2015, Paschek, Dietmar, Golub, Benjamin, Ludwig, Ralf

We report results of molecular dynamics (MD) simulations characterising the hydrogen bonding in mixtures of two different protic ionic liquids sharing the same cation: triethylammonium-methylsulfonate (TEAMS) and triethylammonium-triflate (TEATF). The triethylammonium-cation acts as a hydrogen-bond donor, being able to donate a single hydrogen-bond. Both, the methylsulfonate- and the triflate-anions can act as hydrogen-bond acceptors, which can accept multiple hydrogen bonds via their respective SO3-groups. In addition, replacing a methyl-group in the methylsulfonate by a trifluoromethyl-group in the triflate significantly weakens the strength of a hydrogen bond from an adjacent triethylammonium cation to the oxygen-site in the SO3-group of the anion. Our MD simulations show that these subtle differences in hydrogen bond strength significantly affect the formation of differently-sized hydrogen-bonded aggregates in these mixtures as a function of the mixture-composition. Moreover, the reported hydrogen-bonded cluster sizes can be predicted and explained by a simple combinatorial lattice model, based on the approximate coordination number of the ions, and using statistical weights that mostly account for the fact that each anion can only accept three hydrogen bonds.

2019, Dilla, Martin, Moustakas, Nikolaos G., Becerikli, Ahmet E., Peppel, Tim, Springer, Armin, Schlögl, Robert, Strunk, Jennifer, Ristig, Simon

In this study we assess the general applicability of the widely used P25-TiO2 in gas-phase photocatalytic CO2 reduction based on experimentally determined reactivity descriptors from classical heterogeneous catalysis (productivity) and photochemistry (apparent quantum yield/AQY). A comparison of the results with reports on the use of P25 for thermodynamically more feasible reactions and our own previous studies on P25-TiO2 as photocatalyst imply that the catalytic functionality of this material, rather than its properties as photoabsorber, limits its applicability in the heterogeneous photocatalytic CO2 reduction in the gas phase. The AQY of IrOx/TiO2 in overall water splitting in a similar high-purity gas-solid process was four times as high, but still far from commercial viability.

2019, Dilla, Martin, Jakubowski, Alina, Ristig, Simon, Strunk, Jennifer, Schlögl, Robert

Although the photocatalytic reduction of CO2 to CH4 by using H2O as the oxidant presupposes the formation of O2, it is often not included in the product analysis of most of the studies dealing with photocatalytic CO2 reduction or it is reported to be not formed at all. The present study aims to clarify the absence of O2 in the photocatalytic gas phase CO2 reduction on TiO2. By modifying P25-TiO2 with IrOx co-catalysts it was possible to observe photocatalytic water splitting, i.e. the formation of gaseous O2 and H2 in almost stoichiometric amounts, without the use of sacrificial agents, while bare P25-TiO2 showed no activity in H2 and O2 formation under similar reaction conditions. Investigating the effect of improved H2O oxidation properties on the photocatalytic CO2 reduction revealed that the CH4 formation on P25 from CO2 was completely inhibited as long as the H2O splitting reaction proceeded. Furthermore, we found that a certain amount of O2 is consumed under conditions of photocatalytic water oxidation. A quantification showed it to be in the same order of magnitude as the oxygen which is missing as a byproduct from photocatalytic CO2 conversion. A detailed interpretation of the results in the context of the general understanding of the photocatalytic CO2 reduction with H2O on TiO2 allows the hypothesis that P25-TiO2 undergoes a stoichiometric reaction, meaning that the CH4 formation is not based on a true catalytic cycle and runs only as long as TiO2 can consume oxygen.

2015, Ludwig, Ralf

We could show by means of dispersion-corrected DFT calculations that the interaction energy in protic ionic liquids can be dissected into Coulomb interaction, hydrogen bonding and dispersion interaction. The H-bond energy as well as the dispersion energy can be quantified to be 50 kJ mol−1 each representing ten percent of the overall interaction energy. The dispersion interaction could be dissected into two portions. One third could be related to the dispersion interaction within an ion-pair enhancing the H-bond strength, two thirds stem from dispersion interaction between the ion-pairs. This distribution of dispersion interaction is reflected in the far infrared (FIR) spectra. The H-bond band is shifted weaker than the low frequency band where the latter indicates diffuse cation–anion interaction and H-bond bending motions. Finally, we can dissect the different types of interaction energies indicating their characteristic influence on vibrational modes in the FIR.

