Roland Dittmeyer

Membrane reactors for hydrogenation and dehydrogenation processes based on supported palladium

Abstract Membrane reactors applied to catalytic reactions are currently being studied in many places world-wide. Significant developments in membrane science and the vision of process intensification by multifunctional reactors have stimulated a lot of academic and industrial research, which is impressively demonstrated by more than 100 scientific papers on catalytic membrane reactors being published per year. Palladium as a noble metal with exceptional hydrogen permeation properties and, at the same time, broad applicability as a catalyst, first of all for hydrogenation, is part of many of these developments. This paper discusses two different membrane reactor concepts which both rely on supported palladium, on the one hand as a permselective membrane material, and on the other hand as base component of a membrane-type hydrogenation catalyst. Dense palladium composite membranes can be used for hydrogen separation from packed-bed catalysts in gas-phase hydrocarbon dehydrogenation reactions. Mesoporous membranes containing dispersed bimetallic Pd/X-clusters can be employed as so-called catalytic diffusers for liquid-phase hydrogenation, e.g. of nitrate and nitrite in water. The principles of both concepts are introduced, recently obtained experimental data are evaluated in connection with literature results, and the perspectives for further development are highlighted.

Catalytic dehydrogenation of hydrocarbons in palladium composite membrane reactors

Abstract Different methods for the preparation of hydrogen permselective palladium composite membranes on asymmetric ceramic and porous stainless-steel tubes were tested. Electroless plating, electroplating, chemical and physical vapor deposition, as well as high velocity oxy-fuel spraying were investigated. Electroless plating was confirmed to be a suitable method for coating ceramic supports. Promising methods for the preparation of composite palladium–steel membranes are high velocity oxy-fuel spraying and a combined method of electroplating and electroless plating. Successfully coated membranes were employed for hydrogen separation in lab-scale membrane reactors during the dehydrogenation of ethylbenzene to styrene and propane to propylene. In both cases, the removal of hydrogen significantly increased the olefin yield, as compared to the corresponding conventional packed-bed reactor results. A relative increase of styrene yield above 15% was observed using a palladium–ceramic membrane. The relative increase of propylene yield was close to 18% for the same type of membrane. Employing a stainless steel-based membrane even further increased the yield of propylene. However, in contrast to the virtually defect-free ceramic-based membranes, a certain amount of other components passed the palladium–steel membrane along with hydrogen, amounting to roughly 20% of the total exit gas flow.

Studies on the use of catalytic membranes for reduction of nitrate in drinking water

Hydrogenation of nitrate using supported bimetallic Pd-Cu or Pd-Sn catalysts provides a promising new alternative for denitrification of drinking water with several advantages over established technologies such as reverse osmosis or biological denitrification (Sell et al., 1992). One of the major problems of this technology is the formation of undesired side products, i.e. ammonium and nitrite, both subject to a limiting value far below the admissible nitrate content. Previous studies have shown that the particle size, among other effects, is a key parameter determining the selectivity of the catalyst, pointing to a detrimental effect of pore diffusion limitation (Hahnlein et al., 1998). Moreover, activity and selectivity depend on the concentration of dissolved hydrogen, i.e. high concentration means high activity, but at the same time favoring of ammonium formation due to further hydrogenation. In this paper a porous catalytic membrane acting as a hydrogen diffuser is proposed as a means to solve these problems by creating an efficient three-phase contact between the catalytic surface, dissolved nitrate, and hydrogen gas. Such a design offers the potential to control the activity and selectivity of the process through controlled dosage of hydrogen. Experimental data on the hydrogenation of aqueous nitrate using commercial ceramic membranes modified by insertion of palladium and copper or tin into the pore-structure through impregnation and MOCVD (metalorganic chemical vapor deposition) are presented and discussed together with characterization results obtained by SEM/EDAX and XPS.

Porous, catalytically active ceramic membranes for gas-liquid reactions: a comparison between catalytic diffuser and forced through flow concept

Abstract Catalytically active membranes can be applied for three-phase reactions (liquid, gas and solid catalyst) and have advantages over conventional particle catalysts. Catalytically active components are deposited in the thin fine-porous membrane layer of an asymmetrical ceramic membrane. One reactant is dissolved in the liquid and diffuses through the porous structure of the membrane to the active inner surface, the other reactant is fed through the support to the catalytic layer from the other side of the membrane. Thereby, an effective contact between the two reactants and the solid catalyst is established. Under these conditions catalytically active membranes can typically be applied for hydrogenation or oxidation processes. Alternatively, if pore diffusion needs to be eliminated the dissolved reactants can be pumped through an asymmetric ceramic membrane or just a ceramic support coated with catalytically active metals. Thus, a very short contact time can be achieved. The following paper compares these two concepts—the catalytic diffuser and the forced through flow concept—and discusses their application for hydrogenation processes, like the catalytic nitrate/nitrite reduction in water and the dechlorination of chlorinated hydrocarbons. The developed catalytic membranes are not limited to these reactions, but are applicable for a number of multiphase reactions. A preparation method in order to make ceramic membranes catalytically active and different characterization methods will be described.

Tubular Inorganic catalytic membrane reactors: advantages and performance in multiphase hydrogenation reactions

The tubular inorganic catalytic membrane (TICM) reactor is a novel multiphase solution for advanced chemical or environmental technologies. The advantages of the use of this technology are discussed with reference to two types of applications: (i) water remediation technologies by catalytic hydrogenation (nitrate removal and chlorinated hydrocarbon dehalogenation) and (ii) H2O2 synthesis by H2+O2 reaction, both occurring over Pd-type catalysts supported over porous inorganic membranes. Some examples of the behaviour of these catalytic membranes are given in long-term tests in nitrate reduction, in chloroform dehalogenation and in H2O2 synthesis.

