Kinetic studies deactivator concentration

A model developed by Leksawasdi et al. [11,12] for the enzymatic production of PAC (P) from benzaldehyde (B) and pyruvate (A) in an aqueous phase system is based on equations given in Figure 2. The model also includes the production of by-products acetaldehyde (Q) and acetoin (R). The rate of deactivation of PDC (E) was shown to exhibit a first order dependency on benzaldehyde concentration and exposure time as well as an initial time lag [8]. Following detailed kinetic studies, the model including the equation for enzyme deactivation was shown to provide acceptable fitting of the kinetic data for the ranges 50-150 mM benzaldehyde, 60-180 mM pyruvate and 1.1-3.4 U mf PDC carboligase activity [10]. 

How can these bulky, extremely weakly coordinating anions prevent catalyst deactivation A comparative kinetic study of catalysts with different anions provided a plausible answer [19]. With PFg as a counterion, the rate dependence on olefin concentration was first order, whereas the rate order observed for the corresponding BArp complex was close to zero. This striking difference may be explained by the stronger coordination of PFs or formation of a tight anion pair, which slows down the addition of the olefin to the catalyst to such an extent that it becomes rate-limiting. In contrast, the essentially noncoordinating BArp ion does not interfere with olefin coordination, and the catalyst remains saturated with olefin even at low substrate concentration. The slower reaction of the PFg salt with the olefin could... 

The elimination of the sodium hydride was explained by the process given by Margeri-son and Nyss 289). Following Schmitt 296), Comyn and Glasse also proposed 309) that reaction of the anions formed in the a-methylstyrene system would yield deactivated species via reaction with the solvent, THF. Their kinetic study showed 310) that the process given in Eq. (68) was second order in monomer and first order in active centers, which are not consumed in the reaction. The sequence shown as Eq. (69) was found to be first order in active center concentration and in the dimer which is the product of Eq. (68). 

The ratio of the reactivities of ions and ion pairs (kp+/kp ) are also included in Table 13. They were determined from kinetic studies of the apparent rate constants at either different acid concentrations which vary the extent of dissociation into free ions, or in the presence of tetrabutylam-monium salts with common counteranions such as perchlorates and triflates. This results in ratios of the reactivities of ions and ion pairs of approximately 6 to 24. However, addition of an equimolar amount of salt to triflic acid may lead conjugation of acid with anions [215], with complete deactivation of the system. Therefore, the lower rate constants of propagation for ion pairs may be partially due to removal of the acid from the system. Thus, the values reported in Table 13 can be considered the upper limit of kp+/kp. The true ratio might be lower, with very similar reactivities for ions and ion pairs as in model systems [4]. Miscalculations of the ratio of reactivities of ions and ion pairs has led to unrealistic values of activation parameters calculated for propagation by ions (Ep = 51 kJmol-, ASP = +54 Jmol- K-1) and ion pairs (Ep = 21 kJmol- ASP = -84-mol-,-K l) [17] the latter values are similar to the overall activation parameters for ionic propagation and are quite reasonable. Extrapolation of Kunitake s data to - 80° C shows ion pairs being 30 times more reactive than ions [17], which contradicts the available experimental data [213]. 

Catalyst deactivation by coke formation can occur through a more or less reversible mechanism. We have applied a transient approach to model the reversible behavior of the deactivation, and to separate the deactivation from the main reaction kinetics. The deactivation of a Pt-Sn/AbOs catalyst was studied during propane dehydrogenation. The gas composition and temperature were varied during the experiments, which allowed us to model the deactivation by assuming one reversible and one irreversible type of coke. It was found that the deactivation increased with the propene concentration but was independent of the partial pressure of propane. Hydrogen decreased the deactivation rate and could even activate the catalyst by removing reversible coke. 

Additionally, in a microreactor the intrinsic kinetics and deactivation behavior of SCR catalysts is studied with flows up to 1.5m h . In both test facilities it is possible to vary all process parameters temperature, the ammonia to nitric oxide feed ratio, the nitric oxide and sulfur dioxide concentrations, the space velocity, and the catalyst geometry. These techniques provide information for somewhat small areas and therefore should always be performed to complement bench- or laboratory-scale activity and selectivity measurements. 

