The Catalyst Cycle

In a homogeneously catalyzed reaction the determination of the kinetic factors for the process is usually straightforward. In a solution, reactants and the soluble catalysts are uniformly distributed throughout the reaction medium and the reaction rate can be expressed as a function of the concentrations of these substances. A heterogeneously catalyzed process is more complex because the catalyst is not uniformly distributed throughout the reaction medium. Consider a two phase system, either vapor/solid or liquid/solid, with the solid phase the catalyst. In such a system several steps are needed to complete the catalytic cycle  

The desired reaction takes place only in Step 3, but Steps 2 and 4 also involve chemical changes so any rate data obtained from such reactions includes all three steps. Any activation energy measurements also apply to the three step combination. The situation is further clouded by the fact that while Steps 1 and 5 

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Normally, copper-catalysed Huisgen cycloadditions work with terminal alkynes only. The formation of a Cu-acetylide complex is considered to be the starting point of the catalyst cycle. However, the NHC-Cu complex 18 was able to catalyse the [3-1-2] cycloaddition of azides 17 and 3-hexyne 23 (Scheme 5.6). 

TaniaPhos active catalyst discussion As shown by Salzer (2) such complexes with half sandwich stracture result in the catalyst cycle into a hydride species where the pentadienyl moiety can be hydrogenolyticaUy liberated (2, 6). This was verified in the case of BINAP complexes (2, diss. Podewils, Geyser). In accordance to this fact and other mechanistic aspects from Noyori s work (3, 5) it is likely that the pre-catalyst species undergoes the same reaction pathway and that the reactive part of the pre-catalyst, the pentadienyl moiety, will be liberated under hydrogenolytic conditions as shown below in Scheme 23.9 ... 

Several combinations of promoters showed particular advantageous results. For example, addition of small amounts of group VI-B elements to tin, lithium or phosphine promoted reactions sharply increase the catalyst activity (Table III). Only the metals or the carbonyls are active for this function. It is presumed that part of the function of these elements is to assist in the reduction part of the catalyst cycle especially in the initial phase of the reaction. 

In the case of phosphine, the active catalyst is presumably either bisphosphine dicarbonyl or the phosphine tricarbonyl complex. Kinet-ically the bis-phosphine nickel complex cannot be the predominant species. However, in the presence of very high phosphine concentration it may have an important role in the catalyst cycle. After ligand loss and methyl iodide oxidative addition, both complexes presumably give the same 5 coordinate alkyl species. 

T,he hydroformylation reaction or oxo synthesis has been used on an industrial scale for 30 years, and during this time it has developed into one of the most important homogeneously-catalyzed technical processes (I). A variety of technical processes have been developed to prepare the real catalyst cobalt tetracarbonyl hydride from its inactive precursors, e.g., a cobalt salt or metallic cobalt, to separate the dissolved cobalt carbonyl catalyst from the reaction products (decobaltation) and to recycle it to the oxo reactor. The efficiency of each step is of great economical importance to the total process. Therefore many patents and papers have been published concerning the problem of making the catalyst cycle as simple as possible. Another important problem in the oxo synthesis is the formation of undesired branched isomers. Many efforts have been made to keep the yield of these by-products at a minimum. 

The rest of the catalyst cycle is identical to that illustrated for the rhodium complex-catalyzed reactions in Scheme 1. It has been proposed that the asymmetric induction occurs during the formation of alkyl-Pt(CO)L2 intermediate through olefin insertion into the Pt-H bond [13]. 

In 1970, Chauvin and Herisson presented a study of the co-metathesis of cycloalkene/alkene mixtures using a WOCLj/SnBuj pro-catalyst mixture [12]. Whilst the fully quantitative analysis of the product mixtures was made complicated by the range of techniques that were required for the low, medium and high molecular weight products (mono alkenes, telomers and polymers), it became clear that product ratios were not consistent with what would be predicted by either mechanism in Scheme 12.14. The analysis and associated mechanistic interpretation were seminal and worthy of consideration in some detail here. The key point is that both mechanisms in Scheme 12.14 are pairwise, i.e. each turnover of the catalyst cycle involves two alkenes that undergo concerted alkylidene exchange. When a single alkene, e.g. pent-2-ene (C5), is considered, the products of alkylidene metathesis... 

Reactions orders between zero and one in accordance with one-plus rate equations are very common in enzyme catalysis, even if the cycle is more complex and involves additional reactants or products. The plots just described thus are more broadly applicable. On the other hand, straight lines in such plots are only evidence of saturation kinetics, not an indication that the catalyst cycle has only one intermediate. 

