Group IIIA Halometallate Ionic Liquids: Speciation and Applications in

Review pubs.acs.org/acscatalysis

Group IIIA Halometallate Ionic Liquids: Speciation and Applications in Catalysis Rajkumar Kore,† Paula Berton,†,‡ Steven P. Kelley,†,‡ Pavankumar Aduri,§ Sanjeev S. Katti,§ and Robin D. Rogers*,†,∥ †

Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States Department of Chemistry, McGill University, Montreal, Quebec H3A 0B8, Canada § Reliance Industries Limited, Navi Mumbai, Maharashtra 400701, India ∥ 525 Solutions, Inc., 720 second Street, Tuscaloosa, Alabama 35401, United States ‡

ABSTRACT: Group IIIA halometallate ionic liquids (ILs) present fascinating properties for the field of catalysis, particularly through the ability to tune their Lewis acidity solely by changing the metal complex speciation. In this Review, we present a critical perspective on the use of Group IIIA halide-derived ILs in catalysis, focusing on the effect of speciation of the metal-containing ions on various acid-catalyzed reactions, some of which are applied industrially. We summarize all applications of Group IIIA halometallates in catalysis (where they are notably well-represented in reactions of importance in petroleum refining and processing), compare the authors’ investigations or assumptions with regard to chemical speciation, and present examples of how the tunability of these materials is used to overcome their initially perceived drawbacks. Further, advances in the field of halometallate ILs such as the role of the cations in the IL, IL analogues, and heterogenization strategies are discussed. High selectivity, reactivity, and stability are the cornerstones of the ideal catalyst, and the journey of catalysis research toward the ideal catalyst will be possible only with rational catalyst design and innovative thinking. KEYWORDS: chloroaluminate ionic liquids, group IIIA halometallate ionic liquids, Lewis acidic ionic liquids, speciation, catalysis

1. INTRODUCTION Halometallate compounds are useful as catalysts in many industrially important processes. Petroleum refinery processes in particular, such as alkylation, acylation, isomerization, polymerization, and Diels−Alder reactions, are conventionally catalyzed by aluminum chloride (AlCl3)/hydrofluoric acid (HF)-based catalysts.1−4 However, due to shortcomings including environmental concerns,5 corrosiveness, difficulties in isolation, and low selectivity,6,7 these catalysts are now less preferred. To overcome these difficulties, heterogeneous catalysts such as zeolites are being used, but these have the disadvantage of catalyst deactivation during the process due to coke formation.8 Halometallate ionic liquids (ILs), formed by reacting a metal halide with an organic halide salt, show interesting properties such as strong Lewis acidity,9,10 paramagnetism,11 and novel electrochemical properties,12 depending on the identity and concentration of the metal ions.13 As a consequence of their tunable properties, these ILs have been historically explored in electrochemistry, catalysis, and separation processes, although other uses (e.g., as soft materials or for biomass processing) are also reported.11,13−16 Halometallate ILs provide the high Lewis acidity of conventional homogeneous catalysts in a liquid form which is often immiscible with the product phase, thus © 2017 American Chemical Society

eliminating the need for additional solvents and often for deacidification steps.13,17 The history of halometallate ILs dates back to 1948, when a chloroaluminate IL, resulting from the mixture of the solids pyridinium chloride ([PyrH]Cl) and AlCl3, was utilized for electroplating.18,19 As summarized by Wilkes, extensive investigation was carried out on molten chloroaluminate salts during the 1960s and 70s.20 During that period, the U.S. Air Force Academy selected the alkali chloride-AlCl3 system as a low melting molten salt electrolyte for batteries, while the research group led by King adopted [C2Pyr]Br-AlCl3 ([C2pyr] = N-ethylpyridinium) mixture, both systems now known as ILs.21 Later, the molten salt systems [C4Pyr]Cl-AlCl3 ([C4pyr] = N-butylpyridinium) and dialkylimidazolium chloride-AlCl3 were made in wide composition ranges and investigated as new electrolytes and electrodeposition media.22,23 Air-stable chlorogallate, chloroindate, and chlorothallate ILs have since been reported, overcoming drawbacks observed in AlCl3-based ILs, including hygroscopicity and air-sensitivity.14,24−26 Received: June 1, 2017 Revised: August 24, 2017 Published: August 28, 2017 7014

DOI: 10.1021/acscatal.7b01793 ACS Catal. 2017, 7, 7014−7028

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ACS Catalysis Table 1. Anionic Speciation in Group IIIA Chlorometallate ILs (Arrows Indicate Increasing Lewis Acidity)

a

n.d.: not determined; bppt: precipitate; cnot reported.

of organic transformations, some of which are applied industrially. Further, an overview of the upcoming concept of adduct-based ILs and recent efforts in heterogenization of chloroaluminate ILs are discussed in this Review.

Among halometallate ILs, those containing metals from Group IIIA exhibit unique, tunable chemistry due to the anionic speciation, resulting in distinctive properties (e.g., low melting point, viscosity, acidity, etc).14 Mainly represented by chloroaluminate ILs, these type of ILs show great potential as Lewis acids and have been evaluated as industrial scale replacements of toxic, corrosive, inorganic acids, for example, HF or sulfuric acid (H2SO4).27 It is noteworthy that Chauvin developed chloroaluminate IL-based industrial processes such as liquid alkylation of isobutane28 and dimerization of alkenes,29 classes of coupling reactions which resulted in a Nobel Prize in 2005.30 Furthermore, it was recently announced that Chevron is developing a pilot plant for their liquid-phase alkylation process using a chloroaluminate IL to produce high-octane fuels from C4 paraffin.31 Although this example of commercialization is promising, this is not the rule but the exception, and most of the technologies based on ILs in catalytic reactions never reach full applicability in industry. One of the main factors contributing to the difficulty in scaleup of these processes is the lack of understanding of the chemistry involved in these catalytic reactions. The catalytic properties of the Group IIIA halometallate ILs rely mainly on the anionic species, and the characterization of the anion is crucial although tedious. A great amount of information has been collected on the speciation of halometallate ILs, most recently summarized in a critical overview of metal ion speciation by Estager et al.13 In the present Review, we aim to build on such fundamental studies by exploring in depth the catalytic behavior of Group IIIA halometallate ILs. We present a systematic correlation of anion speciation of Group IIIA halometallate ILs and their catalytic properties for a wide range

