Reactions Catalyzed by Solids

Chapter 17 Heterogeneous Reactions—Introduction /369 Chapter 18 Solid Catalyzed Reactions /376 Chapter 19 The Packed Bed Catalytic Reactor /427 

Chapter 20 Reactors with Suspended Solid Catalyst, Fluidized Reactors of Various Types /447 

Chapter 22 G/L Reactions on Solid Catalyst Trickle Beds, Slurry Reactors, Three Phase Fluidized Beds /500 

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Rate of Reaction Rate equations of fluid reactions catalyzed by solids are of two main types ... 

Gas-phase reactions catalyzed by solid catalysts are normally carried out in gas-particle operation in either fixed or fluidized beds. The possibility of using gas-liquid-particle operations for such reactions is, however, of interest in certain cases, particularly if the presence of a liquid medium for the transfer of heat or mass is desirable. 

The rate also decreases with an increase in the chain length of the alkene molecule (hex-l-ene > oct-1-ene > dodec-l-ene). Although the latter phenomenon is attributed mainly to diffusion constraints for longer molecules in the MFI pores, the former (enhanced reactivity of terminal alkenes) is interesting, especially because the reactivity in epoxidations by organometallic complexes in solution is usually determined by the electron density at the double bond, which increases with alkyl substitution. On this basis, hex-3-ene and hex-2-ene would be expected to be more reactive than the terminal alkene hex-l-ene. The reverse sequence shown in Table XIV is a consequence of the steric hindrance in the neighborhood of the double bond, which hinders adsorption on the electrophilic oxo-titanium species on the surface. This observation highlights the fact that in reactions catalyzed by solids, adsorption constraints are superimposed on the inherent reactivity features of the chemical reaction as well as the diffiisional constraints. 

P2.03.05. REACTION CATALYZED BY SOLIDS. LANGMUIR-HINSHELWQOD MECHANISM. 

Cumene is an important intermediate in the manufacture of phenol and acetone. The feed materials are benzene and propylene. This is a Friedel-Crafts alkylation reaction catalyzed by solid phosphoric acid at 175-225 °C and 400-600 psi. The yield is 97% based on benzene and 92% on propylene. Excess benzene stops the reaction at the monoalkylated stage and prevents the polymerization of propylene. The benzene propylene ratio is 8-10 1. 

Figure 22.1 Various ways of running G/L reactions catalyzed by solids.

Catalysis by Solid Acids. Two aspects are considered here. The first aspect is concerned with transesterification reactions catalyzed by solid acids. Unfortunately, little research dealing with this subject has been reported in the literature. The second aspect deals with esterification reactions of carboxylic acids (or FFAs). This second part addresses an important characteristic of inexpensive TG feedstocks, i.e., high FFA content. Ideally, an active solid catalyst should be able to carry out transesterification and esterification simultaneously, thus eliminating pretreatment steps. It is likely that heterogeneous catalysts that perform well in esterification should also be good candidates for transesterification since the mechanisms for both reactions are quite similar. 

Extending the definition of n-type and p-type reactions, as defined by Vol kenshtein (21) to the electron transfer step, it would seem that the only reaction given by Equation 1 is a p-type reaction. This reaction would be accelerated by the increase in the value of free hole concentration. On the other hand, all other reactions besides the one given by Equation 1 are n-type and would be accelerated by the increase in free electron concentration. Hydrocarbon oxidation reactions catalyzed by solid oxides are accompanied by oxidation and reduction of the catalyst and the degree of the stoichiometric disturbance in the semiconductor changes. The catalytic process in the oxidation of 2-methylpropene over copper oxide catalyst in the presence of Se02 can be visualized as ... 

Rates of fluid phase reactions catalyzed by solids also can be represented at least approximately by powers of the concentrations. A more fundamental approach, however, takes into account mechanisms of adsorption and of reaction on the catalyst surface. A few examples of resulting equations are in item 9 of Table 17.2. 

Reactions catalyzed by solid bases were obvious candidates for testing hypotheses on the nature and the mode of action of enzymes. Bredig [40] used aminated cellulose (B2) as a model because an enzyme was thought to consist of "a specific active function and a colloidal carrier". Indeed, cyanohydrin 40 was formed with an enantiomeric excess of 22% Fig. 3 and Table 3 contain a summary of the reported results for base-catalyzed reactions. It is not clear whether the ZnO/ffuctose catalyst (Bl) described by Erlenmeyer [39] is really heterogeneous but it is the first report on using sugars as modifiers. Some reactions are probably just curiosities (39, 41), but two... 