A kinetic study of the acylation of phenol with phenyl acetate was carried out in liquid phase at 160°C over HBEA zeolite samples, sulfolane or dodecane being used as solvents. The initial rates of hydroxyacetophenone (HAP) production were similar in both solvents. However the catalyst deactivation was faster in dodecane, most likely because of the faster formation of heavy reaction products such as bisphenol A derivatives. Moreover, sulfolane had a very positive effect on p-HAP formation and a negative one on o-HAP formation. To explain these observations as well as the influence of phenol and phenyl acetate concentrations on the rates of 0- and p-HAP formation it is proposed that sulfolane plays two independent roles in phenol acylation solvation of acylium ions intermediates and competition with phenyl acetate and phenol for adsorption on the acid sites. Donor substituents of phenyl acetate have a positive effect on the rate of anisole acylation, provided however there are no diffusion limitations in the zeolite pores. 

In a kinetic study, the reaction was found to be first order in MVN and HCN over concentration ranges below 0.04 M in each reagent. This saturation kinetics im-pHes that the catalyst resting state shifts from Ni-[la]-(COD), 5 (Scheme 5), to either 8 or 9. Based on the known stabihty of the 18-electron allylic hydrocyanation intermediates (vide supra) and the exclusive regioselectivity of this reaction, we beheve that complex 9 is the catalyst resting state under most hydrocyanation conditions. Under these saturation conditions, a maximum activity of 2000 tuxnovers/h (turnover=mol of nitrile/mol of nickel) was observed for the Hgand la. One of the minor comphcations of the reaction is the catalyst deactivation which removes Ni(0) from the system by an oxidative addition of HCN to form Ni(CN)2. A practical consequence of this side reaction is that the catalyst life time is reduced to 700-800 turnovers, unless a fresh supply of Ni(COD)2 is introduced into the medium. 

Consider cases (a) and (b) of problem 8. Some separate studies of deactivation in this system indicate that the rate of deactivation is independent of the concentration of reactants or products, but is proportional to the second power of activity itself, with an half-life of 20 h. (Second-order kinetics, independent of concentrations, are often a sign of decay via sintering of the metallic component of the catalyst—here Pt on AI2O3). 

Furthermore, when the coke itself is not determined, only one deactivation function can be derived, from the decay with time of the main reactioa The model may then be biased. There is more, however. Since coke content, which is related to the local concentration of the reacting species, it predicts a deactivation independent of concentration that is, the approach predicts a uniform deactivation in a pellet or a tubular reactor (e.g., for isothermal conditions at least). In reality, nonunifonnity in deactivation, because of coke profiles, does occur in pellets (or tubular reactorsX as will be shown in the next section. The consequences of neglecting coke profiles in kinetic studies, in catalyst regeneration, or in design calculations may be serious (see Froment and Bischoff [12, 13]). 

Hofmann and coworkers (327-330) have reported a series of studies on the deactivation kinetics for the heterogeneously catalyzed disproportionation of ethyl benzene to benzene and diethyl benzene under SCF conditions. Kinetic studies have been conducted in both a loop reactor using a protonated Y-faujasite (Z-14) catalyst (327) and in a continuous concentration-controlled recycle reactor using an HY-zeolite (HYZ) (329,330) and USY-zeolite, H-ZSM-5, and H-mordenite (328) under supercritical conditions T > 373 C, P > A5 bar). Coke extraction by SCFs was found to be strongly dependent on the type of catalyst used, and the Lewis acid centers were determined to play an important role in the coke formation and activity of the catalysts. A simple kinetic model for the catalyst deactivation was proposed (329) for SCF conditions and high ethyl benzene concentration. Based on the relatively high estimated deactivation energy of about 147 kJ/mol and first-order deactivation, the authors concluded that the catalyst deactivates much slower under SCF conditions than under atmospheric pressure. 

