10.1.4 Classification of Catalysts
One common way to classify catalysts is in terms of the type of reaction they catalyze.
Table 10-1 gives a list of representative reactions and their corresponding catalysts. Further discussion of each of these reaction classes and the materials that catalyze them can be found on the DVD-ROM/Web Professional Reference Shelf R10.1.
Table 10-1. Types of Reactions and Representative Catalysts
If, for example, we were to form styrene from an equimolar mixture of ethylene and benzene, we could carry out an alkylation reaction to form ethyl benzene, which is then dehydrogenated to form styrene. We need both an alkylation catalyst and a dehydrogenation catalyst:
10.2 Steps in a Catalytic Reaction
A photograph of different types and sizes of catalyst is shown in Figure 10-4a. A schematic diagram of a tubular reactor packed with catalytic pellets is shown in Figure 10-4b. The overall process by which heterogeneous catalytic reactions proceed can be broken down into the sequence of individual steps shown in Table 10-2 and pictured in Figure 10-5 for an isomerization reaction.
Figure 10-4a. Different shapes and sizes of catalyst.
Courtesy of the Engelhard Corporation.
Figure 10-4b. Catalytic packed-bed reactor—schematic.
Figure 10-5. Steps in a heterogeneous catalytic reaction.
Table 10-2. Steps in a Catalytic Reaction
- Mass transfer (diffusion) of the reactant(s) (e.g., species A) from the bulk fluid to the external surface of the catalyst pellet
- Diffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surface
- Adsorption of reactant A onto the catalyst surface
- Reaction on the surface of the catalyst (e.g.,
) - Desorption of the products (e.g., B) from the surface
- Diffusion of the products from the interior of the pellet to the pore mouth at the external surface
- Mass transfer of the products from the external pellet surface to the bulk fluid
Each step in Table 10-2 is shown schematically in Figure 10-5.
The overall rate of reaction is limited by the rate of the slowest step in the mechanism. When the diffusion steps (1, 2, 6, and 7 in Table 10-2) are very fast compared with the reaction steps (3, 4, and 5), the concentrations in the immediate vicinity of the active sites are indistinguishable from those in the bulk fluid. In this situation, the transport or diffusion steps do not affect the overall rate of the reaction. ln other situations, if the reaction steps are very fast compared with the diffusion steps, mass transport does affect the reaction rate. In systems where diffusion from the bulk gas or liquid to the catalyst surface or to the mouths of catalyst pores affects the rate, changing the flow conditions past the catalyst should change the overall reaction rate. In porous catalysts, on the other hand, diffusion within the catalyst pores may limit the rate of reaction and, as a result, the overall rate will be unaffected by external flow conditions even though diffusion affects the overall reaction rate.
A reaction takes place on the surface, but the species involved in the reaction must get to and from the surface.
There are many variations of the situation described in Table 10-2. Sometimes, of course, two reactants are necessary for a reaction to occur, and both of these may undergo the steps listed above. Other reactions between two substances may have only one of them adsorbed.
With this introduction, we are ready to treat individually the steps involved in catalytic reactions. In this chapter, only the steps of adsorption, surface reaction, and desorption will be considered [i.e., it is assumed that the diffusion steps (1, 2, 6, and 7) are very fast so that the overall reaction rate is not affected by mass transfer in any fashion]. Further treatment of the effects involving diffusion limitations is provided in DVD-ROM/Web Chapters 11 and 12.