Turnover frequency performance

Minimize the effects of transport phenomena If we are interested in the intrinsic kinetic performance of the catalyst it is important to eliminate transport limitations, as these will lead to erroneous data. We will discuss later in this chapter how diffusion limitations in the pores of the catalyst influence the overall activation energy. Determining the turnover frequency for different gas flow velocities and several catalyst particle sizes is a way to establish whether transport limitations are present. A good starting point for testing catalysts is therefore ... 

The strategy for the asymmetric reductive acylation of ketones was extended to ketoximes (Scheme 9). The asymmetric reactions of ketoximes were performed with CALB and Pd/C in the presence of hydrogen, diisopropylethylamine, and ethyl acetate in toluene at 60° C for 5 days (Table 20) In comparison to the direct DKR of amines, the yields of chiral amides increased significantly. Diisopropylethylamine was responsible for the increase in yields. However, the major factor would be the slow generation of amines, which maintains the amine concentration low enough to suppress side reactions including the reductive aminafion. Disappointingly, this process is limited to benzylic amines. Additionally, low turnover frequencies also need to be overcome. 

Ligand to rhodium ratio is 10, catalysis performed in 13 ml of toluene using 1 ml of 1-octene as the substrate at 80 °C and 50 bar CO/H2. Samples were analysed by means of GC and GC-MS analysis. b Average turnover frequencies were calculated as (mol product)(mol catalyst)1 h 1.c 1 ml of 1-propanol added to the catalyst mixture 1 Initial turnover frequency. 1 ml of triethylamine added to the catalyst mixture. 

The catalysis was performed batch-wise (Figure 4.36). After reaching ca. 90% conversion, the bulk phase was replaced and similar turnover frequencies (TOF) of about 25 h"1 were obtained in the following three runs 2, 3 and 4. When the catalyst capsule was removed, no further activity was detected. Furthermore, the Ru content in the bulk phase was always below the detection limit of AAS, which shows good catalyst retention by the membranes used. 

Concentrations of hydroxide beyond those needed for catalyst activation display approximately 0.5 order dependence with regard to turnover frequency. Since these results are for two catalysts performing different functions, it would appear that they correspond to a separate, noncatalytic step in the reaction sequence. 

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. 

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

Gasteiger et al. reviewed the best performing Fe-based catalysts in the literature up to 2004 1. Even the best of these catalysts (Fe on pyrolyzed peryle-netetracarboxylic dianhydride) showed a corrected turnover frequency of 7% and a volume activity density of 0.2% of Ft. More recent work has focused on optimizing the metal, nitrogen, and carbon composition of the materials. 

The results of some initial experiments indicated that the catalytic activity of 1 is strongly temperature-dependent. For instance, at 100 °C, the rates are relatively low but there is an appreciable turnover [turnover frequency (TOE) = 20.5 h" j. Increasing the reaction temperature to 200 °C increases the TOE to 720 h hence, the thermal robustness of the iridium catalyst is pivotal for optimal catalyst performance. The tridentate pincer-type ligands provide a particularly stable platform for metal confinement through covalent Ir—C bonding combined with the terden-tate chelating coordination. 

Lu and coworkers have synthesized a related bifunctional cobalt(lll) salen catalyst similar to that seen in Fig. 11 that contains an attached quaternary ammonium salt (Fig. 13) [36]. This catalyst was found to be very effective at copolymerizing propylene oxide and CO2. For example, in a reaction carried out at 90°C and 2.5 MPa pressure, a high molecular weight poly(propylene carbonate) = 59,000 and PDI = 1.22) was obtained with only 6% propylene carbonate byproduct. For a polymerization process performed under these reaction conditions for 0.5 h, a TOF (turnover frequency) of 5,160 h was reported. For comparative purposes, the best TOF observed for a binary catalyst system of (salen)CoX (where X is 2,4-dinitrophenolate) onium salt or base for the copolymerization of propylene oxide and CO2 at 25°C was 400-500 h for a process performed at 1.5 MPa pressure [21, 37]. On the other hand, employing catalysts of the type shown in Fig. 12, TOFs as high as 13,000 h with >99% selectivity for copolymers withMn 170,000 were obtained at 75°C and 2.0 MPa pressure [35]. The cobalt catalyst in Fig. 13 has also been shown to be effective for selective copolymer formation from styrene oxide and carbon dioxide [38]. 

