Elias Klemm

Solution infiltration of palladium into MOF-5: synthesis, physisorption and catalytic properties

Palladium was infiltrated into the highly porous metal–organic framework MOF-5 {[Zn4O(bdc)3], bdc = benzene-1,4-dicarboxylate} using [Pd(acac)2] (acac = acetylacetonate) as the precursor in chloroform solution via ‘incipient wetness’ impregnation. The specific surface area decreases from 2885 m2 g−1 to 958 m2 g−1 after infiltration. After reduction in hydrogen or under vacuum, the hydrogen adsorption capacity is increased from 1.15 wt% to 1.86 wt% at 1 atm and 77 K. The palladium loaded MOF-5 has a high catalytic activity in styrene hydrogenation comparable to that of palladium on activated carbon, but cis-cyclooctene hydrogenation is considerably slower. Even at room temperature, the catalyst is not stable in air due to the low hydrothermal stability of the MOF-5 support.

Numerical simulations of single phase reacting flows in randomly packed fixed-bed reactors and experimental validation

Abstract Randomly packed fixed-bed reactors are widely used in the chemical process industries. Their design is usually based on pseudo-homogeneous model equations with averaged semi-empirical parameters. However, this design concept fails for low tube-to-particle diameter ratios (=aspect ratios) where local phenomena dominate. The complete three-dimensional (3D) structure of the packing has therefore to be considered in order to resolve the local inhomogeneities. New numerical methods and the increase of computational power allow us to simulate in detail single phase reacting flows in such reactors, exclusively based on material properties and the 3D description of the geometry, thus without the use of semi-empirical data. The successive simulation steps (packing generation, fluid flow and species calculation) and their validation with experimental data are described in this paper. In order to synthetically generate realistic random packings of spherical particles, we apply a Monte-Carlo method. The subsequent numerical simulation of the 3D flow field and coupled mass transport of reacting species is done by means of lattice Boltzmann methods. The simulation results reveal that not only the local behaviour but also integral quantities like the pressure drop depend remarkably on the local structural properties of the packings, a feature which is neglected when using correlations with averaged values.

Selective Catalytic Oxidation of CH Bonds with Molecular Oxygen

Although catalytic reductions, cross‐couplings, metathesis, and oxidation of CC double bonds are well established, the corresponding catalytic hydroxylations of CH bonds in alkanes, arenes, or benzylic (allylic) positions, particularly with O2, the cheapest, “greenest”, and most abundant oxidant, are severely lacking. Certainly, some promising examples in homogenous and heterogenous catalysis exist, as well as enzymes that can perform catalytic aerobic oxidations on various substrates, but these have never achieved an industrial‐scale, owing to a low space‐time‐yield and poor stability. This review illustrates recent advances in aerobic oxidation catalysis by discussing selected examples, and aims to stimulate further exciting work in this area. Theoretical work on catalyst precursors, resting states, and elementary steps, as well as model reactions complemented by spectroscopic studies provide detailed insight into the molecular mechanisms of oxidation catalyses and pave the way for preparative applications. However, O2 also poses a safety hazard, especially when used for large scale reactions, therefore sophisticated methodologies have been developed to minimize these risks and to allow convenient transfer onto industrial scale.

Application of a chiral metal–organic framework in enantioselective separation

A modular approach for the synthesis of highly ordered porous and chiral auxiliary (Evans auxiliary) decorated metal-organic frameworks is developed. Our synthesis strategy, which uses known porous structures as model materials for incorporation of chirality via linker modification, can provide access to a wide range of porous materials suitable for enantioselective separation and catalysis. Chiral analogues of UMCM-1 have been synthesized and investigated for the enantioseparation of chiral compounds in the liquid phase and first promising results are reported.

CFD-calculation of flow, dispersion and reaction in a catalyst filled tube by the lattice Boltzmann method

The behaviour of a reacting, viscous flow inside the complex geometry of a fixed-bed reactor has been studied by means of a lattice Boltzmann automata (LBA). In particular, two tasks have been investigated in detail. The geometrical structures of the fixed bed have been generated with a Monte-Carlo method. This allows to simulate very efficiently the placement of randomly packed spheres in a cylinder and to obtain detailed information of statistical properties, such as the voidage distribution. This geometrical information is the base for the subsequent numerical flow simulation using a lattice Boltzmann automata. This approach allows to predict the local fluid velocity distribution in the bed as well as the transport and reactions of chemical species taking into account the effect of heterogeneous catalysis. Thus, detailed information can be provided for the diffusion/dispersion effects around catalytic particles and ensembles of spheres. It is shown that this approach allows to improve the understanding of the underlying transport and reaction phenomena in these reactors and to obtain a reliable database for their operating behaviour and design.

