Nucleation as a rate-determining step in catalytic gas generation reactions from liquid phase systems.

The observable reaction rate of heterogeneously catalyzed reactions is known to be limited either by the intrinsic kinetics of the catalytic transformation or by the rate of pore and/or film diffusion. Here, we show that in gas generation reactions from liquid reactants, the nucleation of gas bubbles in the catalyst pore structure represents an additional important rate-limiting step. This is highlighted for the example of catalytic hydrogen release from the liquid organic hydrogen carrier compound perhydro-dibenzyltoluene. A nucleation-inhibited catalytic system produces only dissolved hydrogen with fast saturation of the fluid phase around the active site, while bubble formation enhances mass transfer by more than a factor of 50 in an oscillating reaction regime. Nucleation can be efficiently triggered not only by temperature changes and catalyst surface modification but also by a mechanical stimulus. Our work sheds new light on performance-limiting factors in reactions that are of highest relevance for the future green hydrogen economy.

Dehydrogenation of the liquid organic hydrogen carrier system 2-methylindole/2-methylindoline/2-methyloctahydroindole on Pt(111).

Among other N-heterocycles, indole and its substituted derivatives, such as methylindoles, are considered promising Liquid Organic Hydrogen Carriers (LOHCs) for the storage of renewable energy. We used X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and density-functional theory (DFT) to investigate the low temperature adsorption and consecutive dehydrogenation reaction during heating of 2-methylindole, 2-methylindoline, and 2-methyloctahydroindole on Pt(111) and their viability as the LOHC system. In the photoemission experiments, for all Hx-2-methylindoles, we find deprotonation at the NH bond starting between 240 and 300 K, resulting in a 2-methylindolide species. Simultaneously or before this reaction step, the dehydrogenation of 2-methyloctahydroindole via 2-methylindoline and 2-methylindole intermediates is observed. For 2-methyloctahydroindole, we also find π-allyl intermediates above 230 K. Starting at ∼390 K, decomposition of the remaining 2-methylindolide species takes place under the conditions of our surface science experiments. DFT calculations give insight into the relative energies of the various species, reaction intermediates, and their isomers both in the gas phase and on the Pt(111) surface.

Homogeneously-catalysed hydrogen release/storage using the 2-methylindole/2-methylindoline LOHC system in molten salt-organic biphasic reaction systems.

Ir-Complex catalysed hydrogen release/storage using a 2-methylindole/2-methylindoline Liquid Organic Hydrogen Carrier (LOHC) system is shown to be effective in a temperature range of 120 to 140 °C. In the form of a liquid-liquid biphasic reaction system with molten [PPh4][NTf2] as catalyst immobilisation phase, the applied cationic Ir-complex can be easily separated and recycled enabling a small amount of ionic catalyst solution to store/release a large amount of hydrogen.

Carbon Dioxide-Free Hydrogen Production with Integrated Hydrogen Separation and Storage.

An integration of CO2 -free hydrogen generation through methane decomposition coupled with hydrogen/methane separation and chemical hydrogen storage through liquid organic hydrogen carrier (LOHC) systems is demonstrated. A potential, very interesting application is the upgrading of stranded gas, for example, gas from a remote gas field or associated gas from off-shore oil drilling. Stranded gas can be effectively converted in a catalytic process by methane decomposition into solid carbon and a hydrogen/methane mixture that can be directly fed to a hydrogenation unit to load a LOHC with hydrogen. This allows for a straight-forward separation of hydrogen from CH4 and conversion of hydrogen to a hydrogen-rich LOHC material. Both, the hydrogen-rich LOHC material and the generated carbon on metal can easily be transported to destinations of further industrial use by established transport systems, like ships or trucks.

Evaluation of industrially applied heat-transfer fluids as liquid organic hydrogen carrier systems.

Liquid organic hydrogen carrier (LOHC) systems offer a very attractive method for the decentralized storage of renewable excess energy. In this contribution, industrially well-established heat-transfer oils (typically sold under trade names, e.g., Marlotherm) are proposed as a new class of LOHC systems. It is demonstrated that the liquid mixture of isomeric dibenzyltoluenes (m.p. -39 to -34 °C, b.p. 390 °C) can be readily hydrogenated to the corresponding mixture of perhydrogenated analogues by binding 6.2 wt% of H2. The liquid H2 -rich form can be stored and transported similarly to diesel fuel. It readily undergoes catalytic dehydrogenation at temperatures above 260 °C, which proves its applicability as a reversible H2 carrier. The presented LOHC systems are further characterized by their excellent technical availability at comparably low prices, full registration of the H2 -lean forms, and excellent thermal stabilities.

Low melting Li/K/Cs acetate salt mixtures as new ionic media for catalytic applications--first physico-chemical characterization.

