Statistical Models of Transport and Reaction in Porous Media and Their Applications in Catalysis

Abstract

Most industrial catalysts have a complex, tortuous porous structure. In the past, transport and reaction in such catalysts were described by continuum-type diffusion-reaction models utilizing effective transport and reaction constants and ad hoc parameters such as the tortuosity factor. In recent years it has become clear that such modeling approaches are often inadequate and in many instances lead to qualitatively wrong conclusions. This has prompted the development of a new class of models based on a statistical description of the catalyst's pore structure and on microscopic modeling of the transport and reaction processes at the single pore level. Only one class of such statistical models are discussed here, namely, those describing transport and reaction in reactive porous media, which are simultaneously undergoing morphological changes due to the reaction processes themselves. The best known such models are these which describe catalyst deactivation due to active site coverage and pore blockage. Many other phenomena and processes in catalysis are also amenable to such a theoretical description. These include the impregnation of porous supports by catalytically active metals and loss of catalytic activity due to sintering, attrition and break-up of catalyst particles due to thermal and mechanical factors and irreversible sorption of organic macromolecules in catalysts with restricted porous structures (i.e., zeolites and pillared clays). A close relationship and analogy also exists between statistical models of transport and reaction in porous catalysts and the more general models describing fluid-solid reactive systems. The similarities and differences between these models are discussed in this paper.