Catalysts: everything from a single source

Dr Ralf Mayer, Dr Dorit Wolf and Dr Horst-Werner Zanthoff discuss the development of heterogeneous catalysts and processes.

Heterogeneous catalysis has an enormous range of applications. Whether carrying out hydrogenation or dehydrogenation, hydro-genolysis or oxidation reactions, heterogeneous catalysts based on precious metals are indispensable.Typically, these reagents are very fine, black palladium, platinum, rhodium, iridium or ruthenium powders. The amorphous structures of these powders are still difficult to define and characterise - many of the reaction mechanisms have yet to be elucidated. This is made more difficult because the catalysts do not structure until they are subject to reaction conditions. In addition, an enormous number of variables influence these complex systems, including a wide variety of metallic salt solutions. The type and composition of the carrier materials used as raw materials - which include aluminium, silicon oxide, calcium carbonate or carbon black - also play an important role.

Activated carbon is the carrier used most frequently. Various natural sources are available for activated carbon, including wood, peat or coconut shells, which display various morphologies of their own. All this has an effect on the catalysts. The rest of the functional groups are established through the appropriate carbonisation and activation processes of the carbon.

Catalysis conditions have a direct impact on the development of the catalytic process and help anchor the metal components to the carrier. The oxidation condition, particle size and distribution of the metal components are determined by the conditions present when the metal components are added, and by later reduction. Even the way the reactor is filled plays a role. To generate custom-tailored catalysts for the individual application demands a special kind of expertise - some still call it the 'black art' - because many of the methods rely on intuition and experience. This knowledge will continue to be vital in the future.

Degussa has developed new methods and tools in its Catalysis Project House that allow for a rapid and systematic search for catalysts. The focus of these efforts is a prediction of the optimal combination of catalyst design, substrate and reaction type to meet the needs of customers.

The laboratory equipment required for this includes miniaturised reactor systems for the automatic, parallel production and testing of catalysts. It is integrated into an information management system that controls the process of collecting and managing rough data, data archiving and preparing data for inquiries.

A special profile of characteristics (e.g. oxidation state, size and dispersion of the metal particles, as well as the morphology and porosity of the carrier material) is set by means of a multi-stage production process.

Due to the fact that catalytic reactions are characterised by the complexity of the links between process parameters and by the presence of a host of different substrate structures, the design of the optimal catalyst cannot be predicted without experimentation. For this stage, Degussa also provides equipment suitable for fast and accurate catalyst selection.

In the past, selection involved combining as many ingredients as possible for the best result. It can be appreciated how difficult this is when it is realised that as few as six to 10 ingredients could quickly produce several hundred or even several thousand possible combinations. With the development of high-throughput testing, the groundwork was laid for studying the effect of a large number of catalysts in a short period of time. What was missing was the ability to produce a wide variety of catalysts just as quickly.

In co-operation with the Swiss company Chemspeed, Degussa has built preparation equipment that allows the automated production of several catalysts at once. Based on the 'accelerator synthesiser' system, the plant can accept up to 24 glass reactors.

Process steps such as solid and liquid dosing, agitation, mixing, filtration and washing are performed at small scale.

Apart from actual powder preparation, other technical processing steps still had to be adapted to the small scale. Some of them, such as spray-drying, could not be adapted, so an alternative was sought that could yield identical results. The solution was found in another area of the former project house: biocatalysis.

It turned out that catalysts can be processed just as well when they are freeze-dried - a process that biologists typically use for drying strains of bacteria. Moreover, freeze-drying can be carried oout on with several catalysts at once.

After production and drying, the only process left was the heating of the catalysts in air at temperatures of up to 950°C. In this process, the catalysts must be continually moved so that they do not stick together.

In this case, too, there was no device available that could handle great numbers of small quantities; so a multiple rotary kiln was developed for processing catalysts simultaneously at different temperatures and gas compositions.

