Overall turnover number

After ten consecutive runs the overall turnover number reaches up to 3500 mol 1-octene converted per mol Rh-catalyst. In agreement with these recycling experiments, no Rh could be detected in the product layer by AAS or ICP, indicating leaching of less then 0.07 %. In all experiments, very good selectivities for the linear aldehyde were obtained, thus proving that the attachment of the guanidinium moiety onto the xanthene backbone had not influenced its known positive effect on... 

On occasion these over ligated complexes are materials that can be identified in solution or perhaps more tellingly isolated from catalytic reactions. Such reactions can often be reversed by removal of the excess reagent. Such processes are not considered in the context of this book as resulting in loss of overall turnover number. 

Cofactor regeneration, an economically essential step in the synthetic use of ADH, was accomplished within the PVA matrix using isopropanol as cosubstrate for the ADH itself or for a second alcohol dehydrogenase from Thermoanaerobium hrockii (E.C. 1.1.1.2). An overall turnover number of 10 was achieved, which is a promising magnitude for technical application. However, while the presence of the cosubstrate in the gel-stabilized two-phase system improved the solubility of substrates in the gel phase and consequently enhanced the productivity of the... 

At the end of each cycle the aqueous phase, containing the produced hydrogen peroxide, was removed by decantation and replaced with pure water. The organic layer, containing the catalyst, the quinone and the acid co-catalyst, was then reused in a following cycle. After 10 consecutive cycles (carried out in 10 hours) an overall turnover number (TON) of 4500 moles of hydrogen peroxide per mole of palladium was achieved and the catalyst did not show any appreciable loss of activity. 

Then when CoTTP-py-NHCO-GC modified electrode is obtained, the catalytic activity towards the electrochemical reduction of CO2 showed a current efficiency of 92% for CO production and also an overall turnover number of 10 at -1.1 V vs. SCE. As mentioned before the CoTTP in CoTTP-py-NHCO-GC electrode is in the form of a five coordinate complex with a pyridine ligand, with a vacant site at the trans position to pyridine. It is expected that CO2 reduction occurs at vacant site to which pyridine gives specific reactivity through the called trans effect. The axial pyridine ligand could favor an electron transfer from Co of the complex to CO2, since it increases the electron transfer ability of Co, as in the case of an O2 carrier, and facilitates the reduction of C02 . ... 

The data most frequentiy collected and reported in catalyst performance evaluations are activity or turnover number, selectivity to the desired product(s), overall yield, catalyst life, and the identities and yields of by-products produced. These data are used to further catalyst or process development research efforts, to monitor catalyst manufacture, and to provide quaUty assurance information to catalyst users. 

The overall rate constant for conversion of the E S complex to products E + P is called the turnover number because it represents the number of substrate molecules the enzyme turns over into product per unit time. A value of about 103 per second is typical. 

Whilst trying to be comprehensive, we have also intended to introduce a strong applied flavor to this summary. In the industrial case, catalyst performance is critically judged on overall efficiency, namely catalyst productivity and activity as well as enantioselectivity. As a result, turnover numbers (TONs) and turnover frequencies (TOFs) have been included or calculated whenever possible and meaningful. 

At one extreme diffusivity may be so low that chemical reaction takes place only at suface active sites. In that case p is equal to the fraction of active sites on the surface of the catalyst. Such a polymer-supported phase transfer catalyst would have extremely low activity. At the other extreme when diffusion is much faster than chemical reaction p = 1. In that case the observed reaction rate equals the intrinsic reaction rate. Between the extremes a combination of intraparticle diffusion rates and intrinsic rates controls the observed reaction rates as shown in Fig. 2, which profiles the reactant concentration as a function of distance from the center of a spherical catalyst particle located at the right axis, When both diffusion and intrinsic reactivity control overall reaction rates, there is a gradient of reactant concentration from CAS at the surface, to a lower concentration at the center of the particle. The reactant is consumed as it diffuses into the particle. With diffusional limitations the active sites nearest the surface have the highest turnover numbers. The overall process of simultaneous diffusion and chemical reaction in a spherical particle has been described mathematically for the cases of ion exchange catalysis,63 65) and catalysis by enzymes immobilized in gels 66-67). Many experimental parameters influence the balance between intraparticle diffusional and intrinsic reactivity control of reaction rates with polymer-supported phase transfer catalysts, as shown in Fig. 1. 

Figure 53 shows relative rates of C02 formation under steady-state conditions that were recorded with various single-crystal surfaces of Pd as well as with a polycrystalline Pd wire (173). It must be noted that with these experiments no determination of the effective surface areas was performed so that no absolute turnover numbers per cm2 are obtained. Instead, the reaction rates were normalized to their respective maximum values. As can be seen from Fig. 53, all data points are close to a common line which indicates that, in fact, with this reaction the activity is influenced very little by the surface structure. As has been outlined in Section II, the adsorption of CO exhibits essentially quite similar behavior on single-crystal planes with varying orientation. Since the adsorption-desorption equilibrium of CO forms an important step in the overall kinetics of steady-state C02 formation, this effect forms at least a qualitative basis on which the structural insensitivity may be made plausible. 

Zn11 enhances the acidity of the coordinated water and therefore vastly enhances the concentration of coordinated OH- at pH 7.5. It coordinates both OH- and C02 in close proximity its sufficient lability towards ligand exchange and ability to change coordination numbers from 4 to 5 lead to fast overall turnover (step 2 in Figure 9.5). Water then attacks the five-coordinated Zn complex and HC03 is released,... 

A simple calculation reveals that the picture cannot be quite as simple. Carbonic anhydrase has an exceptionally high overall rate of reaction, its turnover number kcat is -5 x 105 s-1 consequently, the rate constants of individual steps must be greater than this number. The acid dissociation of a Zn11 aqua species seems to be inconsistent with this requirement. The dissociation constant fQ can be written as the ratio of forward k and backward kh rate constants [Eq. (9.20)]. 

Ru-MeO-Biphep (Ru-96a) was used by Roche to reduce 98 to the corresponding P-hydroxy ester with >98% ee at 240-kg scale. Turnover numbers of 50,000 were achieved in this reduction. A process was developed by PPG-Sipsy to reduce 99 with Ru-MeO-Biphep for Pfizer in an approach to candoxatril. The olefin was reduced in >99% ee at 230-kg scale (S/C = 1000-2000).118 Although catalysts that contain DuPhos had been determined to be more effective based on overall yields and isomerization to an enol by-product, the Ru-MeO-Biphep catalyst was preferred as a result of catalyst availability at scale and more favorable licensing agreements.119... 

This led them to act in a number of different directions. First and foremost, they had to lower their operating costs by cutting down on excess personnel and taking the measures needed to increase the productivity of each company. At the same time, they had to reduce, in a concerted way if possible, the overcapacities affecting the hardest-hit sectors. Finally, it seemed advisable to redirect production into areas that were less sensitive to economic change. This meant increasing the share of specialties in relation to commodities in overall turnover. 

The carbonylation step is performed in a mixture of an organic solvent and hydrochloric acid. As catalyst, PdCl2(PPh3)2 is used. The economic feasibility of the overall process is to a large extent determined by the ability to recycle the palladium catalyst. Careful reaction design makes possible total catalyst turnover numbers (TONs) high above 10000. 

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