Identification of acidic species in chloroaluminate ionic liquid catalysts

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 Bronsted 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 Al-27 NMR spectrum, while Bronsted acidity arises from Al2Cl6OH- (chemical shift at 97 ppm). The peak at 94 ppm in the Al-27 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. (C) 2014 Elsevier Inc. All rights reserved.