Catalysts rate depending

Hydrosilylation.—This reaction is catalysed by the usual homogeneous catalysts. In some cases the mechanism involves insertion of the alkene into a metal-hydrogen bond, as in hydrosilylation of butadiene in the presence of PdL(PPh3)2, with L = p-benzoquinone or maleic anhydride. In other cases concerted addition of the silicon hydride to the carbon-carbon double bond is indicated, as in hydrosilylations catalysed by rhodium(i) catalysts such as RhCl(PPh3)3. In the reaction of silanes with hex-l-ene in the presence of this catalyst, rates depend on the stability of the intermediate adduct RhClH(SiR3)(PPh3)2 such an adduct was isolated in one case. Hydrosilylation of ethylene by trimethylsilicon hydride... 

The equivalent nickel content of the feed to the FCCU can vary from <0.05 ppm for a weU-hydrotreated VGO to >20 ppm for a feed containing a high resid content. The nickel and vanadium deposit essentially quantitatively on the cracking catalyst and, depending on catalyst addition rates to the FCCU, result in total metals concentrations on the equiUbrium catalyst from 100 to 10,000 ppm. 

A general observation which has emerged from electrochemical promotion studies is that over wide ranges of catalyst work function catalytic rates depend exponentially on catalyst work function O ... 

Hence, the rate depends only on the ratio of the partial pressures of hydrogen and n-pentane. Support for the mechanism is provided by the fact that the rate of n-pentene isomerization on a platinum-free catalyst is very similar to that of the above reaction. The essence of the bifunctional mechanism is that the metal converts alkanes into alkenes and vice versa, enabling isomerization via the carbenium ion mechanism which allows a lower temperature than reactions involving a carbo-nium-ion formation step from an alkane. 

The adsorption and accumulation of various impurities from the electrolyte or surrounding atmosphere on the catalyst surface. The rate of accumulation of impurities on the catalyst surface depends on its activity for adsorption, which often is parallel to its catalytic activity. 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

Relatively detailed study has been done for the reaction pathways over Au/Ti02 catalysts mainly because of simplicity in catalytic material components. The rate of PO formation at temperatures around 323 K does not depend on the partial pressure of C3H6 up to 20vol% and then decreases with an increase, while it increases monotonously with the partial pressure of O2 and H2 [57]. A kinetic isotope effect of H2 and D2 was also observed [63]. These rate dependencies indicate that active oxygen species are formed by the reaction of O2 and H2 and that this reaction is rate-determining [57,63,64]. 

We find that the rates of reaction for the various amines examined in (28) are governed by an extremely complex set of equilibria. For example, when R = n-Pr, n-Bu or s-Bu, the rate of reaction exhibit first order dependence on [EtjSiH] at constant amine concentration. However, the rate of reaction exhibits inverse non-linear dependence on [n-PrNH2] and [n-BuNH2], but positive non-linear dependence on [s-BuNH2] at constant [Et-jSiH]. Furthermore, if R t-Bu, then the rate of reaction is almost independent of both [t-BuNH2] and [EtjSiH]. Studies of the rate dependence on catalyst concentration for reaction (28) where R NH2 is n-BuNH2 reveal relative catalyst activities that are inversely dependent on [Ruo(C0) 2]. Similar studies with R NH2 = t-BuNH2 reveal that the rate or reaction is linearly dependent on [Ru3(CO) 2]. Piperidine is unreactive under the reaction conditions studied. 

Improved Filtration Rate Filterability is an important powder catalyst physical property. Sometimes, it can become more important than the catalyst activity depending on the chemical process. When a simple reaction requires less reaction time, a slow filtration operation can slow down the whole process. From a practical point of view, an ideal catalyst not only should have good activity, but also it should have good filtration. From catalyst development point of view, one should consider the relationship between catalyst particle size and its distribution with its catalytic activity and filterability. Smaller catalyst particle size will have better activity but will generally result in slower filtration rate. A narrower particle size distribution with proper particle size will provide a better filtration rate and maintain good activity. 

In the low catalyst concentration range, polymerization rate is increased with increased amounts of catalyst however, the exact rate dependence on catalyst concentration has not been established. In general, the rate of copolymerization of butadiene with styrene is increased with increased polymerization temperature, increased Ba/Mg mole ratio, increased buta-diene/styrene comonomer feed ratio, and increased dielectric constant of the polymerization solvent. 

