Physical deactivation effects

A sulfur moiety introduces a hydroperoxide deactivating effect into the stabilizer molecule. Moreover, the condensation of low molecular weight compounds with halides of sulfur represents an easy synthetic approach to the increase of the molecular weight and consequently to the improvement of the physical persistency of stabilizers. Efficient AO were prepared by this way from alkylphenols, aromatic amines or phenothiazine [21,174]. Most of them contain two active units connected with a sulfide/disulfide bridge and should be listed rather among high molecular weight stabilizers. Antioxidant 136 (n > m), prepared from pyrocatechol and... 

Other work has shown that Pt Rh are more S resistant than Pd [3,13], and also more Pb resistance than Pd [6,12-14], Our hypothesis proposes that Pb will form Pb-S-Pd compounds which deactivate the Pd activity, during aging. Because the NOx reduction is greatly dependent on the Pd dispersion, and is effective only when Pd particles or the available Pd surface active sites are close to each other, any steric hinderance generated on the Pd surface that would block the N-N bond formation will compromise the NOx reduction activity. When the Pb-S-Pd compound is formed, illustrated hypothetically below, the closeness of two NOx molecules required so that the N-N bond can be formed (N0+N0->N>+02) is no longer feasible. Consequently, the NOx reduction function of the Pd catalyst is hindered by the physical steric effect. Since neither HC or CO oxidations require formation of a C-C bond, or the proximity of two same molecules, to complete their reactions, the Pb-S-Pd compound intereference is therefore less predominant. 

While the understanding of the other modes of physical deactivation is quite limited, certain useful conclusions can be made on the effect of physical deactivation on the catalyst activity as related to reactor design, provided that the physical deactivation takes place uniformly throughout the pellet. Therefore, this general case will be treated first before proceeding to sintering. 

Physical Deactivation and Sintering 225 Table 6.4 Reactor Point Effectiveness Uniform Sintering te = k/(0... 

The intrinsic rate of a catalytic reaction measured using fine powders is 10" mole/(min volume of powder). These fine powders are compressed into pellets and the observed rate changes to one tenth of the intrinsic rate under identical reaction conditions. After 100 hrs of use, the observed rate is 5 X 10" mole/(min volume). What is the ratio of active catalyst surface area to the initial active surface area Is the assumption of negligible effects of physical deactivation on the concentration profile justified If so, what is E/En ... 

Other groups such as esters, silylethers, and imides are also successfully incorporated through ADMET depolymerization with 14 (Fig. 8.21).49 For an ester functionality, at least two methylene spacer units must be present between die olefin site and die functional group in order to achieve depolymerization. This is due to die negative neighboring group effect, a deactivation of the catalyst by coordination of the functionality heteroatoms to die catalyst.50 By physically... 

Some deactivation processes are reversible. Deactivation by physical adsorption occurs whenever there is a gas-phase impurity that is below its critical point. It can be reversed by eliminating the impurity from the feed stream. This form of deactivation is better modeled using a site-competition model that includes the impurities—e.g., any of Equations (10.18)-(10.21)— rather than using the effectiveness factor. Water may be included in the reaction mixture so that the water-gas shift reaction will minimize the formation of coke. Off-line decoking can be... 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

Once the two salts are mixed in solution (acetone is a common solvent for this), the sodium chloride precipitates and is removed by filtration. The solvent is then removed under reduced pressure and, since salts have no vapour pressure, the ionic liquid remains in the flask. The problem with this reaction is that it is almost impossible to remove the last traces of chloride ions. The chloride not only influences the physical properties of the liquid such as melting point and viscosity, but is also a good nucleophile and can deactivate catalysts and affect reproducibility. A great deal of effort has been directed towards removal of the chloride contamination, including washes and chromatography, but none have proved to be completely effective [9], This has led to the development of some alternative synthetic routes. Simply exchanging Na[BF4]... 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

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