Dedicated to Süd-Chemie on the occasion of its 150th anniversaryDiscovering new catalysts is crucial to the development of sustainable chemical processes for industrial applications as well as for broadening the spectrum of synthetic methodologies and techniques in chemistry. The prerequisite for the directed design of catalysts is understanding how the kinetics, in other words, the activation barrier, in the mechanism of catalysis is controlled by the structural parameters.[1] To identify rate-determining elementary steps and to develop models, comprehensive experimental kinetic data of a broad variety of substrates are necessary. Microfluidic devices integrating chemical synthesis and analysis on the same chip [2][3][4][5] are one promising approach for parallelized highthroughput (ht) kinetic measurements of catalysts with minute material consumption.[6] A unique technique for studying configurational changes in chiral compounds is dynamic chromatography [7] combining molecular interconversion and analysis under precisely controlled conditions. Recently, we reported [8] a unified equation to directly access reaction rate constants of first-order reactions of such experiments. However, commonly used (micro)reactors, in which the reaction, separation, and quantification of conversion are performed consecutively, are limited to the study of single reactions, because competing reactions lead to undefinable reaction kinetics. Here we show for the hydrogenation over highly active Pd nanoparticles and the ring-closing metathesis over the Grubbs second generation catalyst that the synchronous combination of catalysis and separation makes it possible to efficiently perform ht reaction rate measurements (147 reactions per hour) of substrate libraries. The catalytic systems for these multiphase reactions (gas-liquid-solid) were prepared by embedding the catalysts in polysiloxanes, which serve as both solvent and selective stationary separation phase. Furthermore this system can be used for cascade reactions or for preparative synthesis to produce the target compounds.Typically, multiphase catalytic systems, which play a predominant role in industrial processes, are difficult to investigate because the interaction of the substrate with the catalyst is controlled by the mass transfer between the different phases, and therefore the apparent reaction rate is composed of the inherent reaction rate and diffusion rates. To reduce this effect, the interfacial area must be increased. Microstructured reaction systems intrinsically have a high specific interfacial area per volume, only dependent on the radius of the reaction channels; that is, for capillaries with inner diameters between 250 and 100 mm the specific interfacial area per volume ranges from 16 000 to 40 000 m 2 m À3 . Microfluidic systems are currently revolutionizing chemical synthesis, [9][10][11][12][13] because physical processes can be more easily controlled, low operation volumes minimize reagent consumption, and detection is integrated.[14] However, there are s...

The hydrogenation of 1-acetylcyclohexene, cyclohex-2-enone, nitrobenzene, and trans-methylpent-3-enoate catalyzed by highly active palladium nanoparticles was studied by high-throughput on-column reaction gas chromatography. In these experiments, catalysis and separation of educts and products is integrated by the use of a catalytically active gas chromatographic stationary phase, which allows reaction rate measurements to be efficiently performed by employing reactant libraries. Palladium nanoparticles embedded in a stabilizing polysiloxane matrix serve as catalyst and selective chromatographic stationary phase for these multiphase reactions (gas-liquid-solid) and are coated in fused-silica capillaries (inner diameter 250 microm) as a thin film of thickness 250 nm. The palladium nanoparticles were prepared by reduction of palladium acetate with hydridomethylsiloxane-dimethylsiloxane copolymer and self-catalyzed hydrosilylation with methylvinylsiloxane-dimethylsiloxane copolymer to obtain a stabilizing matrix. Diphenylsiloxane-dimethylsiloxane copolymer (GE SE 52) was added to improve film stability over a wide range of compositions. Herein, we show by systematic TEM investigations that the size and morphology (crystalline or amorphous) of the nanoparticles strongly depends on the ratio of the stabilizing polysiloxanes, the conditions to immobilize the stationary phase on the surface of the fused-silica capillary, and the loading of the palladium precursor. Furthermore, hydrogenations were performed with these catalytically active stationary phases between 60 and 100 degrees C at various contact times to determine the temperature-dependent reaction rate constants and to obtain activation parameters and diffusion coefficients.

N-alkylated trans-diaziridines are an intriguing class of compounds with two stereogenic nitrogen atoms which easily interconvert. In the course of our investigations of the nature of the interconversion process via nitrogen inversion or electrocyclic ring opening ring closure, we synthesized and characterized the three constitutionally isomeric diaziridines 1,2-di-n-propyldiaziridine 1, 1-isopropyl-2-n-propyldiaziridine 2, and 1,2-diisopropyldiaziridine 3 to study the influence of the substituents on the interconversion barriers. Enantiomer separation was achieved by enantioselective gas chromatography on the chiral stationary phase Chirasil-beta-Dex with high separation factors alpha (1-isopropyl-2-n-propyldiaziridine: 1.18; 1, 2-diisopropyldiaziridine: 1.24; 100 degrees C 50 kPa He) for the isopropyl substituted diaziridines. These compounds showed pronounced plateau formation between 100 and 150 degrees C, and peak coalescence at elevated temperatures. The enantiomerization barriers DeltaG(double dagger) and activation parameters DeltaH(double dagger) and DeltaS(double dagger) were determined by enantioselective dynamic gas chromatography (DGC) and direct evaluation of the elution profiles using the unified equation implemented in the software DCXplorer. Interestingly, 1-isopropyl-2-n-propyldiaziridine and 1,2-diisopropyldiaziridine exhibit similar high interconversion barriers DeltaG(double dagger) (100 degrees C) of 128.3 +/- 0.4 kJ mol(-1) and 129.8 +/- 0.4 kJ mol(-1), respectively, which indicates that two sterically demanding substituents do not substantially increase the barrier as expected for a distinct nitrogen inversion process.

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NiAl nanoparticles are of considerable importance because of interest in examining the effects of size, surface area, and composition on their physical and catalytic properties. Recently, a new method for the`bottom up' wet chemical preparation of nickel aluminides has been reported. The ability to leach the aluminum from this system provides an entrance to the preparation of skeletal-type metal catalysts similar to those produced in Raney-type systems. Furthermore, it is believed that these nanoparticulate catalysts should provide high surface areas and high activities, while the presence of aluminum within the bulk of these catalysts provides additional stability. Here, we present the results of studies conducted on this system which compare their properties and behavior with traditional bulk Raney nickel systems. Additionally, we show that we are able to alter the properties of these nanoparticles by changing the stoichiometric ratio of nickel and aluminum. The properties of all systems have been analyzed through the use of nitrogen adsorption, X-ray diffraction, and elemental analysis. Finally, the catalysts generated have been compared for their activity in the hydrogenation of butyronitrile.

Review: 46 refs.

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