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Development of tools and methods for the high-throughput preparation of commercial heterogeneous catalysts
Scientific Bases for the Preparation of Heterogeneous Catalysts E.M. Gaigneaux et al. (Editors) © 2006 Elsevier B.V. All rights reserved.
235
Development of tools and methods for the highthroughput preparation of commercial heterogeneous catalysts Ralf W. Mayer, Thomas Quandt, Klaus Schimmer, Hans Lansink Rotgerink, and Thomas Tacke Degussa A G, Exclusive Synthesis & Catalysts, Business Line Catalysts, Rodenbacher Chaussee 4,63457 Hanau, Germany
1. Abstract The usage of high-throughput preparation and screening tools is very beneficial for accelerating the development of heterogeneous catalysts. But a major point in industrial R&D is a smooth and trouble-free scale-up and commercialization after the catalyst is developed. Hence, the catalysts prepared in the highthroughput scale have to represent already the very complex structures and characteristics of commercial heterogeneous catalysts. Therefore, preparation methods and equipment had to be developed that take into account a high degree of parallelisation with reasonable sample amount to be representative for commercially produced catalysts. This development and its successful application to different types of industrially relevant catalysts are presented in this article. 2. Introduction In the development of heterogeneous catalysts for industrial applications a variety of different parameters, such as metal impregnation, metal precipitation, drying, thermal treatment, etc. have to be optimized. Tn addition, mechanical strength and thermal stability of the catalyst, as well as economic factors like regenerability and manufacturing costs have to be considered early on in the catalyst development project. With regard to the chemical process conversion, selectivity, and durability are most important, but also other major aspects like
236
Mayeretet al. R.W. Mayer
the avoidance of specific side-products that are difficult to separate from the desired product have to be considered as well. In some cases additional limitations can occur due to IP rights or the customers' wish to use existing reactor facilities with given restrictions in temperature, pressure, or feed flows. Moreover, environmental aspects have to be considered, not only regarding the efficiency of the catalytic process, but also the catalyst and its compounds. And as a last major issue, the manufacturing capabilities of the catalyst producer need to be taken into account when a new catalyst formulation for an industrial application shall be developed successfully. The present work investigates whether high-throughput tools and methods can be applied and how they have to be modified to meet the requirements of an industrial development of heterogeneous catalysts: an accelerated preparation but adequately regarding the above mentioned issues. 3. High-Throughput Experimentation for Industrial Purposes 3.1. The need for development
high-throughput preparation
in industrial catalyst
One can raise the question, whether the application of high-throughput tools and methodology in the development of commercial catalysts is necessary and reasonable, particularly in the case, where a catalytic lead structure has already been identified. The clear answer is yes. To meet all the requirements that might appear within a catalyst development project for industrial applications, further studies and development efforts have to be carried out, even if a suitable catalyst candidate exists. Beside the catalyst composition, there are many other factors that will influence the performance of the catalyst, e.g.: • the precursors of the used compounds including the support material • raw material quality, costs, and availability on large scale • the preparation route (impregnation, co-precipitation, sequences, etc.) • the thermal treatment (temperature, duration, atmosphere, furnace type) • process stability of the preparation recipe from lab to commercial scale • the particle morphology and surface structure • the reactor type and mode of operation. Although each of these points will effect the catalytic performance individually, the challenge is the combination of these factors. If, for example, an organic salt is chosen as precursor for an impregnation catalyst instead of the nitrate of the desired compound, the particle morphology of this compound will change. But in addition, the thermal treatment will also be different, because the nitrates and organic salts will need different calcination conditions. This difference in
Development of tools and methods for for high-throughput preparation... preparation...
