Homogenous catalytic mechanism

FIGURE 2.20. EC homogeneous catalytic mechanism with the follow-up reaction as ratedetermining step. Variation of the current ratio ip/yfp with the kinetic parameter X, for a series of values of the excess factor, y. From left to right, log y — 0, 0.5, 1, 1.5, 2. 

When pc —> oo, the catalytic loop is complete. The reaction sequence and the current-potential responses are the same as in the two-electron ECE homogeneous catalytic mechanism analyzed in the preceding subsection. When pc —> 0, deactivation prevails, and if the first electron transfer and the deactivation steps are fast, the same irreversible current-potential responses are obtained as in a standard EC mechanism. 

In certain cases, the assumption of a pseudo-homogeneous catalytic mechanism might be valid [19], and the reaction rate equation would depend only on the concentrations in the liquid phase. 

By combining the homogenous catalytic mechanism with interfacial reaction and assuming the addition of olefin to the active catalyst species as the rate-controlling step, a semi-empirical kinetic model could also be proposed, which was also in good agreement with the experimental data. 

Homogeneous catalysts. With a homogeneous catalyst, the reaction proceeds entirely in the vapor or liquid phase. The catalyst may modify the reaction mechanism by participation in the reaction but is regenerated in a subsequent step. The catalyst is then free to promote further reaction. An example of such a homogeneous catalytic reaction is the production of acetic anhydride. In the first stage of the process, acetic acid is pyrolyzed to ketene in the gas phase at TOO C ... 

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

So turning to our present volume, we refer first to the last article in the volume by D. G. H. Ballard. He concludes that our knowledge of catalytic mechanisms is limited because the majority of useful catalysts for practical reasons are heterogeneous and therefore unsuitable for mechanistic studies. Ballard s article well illustrates the fact that where all the techniques are available to establish structure (as they are in homogeneous organometallics), kinetic studies take on a new depth and progress is rapid. 

CH ligands, (2) to initiate homogeneous catalytic reactions such as hydrogenation, hydroformylation, and the water gas shift reaction, and (3) to study the mechanism of thermal reactions by the photochemical preparation of possible intermediates. 

In the various homogeneous catalytic schemes, the solvent may be coordinated to the metal or may simply be present as bulk solvent. When a ligand leaves the coordination sphere of a metal, it may be replaced by a molecule of solvent in a process that is either associative or dissociative. There is no general way to predict which type of mechanism is operative, so in some cases the substitution reactions will be described as they relate to specific processes. Because substitution reactions have been described in Chapter 20, several other types of reactions that constitute the steps in catalytic processes will be described in greater detail. 

Experimental and Theoretical Studies of Mechanisms in the Homogeneous Catalytic Activation of Carbon Monoxide... 

With the recent development of zeolite catalysts that can efficiently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol. Two possible mechanisms for methanol formation are suggested by literature discussions of Fischer-Tropsch catalysis (9-10). These are shown in Schemes 1 and 2. 

The mechanisms of the homogeneous catalytic hydrodehalogenation have been examined by the following methods ... 

RhCl(PPh3)3 has been used for the homogeneous hydrogenation of various diene-based polymers, and its catalytic mechanism is understood to a considerable extent. Parent et al. [81] proposed a mechanism which has been found to be consistent with the kinetic data for various diene-based polymer hydrogenation systems and an understanding of the coordination chemistry of RhCl(PPh3)3 in solution. The main points comprising the mechanism are outlined as follows ... 

Although Ziegler-type catalysts have been widely investigated for the homogeneous hydrogenation of polymers, their catalytic mechanism remains unknown. One possible reason for this may be the complexity of the coordination catalysis and the instability of the catalysts. Metallocene catalysts are highly sensitive to impurities, and consequently it is very difficult to obtain reproducible experimental data providing reliable kinetic and mechanistic information. 

FIGURE 2.21. Homogeneous catalytic EC mechanism. Passage from control by forward electron transfer to control by follow-up reaction upon increasing the mediator concentration. y = 5, k-e/kc = 1000M TZTke/Fv = 100AT1. 

Homogeneous Catalytic EC Mechanism The system is governed by the following dimensionless equations (we need not consider equations involving p, since as in all preceding cases, p = 1 — q), where two additional normalized rate parameters are introduced ... 

The metallacyclobutane mechanism of olefin metathesis has been discussed in Sections 1.3 and 3.1.7. For metathesis of acetylenes carbyne complexes are generally required (Figure 3.44), and both heterogeneous and homogeneous catalytic systems have been developed for this purpose. 

Haynes, A. (2005) The use of high pressure spectroscopy to study catalytic mechanism, in Mechanisms in Homogeneous Catalysis, A Spectroscopic Approach (ed. B. Heaton), Wiley-VCH Verlag GmbH, Weinheim, pp. 107-50. 

High pressure infrared (HP IR) spectroscopy has now been used for over 30 years for the study of homogeneous transition metal catalysed processes. The technique is particularly useful for reactions involving carbon monoxide, for which transition metal carbonyl complexes are key intermediates in the catalytic mechanisms. Such complexes have one or more strong r(CO) absorptions, the frequencies and relative intensities of which provide information about the geometry and electronic character of the metal center. As well as probing the metal species, HP IR spectroscopy can also be used to monitor the depletion and formation of organic reactants and products if they have appropriate IR absorptions. 

In principle, the mechanism of homogeneous hydrogenation, in the chiral as well as in the achiral case, can follow two pathways (Figure 9.5). These involve either dihydrogen addition, followed by olefin association ( hydride route , as described in detail for Wilkinson s catalyst, vide supra) or initial association of the olefin to the rhodium center, which is then followed by dihydrogen addition ( unsaturate route ). As a rule of thumb, the hydride route is typical for neutral, Wilkinson-type catalysts whereas the catalytic mechanism for cationic complexes containing diphosphine chelate ligands seems to be dominated by the unsaturate route [1]. 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

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