On Structured Catalysts and Reactors

Honeycomb monolith reactors have long dominated catalytic automotive exhaust gas after-treatment and many other processes for environmental protection. In recent years, however, alternative spatially structured catalyst carriers have emerged, including open-cell foams and the new generation of additively manufactured Periodic Open Cellular Structures (POCS). Compared with conventional catalyst pellets, all these structured systems offer additional degrees of freedom that can be exploited to mitigate—and in some cases remove—limitations associated with mass, momentum, and heat transport in catalytic reactors. As a result, they are particularly well suited for the intensification of fast, diffusion-limited processes, such as those encountered in environmental catalysis for pollutant abatement.

More recently, it has been recognized that structured catalysts also hold significant potential for intensifying heat-transfer-limited processes, including those involved in the production of sustainable synthetic fuels and energy vectors, such as hydrogen. In the past few years, structured reactors have furthermore attracted growing attention in the context of process electrification, a key enabling strategy for the decarbonization of the industrial sector.

This lecture will provide a perspective on the historical evolution of structured catalysts and reactors, with particular emphasis on the contributions made over the past thirty years by researchers at Politecnico di Milano. Experimental and computational results will be discussed, focusing on:

- interphase mass and heat transfer in structured catalysts, with examples ranging from industrial monoliths for NH₃-SCR to foam catalysts for methane partial oxidation;

- the trade-off between external mass transfer and pressure drop in foam and POCS catalysts;

- the development of thermally conductive structured catalysts, including pilot-scale demonstrations of aluminum monoliths for o-xylene partial oxidation, copper foams for methane steam reforming, and aluminum POCS for the Fischer–Tropsch synthesis;

- the direct Joule heating of SiC foam and POCS catalysts for the electrification of key endothermic reactions for the energy sector, like steam methane reforming, ammonia cracking, and Reverse Water Gas Shift.