237
calcination will consequently lead to an additional change in the particle morphology of this compound, but also influence the surface structure of the support material. Thus, changing one of these parameters will influence several characteristics of the catalyst and consequently its performance. As a consequence and because there is no sufficient theoretical knowledge, empirical studies have to be performed to find the best catalyst for the desired reaction under the given specific reaction conditions. Hence, a large number of different catalysts must be prepared and tested, what will be done best by a parallel, high-throughput approach. 3.2. Boundary conditions and requirements to the equipment for industrial high-throughput preparation 3.2.1. Sample amount If one agrees to the above explained necessity to prepare a large number of catalysts to find the optimal product for a certain application, the next question will be, how small or large the prepared sample can or must be. Of course, the principle of high-throughput experimentation is to apply a combination of highest possible number of samples with the smallest possible amount of sample. There have been attempts to prepare heterogeneous impregnated catalysts in a very small scale like wafers, on which a layer of support material is impregnated with a certain number of metal salt solutions [1,2]. But for industrial development purposes, when a subsequent scale-up and commercialization of the small-scale high-throughput catalyst is predetermined, the small-scale sample must already represent the characteristics and complex structure of a commercially produced catalyst, e.g. • metal (-oxide) dispersion and distribution • content of amorphous and crystalline phases • composition of the different phases • characteristics of the support material (if applied) • oxidation state of the active compound In short, like LePage points out, one must avoid creating a structure that is only a laboratory curiosity which for technical or economic reasons can not be manufactured on industrial scale [3]. Due to our scale-up studies, a sample amount in the range of 5 to 15 grams is considered as necessary to prepare a small-scale catalyst that represents sufficient characteristics and properties of a manufactured one, but still allows one to parallelize and accelerate the preparation. However, the degree of parallelization is limited due to this sample amount, but still a number of 4 to 30 catalysts per day, depending on the type of catalyst, can be prepared, which is completely in-line with the test capacities.
238
Mayeretet al. R.W. Mayer
3.2.2. High-throughput preparation equipment As stated above, every catalyst prepared in small-scale has to be representative for a commercially produced one and include all basic characteristics and features. Thus, the preparation equipment must reflect the production capabilities as much as possible. All equipment described below was chosen and developed to meet these requirements. A high-throughput preparation system must be able to apply all fundamental preparation steps for heterogeneous catalysts: • liquid and solid dosing and dispensing • stirring and heating • filtration • washing • drying In addition, a pH control and the possibility to apply different temperatures in two or more preparation zones will be favorable. Moreover, the dosing system must be able to dose liquids as well as solids while the reaction vessels are stirred and heated, i.e. when solvent vapor is present and a movement takes place in the vessel. A suitable automated preparation system providing almost all features was found in the "Accelerator Synthesizer" platform from Chemspeed Technologies. Equipped with 24 lOOml-glass-vessels, two heating zones, solid and liquid dosing unit, evaporator, and "filtration-to-waste" option, the system allows conducting the necessary basic preparation steps with the desired sample size. A pH control system was additionally developed by Degussa. For the preparation of co-precipitated, e.g. mixed metal oxide, catalysts, the drying step cannot be carried out satisfactorily in the preparation robot. Usually a spray drying step is applied in the production of this kind of catalyst, because the liquid phase of the precipitate suspension still contains dissolved salts that are essential for the catalytic performance. Hence, the suspension must not be filtered off nor can be dried by evaporation due to crystallization reasons. Since there are no laboratory spray-dryers available for that sample size, another method had to be implemented and was found in the freeze drying of these materials [4]. With this method almost the same is done like in spray drying but on another time-scale. Where a spray drier evaporates the water very quickly and thereby prevents the crystallization of the still dissolved salts, the freeze drier literally at first freezes the solution and no crystallization can occur while the water is sublimated. Hence, an identical product is obtained. Another important step in the preparation of particularly mixed metal oxide catalysts is the calcination of the co-precipitated powder after drying. By heating the material to temperatures above 300-400°C in air, the deployed salts are decomposing and the desired metal oxide structure is obtained. Due to this
Development of tools and methods for for high-throughput preparation... preparation...
239
decomposition, there will be no internal structure in the material that could give any physical stability under calcination conditions, and agglomeration and layering of the catalyst together with a plugging might occur. For this reason, a rotary kiln is applied in the manufacturing of these catalysts. To copy this production method for high-throughput preparation, a small-scale 5-fold rotary kiln was constructed [5]. The catalysts are filled into small quartz tubes that are put into separate heating chambers in a big drum-like cylinder. By rotating this drum a continuous movement of the catalyst is ensured during calcination. Moreover, different gas phases as well as different temperatures can be applied. Thus, all essential preparation steps can be applied in small-scale with the utmost consideration of manufacturing procedures and production steps. 3.2.3. High-throughput testing Although the testing of catalysts is not in the main focus of this article, some points have to be mentioned. The development of industrial catalysts does not focus only on the revelation of new catalytic materials for a certain reaction, but has also to address questions about side-products, economics, durability and stability, etc., which are very important for industrial processes. Therefore, the testing of the prepared catalysts has to be very specific with regard to reaction conditions and product analysis. For that purpose, well-established and high reliable multi-channel tubular reactors are used including a sophisticated and accurate GC analysis of the products and side-products [6]. After this screening phase, the most promising catalysts will then be tested in a mini plant or pilot plant, respectively. 4. Results 4.1. Scale-down As a major step in the evaluation of the above mentioned high-throughput tools and techniques, a scale-down of different types of catalysts for several applications was performed. For that purpose, two well established commercial catalysts, one of the mixed metal oxide type for selective olefin oxidation and one impregnated catalyst for ethylene aeetoxylation to vinyl acetate monomer (VAM), respectively, were prepared in the small-scale and their catalytic performance was compared. As shown in Fig. 1 with the selective oxidation catalyst, the scale-down of this catalyst was successful, since both, the commercial and the high-throughput prepared catalyst are showing identical performances. Regarding the calcination procedure one can point out, that only if this step is carried out in the 5-fold rotary kiln, equal catalysts were obtained.
