Catalysts general features

VI. Some General Features of Propagation Centers in One-Component Polymerization Catalysts... 

Although bulk gold is poorly active as a catalyst, gold NPs attached to a variety of support materials exhibit unique catalytic properties in oxidation. The general features can be summarized as follows. 

Hydrogenolysis reactions of hydrocarbons on metal catalysts have been investigated in some detail. Extensive studies have been conducted on both alkanes and cycloalkanes. While a number of questions still remain with regard to mechanistic and kinetic details of the reactions, the general features seem reasonably clear. 

General Features of Late Transition Metal a-Diimine Polymerization Catalysts. 184... 

The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

Figure 5A shows that the selectivity towards butane formation (i.e. the rate of formation of butane relative to that of the butenes) decreases as the Co/Mo ratio increases in the unsupported catalysts. Similar results have previously been reported for alumina supported Co-Mo catalysts (37, 38) and this behavior does therefore appear to be a quite general feature of Co-Mo catalysts. The large change in the selectivity is observed (Figure 5B) to be related to a greater promotion of the HDS reaction rate compared... 

The following general features were found for Ir-catalyzed allylic substitutions with achiral catalysts [6, 11]. Points (b)-(e) are illustrated by Scheme 9.2. 

Oxidation of primary, secondary and benzylic alcohols with TBHP or CHP, mainly catalyzed by Mo and Zr derivatives, were performed by different authors. As an example, Ishii, Ogawa and coworkers reported the conversion of secondary alcohols such as 2-octanol to ketones mediated by catalyst 39 and TBHP. The oxidation of cyclic alcohols depended on steric factors. Zirconium alkoxides may act as catalysts in the conversion of different alcohol typologies with alkyl hydroperoxides . Secondary alcohols, if not severely hindered, are quantitatively converted to the corresponding ketones. The selectivity for equatorial alcohols is a general feature of the system, as confirmed by the oxidation of the sole cis isomer 103 of a mixture 103-bl04 (equation 68). Esters and acids could be the by-products in the oxidation of primary alcohols. 

This paper is a summary of our current understanding of this system. In particular, we will be discussing the observations in terms of selectivity with respect to the availability of reactive lattice oxygen. The organization of the paper is as follows. First, the general features of the reaction scheme for alkane oxidation on vanadate catalysts will be presented. This is followed by a discussion of results on the effect of ease of removal of oxygen from the lattice on the selectivity, and then a discussion on the importance of the atomic arrangement of the active sites. 

This type of duality of action is presumably present in other situations, such as the Fries rearrangement (78), the Friedel-Crafts reaction with acid chlorides (65) or acid anhydrides (21), and the catalytic chlorination of nitrobenzene (17). In these reactions it appears that the uncoordinated Lewis acid is the effective catalyst. The same situation is illustrated by recent work on aromatic amination (32, 33) and halogenation (57, 58, 71) and seems to be general feature of Lewis acid-catalyzed electrophilic reactions of aromatic compounds containing suitable donor groups. 

Comparison of these results with those presented for ethylene in Table 7 show that, for each catalyst, the two reactions show close similarities suggesting that the general features of the mechanisms are the same for both reactions. There are some important differences in detail between the two reactions, particularly with palladium and iridium. With palladium, olefin exchange occurs more readily with propene than with ethylene. This may be due to easier desorption of propene or possibly due to an alternative mechanism such as... 

The metal-catalysed hydrogenation of the higher olefins exhibit general features which are similar to those observed with the n-butenes. Thus, for example, the hydrogenation of hex-1-ene over Adams platinum catalyst [144] is accompanied by very low amounts of double-bond migration the relative rates of isomerisation and hydrogenation are in the ratio 0.03 1. Similarly, in the liquid phase hydrogenation of the n-pentenes over platinum—charcoal and iridium—charcoal [145], little or no isomerisation... 

All these facts and unsolved problems require that the rate equations of type (2) be taken as semi-empirical expressions. They may be directly utilised for engineering purposes with higher certainty than eqn. (1), but they reflect the actual reaction mechanism only in general features. However, the constants are a good source of values for comparison of reactivities and adsorptivities of related reactants on the same catalyst. Such interpretations of experimental data are usually quite meaningful as is confirmed by successful correlations of the constants with other independent quantities. 

Much less information is available on the deamination and related reactions over solid catalysts than on some other elimination reactions but it suffices for comprehension of the general features. 

This diversity of sites explains why the molecular weight distribution (MWD) of polymers produced by Cr/silica is broad (71). Model calculations which assume a single type of active site usually predict Mw/Mn 2,4 but in reality Mw/Afn = 6-15 is common, and 20-30 can be achieved with catalyst modifications. The distribution is also broader than that generally obtained from Ziegler catalysts, for which Mw/Afn = 3-6 under similar conditions. Experience with organometallic compounds suggests that a broad MWD may be a general feature of catalysts which terminate by -elimination. 

Before examining the more recent developments involving single-site metal catalysts for the copolymerization of C02 and epoxides, it may be worthwhile discussing some general features of these processes, and also defining some commonly used terms. First of all, the reaction sequences depicted in Scheme 8.1 are not... 

To keep the mathematics as simple as possible, we treat the catalyst pellet as an infinitely flat plate (b = 0 in eq 139). The solution of eq 139 depends on whether the reactant concentration will drop to zero at some point Xo inside the pellet, in the case that the reaction rate is strongly influenced by diffusion, or will be finite everywhere in the pellet interior, if there is only a moderate effect of diffusion. This is a general feature of zero-order reactions which arises from the assumption that the reaction will proceed at a constant rate until the reactant is completely exhausted. 

The kinetics of catalytic reactions on nonuniform surfaces have been discussed by Roginskii (330,331) certain general features of his discussion will be presented here. The rate of a complex multistage heterogeneous catalytic reaction is controlled by the rate of the slowest step. The slowest step may be the adsorption of the reactants, the chemical reactions on the surface, desorption of the products or diffusion of reactants or products through the gaseous phase near the surface of the catalyst. 

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