2. SPECIATION IN GROUP IIIA HALOMETALLATE IONIC LIQUIDS In contrast to aqueous media, in which metal-aquo complexes dominate the speciation, it is accepted that the nature of metal and mole fraction of metal chloride to organic chloride salt (χMClx) are the two main factors that tune the anionic speciation in Group IIIA halometallate ILs and consequently determine their catalytic activity (Table 1).32−38 The Lewis acidity is modified by the Group IIIA metal present in the anion, following the trend Ga > Al > In. Besides the identity of the metal, in all cases for an individual metal halide, Lewis acidity increases with increasing molar fraction of metal salt (Table 1),10 a phenomenon that can be assigned to the existence of polynuclear halometallate species ([MxCly]z‑).15 Different chloroaluminate anions ([AlCl4]−, [Al2Cl7]−, and [Al3Cl10]−) are detected at different molar ratios of AlCl3 to 1alkyl-3-methylimidazolium chloride ([Cnmim]Cl, where n = 2 or 4). When χMClx is less than 0.5, the melt becomes basic and the dominant metal complex is [AlCl4]−. The neutral melt (χAlCl3 = 0.5) also contains mainly [AlCl4]− anions, whereas the acidic melt (χAlCl3 > 0.5) is mainly composed of [Al2Cl7]− (Table 1), although [AlCl4]−, [Al3Cl10]−, and other anions with higher acidity are also present in small quantities, making strong Lewis acids.39 In addition, the acidity of protons can be enhanced when mineral acids (e.g., hydrochloric acid, HCl) are added to chloroaluminate ILs such as [C2mim]Cl-AlCl3. This 7015


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in viscous liquids. Due to its high metal concentration, the liquid becomes dense, has high heat capacity, high ionic conductivity, and exhibits a wide range of Lewis acidity and redox potentials. The most significant feature that makes this class of compounds similar to chloroaluminate ILs is the dependence of Lewis acidity on the metal chloride concentration (χMCl3), and, like the ILs, Ga-based systems exhibit slightly higher Guttmann acceptor numbers (AN, used to quantify the Lewis acidity of metal salts) than Al- or In-based systems. Adductbased ILs are found to be stronger Lewis acids than chloroaluminate ILs and still offer the major benefits of chlorometallate ILs, such as low vapor pressure and high conductivity.10,51 In recent findings, adduct-based ILs have displayed very high Lewis acidity with Gutmann Acceptor Numbers with AN = 120−182,52 compared to halometallate ILs prepared by metathesis of salt with metal halide, having AN = 32−107.10 Although adduct-based ILs are currently the focus of many investigations, these were first reported during the 1990s, when mixtures of AlCl3 and dimethylformamide (DMF) or dichloromethane were used to catalyze Friedel−Crafts acylation reactions.53,54 The difference between catalytic activity of AlCl3 and the DMF-AlCl3 coordination complex is exemplified with the use of these coordination compounds during the synthesis of substituted benzophenones, where two distinct products were obtained.55 These IL analogues, prepared by addition of excess anhydrous metal halide to donor compounds under solventless conditions, act as strong liquid Lewis acids formed by ligand-assisted heterolytic cleavage of metal halides into ionic compounds.56 It has been known that metal salts such as AlCl3, which are composed of bridged metal centers, may be cleaved heterolytically in the presence of a neutral ligand to give salts of discrete ionic metal complexes, rather than neutral adducts.57 Using Raman spectroscopy, it was found that these mixtures are constituted of cationic, anionic, and neutral complexes (Scheme 1).58,59 Yet, the reports on speciation in this class of IL analogues are scarce, and the species existing in these are still to be elucidated.

enhanced acidity (above 100% H2SO4) is due to the reaction between dissolved HCl and acidic chloroaluminate ions (e.g., [Al2Cl7]−) that releases protons with extremely low solvation and high reactivity.15 While still to be confirmed, the enhanced acidity is probably due to simultaneous existence of acidity of the protons in the melt and the acidity of electron-deficient Alcenters in polynuclear anions. The addition of different metals can also be used to modify the speciation of chloroaluminates (Table 1). Hexacoordinated anions, [M(AlCl4)3]−, are formed when MCl2 (M = Mn(II), Co(II), Ni(II)) are added to the acidic [C2mim]Cl-AlCl3 IL,24,40 while the species [M(AlCl4)n]n−3 and [M(AlCl4)n]n−2 are formed when ytterbium chloride (YbCl3) is dissolved in an acidic [C2mim]Cl-AlCl3 IL.41 Titanium tetrachloride (TiCl4) dissolved in [C4mim][Al2Cl7] is hypothesized to form [Ti(Al2Cl7)4]2−.42 A recent report describes how Lewis acidity was increased when neutral copper(I) chloride (CuCl) was added to the IL [Et3NH][AlCl4] ([Et3NH] = triethylammonium) and resulted in the formation of the new mixed metal species [AlCl4CuCl]−, as confirmed by 27Al NMR and ESIMS.43,44 This, however, is an unusual case where the mixed metal species was identified, and the effects of transition metal salts on the speciation and catalytic properties of chloroaluminate ILs and metal salts are still to be fully explored. Measurement of the metal speciation in halometallate ILs has been a subject of inquiry, and a number of spectroscopic techniques have been adopted. Among these, 27Al NMR, FTIR, and Raman spectroscopy are the prominent and most widely accepted techniques to determine speciation.13 For example, in the [C2mim]Cl-AlCl3 melts, Raman spectroscopy played a major role in elucidating the presence of [AlCl4]− or [Al2Cl7]−, irrespective of the nature of the counterion. 27Al NMR measurements and longitudinal magnetization techniques have been employed for determining the lifetime of [Al2Cl7]− species. Other techniques, such as extended X-ray absorption fine structure (EXAFS) or X-ray photoelectron spectroscopy (XPS), are less preferred due to complexity of analysis. Mass spectrometry (MS) is widely used, likely because of the precision it offers for assigning formulas to different metal species, but it is argued to be unsuitable as a sole means of determining speciation because of how the technique perturbs the speciation of the IL.13 During the MS ionization process, species can be formed such as [MCl5]− (M = Zr or Hf),45 or binuclear anions, such as [In2Cl7]− which have not been detected in the liquids themselves by any other method. Still, because of its ability to distinguish chemically similar species with different molecular weights, MS may be especially useful in combination with other techniques for determining how different halides are distributed in halometallate ILs with mixed halide ions. This approach has been used for Bi3+-based halometallate ILs46 but has not yet been applied to any Group IIIA mixed halide halometallates. The pursuit of stronger Lewis acidic ILs has resulted in exploration of a new class of halometallate ILs known as adduct-based ILs.47 These systems can be considered as analogues of deep eutectic solvents such as choline chloride with ZnCl2/urea48,49 (DES, defined as low melting liquids made by combining a salt with a neutral molecule or another salt50). Following the same concept of DES preparation, reacting an excess of neat AlCl3 or GaCl3 (χMCl3= 0.60) with substoichiometric amounts of simple donors (O-donors such as urea, AcA, dimethylacetamide, P888O; S-donors such as thiourea; P-donors such as trioctylphosphine (P888)) results

