[Show abstract][Hide abstract] ABSTRACT: Liquid organic hydrogen carriers (LOHC) are potential compounds that can facilitate chemical energy storage and hydrogen logistics using reversible hydrogenation. For the process development, the physical solubility of hydrogen in potential LOHCs is required. In this work, solubility of hydrogen in the potential LOHC systems toluene/methylcyclohexane, N-ethylcarbazole/perhydro-N-ethylcarbazole, and dibenzyltoluene/perhydrodibenzyltoluene was measured using the static isochoric saturation method. The data were measured at low pressures up to 10 bar within the temperature range of (293 to 373) K. Hydrogen solubility in hydrogenated forms of the LOHCs was found to be higher compared to the dehydrogenated forms. Solubility in all substances increased with increasing temperature within the whole temperature range under consideration. The temperature dependency of the Henry coefficient of hydrogen in the solvents was correlated using the Benson and Krause correlation.
Full-text · Article · Dec 2015 · Journal of Chemical & Engineering Data
[Show abstract][Hide abstract] ABSTRACT: The application of polymer-based spherical activated carbon (PBSAC) as catalyst support offers unprecedented advantages in the handling of the catalyst in all stages of its application due to its superior fluid dynamic properties and dust-free character, minimizing post processing times and facilitating catalyst recycling. The preparation of PBSAC-based Pd catalysts has been addressed using PBSAC of different particle sizes and degrees of activation. Prior to Pd deposition, the support was oxidized in nitric and sulfuric acidic media under different operating conditions in order to introduce surface charges for the ion exchange. This method was successfully applied as, even for the mildest functionalization conditions, full deposition of Pd was achieved. The synthesized Pd-PBSAC catalysts were successfully employed in the hydrogenation of cinnamic acid achieving its complete conversion at an optimum effective rate constant of 0.122m3kgPd-1s-1.
No preview · Article · Dec 2015 · Chemical Engineering & Technology
[Show abstract][Hide abstract] ABSTRACT: Ionic liquids (ILs) have recently been proposed as modifying agents to tune the selectivity and the activity of electrocatalysts. In order to explore the origin of IL-induced effects on electrocatalytic reactions, we have investigated (i) the interaction of [C1C2Im][OTf]-water mixtures with Pt(1 1 1) single crystal electrodes and (ii) the influence of [C1C2Im][OTf] on methanol oxidation in acidic aqueous solution using cyclic voltammetry (CV) and electrochemical IR spectroscopy. We show that [C1C2Im][OTf] interacts strongly with the Pt(1 1 1) surface in the potential range from -0.2 VAg/AgCl to 0.65 VAg/AgCl, leading to partial blocking of the surface, partial suppression of hydrogen adsorption, and partial suppression of OHads formation. The interaction of the IL with Pt occurs in this potential range via specific adsorption of the [OTf]- anions which bind to Pt(1 1 1) via the SO3- group in a tilted adsorption geometry. The adsorption process is largely reversible as a function of the electrode potential, but shows a complex kinetics and becomes very slow at high IL coverage. Also, [C1C2Im][OTf] has a strong effect on the electro-oxidation of methanol in acidic solution. Adsorption of [OTf]- leads to partial blocking of the indirect oxidation pathway to CO. The onset potential for CO oxidation is found to be shifted to more positive values indicating hindered access of H2O to the electrode surface. CO and the [C1C2Im][OTf] form a mixed co-adsorption layer which is characterized by a strongly red-shifted CO stretching band. The effect is assigned to an IL-induced ligand effect in the mixed adsorbate layer and suggests that the direct modification of active centers by surface-bound IL species plays an important role in IL-modified electrocatalytic materials.
[Show abstract][Hide abstract] ABSTRACT: Although N-ethylcarbazole is considered as a most promising potential liquid organic hydrogen carriers (LOHC) substance, a major drawback for stationary and particularly mobile applications is its high melting point of 343 K. Study of other possible N-alkylcarbazole-derivatives having lower melting points but keeping a high storage density is of practical importance. This contribution presents thermochemical properties (enthalpy of formation, enthalpy of vaporization, enthalpy of sublimation, and fusion enthalpy) for N-alkylcarbazoles (with alkyl = isopropyl, n-propyl, and n-butyl) derived from experiments in order to investigate the applicability of the carbazole derivatives as potential LOHCs. Additionally, high-level quantum chemical calculations were applied to determine molar enthalpies of formation of the gaseous carbazole derivatives and thus validated the experimental findings. Using a combination of the quantum-chemical calculations with vaporization enthalpy data measured in this work, the standard molar liquid phase enthalpies of formation were derived for alkylcarbazole derivatives. Results of this study were applied for thermodynamic analysis of the liquid-phase reversible hydrogenation/dehydrogenation processes with N-alkylcarbazoles.
