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Quantitative characterization of Lewis acidity and activity of chloroaluminate ionic liquids Xuan Zhang, Rui Zhang, Haiyan Liu, Xianghai Meng, Chunming Xu, Zhichang Liu, and Peter A.A. Klusener Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02465 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 3, 2016
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Quantitative characterization of Lewis acidity and activity of chloroaluminate ionic liquids Xuan Zhanga, †, Rui Zhanga, †, Haiyan Liua, Xianghai Menga, Chunming Xua, Zhichang Liua, *, Peter A.A. Klusenerb a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, China b
Shell Global Solutions International B.V., Amsterdam 1031 HW, The Netherlands
KEYWORDS: Chloroaluminate ionic liquids; Pyridine; Quantitative characterization; Lewis acidity; Catalytic activity.
ABSTRACT: Pyridine can act as a basic indicator for both Brønsted and Lewis acids forming different types of bonds. Infrared spectroscopy, in combination with
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Al NMR spectroscopy,
was employed to study the interaction mechanism of pyridine and chloroaluminate ionic liquids (ILs). The acid type, acid amount, acidity and even activity of chloroaluminate ILs could be determined. An in-situ infrared complexometric titration was developed as a quantitative characterization method for determining the different acidities and catalyst activity. The method is based on the principle that Lewis acidic chloroaluminate [Al2Cl7]ˉ anion reacts with pyridine to form the [AlCl4]ˉ anion and pyridine-AlCl3 ([PyAlCl3]) complexes, while Brønsted acid forms a
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pyridinium salt. Both product types showed characteristic shifts of the pyridine peaks in the infrared spectrum.
INTRODUCTION Alkylate is an ideal blending component for gasoline, and its blending percentage in the
gasoline pool becomes higher with the upgrading standard of gasoline1. Concentrated sulfuric acid and hydrofluoric acid are commercial catalysts for isobutane alkylation, but both catalysts have some drawbacks.2, 3 Acidic ILs have been successfully used as catalysts in various acidcatalyzed reactions,4-8 such as alkylation,9-12 polymerization13 and esterification.14,
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The
excellent catalytic performance makes them qualified substitutes for concentrated sulfuric acid and hydrofluoric acid. Chloroaluminate ILs combine both Brønsted acidity and Lewis acidity and are effectively used to catalyze isobutane alkylation.16-18 An IL-catalyzed isobutane alkylation plant with a scale of 100 kt/y was successfully started up in 2013.19 The reactions catalyzed by chloroaluminate ILs mainly follow a carbonium ion mechanism. During reactions, Brønsted acid in chloroaluminate ILs triggers the generation of carbocations, while Lewis acid maintains the carbocations' chain reaction and serves as the basis for the Brønsted acidity.19 It is the synergistic action between Brønsted and Lewis acids that enables isobutane alkylation. The composition and structure of chloroaluminate ILs, especially those of the anions, determine the acidity type and strength. Thus by studying the anion composition, structure and consequently the qualitative and quantitative characterization of the Lewis and Brønsted acids in chloroaluminate ILs, the aim is to better understand the reaction mechanism, the catalytic activity and to build correlations between the acidity and activity of these ILs. Chloroaluminate ILs are sensitive to water; and trace amounts of water, such as moisture in the air, can lead to the formation of HCl and oxygen-containing
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compounds.20 This water sensitivity requires serious attention in the quantitative characterization and the acidity should be detected under the protection of dry nitrogen. Several studies of the structure and acidity of chloroaluminate ILs have been reported. Nuclear magnetic resonance (NMR),21-23 infrared spectroscopy (IR)24, 25 and mass spectrometry (MS)26, 27 have been applied to determine the structure of chloroaluminate ILs. Gale et al. employed Raman spectroscopy to study 1-butylpyridinum-xAlCl3 ILs.28 They found that the relative peak areas of [Al2Cl7]ˉ to [AlCl4]ˉ can be used to determine the acidity of 1-butylpyridinum-xAlCl3 ILs. However, the Raman signal of [Al2Cl7]ˉ to [AlCl4]ˉ often interfere with the fluorescence signals arising from the impurities in the organic part of ILs. Gray et al.21 and Wilkes et al.22 used
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Al NMR spectroscopy to study the relationship
between the type and content of anions in ILs and the structure of ILs. It was found that anions in chloroaluminate ILs existed as [Al2Cl7]ˉ and [AlCl4]ˉ, whereby the half-peak width of the
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Al
NMR spectra signal narrows with decreasing [Al2Cl7]ˉ / [AlCl4]ˉmolar ratio. Thus the relative content of [Al2Cl7]ˉ can be estimated from the half-peak width. Consistent results were obtained in our own experiments.29 However, this method is unsuitable for the determination of the absolute content of [AlCl4]ˉ or [Al2Cl7]ˉ in ILs. Smith et al. studied the acidity of HCl in chloroaluminate ILs by ultraviolet-visible (UV-vis) spectroscopy using aromatic hydrocarbons as indicators.30, 31 Thomazeau et al. also determined the acidity of imidazolium ILs containing different anions by UV-vis spectroscopy using a series of Hammett indicators.32 Osteryoung et al.33,
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and Swadźba-Kwaśny et al.35,
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measured the Gutmann Acceptor
Number (AN) of chlorometallate (III) ILs using the chemical shift in 31P NMR spectroscopy of triethylphosphine oxide (TEPO) as probe molecule. TEPO has the ability to form a complex with
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even very weak Lewis acids. However, the affinity of TEPO to AlCl3 is so high that it is not a suitable probe molecule to differentiate among different chloroaluminates including different molar ratios of AlCl3. Taylor et al. studied the Lewis acidity and hydrogen bond basicity of halometallate-based ILs using the method of X-ray photoelectron spectroscopy (XPS).37 The XPS method can predict the solvent parameters and identify the anion speciation with varying metal halide content. However, the XPS method is not measuring the bulk property, but the surface of the ILs layer on the waiver. Knözinger et al.38 and Kou et al.39 used pyridine or acetonitrile as probe molecules and determined the acidity of solid acids and imidazolium ILs by infrared spectroscopy. Although this method is applicable for the determination of acid type and relative acidity of ILs, even for partially deactivated ILs, there are serious concerns and disadvantages. Firstly, determination is carried out by the traditional KBr tabletting method and the shifts of the Brønsted acid (1540 cm1
) and Lewis acid (1450 cm-1) peaks are examined after the interaction between pyridine and ILs
at a specific ratio; so it will take a lot of effort to make a titration curve. Secondly, the method does not determine the interaction mechanism between pyridine and different acid types in chloroaluminate ILs and ignores the possibility of formation of various Al compounds of pyridine and ILs. For chloroaluminate ILs such as Et3NHCl-xAlCl3 (x is the molar ratio of AlCl3 to Et3NHCl), the structure of anions in ILs changes according to the variation of x. For x increasing from 0 to 2 the anions will change from Clˉ to [AlCl4]ˉ to [Al2Cl7]ˉ, whereby the anions always will be present as equilibrium mixtures, with [Al2Cl7]ˉ is predominant when x=2, [Al2Cl7]ˉ and [AlCl4]ˉ are mainly present when 1
