Heterogeneous catalytic-type systems

Recently, however, the implementation of several forms of modern, high resolution spectroscopy has made it possible to characterize, in relatively detailed fashion, the evolving chemistry of heterogeneous catalytic-type systems. In some cases, in fact, it has been possible to map the behavior of actual commercial catalysts, even following use. For example, much of our present understanding of the functionality of hydrodesulfurization catalysis has been a direct result of investigations of commercial systems with modern surface spectroscopies, as is also the case in the development of the basis reference for metals-support interactions (1). 

In this chapter we will discuss the results of the studies of the kinetics of some systems of consecutive, parallel or parallel-consecutive heterogeneous catalytic reactions performed in our laboratory. As the catalytic transformations of such types (and, in general, all the stoichiometrically not simple reactions) are frequently encountered in chemical practice, they were the subject of investigation from a variety of aspects. Many studies have not been aimed, however, at investigating the kinetics of these transformations at all, while a number of others present only the more or less accurately measured concentration-time or concentration-concentration curves, without any detailed analysis or quantitative kinetic interpretation. The major effort in the quantitative description of the kinetics of coupled catalytic reactions is associated with the pioneer work of Jungers and his school, based on their extensive experimental material 17-20, 87, 48, 59-61). At present, there are so many studies in the field of stoichiometrically not simple reactions that it is not possible, or even reasonable, to present their full account in this article. We will therefore mention only a limited number in order for the reader to obtain at least some brief information on the relevant literature. Some of these studies were already discussed in Section II from the point of view of the approach to kinetic analysis. Here we would like to present instead the types of reaction systems the kinetics of which were studied experimentally. 

In seeking interesting applications of FT-IR/PAS one usually looks for samples of maximum suface area and high opacity. Not surprisingly many heterogenous catalytic systems qualify. In the first stage of such an investigation one prefers to examine a sample system that has been previously characterized successfully by conventional transmission-absorbance type spectral measurements. 

Different approaches have been used in the preparation of heterogeneous Sharpless-type catalytic systems for the asymmetric epoxidation of allylic alcohols, although in most cases the chiral induction was modest (50-60%). Li and coworkers described the preparation of an organic-inorganic hybrid chiral catalyst grafted onto the surface of silica and in mesopores of MCM-41, and its successful application in asymmetric epoxidation . Enantiomeric excesses were higher than 80% with conversions in the range 22-76%. 

A solid-phase sulfur oxidation catalyst has been described in which the chiral ligand is structurally related to Schiff-base type compounds (see also below). A 72% ee was found using Ti(OPr-i)4, aqueous H2O2 and solid-supported hgand 91 . More recently, a heterogeneous catalytic system based on WO3, 30% H2O2 and cinchona alkaloids has been reported for the asymmetric oxidation of sulfides to sulfoxides and kinetic resolution of racemic sulfoxides. In this latter case 90% ee was obtained in the presence of 92 as chiral mediator. ... 

Self-sustained reaction rate oscillations have been shown to occur in many heterogeneous catalytic systems Cl—8]. By now, several comprehensive review papers have been published which deal with different aspects of the problem [3, 9, 10]. An impressive volume of theoretical work has also been accumulated [3, 9, ll], which tries to discover, understand, and model the underlying principles and causative factors behind the phenomenon of oscillations. Most of the people working in this area seem to believe that intrinsic surface processes and rates rather than the interaction between physical and chemical processes are responsible for this unexpected and interesting behavior. However, the majority of the available experimental literature (with a few exceptions [7, 13]) does not contain any surface data and information which could help us to critically test and further Improve the hypotheses and ideas set forth in the literature to explain this type of behavior. 

Among the many mathematical models of fluidized bed reactors found in the literature the model of Werther (J ) has the advantage that the scale-dependent influence of the bed hydrodynamics on the reaction behaviour is taken into account. This model has been tested with industrial type gas distributors by means of RTD-measurements (3)and conversion measurements (4), respectively. In the latter investigation (4) a simple heterogeneous catalytic reaction i.e. the catalytic decomposition of ozone has been used. In the present paper the same modelling approach is applied to complex reaction systems. The reaction system chosen as an example of a complex fluid bed reaction is the synthesis of maleic anhydride (Figure 1). 

Wesslau was the first to demonstrate that, along with certain heterogeneous catalytic systems for polyethylene, there is a certain MWD dependence on the type and number of ligands altogether distributed between catalyst and cocatalyst. With TiCl -aluminium alkyls it was concluded > that, by varying the type of aluminum alkyl, polymers are obtained with a very similar MWD curve, but with the maximum shifted according to the nature of the alkyl group. Instead, different results were found by Russian researchers 59-ieo)... 

A heterogeneous catalytic reaction occurs at or very near the fluid-solid interface. The principles that govern heterogeneous catalytic reactions can be applied to both catalytic and noncatalytic fluid-solid reactions. These two other types of heterogeneous reactions involve gas-liquid and gas-Hquid-solid systems. Reactions between gases and liquids are usually mass-transfer limited. 

H-Mordenite catalyzes the smooth conversion of simple aldehydes and alcohols to form acetals at 30° in the liquid phase. From the examples in Table XXVII, it is apparent that in these heterogeneous catalytic systems, acetal formation is dependent on the structures of both the aldehyde and the alcohol involved. Thus, for a given aldehyde, yields of acetal decreased in the order primary > secondary > tertiary that is, branching at the a-carbon of the alcohol reduced the equilibrium conversion to acetal. In the isobutyraldehyde reactions, an extremely sharp drop in conversion was observed upon changing from isopropanol to fert-butanol as reactant. This observation suggests that, in addition to the increased steric interactions between organic reactants encountered in the tert alcohol system, molecular sieving-type interactions within the narrow mordenite pore system are operative. 

Traditionally, the Michael reaction is catalyzed by mild to moderately strong bases such as potassium t-butoxide, diisopropylamine [38], and tetramethylguani-dine [39] as homogeneous catalysts. The main disadvantages of this type of catalyst are the production of significant amounts of multiple Michael adducts, which are difficult to separate, and problems associated with catalyst recovery. These problems, plus recent interest in environmentally friendly solid catalysts, has led to the development of heterogeneous catalytic systems for this reaction. Examples of heterogeneous catalysts are those based on CsF and KF on alumina... 

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