Decay of catalysts

In reactions involving oxygen and hydrocarbons, the catalyst surface is necessarily reduced to some extent, due to the steady-state occurrence of reduction and reoxidation processes. The above results suggest that ideally, the surface should be as dose as possible to full oxidation. The main objective of the present contribution is to understand why donors of spillover oxygen can keep the surface in a higher oxidation state and thus protect selective sites on the surface and prevent the decay of catalysts to form segregated phases... 

The curves shown in Fig.7 indicate that Cc in the riser inlet zone increases sharply as feedstock Qr increases, causing a rapid decay of catalyst activity. The sharpness of catalyst decay becomes less significant as Cc or tct further increases. There is little difference in a at the riser outlet among different feedstocks. Although has less impact on catalyst activity than Qal, the conversion of nesid oil cracking usually becomes lower than that of VGO cracking because of the rapid building-up of the on the catalyst and quick loss of catalyst activity in a riser zone. 

Various investigators have tried to obtain information concerning the reaction mechanism from kinetic studies. However, as is often the case in catalytic studies, the reproducibility of the kinetic measurements proved to be poor. A poor reproducibility can be caused by many factors, including sensitivity of the catalyst to traces of poisons in the reactants and dependence of the catalytic activity on storage conditions, activation procedures, and previous experimental use. Moreover, the activity of the catalyst may not be constant in time because of an induction period or of catalyst decay. Hence, it is often impossible to obtain a catalyst with a constant, reproducible activity and, therefore, kinetic data must be evaluated carefully. 

The ageing and decay characteristics of catalysts are of immense importance in defining the economics of processes. The simplest criterion that can be applied is that of total productivity during the life of the catalyst and also loss of productivity during the shut down required for catalyst replacement. Figure 2 illustrates notional performances for two catalysts A and B in hypothetical processes in which productivity is simply a measure of quantity of product produced. Catalyst A has a lower initial productivity but is more stable in use and dies off at a much lower rate than catalyst B, which has a high initial productivity which falls relatively... 

Carboxylic acids, ketonization of, 35-37 Catalysts, see also specific substances activity of, 160, 161 aging and decay of, 228-230 cost per ton of product, 224, 233 for coupled heterogeneous reactions, 26-28... 

When the amount of coke formed as a function of time on stream is compared to the decrease in catalytic activity (see Fig. 3), two regimes of deactivation can be noticed for the strongly deactivating catalysts, i e, a slow initial deactivation which is followed by a rapid loss of activity This first phase is characteristic of a slow transformation of the reactive carbon into less reactive coke. The second phase is attributed to carbon formed on the support which accumulates there and rapidly covers the Pt particles when its amount reaches a critical value causing the sudden decay of catalytic activity. 

Finally, experimental procedures differing from that described in the preceding examples could also be employed for studying catalytic reactions by means of heat-flow calorimetry. In order to assess, at least qualitatively, but rapidly, the decay of the activity of a catalyst in the course of its action, the reaction mixture could be, for instance, either diluted in a carrier gas and fed continuously to the catalyst placed in the calorimeter, or injected as successive slugs in the stream of carrier gas. Calorimetric and kinetic data could therefore be recorded simultaneously, at least in favorable cases, by using flow or pulse reactors equipped with heat-flow calorimeters in place of the usual furnaces. 

As mentioned above one of the fundamental attributes ascribed to homogeneous catalysts is superior activity at low temperature. However, even within classes of such catalysts, improvements in catalyst activity can be made allowing operation at lower temperatures, thus reducing or avoiding completely this mode of catalyst decay. One such example can found in recent advances in palladium catalysed ethene carbonylation (Equation 1.1). 

In the case of cobalt ions, the inverse reaction of Co111 reduction with hydroperoxide occurs also rather rapidly (see Table 10.3). The efficiency of redox catalysis is especially pronounced if we compare the rates of thermal homolysis of hydroperoxide with the rates of its decomposition in the presence of ions, for example, cobalt decomposes 1,1-dimethylethyl hydroperoxide in a chlorobenzene solution with the rate constant kd = 3.6 x 1012exp(—138.0/ RT) = 9.0 x 10—13 s—1 (293 K). The catalytic decay of hydroperoxide with the concentration [Co2+] = 10 4M occurs with the effective rate constant Vff=VA[Co2+] = 2.2 x 10 6 s— thus, the specific decomposition rates differ by six orders of magnitude, and this difference can be increased by increasing the catalyst concentration. The kinetic difference between the homolysis of the O—O bond and redox decomposition of ROOH is reasoned by the... 

