Dieter Hönicke

Miniaturization of screening devices for the combinatorial development of heterogeneous catalysts

The present work is focused on the determination of the advantages, bottlenecks and challenges of miniaturized screening systems which are essential to the success of combinatorial high-throughput methodologies in heterogeneous catalysis. Two different reactor configurations with different degrees of miniaturization were developed for the parallel and fast screening of heterogeneously catalyzed gas phase reactions: a monolithic reactor system acting as a multichannel reactor and a microreaction system based on microfabrication techniques. In both cases, a scanning mass spectrometry technique was successfully applied for quantitative product analysis within 60 s per catalyst. Due to its flexibility and high spatial resolution, this three dimensional scanning MS can be used with different and highly parallel reactor arrays. Many experiments were carried out to study the efficiency and reliability of the different screening systems, with the oxidation of methane, the oxidation of CO, and the oxidative dehydrogenation of i-butane as model reactions. Moreover, chip modules in silicon–glass technology having a number of parallel microchannels were developed, each of them containing a different catalyst. Using this approach, “catalysis-on-a-chip” proved in methane oxidation was possible. Finally, a multibatch reactor consisting of a number of parallel mini autoclaves was developed and tested in the liquid-phase hydrogenation of citral in order to overcome the lack of parallel and fast screening procedures for heterogeneously catalyzed gas–liquid reactions widely spread in the chemical industry.

Hydrogenation of crotonaldehyde on Pt/TiO2 catalysts: Influence of the phase composition of titania on activity and intramolecular selectivity

The hydrogenation of crotonaldehyde in the gas phase at 413 K and 2 MPa over Pt/TiO2 catalysts reduced at 473 K (LTR) or 773 K (HTR) was investigated in order to examine if the catalytic properties could be altered by the phase composition of TiO2. The catalysts prepared by ion-exchange or sol-gel technique were characterized by physisorption, chemisorption, EPR, TEM and XRD measurements. It was found that the phase composition of the support has a strong influence on the activity which decreased with increasing anatase fraction (ξANA). Furthermore, the specific activities on a per g Pt basis were higher in the case of the HTR catalysts than those of their LTR counterparts. The selectivity to crotyl alcohol ranged from 30 to 40% at crotonaldehyde conversions up to 50% independent of the catalyst support used. However, at higher conversions up to 80% the highest selectivity of 53% to crotyl alcohol was obtained over a Pt catalyst with ξANA = 65%. Both the observed behavior in selectivity which corresponds to the TOF data for product formation on individual Pt catalysts as well as the catalytic properties of physical mixtures of the catalysts suggest that the differences in selectivities are connected with different degrees of competitive adsorption and readsorption of the products which depend on the TiO2 phase composition. In addition, the solgel derived Pt catalyst gave a selectivity of 51% for crotyl alcohol which shows that the SMSI effect could be more pronounced by the close contact of Pt with the anatase matrix.

Microfabricated components for heterogeneously catalysed reactions

Microsystems offer considerable industrial potential for unit operations, e.g. heat transfer, mass transport, and mixing of gases and liquids. The development of techniques for performing chemical reactions in microsystems has high priority. Only a few examples are known of using microreactors for performing homogeneous chemical reactions and heterogeneously catalysed liquid-phase enzyme reactions. However, heterogeneously catalysed gas-phase reactions in microreactors have not been reported at all. This is due to the lack of surfaces having enough catalytically active sites. The present article describes a method for treating the surfaces of channels in a microreactor in order to achieve adequate numbers of catalytically active and selective sites - which is the main prerequisite for the performance of heterogeneously catalysed reactions. The experimental procedure and the measurements of characteristic parameters are described. An envisaged scheme for a chemical microsystem similar to a chemical microplant is presented, and its scaling up via replication is addressed.

