, and respectively. 6.8%, respectively. When the [BMIM]Cl-nZnCl2, [Et3NH]Cl-nZnCl2, and [BPy]Cl-nZnCl2 ionic liquids were used as the catalysts, after reacting at 80 °C for 8 h, the conversions of toluene increased from 95.2% to 99.7%,

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When the [BMIM]Cl-nZnCl2, [Et3NH]Cl-nZnCl2, and [BPy]Cl-nZnCl2 ionic liquids were used as the catalysts, after reacting at 80 ◦C for 8 h, the conversions of toluene increased from 95.2% to 99.7%, 98.2% to 99.6%, and 98.5% to 99.4%, respectively, with increasing the n values from 1 to 2.5 (Figure2b). Over the [BMIM]Cl- nZnCl2 catalysts, the selectivities of o-chlorotoluene increased from 63.4% to 65.4% and then decreased to 55.9% with increasing the n values from 1 to 2 and 2.5 while over [Et3NH]Cl-nZnCl2 catalysts, the selectivities of o-chlorotoluene increased from 61.2% to 63.9% and then decreased to 59.8%. When the [BPy]Cl-nZnCl2 ionic liquids were used as the catalysts, the selectivities of o-chlorotoluene decreased from 62.9% to 56.4%. The selectivities of p-chlorotoluene decreased from 26.1% to 21.1%, 27.2% to 25.0%, and 25.4% to 23.6% with increasing the n values. However, the selectivities of dichlorotoluenes increased from 2.6% to 20.8%, 3.8% to 11.3%, and 4.3% to17.9%. The selectivities of m-chlorotoluene and benzyl chloride were less than 4.0% and 5.7%, respectively. When the [BMIM]Cl-nCuCl, [Et3NH]Cl-nCuCl, and [BPy]Cl-nCuCl ionic liquids were used as the catalysts, after reacting at 80 ◦C for 8 h, with increasing the n values of the ionic liquids from 1 to 2.5, the conversions of toluene increased from 89.6% to 94.9%, 90.4% to 93.2%, and 86.6% to 89.0%, respectively (Figure2c). The selectivities of o-chlorotoluene decreased from 58.9% to 44.7%, 48.9% to 47.8%, and 56.2% to 50.1%. The selectivities of p-chlorotoluene decreased from 29.6% to 16.8%, 26.8% to 20.0%, and 34.4% to 21.9%. However, the selectivities of benzyl chloride increased from 6.1% to 31.7%, 20.1% to 27.9%, and 5.8% to 22.4%. The selectivities of m-chlorotoluene and dichlorotoluenes were less than 1.9% and 6.4%, respectively. The results showed that the conversions of toluene over the ionic liquid catalysts 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, being consistent with that of their Lewis acid strengths. The ZnnCl 2n+1 anion-containing ionic liquids with high Lewis acid strength showed high catalytic activity for the − chlorination of toluene with gaseous chlorine. The AlnCl 3n+1 anion-containing ionic liquids favored the chlorination of the resultant monochlorotoluene to dichlorotoluenes as compared to the other ionic liquid catalysts. The explanation for this could be that the co-presence of Brönsted and Lewis acid sites − was beneﬁcial to the formation of dichlorotoluenes. For the CunCl n+1 anion-containing ionic liquids, − − their catalytic activities for the chlorination of toluene were less than the ZnnCl 2n+1 and AlnCl 3n+1 − anion-containing ionic liquids. The explanation for this could be that the CunCl n+1 anion-containing ionic liquids with low Lewis acid strength gave low catalytic activity for the chlorination reaction. − However, benzyl chloride was formed in a large scale while the CunCl n+1 anion-containing ionic liquids were used as the catalysts. The results revealed that the chlorination of toluene to chlorinated − toluene and the chlorination of toluene to benzyl chloride are competitive reactions. The ZnnCl 2n+1 − and AlnCl 3n+1 anion-containing ionic liquids with high Lewis acid strength favored the chlorination of toluene to chlorinated toluene via the electrophilic substitution reaction, suppressing the formation of benzyl chloride via the free radical reaction. It was also found that the toluene conversions and the product selectivities over these ionic liquid catalysts with different cation counterparts but with the same anion counterpart were similar to each other, indicating that the cation counterparts of these ionic liquids had little effect on their catalytic activities in the chlorination reaction.

