Catalyst behavior, activation temperature

No differences in operability and catalyst behavior (activity and deactivation) in the two plants were discernible. The expected catalyst lifetime in a commercial plant, calculated from the movement of the temperature profile down the catalyst bed with time, in both cases will be more than 16,000 hrs under the design conditions. 

Another part of our investigation deals with the effect of heat treatment on the leaching behavior of palladium on activated carbon catalysts. Heat treatment is a known technique to increase the performance of catalysts. (3) Therefore, standard carbon supported palladium catalysts were exposed to different temperatures ranging from 100 to 400 °C under nitrogen. The catalysts were characterized by metal leaching, hydrogenation activity and CO-chemisorption. 

In addition, we investigated a nonlinear-like effect (NLE), activity, temperature dependence, and kinetics of hydroxy[2.2]paracyclophane ketimine ligands with the 1,2-addition reaction of diethylzinc to cyclohexylcarbaldehyde. A linear correlation between the enantiomeric excess of AHPC ketimine ligands bearing a phenylethyl side group and the product was observed with 0.5 mol% of catalyst loading. When the catalyst loading of (Sp,S)/(Rp,R)-4a was increased to 4 mol%, a precipitate of the inactive heterochiral species was formed and resulted in a positive nonlinear like effect (Fig.2.1.3.6), while a linear behavior is observed with 5b (Fig. 2.1.3.7). The enantiomeric ratio was found to have linear temperature dependence. [16]... 

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. 

This supramolecular dendritic assembly serves as a valuable soluble model for the interaction of perfluoro-tagged catalysts with insoluble supports such as fluorous silica gel and clearly reveals the ligand diffusion from the complex at elevated temperatures. This behavior can also explain the high catalytic activity of the heterogeneous FPS system. 

Figure 4 shows the variation of surface acid-base properties of various Pt/C catalysts, reflected by the pHzpc, as a function of the activation temperature. The observed behavior is qualitatively similar to that reported in Figure 3. Figure 5 shows that an increase of acidic or basic surface groups (Ca, Cb) produces a corresponding shift of the pHzpc toward acidic or basic regions, respectively. Thus, the pHzpc clearly reflects the concentration of surface functional groups. 

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. 

Whatever the mechanism, why should the behavior of the catalyst be so dependent on the activation temperature From about 400 C to 900 C, where both activity and RMIP dramatically increase, the only significant change on the silica known to occur is the declining silanol population. Perhaps these hydroxyls coordinate to the active centers, blocking ethylene and thus poisoning the catalyst much as free water would do in the reactor In some detailed experiments Krauss et.al. " have indeed found, by measuring the amount and aH of chemisorption by CO, N2, and O2, an inverse correlation between the coordinative unsaturation of Cr(II) centers and the surrounding liydroxyl population. 

The behavior shown in Figure 28, a leveling of catalyst activity with increased chromium loading, could be viewed as a consequence of mass transport limitations. However, the activity can still be increased or decreased according to other preparation variables and reaction conditions. For example, activity is improved by increasing the activation temperature, by the addition of cocatalyst, by the reduction of Cr(VI) in CO, or by increasing the ethylene concentration in the reactor. The activity can also be lowered by poisons. 

The presence of titania also changed the dependence of catalyst performance on activation temperature. As shown in Figures 41 and 43, the amount of each site type on Cr/silica was found to vary with temperature, but not the character of each site itself. That is, raising the activation temperature did not produce new site behavior, but only redistributed the population of chromium within the same site types. When titania is added, however, this statement no longer holds true the site character does change with temperature. 

The data of Table 55 show how the polymer composition varied with activation temperature. Such observations have been reported from this and other laboratories for catalysts made with several different organo-chromium compounds [301,640,644,654], and most recently by Bade et al. [311], who used chromium allyl to make their catalyst. Presumably, the calcination temperature of the silica resulted in the formation of two very different species. Cr(DMPD)2 reacted with silica treated at 250 and at 400 °C to yield di-attached or coordinated species, whereas it reacted differently with silica treated at 600 °C, because on that support only a single oxide attachment can form. Clearly, the higher OH group population has a major effect on the behavior of the site. 

Figure 2 shows transient activity data for NO reduction by CO for two 4% rhodium sil ica/catalysts, one of which was reduced at 200°C and the other which was reduced at 300°C. As can be seen, there is virtually no difference at all at any point in time in the activity of the two catalysts. Thus, once again reduction temperature does not seem to effect the catalyst behavior at Least in this temperature range. 

For the Zinc-23 catalyst, low carbon balances were observed. This suggests the possibility that carbon was deposited on the catalyst during the reaction, potentially contributing to a loss of activity. Temperature-programmed oxidation from 75 to 430 C was performed at the conclusion of the reaction, and significant desorption of CO2 was observed. Afterwards, the OMR reaction was re-started and the catalyst resumed its activity before this oxidation treatment after a short transient. Similar behavior was observed for the Zirconia-41 catalyst, which did not have low carbon balances. 

At this time it had become possible to determine experimentally total surface area and the distribution of sizes and total volume of pores. Wheeler set forth to provide the theoretical development of calculating the role of this pore structure in determining catalyst performance. In a very slow reaction, reactants can diffuse to the center of the catalyst pellet before they react. On the other hand, in the case of a very active catalyst containing small pores, a reactant molecule will react (due to collision with pore walls) before it can diffuse very deeply into the pore structure. Such a fast reaction for which diffusion is slower than reaction will use only the outer pore mouths of a catalyst pellet. An important result of the theory is that when diffusion is slower than reaction, all the important kinetic quantities such as activity, selectivity, temperature coefficient and kinetic reaction order become dependent on the pore size and pellet size with which a pellet is prepared. This is because pore size and pellet size determine the degree to which diffusion affects reaction rates. Wheeler saw that unlike many aspects of heterogeneous catalysis, the effects of pore structure on catalyst behavior can be put on quite a rigorous basis, making predictions from theory relatively accurate and reliable. 

These complexes showed higher thermal stabihty in toluene at 80 °C than the Hoveyda first-generation catalyst, with half-lives ranging from 3 to 6 h, depending on the nature of the Schiff base-derived hgand. They also showed latent catalyst behavior, as only moderate-to-low olefin metathesis activity was observed at room temperature in CM and RCM [44]. On the other hand, these complexes were active in the ROMP of cyclooctene and cyclopentene. The NHC-containing catalyst was found to be especially efficient, leading to a TOP of 667 min at room temperature [43]. 

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