Shaping micro- and macroscopic properties of ionic liquid-solute systems: multifunctional task-specific (solvent, catalyst, water scavenger) agents
Final Report Abstract
The project “Shaping micro- and macroscopic properties of ionic liquid-solute systems: multifunctional task-specific (solvent, catalyst, water scavenger) agents” set out with the initial hypothesis that the esterification (a condensation reaction) of an alcohol and a carboxylic acid should benefit from the application of a multi-functional task-specific ionic liquid (IL). The multifunctional role of the IL is acting as not only the solvent and water scavenger, but also as organo-catalyst: specific interactions, in particular by hydrogen bonding of the anion with the reactants were thought to activate the reactants. The methodology included the study of the effect of structural variations of homologous series of ILs on the kinetics and thermodynamics of the reaction. Furthermore, temperatureand concentration-dependent physico-chemical and spectroscopic investigations of binary (IL/solute) mixtures were conducted to understand the interactions with the reactants (benzyl alcohol, acetic acid), the by-product water and in certain cases an auxiliary acid catalyst. In summary, the study demonstrated that in the absence of an auxiliary acid catalyst, 1. all ILs decrease the rate of reaction compared to the solvent-free system, 2. the rate of reaction decreases with increasing basicity of the IL anion, and 3. the equilibrium yield decreases linearly with increasing amounts of IL present. 4. A beneficial water-scavenging effect was not observed in the esterification. Studies on the physico-chemical properties (and AIMD calculations) of an acetate-based IL and its mixture with water conducted in cooperation with Prof. Kirchner show that the anion-water hydrogen bond is not sufficiently strong to physically remove water from the reaction equilibrium. Instead, a fluctuating and highly structured ion-water arrangement is formed, as also found for methanesulfonate-water mixtures. 5. Spectroscopic investigations clearly show that a basic anion of the IL beneficially enhances the nucleophilicity of the alcohol, but simultaneously decreases detrimentally the electrophilicity of the carboxylic acid, as both reactants are hydrogen bond donors. Hence, the IL itself cannot act as an organo-catalyst in this reaction. In the presence of an auxiliary acid catalyst, three categories of ILs were identified. 4. ILs bearing a basic anion (acetate). Acid catalysts such as H3PO4, H2SO4 or CH3SO3H are “neutralized” by the IL (ion exchange takes place). Rate, yield and selectivity can only be affected when super-stoichiometric amounts of acid are added to the IL. 5. ILs bearing an anion of medium basicity ([CH3SO3]-, Cl-). Increasing amounts of acid catalyst (H3PO4, H2SO4 or CH3SO3H) increase the rate of reaction. The equilibrium yield decreases linearly with the amount of IL present. 6. ILs bearing an anion of low basicity ([NTf2]-, [OTf]-). A very high catalytic activity is obtained at low acid catalyst concentrations (< 1 mol% of CH3SO3H), and the equilibrium yield is not dependent on the amount of IL present. In these instances, the proton is not strongly solvated and hence capable to undergo catalytic activation of the reactants. In an extension to the above study on 1,3-dialkylimidazolium-based ILs, acido-basic methylimidazolium-based ionic liquids (ABILs) were studied. ABILs composed of an acid with a lower pKa gave higher reaction rates due to the higher activity (lowest solvation of the proton), but also lower selectivities (formation of dibenzyl ether). Irrespective of the composition of the ABIL, increasing amounts decreased the equilibrium yield, as found for conventional ILs. Overall, it can be stated that effect if ILs on the esterification (and similar acid-catalyzed condensation reactions) is well understood now. Proton activity is dependent on the basicity of the IL anion. If the anion possesses a low basicity (i.e. low solvation of the proton), a very effective reaction medium results giving high reaction rates and yields of ester (80%, 3 h, 100°C). Several strong and sustainable cooperations were formed within the SPP 1191 (Verevkin, Kirchner, Imhof, Bordusa, Giernoth, Lenzer). The DFG funding enabled my own habilitation and the dissertation of 5 PhD and 11 MSc students from related projects in the years 2006 – 2013. To-date, a total of 25 publications, 5 book chapters and 3 patents have resulted.