The aqueous biphasic processes require a minimum solubility of the reactants S in the catalyst phase [196, 205]. Therefore, hydroformylation of higher olefins (approx. > Cg) or functionally substituted olefins is more difficult but offers various advantages, such as the simplification of reaction sequences and reduced expenditure for the catalyst cycle. So far, work on these biphasic processes for the conversion of higher olefins, except for Kuraray s recent devel-... 

A catalyst was subjected to a series of 11 reaction cycles where the number of propane pulses was increased by one on each cycle hence, in cycle 1, only one pulse of propane was passed over the catalyst, while cycle 10 had ten pulses of propane passed over the catalyst. Cycle 11 was a repeat of cycle 1 with only one pulse of propane passed over the catalyst. The data indicated that changes were occurring with respect to the amount of carbon deposited and the ability of the oxygen to remove it effectively. In Figure 1 it can be seen that the amoimt of carbon deposited increases with amount of propane passed over the catalyst however, the amount of carbon removed by the oxygen treatment at 873 K, although increasing, increases at a slower rate. 

Due to the high fouling rate, the catalyst cycle length was lower than expected. 

CO oxidation in O2 excess on the fresh model catalyst. XPS analysis indicates stabilization of Pt oxide in the case of Pt/ceria [98]. In contrast, similar measurements on Pt/alumina show that these Pt particles are metallic and the chemical state does not change significantly after several CO oxidation cycles in O2 excess [98]. The long-term lean CO oxidation experiments on Pt/ceria indicate that the catalyst activity gradually changes as a function of CO oxidation experiment time. Extensive CO reduction causes an up-shift of both T50 and E a, which is attributed to C deposition on the catalyst [9], probably via CO disproportionation at CO excess. A lower value of T50 and E can, however, be restored by running a H2 oxidation over the catalyst (cycles 5-7) around stoichiometric conditions a = 0.67). The influ-... 

The concentration of PTC has significant effect on the activity as well as selectivity to PAA. Both the activity and selectivity increased with the concentration of PTC and infact there was no conversion of benzylchloride without the PTC (Figure. 1). This is expected, as one of the reactants (NaOH) which is needed for the reductive elimination step of the catalyst cycle being in another phase. 

Tn 1955 Pines and Schaap (1) discovered that toluene was alkylated by ethylene in the presence of sodium or potassium metal or, more specifically, their organometallic derivatives. This reaction requires a high temperature (about 200°C) and considerable olefin pressure the organometallic catalyst is essentially insoluble in the reaction medium. The catalyst cycle—for example, in the side-chain ethylation of toluene— involves a benzyl carbanion which adds to ethylene to form a primary alkyl carbanion. The latter immediately abstracts a proton from the excess toluene reactant to form n-propylbenzene and to reform the energetically-favored benzylic anion in a catalytic cycle. 

Newly developed second-stage catalysts which increase their ability to resist either sulfur spikes in the second-stage feed or overall higher sulfur levels will greatly enhance the economics of the process. Criterion26 developed a more poison-resistant catalyst that in one plant extended the catalyst cycle length to 3.5 years from 6 months attained by the previous nickel catalyst. 

Mechanistic studies of rhodium-catalyzed hydroformylation of olefins have shown that the basic feature of the catalyst cycle is more or less the same as that of the cobalt-catalyzed reaction.When unmodified rhodium carbonyls, e.g., Rh4(CO)i2 and RhefCOlie, are used as catalysts, there is an equilibrium among Rh4(CO)i2, Rh6(CO)i6, and HRh(CO)n (n = 3 or 4) in the presence of carbon monoxide and hydrogen, which complicates the mechanistic study. Nevertheless, HRh(CO)n (n = 3) is postulated as the active catalyst species, and the... 

The catalyst cycle [8] consists of four stages (see Scheme 4) ... 

Reaction condition optimization considers reaction severity in terms of temperature and pressure profiles in accordance with catalyst performance in the entire run length. Optimizing reaction conditions, selecting better catalysts, and maintaining catalyst performance in operation have significant effects on both yields and energy efficiency. Consider reaction temperature as an example. In the catalyst cycle, the catalyst performance deteriorates, which affects the reaction conversion. To compensate, the reaction temperature may be increased. However, more severe reaction conditions require more heat from hot utilities such as fired heaters, while severe conditions also produce more desirable products as well as undesirable by-products. The question is how to determine the optimal reaction temperature, which is a function of reaction conversion, production rate, and energy use. 

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