Structured and Micro-Structured Catalysts and Reactors

Dear Colleagues,

Structured catalysts and reactors (SC&R) are being successfully employed in a number of processes, mostly related with Environmental Catalysis applications. These well-stablished technologies include the control of pollutants emissions from transportation vehicles, of NOx emissions from stationary sources or the removal of volatile organic compounds and the catalytic combustion for power generation plants. The selective catalytic reduction of NOx in power and nitric acid production plants, refineries and waste incinerators is an excellent example of the use of SC&R.

SC&R are characterized by relatively large void fractions and many more ordered paths of the fluids compared to conventional packed beds of particulate catalysts. These facts together with the laminar regime that typically governs the flow of the fluids lead up to two orders of magnitude lower pressure drops in structured reactors than in fixed-bed reactors. This opens the possibility of using SC&R in processes involving very high flow rates of the reactants (environmental applications) or requiring very short contact times, such as in the selective oxidation of hydrocarbons.

In the case of the micro-structured systems, such as microreactors and catalytic-wall microchannel reactors, the very small characteristic dimensions (typically below 1 mm) prevailing in these devices allow a significant enhancement of the mass and heat transport rates. This results in an incomparable intensification of the process with an excellent temperature control, and improved product quality and process safety.

Structured catalysts normally consist in a ceramic or metallic substrate that can adopt several configurations, such as parallel channels monoliths, open cell foams, stacked wire meshes, and microchannel reactors. The substrate provides structural entity and determines the flow pattern of the fluids inside the reactor. The catalyst, typically composed of a porous support, the active component, and eventually modifiers to tune some properties or provide new ones, is incorporated in the form of a thin layer that coats the substrate walls or even forming micro-packed-beds inside the substrate cavities.

Structured catalysts based on ceramic substrates are being traditionally employed in a number of applications. On the other hand, the use of metallic substrates is being increasingly considered for reactions with a strong thermal effect for which a very good heat transfer capacity is required in order to avoid problems associated to hot spots and temperature gradients. Nevertheless, preparing structured catalysts on metallic substrates remains still challenging due to the difficulties associated to the deposition of the catalyst on the substrate surface. The formation of homogeneous and adherent thin layers of catalyst on these substrates is very difficult and the procedures employed are in many cases halfway between science and art. These difficulties are related with the very different characteristics of the substrate and the catalyst, the modifications suffered by the catalyst during the deposition process, and the increasing complexities of the shapes of the new substrate designs and of the new technology employed, such as in the case of additive manufacturing.

In this context, the aim of this Special Issue is to collect a series of novel contributions in the field of SC&R and microreactors that allow updating the state-of-the-art. The special issue is devoted although not limited to parallel channels monoliths, open cell foams, stacked wire meshes and microchannel reactors. Topics to be covered are SC&R preparation and characterization, microreactors fabrication and applications for process intensification, as well as modeling and simulation of SC&R and microreactors.

Prof. Luis M. Gandía
Prof. Mario Montes
Prof. José Antonio Odriozola

Guest Editors

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