Heterogeneously catalyzed reactions

M. Ehsasi et al. Steady and nonsteady rates of reaction in a heterogeneously catalyzed reaction Oxidation of CO on platinum, experiments and simulations. J Chem Phys 97 4949-4956, 1989. 

In comparison to heterogeneous catalyzed reactions, homogeneous catalysis offers several important advantages. The catalyst complex is usually well defined and can be rationally optimized by ligand modification. Every metal center can be active in the reaction. The reaction conditions are usually much milder (T usually < 200 °C), and selectivities are often much higher than with heterogeneous catalysts. 

Instead ef the name metathesis, the term disproportionation is frequently applied to the reaction, and sometimes the term dismutation. For historical reasons the name disproportionation is most commonly used for the heterogeneously catalyzed reaction, while the homogeneously catalyzed reaction is usually designated as metathesis. The name disproportionation is correct in the case of the conversion of acyclic alkenes according to Eq. (1) however, this name is inadequate in most other situations, such as the reaction between two different alkenes, and reactions involving cycloalkenes. Similar objections apply to the name dismutation. The name metathesis is not subject to these limitations and, therefore, is preferred. 

A quite analogous treatment may be applied to the kinetics of heterogeneously catalyzed reactions. Consider a surface S that contains so active sites to which reactant A, a gaseous or solute species, may bind reversibly with an equilibrium constant KA ... 

As is well known chemical reactions are accelerated by increasing the temperature. This also applies to heterogeneously catalyzed reactions taking place on the surface of polar sorbents such as aluminium oxide or silica gel (Tables 2.1 and 2.2). Such reactions have also been reported on the moderately polar NH2 layers. Alperin et al. have described the activation of cellulose to yield specific information concerning the substances chromatographed [1]. 

Equation (1.20) is frequently used to correlate data from complex reactions. Complex reactions can give rise to rate expressions that have the form of Equation (1.20), but with fractional or even negative exponents. Complex reactions with observed orders of 1/2 or 3/2 can be explained theoretically based on mechanisms discussed in Chapter 2. Negative orders arise when a compound retards a reaction—say, by competing for active sites in a heterogeneously catalyzed reaction—or when the reaction is reversible. Observed reaction orders above 3 are occasionally reported. An example is the reaction of styrene with nitric acid, where an overall order of 4 has been observed. The likely explanation is that the acid serves both as a catalyst and as a reactant. The reaction is far from elementary. 

For enzymatic and other heterogeneously catalyzed reactions, there may be competition for active sites. This leads to rate expressions with forms such as... 

Example 7.6 Suppose a liquid-solid, heterogeneously catalyzed reaction is conducted in a jacketed, batch vessel. The reaction is A B. The reactants are in the liquid phase, and the catalyst is present as a slurry. The adiabatic temperature rise for complete conversion is 50 K. The reactants are charged to the vessel at 298 K. The jacket temperature is held constant at 343 K throughout the reaction. The following data were measured ... 

Example 10.1 Consider the heterogeneously catalyzed reaction A —> P. Derive a plausible form for the intrinsic kinetics. The goal is to determine a form for the reaction rate that depends only on gas-phase concentrations. 

Reversible Unimolecular Reactions. The intrinsic reaction steps in heterogeneously catalyzed reactions are usually reversible. The various limiting cases can be found by taking limits before redefining the constants, e.g., take limits on Equation (10.11), not Equation (10.12). However, a more direct route is to assume that the fast steps achieve equilibrium before deriving the counterpart to Equation (10.11). 

Example 10.3 Suppose that adsorption is much slower than surface reaction or desorption for the heterogeneously catalyzed reaction A < > P. 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

Our goal is to bridge the gap between studies of well-characterized surfaces in UHV and studies of high pressure, complex, heterogeneously catalyzed reactions. To achieve this goal, we... 

Generally, whenever fluids are processed in a confined space, two different types of phenomena are observed surface and volume effects. An example of a surface effect is a heterogeneously catalyzed reaction occurring at the walls of the vessel, whereas the motion of a fluid due to gravitational forces would be described as a volume effect. In brief, it can be stated that the surface effects gain in importance compared with the volume effects when the size of a reactor decreases. In particular, the reduction of length scale leads to a... 

