| Summary: | The acceptor properties of mixed chlorometallate ionic liquids for isobutane-butene alkylation (C<sub>4</sub> alkylation) reaction were studied. These ionic liquids were prepared by mixing metal chlorides with either triethylamine hydrochloride or 1-butyl-3-methylimidazolium chloride in various molar ratios. Using triethylphosphine oxide as a probe, Gutmann Acceptor Numbers (AN) of the catalysts were determined, and the Lewis acidity of mixed chlorometallate ionic liquids was quantitatively measured. Additionally, AN value was developed to determine the relationship between Lewis acidity and catalytic selectivity. The favorite AN value for the C<sub>4</sub> alkylation reaction should be around 93.0. The [(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>NH]Cl⁻AlCl<sub>3</sub>−CuCl appears to be more Lewis acidity than that of [(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>NH]Cl⁻AlCl<sub>3</sub>. The correlation of the acceptor numbers to speciation of the mixed chlorometallate ionic liquids has also been investigated. [AlCl<sub>4</sub>]<sup>−</sup>, [Al<sub>2</sub>Cl<sub>7</sub>]<sup>−</sup>, and [MAlCl<sub>5</sub>]<sup>−</sup> (M = Cu, Ag) are the main anionic species of the mixed chlorometallate ILs. While the presence of [(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>N·M]<sup>+</sup> cation always decreases the acidity of the [(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>NH]Cl−AlCl<sub>3</sub>−MCl ionic liquids.
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