Scheme 1. Existence of Cationic, Anionic, and Neutral Species in Equilibrium in IL Analogues. Reproduced with Permission from Ref 59. Copyright 2015 Royal Society of Chemistry

3. GROUP IIIA HALOMETALLATE IONIC LIQUIDS IN CATALYTIC APPLICATIONS In a meticulous march for catalyst design, there are certain milestones such as reactivity, selectivity, cost, and environmental and energy concerns that take a common catalyst toward an ideal catalyst. Halometallate ILs are classified as 7016


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[Al2Cl7]− (assumed by the author,66 Table 2), which has an active role in this type of reaction.67 Mechanistic investigation confirms that the alkylation of benzene using chloroaluminate ILs follows the mechanism of Friedel−Crafts Lewis acid catalyzed reactions.67 In this mechanism, 1-dodecene forms a carbonium ion with Al3+ and further reacts with benzene, leading to an alkylated product from an unstable intermediate complex. The acidic chloroaluminate ILs can provide highly polar, noncoordinating environments for substrates due to the high concentration of [Al2Cl7]− species (χAlCl3 > 0.5) and solubility of arenes. The primary advantages of employing chloroaluminate ILs as catalysts include better reaction rate, higher conversion, and higher product selectivity (Table 2).44,68 In the case of alkylation of activated aromatic substrates such as catechols and phenols, the milder halometallate IL [C4mim]Cl-InCl3 at 0.67 mole fraction can be used,69 since relatively mild reaction conditions are required compared to alkylation of benzene with 1-dodecene. The same case is observed with the alkylation of benzene with benzyl chloride, which requires relatively mild reaction conditions since Cl− is a good leaving group, forming a reactive benzylic carbocation. The alkylation of benzene with benzyl chloride in [C2mim]ClAlCl3 (χ = 0.67) is highly selective due to the active [Al2Cl7]− (assumed by the authors, Table 2).63,70 However, the yields were greatly influenced by stirring rate, indicating the reaction is affected by mass-transfer limitations due to inadequate contact between the viscous IL catalyst and the reactant molecules.63 The addition of a second halide, forming mixed halide anions, is another strategy to obtain medium Lewis acidity while adding molecular polarizability: The addition of AlCl3 to [C4mim]Br (χ = 0.67) was found to give a more effective catalyst compared to [C4mim][Al2Cl7] or the system formed when AlCl3 was added to [C4mim]I (the species assumed by the authors to be formed were [C4mim][Al2Cl6Br] and [C4mim][Al2Cl6I], respectively, Table 2).64 A decrease in catalytic activity was observed for cations with longer alkyl chains ([C8mim]+ and [C12mim]+, Table 2), which was attributed to differences in molecular polarizability and Lewis acidity. However, the authors used IR probes that showed negligible differences in acidity and gave polarizability results that were the opposite of the expected behavior (smaller ions were considered more polarizable when in fact they should be less71). Other factors such as differences in physical properties and a lower concentration of [Al2Cl6Br]− due to the higher molar volume of these ILs are likely more important, especially since these ILs do not dissolve in benzene, the reaction solvent. Attempts to treat ILs using approaches and theory for homogeneous solutions of metal complexes must be done with caution. Catalytic activity was increased by adding both the Brønsted acid HCl as well as the Lewis acidic [C2mim]Cl-AlCl3 IL (χ = 0.67) in the reaction mixture when compared to the sole use of [C2mim]Cl-AlCl3 as catalyst.72 As with halometallate ILs, the pH of the solution and the composition of the IL determine the Brønsted acidity of the system, since it was observed that protons act as superacids in the presence of chloroaluminate ILs.73 The most reluctant substrates can be alkylated in these conditions. Significant catalytic activity was achieved in the alkylation of benzene by using Ga-based ILs.74 The adduct of trioctylphosphine oxide (P888O) with GaCl3 at 0.6 mole fraction showed higher catalytic activity than the [C2mim][Ga2Cl7] IL (Table 2). Although higher reaction rates were

Lewis acidic ILs, and there are several factors that make these ILs interesting substitutes for conventional catalysts, including (i) controllable acidity, (ii) ability to dissolve a wide range of materials, and (iii) easy isolation of the product (Figure 1). In addition, as with other ILs, halometallate ILs can be used as solvents.

Figure 1. Halometallate ionic liquids as catalysts.

As discussed above, the speciation, and thus Lewis acidity, of halometallate ILs depends upon the electrophilicity of the metal ions as well as halide salts, the composition of metal salts [MXn]− (M = metal, and X = halogen),60,61 and the addition of different metals to chloroaluminates, resulting in new anionic species with different acidity.43,44 It is also expected that the selection of the cation influences the catalytic activity of the resulting IL, although, as will be discussed in the next section, most of the studies focus on imidazolium-based ILs. A summary of the reported Lewis acid-catalyzed reactions carried out using halometallate ILs and adduct-based halometallate ILs is presented in Table 2. 3.1. Alkylation. In Friedel−Crafts alkylations, substrates such as benzene react with olefins such as 1-dodecene in the presence of acidic catalysts, traditionally HF or AlCl3, to generate alkylated products. Although widely used in industry, these still present important drawbacks that include toxicity, low conversion and selectivity, rapid catalyst deactivation, and high reactant and energy feed.62 This type of reaction was the first one proposed using an acidic IL ([C2mim]Cl-AlCl3, Table 2),63 and the increase of the mole fraction of AlCl3 in the chloroaluminate IL (increasing the Lewis acidity) resulted in an increase of the rate for the alkylation of benzene with 1dodecene.64,65 A moderately Lewis acidic IL system (χAlCl3 = 0.55) is sufficient to generate the electrophile, a rate-determining step in this type of reaction.64 For example, [C4mim]Cl-AlCl3 ([C4mim] = 1-butyl-3-methylimidazolium) IL at χAlCl3 = 0.5 showed no activity, but 97% conversion and 33% product selectivity were found using the IL at χAlCl3 = 0.67, containing 7017