No preview · Article · Nov 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Liquid Organic Hydrogen Carrier (LOHC) systems offer a very attractive way for storing and distributing hydrogen from electrolysis using excess energies from solar or wind power plants. In this contribution, an alternative, high-value utilization of such hydrogen is proposed namely its use in steady-state chemical hydrogenation processes. We here demonstrate that the hydrogen-rich form of the LOHC system dibenzyltoluene/perhydro-dibenzyltoluene can be directly applied as sole source of hydrogen in the hydrogenation of toluene, a model reaction for large-scale technical hydrogenations. Equilibrium experiments using perhydro-dibenzyltoluene and toluene in a ratio of 1:3 (thus in a stoichiometric ratio with respect to H2) yield conversions above 60%, corresponding to an equilibrium constant significantly higher than 1 under the applied conditions (270 °C).
Preview · Article · Nov 2015 · International Journal of Hydrogen Energy
[Show abstract][Hide abstract] ABSTRACT: The Liquid Organic Hydrogen Carrier (LOHC) concept offers an efficient route to store hydrogen using organic compounds that are reversibly hydrogenated and dehydrogenated. One important challenge towards application of the LOHC technology at a larger scale is to minimize degradation of Pt-based dehydrogenation catalysts during long-term operation. Herein, we investigate the regeneration of Pt/alumina catalysts poisoned by LOHC degradation. We combine ultrahigh vacuum (UHV) studies on Pt(111), investigations on well-defined Pt/Al2O3 model catalysts, and near-ambient pressure (NAP) measurements on real core-shell Pt/Al2O3 catalyst pellets. The catalysts were purposely poisoned by reaction with the LOHC perhydro-dibenzyltoluene (H18-MSH) and with dicyclohexylmethane (DCHM) as a simpler model compound. We focus on oxidative regeneration under conditions that may be applied in real dehydrogenation reactors. The degree of poisoning and regeneration under oxidative reaction conditions was quantified using CO as a probe molecule and measured by infrared reflection-absorption spectroscopy (IRAS) and diffuse reflectance Fourier transform IR spectroscopy (DRIFTS) for planar model systems and real catalysts, respectively. We find that regeneration strongly depends on the composition of the catalyst surface. While the clean surface of a poisoned Pt(111) single crystal is fully restored upon thermal treatment in oxygen up to 700K, contaminated Pt/Al2O3 model catalyst and core-shell pellet were only partially restored under the applied reaction conditions. Whereas partial regeneration on facet-like sites on supported catalysts is more facile than on Pt(111), carbonaceous deposits adsorbed at low-coordinated defect sites impede full regeneration of the Pt/Al2O3 catalysts.
No preview · Article · Nov 2015 · Applied Surface Science
[Show abstract][Hide abstract] ABSTRACT: We herein present an easy approach to increase drastically the catalytic activity of Ru/γ-Al2O3 catalysts in ammonia decomposition by surface modification with alkali hydroxides. The best activities have been achieved by a catalyst modification with lithium hydroxide in a Li/Al ratio of 1.7. In contrast, coatings with potassium and caesium hydroxide caused mass transfer limitations in the catalytic material already at low amounts (ratios alkali/Al <0.2-0.3). MAS ssNMR and XRD studies indicate the in-situ formation of alkali aluminates resulting in a basic modification of the support.
