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.
Highlights
Acidity of Ionic LiquidPyridine is commonly used as a molecular probe to determine the Lewis and Brönsted acidities
Introduction oChlorotoluene as an important raw material has been widely used in the production of pharmaceuticals, pesticides, spices, and dyes [1,2,3,4,5]. o-Chlorotoluene has been commercially produced by the chlorination of toluene with gaseous chlorine over conventional Lewis acid catalysts, such asFeCl3 and AlCl3, accompanied by the formation of m-chlorotoluene, p-chlorotoluene, benzyl chloride, and dichlorotoluenes with high yields [6,7,8]
The results showed that the conversions of toluene over the ionic liquid catalysts were in an order of Znn Cl− 2n+1 anion-containing ionic liquids > Aln Cl− 3n+1 anion-containing ionic liquids > Cun Cl− n+1 anion-containing ionic liquids, being consistent with that of their Lewis acid strengths
Summary
Pyridine is commonly used as a molecular probe to determine the Lewis and Brönsted acidities. −1 at 1538 cm attributed to the Brönsted acid sites were observed, due to the interaction between between AlCl3 and water from air [22]. Lewis acid sites the Znn Cl 2n+1 , and Cun Cl n+1 anion-containing ionic liquids. 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. 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.
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