Peter S. Schulz

Influence of Different Anions on the Surface Composition of Ionic Liquids Studied Using ARXPS

Angle-resolved X-ray photoelectron spectroscopy has been used to study the influence of different types of anions on the surface composition of ionic liquids (ILs). We have investigated nine ILs with the same cation, 1-octyl-3-methylimidazolium [C(8)C(1)Im](+), but very different anions. In all cases, an enrichment of the cation alkyl chains is found at the expense of the polar cation head groups and the anions in the first molecular layer. This enhancement effect decreases with increasing size of the anion, which means it is most pronounced for the smallest anions and least pronounced for the largest anions. A simple model is proposed to explain the experimental observations.

Surface Studies on the Ionic Liquid 1-Ethyl-3-Methylimidazolium Ethylsulfate Using X-Ray Photoelectron Spectroscopy (XPS)

Surface studies of ionic liquids are particularly important for all kinds of multiphasic operations employing ionic liquids, e.g. biphasic homogeneous catalysis or supported ionic liquid phase catalysis. Using X-ray photoelectron spectroscopy (XPS), the surface composition of the model system 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtOSO3] was investigated. By comparing two different samples of this ionic liquid from two different origins, we observed a decisive influence of silicon containing impurities on composition and structure of the surface. For the case of the impurities containing ionic liquid, our angle-dependent XPS data are in agreement with a model of a surface layer consisting of highly oriented ionic liquid molecules. From a fundamental point of view, our study may be of general relevance for the understanding of the chemistry of liquid surfaces in general.

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.

Transesterification of methylsulfate and ethylsulfate ionic liquids—an environmentally benign way to synthesize long-chain and functionalized alkylsulfate ionic liquids

A new environmentally friendly method to synthesize long-chain and functionalized alkylsulfate ionic liquids is reported. The two-step synthesis comprises the synthesis of a methylsulfate or ethylsulfate ionic liquid by direct alkylation in the first step. In the second step, this intermediate is transformed in a transesterification reaction, using different functionalized and non-functionalized alcohols, to the corresponding new alkylsulfate melts. The entire reaction sequence is halide-free and liberates methanol or ethanol as the only by-products. Moreover, it is carried out in a solvent-free manner and scale-up is straight forward.

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.

Ligand effects on the surface composition of Rh-containing ionic liquid solutions used in hydroformylation catalysis.

In hydroformylation, multiphase catalysis is a well-established and industrially realized method for effective catalyst separation and recycling. Aqueous phase liquid–liquid biphasic catalysis was developed through the pioneering discovery of the highly water-soluble ligand tris(3-sodium sulfonatophenyl)phosphine (tppts, 1) by Kuntz in 1976. This ligand concept enabled the industrial realization of aqueous hydroformylation for propene hydroformylation in the Ruhrchemie–Rh ne–Poulenc process. The process went on stream in 1984, and is still operating today (550000 tons per year). However, the limited solubility of higher olefins (>C4) in water has prompted much research activity into alternative polar catalyst media for liquid–liquid multiphase hydroformylation. Among these endeavors, the use of lowmelting salts, so-called ionic liquids (ILs), has attracted particular interest as many ionic liquids show sufficiently high solubilities for higher olefins to allow reasonable reaction rates. First reports on the application of ionic liquids in Rh-catalyzed hydroformylation were published by Chauvin s group in 1995. Already in this first paper, the use of sulfonated triphenylphosphine ligands was highlighted as a crucial precondition to avoid Rh-leaching into the organic product phase. Later, sulfonated triphenylphosphine ligands were also combined with imidazolium counter ions. ColeHamilton and co-workers suggested, for example, [C3mim]2ACHTUNGTRENNUNG[PhP ACHTUNGTRENNUNG(C6H4SO3)2] as a suitable ligand for hydroformylation reactions in the biphasic system ionic liquid/scCO2. [7] The same ligand system was applied recently to an even more efficient catalytic system using the ionic catalyst solution in the form of a supported ionic liquid phase (SILP). Such SILP catalytic systems have also been very successfully applied in continuous gas-phase reactions where the ionic liquid film supported on a highly porous inorganic support is contacted directly with the gas-phase of the reactants to perform continuous hydroformylation using a continuous fixed-bed reactor. In general, hydroformylation in ionic liquids has produced a huge amount of scientific activity over the last 15 years and particular progress was made by the use of regioselective ionic ligand systems and by the application of ionic liquids carrying halide-free, cheap and hydrolytic stable anions. The field has been recently summarized comprehensively in an excellent review by Haumann and Riisager. In multiphase catalysis, interface processes such as substrate diffusion into the catalyst phase, the reaction rate at the phase boundary (in comparison to reaction rate in the bulk), and product diffusion back into the organic phase play a crucial role for the overall performance of the system. Despite this obvious fact, experimental investigations into the chemical nature of the liquid surface of catalytic systems are lacking so far. This is even more surprising in the light of recent theoretical findings by the group of Wipff, which demonstrated that the composition of the catalytic interface may well be very different from the average chemical composition of two adjacent bulk liquid phases. This fact is of high relevance for the detailed understanding of all above-mentioned interface transport and reaction processes. Moreover, the authors demonstrated that the surface-active character of a given type of an IL soluble complex was similar at the IL–vacuum interface and at the IL interface with weakly polar organic solvents. The [a] Dipl.-Chem. C. Kolbeck, Dipl.-Chem. T. Cremer, Dr. F. Maier, Prof. Dr. H.-P. Steinr ck Chair of Physical Chemistry II Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91054 Erlangen (Germany) Fax: (+49)9131-852-8867 E-mail : steinrueck@chemie.uni-erlangen.de [b] Dipl.-Chem. N. Paape, Dr. P. S. Schulz, Prof. Dr. P. Wasserscheid Lehrstuhl f r Chemische Reaktionstechnik Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91058 Erlangen (Germany) [] These authors contributed equally to this work.