2.3. Effect of Catalyst Loading

Among the ionic liquids, the [BMIM]Cl-2ZnCl2 ionic liquid exhibited the highest catalytic activity for the formation of o-chlorotoluene. Therefore, it was selected as the model catalyst to investigate the effect of other experimental parameters on the toluene chlorination reaction. The conversions of toluene and the selectivities of products in the catalytic chlorination reaction over the [BMIM]Cl-2ZnCl2 ionic liquid catalyst with different loadings are shown in Figure3. When the mole ratios of [BMIM]Cl-2ZnCl2 ionic liquid to toluene were 1:100, 3:100, and 5:100, the conversions of toluene were 84.4%, 99.7%, and 99.7% and the selectivities of o-chlorotoluene were 65.0%, 65.4%, and 62.1%, respectively. The selectivities of p-chlorotoluene decreased from 26.8% to 24.1% upon increasing the catalyst loading while the selectivities of dichlorotoluenes increased from 2.2% to Catalysts 2018, 8, 532 8 of 15

Catalysts 2018, 8, x FOR PEER REVIEW 8 of 15 10.9%. Only a small amount of m-chlorotoluene and benzyl chloride were formed with the selectivities not only favored the chlorination reaction between toluene and gaseous Cl2, but also caused the of less thanCatalysts 4.4% 2018 and, 8, x FOR 1.7%, PEER REVIEW respectively. The results showed that high catalyst loading8 of 15 not only chlorination of monochlorotoluenes to dichlorotoluenes. favored the chlorination reaction between toluene and gaseous Cl2, but also caused the chlorination of not only favored the chlorination reaction between toluene and gaseous Cl2, but also caused the monochlorotoluenes to dichlorotoluenes. chlorination of monochlorotoluenes to dichlorotoluenes . 100 100 80 80 60

40 40

20 20

Conversion selectivities and (%)

0 0 1 1 22 33 4 4 5 5 Catalyst loadings (mol/100 mol toluene) Catalyst loadings (mol/100 mol toluene) Figure 3. Chlorination of toluene with gaseous Cl2 over the [BMIM]Cl-2ZnCl2 ionic liquid catalyst FigureFigure 3. Chlorination 3. Chlorination of of toluene toluene with with gaseous gaseous ClCl22 over the the [BMIM]Cl [BMIM]Cl-2ZnCl-2ZnCl2 ionic2 ionic liquid liquid catalyst catalyst with different loadings. ■, conversion of toluene; ●, selectivity of o-chlorotoluene; ▲, selectivity of withwith different different loadings. loadings. ,■ conversion, conversion ofof toluene; ●, ,selectivity selectivity of ofo-chlorotoluene;o-chlorotoluene; ▲, selectivity, selectivity of of m-chlorotoluene; ▼, selectivity of p-chlorotoluene; ◄, selectivity of benzyl chloride; ►, selectivityN of m-chlorotoluene;m-chlorotoluene;dichlorotoluenes.H, ▼ selectivity, selectivity Reaction ofconditions: ofp -chlorotoluene;p-chlorotoluene; toluene, 0.5 mol◄J,, selectivity; selectivitychlorine, 2 5of mLof benzyl benzylmin−1 ;chloride; reaction chloride; t emperature,►, selectivityI, selectivity of of dichlorotoluenes.dichlorotoluenes.80 °C; reaction Reaction Reaction time, 8 conditions:h. conditions: toluene, toluene, 0.5 mol mol;; chlorine, chlorine, 25 25 mL mL min min−1; reaction−1; reaction temperature, temperature, 80 ◦C;80 reaction°C; reaction time, time, 8 h. 8 h. 2.4. Effect of Reaction Temperature 2.4. Effect2.4. Effect of Reaction ofThe Reaction conversions Temperature Temperature of toluene and the selectivities of products in the catalytic chlorination reaction Theover conversions the [BMIM]Cl of- 2ZnCltoluene2 ionic and liquidthe selectivities catalyst at of different products reaction in the temperatures catalytic chlorination are shown reaction in The conversionsFigure 4. of toluene and the selectivities of products in the catalytic chlorination reaction over the [BMIM]Cl-2ZnCl2 ionic liquid catalyst at different reaction temperatures are shown in over the [BMIM]Cl-2ZnCl2 ionic liquid catalyst at different reaction temperatures are shown in Figure4. Figure 4.

