Analysis of the Relationship between the Composition of a Bronsted Acidic Task-Specific Ionic Liquid: 1-(4-Sulfonic Acid)-butyl-3-Methylimidazolium Hydrogen Sulfate ([bsmim] [HSO 4 ]), and Its Behavior on Reactive Systems

: Ionic liquids (IL) are proposed as an alternative to conventional organic solvents. Among them, there is a subgroup called task-specific ionic liquid (TSIL), in which a functional group is covalently attached to the cation or to the anion. This is allows direction towards a specific action of its chemical properties in a reactive system. In recent years, 1-(4-sulfonic acid)-3-methyl butylimid-azolium [bsmim] with bisulfate counterion [HSO 4 ] has been one of the most studied TSIL. This work aims to contribute to a better comprehension of the relationship between the structure of this TSIL and its catalytic behavior. The objective is to identify which part of the structure of the TSIL is re-sponsible for the catalytic action observed in specific reactive systems and to understand how the addition of a functional group such as -SO 3 H influences the structure and therefore its behavior. 1-butyl-3-methylimidazolium [bmim] was also synthesized with the same counterion. Both ILs were tested in two reactive systems—esterification reactions and synthesis of dibenzoxanthenes. The yields obtained were compared with respect to the action of sulfuric acid. Furthermore, cyclic volt-ammetry was used to identify the species present in the ILs. The results indicate that TSIL shows an improved catalytic activity in both types of reactions in comparison with bmim HSO 4 . The relationship between the voltammetric results and the final performance allowed the interpretation of the behavior of these ILs. The IL synthesis and purification method influence on the present species and may affects the results of the reactive system.


Introduction
In recent years, the need for design and development of sustainable and environmentally friendly chemical processes has grown. Thus, ionic liquids (ILs) have emerged as alternative catalysts and solvents for a wide variety of processes.
ILs have certain advantages over traditional molecular solvents, such as low vapor pressure, high catalytic activity, thermal stability, and the possibility of varying their structure to manipulate certain properties, such as solubility and their behavior, in different reactive systems [1][2][3].
There is a new tendency to obtain greener ILs. Thus, a new route has emerged to design so-called task-specific ionic liquids (TSILs), in which a functional group is covalently attached to the cation or alternatively to the anion. In this direction, functionalized TSILs with a sulfonic acid group have been synthetized, offering a new possibility for  bronsted acid catalysts that are more environmentally friendly. These TSILs allow the development of reactions with a phase transfer and the possibility of catalyst recovery and recycling.
The objective of our work is to relate the behavior of these TSILs in different reactive systems to their chemical structure. The aim is to optimize the selection of TSIL to achieve a specific property and analyze the effect of the decoration of the structure of imidazolium cation with a sulfonic group. In this direction, the synthesis of a TSIL was developed by decorating the structure of the 1-butyl-3-methyl imidazolium (bmim) cation with an acid sulfonic group, to obtain 1-(4-sulfonic acid)-butyl-3-methyl imidazolium (bsmim), with bisulfate counterion [HSO4]. Both ILs ([bsmim] [HSO4] and [bmim] [HSO4]) were characterized by cyclic voltammetry. This tool was used to identify the species present in ILs which are synthetized from strong acids [4]. According to what was observed and to correlate results, the ILs were applied in two reactive systems: synthesis of dibenzoxanthenes and esterification reactions. The results of both reactions are discussed and compared with respect to the yields obtained when sulfuric acid (H2SO4) was used.

Synthesis of ILs
The synthesis of the ILs was carried out following the technique reported in our recently submitted work [5].

Reaction of Dibenzoxanthenes
The general reaction is shown in Scheme 1. It was carried out by mixing 10 mmol of β-Naphthol, 5 mmol of aldehyde (benzaldehyde or 4-nitrobenzaldehyde), and 0.25 mmol of TSIL into a round-bottom flask under reflux and agitation, for 2 h, at 100 °C. The reaction was monitored by silica gel TLC plates using hexane/acetate. After cooling to room temperature, 10 mL of water was added, and the mixture was stirred for 5 min. The solid obtained was filtered, recrystallized using 10 mL of ethanol, and dried in a vacuum pump. The purity of it was determined by FT-IR spectroscopy and melting point.

Esterification Reactions
The reactions (Scheme 2) were carried out in a round-bottom flask under reflux and agitation, at 60 °C. 0.5 mmol of catalyst was added, using a 1:2 ratio of carboxylic acid/alcohol. The reaction was monitored by gas chromatography for a range of 1-24 h depending on the progress of the reaction, taking the constant area of the chromatographic peak corresponding to the product as a criterion. The product was quantified by the internal standard method using 1-bromo octane.  Table 1 shows the percentage of yield obtained for each aldehyde used in both ILs and H2SO4.

Esterification Reactions
For this reaction, the carboxylic acid was kept fixed, and the structure of the alcohol was varied. The yields obtained for both ILs and H2SO4 are shown in Table 2. For alcohols with molecular weight less than 100 g/mol (ethanol-isoamyl alcohol), the average yield is 93.5% for both ILs and 73% for H2SO4, and for octanol the average yield was 51% compared to 35% obtained for mineral acid. For both ILs, the formation of two phases was observed, one corresponding to the IL and the other to the product. Consequently, these systems present the great advantage of recycling both catalysts. The esters produced show immiscibility in the ILs, therefore the esterification begins as a homogeneous process and ends as two-phase (product and excess of reactant in the upper phase and the catalyst in the lower aqueous phase).
This differential solubility allows the facilitation of the separation of the products by simple decantation. Thus, it was possible to recycle and reuse the catalysts. The ILs were separated from the water produced during the synthesis. They were washed three times with ethyl acetate to eliminate remaining traces of the reaction. The results after four recycling cycles to obtain isoamyl acetate for each IL are shown in Table 3.

Conclusions
The voltametric results of TSIL, [bsmim] [HSO4] indicate that in addition to the ionic pair, free H2SO4 of the autoprotolysis equilibrium is present, because the method of obtaining the TSIL involves a protonation equilibrium of the zwitterion (4-(1-methyl imidazolium-3-yl)-1-butane sulfonate (Zbsmim)) with H2SO4. On the other hand, for [bmim] [HSO4], the bisulfate anion is forming part of the cation-anion ionic network in the IL's structure, so it would not be available to catalyze. In this case, the catalysis could be due to the presence of the bisulfate sodium specie because the method of obtaining this IL involves the metathesis reaction between this salt and IL [bmim] [Br] (the voltametric results were recently sent for revision [5]). From the results observed in the reactive systems for both ILs, we can conclude that in general there is an increase in the yield of between 15-30% for both reactions with respect to the action of the inorganic catalyst. The ILs are suitable for both systems. The TSIL [bsmim] [HSO4] allows the obtaining of better performance in both reactive systems with the possibility of its recycling and reusing. This TSIL is a reservoir of H2SO4, available for controlled delivery according to the demand of the reactive systems.