Catalytic degradation of polyethylene using thermal gravimetric analysis and a cycled-spheres-reactor

Abstract Commercial samples of pure polyethylene were decomposed over H-ZSM-5 and Y-type zeolites using thermal gravimetric analysis (TGA) and a laboratory-scale-plant, so-called the cycled-spheres-reactor. By the TGA measurements, the activity and the deactivation behavior of the zeolite catalysts were determined. The plastic to catalyst ratio was varied to find out the optimal value for catalyst screening and for the operation of the cycled-spheres-reactor. In addition, the deactivation behavior of the zeolite catalysts was investigated. Y-type zeolites revealed lower activity and faster deactivation behavior than H-ZSM-5. Higher module of H-ZSM-5 and Y-type zeolite showed slower deactivation, but lower activity than lower module of those. Experiments in the cycled-spheres-reactor proved the results of the TGA measurements in terms of activity. The main products in the non-catalytic degradation were waxes, and when catalysts were applied, a high yield of oils was obtained at the expense of waxes. The product spectra of product oils obtained with catalysts lay mainly in the range C4–C10.

Kinetic investigations of the deactivation by coking of a noble metal catalyst in the catalytic hydrogenation of nitrobenzene using a catalytic wall reactor

Abstract The vapour phase hydrogenation of nitrobenzene to aniline is a highly exothermic reaction deactivated by coking of the palladium catalyst supported on α-alumina carrier. For studying the deactivation of the catalyst a catalytic wall reactor was used in order to ensure isothermal reaction conditions for the kinetic measurements. Furthermore, the catalytic wall reactor allowed the determination of axial coke profiles by total carbon analysis of different wall segments. On the assumption that the main reaction and the deactivation of the catalyst can be assumed separable both the steady state and the unsteady state kinetics were studied. Nitrobenzene was identified as the relevant coke precursor whereas aniline has neither an influence on the main reaction nor on the deactivation. It could be shown that the hydrogenation of nitrobenzene to aniline follows a Langmuir–Hinshelwood mechanism considering the surface reaction of the adsorbed nitrobenzene molecule and one adsorbed hydrogen atom as the rate determining step. The differentiation of coke on the active sites and coke on the support must be taken into account to model the kinetics of coke formation with sufficient accuracy.

The Synthesis of Cresol from Toluene and N2O on H[Al]ZSM-5: Minimizing the Product Diffusion Limitation by the Use of Small Crystals

The direct hydroxylation of toluene with nitrous oxide to cresol has been studied on two different H[Al]ZSM-5 zeolites with an Si/Al ratio of around 25 and different crystal sizes (30-70 nm and 1-3 μm). The samples were activated by calcination and characterized by X-ray diffraction, temperature programmed desorption of ammonia, adsorption of nitrogen and transmission electron microscopy. For the two different crystal sizes, different macroscopic cresol yields and time on stream behaviours are observed. The sample having larger crystals shows a decrease in toluene conversion with increasing reaction temperature. For the smaller crystals an increase in toluene conversion, selectivity to cresol and amount of para-cresol in the cresol fraction with increasing reaction temperature is observed. The para-cresol selectivity is lower on the sample with the longer diffusion path. The findings are explained by product diffusion limitation caused by high reactivity and strong adsorption of the polar product cresol on H[Al]ZSM-5, resulting in a rapid deactivation of the larger crystals.

A Rechargeable Hydrogen Battery Based on Ru Catalysis

Apart from energy generation, the storage and liberation of energy are among the major problems in establishing a sustainable energy supply chain. Herein we report the development of a rechargeable H2 battery which is based on the principle of the Ru-catalyzed hydrogenation of CO2 to formic acid (charging process) and the Ru-catalyzed decomposition of formic acid to CO2 and H2 (discharging process). Both processes are driven by the same catalyst at elevated temperature either under pressure (charging process) or pressure-free conditions (discharging process). Up to five charging-discharging cycles were performed without decrease of storage capacity. The resulting CO2/H2 mixture is free of CO and can be employed directly in fuel-cell technology.

Deactivation kinetics in the hydrogenation of nitrobenzene to aniline on the basis of a coke formation kinetics : investigations in an isothermal catalytic wall reactor

The heterogeneously catalysed gas-phase hydrogenation of nitrobenzene to aniline on an α-alumina supported palladium catalyst is a highly selective reaction which however is deactivated by coking. A catalytic wall reactor (CWR) was proved to be a powerful tool for the determination of a deactivation kinetics on the basis of a coke formation kinetics. Nitrobenzene was identified as coke precursor, whereas aniline had no influence neither on the kinetics of the nitrobenzene hydrogenation nor on the kinetics of coking. The coking of the catalyst was investigated by monitoring axial coke profiles along the coated wall of the CWR with progressing time-on-stream. Analysing the coke formation at the coated section close to the inlet allows a fast determination of the fundamental dependencies of coke formation on partial pressures of reactants and on temperature. For the first coated section partial pressures correspond to the well-defined inlet values and do not depend on time. Fitting coke profiles and the nitrobenzene outlet concentrations allowed the conclusion that it is indispensable to differentiate between coke on the support and coke on the active site. Furthermore, both coke on the support and coke on the active site were found to be responsible for deactivating the hydrogenation of nitrobenzene. Thus, it was concluded that also the support is involved in hydrogenation of nitrobenzene due to hydrogen spillover.