240
R. W.Mayer Mayeretetal.al. R.W.
m«
A
AA
*
i
• *
Conversion
Fig. 1: Comparison of commercial reference catalyst (A.) for selective oxidation with highthroughput prepared ones (5-fold rotary kiln: Xa, muffle oven: vH ).
For the VAM catalyst, as an example for an impregnated catalyst, the scaledown was successful, too, like visible in Fig. 2, where the catalytic performance in VAM synthesis is shown for the two differently prepared catalysts for several reaction conditions.
A A
• • n °
1 *
*
time on stream [h]
Fig. 2: Comparison of commercial (X) vs. high-throughput prepared (X) catalyst for VAM synthesis at different reaction conditions (shape).
Development of tools and methods for preparation... for high-throughput preparation...
241
4.2. Further optimization For the further optimization of the catalysts, the usual generation approach was used after the basic preparation steps were qualified: As a start, one generation of 50 to 100 catalysts that can be based either on statistical or evolutionary planning is prepared. After testing of all catalysts of one generation, the next generation is planned based on the obtained results. Fig. 3 shows the catalytic performance of the catalysts for selective oxidation of the first and second generation. The improvements made from one generation to the other are clearly visible.
Conversion
Fig. 3: Comparison of the catalytic performance (selectivity vs. conversion) of the catalysts of the first (<—) and second (t) generation prepared hy means of high-throughput equipment.
Regarding the further development of the VAM catalysts, a first set of catalysts was prepared with different dopants. As Fig. 4 shows, some dopants could be identified that are improving the catalytic performance compared to the undoped reference. -
-|
-|
Ref.
-
A1 A2
B
C1 C2 C3
D
E1 E2
F
G1 G2 Ga1 H
J
K
L1 L2
L3 M l
M2
Dopands
Fig. 4: Space-Time-Yield of VAM catalysts with different dopants
N
0 1 02
P
Q1 Q2
R
S
242
Mayeretet al. R.W. Mayer
5. Conclusions A main conclusion of this development project is that a high-throughput preparation of heterogeneous catalysts can be applied for commercial purposes. There is a need for an accelerated preparation of the catalysts because besides the composition of the catalysts the influence of the other preparation steps on the catalytic performance is particularly of interest for industrial processes. With the right preparation equipment, catalysts can be prepared in small-scale that represent most of the characteristics and features of commercially produced ones. However, the sample size should not be below a certain limit to assure this similarity, hence a limit in parallelism occurs, too. But still a factor of 4 to 8 depending on the catalyst type is achieved compared to manual preparation. Together with meanwhile well-established high-throughput test units, these tools and techniques are accelerating the development of heterogeneous catalysts. 6. Acknowledgment Uwe Rodemerck and Mariana Stoyanova with the Leibniz Institute for Catalysis, Branch Berlin, (former Institute for Applied Chemistry BerlinAdlershof, ACA) are thankfully acknowledged for their dedicated work in testing the catalysts. Part of this work was co-funded by the Federal Ministry for Research and Education of Germany under the reference number 03X2003. References 1. J.S. Paul, J. Urschey, P.A. Jacobs, W.F. Maier, F. Verpoort, J. Catal., 220 (2003) 136 2. US Patent Application No. 2004/0028815, assigned to Engelhard Corp. (2004) 3. J.F. LePage, 1999. Developing Industrial Catalysts. In: G. Ertl, H. Knozinger, J. Weitkamp (Eds.), Preparation of Solid Catalysts. Wiley-VCH, Weinheim, Germany, p. 3-10 4. PCT Patent Application No. WO2005/058499, assigned to Degussa AG (2005) 5. PCT Patent Application No. WO2005/039765, assigned to Degussa AG (2005) 6. U. Rodemerck, D. Wolf, O.V. Buyevskaya, P. Claus, S. Senkan, M. Baerns, Chem. Eng. J., 82 (2001) 3
235
Development of tools and methods for the highthroughput preparation of commercial heterogeneous catalysts Ralf W. Mayer, Thomas Quandt, Klaus Schimmer, Hans Lansink Rotgerink, and Thomas Tacke Degussa A G, Exclusive Synthesis & Catalysts, Business Line Catalysts, Rodenbacher Chaussee 4,63457 Hanau, Germany
1. Abstract The usage of high-throughput preparation and screening tools is very beneficial for accelerating the development of heterogeneous catalysts. But a major point in industrial R&D is a smooth and trouble-free scale-up and commercialization after the catalyst is developed. Hence, the catalysts prepared in the highthroughput scale have to represent already the very complex structures and characteristics of commercial heterogeneous catalysts. Therefore, preparation methods and equipment had to be developed that take into account a high degree of parallelisation with reasonable sample amount to be representative for commercially produced catalysts. This development and its successful application to different types of industrially relevant catalysts are presented in this article. 2. Introduction In the development of heterogeneous catalysts for industrial applications a variety of different parameters, such as metal impregnation, metal precipitation, drying, thermal treatment, etc. have to be optimized. Tn addition, mechanical strength and thermal stability of the catalyst, as well as economic factors like regenerability and manufacturing costs have to be considered early on in the catalyst development project. With regard to the chemical process conversion, selectivity, and durability are most important, but also other major aspects like
