Catalysts reactant size

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

The catalyst used throughout this study was a 1% w/w palladium on alumina supplied by Johnson Matthey. The support consisted of 0-alumina trilobes (S.A. -100 m g ) and the catalyst was sized to <250p for all catalytic studies. The reactants and modifiers (all Aldrich >99 %) were used without further purification. No significant impurities were detected by GC. The gases (BOC, >99.99 %) were used as received. 

To use membrane filtration for residence time decoupling the molecular weight of the chemical catalyst has to be increased, for example, by binding the catalyst to a homogeneously soluble polymer [4]. This allows for separation of reactants and catalysts by size. Due to their similarity to biological catalysts, the term chemzyme (chemical enzyme) [5, 6] has been coined for these polymer-enlarged but still homogeneously soluble chemical catalysts (Fig. 3.1.2) [7]. 

The fact that ATR-IR spectroscopy uses an evanescent field and therefore probes only the volume very close to the IRE has important consequences for its application in heterogeneous catalysis, in investigations of films of powder catalysts. The catalyst particle size and packing affect the size of the detectable signals from the catalyst and bulk phase. Furthermore, if the catalyst layer is much thicker than the penetration depth of the evanescent field, diffusion of reactants and products may influence the observed signals. In fast reactions, gradients may exist within the catalyst layer, and ATR probes only the slice closest to the IRE. 

This sieve effect cannot be considered statically as a factor that only determines the amount of accessible acid groups in the resin in such a way that the boundary between the accessible and non-accessible groups would be sharp. It must be treated dynamically, i.e. the rates of the diffusion of reactants into the polymer mass must be taken into account. With the use of the Thiele s concept about the diffusion into catalyst pores, the effectiveness factors, Thiele moduli and effective diffusion coefficients can be determined from the effect of the catalyst particle size. The apparent rates of the methyl and ethyl acetate hydrolysis [490] were corrected for the effect of diffusion in the resin by the use of the effectiveness factors, the difference in ester concentration between swollen resin phase and bulk solution being taken into account. The intrinsic rate coefficients, kintly... 

The industrial rates obtained earlier from the pseudohomogeneous model actually include diffusional limits and are suitable for the specific reactor with the specific catalyst particle size for which the data was extracted. Such pseudohomogeneous models do not account explicitly for the catalyst packing of the reactor. On the other hand, heterogeneous models account for the catalyst explicitly by considering the diffusion of reactants and of products through the pores of the catalyst pellet. 

Hydrodesulfurization catalysts are normally used as extrudates or as porous pellets, but the particle size and pore geometry have an important influence on process design-especially for the heavier feedstocks. The reaction rates of hydro-desulfurization catalysts are limited by the diffusion of the reactants into, and the products out of, the catalyst pore systems. Thus, as the catalyst particle size is decreased, the rate of desulfurization is increased (Figure 5-9) (Frost and (Nottingham, 1971) but the pressure differential across the catalyst bed also diminishes and a balance must be reached between reaction rate and pressure drop across the bed. 

Liquid-solid mass transport (liquid reactant) Amount of catalyst Catalyst particle size Concentration of reactant in liquid phase Temperature Agitation rate Reactor design Viscosity Relative densities Concentration of gas-phase reactant Concentration of active components on catalyst... 

Chemical reaction (with insignificant pore diffusion resistance) Temperature Amount of catalyst Reactant concentrations Concentration of active components on catalyst Stirring rate Reactor design Catalyst particle size... 

A prominent trade-off in fixed-bed reactor design concerns the catalyst particle size. What is the basis for the choice of a certain particle size When the catalyst performance is to be optimized, the application of the Thiele model helps to provide an answer (Figure 7). The Thiele modulus accounts for the competition between the chemical reaction and the limitation of transport of reactants by diffusion in a porous catalyst particle. It is defined as the square root of the ratio of the characteristic diffusion time fo = L /D and the characteristic reaction time (r. For a... 

Of all the reaction variables involved in a heterogeneously catalyzed reaction, the most important is the nature of the catalyst to be used. Factors associated with catalyst preparation and selection will be discussed in Sections II and III. The relative importance of the other reaction parameters will depend on a number of factors. Reactions that run in a continuous or flow system have different requirements from those run in a batch mode. Generally, parameters such as the quantity of catalyst, the size of the catalyst particles, the temperature of the system, the concentration of the substrate(s), and, when gaseous reactants are used, the reaction pressure, are important variables in heterogeneously catalyzed reactions. In flow reactions the catalyst substrate contact time can frequently have a significant impact on the outcome of the reaction. In liquid phase batch processes catalyst agitation can also play an important role. The one constant parameter in almost all liquid phase reactions is the presence of a solvent, the nature of which is an important factor in heterogeneously catalyzed liquid phase reactions. 

The catalytic application of montmorillonites may either be related to their acidity [5-7], or their swelling properties [8-9], Swelling, induced by careful selection of both the reaction medium and interlayer cations, enables the reactant molecules to enter the interlamellar region and undergo catalytic reaction on interstitial active sites. Moreover, the layered structure of montmorillonite may induce a substrate selectivity depending on the reactant size. Therefore, clay intercalated metal catalysts offer potential applications in the preparation of fine chemicals. 