In order to expand the temperature limits of Supported Ionic Liquid Phase (SILP) or Solid Catalyst with Ionic Liquid Layer (SCILL) systems to higher operation temperatures, the mixture of lithium acetate, potassium acetate, and caesium acetate (molar ratio of 0.2/0.275/0.525) has been studied in detail. Physico-chemical properties of the bulk melt are reported together with stability data of the modern salt on various solid support materials showing attractive properties for many potential high temperature applications.

Oxidative depolymerization of lignin in ionic liquids.

Beech lignin was oxidatively cleaved in ionic liquids to give phenols, unsaturated propylaromatics, and aromatic aldehydes. A multiparallel batch reactor system was used to screen different ionic liquids and metal catalysts. Mn(NO(3))(2) in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM][CF(3)SO(3)] proved to be the most effective reaction system. A larger scale batch reaction with this system in a 300 mL autoclave (11 g lignin starting material) resulted in a maximum conversion of 66.3 % (24 h at 100 degrees C, 84x10(5) Pa air). By adjusting the reaction conditions and catalyst loading, the selectivity of the process could be shifted from syringaldehyde as the predominant product to 2,6-dimethoxy-1,4-benzoquinone (DMBQ). Surprisingly, the latter could be isolated as a pure substance in 11.5 wt % overall yield by a simple extraction/crystallization process.

Determination of glucose and cellobiose dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate using Fourier transform infrared spectroscopy.

The conversion of biogenic carbohydrate feedstock to chemicals or energy equivalents is a promising approach to solve the problem of limited fossil fuel reserves. Some concepts to accomplish these transformations are based on ionic liquids (ILs) due to their ability to dissolve biopolymers, such as cellulose, and even complex biopolymer mixtures, such as wood. However, concerning control of such conversions, a reliable tool for process analytics is required. In this paper we demonstrate the applicability of Fourier transform infrared (FT-IR) spectroscopy to perform quantitative concentration measurements of glucose and cellobiose as two examples of carbohydrates dissolved in the room-temperature ionic liquid [EMIM][OAc] (1-ethyl-3-methylimidazolium acetate). For this purpose, binary mixtures in the range 0-20 wt% have been studied. A previously developed method for the data analysis, which was based on the Beer-Lambert relation, has been universalized by employing empirical correlations between the measured quantity (i.e., extinction) and the carbohydrate concentration. In the entire spectral range under investigation (500-4000 cm(-1)) numerous individual wave-numbers have been identified, allowing quantitative measurements with high accuracy and precision.

Chloroalkylsulfonate ionic liquids by ring opening of sultones with organic chloride salts.

An efficient route to prepare ionic liquids with chloroalkylsulfonate anions is presented; the synthesis proceeds in a one-step ring-opening reaction of sultones with an organic chloride salt and provides a very attractive access to new anion functionalised ionic liquids.

Quantitative analysis of alpha-D-glucose in an ionic liquid by using infrared spectroscopy.

The applicability of infrared (IR) spectroscopy for quantitative concentration measurements in mixtures of carbohydrates and ionic liquids (ILs) is investigated. The compound 1-ethyl-3-methylimidazolium acetate, [EMIM][OAc]-an "enzyme-friendly" ionic liquid with great application potential in the dissolution of various biomasses-is used as solvent in combination with alpha-D-glucose. Our study establishes a new way to monitor the concentration of sugars in ILs, thus providing a convenient method to follow the kinetics of, for example, enzymatic reactions in [EMIM][OAc]. As a first step, we present the IR spectrum of pure [EMIM][OAc] (this constitutes the first vibrational study of this particular IL). Although numerous lines overlap in the fingerprint region of the spectrum, characteristic features can be assigned to the corresponding vibrational modes of both ions. Secondly, we study different mixtures of the IL with alpha-D-glucose (in the concentration range: 0-20 mass % glucose) and analyze them by means of IR spectroscopy, followed by computational methods, thus demonstrating the great potential of this spectroscopic technique in quantitative measurements.

Synthesis and properties of ionic liquids derived from the 'chiral pool'.

New chiral ionic liquids have been synthesised which are directly derived from the 'chiral pool' and therefore readily available in kg scale; NMR-measurements indicate that these liquids may be interesting solvents for enantioselective reactions and useful in chiral separation techniques.

Activation, Tuning, and Immobilization of Homogeneous Catalysts in an Ionic Liquid/Compressed CO2 Continuous-Flow System.

A new immobilization scheme for enantioselective catalysts was developed by using a combination of ionic liquids and compressed CO2 . Under continuous flow conditions, stable conversion and asymmetric induction was achieved over more than 60 h in the enantioselective Ni-catalyzed hydrovinylation of styrene. While the ionic liquid dissolves and activates the organometallic catalyst in a tuneable manner, the presence of compressed CO2 greatly facilitates mass transfer and gives easy access to continuous processes.