Iron turnover frequency

The cyclodimerization of 1,3-butadiene was carried out in [BMIM][BF4] and [BMIM][PF(3] with an in situ iron catalyst system. The catalyst was prepared by reduction of [Fe2(NO)4Cl2] with metallic zinc in the ionic liquid. At 50 °C, the reaction proceeded in [BMIM][BF4] to give full conversion of 1,3-butadiene, and 4-vinyl-cyclohexene was formed with 100 % selectivity. The observed catalytic activity corresponded to a turnover frequency of at least 1440 h (Scheme 5.2-24). 

A comparison of the results of a theoretical treatment of the transient behaviour of iron catalysts with experimental data shows that the low turnover frequencies found for those catalysts cannot be the result of a low rate constant for the propagation reaction. To obtain accurate data for the transient period, which lasted less than 20 s, a reaction system with very little axial dispersion was built. 

A comparison of the theoretical treatment of the rate of synthesis of hydrocarbons with the experimental results, clearly shows that the low turnover frequencies that are measured on iron catalysts cannot be explained by a low rate constant of propagation. This result is clearly in contrast with the conclusions of Dautzenberg et al. for ruthenium. This does not signify, however, that the propagation has to be rate determining on ruthenium. 

Another test of validity is to check the performance of the model against experimental rate data obtained far from equilibrium. The microkinetic model presented in Table 7.3.1 predicts within a factor of 5 the turnover frequency of ammonia synthesis on magnesia-supported iron particles at 678 K and an ammonia concentration equal to 20 percent of the equilibrium value. This level of agreement is reasonable considering that the catalyst did not contain promoters and that the site density may have been overestimated. The model in Table 7.3.1 also predicts within a factor of 5 the rate of ammonia synthesis over an Fe(lll) single crystal at 20 bar and 748 K at ammonia concentrations less than 1.5 percent of the equilibrium value. 

When the total concentration in ruthenium was decreased, the catalytic activity fell off indicating that cluster catalysis was occurring (46). Moreover, Laine has found a synergistic effect between ruthenium and iron. Indeed, whereas the turnover frequencies displayed by [Fe3(CO),2] and [Ru3(CO),2] for the hydroformylation of pent-l-ene were, respectively, 38 and 40 hour , a 1 1 mixture of [Fe3(CO),2] and [Ru3(CO),2] gave a value of 230 hour . 

In the catalysis community, it is generally accepted that there are two types of support materials for heterogeneous oxidation catalysts [84]. One variety is the reducible supports such as iron, titanium, and nickel oxide. These materials have the capacity to adsorb and store large quantities of molecules. The adsorbed molecules diffuse across the surface of the support to the catalyst particle where they are activated to a superoxide or atomically bound state. The catalytic reaction then takes place between the reactant molecules and the activated on the catalyst particle. Irreducible supports, in contrast, have a very low ability to adsorb O. Therefore, can only become available for reaction through direct adsorption onto the catalyst particle. For this reason, catalysts deposited on irreducible supports generally exhibit turnover frequencies that are much lower than those deposited on reducible supports [84]. More recent efforts in our laboratory are focused on characterizing catalyst support materials that are commonly used in industry. These studies are aimed at deciphering how specific catalyst and support material combinations result in superior catalytic activity and selectivity. 

Kinetic parameters of supported iron and zinc oxides for the water-gas shift reaction including activation energy (E.), surface area (S.A.) power law exponents (1, m, and q), and the turnover frequency at 653 K (TOF)... 

Note that this is the reaction rate or activity. However, this definition takes into account the reaction medium, be it volume, surface, or interface, and not exactly the active sites. Not all mass or surface is active, but part of its outer surface has active sites, which are truly the sites where the chemical reaction occurs. Therefore, rj in fact represents the apparent rate. An important example of reaction that allows to differentiate the apparent from the true rate is the hydrogenation of carbon monoxide to form methane, which is conducted with different catalysts. With iron and cobalt catalysts, the rate per unit of mass of catalyst, used as reference, has shown controversial values. The activity of the catalysts in the Fischer-Tropsch synthesis to form hydrocarbons would decrease according to the order Fe > Co > Ni. However, when the rate per active site was defined, the order of activity was different, i.e., Co > Fe > Ni. This controversy was resolved by Boudart, who defined the intrinsic activity, i.e., the rate per active site. To make it more clear, the turnover frequency (TOF) was defined. Thus, the intrinsic activity is determined, knowing the active sites, i.e. ... 

It should be clear from this discussion that the working, active, and selective catalyst is a complex, multicomponent chemical system. This system is finely tuned and buffered to carry out desirable chemical reactions with high turnover frequency and to block the reaction paths for other thermodynamically equally feasible but unwanted reactions. Thus, an iron catalyst or a platinum catalyst is composed not only of iron or platinum but of several other constituents as well to ensure the necessary surface structure and oxidation state of surface atoms for optimum catalytic behavior. Additives are often used to block sites. 

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