The averaging technique characteristic of the second approach may apply to the case of a tubular reactor where the ratio of the characteristic catalyst particle size to the diameter of a single tube is close to unity, but it is invalid, as will be shown, in the general case of fixed-bed reactors. This approach keeps out of a researcher s field of vision the problem of the reactor stability to local perturbations. At the same time, the technologist is often faced with hot spots in the catalyst bed of a fixed-bed reactor, which make its operation imperfect and even lead to an emergency situation in a number of cases, Until recently, nonuniformity of the fields of external parameters (e.g., nonuniform packing of the catalyst bed or nonuniformity of reactant stream velocity ) was considered the only cause of these phenomena. The question naturally arises whether the provision for uniformity of external conditions guarantees the uniformity of temperature and concentration profiles at the reactor cross-section. The present paper seeks to answer this question, which, as a matter of fact, has not yet been posed in such a form in the theory of chemical reactors. 

The intrinsic activity depends on the chemical and physical properties of the active component. For unsupported catalysts, the most important properties are the composition and structure of the catalyst surface and the presence, or absence, of special sites such as Br0nsted or Lewis acid centers, anion or cation defects, and sites of high coordination. For supported catalysts, the size and morphology of the dispersed phase are of additional importance. If intraparticle transport of reactants occurs with a characteristic time that is short compared to that of the reaction, then the observed and intrinsic rates of reaction will be identical. When the characteristic time for intraparticle mass transport is less than that for reaction, the observed rate of reaction per unit mass of catalyst becomes less than the intrinsic value, and the reaction kinetics are dominated by the effects of intraparticle mass transport. The factors governing intraparticle transport are the diffusivities of the reactants and products and the characteristic distance for diffusion. 

Heterogeneously catalyzed reactions. Macroscopic fluid models are combined with microscopic transport models in the catalyst particles to describe how concentration changes with time and position in a catalytic reactor. Special considerations must be given to the selection of experimental temperature and catalyst particle size to minimize (and hopefully eliminate) internal transport limitations on the catalytic reaction rate. The next requirement is that the flow pattern in the reactor Is accurately represented by the well-mixed or plug-flow assumption. The subsequent discussion applies to gas-phase reactants. 

As described above, the SCF had an effect not only on the diffusion of the reactant, but also on the desorption of adsorbed species. Figure 4.8-5 shows the influence of catalyst particle size on the alkene content in the supercritical phase reaction. It was found that a change of catalyst particle size had little effect on the alkene content in the product. This suggests that the supercritical fluid had a more obvious effect on the desorption of the produced alkenes. The diffusion rate may have been higher than the adsorption rate of the alkenes, at least under these experimental conditions. 

The assumption of neglecting the terms related to the intra- and extra-particle diffusion has been checked by carrying out kinetic runs with different catalyst particle size and different reactant flow rate as well as by theoretical estimates. 

In slurry systems, similar to fluidized beds, the overall rate of chemical transformation is governed by a series of reaction and mass-transfer steps that proceed simultaneously. Thus, we have mass transfer from the bubble phase to the gas-liquid interface, transport of the reactant into the bulk liquid and then to the catalyst, possible diffusion within the catalyst pore structure, adsorption and finally reaction. Then all of this goes the other way for product. Similar steps are to be considered for heat transfer, but because of small particle sizes and the heat capacity of the liquid phase, significant temperature gradients are not often encountered in slurry reactors. The most important factors in analysis and design are fluid holdups, interfacial area, bubble and catalyst particle sizes and size distribution, and the state of mixing of the liquid phase. ... 

Equation f exhibits a maximum value with respect to pore size, and so there is an optimum catalyst pore size that should be used, as shown in Figure 2. The reason is that very small pores hinder, or even block the diffusion of reactant into the catalyst pellet, but they do contain a large surface area. Very large pores, on the other hand, do not cause any hindered diffusion, but they do not have much surface area. Figure 3 shows the agreement of the model with data from a particular system. 

If the two competing reactions are of the same kinetic order (e.g., first order), then the catalyst pore structure will not affect Type II selectivity. This is because at each point in the pore structure the two reactions will proceed at the same relative rate, ki/kt, independent of the partial pressure of A. Hence the yield of the desired product B will be ki/(ki + fcs), independent of the catalyst pore size. (We define yield here as the number of moles of B formed per mole of A reacted.) If, hbwever, the two reactions are of different kinetic order, then selectivity may depend on pore size, since the decrease in partial pressure of A within the pore structure will affect the two competing reaction rates differently. As shown in Fig. 5, the concentration of reactant A will decrease markedly within the catalyst pore structure for a rapid reaction on a catalyst with small pores. If the reaction A—>B were first order... 

Figure 5.4 A schematic illustration of the shape selectivity concept, classically referring to a significant modification in the distribution of products in a catalytic reaction due to the localisation of the active sites within a confined space of the catalyst. Reactant shape selectivity (a) occurs when some but not all the components in a reaction mixture can reach the inner active sites and react. When the size and shape of the inner void modify the distribution of products by limiting the number of possible transition states, the effect is called transition state shape selectivity (b). And, finally, when a molecule can be formed but cannot be desorbed due to its size and shape, the product distribution is altered by product shape selectivity (c). In addition to these broad concepts, thermodynamic effects are also of importance. Reproduced by permission from Macmillan Publishers Ltd Nature, B. Smit and T.L.M. Maesen, 451, 671 (20). Copyright (2008) Macmillan Publishers Ltd.

When malononitrile was used as a substrate, a high conversion of 98% was obtained after 12 h, whereas almost no activity was observed in the conversion of ethyl cyanoacetate and cyano-acetic acid tert-butyl ester, suggesting that the catalyst was size selective. The observation of reactant shape selectivity revealed that the catalytic turnovers take place in the MOF channels rather than at the outer surface of the MOF particles. Upon desolvation, for example, by the removal of water... 

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