100

100 80

80 60

60 40

40 20

Conversion selectivities and (%)

0 20 60 80 100

Conversion selectivities and (%) o Reaction temperature ( C)

Figure 4. Chlorination0 of toluene with gaseous Cl2 over the [BMIM]Cl-2ZnCl2 ionic liquid catalyst at 60 80 100 different reaction temperatures. ■, conversion of toluene; ●, selectivity of o-chlorotoluene; ▲, o Reaction temperature ( C)

FigureFigure 4. Chlorination 4. Chlorination of tolueneof toluene with with gaseous gaseous ClCl22 over the the [BMIM]Cl [BMIM]Cl-2ZnCl-2ZnCl2 ionic2 ionic liquid liquid catalyst catalyst at at ■ ▲ differentdifferent reaction reaction temperatures. temperatures., conversion , conversion of toluene; of toluene;, selectivity ●, selectivity of o-chlorotoluene; of o-chlorotoluene;N, selectivity , of m-chlorotoluene; H, selectivity of p-chlorotoluene; J, selectivity of benzyl chloride; I, selectivity −1 of dichlorotoluenes. Reaction conditions: toluene, 0.5 mol; chlorine, 25 mL min ; [BMIM]Cl-2ZnCl2, 3 mol/100 mol toluene; reaction time, 8 h. Catalysts 2018, 8, x FOR PEER REVIEW 9 of 15

selectivity of m-chlorotoluene; ▼, selectivity of p-chlorotoluene; ◄, selectivity of benzyl chloride; ►, Catalystsselectivity2018, 8, 532 of dichlorotoluenes. Reaction conditions: toluene, 0.5 mol; chlorine, 25 mL min−1; 9 of 15

[BMIM]Cl-2ZnCl2, 3 mol/100 mol toluene; reaction time, 8 h.

◦ TolueneToluene was was almost almost completely completely converted converted when when the the reaction reaction temperature temperature was was raised raised to to 80 80 °CC.. TheThe selectivities selectivities of of oo--chlorotoluenechlorotoluene gradually gradually increased increased from from 64.3% 64.3% to to 65.4% 65.4% upon upon increasing increasing the the ◦ ◦ reactionreaction temperatures temperatures from from 60 60 to to 80 80 °C.C. Further Further increasing increasing the thereaction reaction temperature temperature to 100 to ° 100C, theC, selectivitythe selectivity decreased decreased to 62.0%. to 62.0%. The selectivities The selectivities of p-ch oflorotoluenep-chlorotoluene decreased decreased from 26.5 from% to 26.5% 23.5% to w23.5%hile the while selectivities the selectivities of dichlorotoluene of dichlorotolueness increased increased from 4.4 from% to 1 4.4%0.7% toupon 10.7% increasing upon increasing the reaction the ◦ temperaturesreaction temperatures from 60 to from 100 60°C. to The 100 selectivitiesC. The selectivities of m-chlorotoluene of m-chlorotoluene and benzyl and chloride benzyl were chloride less thanwere 4.0 less% thanand 1.6%, 4.0% andrespectively 1.6%, respectively.. The results The showed results that showed the [BMIM]Cl that the [BMIM]Cl-2ZnCl-2ZnCl2 ionic liquid2 ionic catalyst liquid hadcatalyst stable had activity stable activity for the for catalytic the catalytic chlorination chlorination of toluene of toluene to too-chlorotolueneo-chlorotoluene at at the the reaction reaction ◦ temperaturestemperatures ranging ranging from from 60 60 to to 100 100 °C.C. 2.5. Effect of Reaction Time 2.5. Effect of Reaction Time The toluene conversions and the product selectivities at 80 ◦C and different reaction time The toluene conversions and the product selectivities at 80 °C and different reaction time periods are shown in Figure5. Upon prolonging the reaction time to 8 h, toluene was almost periods are shown in Figure 5. Upon prolonging the reaction time to 8 h, toluene was almost completely converted. When the reaction time periods ranged from 2 to 8 h, the selectivities of completely converted. When the reaction time periods ranged from 2 to 8 h, the selectivities of o-chlorotoluene, m-chlorotoluene, p-chlorotoluene, and dichlorotoluenes were around 65%, 5%, 26%, o-chlorotoluene, m-chlorotoluene, p-chlorotoluene, and dichlorotoluenes were around 65%, 5%, 26%, and 4%, respectively. The selectivity of benzyl chloride was less than 2.3%. The results showed that and 4%, respectively. The selectivity of benzyl chloride was less than 2.3%. The results showed that reaction time had little effect on the product selectivity. reaction time had little effect on the product selectivity.

