Comprehensive analysis of chloroaluminate and composite ionic liquids using direct analysis in real time mass spectrometry

Abstract

<p indent=0mm>A composite chloroaluminate ionic liquid (IL) modified with copper(I) chloride has attracted considerable attention due to its high catalytic activity and selectivity for isobutane alkylation, where the bimetallic anions of Cu(I)-Al(III) have been proposed to play important roles. Characterization of chloroaluminate and composite ionic liquids (CIL) is crucial for a better understanding of alkylation mechanism and catalysts design. In this work, a plasma-based ion source, direct analysis in real time (DART), was introduced to achieve soft but effective ionization of chloroaluminate IL and CIL with minimized hydrolysis, fragmentation and salt effects. Combined with the isotope patterns strategy and high-energy collision dissociation (HCD) technology, this developed high resolution mass spectrometry (HRMS) method, DART-Orbitrap-HRMS, was not only able to identify the accurate chemical composition of chloroaluminate IL and CIL, but also capable of structure analysis. Based on the comparison of chloroaluminate IL and CIL, it showed that similar acidic anions in both ILs were [AlCl₄]⁻, [Al₂Cl₇]⁻ and [Al₃Cl₁₀]⁻. The remarkable difference was the verification of Cu(I)-Al(III) bimetallic anions as [CuAlCl₅]⁻ and [CuAl₂Cl₈]⁻ merely in CIL, which were proposed to count for the high catalytic activity. Moreover, Cu(I) cation ion that might be related to the alkylation selectivity was characterized as [(Et₃NH)₂CuCl₂]⁺ in CIL. Structure analysis of the two bimetallic anions were then achieved by HCD technology. All the results have provided substantial support for the structure-activity study of CIL, setting the stage for in-depth research on IL-catalyzed isobutane alkylation.