[Show abstract][Hide abstract] ABSTRACT: Ionic liquid (ILs) are flexible reaction media and solvents for the synthesis of metal nanoparticles (NPs). Here, we describe a new preparation method for metallic NPs in nanometer thick films of ultraclean ILs in an ultrahigh vacuum (UHV) environment. CO-covered Pd NPs are formed by simultaneous and by sequential physical vapor deposition (PVD) of the IL and the metal in the presence of a low partial pressure of CO. The film thickness and the particle size can be controlled by the deposition parameters. We have followed the formation of the NPs and their thermal behavior by time-resolved IR reflection absorption spectroscopy (TR-IRAS) and by temperature-programmed IRAS (TR-IRAS). Codeposition of Pd and [C1C2Im][OTf] in CO at 100 K leads to the growth of homogeneous multilayer films of CO-covered Pd aggregates in an IL matrix. The size of these NPs can be controlled by the metal fraction in the co-deposit. With increasing metal fraction the size of the Pd NP also increases. At very low metal content, small Pd carbonyl-like species are formed, which bind CO in on-top geometry only. Upon annealing, the [OTf]- anion coadsorbs at the NP surface and partially displaces CO. Coadsorption of CO and IL is indicated by a strong red-shift of the CO stretching bands. While the weakly bound on-top CO is mainly replaced below the melting transition of the IL, coadsorbate shells with bridge-bonded CO and IL are stable well above the melting point. Larger three-dimensional Pd NPs can be prepared by PVD of Pd onto a solid [C1C2Im][OTf] film at 100 K. Upon annealing, on-top CO desorbs from these NPs below 200 K. Upon melting of the IL film, the CO-covered Pd NPs immerse into the IL and again form a stable coadsorbate shell that consists of bridge-bonded CO and the IL.
[Show abstract][Hide abstract] ABSTRACT: This contribution deals with pore diffusion influences on the dehydrogenation kinetics of perhydro-N-ethylcarbazole (H12-NEC). The reaction is of high interest in the context of hydrogen storage in the N-ethylcarbazole (NEC)/perhydro-N-ethylcarbazole (H12-NEC) Liquid Organic Hydrogen Carrier (LOHC) system. The hydrogen content of H12-NEC is 5.8 wt% and total dehydrogenation releases for each mL of H12-NEC more than 600 mL of H2. Further optimization of H12-NEC dehydrogenation catalysis requires a better understanding of the role of mass transfer effects. Pore diffusion effects have been studied by preparing egg-shell catalysts (Pt/γ-alumina layer on α-alumina core) of different active layer thicknesses (24 -88 μm). It has been found that even at very thin catalyst layers (24 μm) the kinetic regime is limited to 235 °C, thus pore diffusion effects the dehydrogenation in almost all commercial catalysts strongly. This journal is
No preview · Article · Oct 2015 · Energy & Environmental Science
[Show abstract][Hide abstract] ABSTRACT: Herein, we report a remarkable finding that biomass oxidation to formic acid (FA) in water-organic biphasic reaction systems is far more selective than the same reaction in a monophasic aqueous media. While literature claims that the yield of FA from carbohydrates and biomass is limited to less than 68%, even for simple substrates such as glucose or glycerol, we demonstrate in this study that FA yields of up to 85% can be obtained from glucose. Using our biphasic reaction protocol, even raw lignocellulosic biomass, such as beech wood, leads to FA yields of 61%. This is realized by applying polyoxometalate H8PV5Mo7O40 as a homogeneous catalyst, oxygen as the oxidant and water as the solvent in the presence of a long-chain primary alcohol as an in-situ extracting agent. The new, liquid-liquid biphasic operation opens a highly effective way to produce pure FA, a liquid syngas equivalent, from wood in a robust, integrated, and low-temperature process. This journal is
No preview · Article · Oct 2015 · Energy & Environmental Science
[Show abstract][Hide abstract] ABSTRACT: We investigated the surface reaction of the liquid organic hydrogen carrier dicyclohexylmethane (DCHM) on Pt(111) in ultrahigh vacuum by high-resolution X-ray photoelectron spectroscopy, temperature-programmed desorption, near-edge X-ray absorption fine structure, and infrared reflection-absorption spectroscopy. Additionally, the hydrogen-lean molecule diphenylmethane and the relevant molecular fragments of DCHM, methylcyclohexane, and toluene were studied to elucidate the reaction steps of DCHM. We find dehydrogenation of DCHM in the range of 200-260 K, to form a double-sided π-allylic species coadsorbed with hydrogen. Subsequently, ∼30% of the molecules desorb, and for ∼70%, one of the π-allyls reacts to a phenyl group between 260 and 330 K, accompanied by associative hydrogen desorption. Above 360 K, the second π-allylic species is dehydrogenated to a phenyl ring. This is accompanied by C-H bond scission at the methylene group, which is an unwanted decomposition step in the hydrogen storage cycle, as it alters the original hydrogen carrier DCHM. Above 450 K, we find further decomposition steps which we assign to C-H abstraction at the phenyl rings.