100

ersion selectivities and (%) 20

Conv

0 0 2 4 6 8

Reaction time (h)

FigureFigure 5. 5. CChlorinationhlorination of of t tolueneoluene with with gaseous Cl 2 overover the the [BMIM]Cl [BMIM]Cl-2ZnCl-2ZnCl22 ionicionic liquid liquid catalyst catalyst at at differentdifferent reaction reaction times. times. ■, ,conversion conversion of of toluene; toluene; ●, ,selectivity selectivity of of o-ochlorotoluene;-chlorotoluene; ▲N, ,selectivity selectivity of of mm--chlorotoluene;chlorotoluene; ▼H,, selectivity selectivity of of pp--chlorotoluene;chlorotoluene; ◄J,, selectivity selectivity of of benzyl benzyl chloride; chloride; ►I,, selectivity selectivity of of −1−1 dichlorotoluenes.dichlorotoluenes. Reaction Reaction conditions: conditions: toluene, toluene, 0.5 0.5 mol mol;; chlorine, chlorine, 25 25 mL mL min min; [BMIM]Cl; [BMIM]Cl-2ZnCl-2ZnCl2, 32 , mol3 mol/100/100 mol mol toluene; toluene; reaction reaction temperature, temperature, 80 80°C.◦ C.

22.6..6. Recycling Recycling Performance Performance To investigate the recycling performance of the [BMIM]Cl-2ZnCl ionic liquid catalyst, To investigate the recycling performance of the [BMIM]Cl-2ZnCl2 ionic2 liquid catalyst, the recyclingthe recycling experiment experimentss were werealso conducted. also conducted. After reaction, After reaction, the reaction the reaction mixture mixture and the and ionic the liqui ionicd catalystliquid catalyst were separated were separated with a withfunnel. a funnel. The recovered The recovered ionic liquid ionic liquidwas reused was reused as the ascatalyst the catalyst for next for run.next Figure run. Figure 6 shows6 shows the recycling the recycling performance performance of the of ionic the ionicliquid liquid catalyst. catalyst.

