Catalysts, general multicomponent

Even though the two-stage process recommends itself for the flexibility it offers for the optimization of each oxidation step, the single-pass option is attractive as it requires lower investment for fewer reactors and other equipment. The catalysts claimed in the patents are generally multicomponent metal oxides based on Mo with Te as the promoter. However the yields are lower (50-60%) and deactivation faster owing to the volatility of Te oxide. 

Recent examples of various transition-metal catalysts used for acetylene polymerization are shown in Table 15.1. The central metals of effective catalysts are usually restricted to Mo, W, Nb, Ta, and Ru. All these catalysts can be classified into single-, two-, and three-component catalysts. In general, multicomponent catalysts show high activity but the system is complicated, whereas single-component catalysts, sometimes called well-defined catalysts, are often simpler. 

The oxidation catalysts are generally multicomponent materials [7,17,20,26,32,33,35,38]. The main component is considered to be the catalytic... 

We see, then, that pressure gradients must necessarily exist in catalyst pellets to free the fluxes from the constraints Imposed by Graham s relation (11,42), or Its generalization = 0 in multicomponent systems. Without this freedom the fluxes are unable to adjust to the demands... 

A general problem existing with all multicomponent catalysts is the fact that their catalytic activity depends not on the component ratio in the bulk of the electrode but on that in the surface layer, which owing to the preferential dissolution of certain components, may vary in time or as a result of certain electrode pretreatments. The same holds for the phase composition of the surface layer, which may well be different from that in the bulk alloy. It is for this reason that numerous attempts at correlating the catalytic activities of alloys and other binary systems with their bulk properties proved futile. 

The Langmuir Equation for the Case Where Two or More Species May Adsorb. Adsorption isotherms for cases where more than one species may adsorb are of considerable significance when one is dealing with heterogeneous catalytic reactions. Reactants, products, and inert species may all adsorb on the catalyst surface. Consequently, it is useful to develop generalized Langmuir adsorption isotherms for multicomponent adsorption. If 0t represents the fraction of the sites occupied by species i, the fraction of the sites that is vacant is just 1 — 0 where the summation is taken over all species that can be adsorbed. The pseudo rate constants for adsorption and desorption may be expected to differ for each species, so they will be denoted by kt and k h respectively. 

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

Generally, variations of the chemical nature of the catalyst have not, so far, yielded as much information as have the systematic variations of the compositions of multicomponent catalysts. Most mixed catalysts in industry are heterogeneous mixtures. It would go beyond the scope of the present article to review them. But it is noticeable that, in general, two kinds of promoter action can be distinguished (1) Structural promotion in which the activation energy of the active constituent... 

Choosing divalent and trivalent cations and determining the composition is the most important in designing the multicomponent bismuth molybdate catalyst system. Catalytic activities of typical tri- and tetracomponent bismuth molybdate catalysts having multiphase structure were reported for the oxidation of propylene to form acrolein (35, 36, 40-43, 97, 98). A typical example of the activity test is shown in Fig. 6. Summarizing the results shown in Fig. 6 and reported previously (30, 43, 44), the following trends are generally found. 

During their technical application, molecular sieve catalysts are generally used under the conditions of multicomponent adsorption and diffusion. Selective measurement of the diffusivity of individual components is therefore of both theoretical and practical relevance. The traditional way to perform... 

Oxtoby DW, Kashchiev D (1994) A general relation between the nucleation work and the size of the nucleus in multicomponent nucleation. J ChemPhys 100 7665-7671 Park PW, Ledford JS (1997) Characterization and CH4 oxidation activity of Cr/Al203 catalysts. Langmirir 13 2726-2730... 

Adsorption characteristics of co-precipitated samples with the composition Fe(0H)3 -Cd(OH)2 and Fe(OH)3 - Cr(OH)3 confirm this statement and generality of the regularity found follows from the structural formation mechanism considered. This regularity inherent in all two- and multicomponent systems with different pH of initial and complete precipitation of components opens a broad perspective for scientifically justified and purposeful selection of conditions for the synthesis of adsorbents with the given structure and catalysts with the component precipitated first being a carrier and the other, an active phase. 

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