Turnover number performance

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. 

Beckmann rearrangements of several ketoximes were performed in room-temperature ionic liquids based on l,3-dialkylimida2olium or alkylpyridinium salts containing phosphorus compounds (such as PCI5) by Deng and Peng [59] (Scheme 5.1-31, BP = 1-butylpyridinium). Turnover numbers of up to 6.6 were observed, but the authors did not mention whether the ionic liquid could be reused. 

Despite those challenges, both Johnson [161] and Grela [162] performed several cross metathesis reactions with vinylhalides using phosphine free catalysts. Turnover numbers (TON) above 20 were very few, while in many cases the TON stayed below ten. The diastereoselectivity of CMs with vinylhalides is shghtly in favour of the Z product which is similar to their acrolein-counterparts. 

Attempts to perform hydrodefluorination on other polyfluoroarenes using 9c met with mixed success (Table 8.4). Pentafluoropyridine proved to be highly active with a turnover number of 13.6, although the distribution of products revealed up to three hydrodefluorination reactions (Table 8.4, entry 4). Substrates with a lower fluorine content, such as CF.CH, and 1,2- or 1,4-CF,H were unreactive. 

Selecting a rigorous and convenient quantitahve parameter characterizing the catalyhc achvity, A, is of prime importance when studying electrocatalytic phenomena and processes. The parameter usually selected is the current density, i (in AJan ), at a specified value of electrode poteuhal, E. The current density is referred to the electrode s true working surface area [which can be measured by the Brunauer-Emmett-TeUer (BET) or other methods]. Closely related to this true current density is another parameter, known as the turnover number y (in s ), and indicating the number of elementary reachon acts performed or number of electrons transferred in unit time per surface atom (or catalytic surface site) of the catalyst. 

This catalytic system was further studied by Strohmeier and Steigerwald, who performed reactions at 10 bar without solvent to achieve hydrogenation of a series of aldehydes (Table 15.1) [2]. Turnover numbers (TON) of up to 8000 were achieved in the case of the hydrogenation of benzaldehyde. The chemoselectivity of this catalyst towards carbonyl hydrogenation over alkene hydrogenation was... 

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. 

The performance concerning selectivity, activity, productivity and so on, must fulfill the requirements of a given product As a rule of thumb, (enantio) selectivity should be >90-95% turnover numbers (TON) should be >200 for small volume products and reactions with high added value and >10000 for large-volume products and reaction times should not exceed 5-10 h. Furthermore, functional group tolerance will often be important as most substrates will have other functional groups which can either be reduced or which can interfere with the catalyst via complexation. 

Figure 1.33 illustrates asymmetric hydrogenation of a functionalized imine with a XYLIPHOS-Ir catalyst, occurring with a catalyst turnover number of 2,000,000. The presence of I under acidic conditions is crucial to achieve high catalytic performance. (5)-Metolachlor, a herbicide, is industrially produced in a > 10,000-ton quantity per year by this reaction. 

Che has reported that both achiral and chiral rhodium catalysts function competently for intramolecular aziridination reactions of alkyl- and arylsulfonamides (Scheme 17.29) [59, 97]. Cyclized products 87 are isolated in 90% yield using 2 mol% catalyst, PhI(OAc)2, and AI2O3. Notably, reactions of this type can be performed with catalyst loadings as low as 0.02 mol% and display turnover numbers in excess of 1300. In addition, a number of chiral dimeric rhodium systems have been examined for this process, with some encouraging results. To date, the best data are obtained using Doyle s Rh2(MEOX)4 complex. At 10 mol% catalyst and with a slight excess of Phl=0, the iso-... 

Homogeneous catalysis is, in the first instance, a synthetic approach. Therefore, a high turnover number is highly desirable for pragmatic and economic reasons. Nevertheless, experiments should be conducted at rather low substrate to precursor ratio as well as rather high substrate to precursor ratio. This type of approach is particularly well suited to semi-batch investigations. A wide variation in substrate to precursor ratio is readily performed with a minimum of resources. It may even be useful to provide perturbations in product, temperature and pressure. 

Major challenges remain in catalyst development. There will be continuing efforts to increase turnover numbers and rates and to perform these reactions under environmentally friendly conditions. Transition metals, especially Cu, Rh, Ru, Co, Mo, and Zn are effective today will other metals with attendant ligands be found whose electronic and steric properties are superior to those currently optimized ... 

The lactonization of o-iodobenzyl alcohol 311 was performed in the presence of PdCl2[(P(OEt)3]2 (0.02 mol%) and triethylamine (2.2 equiv.) at 130°C and 5 atm of CO in SCCO2 (200atm) to give phthalide 312 quantitatively (Equation (28)). " In this reaction, the turnover number (TON) in SCCO2 reached 5000 after 18h, which was considerably better than that in toluene (3800). Moreovet, the reaction in SCCO2 reached 4700 TON after 6h as compared to only 1100 TON for that in toluene. 

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

Catalyst structure. The investigation of a large series of Pt/Al203 catalysts showed that catalysts with dispersions > 0.4 give lower optical yields and lower turnover numbers. However, the platinum dispersion is not a sufficient parameter to explain the enantioselectivities observed for the different catalysts. Other factors such as texture of the support, morphology and size distribution of the platinum particles may affect the catalyst performance as well [30,58,59, 61]. 

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. 

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