Andre et al. performed kinetic studies on the polymeriza-tion of DEAAm in THF in the presence of triethylaluminum at -78 °C. The kinetics of this process is very complex. It involves two equilibria activation of monomer and deactivation of chain-ends by EtsAl. In addition, EtsAl interacts with the monomer amide groups and with THF. All these effects are in a delicate balance that depends on the ratio of the concentrations of EtsAl, monomer, and chain-ends. However, the initiator or blocking efficiencies of these systems remained low (f<0.70). Quantum-chemical calculations on up to trimeric models confirm the various equilibria involved."" EtsAl-coordinated, solvated unimers are the most stable species in the presence of EtsAl, whereas unimers and dimers coexist in the absence of ligand. 

The kinetics of a particular catalytic reaction A R are studied at temperature T in a basket reactor (batch-solids and mixed flow of gas) in which the gas composition is kept unchanged, despite deactivation of the catalyst. What can you say about the rates of reaction and deactivation from the results of the following runs Note, to keep the gas concentration in the reactor unchanged the flow rate of reactant had to be lowered to about 5% of the initial value. 

High conversion with an optimal reaction rate [7, 11, 75, 95], increase of the turnover numbers, i.e., the moles of substrate converted per mole of enzyme deactivated [3, 75, 95], and high stereospecificity of the compound of interest are targets of particular interest in the operation of these batch reactors [10,11,48, 77]. The achievement of these goals requires the study of different variables type and concentration of peroxide, substrates and cofactors, enzyme activity and purity, composition of the reaction medium, pH, temperature, or agitation. Such optimization requires a deep knowledge of the system and a mathematical model that represents it satisfactorily. The kinetic model obtained in batch experiments is the... 

Although the above-discussed studies have defined sulfur-poisoning tolerances for conventional nickel catalysts used in steam reforming of natural gas and naptha, they have not considered in sufficient detail the kinetics of poisoning at above-threshold concentrations nor the effects of catalyst and/or gas compositions on rate of deactivation and tolerance level. Nor is there any previous report on the effects of sulfur on product distribution (i.e., relative rates of production of H 2, CO, CH4) in steam reforming of hydrocarbons. 

Bimolecular deactivation (pathway vii, Fig. 1) of electronically excited species can compete with the other pathways available for decay of the energy, including emission of luminescent radiation. Quenching of this kind thus reduces the intensity of fluorescence or phosphorescence. Considerable information about the efficiencies of radiative and radiationless processes can be obtained from a study of the kinetic dependence of emission intensity on concentrations of emitting and quenching species. The intensity of emission corresponds closely to the quantum yield, a concept explored in Sect. 7. In the present section we shall concentrate on the kinetic aspects, and first consider the application of stationary-state methods to fluorescence (or phosphorescence) quenching, and then discuss the lifetimes of luminescent emission under nonstationary conditions. 

Kinetics of Catalyst Deactivation. In order to study the kinetics of the deactivation of stabilized catalyst, we carried out several sets of experiment varying pressure, with constant space velocity and with constant contact time, respectively. We assumed that reaction rate of light naphtha conversion conforms to first-order kinetics with respect to light naphtha concentration and that the decreasing rate of active site, which is caused by coke deposition, is expressed by first order. Then catalyst activity is described as exponential deactivation (S). 

Actually, studies on the propylene polymerization at atmospheric pressure carried out in our laboratories 101 > have demonstrated that R0 and the deactivation rate depend, in a complex manner, on both the organoaluminum and external donor concentrations (see Sect. 6.1.2 and 6.1.3). The kinetic curves obtained cannot be reduced to a single model for the deactivation of active centers according to a simple 1 st and 2nd order law, but rather they seem to follow a more complicated behavior. This is not surprising if one considers that the decay of polymerization rate is probably the effect of an evolution, in time, of a plurality of different catalytic species having different stability, reactivity and stereospecificity (see Sect. 6.3). 

Deactivation Kinetics A separable form of rate equation was used for the poisoning reaction, first order with poison concentration and active sites, respectively. Similar forms have been used elsewhere (Richardson, 1971 Baiker et al -, 1966 Zrncevic and Gomzi, 1983) From previous studies done on the thiophene... 

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