Catalysts 2018, 8, 532 10 of 15

Catalysts 2018, 8, x FOR PEER REVIEW 10 of 15

100 a

)

% ( 80

Conversion selectivities and

0 1 2 3 4 5

Recycling times

1540 1450 b

Absorbance Fresh

after 5 recycling times

1600 1500 1400 1300 1200 1100 -1 Wavenumber (cm )

Figure 6. (a) Recycling performance of thethe [BMIM]Cl-2ZnCl[BMIM]Cl-2ZnCl22 ionicionic liquid liquid catalyst catalyst in in the c chlorinationhlorination 2 ■ ▲ of t tolueneoluene with gaseous Cl 2.. , ,conversion conversion of of toluene; toluene; ●, ,selectivity selectivity of of oo--chlorotoluene;chlorotoluene; N,, selectivity selectivity of m--chlorotoluene;chlorotoluene; ▼H, selectivity of p--chlorotoluene;chlorotoluene; ◄J, selectivity of of benzyl benzyl chloride; chloride; ►I, selectivity −1−1 of dichlorotoluenes. Reaction Reaction conditions: conditions: toluene, toluene, 0.5 0.5 mol mol;; chlorine, chlorine, 2 255 mL mL min min ; [BMIM]Cl; [BMIM]Cl-2ZnCl-2ZnCl2, 23, mol3 mol/100/100 mol mol toluene; toluene; reaction reaction temperature, temperature, 80 80 °C◦;C; reaction reaction time, time, 8 8 h; h; (b (b) )FT FT–IR–IR spectra spectra of

pyridine/fresh [BMIM]Cl [BMIM]Cl-2ZnCl-2ZnCl2 2ionicionic liquid liquid and pyridine/spent pyridine/spent [BMIM]Cl [BMIM]Cl-2ZnCl-2ZnCl22 ionicionic liquid recyclrecycleded for 5 times. The v volumeolume ratio of pyridine to ionic liquid is 1:1.

As shown in Figure Figure 66a,a, thethe conversionsconversions ofof toluenetoluene slightlyslightly decreaseddecreased fromfrom 99.7%99.7% toto 92.5%92.5% whenwhen the [BMIM]Cl-2ZnCl[BMIM]Cl-2ZnCl2 ionic2 ionic liquid liquid catalyst catalyst was recycled was recycled ﬁve times. five The selectivities times. The of selectivitieso-chlorotoluene of oslightly-chlorotoluene decreased slightly from 65.4%decreased to 62.6% from while 65.4% the to selectivities 62.6% while of thep-chlorotoluene selectivities of slightly p-chlorotoluene increased slightlyfrom 26.0% increased to 29.0%. from The 26.0 selectivities% to 29.0 of%m. T-chlorotoluene,he selectivities benzyl of m-chlorotoluene, chloride, and dichlorotoluenesbenzyl chloride, wereand dichlorotoluenearound 3.5%, 2.0%,s were and 4.0%,around respectively. 3.5%, 2.0%, The and results 4.0%, showed respectively. that the The [BMIM]Cl-2ZnCl results showed2 ionic that liquid the [BMIM]Clcatalyst had-2ZnCl good2 recyclingionic liquid performance catalyst had for good the chlorinationrecycling performance reaction. for the chlorination reaction. When pyridine was mixed with the fresh [BMIM]Cl-2ZnCl2 ionic liquid catalyst or the spent [BMIM]Cl-2ZnCl2 ionic liquid catalyst was recycled 5 times, the IR absorption peaks at 1449 cm−1

