Activity, catalyst decay

The vanadium-based catalyst systems deteriorate with time and decrease in the number of catalytic centers as the polymerizations progress. The rate of decay is affected by conditions used for catalyst preparation, compositions of the catalysts, temperature, solvents, and Lewis bases. It is also affected by the type and concentration of the third monomer. Additions of chlorinated compounds to the deactivated catalysts, however, help restore activity. Catalyst decay can also be overcome by continually feeding catalyst components into the polymerization medium. ... 

In addition, steady-state polymerization conditions are required in order to determine meaningful kinetic data from an experimental catalyst such as catalyst activity, catalyst decay rates and catalyst reactivity ratios for comonomers such as 1-butene, 1-hexene and 1-octene. 

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. 

For all samples, a strong dependence of the temperature of pretreatment on the initial activity and on the catalyst decay was observed. For example, the initial conversion of CS1.9P pretreated at 473 K, was reduced by about one half after a pretreatment at 573K and the deactivation was more pronounced. 

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). 

The sterically encumbered R2 substituents give steric protection to the oxygen-donors that are attached to the metal centers from coordination with Lewis acids such as MAO, or from another molecule of the catalytically active cationic species, which are supposed to be highly electrophilic. The coordination increases steric congestion near the polymerization center, which at least hampers ethylene coordination to the metal. Even worse, it may cause catalyst decay by, for instance, loss of the ligand. 

Moreover, the catalyst deactivation must also be considered in order to use these solid materials in industrial processes. Figure 13.8 shows the variation of catal54ic activity (2-butene conversion) with the time on stream obtained under the same reaction conditions on different solid-acid catalysts. It can be seen how all the solid-acids catalysts studied generally suffer a relatively rapid catalyst deactivation, although both beta zeolite and nafion-sihca presented the lower catalyst decays. Since the regeneration of beta zeolite is more easy than of nafion, beta zeolite was considered to be an interesting alternative. ... 

Reaction conditions permitting a catalyst to pass through many catalytic rounds. Multiple-turnover conditions are usually obtained by maintaining the substrate concentration in excess over the concentration of active catalyst. This technique usually allows one the opportunity to evaluate the catalytic rate constant ka,t, which is the first-order decay rate constant for the rate-determining step for each cycle of catalysis, and one can evaluate the magnitude of other parameters such as the substrate s dissociation constant or Michaehs constant. 

Aramendia et al. (22) investigated three separate organic test reactions such as, 1-phenyl ethanol, 2-propanol, and 2-methyl-3-butyn-2-ol (MBOH) on acid-base oxide catalysts. They reached the same conclusions about the acid-base characteristics of the samples with each of the three reactions. However, they concluded that notwithstanding the greater complexity in the reactivity of MBOH, the fact that the different products could be unequivocally related to a given type of active site makes MBOH a preferred test reactant. Unfortunately, an important drawback of the decomposition of this alcohol is that these reactions suffer from a strong deactivation caused by the formation of heavy products by aldolization of the ketone (22) and polymerization of acetylene (95). The occurrence of this reaction can certainly complicate the comparison of basic catalysts that have different intrinsic rates of the test reaction and the reaction causing catalyst decay. 

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. 

This observation was not so obvious on coke yields because the coke production is a contribution of mnltiple mechanisms and reactions. Thus, the coke yields are quite similar, probably because the catalytic coke is decreased while the contaminant coke is increased. The coke remarks are also observed on the CPS samples taking into account that the dehydrogenation degree is not strongly affected by the extended ReDox cycles, becanse the lower catalysts decay is limiting the effect of the required mass of catalyst (C/0 ratio). Thus, the small decrement of the coke yield on the CPS samples is possibly related to the descent of the catalyst (less specific area) leaving less available space for coke adsorption and less activity for catalytic coke production. It is clear that prolonging the deactivation procednres is not beneficial as far as the metal effects are concerned. 

If catalyst decay over the reaction time can be seen as a decreasing number of active sites, then often a first-order rule holds true... 

There is a complex and little understood relationship between coke content, catalyst activity, and the chemical nature of the coke. For instance, the H/C ratio of coke depends on how the coke was formed its exact value will vary from system to system (Cumming and Wojciechowski, 1996). And it seems that catalyst decay is not related in any simple way to the hydrogen-to-carbon atomic ratio of the coke, or to the total coke content of the catalyst, or any simple measure of coke properties. Moreover, despite many and varied attempts, there is currently no consensus as to the detailed chemistry of coke formation. There is, however,... 

Fig. 5.12 Catalyst decay experiments effect of water and sulfur on the isomerization activity of the new sulfated zirconia formulation (SZNbPt) and conventional sulfated zirconia (ZS).