WiESSMEiER G., Honicke, D., Micro-fabricated components for heterogeneously catalyzed reactions, J. Micromedi. Microeng. 6 (1996) 285-289. 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

Both heterogeneous and homogeneous catalysts have been found which allow the hydroamination reaction to occur. For heterogeneously catalyzed reactions, it is very difficult to determine which type of activation is involved. In contrast, for homogeneously catalyzed hydroaminations, it is often possible to determine which of the reactants has been activated (the unsaturated hydrocarbon or the amine) and to propose reaction mechanisms (catalytic cycles). 

In the present work, a Cu-13X zeolite sample was ceramized and used as a catalyst for the wet oxidation of phenol solutions and olive oil mill wastewaters (OOMW). The material showed good catalytic activity for the abatement of phenol and poly-phenols, excellent stability and no leaching of the active species. In this way a real heterogeneously catalyzed reaction was performed. Moreover, the catalyst was reused without special reactivation treatments for different consecutive reaction cycles. 

Heterogeneously catalyzed reactions are rather complex processes. In a two-phase system, either liquid/solid or gas/solid, several steps are needed to complete the catalytic cycle ... 

There has been a limited number of reports of heterogeneously catalyzed reactions in the liquid phase under the action of microwave irradiation. 

Chemat et al. [35] examined rates of esterification of stearic acid by butanols as a model reaction to compare differences between homogeneously and heterogeneously catalyzed reactions, Scheme 10.1. Particular attention was paid to whether the effect of microwaves can be advantageously used to improve yields and to accelerate the rate of esterification. 

Several other miscellaneous heterogeneously catalyzed reactions have been performed in the liquid phase. Hexane was successfully oxyfunctionalized with aqueous hydrogen peroxide by use of the zeolite TS-1 catalyst [50] and microwave-promoted acetalization of a number of aldehydes and ketones with ethylene glycol proceeded readily (2 min) in the presence both of heterogeneous (acidic alumina) and homogeneous (PTSA, Lewis acids) catalysts [51], Scheme 10.7. 

Spiro [27] has derived quantitative expressions for the catalytic effect of electron conducting catalysts on oxidation-reduction reactions in solution in which the catalyst assumes the Emp imposed on it by the interacting redox couples. When both partial reaction polarization curves in the region of Emp exhibit Tafel type kinetics, he determined that the catalytic rate of reaction will be proportional to the concentrations of the two reactants raised to fractional powers in many simple cases, the power is one. On the other hand, if the polarization curve of one of the reactants shows diffusion-controlled kinetics, the catalytic rate of reaction will be proportional to the concentration of that reactant alone. Electroless metal deposition systems, at least those that appear to obey the MPT model, may be considered to be a special case of the general class of heterogeneously catalyzed reactions treated by Spiro. 

This study was carried out to simulate the 3D temperature field in and around the large steam reforming catalyst particles at the wall of a reformer tube, under various conditions (Dixon et al., 2003). We wanted to use this study with spherical catalyst particles to find an approach to incorporate thermal effects into the pellets, within reasonable constraints of computational effort and realism. This was our first look at the problem of bringing together CFD and heterogeneously catalyzed reactions. To have included species transport in the particles would have required a 3D diffusion-reaction model for each particle to be included in the flow simulation. The computational burden of this approach would have been very large. For the purposes of this first study, therefore, species transport was not incorporated in the model, and diffusion and mass transfer limitations were not directly represented. 

Propose a rate law based on the Langmuir-Hinshelwood model for each of the following heterogeneously catalyzed reactions ... 

The principal classes of reported heterogeneously catalyzed reactions and the synthesis of biologically active molecules by heterogeneous diastereoselective catalysis are covered in a recent review by De Vos and coworkers [86],... 

In heterogeneous catalyzed reaction, the surface of a solid serves as a catalyst while the reactants are in gaseous or liquid phase. One or more reactants get adsorbed on the surface of the solid. This phenomenon is similar to the formation of intermediate complex in homogeneous catalysis. In many cases, heat of adsorption leads to the activation of reactants and the reaction takes place easily. The surface thus provides an alternative path for the reaction to proceed with an accelerated rate. The reaction consists of the following four consecutive steps. 

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