Reinhard Schomäcker

Oxidative Dehydrogenation of Ethane over Multiwalled Carbon Nanotubes

C-H activation; carbon nanotubes; dehydrogenation; oxidation; surface modification The suitability of nanocarbons as catalysts for oxida-tive dehydrogenation (ODH) reactions has been investi-gated for numerous hydrocarbon substrates, such as ethylbenzene, 1-butene, isobutene, n-butane, and pro-pane.

On the nanoparticle synthesis in microemulsions: detailed characterization of an applied reaction mixture

Abstract In this study we present well-characterized non-ionic (w/o)-microemulsions as suitable reaction media for the synthesis of nanoparticles. For oil-rich three component systems of water, cyclohexane and Marlipal surfactants, we determined the phase behavior, the viscosity, the droplet size and the interdroplet exchange rates. The general influence of the particular microemulsion properties on the mechanism of nanoparticle formation is discussed on the basis of the LaMer model for particle formation. As the nucleation, particle growth and aggregation are controlled by the droplet exchange process, all these steps are retarded in comparison to a homogeneous solution. This retardation causes a particle growth that is not accompanied by aggregation of nuclei. Therefore, a higher number of nuclei is maintained during the precipitation that grows to smaller particles than have been expected for a precipitation in a homogeneous solution. Therefore, we assume that for all precipitation reactions, where the growth step is diffusion controlled and aggregation of nuclei reduces the number of particles, the application of a microemulsion as precipitation medium yields much smaller particles than obtained from homogeneous solutions.

Hydroformylation of 1-Dodecene with Water-Soluble Rhodium Catalysts with Bidentate Ligands in Multiphase Systems

A hydrophilic metal–ligand complex formed from the precursor [dicarbonyl(acetylacetonato)rhodium(I)] {[Rh(acac)(CO)2]} and the bidentate ligand [2,7‐bis(SO3Na)‐4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene] (SulfoXantPhos), was found to be a suitable candidate as a catalyst complex for the hydroformylation of 1‐dodecene in multiphase systems formulated from water, 1‐dodecene, and a nonionic surfactant. To improve the solubilization of the olefin in the aqueous phase, surfactants were added. The multiphase system acted as a tunable solvent, through which not only the interfacial area was increased during the reaction but also the phase separation behavior could be manipulated through temperature changes, thus allowing an easy separation of the expensive rhodium complex from the organic phase after the reaction. The influence of different process parameters such as the type of surfactant, type of ligand, and the metal/ligand ratio was investigated and discussed. Also the influence of the phase state on the reaction was determined. Under optimized reaction conditions, turnover frequencies of >300 h−1 and selectivities of 98:2 towards the linear product could be achieved.

Mesoporous Carbon Nitride-Tungsten Oxide Composites for Enhanced Photocatalytic Hydrogen Evolution

Composites of mesoporous polymeric carbon nitride and tungsten(VI) oxide show very high photocatalytic activity for the evolution of hydrogen from water under visible light and in the presence of sacrificial electron donors. Already addition of very small amounts of WO3 yields up to a twofold increase in the efficiency when compared to bulk carbon nitrides and their composites and more notably even to the best reported mesoporous carbon nitride-based photocatalytic materials. The higher activity can be attributed to the high surface area and synergetic effect of the carbon nitrides and the WO3 resulting in improved charge separation through a photocatalytic solid-state Z-scheme mechanism.

Enzyme Catalysis in Reverse Micelles

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviour and the corresponding activity of enzymes the application of these media can lead to favourable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes.