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ACS Catalysis Table 2. Summary of Reactions Catalyzed by Group IIIA Halometallate ILs

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ACS Catalysis Table 2. continued

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a

Selectivity expressed as ratio of endo to exo product. *IL used in more than a catalytic amount.

at χ = 0.52 ([AlCl4]−, [AlCl3I]−, and [AlCl2I2]− anions confirmed by 27Al NMR, Table 2). With increasing mole fraction of AlCl3 from 0.48 to 0.58 in [C4mim]Br-AlCl3, catalytic activity was also increased due to the strong acidic [Al2Cl6Br]−species generated at 0.58 mole fraction (confirmed by 27Al NMR). It should be noted that with longer reaction time the strongly acidic, active [Al2Cl6Br]− anion deactivates. When additives such as benzene (0.8 wt%) were used in the alkylation reaction of isobutane with 2-butene using [Et3NH]Cl-AlCl3 at χ = 0.67 ([Al2Cl7]− species confirmed by 27 Al NMR), the catalytic activity increased significantly.76

achieved when small ligands like urea or DMA were added to high molar ratios of GaCl3, a bulkier, lipophilic ligand (e.g., P888O) was needed to induce phase-separation of the catalyst from the reaction mixture. Alkylation of isobutane with 2-butene was catalyzed by using different Lewis acidic chloroaluminate ILs with mixed halides at χ = 0.52, which resulted in different catalytic activity.75 [C4mim]Br-AlCl3 at χ = 0.52 ([AlCl3Br]− anion species confirmed by 27Al NMR) was found to be a more effective catalyst than [C4mim]Cl-AlCl3 at χ = 0.52 ([AlCl4]− and [Al2Cl7]− anions confirmed by 27Al NMR) or [C4mim]I-AlCl3 7021


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ACS Catalysis 3.2. Acylation. In Friedel−Crafts acylation reactions, substrates such as benzene react with an acylating agent such as acetyl chloride in the presence of strong Lewis acid catalysts (usually AlCl3) to generate acylated products. Disadvantages include rapid catalyst deactivation, as well as high reactant and energy feed.62 Wilkes et al. carried out a detailed investigation of Friedel−Crafts acylation of benzene with acetyl chloride catalyzed by [C2mim]Cl-AlCl3 ILs with different χAlCl3.63 As expected, the rate of reaction depends on the Lewis acidity of the IL (ILs of greater acidity demonstrated greater catalytic activity), which is, in turn, dependent on the IL composition. For example, [C2mim]Cl-AlCl3 (χ = 0.5) (containing [AlCl4]− species, assumed by author) showed no activity, but 100% yield was found using the IL at χ = 0.67 (containing [Al2Cl7]− species, assumed by author, Table 2), highlighting the importance of anionic speciation. The proposed mechanism for acylations of aromatic compounds catalyzed by the chloroaluminate ILs involves two steps.63 First, acetyl chloride reacts with the [Al2Cl7]− species, and an acylium carbocation is generated which further reacts with aromatic compounds, leading to the acylated product. Acylation of anthracene with acetyl chloride using [C2mim]Cl-AlCl3 (χ = 0.67) (containing [Al2Cl7]− species, assumed by author), resulted in good yields with high selectivity for diacylated products (Table 2).77 An initial monoacylation was observed, followed by its disproportionation to the desired diacetylanthracenes due to the presence of a Brønsted superacid in the system. Good yields and high selectivity were also obtained during the acylation of anthracene with oxalyl chloride in the presence of [C4mim]Cl-AlCl3 (χ = 0.67) (containing [Al2Cl7]− species assumed by author), although no activity was observed with χ = 0.5 ([AlCl4]− as the predominant anion).78 Interestingly, the catalyst was reused up to five cycles without substantial loss of catalytic activity. Acylation of a range of substituted indoles was reported in the presence of [C2mim]Cl-AlCl3 (χ = 0.67).79 Though the products were generated with moderate to high yields, tedious workup and destruction of the IL were the shortcomings of the process. 3.3. Oligomerization. Oligomerization is industrially used to upgrade the quality of lubricating oil following the oligomerization of α-olefins as their base stock using acid catalyst (usually AlCl3, HF, and H2SO4).80 After Nobel laureate Chauvin’s “Dimersol Process,” intensive investigation of chloroaluminate IL-catalyzed oligomerization processes was carried out, as summarized in Table 2. Initial attempts to improve selectivity led to the design of alkylimidazolium and pyridinium-chloroaluminate ILs.29 When the reaction is catalyzed by halometallate ILs containing [Cnmim]+ or [CnPyr]+ cations, the degree of oligomerization increases with the alkyl chain length,81 facilitating product isolation, reusability of catalyst, and prevention of isomerization. The basic motivations of performing oligomerization in chloroaluminate ILs are the easy separation of products and an increase in the product selectivity.82 In oligomerization of 1-propene using a mixture of a Ni complex and [C4mim]Cl-EtAlCl2 (χ = 0.7) as the chloroaluminate IL catalyst, significant catalytic activity was observed compared to [C4mim]Cl-AlCl3 (χ = 0.6) IL (Table 2). Furthermore, when a mixture of [C4mim][AlCl4], AlCl3, EtAlCl2, and TiCl4 was used as a catalyst for oligomerization of 1-butene, significant catalytic activity was observed when compared to a mixture of [C4mim][AlCl4], AlCl3, and EtAlCl2 as a catalyst system (Table 2). Anion speciation was not