Catalysts 2018, 8, 532 11 of 15

CatalystsWhen 2018,pyridine 8, x FOR PEER was REVIEW mixed with the fresh [BMIM]Cl-2ZnCl2 ionic liquid catalyst or the11 spent of 15 −1 [BMIM]Cl-2ZnCl2 ionic liquid catalyst was recycled 5 times, the IR absorption peaks at 1449 cm werewere observedobserved inin bothboth samplessamples (Figure(Figure6 b),6b), indicating indicating that that the the [BMIM]Cl-2ZnCl [BMIM]Cl-2ZnCl22 ionicionic liquidliquid catalystcatalyst waswas stablestable inin thethe chlorinationchlorination ofof toluene.toluene. ItIt waswas reportedreported thatthat K-LK-L zeolitezeolite catalystscatalysts couldcould catalyzecatalyze thethe toluenetoluene chlorinationchlorination withwith gaseousgaseous ◦ ClCl22 at 70–8070–80 °C to oo-chlorotoluene-chlorotoluene with selectivitiesselectivities in aa rangerange ofof 20–49%20–49% [[6,8].6,8]. WhenWhen NaK-LNaK-L zeolitezeolite catalyzedcatalyzed the toluene chlorination with with the the use use of of the the expensive expensive SO SO2Cl2Cl2 as2 as the the chlorinating chlorinating agent agent at at50 50°C◦, C,p-chlorotoluenep-chlorotoluene was was favorably favorably formed formed with with the p the-chlorotoluene/p-chlorotoluene/o-chlorotolueneo-chlorotoluene ratio ratioof more of morethan 3:1 than [7]. 3:1 As [7 compared]. As compared with the with previous the previous work, work,it wasit clear was that clear the that [BMIM]Cl the [BMIM]Cl-2ZnCl-2ZnCl2 ionic2 liquidionic liquidcatalyst catalyst exhibited exhibited excellent excellent catalytic catalytic activity activity for for the the toluene toluene chlorination chlorination with with gaseous gaseous Cl2 toto oo-chlorotoluene.-chlorotoluene.

2.7.2.7. ReactionReaction RoutesRoutes ChlorinationChlorination of of toluene toluene is a is typical a typical electrophilic electrophilic substitution substitution reaction. reaction. The methyl The m groupethyl of group toluene of astoluene the electron-donating as the electron-donating substitute substitute can activate can the activate phenyl the ring, phenyl being beneﬁcial ring, being to benefithe electrophiliccial to the phenylelectrophilic chlorination phenyl reaction chlorination [7,8]. o -Chlorotoluene reaction [7,8]. ando-Cphlorotoluene-chlorotoluene and are easilyp-chlorotoluene formed in the are toluene easily chlorinationformed in the reaction toluene because chlorination methyl reaction is a usual becauseortho /methylpara directing is a usual group. ortho/para directing group. TheThe reactionreaction routesroutes inin thethe toluenetoluene chlorinationchlorination withwith gaseousgaseous ClCl22 catalyzedcatalyzed byby thethe ionicionic liquidliquid catalystcatalyst areare suggestedsuggested asas SchemeScheme1 1. .

1. Chlorination reaction by the electrophilic substitution.

2. Chlorination reaction by the free radical substitution

SchemeScheme 1. 1. The reaction routes inin thethe chlorinationchlorination ofof toluenetoluene withwith gaseousgaseous chlorinechlorine overover thethe ionicionic liquidliquid catalyst.catalyst.

− Firstly,Firstly, chlorinechlorine molecules were polarized polarized by by the the ionic ionic liquid liquid catalyst catalyst to to form form Cl Cl− anionsanions and and Cl+ Clcations+ cations.. The The positive positive Cl+ Clcation+ cation as asan anelectrophile electrophile attacked attacked the the phenyl phenyl ring ring of of toluene toluene to to form aa − ππ-complex-complex whilewhile thethe ClCl− anionanion was was harbored harbored by the ionic liquid catalyst to form a chlorinated anion, − ILIL…Cl... Cl−. Then,. Then, the the π-πcomplex-complex evolved evolved into into a a σσ--complex.complex. Finally, Finally, the the reactive intermediate intermediate was was converted to monochlorotoluene by the deprotonation. The resulting proton reacted with the IL…Cl− anion to form HCl and the Lewis acidic ionic liquid catalyst was recovered. The monochlorotoluenes could be further chlorinated to form dichlorotoluenes. Benzyl chloride was produced as a by-product via the free-radical reaction [6].