Wojdechowski, B.W. (1998) The reaction mechanism of catalytic cracking quantifying activity, selectivity, and catalyst decay. Catal. Rev. — Sci. Eng., 40, 209. 

Potential pitfalls exist in ranking catalysts based solely on correlations of laboratory tests (MAT or FFB) to riser performance when catalysts decay at significantly different rates. Weekman first pointed out the erroneous conversion ranking of decaying catalysts in fixed bed and moving bed isothermal reactors (1-3). Phenomena such as axial dispersion in the FFB reactor, the nonisothermal nature of the MAT test, and feedstock differences further complicate the catalyst characterization. In addition, differences between REY, USY and RE-USY catalyst types exist due to differences in coke deactivation rates, heats of reaction, activation energies and intrinsic activities. 

An important conclusion from these simulations is that neither the FFB nor the MAT test simulates riser behavior, since neither of these tests reflects the intrinsic catalyst activity and decay. These intrinsic parameters are masked by backmixing in the FFB test and by a high temperature drop in the adiabatic MAT unit. In addition, the time averaged values of the long contact time FFB and MAT tests overemphasize the low catalyst activity at long contact times, while the short contact time riser is only exposed to initial activity and catalyst decay. 

In spite of the presence of Nd-clusters, partial alkylation and micro heterogeneities the number of active Nd-species seems to be fairly constant during the course of a polymerization. Otherwise neither consistent polymerization kinetics (particularly lst-order monomer consumption up to high monomer conversion) nor linear increases of molar mass during the whole course of the polymerization would be observed in so many studies. It therefore can be concluded that the fraction of active Nd as well as the number of active catalyst species are fixed either at an early stage of the polymerization or even prior to initiation of the polymerization. It can be speculated whether the fixation of the number of active species occurs during catalyst prefor-mation/activation or even during the preparation of the Nd compound. In contrast to this consideration Jun et al. report on the decay of active cen-... 

Corma el al. (126) found that PtNaY was an active but rather unstable catalyst for methylcyclohexane dehydrogenation to toluene. These workers studied both the dehydrogenation and the catalyst decay kinetics. It was concluded that the reaction occurs via a series of consecutive partial dehydrogenation steps, the first of which was rate determining. Further, catalyst deactivation was caused by coke deposition from partially unsaturated precursor molecules. 

Bartholomew and co-workers also measured the loss of catalytic activity with time of Ni and Co bimetallics (157, 194), Ni-molybdenum oxide (23, 113), and borided Ni and Co catalysts (161) during methanation in the presence of 10 ppm H2S. Typical activity versus time plots are shown in Figs. 25 and 26. Activity is defined as the ratio of the mass-based rate of methane production at any time t divided by the initial rate. The activitytime curves are generally characteristic of exponential decay some catalysts decay more slowly than others, but all catalysts suffer at least two orders of magnitude loss in activity within a period of 100-150 hr. Accordingly, it does not appear that other metals or metal oxides in conjunction with Ni significantly change the sulfur tolerance defined in terms of steady-state activity of Ni. These materials can, however, influence the rate at which the... 

The rapid loss of catalyst activity in catalytic cracking has had a profound influence on the engineering and commercialization of cat cracking. In this work we propose a plausible chemical mechanism for catalyst decay and derive an expression for the kinetics of the decay process based on this mechanism. 

From these considerations we can see an outline of the kinetics and mechanism of catalyst decay. While the catalyst remains in the presence of the reactant-product stream, on each active site the processes which dominate are the "fiuitful" processes of the attached carbenium ions, involving protolysis, P-cracking, disproportionation, and the reversible adsorption-desorption of product olefins. These events, in combination, constitute the dKiin mechanism of cracking 4) and yieldthe major products of the "cracking" reactioa None of these processes results in an irreversible reduction of catalyst activity, although the various carbenium ions present will undergo various mainline reactions at different rates. 

Using this picture of the chemical events involved in catalyst decay, we can quantify the consequences of such behaviour without any prior knowledge of the activity of each of the many species which may be formed in this way. To do this, we use the concept and the mathematics of Markov chains, a mathematical construct which allows for the wide variety of processes described above to be systematized and considered in quantitative terms. 

Nevertheless, it is possible that such analytical fittings of the catalyst decay curve are too oversimplified to take into account the complexity of the phenomena which take place during polymerization. On the other hand, the kinetic studies are only able to measure the average constants of the reaction and not those for each individual species. Thus, although the mechanism of deactivation of the active centers, or part thereof, has clearly been shown to be of a chemical nature, it can only be explained in hypothetical terms. In agreement with the 2nd order decay law they had proposed, Keii and Doi98 99) speculated on a bimolecular disproportionation of the active species with a consequent reduction of Ti3+ to Ti2 1 due to the action of the cocatalyst. 

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