Catalysts, solid effectiveness

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

Although NEMCA is a catalytic effect taking place over the entire catalyst gas-exposed surface, it is important for its description to also discuss the electrocatalytic reactions taking place at the catalyst-solid electrolyte-gas three phase boundaries (tpb). This means that the catalyst-electrode must also be characterized from an electrochemical viewpoint. When using YSZ as the solid electrolyte the electrochemical reaction taking place at the tpb is ... 

Phase-transfer catalysis is a special type of catalysis. It is based on the addition of an ionic (sometimes non-ionic like PEG400) catalyst to a two-phase system consisting of a combination of aqueous and organic phases. The ionic species bind with the reactant in one phase, forcing transfer of this reactant to the second (reactive) phase in which the reactant is only sparingly soluble without the phase-transfer catalyst (PTC). Its concentration increases because of the transfer, which results in an increased reaction rate. Quaternary amines are effective PTCs. Specialists involved in process development should pay special attention to the problem of removal of phase-transfer catalysts from effluents and the recovery of the catalysts. Solid PTCs could diminish environmental problems. The problem of using solid supported PTCs seems not to have been successfully solved so far, due to relatively small activity and/or due to poor stability. 

Figure 47.2. (a) Effect of residence time 156 s, fresh catalyst (solid symbol) 80 s, catalyst used once (open symbol) and (b) effect of catalyst particle size in citral hydrogenation at 25°C, 6.1 bar total pressure, residence time 156 s, solvent ethanol, 0.1 g catalyst Ni/Si02, initial citral concentration 0.02 M. 

Effective Thermal Conductivities of Porous Catalysts. The effective thermal conductivity of a porous catalyst plays a key role in determining whether or not appreciable temperature gradients will exist within a given catalyst pellet. By the term effective thermal conductivity , we imply that it is a parameter characteristic of the porous solid structure that is based on the gross geometric area of the pellet perpendicular to the direction of heat transfer. For example, if one considers the radial heat flux in a spherical pellet one can say that... 

It is anticipated that the equilibrium filter cake mass would depend strongly on the axial velocity through the cross-flow filter assembly. The shear rate at the filter surface will increase the entrainment of the catalyst solids for a given permeate flow rate. Therefore, for each differential pressure condition, the axial velocity will be varied in order to quantify the effect of the wall shear on the filter cake resistance term. 

A favorable combination of valence forces of both components seems to be the basic principle of the nickel-molybdenum ammonia catalyst. It has been found (50) that an effective catalyst of this type requires the presence of two solid phases consisting of molybdenum and nickel on the one hand and an excess of metallic molybdenum on the other. Similar conditions prevail for molybdenum-cobalt and for molybdenum-iron catalysts their effectiveness depends on an excess of free metal, molybdenum for the molybdenum-cobalt combination and iron for the molybdenum-iron combination, beyond the amounts of the two components which combine with each other. A simple explanation for the working mechanism of such catalysts is that at the boundary lines between the two phases, an activation takes place. In the case of the nickel-molybdenum catalyst, the nickel-molybdenum phase will probably act preferentially on the hydrogen and the molybdenum phase on the nitrogen. 

Hydrogen peroxide is an inexpensive oxidant, but it requires a catalyst to effect oxidation of an alcohol to the ketone. Removal of the catalyst then becomes an issue. Ronny Neumann of the Weizmann Institute of Science reports (J. Am. Chem. Soc. 2004,126, 884) the development of a hybrid organic-tungsten polyoxometalate complex that is not soluble in organic solvents, but that nonetheless catalyzes the hydrogen peroxide oxidation of alcohols to ketones. The solid catalyst is removed by filtration after the completion of the reaction. The catalyst retained its activity after five recyles. 