Fig. 3. Reactions catalyzed by solid basic catalysts. The number of the modified catalyst (see Table 3) and the best optical yield are given as (Nr/ee).

In this section, these influences will be described. Besides the acidic properties, the absorption properties of solid heteropolyacids for polar molecules are often critical in determining the catalytic function in pseudoliquid phase behavior. This is a new concept in heterogeneous catalysis by inorganic materials and is described separately in Section VI. With this behavior, reactions catalyzed by solid heteropoly compounds can be classified into three types surface type, bulk type I, and bulk type II (Sections VII and IX). Softness of the heteropolyanion is important for high catalytic activity, although the concept has not yet been sufficiently clarified. 

Zamaraev and Thomas provide a concise summary of work done with a family of classic catalytic test reactions—dehydration of butyl alcohols—to probe the workings of acidic molecular sieve catalysts. This chapter echoes some of the themes stated by Pines and Manassen, who wrote about alcohol dehydration reactions catalyzed by solid acids in the 1966 volume of Advances in Catalysis. 

In comparison with molecular catalysts, solid catalysts can be isolated from the reaction mixtures by filtration or used in continuous processes this is both environmentally friendly and useful in laboratory-scale experiments. The most important reactions catalyzed by solid superbases are isomerization reactions and the alkylation of substituted arenes in the side chain (Scheme 2). They proceed at room temperature or below with high yield (typically >99%). The surperbase-cata-lyzed alkylation of aromatic compounds complements the acid-type Friedel-Crafts alkylation and acylation, because the latter results in ring alkylation, whereas the former results in side-chain alkylation. 

The Pechmann reaction catalyzed by solid acid catalysts (e.g. zeolite H-BEA) in refluxing toluene has been reported by H. van Bekkum et al. (64). The reaction proceeds via transesterification and intramolecular hydroxyalkylation, followed by dehydration (Eq. 15.4.2). 

The legacy of Temkin is a rich one. While strong on theory, Temkin was also a gifted and exacting experimentalist. He and his co-workers proposed and built in 1950 a continuous flow reactor that operates in a gradientless manner to measure directly the rate of reactions catalyzed by solids. 

For the moment, let us focus our attention on gas-phase reactions catalyzed by solid smfaces. For a catalytic reaction to occur, at least one and frequently all of the reactants must become attached to the smface. Tliis attachment,is known as adsorption and takes place by two different processes physical adsorption and chemisorption. Physical adsorption is similar to condensation. The process is exothermic, and the heat of adsorption is relatively small, being on the order of 1 to 15 kcal/g mol. The forces of attraction between the gas molecules and the solid smface are weak. These van der Waals forces consist of interaction between permanent dipoles, between a permanent... 

Obviously, many steps are involved, and any step can be the rate-determining one. In addition, if the reaction is highly endothermic or exothermic (typical of oxidation, hydrogenation reactions), then heat has to be supplied or removed from the reactor. Sometimes the rate of heat transfer may control the overall rate of the reaction. In gas-liquid reactions catalyzed by solid particles, the suspension of catalyst particles can sometimes control the overall rate of reaction. As a first step in the process design portfolio, the rate-controlling step has to be determined, as described below. 

Yamato, T., Hideshima, C., Prakash, G. K. S., and Olah, G. A. 1991. Organic reactions catalyzed by solid superacids. 5. Perfluorinated sulfonic acid resin (Nafion-H) catalyzed intramolecular Friedel-Crafts acylation. /. Org. Chem. 56 3955-3957. 

For an excellent review on oxidation reactions catalyzed by solid ba.ses. see ref [281]. 

The principles of homogeneous reaction kinetics and the equations derived there remain valid for the kinetics of heterogeneous catalytic reactions, provided that the concentrations and temperatures substituted in the equations are really those prevailing at the point of reaction. The formation of a surface complex is an essential feature of reactions catalyzed by solids and the kinetic equation must account for this. In addition, transport processes may influence the overall rate heat and mass transfer between the fluid and the solid or inside the porous solid, > that the conditions over the local reation site do not correspond to those in the bulk fluid around the catalyst particle. Figure 2.1-1 shows the seven steps involved when a molecule moves into the catalyst, reacts, and the product moves back to the bulk fluid stream. To simplify the notation the index s, referring to concentrations inside the solid, will be dropped in this chapter. 

Kinetic equations for reactions catalyzed by solids based on the chemisorption mechanism may alwa)rs be written as a combination of three groups ... 

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