discussed in this report, so it is uncertain to what extent the properties of the catalyst depend on the presence of the transition metal ion vs its effect on the speciation of the chloroaluminate anions. In the mechanism for oligomerization of olefins, Al3+ stabilizes the olefin carbocation, and this intermediate reacts further with olefins, leading to dimers, trimers, and so on. Different cations were combined with the anion [Al2Cl7]− and used for the synthesis of poly(α-olefins) from 1-decene, as replacements for boron trifluoride (BF3).59 The highest conversions were achieved with the lipophilicity of the cation following the order: [C2mim]+ < [P4444]+ ∼ [C8mim]+ < [P66614]+ (Table 2). Interestingly, the product distribution could be tuned with the cation: The heaviest oligomers were obtained with the more lipophilic cations, whereas [C2mim][Al2Cl7] produced higher amounts of lighter oligomers. When comparing the effectiveness of adduct-based halometallate ILs, they have similar or lower catalytic activity than conventional chloroaluminate ILs but higher selectivity toward lighter oligomers. Upon increasing the mole fraction of GaCl3 from 0.5 to 0.75 in Ur-GaCl3 adduct-based ILs, the product selectivity increased. In adduct-based ILs of GaCl3 and AlCl3, the AlCl3 adduct-based IL exhibited slightly higher catalytic activity. Among the other adduct-based ILs, those based on P888O-AlCl3 were the best catalysts for synthesis of poly(αolefins). 3.4. Diels−Alder Reactions. The Diels−Alder reaction of cyclopentadiene and methyl acrylate is generally catalyzed by a Lewis acid (usually ZnCl2, BF3, or AlCl3). When applicable to this type of reaction, the composition of halometallate ILs shows a substantial effect on selectivity and reactivity.83 The Diels−Alder reaction of cyclopentadiene and methyl acrylate using [C2mim]Cl-AlCl3 indicates that the product yield increases with the advancement of time while the stereochemical selectivity (endo/exo ratio ∼19/1) remains unchanged (Table 2). Remarkably, the use of acidic [C2mim]Cl-AlCl3 (χ = 0.67) as a reaction medium increases the stereochemical selectivity 4 times compared to the basic ionic melt (χ = 0.5) (endo/exo ratio ∼5/1). A computational study of the Diels−Alder reaction in chloroaluminate(III) ILs confirmed that the rate of reaction is faster in acidic conditions.84 The reaction rate and endo/exo selectivity in the case of [C2mim]Cl-AlCl3 (χ = 0.67) as a catalyst and reaction medium proved to be superior to the conventional reaction media (water, methanol, ethanol, benzene, formamide, and ethylammonium nitrate).83 By employing the recoverable and reusable IL reaction medium, problems associated with the organic solvent and catalyst wastes were eliminated. Recently, mixed Lewis acidic ILs involving the borenium cation and Group IIIA halometallate anions85 were investigated for the Diels−Alder reaction of cyclopentadiene and ethyl acrylate.52 There is a cumulative effect of anion and cation to increase acidity, and the cation plays the predominant role. This was confirmed when a borenium cation coupled with even the nonacidic [AlCl4]− anion resulting in an acidic IL. Recently, P 888O-MCl3 adduct-based systems (χMCl3 = 0.5) were investigated for the Diels−Alder reaction of cyclopentadiene and ethyl acrylate.86 Out of P888O-AlCl3, P888O-GaX3, and P888O-InCl3 adduct systems, only P888O-InCl3 system showed 14% catalytic activity. 3.5. Hydrogenation. Hydrogenation of arenes is performed with several catalysts (transition-metal complexes or by supported metal catalysts) and has a large number of 7022


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tolerate moisture without undergoing degradation, although the Lewis acidity of the resulting IL is decreased.90 3.7. Condensation. The Knoevenangel reaction of benzaldehyde and diethylmalonate was carried out using [C4mim]Cl-AlCl3 and [C4pyr]Cl-AlCl3, and higher yields were found with the imidazolium-based IL (Table 2).95 During the reaction, the obtained benzylidene malonate reacted again with unreacted diethylmalonate to produce the Michael adduct, which is a side product in the reaction. The fate of the two competing reactions was controlled by tuning the Lewis acidity of these ILs (by changing mole fraction from 0.55 to 0.67), where with increasing the Lewis acidity, a decrease of the Knoevenangel product selectivity was observed. The ratio of Knoevenagel to Michael product (K/M) was found to be highest when the molar ratio of aldehyde, diethylmalonate, and [C4mim]Cl-AlCl3 (χ = 0.67) IL was 1.0:1.0:0.5. Chloroindate ILs were used as catalysts as well as reaction media in the tetrahydropyranylation of different alcohols (pentan-1-ol, ethylene glycol, and menthol).96 When 5 mol % of InCl3 catalyst was used along with [C4mim][InCl4] IL as solvent in the tetrahydropyranylation of 1-pentanol, the catalytic activity was increased (from 54 to 87%) compared to the reaction using only [C4mim][InCl4]. A biphasic system was obtained after the reaction, and the product was separated by simple decanting. [C4mim][InCl4] was also used for the Biginelli one-pot condensation reaction of benzaldehyde, ethylacetoacetate, and urea. The reaction was completed in a short time (25 min) with 98% yield, and the IL was reused after product separation.97 When applied to the synthesis of 3,4dihydropytimidinones/-thiones, high yields and short times were observed. The IL was recovered from both reactions after product separation and was reused for five to six cycles without any significant loss in catalytic activity. Interestingly, interactions between the anion with the aromatic imidazolium hydrogen atoms were reported for the IL [C4mim][InCl4], although the role of the cation in the catalytic activity is still to be elucidated. 3.8. Isomerization. Isomerization of straight chain alkanes to branched chain alkanes is a main reaction for the improvement of the octane value of gasoline.98,99 The isomerization of n-pentane using [Et3NH]Cl-AlCl3 was reported, with an increase in the conversion and selectivity occurring when using this IL as catalyst.100 The control of the isomerization is of high importance in, for example, the synthesis of tetrahydrodicyclopentadiene (THDCPD); its exoisomer is used in the high energy density Fuel Jet Propellants (JP-10), as fuel for short-range missiles and aircraft, and in other industrial applications.101 The isomerization of endoTHDCPD to exo-THDCPD was conducted by employing four AlCl3-based halometallate ILs, [PyrH]Cl-AlCl3, [C4mim]ClAlCl3, [C16mim]Cl-AlCl3, and [Et3NH]Cl-AlCl3 (Scheme 2).102 Although catalytic conversion was observed with the four ILs, the isomerization percent followed the order [PyrH]Cl-AlCl3 (χ = 0.6) > [Et3NH]Cl-AlCl3 (χ = 0.6) > [C4mim]Cl-AlCl3 (χ = 0.6) > [C16mim]Cl-AlCl3 (χ = 0.6) (Table 2). This same order was observed for the Lewis acidity of these ILs, measured by FT-IR. Isomerization of the endoisomer to the exo-isomer in the presence of halometallate ILs at 0.67 mole fraction resulted in good yields and high selectivity, but no activity was found with basic ILs at 0.5 mole fraction (Table 2). Increasing the mole fraction of AlCl3 (from 0.65 to 0.75) and the reaction time (from 1 to 6 h) improved the conversion, and the increase in temperature up to 70 °C

