Structured catalysts, transport

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

The majority of the early MRI studies specific to catalysis addressed the heterogeneity in structure and transport within catalyst pellets. In-plane spatial resolution achieved in these investigations was approximately 30 pm, and the pellets themselves were of typical dimension 1-5 mm. In the majority of cases, investigations addressed the pure (usually oxide) support so that the quantitative nature of the data obtained was not lost because of the presence of metal (which introduces an unknown degree of nuclear spin relaxation time contrast into the images). 

The effective diffusivity Dn decreases rapidly as carbon number increases. The readsorption rate constant kr n depends on the intrinsic chemistry of the catalytic site and on experimental conditions but not on chain size. The rest of the equation contains only structural catalyst properties pellet size (L), porosity (e), active site density (0), and pore radius (Rp). High values of the Damkohler number lead to transport-enhanced a-olefin readsorption and chain initiation. The structural parameters in the Damkohler number account for two phenomena that control the extent of an intrapellet secondary reaction the intrapellet residence time of a-olefins and the number of readsorption sites (0) that they encounter as they diffuse through a catalyst particle. For example, high site densities can compensate for low catalyst surface areas, small pellets, and large pores by increasing the probability of readsorption even at short residence times. This is the case, for example, for unsupported Ru, Co, and Fe powders. 

SEM and TEM images give detailed information about the porous structure of a supported heterogeneous catalyst (pore size distribution, typical sizes of the particles, etc.). The information from SEM and TEM images can be used in the reconstruction of porous catalytic medium. In the digitally reconstructed catalyst, transport (diffusion, permeation), adsorption, reaction, and combined reaction-diffusion processes can be simulated (Stepanek et al., 2001a). Parametric studies can be performed, and the resulting dependencies can serve as a feedback for the catalyst development. 

The described treatment of mass transport presumes a simple, relatively uniform (monomodal) pore size distribution. As previously mentioned, many catalyst particles are formed by tableting or extruding finely powdered microporous materials and have a bidisperse porous structure. Mass transport in such catalysts is usually described in terms of two coefficients, a effective macropore diffusivity and an effective micropore diffusivity. 

Considerations based on the known physical phenomena can guide the choice of catalyst porosity and porous structure, catalyst size and shape and reactor type and size. These considerations apply both to laboratory as well as to large-scale operations. Many comprehensive reviews and good books on the problem of reactor design are available in the literature. The purpose of this book is to teach the reader the mathematical tools that are available for calculating interaction between the transport phenomena and true chemical kinetics, allowing optimization of catalyst performance. The discussed theories are elucidated with examples to provide training for application of the mathematics. 

Dimensional analysis of the coupled kinetic-transport equations shows that a Thiele modulus (4> ) and a Peclet number (Peo) completely characterize diffusion and convection effects, respectively, on reactive processes of a-olefins [Eqs. (8)-(14)]. The Thiele modulus [Eq. (15)] contains a term ( // ) that depends only on the properties of the diffusing molecule and a term ( -) that includes all relevant structural catalyst parameters. The first term introduces carbon number effects on selectivity, whereas the second introduces the effects of pellet size and pore structure and of metal dispersion and site density. The Peclet number accounts for the effects of bed residence time effects on secondary reactions of a-olefins and relates it to the corresponding contribution of pore residence time. 

S.C. Reyes and E. Iglesia, Simulation techniques for the characterization of structural and transport properties of catalyst pellets, in Computer-Aided Design of Catalysts E.R. Becker, and C.J. Pereira, cds., Dekker, New York, 1993. 

S.C. Reyes and E. Iglesia, Effective diffusivities in catalyst pellets New model porous structures and transport simulation techniques, J. Catal. 129 457 (1991). 

PI has inspired the development of new equipment as well as new innovative processes. Until now, most efforts were directed toward improved transport properties in chemical reactors to achieve processes, Hmited only by the inherent chemical kinetics. Further activities concern the use of alternative energy resources and nonconventional fluids. The development of structured catalysts and reactors can be considered as a mean for a significant increase of product yield and productivity. It fadhtates the transformation of batch to continuous processes and opens new process windows for the use of high temperature and high reactant concentrations and allows increased process flexibility. 

In the following sections, we first recall the influence of transport processes on the catalyst performance and product selectivity. Different possibilities for reducing or even avoiding transport phenomena are presented. The discussion is mainly focused on structured catalysts and microstructured reactors (MSR) including the energy dissipation for the improved transport performances. The final aim is to achieve the chemical potential of the catalyst at optimum temperature and at high concentration, avoiding any dilution of the reactant. 

In the following two sections, possibilities to obtain high transport rates and to avoid the above-mentioned high energy dissipations using MSR and/or structured catalysts are discussed. 

New windows of opportunity for perfusive structured catalysts (not discussed in this chapter). Flow-through monoliths where the internal mass/heat transfer rates are enhanced due to the presence of convection were also considered. Non-isothermal operation [144] and design rules for maximum transport enhancement [17,145] are among the most relevant results. 

Geometrical, flow and local transport properties of structured catalysts... 

The interest in structured catalysts is mainly related to their peculiar geometrical characteristics, which may result in enhanced flow and transport properties with respect to conventional randomly packed beds of particles. Accordingly, before discussing examples of applications in selective oxidation processes, such properties are briefly outlined in this section for the two main families of structured catalysts, namely honeycombs and foams. 

In fact, it is rather difficult to design or expect a single type of SPP derivative to meet all the requirements. Molecular simulation seems to be an effective way as a guide for the materials designs either to explore the structure-property relationship or to predict some molecular actions. Future works rely on omnifarious efforts, including membranes, catalysts, transport modeling study, and technical assessments. 

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