Preparation of structured egg-shell catalysts for selective oxidations by the ANOF technique

Abstract Shell-type catalysts supported on metal wires were prepared by anodic oxidation with spark discharge (the “ANOF” technique) in the presence of precursors of the catalytically active phase. Aluminium, magnesium and titanium oxide were employed as carriers, whereas nickel (also doped with lithium), chromium or molybdenum were the active components. The carrier oxides displayed regular pore structures determined by the preparation condition and influenced by the precursor of the active component, which was incorporated in the pore system of the carrier during the anodisation process. The nickel-containing catalysts were found to yield 60–90% cyclohexene selectivity in the oxidation of cyclohexane.

Monolithic Microreactors Possessing Regular Mesopore Systems for the Succesful Performance of Heterogeneously Catalysed Reactions

Microreactors represent a novel approach for chemical processing. In this study a monolithic microreactor having channels in micrometer dimension and regular pores in nanometer scale has been developed. The microreactor was tested in the partial hydrogenation of cyclododecatriene to yield cyclododecene. In addition, several particle bed reactors were tested, in order to establish advantages and the superiority of the monolithic microreactor.

Simultaneous Screening of Catalysts in Microchannels: Methodology and Experimental Setup

Given the excellent properties of microfluidic reactors our project aims to develop a fast and parallelized catalyst screening unit. In a first step the channel- walls of microstructured wafers are coated with catalytic active species. For the screening of catalytic properties a stack of 35 catalyst wafers is operated under steady-state conditions and continuous flow. Each wafer is exposed to the same amount of reactant gases at the same time, temperature and pressure. The product analysis is carried out using mass spectrometry for each catalyst wafer in order to gain full chemical information of the product distribution, conversion degrees and selectivities. A fast, sequentially sampling device is used to sample each catalyst wafer at least every 60 minutes.

Selective Oxidation of 1-Butene to Maleic Anhydride— comparison of the Performance between MicroChannel Reactors and a Fixed Bed Reactor

Two types of microchannel reactors made of packed wafer stacks were used for the selective partial oxidation of 1-butene to maleic anhydride with hydrocarbon concentrations to some extent within the explosion range. The microstructured wafers were catalytically activated by anodic oxidation of aluminum wafers followed by an impregnation process with V2O5/P2O5/TiO2. Hereby, channel diameters varied between 80 and 400 μm. Catalytic results in the different microchannel reactors were compared with those of identically prepared catalysts used in fixed bed reactors.

The partial gas‐phase hydrogenation of benzene to cyclohexene on supported and coated ruthenium catalysts

The conversion of benzene to useful products such as cyclohexene is of industrial interest because of the expected surplus of benzene due to its substitution in gasoline by other non‐polluting components in the next years. Therefore, the partial gas‐phase hydrogenation of benzene to cyclohexene at atmospheric pressure was performed in order to develop catalysts as an alternative to those used in liquid‐phase hydrogenation. Two types of ruthenium‐containing catalysts were investigated, viz. supported catalysts with different support materials and coated catalysts with electrolytically formed alumina as support. In order to yield the desired cyclohexene the presence of methanol as a reaction modifier was necessary in the gas phase during the reaction. The hydrogenation on supported Ru catalysts gave selectivities of about 35%, while on coated Ru catalysts selectivities up to 45% were obtained at conversion degrees of 5%. Improved catalyst performance, especially higher selectivity and yield, was obtained at increased partial pressure of methanol and hydrogen and by addition of copper as second metal in the oxide layer of the coated catalysts.

Selective Reactions in Microchannel Reactors

Microchannel reactors made of packed wafer stacks were used for selective hydrogenation of benzene to cyclohexene as well as for partial oxidation of 1-butene to maleic anhydride and ethene to ethene oxide. The microstructured wafers were catalytically activated by anodic oxidation of aluminum wafers followed by an impregnation process with ruthenium for benzene hydrogenation and V2O5/TiO2 for 1-butene oxidation or by physical vapor deposition of silver for ethene epoxidation.

Supported and Coated Nickel Catalysts: Are they Suitable for the Partial Gas Phase Hydrogenation of Benzene to Cyclohexene?

This article investigates the gas phase hydrogenation of benzene on supported and coated porous Ni catalysts. The goal is to determine the selectivity to cyclohexene under suitable reaction conditions in the absence and presence of methanol as a reaction modifier.