applications in chemical processes, particularly for the generation of cleaner diesel fuels.87 Leaching of metal and isolation of products are the main challenges of the process. Stereoselective hydrogenation of anthracene was carried out using [C2mim]Cl-AlCl3 (χ = 0.67) in the presence of an electropositive metal and proton source.88 It is interesting to note that the selectivity was modified by using different electropositive metals. Among the electropositive metals used, Al was found to be effective because the generation of AlCl3 as a byproduct kept the composition of chloroaluminate IL unaffected. This is a typical metal dissolving reduction process in chloroaluminate ILs, and it is analogous to the Birch reduction.89 3.6. Esterification. Esterification of alcohols with acetic acid was carried out using [C4pyr]Cl-AlCl3.90 It was found that a mole fraction of 0.5 was sufficient to catalyze the reaction, with an activity better than the conventional H2SO4 catalyst (Table 2). Chloroaluminate and chloroindate ILs were evaluated as catalysts for biodiesel production from soybean oil by trans-esterification (reactions in which the alkyl group of an ester is exchanged with one from an alcohol).91,92 The IL [C4mim][InCl4] was used for trans-esterification of soybean oil with methanol, but no product was observed without the addition of another catalyst, indicating that the IL by itself was not able to promote the reaction. However, since the IL is a liquid, it can also be used as the solvent and allows the addition of a catalyst, such as the [Sn(3-hydroxy-2-methyl-4-pyrone)2(H2O)2] complex, which does result in good yields for biodiesel. When the IL [C4mim][BF4] was used as a replacement for [C4mim][InCl4], the yields were lower (55% vs 83%), indicating that the Lewis acidic IL has an important role that still needs to be elucidated. After recovering and reusing the IL phases for this reaction, a drastic decrease in the yields was observed, probably due to the decomposition of the Sn complex.92 Due to its higher acidity, higher yields were obtained when [AlCl4]− was used as a catalyst in the synthesis of biodiesel, without the need of a second catalyst.91 The ammonium salt used to make the IL was found to have an effect. [Et3NH]ClAlCl3 (χ = 0.7, Table 2) and methylimidazolium chloride ([C1mimH]Cl-AlCl3) were found to have similar yields (98.5% vs 96.8%, respectively), while hexadecyltrimethylammonium bromide ([C16TA]Br-AlCl3) showed a 20% lower yield. In addition, the yield diminished with increasing alkyl chain length on the imidazolium cation. It was proposed that longer alkyl chains led to increased steric hindrance and reduced mass transfer in the system, limiting the reaction. The biodiesel synthesis using halometallate ILs seems to follow the same mechanism as the Lewis acid-catalyzed transesterification reaction. In this mechanism, the ester group from a triglyceride molecule forms a carbocation with the halometallate and further reacts with methanol leading to the methyl fatty ester product.93 The IL-catalyzed biodiesel process had many advantages compared to H2SO4 and solid acid catalysts, such as lower reaction temperature, lower cost of the catalyst used, high yields, no saponification, and reusability, all demonstrating great potential for more sustainable processes. Still, the major drawback of the IL-catalyzed esterification process is that the water generated as a byproduct might interfere with the reaction in the presence of Group IIIA halometallate ILs, most of which are sensitive to moisture.94 Chloroaluminate ILs with mole fractions of AlCl3 ≤ 0.5 can 7023


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Perhaps the single largest chemical influence of the cation is that it can be protic or aprotic. ILs with protonated cations form a special class, protic ILs, where the strong hydrogen bond donation and Brønsted acidity of the cation plays a key role in their physical and chemical properties.105 Because protic IL cations are generally made from strong bases such as amines, they also frequently engage in strong hydrogen bonding which couples two molecules together rather than full proton transfer.106 This makes protic ILs appealing as alternatives to strong acids with a potentially tunable proton-substrate interaction. In addition, Group IIIA halometallate anions are very weak bases, and their ability to act as liquid media and enhance the acidity of protic cations could open an entire area of catalysis beyond the acidity of the metal ions themselves. Examples exist such as the use of [Et3NH]Cl-AlCl3 for conversion of isobutene to trimethylpentanes,76 where the protic cation was chosen to improve selectivity of trimethylpentanes compared to the previously reported 1,3-dialkylimidazolium chloroaluminate-catalyzed reactions,75 but the possible uses for protic ammonium ILs as tunable strength Brønsted acids are largely unexplored. Even aprotic cations offer many degrees of structural tunability. However, it is evident from Table 2 that only 1,3dialkylimidazolium ILs have been thoroughly investigated for their applications. These were the first family of ILs to be used in this area and represent only a small fraction of reported ILs. The narrow focus of application-based studies of ILs on a small family is not restricted to catalysis; for instance, a series of ILs from an early report by Graetzel et al. have come to dominate the application of ILs as electrolytes in devices,107 even though a huge range of ILs have had their electrochemical properties characterized.108 This unfortunate state reflects a tendency to generalize both the uses and limits of all ILs based on a small subset of well-studied ILs, which are often early discoveries in the field which could likely be improved. Thus, while the IL cation likely has a number of potential uses in Lewis acidcatalyzed reactions beyond being a potential source of protons, these are still rather unexplored in the literature. Given that much fundamental research on ILs is undertaken with applications in mind, we expect that it will be up to researchers discovering new ILs to link application-based studies, such as catalysis, with fundamental chemical characterization in order to advance this area. 4.2. Immobilization of Chloroaluminate ILs. While halometallate catalysts owe many of their advantages to their liquid state, immobilization of these ILs on solid supports is considered a promising strategy to make them more accessible and reusable in processes catalyzed by solid-phase catalysts.109 Furthermore, supporting ILs creates high-surface-area films with very short diffusion distances, avoiding mass transfer limitations and using the volume of the IL more efficiently.110 Highly acidic chloroaluminate ILs can be sheltered in pretreated silica-based support materials to generate supported IL phases (SILPs).110 Chloroaluminate ILs were supported on solid materials via two different approaches; (1) impregnation (noncovalent bonding) and (2) grafting (covalent bonding) of the cation on the surface of a solid.109 Impregnation can be performed by “incipient wetness,” that is, the IL is added to the support until the mixture loses the appearance of dry powder; while grafting of the IL on a solid support implies a covalent binding of the IL to the support. Of these two approaches, grafting of ILs on solid supports is reported to be more effective; when the IL [C4mim]Cl-AlCl3 (χ = 0.67) was