Thermophysical properties of the ionic liquids [EMIM][B(CN)4] and [HMIM][B(CN)4].

In the present study, the thermophysical properties of the tetracyanoborate-based ionic liquids (ILs) 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][B(CN)4]) and 1-hexyl-3-methylimidazolium tetracyanoborate ([HMIM][B(CN)4]) obtained by both experimental methods and molecular dynamics (MD) simulations are presented. Conventional experimental techniques were applied for the determination of refractive index, density, interfacial tension, and self-diffusion coefficients for [HMIM][B(CN)4] at atmospheric pressure in the temperature range from 283.15 to 363.15 K. In addition, surface light scattering (SLS) experiments provided accurate viscosity and interfacial tension data. As no complete molecular parametrization was available for the MD simulations of [HMIM][B(CN)4], our recently developed united-atom force field for [EMIM][B(CN)4] was partially transferred to the homologous IL [HMIM][B(CN)4]. Deviations between our simulated and experimental data for the equilibrium properties are less than ±0.3% in the case of density and less than ±8% in the case of interfacial tension for both ILs. Furthermore, the calculated and measured data for the transport properties viscosity and self-diffusion coefficient are in good agreement, with deviations of less than ±30% over the whole temperature range. In addition to a comparison with the literature, the influence of varying cation chain length on thermophysical properties of [EMIM][B(CN)4] and [HMIM][B(CN)4] is discussed.

An Adaptive Self-Healing Ionic Liquid Nanocomposite Membrane for Olefin-Paraffin Separations

An adaptive self-healing ionic liquid nanocomposite membrane comprising a multi-layer support structure hosting the ionic salt [Ag](+) [Tf(2) N](-) is used for the separation of the olefin propylene and the paraffin propane. The ionic salt renders liquid like upon complexation with propylene, resulting in facilitated transport of propylene over propane at benchmark-setting selectivity and permeance levels. The contacting with acetylene causes the ionic salt to liquefy without showing evidence of forming explosive silver acetylide.

Infrared Spectroscopy of a Wilkinson Catalyst in a Room‐Temperature Ionic Liquid

Homogeneous catalysis in room-temperature ionic liquids (ILs) constitutes a most interesting field of research with high potential in technical applications. As concerns the hydrogenation of unsaturated hydrocarbons, Wilkinsons compound RhCl(PPh(3))(3) represents a catalyst that provides high selectivity and activity. Herein, we demonstrate the application of infrared spectroscopy to the quantitative analysis of the Wilkinson catalyst in the IL 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]). Our study demonstrates for the first time the quantitative, accurate and reproducible determination of the concentration of a rhodium catalyst by means of IR spectroscopy and, moreover, allows the investigation of intermolecular interactions. Spectral features, located mainly in the fingerprint region of the IR spectrum, are identified revealing the influence of the dissolved catalyst on the ILs vibrational structure. In particular, the ring-bending mode of the imidazolium ring shows a frequency shift as a function of catalyst concentration, probably due to hydrogen-bond formation between the IL cation and the Rh complex. The results show the potential of IR spectroscopy both for application as a quick process control technology in catalytic processes and as a tool for better understanding of IL-catalyst interactions.

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.