No preview · Article · Aug 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The surface tension of nine tricyanomethanide ([C(CN)3]−)- and tetracyanoborate ([B(CN)4]−)-based ionic liquids (ILs) carrying a homologous series of the 1-alkyl-3-methylimidazolium ([alkyl-MIM]+) cations [EMIM]+ (ethyl), [BMIM]+ (butyl), [HMIM]+ (hexyl), [OMIM]+ (octyl), and [DMIM]+ (decyl) was measured with the pendant drop method in the temperature range between (283 and 353) K at atmospheric pressure with an estimated uncertainty of 2 % (k = 2). For the probed ILs, the surface tension decreases with increasing temperature and increasing alkyl chain length of the cation. Smaller values for the [B(CN)4]−-based ILs compared to the [C(CN)3]−-based ILs having the same cation were observed. The measured surface tensions agree with the limited number of experimental data found in the literature for the two IL families. A simple prediction based on the surface tension measured at 293 K and the temperature dependence of density showed good agreement with the measured temperature-dependent data. In comparison to other [alkyl-MIM]+-based ILs with anions of varying molecular size, the fairly large surface tensions of the ILs investigated in this study could be attributed to the strong charge delocalization in their relatively small anions.
No preview · Article · Aug 2015 · Journal of Chemical & Engineering Data
[Show abstract][Hide abstract] ABSTRACT: The telomerization of butadiene with methanol was investigated in the presence of different palladium catalysts modified either with triphenylphosphine (TPP) or 1,3-dimesityl-imidazol-2-ylidene (IMes) ligand. When pure butadiene was used as substrate, a moderate selectivity for the Pd-TPP catalyst toward the desired product 1-methoxy-2,7-octadiene (1-Mode) of around 87% was obtained, while the IMes carbene ligand almost exclusively formed 1-Mode with 97.5% selectivity. The selectivity remained unchanged when the pure butadiene feed was replaced by synthetic crack-C4 (sCC4), a technical feed of 45 mol% butadiene and 55 mol% inerts (butenes and butanes). The TPP-modified catalyst showed a lower reaction rate, which was attributed to the expected dilution effect caused by the inerts. Surprisingly, the IMes-modified catalyst showed a higher rate with sCC4 compared to the pure feed. By means of a model-based experimental analysis, kinetic rate equations could be derived. The kinetic modeling supports the assumption that the two catalyst systems follow different kinetic rate equations. For the Pd-TPP catalyst, the reaction kinetics were related to the Jolly mechanism. In contrast, the Jolly mechanism had to be adapted for the Pd-IMes catalyst as the impact of the base seems to differ strongly from that for the Pd-TPP catalyst. The Pd-IMes system was found to be zero order in butadiene at moderate to high butadiene concentrations and first order in base while the nucleophilicity of the base is influenced by the methanol amount resulting in a negative reaction order for methanol.
No preview · Article · Aug 2015 · Journal of Catalysis
[Show abstract][Hide abstract] ABSTRACT: The selective oxidation of complex, water-insoluble and wet biomasses from second and third generation to formic acid including effective catalyst recycling is reported. Additionally, the relevance and limits of potential contaminants are illustrated by different experimental approaches. By using a very robust homogeneous polyoxometalate catalyst in aqueous solution, molecular oxygen as oxidant and an acid as solubilizer, it is possible to convert different lignocellulosic and algae feedstock into formic acid and pure carbon dioxide. The applied green oxidation system benefits from its low reaction temperature (below 100 °C) and its very selective nature. Furthermore, catalyst recycling over three batches has been successfully carried out.
