REACTIONS WITH SOLID CATALYSTS

Most industrial reactors and high pressure laboratory equipment are built using metal alloys. Some of these same metals have been shown to be effective catalysts for a variety of organic reactions. In an effort to establish the influence of metal surfaces on the transesterification reactions of TGs, Suppes et collected data on the catalytic activity of two metals (nickel, palladium) and two alloys (cast iron and stainless steel) for the transesterification of soybean oil with methanol. These authors found that the nature of the reactor s surface does play a role in reaction performance. Even though all metallic materials were tested without pretreatment, they showed substantial activity at conditions normally used to study transesterification reactions with solid catalysts. Nickel and palladium were particularly reactive, with nickel showing the highest activity. The authors concluded that academic studies on transesterification reactions must be conducted with reactor vessels where there is no metallic surface exposed. Otherwise, results about catalyst reactivity could be misleading. 

Summary In concluding the treatment of physical properties of catalysts, let us review the purpose for studying properties and structure of porous solids. Heterogeneous reactions with solid catalysts occur on parts of the surface active for chemisorption. The number of these active sites and the rate of reaction is, in general, proportional to the extent of the surface. Hence it is necessary to know the surface area. This is evaluated by low-temperature-adsorption experiments in the pressure range where a mono-molecular layer of gas (usually nitrogen) is physically adsorbed on the catalyst surface. The effectiveness of the interior surface of a particle (and essentially all of the surface is in the interior) depends on the volume and size of the void spaces. The pore volume (and porosity) can be obtained by simple pycnometer-type measurements (see Examples 8-4 and 8-5). The average size (pore radius) can be estimated by Eq. (8-26) from the... 

The fact that Weiss and Downs have been aide to isolate phenol in the products of their reactions with solid catalysts indicates a hydroxylation mechanism similar to that postulated in the case of vapor phase catalysis, in whidi the formation of the monohydroxylated derivative is the first step. The presence of the hydroxyl group as a substituent in the benzene molecule activates the para and ortho positions so that the introduction of a second oxygen molecule would be expected to result in the formation of quinol (C6H4(OH)2l 4) and catechol (C0H4(OH)21 2) with a preponderance of the former. Quinone which would result from the further oxidation of quinol has been found in the oxidation products from benzene for the case of the homogeneous catalytic reaction. 

Fewer details have been published on the phenomena that occur during alkylation reactions with solid catalysts than with liquid catalysts. With solid catalysts, which are porous, most of the reactants diffuse into the catalyst pores, then various reactions occur, and finally most of the reaction products diffuse out of the pores. For the alkylation of isobutane, the catalyst quickly deactivates regardless of the catalyst chosen or the specific operating conditions employed. Such deactivation is apparently due to rather high molecular weight by-products that fail to diffuse from the pores. The diffusivity values for different molecules in the pores obviously depend on their molecular weight and shapes. For the alkylation of aromatics, solid catalysts have been found that undergo deactivation much slower. On the basis of commercial results, catalysts have been found that do not need to be reactivated or replaced for several years. 

Reactions with solid catalysts are often carried out under conditions of high reactivity. Therefore, as a rule, both external and internal transport rates are rate determining to a certain extent, depending on the value of the catalyst number Ca. This determines in principle the optimum particle size of the catalyst, with respect to the amount of catalyst required. Obviously, for practical reasons one may prefer a different particle size. Particularly when there are severe heat transfer requirements, a fine suspended catalyst may be desirable. 

Several approaches have been reported for the reactions with solid catalysts [11]. Catalytically active metals may be used to cover the inner walls of a microchannel [12-14] or catalysts can be loaded on polymer beads in the microreactor channel [15]. These methods of catalyst deployment in the microchannel benefit from a high surface area-to-volume ratio. Otherwise, regeneration of the catalyst causes great... 

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