Durability enhanced ionic liquid catalyst for Friedel–Crafts reaction between benzene and 1-dodecene: insight into catalyst deactivation

Abstract

Friedel–Crafts alkylation of benzene with 1-dodecene, which is an important reaction of synthetic detergent, was studied using the catalyst [bmim][TFSI]/AlCl₃ (1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide/AlCl₃). These ionic liquid catalysts show biphasic behaviors at a specific condition. Active site species and Lewis acidity were determined by NMR and acetonitrile probe FT-IR. NMR spectroscopy investigations indicated that “coordinated” and “uncoordinated” [TFSI]⁻ existed in the ionic liquid catalyst system simultaneously, while tetrahedral 4-coordinate Al₂Cl₇⁻ was the main active species for Friedel–Crafts alkylation. The complex [AlCl_x(TFSI)_y]⁻ tends to decompose and release 4-coordinated Al₂Cl₇⁻ and [TFSI]⁻. The phase behavior study of the ionic liquid [bmim][TFSI]/AlCl₃ demonstrated that the upper phase could be used as an efficient heterogeneous catalyst when AlCl₃/IL ≥ 1.5. Due to the presence of the anion [TFSI]⁻ around the active sites, especially [TFSI]⁻ at the “uncoordinated” state, the ionic liquid formed a stable and hydrophobic chemical environment, which enhanced catalyst durability. The influence of various reaction conditions including catalyst reusability on the alkylation reaction was studied, and the potential reasons for catalyst deactivation were discussed. The highest 2-LAB selectivity was more than 50% when 1-dodecene conversion was nearly 100%. Compared with liquid or solid acid catalyst, ionic liquid catalysts have many advantages concerning energy and the environment.