The rate also varies with butadiene concentration. However, the order of the rate dependence on butadiene concentration is temperature-de-pendent, i.e., a fractional order (0.34) at 30°C and first-order at 50°C (Tables II and III). Cramer s (4, 7) explanation for this temperature effect on the kinetics is that, at 50°C, the insertion reaction to form 4 from 3, although still slow, is no longer rate-determining. Rather, the rate-determining step is the conversion of the hexyl species in 4 into 1,4-hexadiene or the release of hexadiene from the catalyst complex. This interaction involves a hydride transfer from the hexyl ligand to a coordinated butadiene. This transfer should be fast, as indicated by some earlier studies of Rh-catalyzed olefin isomerization reactions (8). The slow release of the hexadiene is therefore attributed to the low concentration of butadiene. Thus, Scheme 2 can be expanded to include complex 6, as shown in Scheme 3. The rate of release of hexadiene depends on the concentra-... 

No evidence of ruthenium metal formation was found in catalytic reactions until temperatures above about 265°C (at 340 atm) were reached. The presence of Ru metal in such runs could be easily characterized by its visual appearance on glass liners and by the formation of hydrocarbon products (J/1J) The actual catalyst involved in methyl and glycol acetate formation is therefore almost certainly a soluble ruthenium species. In addition, the observation of predominantly a mononuclear complex under reaction conditions in combination with a first-order reaction rate dependence on ruthenium concentration (e.g., see reactions 1 and 3 in Table I) strongly suggests that the catalytically active species is mononuclear. 

As shown in Table I, at 0.1 mM Ru (C0) 2 concentration, CO pressure has little if any effect on activity. On the other hand, at fixed pressure, the concentration of ruthenium carbonyl has a dramatic effect on activity (see Figure 2). At 0.1 mM Ru CCO), ruthenium carbonyl is very active for the WGSR, small decreases in catalyst concentration lead to substantial increases in activity, and no activity dependenee on CO pressure is observed. At concentrations of 0.5 mM or more, less activity is observed, changes in concentration cause smaller effects in activity and rate dependence on pressure is manifested. Diffusion effects have been shown to be unimportant (26). 

Peptide hydrolysis by platinum(II) (436) and palladium(II) complexes (437). In the latter case there is selective hydrolysis of the unactivated peptide bond in iV-acetylated L-histidylglycine the hydrolysis rate depends on the steric bulk of the catalyst. 

In this case, the actual redox step is preceded by the formation of an adduct or a complex between the catalyst, the substrate and dioxygen. The order of these reaction steps is irrelevant as long as the rate determining step is Eq. (8). If Eqs. (6) and (7) are rapidly established pre-equilibria the reaction rate depends on the concentrations of all reactants. In some instances, the rate determining step is the formation of the MS complex and the reaction rate is independent of the concentration of dioxygen. 

Heterogeneously catalyzed hydrogenation is a three-phase gas-liquid-solid reaction. Hydrogen from the gas phase dissolves in the liquid phase and reacts with the substrate on the external and internal surfaces of the solid catalyst Mass transfer can influence the observed reaction rate, depending on the rate of the surface reaction [15]. Three mass transfer resistances may be present in this system (Fig. 42.1) ... 

The comparison of hydrogen consumption in the rhodium-catalyzed enantiomeric hydrogenation of a yS-dehydroamino acid using Et-Duphos (Et-Du-PHOS = l,2-bis(2,5-diethyl-phospholanyl)benzene)) as the chiral ligand shows the huge differences in rate, depending on the manner in which the catalyst was prepared (Fig. 44.1) [10b,c]. 

For hydrogenation to take place, the substrate usually needs to bind to the metal complex, although exceptions are known to this rule [25]. Substrate inhibition can occur in a number of ways, for example if more than one molecule of substrate binds to the metal complex. At low concentration this may be a minor species, whereas at high substrate concentration this may be the only species. One example of this is the hydrogenation of allyl alcohol using Wilkinson s catalyst. Here, the rate dependence on the substrate concentration went through a maximum at 1.2 mmol IT1. The authors propose that this is caused by formation of a complex containing two molecules of allyl alcohol (Scheme 44.1) [26],... 

The global rates of heat generation and gas evolution must be known quite accurately for inherently safe design.. These rates depend on reaction kinetics, which are functions of variables such as temperature, reactant concentrations, reaction order, addition rates, catalyst concentrations, and mass transfer. The kinetics are often determined at different scales, e.g., during product development in laboratory tests in combination with chemical analysis or during pilot plant trials. These tests provide relevant information regarding requirements... 

A second order reaction is conducted in a CSTR with a slurried porous catalyst whose specific rate depends on time on stream according to k = 5/(1+0.02t) (1)... 

Background Reaction rates depend on several criteria the concentration of the reactants, the nature of the reaction, temperature, and presence of catalysts. The rate of most reactions increases when the concentration of any reactant increases. For the reaction... 

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