In situ surface coverage analysis of RuO2-catalysed HCl oxidation reveals the entropic origin of compensation in heterogeneous catalysis

In heterogeneous catalysis, rates with Arrhenius-like temperature dependence are ubiquitous. Compensation phenomena, which arise from the linear correlation between the apparent activation energy and the logarithm of the apparent pre-exponential factor, are also common. Here, we study the origin of compensation and find a similar dependence on the rate-limiting surface coverage term for each Arrhenius parameter. This result is derived from an experimental determination of the surface coverage of oxygen and chlorine species using temporal analysis of products and prompt gamma activation analysis during HCl oxidation to Cl(2) on a RuO(2) catalyst. It is also substantiated by theory. We find that compensation phenomena appear when the effect on the apparent activation energy caused by changes in surface coverage is balanced out by the entropic configuration contributions of the surface. This result sets a new paradigm in understanding the interplay of compensation effects with the kinetics of heterogeneously catalysed processes.

Facile one-pot synthesis of Pt nanoparticles /SBA-15: an active and stable material for catalytic applications

Pt/SBA-15 with an enhanced surface area but unchanged pore diameter (compared to pure SBA-15) and a Pt average particle size of ∼9 nm shows a high and stable activity for both gas-phase CO oxidation and liquid-phase cyclooctadiene hydrogenation. No intrinsic change in the structure of the catalyst occurs after several reaction cycles, suggesting that the Pt/SBA-15 presented here is an active and stable catalyst.

A novel technique for preparation of aminated polyimide membranes with microfiltration characteristics

In the wet-chemical treatment of polyimide (PI) membranes with aminic modifiers, the modifier molecules will be covalently bound to the membrane polymer. Using modifiers with high aminic nitrogen the amination is combined with a degradation process, which shifts separation properties from ultrafiltration characteristics (untreated membrane) to microfiltration characteristics (treated membrane). Under optimal treatment conditions the steepness of separating curves of the aminated membranes is comparable with the steepness of separating curves of commercial microfilters. Characteristic data of membranes such as water permeability, amine content, SEM morphology, wettability and the dependence of membrane thickness on treatment conditions were presented. Data show that membrane properties are insensitive to modifier concentration in the treatment bath with respect to degradation reaction but sensitive to reaction rate. From the data of amine content per unit surface area of membrane and amine content per unit membrane weight, it was concluded that the functionalization/degradation process is divided in two steps. In the first step functionalization dominates, whereas in the second step functionalization and degradation are simultaneous processes resulting in an equilibrium state. The expected reaction sequence of degradable functionalization is proposed and discussed. Initial membrane morphology seems to be the key parameter for further investigations to optimize membrane preparation processes.

Rhodium catalyzed hydrogenation reactions in aqueous micellar systems as green solvents

The hydrogenation of itaconic acid and dimethyl itaconate is transferred from methanol to aqueous micellar solutions of several surfactants, e.g., SDS and Triton X-100, in order to facilitate the recovery of the catalyst. The reaction rate and selectivity strongly depends on the chosen surfactant and in some cases also on the surfactant concentration. In the best case the selectivity is the same as in methanol but the reaction rate is still lower because of a lower hydrogen solubility in water. Repetitive semi-batch experiments are chosen to demonstrate that high turn-over-numbers (>1000) can be reached in aqueous micellar solutions. No notable catalyst deactivation is observed in these experiments. The performance of micellar reaction systems is controlled by the partition coefficient of the substrates between the micelles and the continuous aqueous phase which can be predicted using the Conductor-like Screening Model for Real Solvents (COSMO-RS).

Engineering Aspects of Preparation of Nanocrystalline Particles in Microemulsions

Engineering aspects of the preparation of palladium nanoparticles in non-ionic w/o-microemulsions are examined. In order to achieve reproducible synthesis conditions a semi-batch reactor with a standardized design is used. Influences of the stirring rate and of different ways of concentration control on the product properties are observed. For reproducible synthesis it is important to establish appropriate and defined preparation conditions. Monodisperse palladium particles of around 5 nm size are obtained by adding the microemulsion containing the palladium salt at a constant feed rate to the precharged microemulsion containing the reducing agent. A quantitative kinetic model is proposed to describe particle formation in microemulsions. Unknown parameters of the model have been estimated by independent examinations or can be achieved by fitting to the experimental data.