Identification of acidic species in chloroaluminate ionic liquid catalysts

• Lewis and Brønsted acidic species were analyzed in chloroaluminate ILs.
• Brønsted acidic Al2Cl6OH− is an active species for alkylation catalysis.
• Rejuvenation with HCl is explained via reversible reaction with non-active Al2Cl5O−.
• Role of water in alkylation catalysis of chloroaluminate ILs is further elucidated.

Chloroaluminate ionic liquids (ILs), especially composite ionic liquids (CILs) which are ILs modified with copper(I) chloride, are highly active and selective catalysts for the alkylation of 2-butene with isobutane. The Lewis and Brønsted acidic species of these ILs were investigated by NMR spectroscopy. Pyridine is found to be a suitable indicator. Lewis acidity arises mainly from Al2Cl7− having a chemical shift at 102 ppm in the 27Al NMR spectrum, while Brønsted acidity arises from Al2Cl6OH− (chemical shift at 97 ppm). The peak at 94 ppm in the 27Al NMR spectrum is related to Al2Cl5O−. These new insights have improved our understanding of the structure of the active species in chloroaluminate ionic liquid alkylation catalysts.

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