Factors controlling the performance of catalytic dehydrogenation of ethylbenzene in palladium composite membrane reactors

Ethylbenzene dehydrogenation to styrene in a palladium composite membrane reactor was investigated. The reaction kinetics have been studied in a gradientless recycle reactor to develop a suitable kinetic model, which was subsequently used for the simulation of a hypothetical industrial-scale packed-bed membrane reactor. For this simulation, the measured hydrogen permeability of palladium composite membranes, prepared by different methods on asymmetric porous ceramic and porous sinter metal supports, was used. The results demonstrate that the performance of the membrane reactor is controlled both by the membrane permeability and by the reaction kinetics, i.e. 4–27 % savings of the ethylbenzene feed at equal styrene output are predicted for industrially relevant operating conditions. Besides savings of raw materials, a second advantage of the membrane reactor is seen in a reduced ethylbenzene load of the ethyleenzene/styrene fractionation column, thanks to the increased conversion. Moreover, a simplified heat management seems to be possible by utilizing the combustion of the permeated hydrogen to supply the heat required for the dehydrogenation. Concerning membrane permeability, the simulation demonstrates a noticeable contribution of the support to the overall hydrogen transport resistance when going for a tube size suitable for an industrial-scale reactor. Hence, care has to be taken not only of the permeability of the hydrogen permselective layer, but also of the thickness, porosity, and mean pore size of the support. All in all, the simulation results show that a packed-bed multitubular membrane reactor in fact offers the potential for substantially increasing the styrene yield in ethylbenzene dehydrogenation. What is needed are both high hydrogen permeability and high catalyst efficiency. However, a large-scale application obviously means a very big challenge not only in respect of the manufacture of highly permeable and resistive membranes but also in terms of maximizing catalyst efficiency and developing a suitable reactor design.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

In the future, (electro‐)chemical catalysts will have to be more tolerant towards a varying supply of energy and raw materials. This is mainly due to the fluctuating nature of renewable energies. For example, power‐to‐chemical processes require a shift from steady‐state operation towards operation under dynamic reaction conditions. This brings along a number of demands for the design of both catalysts and reactors, because it is well‐known that the structure of catalysts is very dynamic. However, in‐depth studies of catalysts and catalytic reactors under such transient conditions have only started recently. This requires studies and advances in the fields of 1) operando spectroscopy including time‐resolved methods, 2) theory with predictive quality, 3) kinetic modelling, 4) design of catalysts by appropriate preparation concepts, and 5) novel/modular reactor designs. An intensive exchange between these scientific disciplines will enable a substantial gain of fundamental knowledge which is urgently required. This concept article highlights recent developments, challenges, and future directions for understanding catalysts under dynamic reaction conditions.

CVD preparation of catalytic membranes for reduction of nitrates in water

A catalytic membrane contactor for selective hydrogenation of nitrate in water to nitrogen is discussed as a promising new approach to develop a technically feasible catalytic process for nitrate reduction from ground and surface water. Metal organic chemical vapour deposition (MOCVD) was used to place catalytically active metals, i.e., palladium and tin, inside the porous top-layer of asymmetric ceramic membranes (e.g. alumina) with different pore sizes and thicknesses of the top-layer. The influence of the membrane properties, i.e., the material, pore size and thickness of the catalytic layer, and the influence of the MOCVD process parameters on the deposition of the metals was studied and catalytic membranes with different palladium loading and palladium/tin ratio were prepared. These membranes were characterised with respect to the pore structure and the distribution of the active metals. Their catalytic performance in the hydrogenation of nitrate in water was investigated in a laboratory stirred tank membrane reactor under continuous flow conditions to explore the influence of important process parameters. The results show that the catalytic membranes offer a high activity and a fair selectivity to nitrogen which may be further improved by an optimisation of the preparation procedure.

Dynamic Modeling of Chemical Reaction Systems with Neural Networks and Hybrid Models

A common problem in kinetic modeling of complex chemical reactions is that a rigorous description of the reaction system, e.g., based on elementary reactions, is not possible. This is because either the reaction involves too many reactions and intermediates or the reaction mechanism is not known in sufficient detail. Alternative data-driven modeling, e.g., using neural networks, normally demands large amounts of experimental data and has poor generalization capability. In such situations a combined physical and data-driven (i.e. hybrid) model may be attractive, that utilizes the specific advantages of both approaches while avoiding their disadvantages. This paper explains the procedure of hybrid modeling of integral (i.e. time-dependent) data by using examples from chemical kinetics. The benefits of the hybrid models are described in comparison to the limiting cases of purely physical and purely data-driven models. In general, the hybrid model surpasses the purely physical and neural network models in terms of a combined interpolation- and extrapolation-range criterion.

Crystallization Fouling in Experimental Micro Heat Exchangers—Optical and Thermal Investigations

The intention of this work is the basic investigations of the fouling behavior in micro heat exchangers. Therefore fouling experiments with calcium carbonate (CaCO3) in an experimental micro heat exchanger were carried out and observed with a digital microscope. The investigations included local temperature measurements confirmed by computational fluid dynamics simulation as well as optical visualization of the fouling process inside the microchannels. The detected fouling resistances R f were in the range of 10−5–10−4 m2 K W−1. Cleaning in place was possible and also optically observed.