236
Mayeretet al. R.W. Mayer
the avoidance of specific side-products that are difficult to separate from the desired product have to be considered as well. In some cases additional limitations can occur due to IP rights or the customers' wish to use existing reactor facilities with given restrictions in temperature, pressure, or feed flows. Moreover, environmental aspects have to be considered, not only regarding the efficiency of the catalytic process, but also the catalyst and its compounds. And as a last major issue, the manufacturing capabilities of the catalyst producer need to be taken into account when a new catalyst formulation for an industrial application shall be developed successfully. The present work investigates whether high-throughput tools and methods can be applied and how they have to be modified to meet the requirements of an industrial development of heterogeneous catalysts: an accelerated preparation but adequately regarding the above mentioned issues. 3. High-Throughput Experimentation for Industrial Purposes 3.1. The need for development
high-throughput preparation
in industrial catalyst
One can raise the question, whether the application of high-throughput tools and methodology in the development of commercial catalysts is necessary and reasonable, particularly in the case, where a catalytic lead structure has already been identified. The clear answer is yes. To meet all the requirements that might appear within a catalyst development project for industrial applications, further studies and development efforts have to be carried out, even if a suitable catalyst candidate exists. Beside the catalyst composition, there are many other factors that will influence the performance of the catalyst, e.g.: • the precursors of the used compounds including the support material • raw material quality, costs, and availability on large scale • the preparation route (impregnation, co-precipitation, sequences, etc.) • the thermal treatment (temperature, duration, atmosphere, furnace type) • process stability of the preparation recipe from lab to commercial scale • the particle morphology and surface structure • the reactor type and mode of operation. Although each of these points will effect the catalytic performance individually, the challenge is the combination of these factors. If, for example, an organic salt is chosen as precursor for an impregnation catalyst instead of the nitrate of the desired compound, the particle morphology of this compound will change. But in addition, the thermal treatment will also be different, because the nitrates and organic salts will need different calcination conditions. This difference in
Development of tools and methods for for high-throughput preparation... preparation...
237
calcination will consequently lead to an additional change in the particle morphology of this compound, but also influence the surface structure of the support material. Thus, changing one of these parameters will influence several characteristics of the catalyst and consequently its performance. As a consequence and because there is no sufficient theoretical knowledge, empirical studies have to be performed to find the best catalyst for the desired reaction under the given specific reaction conditions. Hence, a large number of different catalysts must be prepared and tested, what will be done best by a parallel, high-throughput approach. 3.2. Boundary conditions and requirements to the equipment for industrial high-throughput preparation 3.2.1. Sample amount If one agrees to the above explained necessity to prepare a large number of catalysts to find the optimal product for a certain application, the next question will be, how small or large the prepared sample can or must be. Of course, the principle of high-throughput experimentation is to apply a combination of highest possible number of samples with the smallest possible amount of sample. There have been attempts to prepare heterogeneous impregnated catalysts in a very small scale like wafers, on which a layer of support material is impregnated with a certain number of metal salt solutions [1,2]. But for industrial development purposes, when a subsequent scale-up and commercialization of the small-scale high-throughput catalyst is predetermined, the small-scale sample must already represent the characteristics and complex structure of a commercially produced catalyst, e.g. • metal (-oxide) dispersion and distribution • content of amorphous and crystalline phases • composition of the different phases • characteristics of the support material (if applied) • oxidation state of the active compound In short, like LePage points out, one must avoid creating a structure that is only a laboratory curiosity which for technical or economic reasons can not be manufactured on industrial scale [3]. Due to our scale-up studies, a sample amount in the range of 5 to 15 grams is considered as necessary to prepare a small-scale catalyst that represents sufficient characteristics and properties of a manufactured one, but still allows one to parallelize and accelerate the preparation. However, the degree of parallelization is limited due to this sample amount, but still a number of 4 to 30 catalysts per day, depending on the type of catalyst, can be prepared, which is completely in-line with the test capacities.
