Carbon materials are used in a wide range of applications in different areas, such as pollutant removal (active carbon), gas separation (carbon molecular sieves) and chemical reactions (as catalysts and as catalyst supports); in fine chemistry they are among the workhorses. Challenges regarding mechanical properties, reproducibility and quality control in large-scale production of carbons limit the physical form to granules and extrudates. These morphologies force reactor design to essentially packed bed and slurry reactors. These are not always optimal.
Instead of accepting the random and chaotic behaviour of classical reactors one can attempt to build reactors of a fully regular spatial structure. Such a structure may be designed in full detail up to the local geometry of the catalyst species. This offers control of the local environments, allowing the fluid mechanics to be simplified to something well-behaved, which is often laminar. The engineer can easily direct the interaction of transport phenomena and reaction. These reactors are referred to as structured reactors. In nearly all respects structured catalysts and reactors outperform random particles and random/chaotic reactors.
Structured reactors exist in many different shapes. Monoliths, the most popular structured reactors, consist of large numbers of parallel channels. A very different type is the solid foam structure. Foams are 3D cellular materials made of interconnected pores, forming a spongy network. They also combine a high porosity (up to 97%) with a high surface area. They are produced in a large variety of materials, including carbon.
Catalyst bodies can also be made from knitted threads or woven in fabrics, felts etc. In particular glass fibres, sintered metal fibers and carbon fibers are applied.
In the lecture the focus will be on monoliths. Design and development of a satisfactory monolithic reactor is highly dependent on the quality of the synthesis protocols, as will be illustrated in particular with an example of biocatalysis.
Integral carbon monoliths are prepared by extrusion of the carbon precursor, mixed with additives to facilitate extrusion.
Carbon coated monoliths can be produced by different methods
– Melting of the carbon precursor: a monolithic structure is heated with a pitch carbon, resulting in penetration in the pores of the monolith structure. During heating carbonization occurs, permanently solidifying the carbon on the monolith.
– Dipcoating: a liquid polymer (solution) is introduced into the channels, and converted into solid carbon by heating in an inert atmosphere, sometimes after pre-oxidation to induce cross-linking, avoiding mobility during heating.
– CVD: chemical vapour deposition of (fragments of) hydrocarbons by radical chemistry or catalytic growth on a ceramic or metallic monolith.
– Carbon nanofibers (CNFs): fiber growth over deposited metal particles (usually Ni or Fe), deposited by impregnation on the washcoated monolith, followed by heating in a gas flow with carbon precursor.
Reactor configurations such as the Monolithic Loop Reactor and the Monolithic Stirrer Reactor will be discussed illustrating the high degree of precision and the convenience that can be achieved in reactor design.
It is concluded that carbon/monolith systems, especially those based on CNFs, have a high potential in liquid phase catalysis and biocatalysis, due to the excellent accessibility of the active phase, present at the outside of the fibres, without any microporosity interfering with the reaction.
1) “Structured catalysts and reactors” (Cybulski, A. and Moulijn, J.A. eds.), Chemical Industries – A series of reference books and textbooks, CRC Taylor & Francis, Boca Raton, Vol. 110, (2006).
2) F. Kapteijn, J. J. Heiszwolf, T. A. Nijhuis, and J. A. Moulijn, CATTECH 3, 24 (1999)
3) J. A. Moulijn, M. T. Kreutzer, T. A. Nijhuis and F. Kapteijn, Advances in Catalysis, in press
4) K. M. de Lathouder, J. J. W. Bakker, M. T. Kreutzer, S. Wallin, F. Kapteijn and J. A. Moulijn, Chem. Eng. Research and Design 2006, 84, 390-398.