Applicability of Monomolecular Rate Theory to Xylene Isomerization Selectivity Kinetics over Fresh AP Catalyst. The kinetics of liquid-phase xylene isomerization over fresh zeolite containing AP catalyst are effectively interpreted by pseudomonomolecular rate theory. The agreement between the experimental data (data points) and predicted reaction paths (solid lines) for operation at 400° and 600°F is shown in Figure 2. The catalyst used was in the form of extrudates comprised of the zeolite component and an A1203 binder. Since xylene disproportionation to toluene and trimethylbenzenes was low, selectivity data were obtained by mere normalization of the xylene compositions (2 axyienes = 1.0). 

In practice, the thickness of liquid films in trickle beds has been estimated to vary between 0.01 and 0.2 mm (0.004 and 0.008 in). The dynamic liquid holdup fraction is 0.03 to 0.25, and the static fraction is 0.01 to 0.05. The high end of the static fraction includes the liquid that partially fills the pores of the catalyst. The effective gas-liquid interface is 20 to 50 percent of the geometric surface of the particles, but it can approach 100 percent at high liquid loading. This results in an increase of reaction rate as the amount of wetted surface increases (i.e., when the gas-solid reaction rate is negligible). 

In heterogeneous catalysis a fluid (liquid or gas) is brought into contact with a solid catalyst, which effects... 

It can be readily understood that the structure of the oxide, from which the reduced catalyst is prepared, plays an important role for the properties of the catalyst. This dependence has been proved experimentally by the influence which the rate of cooling of the oxides of a given catalyst composition shows upon the catalytic properties of the reduced catalyst. This effect can be interpreted by considering that in the reduced catalyst the promoters are distributed all over the surface and that it is, of course, highly important how they are distributed. This distribution cannot be independent of the way in which the promoters are present in the oxidic state, whether in solid solution in the magnetite, as separate crystals or as amorphous glassy layers. 

Several advantages and disadvantages of a trickle-bed reactor are listed in Table 1-5. The commercial trickle-bed reactors are operated under plug-flow conditions. The catalysts are effectively wetted. These factors allow high conversion to be achieved in a single reactor. The liquid-to-solid ratio (or liquid holdup) in a trickle-bed reactor is small, thus minimizing the importance of homogeneous... 

First, let us examine the criteria applicable to diffusion effects in the gas phase, i.e., the spaces and channels over or between catalyst particles. When the catalyst solids are not porous but have all their active surfaces located in their geometric contours, diffusion in the outside gas space will be the only existing diffusion problem. However, even when the catalyst particles are subject to internal diffusion effects, the external gas space conditions need still be examined separately. The criteria will be examined assuming the reaction to be of first order, keeping in mind that deviation from exact first-order kinetics does not alter the diffusion picture by considerable magnitudes, as was seen above. 

For gas space diffusion conditions, the criteria are clearly contained in formulas (12) and (14), in that diffusion effects will be absent or negligible if the measured reaction rate dn/dt obtained from the geometric (external) surfaces of catalyst solid is such that from equation (12)... 

It is possible to formulate an expression for effective rate by analyzing relative rates of these different steps. Numerous reaction-engineering textbooks (for example, Levenspiel, 1972 Doraiswami and Sharma, 1984) discuss the formulation of effective rates and, therefore, it will not be discussed here. Such models of effective rate can be incorporated in the CFD framework by suitably modifying the source term (Eq. (5.32)). In many solid catalyzed processes, an effective rate of continuous phase reactions can be defined in terms of catalyst (solid) loading. In such cases, it is not necessary to model the surface reactions rigorously. The reaction sources appearing in the continuous phase can be directly formulated from a knowledge of volume fraction of solid phase. Some of these examples are discussed in Part III and IV. 

Fortunately, solid sucrose suspended in a liquid polyether is totally propoxylated in the presence of a tertiary amine used as catalyst. This effect is explained by the strong solvating ability of the tertiary amines. 

The Fries rearrangement provides another important route to ketones. For example, the rearrangement of phenyl acetate to hydroxyacetophenones can be used on route to p-hydroxyacetanilide, which is an important painkiller (paracetamol). The use of solid catalyst to effect this reaction has had limited success however, typically due to low selectivity and easy catalyst deactivation. 

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