Microwave accelerated heterogeneous catalysis in flowthrough reactors
Final Report Abstract
By our experimental and theoretical studies we could demonstrate, that the formation of hot spot effects in Pd loaded polymer/carrier composites operated in a microwave field at high flow rates of the reaction mixture is not realistic and an enhancement of chemical reaction rates can not be expected by microwave radiation in this case. At low flow rates hot spot effects can be possible. Nevertheless, the modelling effort accomplished in this work has shown how some properties of the composite materials, which can be easily manipulated, could enhance the temperature gradients achievable, a first step towards a process intensification by the combination of polymer-glass composite materials and microwave irradiation. As a matter of fact, the conclusions derived from the model could be applied to a more general concept, an intensification of chemical transformations using heterogeneous catalysts, microwave irradiation and microreaction technology. In summary, it was demonstrated that the PASSflowTM technique can be successfully utilized in a catalytic process using transition metal catalysts. This study reveals that this technique can be extended to microwave heated operations. The design of the PASSflowTM reactors and their materials of construction are well suited for this application. Using microwave transparent reactor walls and inner structures catalytic metal sites and the polymer/glass composite can be heated effectively. Forced convective flow inside the reactors is beneficial for traditional heating as well as for microwave heating. The influence of catalyst distribution inside the composites was investigated for the transfer hydrogenation of ethylcinnamate as a model reaction. It was observed that the reactivity of prepared catalysts is different in traditional and microwave heating, depending on the flow rate of the reaction mixture. The reason for different performance is related to the presence of two different effects. On the one hand a high active site dilution produces a decrease in the nanoparticle size leading to a higher surface area and coupled with this to a higher catalytic activity. On the other hand the same dilution produces a decrease in the Pd content of the catalysts leading to a small rate enhancement by hot spot effects. At low Pd content the amount of microwave energy that can be absorbed and dissipated by the catalyst drops. Based on the studies presented here the successful application of microwaves in flow through micro reactors depends strongly on the operation parameters used in the experiments and the specific properties of the catalytic composite materials used. At high flow rates mass transfer is enhanced and the rate of chemical reactions can be improved by better mass transfer. This in combination with microwave heating conflicts, because at high flow rate a great heat loss of possible hot spots may occur by the flowing solvent. So application of microwaves is more useful at lower flow rates where the heat stays on the active sites. But at very low flow rates mass transfer is reduced, leading to lower rates. As a result both the flow rate as well as the material properties have to be adjusted to reach satisfying results. With respect to scale –up the appropriate reactor design has to be considered. In the case of microwave application large diameters seem not very promising if a strongly microwave absorbing solvent or highly metal loaded catalysts are used because penetration depth is limited. With our monolithic composites based on microwave transparent supports and controlled noble metal particle preparation, reactor diameters with several cm seem possible. Uniform heating of the catalyst by microwaves and radial mixing of the liquid passing through the reactor by the irregular shaped channels in the composites can lead to a homogeneous heat distribution in the bulk liquid. By this combination a successful combination of microwave heating and flow through operation looks promising.
Publications
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“Microwave-Assisted Heterogeneous Catalysis in Flow-Through Reactors”. Symposium on Microwave Accelerated Synthesis (MAS´05). September 2005, Düsseldorf, Germany
Cecilia, R., Kunz, U., Mennecke, K., Kirschning, A., Glasnov, T. and Kappe, C.O.
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“Particle size control in Pd polymer supported Catalysts and their application to microwave assisted synthesis”. 5th Congress of the Spanish Catalysis Society (SECAT ´05). June 2005, Madrid, Spain
Cecilia, R., Kunz, U., Mennecke, K., Kirschning, A., Glasnov, T. and Kappe, O.
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„Monolithic polymer/carrier materials: versatile composites for fine chemical synthesis“. 2nd International Conference on Structured Catalysts and Reactors (ICOSCAR-2). October 2005, Delft, The Netherlands
Kunz, U., Kirschning, A., Wen, H.L., Solodenko, W., Cecilia, R., Kappe, C.O and Turek.T.
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„Monolithic polymer/carrier materials: versatile composites for fine chemical synthesis“. Cat. Today. 105, (2005) 318
Kunz, U., Kirschning, A., Wen, H.L., Solodenko, W., Cecilia, R., Kappe, C.O and Turek, T.
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“Catalytic Flow-Through Microreactors for Microwave Assisted Organic Syntheses”. ACHEMA 2006. May 2006, Frankfurt am Main, Germany
Cecilia, R., Kunz, U., Turek, T., Mennecke, K., Kirschning, A., Glasnov, T. and Kappe, C.O.