Researchers performed the biphasic hydrogenation of cyclohexene with Rh(cod)2 BF4 (cod = cycloocta-1,5-diene) in ILs. They observed roughly equal reaction rates, reported as turnover frequencies of ca. 50 h in either [bmim][BF4] or [bmim][PF6]. The presumption here was that the [bmim][BF4] was free from chloride. In a separate report, the same group showed that RuCl2(Ph3P)3 in [bmim][BF4] was an effective catalyst for the biphasic hydrogenation of olefins, with turnover frequencies up to 540 h Similarly, (bmim)3-Co(CN)5 dissolved in [bmim][BF4] catalyzed the hydrogenation of butadiene to but-l-ene, with 100% selectivity at complete conversion. 

It is far from easy to distinguish structural, electronic and synergistic promotion effects. Structural promotion is, in this respect, the most easily to observe. Most synergistic elfects are also widely discussed in the literature in enhancing the catalytic performance of supported cobalt nanoparticles. Instead, promotion as a result of electronic effects are much more difficult to detect. The main reason is that one has to discriminate between the number of surface cobalt sites and the intrinsic activity of a surface cobalt site (turnover frequency). This is especially difficult in view of the complexity of the catalyst material. It also requires spectroscopic tools, which are able to detect changes in the electronic structure of the supported cobalt nanoparticles. 

Every (bio)catalyst can be characterized by the three basic dimensions of merit -activity, selectivity and stability - as characterized by turnover frequency (tof) (= l/kcat), enantiomeric ratio (E value) or purity (e.e.), and melting point (Tm) or deactivation rate constant (kd). The dimensions of merit important for determining, evaluating, or optimizing a process are (i) product yield, (ii) (bio)catalyst productivity, (iii) (bio)catalyst stability, and (iv) reactor productivity. The pertinent quantities are turnover number (TON) (= [S]/[E]) for (ii), total turnover number (TTN) (= mole product/mole catalyst) for (iii) and space-time yield [kg (L d) 11 for iv). Threshold values for good biocatalyst performance are kcat > 1 s 1, E > 100 or e.e. > 99%, TTN > 104-105, and s.t.y. > 0.1 kg (L d). ... 

The turnover frequency allows performance comparison between different catalyst systems, biological and/or non-biological. Its threshold is at 1 event per second per active site. According to the definition, a turnover frequency can be determined only if the number of active sites is known (Chapter 9, Section 9.2.3). For an enzyme reaction obeying Michaelis-Menten kinetics, Eq. (2.15) holds. 

The turnover number is not used frequently in biocatalysis, possibly as the molar mass of the biocatalyst has to be known and taken into account to obtain a dimensionless number, but it is the decisive criterion, besides turnover frequency and selectivity, for evaluation of a catalyst in homogeneous (chemical) catalysis and is thus quoted in almost every pertinent article. Another reason for the low popularity of the turnover number in biocatalysis, apart from the challenge of dimensionality, is the focus on reusability of biocatalysts and the corresponding greater emphasis on performance over the catalyst lifetime instead of in one batch reaction as is common in homogeneous catalysis (Blaser, 2001). For biocatalyst lifetime evaluation, see Section 2.3.2.3. 

Some of those criteria have been discussed above in the context of the catalyst or the process, such as enantioselectivity or diastereoselectivity instead of simple selectivity to product, and catalyst performance data such as turnover frequency (tof), turnover number (TON), total turnover number (TTN), and space-time-yield (s.ty.). 

The oxidation of 3-carene to 3-caren-5-one (Figure 3.46) is a key step in the synthesis of the pyrethroid ester insecticide Deltamethrin [162,163]. This reaction is performed with air as the oxidant, catalyzed by 2 mol% of a Cr-pyridine complex (the catalyst precursors are CrCl3 and pyridine). Table 3.1 shows the turnover frequencies obtained using various Cr/pyridine ratios. 

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