Catalyst turnover rate

Catalyst Turnover rate in pyridine (moles/moles atom catalyst per hour) Turnover rate in acetone (moles/moles atom catalyst per hour)... 

Table 1, Characterization data on the catalysts turnover rates (molecules exp. atom V1) in the hydrogenation of cyclohexene (chex) and 1-hexene (1-hex) over these catalysts (Boudart s data under similar conditions, Pt/Si02 7.67-9.16 [11], Pd/Si02 6.47-8.25 [12])...

The reaction conditions used were quite mild, e.g. 1 atm O2 ambient temperature. Reaction rate was studied both as a function of olefin chain length and as a function of solvent composition. It was found that In a mlcroemulslon the rate of reaction decreased with Increasing chain length. Catalyst turnover rates, after two hours, were Cj q=19.1, C, =15.7 and Ci8 12.4. 

A number of olefins are readily hydrocyanated in the presence of NiL3 or NiL4 [15], but usually catalyst turnover rates demanded (i.e., the number of moles of product formed per mole of catalyst used) and the selectivity tends to be low. It was found that Lewis acids are effective co-catalysts, which enable the reaction pathway and therefore the reaction selectivity to be piloted and accelerate the rate of hydrocyanation [10]. Investigations on the promoting effect of Lewis acids (e. g., AlCL, ZnCL, BPh3 [14, 44]) imply the formation of a 1 1 complex between Lewis acid and NiL4, since at this ratio the reaction rate reaches a maximum [40, 41, 45]. 

Catalyst Turnover Rate (s"1) Rate Constant (s/kPa) Activation Energy (kJ/mol) Preexponential Factor (s/kPa)... 

The Pd-catalyzed coupling reaction of an aryl halide and olefin is a very efficient and practical method for making C—C bonds. The Heck alkenylation of aryl bromides with ethylene was used by Dow Chemical to make high-purity 2- and 4-vinyltoluenes, which are of interest as co-monomers in styrene polymers [159]. The monomer, o-vinyltoluene (99), has a low toxicity and an attractive co-monomer for styrene polymers. o-Vinyltoluene improved heat distortion properties of styrene and polymerization rate. It also minimized color formation or cross-linking and it was difficult to make by other routes [159]. Catalyst turnover, rate, and lifetime were significantly improved. 

In many cases, the addition of Lewis bases capable of coordinating to the metal center during epoxidation catalysis has been found to have a beneficial effect on catalyst turnover rate and number as well as epoxide yield. Commonly used additives include pyridine, imidazole, and pyridine N-oxide derivatives. The proposed roles of N-oxide derivatives in [Cr(salen)] -catalyzed and [Mn(salen)] -catalyzed epoxidation reactions include activation of the intermediate metal-0X0 complex [15,50], dissociation of umeactive p-oxo dimer complexes to reactive monomeric species [19,25], and/or solubilization of the active oxidant in bi-phasic reaction media [51]. 

Borazine reacts with alkynes (HC H or HC=CR) in the presence of RhH(CO)[P(CeH5)3] to give 2-vinyl- or 2-alkenyl-borazines. The product obtained with propyne consists of a 4 1 mixture of (E)-2-(propen-1-yl)borazine and 2-(propen-2-yl)borazine. In the reaction with ethyne at room temperature the catalyst turnover rate is 293 in a 4 h period, while with propyne it is 419 in a 10 h period [10]. 

Systematic efforts to combine the multiple attractive features of POM catalysts with the sorption and high-surface-area attributes of metal organic frameworks (MOFs) have produced several MOFs with POM units residing in the various pores. POMs in polymeric matrices, [50, 51] and most recently POMs in MOF pores that exhibit catalytic activity [52], have been recently reviewed. Research by Maksimchuk, Kholdeeva and co-workers [53, 54] and Gascon, Kapteijn and coworkers [55, 56] are noteworthy. Many of these hybrid (or composite) materials do display the catalytic attributes of the POMs and some of the selective uptake properties of the MOFs. Separate from this evident combining of attractive properties exhibited by both component structures, some POM-MOF type materials also are potential examples of complex behavior because the catalyzed reactions are far from equilibrium and the multiple features impacting catalyst turnover rates aren t related to each other in a simple way they can change with time (conversion of the substrate) and conditions. 

Enzymes are excellent catalysts for two reasons great specificity and high turnover rates. With but few exceptions, all reac tions in biological systems are catalyzed by enzymes, and each enzyme usually catalyzes only one reaction. For most of the important enzymes and other proteins, the amino-acid sequences and three-dimensional structures have been determined. When the molecular struc ture of an enzyme is known, a precise molecular weight could be used to state concentration in molar units. However, the amount is usually expressed in terms of catalytic activity because some of the enzyme may be denatured or otherwise inactive. An international unit (lU) of an enzyme is defined as the amount capable of producing one micromole of its reaction product in one minute under its optimal (or some defined) reaction conditions. Specific activity, the activity per unit mass, is an index of enzyme purity. 