238
Mayeretet al. R.W. Mayer
3.2.2. High-throughput preparation equipment As stated above, every catalyst prepared in small-scale has to be representative for a commercially produced one and include all basic characteristics and features. Thus, the preparation equipment must reflect the production capabilities as much as possible. All equipment described below was chosen and developed to meet these requirements. A high-throughput preparation system must be able to apply all fundamental preparation steps for heterogeneous catalysts: • liquid and solid dosing and dispensing • stirring and heating • filtration • washing • drying In addition, a pH control and the possibility to apply different temperatures in two or more preparation zones will be favorable. Moreover, the dosing system must be able to dose liquids as well as solids while the reaction vessels are stirred and heated, i.e. when solvent vapor is present and a movement takes place in the vessel. A suitable automated preparation system providing almost all features was found in the "Accelerator Synthesizer" platform from Chemspeed Technologies. Equipped with 24 lOOml-glass-vessels, two heating zones, solid and liquid dosing unit, evaporator, and "filtration-to-waste" option, the system allows conducting the necessary basic preparation steps with the desired sample size. A pH control system was additionally developed by Degussa. For the preparation of co-precipitated, e.g. mixed metal oxide, catalysts, the drying step cannot be carried out satisfactorily in the preparation robot. Usually a spray drying step is applied in the production of this kind of catalyst, because the liquid phase of the precipitate suspension still contains dissolved salts that are essential for the catalytic performance. Hence, the suspension must not be filtered off nor can be dried by evaporation due to crystallization reasons. Since there are no laboratory spray-dryers available for that sample size, another method had to be implemented and was found in the freeze drying of these materials [4]. With this method almost the same is done like in spray drying but on another time-scale. Where a spray drier evaporates the water very quickly and thereby prevents the crystallization of the still dissolved salts, the freeze drier literally at first freezes the solution and no crystallization can occur while the water is sublimated. Hence, an identical product is obtained. Another important step in the preparation of particularly mixed metal oxide catalysts is the calcination of the co-precipitated powder after drying. By heating the material to temperatures above 300-400°C in air, the deployed salts are decomposing and the desired metal oxide structure is obtained. Due to this
Development of tools and methods for for high-throughput preparation... preparation...
239
decomposition, there will be no internal structure in the material that could give any physical stability under calcination conditions, and agglomeration and layering of the catalyst together with a plugging might occur. For this reason, a rotary kiln is applied in the manufacturing of these catalysts. To copy this production method for high-throughput preparation, a small-scale 5-fold rotary kiln was constructed [5]. The catalysts are filled into small quartz tubes that are put into separate heating chambers in a big drum-like cylinder. By rotating this drum a continuous movement of the catalyst is ensured during calcination. Moreover, different gas phases as well as different temperatures can be applied. Thus, all essential preparation steps can be applied in small-scale with the utmost consideration of manufacturing procedures and production steps. 3.2.3. High-throughput testing Although the testing of catalysts is not in the main focus of this article, some points have to be mentioned. The development of industrial catalysts does not focus only on the revelation of new catalytic materials for a certain reaction, but has also to address questions about side-products, economics, durability and stability, etc., which are very important for industrial processes. Therefore, the testing of the prepared catalysts has to be very specific with regard to reaction conditions and product analysis. For that purpose, well-established and high reliable multi-channel tubular reactors are used including a sophisticated and accurate GC analysis of the products and side-products [6]. After this screening phase, the most promising catalysts will then be tested in a mini plant or pilot plant, respectively. 4. Results 4.1. Scale-down As a major step in the evaluation of the above mentioned high-throughput tools and techniques, a scale-down of different types of catalysts for several applications was performed. For that purpose, two well established commercial catalysts, one of the mixed metal oxide type for selective olefin oxidation and one impregnated catalyst for ethylene aeetoxylation to vinyl acetate monomer (VAM), respectively, were prepared in the small-scale and their catalytic performance was compared. As shown in Fig. 1 with the selective oxidation catalyst, the scale-down of this catalyst was successful, since both, the commercial and the high-throughput prepared catalyst are showing identical performances. Regarding the calcination procedure one can point out, that only if this step is carried out in the 5-fold rotary kiln, equal catalysts were obtained.