Catalysts 2018, 8, 532 12 of 15 converted to monochlorotoluene by the deprotonation. The resulting proton reacted with the IL ... Cl− anion to form HCl and the Lewis acidic ionic liquid catalyst was recovered. The monochlorotoluenes could be further chlorinated to form dichlorotoluenes. Benzyl chloride was produced as a by-product via the free-radical reaction [6]. − We suggested that the ZnnCl 2n+1 anion-containing ionic liquids with a higher Lewis acid strength favored the chlorination of toluene to o-chlorotoluene. The co-presence of the Lewis and − Brönsted acid sites in the AlnCl 3n+1 anion-containing ionic liquids favored the further chlorination of monochlorotoluene to dichlorotoluene. The reactions of toluene chlorination to chlorotoluene − and benzyl chloride are competitive. Therefore, when the CunCl n+1 anion-containing ionic liquids with weak Lewis acid strength were used as the catalysts, more benzyl chloride was formed via the free-radical substitution reaction.

3. Experimental

3.1. Materials

Liquid Cl2 of industrial grade was purchased from Jiangsu SOPO Co., Ltd. Zhenjiang, China. N-Methylimidazole with the purity of 99% was supplied by Yancheng Medical Factory, Yancheng, China. Anhydrous AlCl3, CuCl, ZnCl2, n-chlorobutane, triethylamine, pyridine, ethyl acetate, toluene, hydrochloric acid, and isopropanol were purchased from the Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All the chemicals were of reagent grade and were used as received without further puriﬁcation.

3.2. Synthesis of Ionic Liquids

3.2.1. Synthesis of 1-Butyl-3-methylimidazolium Chloride ([BMIM]Cl) According to the reported method [23,24], [BMIM]Cl was synthesized as follows. 1.7 mol of n-chlorobutane and 1.5 mol of N-methylimidazole were added into a three-necked flask (500 mL) under stirring. A nitrogen stream at a flow rate of 10 mL min−1 was introduced into the flask. The reaction was carried out under reflux for 48 h. When the reaction mixture was cooled to 0 ◦C, the [BMIM]Cl turned to be solid. The as-synthesized [BMIM]Cl was filtrated and repeatedly washed with ethyl acetate at least two times. The remaining ethyl acetate was evaporated at 80 ◦C from [BMIM]Cl. The [BMIM]Cl sample was dried at 70 ◦C for 24 h in a vacuum oven.

3.2.2. Synthesis of Triethylammonium Chloride ([Et3NH]Cl) The synthesis of [Et3NH]Cl was done according to the method reported in reference [25]. The synthesis procedures are briefly stated as follows. Firstly, triethylamine (1.5 mol) and concentrated hydrochloric acid (1.7 mol) were added into a three-necked flask (500 mL). The reaction was carried out at ambient temperature for 3 h under stirring. After reaction, the reaction mixture was filtrated and repeatedly washed with anhydrous ethanol at least two times. The remaining ethanol was evaporated at 80 ◦C from [Et3NH]Cl. The prepared [Et3NH]Cl sample was dried at 70 ◦C for 24 h in a vacuum oven.

3.2.3. Synthesis of 1-Butylpyridinium Chloride ([BPy]Cl) [BPy]Cl was synthesized according to the method reported in reference [15]. The synthesis procedures are briefly stated as follows. Firstly, pyridine (1.5 mol) and n-chlorobutane (1.7 mol) were added into a 500 mL flask under stirring. A N2 stream continuously flowed through the reaction system at a flow rate of 10 mL min–1. In a dark fume hood, the reaction took place under reflux for 72 h. After reaction, the reaction mixture was cooled to room temperature and [BPy]Cl crystallized. The [BPy]Cl sample was filtrated and washed with ethyl acetate at least twice. The remained ethyl Catalysts 2018, 8, 532 13 of 15 acetate was evaporated at 80 ◦C from the [BPy]Cl sample. The as-prepared [BPy]Cl sample was dried at 70 ◦C for 24 h in a vacuum oven.

3.2.4. Synthesis of Ionic Liquids The ionic liquids were synthesized by using [BMIM]Cl, [Et3NH]Cl, [BPy]Cl, AlCl3, ZnCl2, and CuCl as the raw materials. The preparation method of [BMIM]Cl-nAlCl3 is brieﬂy described as follows. In a nitrogen atmosphere, a certain amount of anhydrous AlCl3 was added slowly into a round-bottom ﬂask containing [BMIM]Cl under stirring. The reaction was carried out in a nitrogen atmosphere at 120 ◦C for 2 h. The [BMIM]Cl-nAlCl3 ionic liquids with the n values of 1, 2, and 2.5 were synthesized by changing the AlCl3/[BMIM]Cl ratios. The synthesis procedures of other ionic liquids were the same as those described above. The synthesized ionic liquid samples were stored in a desiccator.