The rhodium complexes are excellent catalysts for hydrogenation of NBR. At low temperature and pressure, high catalyst concentrations are used to obtain a better rate of reactions. Due to higher selectivity of the reaction, pressure and temperature can be increased to very high values. Consequently the rhodium concentration can be greatly reduced, which leads to high turnover rates. The only practical drawback of Rh complex is its high cost. This has initiated the development of techniques for catalyst removal and recovery (see Section VU), as well as alternate catalyst systems based on cheaper noble metals, such as ruthenium or palladium (see Sections IV.A and B). 

Fueled by the success of the Mn (salen) catalysts, new forays have been launched into the realm of hybrid catalyst systems. For example, the Mn-picolinamide-salicylidene complexes (i.e., 13) represent novel oxidation-resistant catalysts which exhibit higher turnover rates than the corresponding Jacobsen-type catalysts. These hybrids are particularly well-suited to the low-cost-but relatively aggressive-oxidant systems, such as bleach. In fact, the epoxidation of trans-P-methylstyrene (14) in the presence of 5 mol% of catalyst 13 and an excess of sodium hypochlorite proceeds with an ee of 53%. Understanding of the mechanistic aspects of these catalysts is complicated by their lack of C2 symmetry. For example, it is not yet clear whether the 5-membered or 6-membered metallocycle plays the decisive role in enantioselectivity however, in any event, the active form is believed to be a manganese 0x0 complex <96TL2725>. 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

The half-wave potentials of (FTF4)Co2-mediated O2 reduction at pH 0-3 shifts by — 60 mV/pH [Durand et ah, 1983], which indicates that the turnover-determining part of the catalytic cycle contains a reversible electron transfer (ET) and a protonation, or two reversible ETs and two protonation steps. In contrast, if an irreversible ET step were present, the pH gradient would be 60/( + a) mV/pH, where n is the number of electrons transferred in redox equilibria prior to the irreversible ET and a is the transfer coefficient of the irreversible ET. The —60 mV/pH slope is identical to that manifested by simple Ee porphyrins (see Section 18.4.1). The turnover rate of ORR catalysis by (ETE4)Co2 was reported to be proportional to the bulk O2 concentration [Collman et ah, 1994], suggesting that the catalyst is not saturated with O2. 

The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced [(dipor)Co2] state as the redox-active form of the catalyst. The redox equilibrium between the mixed-valence and fully reduced forms is shifted toward the catalytically inactive mixed-valence state, and hence controls the amount of catalytically active species in the catalytic cycle and contributes to the — 60 mV/pH dependence. The fully reduced form is known to bind O2 (probably reversibly) in organic solvents [LeMest et al., 1997 Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically viewed as a pair of Co ions bridged by a peroxide, which are of course quite common in the O2 chemistry of nonporphyrin Co complexes. To obtain the —60 mV/pH dependence of the catalytic turnover rate, a protonation step is required either prior to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason to assume that the two classes of cofacial porphyrin catalysts, with rather different catalytic performance (Fig. 18.15), must follow the same mechanism. 

CO to generate acetic acid in aqueous conditions by means of several catalysts (Table 2.2).26 RhCl3 catalyzed the direct formation of methanol and acetic acid from methane, CO, and O2 in a mixture of perfluorobutyric acid and water with a turnover rate at approximately 2.9 h-1 based on Rh at 80-85°C.27 Under similar conditions, ethane was more active and gave ethanol, acetic acid, and methanol. 

Therefore, recent interest has been focused prevailingly on electrodes modified by a multilayer coverage, which can easily be achieved by using polymer films on electrodes. In this case, the mediated electron transfer to solution species can proceed inside the whole film (which actually behaves as a system with a homogeneous catalyst), and the necessary turnover rate is relatively lower than in a monolayer. 

Figure 3.8. Kinetic data from molecular beam experiments with NO + CO mixtures on a Pd/MgO(100) model catalyst [70]. The upper panel displays raw steady-state C02 production rates from the conversion of Pco = PN0 = 3.75 x 10-8 mbar mixtures as a function of the sample temperature on three catalysts with different average particle size (2.8, 6.9, and 15.6 nm), while the bottom panel displays the effective steady-state NO consumption turnover rates estimated by accounting for the capture of molecules in the support. After this correction, which depends on particle size, the medium-sized particles appear to be the most active for the NO conversion. (Reproduced with permission from Elsevier, Copyright 2000).

Rh(CO)2(acac)(dppp)] catalyst gives rates (100-200 turnovers h 1) and selectivities (80-90%) in the reductive carbonylation of MeOH to acetaldehyde this is comparable to the best Co-based catalysts, but requires a much lower temperature (140 °C) and pressure (70 bar). Addition of Ru to this catalyst results in the in situ hydrogenation of acetaldehyde and production of EtOH.68... 

Figand acceleration (the so-called Criegee effect) is the important feature of asymmetric dihydroxylation using cinchona ligands.193 In particular, bis-cinchona ligands provide remarkable acceleration (Scheme 48). This enables high turnover rates of the osmium catalysts. 

The chiral rhodium porphyrin catalyst (90) shows a high turnover rate, though enantioselec-tivity is modest (less than 60% ee). It is, however, noteworthy that cw-selective cyclopropanation of simple olefins (cisjtrans = 2—14.2/1) was realized for the first time with (90).249 250... 

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