240
R. W.Mayer Mayeretetal.al. R.W.
m«
A
AA
*
i
• *
Conversion
Fig. 1: Comparison of commercial reference catalyst (A.) for selective oxidation with highthroughput prepared ones (5-fold rotary kiln: Xa, muffle oven: vH ).
For the VAM catalyst, as an example for an impregnated catalyst, the scaledown was successful, too, like visible in Fig. 2, where the catalytic performance in VAM synthesis is shown for the two differently prepared catalysts for several reaction conditions.
A A
• • n °
1 *
*
time on stream [h]
Fig. 2: Comparison of commercial (X) vs. high-throughput prepared (X) catalyst for VAM synthesis at different reaction conditions (shape).
Development of tools and methods for preparation... for high-throughput preparation...
241
4.2. Further optimization For the further optimization of the catalysts, the usual generation approach was used after the basic preparation steps were qualified: As a start, one generation of 50 to 100 catalysts that can be based either on statistical or evolutionary planning is prepared. After testing of all catalysts of one generation, the next generation is planned based on the obtained results. Fig. 3 shows the catalytic performance of the catalysts for selective oxidation of the first and second generation. The improvements made from one generation to the other are clearly visible.
Conversion
Fig. 3: Comparison of the catalytic performance (selectivity vs. conversion) of the catalysts of the first (<—) and second (t) generation prepared hy means of high-throughput equipment.
Regarding the further development of the VAM catalysts, a first set of catalysts was prepared with different dopants. As Fig. 4 shows, some dopants could be identified that are improving the catalytic performance compared to the undoped reference. -
-|
-|
Ref.
-
A1 A2
B
C1 C2 C3
D
E1 E2
F
G1 G2 Ga1 H
J
K
L1 L2
L3 M l
M2
Dopands
Fig. 4: Space-Time-Yield of VAM catalysts with different dopants
N
0 1 02
P
Q1 Q2
R
S
242
Mayeretet al. R.W. Mayer
5. Conclusions A main conclusion of this development project is that a high-throughput preparation of heterogeneous catalysts can be applied for commercial purposes. There is a need for an accelerated preparation of the catalysts because besides the composition of the catalysts the influence of the other preparation steps on the catalytic performance is particularly of interest for industrial processes. With the right preparation equipment, catalysts can be prepared in small-scale that represent most of the characteristics and features of commercially produced ones. However, the sample size should not be below a certain limit to assure this similarity, hence a limit in parallelism occurs, too. But still a factor of 4 to 8 depending on the catalyst type is achieved compared to manual preparation. Together with meanwhile well-established high-throughput test units, these tools and techniques are accelerating the development of heterogeneous catalysts. 6. Acknowledgment Uwe Rodemerck and Mariana Stoyanova with the Leibniz Institute for Catalysis, Branch Berlin, (former Institute for Applied Chemistry BerlinAdlershof, ACA) are thankfully acknowledged for their dedicated work in testing the catalysts. Part of this work was co-funded by the Federal Ministry for Research and Education of Germany under the reference number 03X2003. References 1. J.S. Paul, J. Urschey, P.A. Jacobs, W.F. Maier, F. Verpoort, J. Catal., 220 (2003) 136 2. US Patent Application No. 2004/0028815, assigned to Engelhard Corp. (2004) 3. J.F. LePage, 1999. Developing Industrial Catalysts. In: G. Ertl, H. Knozinger, J. Weitkamp (Eds.), Preparation of Solid Catalysts. Wiley-VCH, Weinheim, Germany, p. 3-10 4. PCT Patent Application No. WO2005/058499, assigned to Degussa AG (2005) 5. PCT Patent Application No. WO2005/039765, assigned to Degussa AG (2005) 6. U. Rodemerck, D. Wolf, O.V. Buyevskaya, P. Claus, S. Senkan, M. Baerns, Chem. Eng. J., 82 (2001) 3