Turnover frequency oxidation reactions

Table 11.2 and assume A=100, which is rather conservative value, to compute J via Eq. (11.32) and O via Eq. (11.22). The results show t p 0.91 which implies that the O2 backspillover mechanism is fully operative under oxidation reaction conditions on nanoparticle metal crystallites supported on ionic or mixed ionic-electronic supports, such as YSZ, Ti02 and Ce02. This is quite reasonable in view of the fact that, as already mentioned an adsorbed O atom can migrate 1 pm per s on Pt at 400°C. So unless the oxidation reaction turnover frequency is higher than 103 s 1, which is practically never the case, the O8 backspillover double layer is present on the supported nanocrystalline catalyst particles. 

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

The catalyst activity, given as average turnover frequency (TOF mol product per mol catalyst per reaction time units h-1), affects the production capacity. For hydrogenations, TOFs should be >200h 1 for small-scale products, and >10000 IT1 for large-scale products. Due to lower catalyst costs and often higher added values, lower TON and TOF values are acceptable for enantioselective oxidation and C-C bond-forming reactions. 

From Srivatsava et al (248). Reaction conditions catalyst (TS-1 Si/Ti = 36, Ti-MCM-41 Si/Ti = 46), 100 mg co-catalyst, 0.0072 mmol epoxide, 18 mmol CH2CI2, 20 mL CO2, 6.9 bar. DMAP N,N-dimethylaminopyridine EC epichlorohydrin PO propylene oxide SO stytene oxide BO a-butylene oxide TOF turnover frequency (moles epoxide converted per mole of Ti per hour. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

Figure 15.25 Turnover frequency and selectivity of CO oxidation at 333 K in the PROX reaction, (a) Turnover frequency of CO oxidation at 33 K in the PROX reaction on various Pt catalysts (b) CO oxidation selectivity at 333 K in the PROX reaction on various Pt catalysts.

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]. 

In 1992, Hari Prasad Rao and Ramaswamy reported on the oxyfunctionalization of alkanes with H2O2 using a vanadium silicate molecular sieve s . With this catalyst acyclic and cyclic alkanes were oxidized to a mixture of the corresponding alcohols (primary and secondary ones), aldehydes and ketones. Unfortunately, most of the early attempts were of rather limited success due to low turnover frequencies and radical producing side reactions as observed, for example, by Mansuy and coworkers in 1988. ... 

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. 

According to the International Union of Pure and Applied Chemistry (IUPAC O)) the turnover frequency of a catalytic reac tion is defined as the number of molecules reacting per active site in unit time. The term active sites is applied to those sites for adsorption which are effective sites for a particular heterogeneous catalytic reaction. Because it is often impossible to measure the amount of active sites, some indirect method is needed to express the rate data in terms of turnover frequencies In some cases a realistic measure of the number of active sites may be the number of molecules of some compound that can be adsorbed on the catalyst. This measure is frequently used in the literature of the Fischer-Tropsch synthesis, where the amount of adsorption sites is determined by carbon monoxide adsorption on the reduced catalyst. However, it is questionable whether the number of adsorption sites on the reduced catalyst is really an indication of the number of sites on the catalyst active during the synthesis, because the metallic phase of the Fischer-Tropsch catalysts is often carbided or oxidized during the process. 

The physical meaning of the term /i jj, is the current density achieved if all surface reactions are exergonic (i.e. the highest possible turnover frequency per site). The term jxxmii is dependent on the number of active sites per area and potential independent surface reactions. This means that / miii is dependent on the catalyst material. However, if similar surfaces are compared (e.g. a set of 110 rutile oxide surfaces), the number of active sites per area only varies with a few presents, as the lattice constants are very similar, and y limit can effectively be considered material independent, unlike exchange current density. In that case, trends in fy°ER should correlate with trends in activity, ya. 

Another class of ruthenium catalysts, which has attracted considerable interest due to their inherent stability under oxidative conditions, are the polyoxome-talates [161]. Recently, Mizuno et al. [162] reported that a mono-ruthenium-sub-stituted silicotungstate, synthesized by the reaction of the lacunary polyoxometa-late [SiWu039]8- with Ru3+ in an organic solvent, acts as an efficient heterogeneous catalyst with high turnover frequencies for the aerobic oxidation of alcohols (see Fig. 4.63). Among the solvents used 2-butyl acetate was the most... 

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