2016, Kerlé, Daniela, Namayandeh Jorabchi, Majid, Ludwig, Ralf, Wohlrab, Sebastian, Paschek, Dietmar

We have determined the temperature dependence of the solvation behavior of a large collection of important light gases in imidazolium-based ionic liquids with the help of extensive molecular dynamics simulations. The motivation of our study is to unravel common features of the temperature dependent solvation under well controlled conditions, and to provide a guidance for cases, where experimental data from different sources disagree significantly. The solubility of molecular hydrogen, oxygen, nitrogen, methane, krypton, argon, neon and carbon dioxide in the imidazolium based ionic liquids of type 1-n-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][NTf2]) with varying alkyl side chain lengths n = 2, 4, 6, 8 is computed for a temperature range between 300 K and 500 K at 1 bar. By applying Widom's particle insertion technique and Bennet's overlapping distribution method, we are able to determine the temperature dependent solvation free energies of those selected light gases in simulated imidazolium based ionic liquids with high statistical accuracy. Our simulations demonstrate that the magnitude of the solvation free energy of a gas molecule at a chosen reference temperature and that of its temperature-derivatives are intimately related to one another. We conclude that this "universal" behavior is rooted in a solvation entropy-enthalpy compensation effect, which seems to be a defining feature of the solvation of small molecules in ionic liquids. The observations lead to simple analytical relations, determining the temperature dependence of the solubility data based on the absolute solubility at a certain reference temperature. By comparing our results with available experimental data from many sources, we can show that our approach is particularly helpful for providing reliable estimates for the solvation behavior of very light gases, such as hydrogen, where conflicting experimental data exist.

2016, Strauch, Matthias, Bonsa, Anne-Marie, Golub, Benjamin, Overbeck, Viviane, Michalik, Dirk, Paschek, Dietmar, Ludwig, Ralf

We describe a method for the accurate determination of deuteron quadrupole coupling constants χD for N–D bonds in triethylammonium-based protic ionic liquids (PILs). This approach was first introduced by Wendt and Farrar for O–D bonds in molecular liquids, and is based on the linear relationship between the deuteron quadrupole coupling constants χD, and the proton chemical shifts δ1H, as obtained from DFT calculated properties in differently sized clusters of the compounds. Thus the measurement of δ1H provides an accurate estimate for χD, which can then be used for deriving reorientational correlation-times τND, by means of NMR deuteron quadrupole relaxation time measurements. The method is applied to pure PILs including differently strong interacting anions. The obtained χD values vary between 152 and 204 kHz, depending on the cation–anion interaction strength, intensified by H-bonding. We find that considering dispersion corrections in the DFT-calculations leads to only slightly decreasing χD values. The determined reorientational correlation times indicate that the extreme narrowing condition is fulfilled for these PILs. The τc values along with the measured viscosities provide an estimate for the volume/size of the clusters present in solution. In addition, the correlation times τc, and the H-bonded aggregates were also characterized by molecular dynamics (MD) simulations.

2019, Zhivonitko, Vladimir V., Bresien, Jonas, Schulz, Axel, Koptyug, Igor V.

Metal-free H2 activations are unusual but interesting for catalytic transformations, particularly in parahydrogen-based nuclear spin hyperpolarization techniques. We demonstrate that metal-free singlet phosphorus biradicaloid, [P(μ-NTer)]2, provides pronounced 1H and 31P hyperpolarization while activating the parahydrogen molecules. A brief analysis of the resulting NMR signals and the important kinetic parameters are presented.

2015, Knorr, Anne, Fumino, Koichi, Bonsa, Anne-Marie, Ludwig, Ralf

The subtle energy-balance between Coulomb-interaction, hydrogen bonding and dispersion forces governs the unique properties of ionic liquids. To measure weak interactions is still a challenge. This is in particular true in the condensed phase wherein a melange of different strong and directional types of interactions is present and cannot be detected separately. For the ionic liquids (2-hydroxyethyl)-trimethylammonium (cholinium) bis(trifluoro-methylsulfonyl)amide and N,N,N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)amide which differ only in the 2-hydroxyethyl and the propyl groups of the cations, we could directly observe distinct vibrational signatures of hydrogen bonding between the cation and the anion indicated by ‘jumping and pecking’ motions of cholinium. The assignment could be confirmed by isotopic substitution H/D at the hydroxyl group of cholinium. For the first time we could also find direct spectroscopic evidence for H-bonding between like-charged ions. The repulsive Coulomb interaction between the cations is overcome by cooperative hydrogen bonding between the 2-hydroxyethyl functional groups of cholinium. This H-bond network is reflected in the properties of protic ionic liquids (PILs) such as viscosities and conductivities.