Scheme 2. Proposed Reaction Mechanism for the Isomerization of endo-THDCPD Using Acidic Catalysts. Reprinted with Permission from Ref 102. Copyright 2011 Elsevier

increased the conversion (from 14.5 to 39.5%). The chloroaluminate IL catalyst was recycled and reused for four cycles in the isomerization of endo-isomer, without loss in catalytic activity. The isomerization of exo-THDCPD to adamantane using the same three chloroaluminate ILs was also studied. Here also, the conversion followed the order [PyrH]Cl-AlCl3 > [Et3NH]ClAlCl3 > [C4mim]Cl-AlCl3. Isomerization of the exo-isomer to adamantane in the presence of a halometallate IL at 0.6 mole fraction resulted in up to 67% selectivity under optimized conditions.103 3.9. Oxidation. The Baeyer−Villiger oxidation of 2adamantanone with hydrogen peroxide was catalyzed by [C2mim]Cl-GaCl3 at mole fractions of 0.5 to 0.75.104 It was observed that the rate of the reaction was dependent on the Lewis acidity of the IL; ILs of greater acidity demonstrated greater catalytic activity than those of basic, neutral, or even weaker acidity. For example, [C2mim]Cl-GaCl3 at χGaCl3 = 0.5 ([GaCl4]−, assumed by author) showed no activity, but 93% activity was found using this IL at χGaCl3 = 0.67 ([Ga2Cl7]−), and 99% activity was found at χGaCl3 = 0.75 ([Ga3Cl10]−, assumed by author, Table 2). During the reaction, the in situ formation of [GaCl3OH]− species led to the enhancement of activity in the Baeyer−Villiger oxidation of 2-adamantanone. Under the same reaction conditions, when [C2mim]Cl-AlCl3 at χAlCl3 = 0.67 ([Al2Cl7]−, assumed by author) was used, no activity was observed at short times, although 11% activity was found with longer reaction times.

4. METAL-INDEPENDENT MECHANISMS OF IL TUNABILITY 4.1. Roles of the IL Cation. Although the catalytic activity of halometallate ILs is clearly dominated by the identities and ratio of the metal and halide ions, the IL cation provides a handle to fine-tune the activity of a given halometallate system. Indeed, it was the electrochemical stability of the cation which led to the switch from the initially studied alkylpyridinium halometallate melts to the now dominant 1,3-dialkylimidazolium cations. In their review, Estager et al. noted that the IL cation could affect the speciation of the halometallate itself.13 The strength of cation−anion interactions affects the equilibrium distribution of free halide to halometallate species, and in the case of [PbCl3]− ILs, also affects the geometry of the metal complex. 7024


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host chloroaluminate ILs.117 Liquid-phase alkylation of isobutane using raffinate II as a source of alkene was also accomplished by immobilization of acidic ILs grafted on siliceous MCM-41 (MCM = Mobil composition of matter) and porous silica (grade FK 700).118 Imidazolium and phosphonium chloroaluminate-based ILs were supported on a number of solid hosts such as silica (FK 700), Nafion proprietary catalyst SAC-13, and zeolite polymorph H-Beta. Out of these, imidazolium and silica host systems were found to be more efficient. [PyrH]Cl-AlCl3 (χAlCl3= 0.65) impregnated on sodium montmorillonite clay was used as a catalyst for isomerization of endo-THDCPD to exo-THDCPD. The resultant intercalated clay could expand the silicate interlayer spacing, which perhaps enhanced the organic compatibility and ultimately increased the quantitative conversion and selectivity of endo-THDCPD to exo-THDCPD.119

impregnated on silica as a solid support and used in the alkylation of benzene with 1-dodecene, the catalytic activity was found to be reduced (from 46.8 to 10.9% conversion of 1dodecene) compared to the IL grafted on silica. This was assigned to bonding between the anion of the IL and silanol hydroxyl groups on the silica surface (Scheme 3).111 To recover Scheme 3. Impregnation of Chloroaluminate ILs on Silica Support. Reproduced with Permission from Ref 111. Copyright 2002 Royal Society of Chemistry

catalyst using an external magnet, Fe3O4 nanoparticles were coated with silica, and the [AlCl4]− based chloroaluminate IL was supported on the silica via the grafting approach.112 This catalyst was found to be efficient and reusable for the synthesis of a diverse range of dihydropyano[3,2-b]chromenediones via a one-pot, multicomponent, and solvent free reaction. Using polymers as supports for chloroaluminate ILs improved their recyclability in Diels−Alder reactions (Scheme 4).113 Reacting AlCl3 with polymeric pyridinium- and imidazolium-based ILs has been the general strategy to access polymeric chloroaluminate ILs, and these ILs offer ease of neutralization after the catalytic process with minimal loss of activity. In this context, poly(styrene)-supported pyridinium chloroaluminate ILs have been new entries to the family of the heterogeneous catalysts while favorably combining the features of Lewis acidic ILs and advantages of solid polymer supports.114 The support of pyridinium chloroaluminate on a polymeric matrix was found to be advantageous for Knoevenagel reactions. Recently, [C4mim][NTf2]-AlCl3 ([NTf2]−, bis((trifluoromethyl)sulfonyl)imide) was reported to enhance stability and durability in the alkylation of benzene. Uncoordinated [NTf2 ]− provided durability and water resistance by forming a stable and hydrophobic environment.115 In order to combine the advantages of ILs and heterogeneous systems, the catalytic IL system [C4mim][NTf2]-AlCl3 supported on mesoporous material Santa-Barbara Amorphous 15 (SBA-15) has been investigated.116 Due to its ordered structure and high specific surface area, the immobilized catalyst presents advantages such as enhanced selectivity, Lewis acidity, and reusability. Immobilized chloroaluminate ILs were also proven to be suitable candidates for the alkylation of aromatic compounds with olefins in which Lewis acidic strength plays a vital role in the distribution of the products. A wide range of solids such as silica, alumina, zeolite H-Beta, TiO2, and ZrO2 were found to