[Show abstract][Hide abstract] ABSTRACT: Ionic liquids (ILs) are possible working fluids for the separation of carbon dioxide (CO2) from flue gases. For evaluating their performance in such processes, reliable mutual-diffusivity data are required for mixtures of ILs with relevant flue gas components. In the present study, dynamic light scattering (DLS) and Molecular Dynamics (MD) simulations were used for the investigation of the molecular diffusion in binary mixtures of the IL 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][B(CN)4]) with the dissolved gases carbon dioxide, nitrogen, carbon monoxide, hydrogen, methane, oxygen, and hydrogen sulfide at temperatures from 298.15 to 363.15 K and pressures up to 63 bar. At conditions approaching infinite dilution of a gas, the Fick mutual diffusivity of the mixture measured by DLS and the self-diffusivity of the corresponding gas calculated by MD simulations match, which could be generally found within combined uncertainties. The obtained diffusivities are in agreement with literature data for the same or comparable systems as well as with the general trend of increasing diffusivities for decreasing IL viscosities. The DLS and MD results reveal distinctly larger molecular diffusivities for [EMIM][B(CN)4]-hydrogen mixtures compared to mixtures with all other gases. This behavior results in the failure of an empirical correlation with the molar volumes of the gases at their normal boiling points. The DLS experiments also showed that there is no noticeable influence of the dissolved gas and temperature on the thermal diffusivity of the studied systems.
No preview · Article · Jun 2015 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: A high viscosity-index (VI) is crucial for lubricants in industrial gearboxes exposed to changing load or weather conditions. Especially in the field of wind turbine oil, viscosity indices of 150 or higher are demanded to reduce power losses and ensure reliability at the same time. In this context the use of dissolved CO2 to improve viscosity-temperature behavior has been investigated for task-specific, halogen-free ionic liquids and benchmarked against polyalpha olefins. Measuring viscosity and density of the lubricants with dissolved CO2, it was proven that the VI can be increased significantly even at moderate pressures. In addition, measurements and simulation on CO2 solubility and studies on corrosion and tribology under CO2 pressure are presented.
No preview · Article · Apr 2015 · Industrial & Engineering Chemistry Research
[Show abstract][Hide abstract] ABSTRACT: Due to their low vapour pressures, non-flammability, high thermal stabilities and excellent tribological properties Ionic Liquids (ILs) are highly attractive lubricant base oils and additives. However, for practical applications of ILs in lubrication, two requirements are often limiting, the required miscibility with standard mineral oils (≥ 5 wt%) and the complete absence of corrosive halide ions in the ionic liquid. Moreover, the need for full compatibility with standard oil additives reduces the number of potential IL-based lubricant additives even further. In this contribution, an economic halide-free synthesis route to oil-miscible ionic liquids is presented and very promising tribological properties of such ILs as base oil or additive are demonstrated. Therefore sliding tests on bearing steel and XPS analysis of the formed surface films are shown. Corrosion test results of different bearing metals in contact with our halide-free ILs and (salt) water prove their applicability as real life lubricants. In the Sustainable Chemistry and Engineering context, we present a halide-free design approach for ionic performance chemicals that may contribute to significant energy savings due to their enhanced lubrication properties.
[Show abstract][Hide abstract] ABSTRACT: Due to the known corrosion and crystallization issues of LiBr/H2O, the state-of-the-art working pair in sorption heat pump (SHP) systems, research into alternative working pairs is of high practical relevance. We have studied a wide range of ionic liquids (ILs) for this application in order to find potential new systems with enhanced performance. The screening was conducted with a focus on vapor pressure measurements of, in total, 74 examined working pairs. As common vapor-liquid-equilibrium measurements are very precise but rather time-consuming, we developed a new setup allowing a fast relative determination of humidities with very small sample volumes for screening purposes. By this method we identified seventeen IL/H2O working pairs fulfilling the technical relevant criterion of a water vapor pressure pH2O ≤ 10 mbar at T = 308 K with an IL content of less than 80 wt % (wIL < 0.8). Further evaluation of these candidates with respect to their thermal stability and viscosity allowed us to identify [MMIM][HCOO]/H2O, [MMIM][OAc]/H2O, [MMIM][C2H5COO]/H2O, [Me4N][HCOO]/H2O, [Me4N][OAc]/H2O and [Me4N] [C2H5COO]/H2O as the most promising IL/H2O systems for a possible application in SHP systems.Keywords: Ionic liquid; Sorption heat pump; Vapor pressure; Thermal stability