Heterogeneously catalyzed

The commonly accepted mechanism of heterogeneously catalyzed hydrogenation involves activation of both the hydrogen and the C—C multiple bond adsorbed on the metal surface. First one hydrogen atom is transferred to the least hindered position of the multiple bond to give a half-hydrogenated adsorbed species. This reaction is fully reversible and ac-... 

Dehydration of 1-pentanol or 2-pentanol to the corresponding olefins has been accompHshed, in high purity and yields, by vapor-phase heterogeneous catalyzed processes using a variety of catalysts including neutral gamma —Al Og catalyst doped with an alkah metal (23), zinc aluminate (24,25), hthiated clays (26), Ca2(P0 2 montmorillonite clays (28). Dehydration of 2-methyl-1-butanol occurs over zinc aluminate catalyst at... 

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. 

E. V. Albano. On the influence of reactant s induced surface transformations in the behavior of a heterogeneously catalyzed dimer-monomer reaction model. J Chem Phys 709 7498-7505, 1998. 

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. 

Mol et al. (61) analyzed the products formed in the heterogeneously catalyzed metathesis of propene labeled with 14C. With propene labeled in the 2-position there will be, in the case of a transalkylation reaction, an equal distribution of the radioactivity over the products ... 

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 ... 

Ethylene oxide is produced by a heterogeneous catalyzed gas phase oxidation of ethylene with pure oxygen at temperatures of 240-290°C and 5-25 bar [63] ... 

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. 

Research into cluster catalysis has been driven by both intrinsic interest and utilitarian potential. Catalysis involving "very mixed -metal clusters is of particular interest as many established heterogeneously catalyzed processes couple mid and late transition metals (e.g., hydrodesulfurization and petroleum reforming). Attempts to model catalytic transformations arc summarized in Section II.F.I., while the use of "very mixed -metal clusters as homogeneous and heterogeneous catalysis precursors are discussed in Sections I1.F.2. and I1.F.3., respectively. The general area of mixed-metal cluster catalysis has been summarized in excellent reviews by Braunstein and Rose while the tabulated results are intended to be comprehensive in scope, the discussion below focuses on the more recent results. 

Multi-scale modeling problems in heterogeneously catalyzed a-olefin polymerization... 

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... 

Other types of non-micro-channel, non-micro-flow micro reactors were used for catalyst development and testing [51, 52]. A computer-based micro-reactor system was described for investigating heterogeneously catalyzed gas-phase reactions [52]. The micro reactor is a Pyrex glass tube of 8 mm inner diameter and can be operated up to 500 °C and 1 bar. The reactor inner volume is 5-10 ml, the loop cycle is 0.9 ml, and the pump volume adds a further 9 ml. The reactor was used for isomerization of neopentane and n-pentane and the hydrogenolysis of isobutane, n-butane, propane, ethane, and methane at Pt with a catalyst. 

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... 

For this purpose, cylindrical channels have been assumed. In randomly packed fixed beds the porosity is about 0.4, from which the relationship dp = 2.25 d is obtained. Since the focus is on heterogeneously catalyzed gas-phase reactions, it is important to not only ensure comparable conditions from a hydrodynamic point of view, but also as far as chemical reaction kinetics is concerned. Therefore, it is assumed that both reactors contain the same amount of catalyst. 

WiESSMEiER, G., Honigke, D., Heterogeneously catalyzed gas-phase hydrogenation of cis,trans,trans-1,5,9-cyclododecatriene on palladium catalysts having regular pore systems,... 

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

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