3.3. Determination of Lewis and Brönsted Acid Strengths of Ionic Liquids The Lewis acid strength of the ionic liquids was measured according to the blue shift of pyridine infrared (IR) absorption band at ca. 1450 cm−1 and the Brönsted acid amount was determined according to the intensity of the band at ca. 1540 cm−1 [22,26]. For the FT-IR analysis, all the samples were prepared by mixing pyridine and ionic liquid with a volume ratio of 1:1. The FT-IR spectra of the samples were recorded on a FT-IR spectrophotometer (Nicolet Nexus 470, Thermo Fisher Scientiﬁc, Waltham, MA, USA) at ambient temperature.

3.4. Chlorination Reaction 0.5 mol of toluene and a given amount of ionic liquid catalyst were added into a 100 mL four-necked round bottom flask, which was equipped with a thermometer, a chlorine inlet tube, a condenser, and a magnetic stirrer. Before the chlorination reaction, a nitrogen stream at a flow rate of 20 mL min−1 was introduced into the reactor to replace air inside. When the reaction solution was heated to a prescribed reaction temperature under stirring at 100 rpm, gaseous Cl2 dried by passing through a concentrated sulfuric acid solution was introduced into the reactor at 25 mL min−1. The exhaust gas was absorbed in a NaOH aqueous solution. In order to avoid the photon-initiated free-radical chlorination, the chlorination reaction was carried out in a dark fume hood. The compositions of the reaction mixtures were analyzed on a gas chromatograph, equipped with a KR-ELJB capillary column and a flame ionization detector [27]. The conversion of toluene and the selectivities of o-chlorotoluene, p-chlorotoluene, m-chlorotoluene, dichlorotoluene, and benzyl chloride were calculated according to the following equations.

[Toluene]in − [Toluene]out Conversion(Toluene) = × 100% (1) [Toluene]in

[Product]out Selective(Product) = × 100% (2) [Toluene]in − [Toluene]out

4. Conclusions

When the ionic liquids with the cation counterparts, [BMIM], [Et3NH], and [BPy] and the anion − − − counterparts, AlnCl 3n+1, ZnnCl 2n+1, and CunCl n+1 were used as catalysts for the chlorination of toluene with Cl2 to o-chlorotoluene, the Lewis acid sites present in the ionic liquids catalyzed the chlorination reaction. The selectivity of o-chlorotoluene was much higher than those of p-chlorotoluene, − m-chlorotoluene, and benzyl chloride. The ZnnCl 2n+1 anion-containing ionic liquids showed high catalytic activity and selectivity for the chlorination of toluene to o-chlorotoluene. Dichlorotoluenes − were favorably formed when the AlnCl 3n+1 anion-containing ionic liquids were used as catalysts. Catalysts 2018, 8, 532 14 of 15

− When the chlorination reaction was catalyzed by the CunCl n+1 anion-containing ionic liquids, benzyl chloride was easily formed.

Author Contributions: Conceptualization, H.Y.; Investigation, X.Z., Y.F., X.H. and A.W. Project administration, H.Y.; Supervision, H.Y.; Methodology, H.Y. and A.W.; Funding acquisition, A.W. Funding: This research was funded by “National Natural Science Foundation of China, grant number 21506078” and “Science and Technology Department of Jiangsu Province, grant number BY2014123-10”. Acknowledgments: This work was financially supported by the research fund from Science and Technology Department of Jiangsu Province (BY2014123-10), the National Natural Science Foundation of China (21506078) and China Postdoctoral Science Foundation (2016M601739). Conflicts of Interest: The authors declare no conflict of interest.

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