Inga Niedermaier

Carbon Dioxide Capture by an Amine Functionalized Ionic Liquid: Fundamental Differences of Surface and Bulk Behavior

Carbon dioxide (CO2) absorption by the amine-functionalized ionic liquid (IL) dihydroxyethyldimethylammonium taurinate at 310 K was studied using surface- and bulk-sensitive experimental techniques. From near-ambient pressure X-ray photoelectron spectroscopy at 0.9 mbar CO2, the amount of captured CO2 per mole of IL in the near-surface region is quantified to ~0.58 mol, with ~0.15 mol in form of carbamate dianions and ~0.43 mol in form of carbamic acid. From isothermal uptake experiments combined with infrared spectroscopy, CO2 is found to be bound in the bulk as carbamate (with nominally 0.5 mol of CO2 bound per 1 mol of IL) up to ~2.5 bar CO2, and as carbamic acid (with nominally 1 mol CO2 bound per 1 mol IL) at higher pressures. We attribute the fact that at low pressures carbamic acid is the dominating species in the near-surface region, while only carbamate is formed in the bulk, to differences in solvation in the outermost IL layers as compared to the bulk situation.

Organic Reactions in Ionic Liquids Studied by in Situ XPS

We demonstrate the application of in situ X-ray photoelectron spectroscopy (XPS) to monitor organic, liquid-phase reactions. By covalently attaching ionic head groups to the reacting organic molecules, their volatility can be reduced such that they withstand ultra high vacuum conditions. The applied method, which is new for the investigation of organic reactions, allows for following the fate of all elements present in the reaction mixture--except for hydrogen--in a quantitative and oxidation-state-sensitive manner in one experiment. This concept is demonstrated for the alkylation of a tertiary amine attached to an imidazolium or phosphonium moiety by the anion 4-chlorobutylsulfonate ([ClC(4)H(8)SO(3)](-)). In the course of the reaction, the covalently bound chlorine is converted to chloride and the amine to ammonium as reflected by the distinct shifts in the N 1s and Cl 2p binding energies.

Influence of Substituents and Functional Groups on the Surface Composition of Ionic Liquids

We have performed a systematic study addressing the surface behavior of a variety of functionalized and non-functionalized ionic liquids (ILs). From angle-resolved X-ray photoelectron spectroscopy, detailed conclusions on the surface enrichment of the functional groups and the molecular orientation of the cations and anions is derived. The systems include imidazolium-based ILs methylated at the C2 position, a phenyl-functionalized IL, an alkoxysilane-functionalized IL, halo-functionalized ILs, thioether-functionalized ILs, and amine-functionalized ILs. The results are compared with the results for corresponding non-functionalized ILs where available. Generally, enrichment of the functional group at the surface is only observed for systems that have very weak interaction between the functional group and the ionic head groups.

Monitoring of Liquid‐Phase Organic Reactions by Photoelectron Spectroscopy

There are strings attached: after linking the reacting groups to head groups of ionic liquids to drastically lower the vapour pressures of the reactants, ordinary liquid-phase organic reactions can be monitored by in situ X-ray photoelectron spectroscopy. This approach is demonstrated for the nucleophilic substitution of an alkyl amine and an alkyl chloride moiety, which are attached to the cation and anion of ionic liquids, respectively.

Cyclic Thiouronium Ionic Liquids: Physicochemical Properties and their Electronic Structure Probed by X‐Ray Induced Photoelectron Spectroscopy