5. CONCLUSIONS AND FUTURE REMARKS ILs are themselves molecular puzzles which open a new horizon of applications as Lewis acid catalysts when combined with metal salts. These halometallate ILs have the added advantage of fine-tuning their properties by controlling metal halide concentration. Because of their anionic speciation (tunable Lewis acidic properties), halometallate ILs have acquired a wide range of applications. In the petrochemical industry in particular, processes such as alkylation, acylation, oligomerization, isomerization, and hydrogenation reactions require strong acidic conditions conventionally mediated by toxic chemicals. With halometallate ILs, the sustainability of these processes is increased, since the toxicity of the chemicals is lower, and some of these ILs can be reused for several catalytic cycles. In the present review, we have discussed the most important applications of Group IIIA halometallates in catalysis, compared the authors’ investigations or assumptions with regard to chemical speciation, and presented examples of how the tunability of these materials is used to overcome their drawbacks. An overview of the anionic species evaluated for different catalytic reactions (Table 3) indicates that [Al2Cl7]− is the species usually evaluated, although depending on the reaction, other species might be better. In addition, in most of the analyzed publications, the authors report the IL composition (not species) or assumed the speciation based on previous publications, ignoring all the components of the catalytic mixture that might influence on the speciation. A deeper understanding of the speciation can lead to erudite design of halometallate IL. In general, the literature suggests that halometallate ILs have the same mechanisms as conventional Lewis acid catalysts. This is an important advantage of the class of ILs as a whole; they can be used to solve challenges related to the properties of

Scheme 4. Grafted (or Functionalized) Chloroaluminate ILs on Silica (Left) or Polymer (right)

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ACS Catalysis Table 3. Summary of Speciation of Group IIIA Halometallate ILs Used in Catalysis reaction alkylation

acylationa oligomerization

Diels−Alder reactionsa

hydrogenationa esterification trans-esterification condensationa isomerization oxidationa a

metal speciation of halometallate IL used in catalysis [AlCl4]−, [Al2Cl7]−, [AlCl3Br]−, [Al2Cl6Br]−, [AlCl3I]−, [AlCl2I2]−, [Al2Cl6I]− [InCl6]− [GaCl4]−, [Ga2Cl7]− [AlCl2L2]+, [GaCl2L2]+, (L = neutral Lewis base) [AlCl4]−, [Al2Cl7]− [AlCl4]−, [Al2Cl7]−, [GaCl4]−, [Ga2Cl7]− [AlCl2L2]+, [GaCl2L2]+, (L = neutral Lewis base) [C4mim]Cl/EtAlCl2/NiCl2(PPri3)2 (χAl = 0.7) [C4mim][AlCl4]/AlCl3/EtAlCl2 /TiCl4 (χAl = 0.54) [AlCl4]−, [Al2Cl7]− [AlCl4]−, [BCl2(4pic)]+[AlCl4]−, [BCl2(4pic)]+[GaCl4]− [AlCl2(P888O)2]+[AlCl4]−, [GaCl2(P888O)2]+GaCl4]−, [InCl3(P888O)] [Al2Cl7]− [AlCl4]− [InCl4]−, [AlCl4]−, [Al2Cl7]−, [Al2Cl6Br]− [InCl4]−, [AlCl4]−, mixture of [AlCl4]− and [Al2Cl7]−, [Al2Cl7]− [AlCl4]−, [Al2Cl7]− [AlCl4]−, [Al2Cl7]−, [GaCl4]−, [Ga2Cl7]−, [Ga3Cl10]−

IL used in more than a catalytic amount; bold-faced font indicates best species in catalysis.

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conventional catalysts without greatly interfering with their chemistry. As noted in Table 2, halometallate ILs can be used in catalytic amounts in alkylation, oligomerization, esterification, trans-esterification, and isomerization reactions, but they can also be used as reaction media. There is still much scope to improve IL catalysts by understanding the possibilities and limits of use of halometallate ILs. Various strategies such as heterogenization, polymerization, making them hydrophobic, and enhancing and controlling Lewis acidity are being adopted to curb limitations and make them usable in real-world applications. The role of the cation in these ILs is still to be explored in great extent. As global petroleum reserves continue to be used up, catalysis will play an important role, at least in the short term, of powering the chemical industry in efficiently using unconventional fossil fuel resources. Chloroaluminate ILs are based on earth abundant elements which are recycled globally on a huge scale, but the need for specialized handling creates hazards and additional energy burdens. Less air-sensitive, more easily recycled ILs based on indium and gallium are emerging, but these ILs are based on more scarce metals which may have greater impact when obtained from or discharged back into the environment. Investigations into new applications of these ILs should consider where these strengths and weaknesses make a real impact on the sustainability of the process. Finally, there needs to be an awareness that society is pushing away from a reliance on nonrenewable resources altogether, and investigations of these ILs should keep in mind where they will fit if and when there is no longer even a demand for the catalysts they are designed to replace. In the near future, the upcoming technologies will unravel molecular insights of halometallate ILs, gaining more precise control over their properties, and opening new opportunities for the applications of this class of ILs. We believe this work will encourage the exploration of fundamentally novel chemistry while finding better ways to use chloroaluminate ILs and tunable Lewis acidic mixed metal double salt ILs. It is always the molecular-level structural knowledge that has ignited new findings and possibilities of new applications.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rajkumar Kore: 0000-0002-3361-1188 Steven P. Kelley: 0000-0001-6755-4495 Robin D. Rogers: 0000-0001-9843-7494 Notes

The authors declare the following competing financial interest(s): Dr. Robin D. Rogers has partial ownership of 525 Solutions, Inc. The University of Alabama maintains approved Conflict of Interest Management Plans.

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ACKNOWLEDGMENTS The authors would like to thank Reliance Industries Limited, India for their continuing support of related research.

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REFERENCES

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