Proof that this method can yield catalysts that meet the performance characteristics of the catalyst systems used for technical applications was produced in conjunction with the Feed Additives Business Unit for catalysts in the selective oxidation of hydrocarbons. The work of the project house has produced promising methods for improvements in technical catalysts, which have continued to be developed since its conclusion.

scale-up methods

Following completion of the project house in late June 2004, the area of heterogeneous catalysis was integrated into the Catalysts Business Line of the Exclusive Synthesis & Catalysts Business Unit. As a result, the Catalysts Business Line is now offering parallelised production of catalysts not only to Degussa's internal units, but to external customers for the joint development and optimisation of catalysts.

Simply testing a catalyst 'on the small scale' has not yet resulted in reaction engineering usable on the industrial scale. Rather, the catalyst-reactor process system must be raised to a technical and economic optimum. This is why methods have also been developed in the Catalysis Project House to accelerate scale-up at minimal cost.

Due to the many parameters, such as temperature, partial pressure, catalyst stability, byproducts (even trace amounts), substance and heat transport, as well as the catalyst compositions found in the initial screening, a fundamentally different approach is needed compared with high-throughput tests. What is focused on is the maximum flow of information on the experiment, complete mass balance, and the evaluation of long-term effects - a case for intelligent design of experiments and predefinition of optimisation target functions.

kinetic screening

Against this backdrop, Degussa has developed innovative methods for 'kinetic screening,' which allow kinetic links to be quickly recorded. Complete kinetics are obtained through a suitable variation in the reaction parameters during screening. When comparing various catalysts, these methods permit clear conclusions about the structure-effect relationships of a reaction and, as a consequence, faster optimisation of catalysts. At the same time, this procedure forms the basis for plant design and saves time in scale-up. The key to minimising scale-up time, however, is to drastically reduce the time spent in recording kinetics. While several weeks are needed in the conventional case, kinetic screening yields a result in only a few hours or days.

Kinetic screening is based on miniaturisation with targeted scale-down, automation and limited parallelisation from the design of experiments up to data visualisation. It also involves special methods, such as transient measuring technologies for faster model discrimination. Additionally, new modular micro-reactors supply intrinsic kinetic data, even in complex reaction systems, such as in the testing of powder catalysts under continuous operation conditions. Reactors are provided with the essential details for later scale-up, and therefore no time is lost in later project phases.

Flexible laboratory process control systems provide constant process data and can easily be adapted to new tasks in a matter of minutes. This lowers the otherwise considerable cost of experiments.

Advanced online analytics (e.g. Fast-GC) quickly supply kinetic data, and through higher data density, improve the statistical security of the results as compared with conventional methods. Specially developed software minimises the use of human resources to analyse results and generate the desired evaluation data; including yields, selectivities, rate constants, deactivation speeds, etc. With the help of flexible visualisation programmes, these show the chemist and process engineer links between catalyst and process, which reveal possible paths to optimisation.

For Degussa, not only has the work of the project house meant an enormous methodical advance in the development of catalytic processes, it has also already started to bear fruit by advancing the development of a catalyst for the cracking of ethers for the C4 Chemistry Business Unit. Approximately 70 catalysts were subjected to kinetic screening in more than 600 experiments in only four months. This resulted in five new compositions that could be able to meet the technical requirements.

critical parameters

The optimal reaction conditions were also determined and critical parameters were defined for technical use. A potential candidate displayed its qualities under pilot-scale conditions, and could be used in an industrial-scale plant.

Following the successful conclusion of the project house, kinetic screening methodology is being expanded and is available to both internal and external customers of Process Technology and Engineering Chemical Reaction Technology. A close-meshed network and guaranteed exchange of knowledge with the Catalysts Business Line and Degussa Engineering will also ensure custom-tailored solutions in catalytic process development in the future: from the search for catalysts, to process design and process optimisation, to quality control for catalysts - everything from a single source.