In the last 20 years, ionic liquids (ILs) have attracted growing interest as fluids for a broad range of applications, including catalysis, electrochemistry, separation technologies, molecular materials, or performance chemicals. The main part of this research has been carried out using ILs based on 1,3-dialkylimidazolium cations. This is due to the particularly attractive physicochemical properties (e.g. melting points, viscosities, thermal stabilities) of imidazolium salts compared with their ammonium, phosphonium, pyridinium, and sulfonium counterparts. In addition, the 1,3dialkylimidazolium structural motif offers attractive options to modify the steric properties of the ionic head group. Starting from differently alkyl functionalized imidazoles, it is synthetically straight forward to form the ionic liquid cation by means of an alkylation reaction introducing other alkyl groups, eventually with additional functionalities. Though there are numerous publications describing a broad range of imidazolium salts variously substituted at the 1and 3-position, reports dealing with the introduction of alkyl or even substituted alkyl groups in the 2-position of the same type of cation are rather scarce. It is well known that the hydrogen attached to the C2-position of an 1,3-dialkylACHTUNGTRENNUNGimidzazolium ion is relatively acidic. This offers a way to exploit such a reactive site for new synthetic approaches, for example, via the corresponding metal–carbene complexes. On the other hand, this fact limits the stability of 1,3-diACHTUNGTRENNUNGalkylimidazolium ionic liquids in the presence of bases. Alternatively, 1,2-dimethyl-3-alkylimidazolium salts have been proposed to overcome such complication. The latter are easily accessible by alkylation of the commercial 1,2-dimethylimidazole but typically display significantly higher melting points compared to their corresponding 1-alkyl-3-methylimidazolium salts. Kempter and Kirchner have rationalized this important and general observation by molecular dynamics studies. Only a few approaches have been proposed for functionalizing the imidazolium 2-position apart from methyl. Ennis and Handy reacted 1,3-dialkylimidazolium halide salts with NaH followed by alkylation of the in situ formed carbene with alkylhalides. Lee and coworkers introduced a carbonyl group in the 2-position by reacting 1,3-dialkylimidazolium salts with formaldehyde. Rogers and co-workers have developed a synthetic strategy based on imidazolyl-2-carboxylates obtained via alkylation of 1-alkylimidazole with dimethyl carbonate. Recently, a report of the Rogers group has demonstrated the carbenoid nature of 1,3-dialkylimidazolium acetates by reacting the latter with chalcogenides, such as sulfur. Based on this inspiring contribution, in the following we present a functionalization strategy that involves the formal attachment of a thio-alkyl group at the 2-position of the imidazolium ring resulting in ILs with cyclic thiouronium cations exhibiting a remarkable electronic structure of the cations as witnessed by X-ray induced photoelectron spectroscopy (XPS). Basically, the applied synthetic approach based on using imidazolyl-2-thiones as starting material for a consecutive alkylation reaction with alkyl iodides, followed by anion metathesis with lithium bis(trifluoromethyl sulfonyl)imide (Li ACHTUNGTRENNUNG[Tf2N]). The adopted three-step synthetic sequence is depicted in Scheme 1. Note that the chemistry of such system has been pioneered by Merck KGaA. However, the spe-

Perspective: Chemical reactions in ionic liquids monitored through the gas (vacuum)/liquid interface

This perspective analyzes the potential of X-ray photoelectron spectroscopy under ultrahigh vacuum (UHV) conditions to follow chemical reactions in ionic liquids in situ. Traditionally, only reactions occurring on solid surfaces were investigated by X-ray photoelectron spectroscopy (XPS) in situ. This was due to the high vapor pressures of common liquids or solvents, which are not compatible with the required UHV conditions. It was only recently realized that the situation is very different when studying reactions in Ionic Liquids (ILs), which have an inherently low vapor pressure, and first studies have been performed within the last years. Compared to classical spectroscopy techniques used to monitor chemical reactions, the advantage of XPS is that through the analysis of their core levels all relevant elements can be quantified and their chemical state can be analyzed under well-defined (ultraclean) conditions. In this perspective, we cover six very different reactions which occur in the IL, with the IL, or at an IL/support interface, demonstrating the outstanding potential of in situ XPS to gain insights into liquid phase reactions in the near-surface region.

Surface Enrichment in Equimolar Mixtures of Non-Functionalized and Functionalized Imidazolium-Based Ionic Liquids

Abstract For equimolar mixtures of ionic liquids with imidazolium‐based cations of very different electronic structure, we observe very pronounced surface enrichment effects by angle‐resolved X‐ray photoelectron spectroscopy (XPS). For a mixture with the same anion, that is, 1‐methyl‐3‐octylimidazolium hexafluorophosphate+1,3‐di(methoxy)imidazolium hexafluorophosphate ([C8C1Im][PF6]+[(MeO)2Im][PF6]), we find a strong enrichment of the octyl chain‐containing [C8C1Im]+ cation and a corresponding depletion of the [(MeO)2Im]+ cation in the topmost layer. For a mixture with different cations and anions, that is, [C8C1Im][Tf2N]+[(MeO)2Im][PF6], we find both surface enrichment of the [C8C1Im]+ cation and the [Tf2N]− (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO)2Im]+ and [PF6]− are depleted from the surface. We propose that the observed behavior in these mixtures is due to a lowering of the surface tension by the enriched components. Interestingly, we observe pronounced differences in the chemical shifts of the imidazolium ring signals of the [(MeO)2Im]+ cations as compared to the non‐functionalized cations. Calculations of the electronic structure and the intramolecular partial charge distribution of the cations contribute to interpreting these shifts for the two different cations.