Acids are well known as efficient catalysts of various heterolytic reactions (hydrolysis, esterification, enolyzation, etc. [225,226]). They catalyze the heterolytic decay of hydroperoxides formed during oxidation. For example, they catalyze the decomposition of cumyl hydroperoxide into phenol and acetone (important technological reaction) [5]. 

The formation and decay of these product-catalyst-7i-complexes are expected to occur according to the sequence of reactions as outlined in Scheme 12.4. The kinetic constants associated with the occurrence of kHYD and the decay of k0FF> respectively, can both be determined by PHIP-NMR using a process termed dynamic PASADENA (DYPAS) spectroscopy, as has been outlined previously [37]. For this purpose the addition of parahydrogen to the reaction is synchronized with the pulse sequences of the NMR spectrometer, whereby the time for acquiring the NMR spectra is delayed by variable amounts. The results thereof are listed in Table 12.3. A variety of kinetic constants can be determined, and the method is reasonably accurate the margins of error are also indicated in Table 12.3 [37]. 

Table 12.3 Rates of formation and of decay of the interim product-catalyst-re-complexes [44].

Mossbauer Measurements. Co-Mo catalysts cannot be studied directly in absorption experiments since neither cobalt nor molybdenum has suitable Mossbauer isotopes. However, by doping with 57Co the catalysts can be studied by carrying out Mossbauer emission spectroscopy (MES) experiments. In this case information about the cobalt atoms is obtained by studying the 57Fe atoms produced by the decay of 57Co. The possibilities and limitations on the use of the MES technique for the study of Co-Mo catalysts have recently been discussed (8., 25.). 

The plot of residuals versus some measure of the time at which experiments were run can also be informative. If the number of hours on stream or the cumulative volume of feed passed through the reactor is used, nonrandom residuals could indicate improper treatment of catalyst-activity decay. In the same fashion that residuals can indicate variables not taken into account in predicting reaction rates, variables not taken into account as affecting activity decay can thus be ascertained. 

While immobilised polyamino acids can be recovered from a reaction and re-used in subsequent operations, it has been found that after repeated recycling the quality of the catalyst declines, resulting in increased reaction times and reduced stereoselectivity. The quality of the catalyst declines particularly quickly when it is used in the recently developed biphasic epoxidation conditions (see Sect. 4.1.2). This gradual decay of the polyamino acid catalyst led to the development of a regeneration procedure. 

Influence of catalyst load on the degradation of B02 solution. The profile for the color decay of 0.1 mM B02 solution by 0.01 g of C2-Ms at pH 3, in the absence of H2O2, is shown in figure 22. It is observed a 57% of color removal by adsorption after 5 h (but no equilibration was yet reached) whereas the solution was effectively colorless when 4 mM H2O2 was present under similar experimental conditions, as depicted in figure 23 (plot A). 

The long lifetimes and high redox potentials of a range of ruthenium(II) complexes and in particular [Ru(bpy)3] " have important consequences for their use as photoactive redox catalysts. This area of research is extremely active and we now focus on the decay of the excited state of [Ru(bpy)3] + ( [Ru(bpy)3] " ) and its quenching. Braterman et al. have described the electronic absorption spectrum and structure of the emitting state of [Ru(bpy3] +, and the effects of excited state asymmetry. The effects of solvent on the absorption spectrum of [Ru(bpy)3] " have been studied. In H2O, MeCN and mixtures of these solvents, the value of e(450 nm) remains the same ((4.6 0.4) x 10 dm mol cm ). The ground state spectrum is essentially independent of... 

We will use the term deactivation for all types of catalyst decay, both fast and slow and we will call any material which deposits on the surface to lower its activity a poison. 

Pore Diffusion. For a pellet, pore diffusion may strongly influence the progress of catalyst decay. First consider parallel deactivation. From Chapter 18 we know that reactant may either be evenly distributed throughout the pellet Mj < 0.4... 

Additional Factors Influencing Decay. Numerous other factors may influence the observed change in activity of catalyst. These include pore mouth blocking by deposited solid, equilibrium, or reversible poisoning where some activity always remains, and the action of regeneration (this often leaves catalyst with an active exterior but inactive core). 

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