Catalyst poisoning temperature effect

Siace nitroarenes are reported to be catalyst poisons (18), the concentration of DNT ia the reaction medium is kept as low as is practical with regard to production goals and catalyst usage. The pubHshed kinetic studies are of Htde iadustrial value siace they describe batch processes with high DNT catalyst ratios (18—21). The effects of important process variables, such as temperature and pressure, can only be iaferred from descriptions ia the patent Hterature. 

Platinum and rhodium sulfided catalysts are very effective for reductive alkylation. They are more resistant to poisoning than are nonsulfided catalysts, have little tendency to reduce the carbonyl to an alcohol, and are effective for avoidance of dehydrohalogenation in reductive alkylation of chloronitroaromatics and chloroanilines (14,15). Sulfided catalysts are very much less active than nonsulfided and require, for economical use, elevated temperatures and pressures (300-2(KX) psig, 50-l80 C). Most industrial reductive alkylations, regardless of catalyst, are used at elevated temperatures and pressures to maximize space-time yields and for most economical use of catalysts. 

Behm RJ, Jusys Z. 2006. The potential of model studies for the understanding of catalyst poisoning and temperature effects in polymer electrolyte fuel cell reaction. J Power Sources 154 327-342. 

Special care has to be taken, however, that the quinoline titer truly represents the minimum amount of catalyst poison. In most cases this type of base is adsorbed by inactive as well as active sites. Demonstration of indiscriminate adsorption is furnished by the titration results of Roman-ovskii et al. (52). These authors (Fig. 13) showed that introduction of a given dose of quinoline at 430°C in a stream of carrier gas caused the activity of Y-zeolite catalyst (as measured by cumene conversion) to drop with time, reach a minimum value, then slowly rise as quinoline was desorbed. The decrease in catalytic activity with time is direct evidence for the redistribution of initially adsorbed quinoline from inactive to active centers. We have observed similar behavior in carrying out catalytic titrations of amorphous and crystalline aluminosilicates with pyridine, quinoline, and lutidine isomers. In most cases, we found that the poisoning effectiveness of a given amine can be increased either by lengthening the time interval between pulse additions or by raising the sample temperature for a few minutes after each pulse addition. 

The highest possible Insensitivity to Oxygen- and Chlorine-Containing Catalyst Poisons. Keep in mind that the effects of poisons (e.g., oxygen compounds) may become more severe as temperature declines. 

The highest possible insensitivity to oxygen- and chlorine-containing catalyst poisons, which may be present in even the very effectively purified synthesis gas of a modern process (see Sections 3.6.1.5, and 4.3.2). In assessing the newly developed catalyst systems recommended for operation at very low temperatures (see Section 3.6.2.3), it must be kept in mind that the effect of poisons, for example, oxygen compounds, may become more severe as temperature declines (see Fig. 25). 

When gum formation proceeds, the minimum temperature in the catalyst bed decreases with time. This could be explained by a shift in the reaction mechanism so more endothermic reaction steps are prevailing. The decrease in the bed temperature speeds up the deactivation by gum formation. This aspect of gum formation is also seen on the temperature profiles in Figure 9. Calculations with a heterogenous reactor model have shown that the decreasing minimum catalyst bed temperature could also be explained by a change of the effectiveness factors for the reactions. The radial poisoning profiles in the catalyst pellets influence the complex interaction between pore diffusion and reaction rates and this results in a shift in the overall balance between endothermic and exothermic reactions. 

Since sulfur is the most effective of all catalyst poisons, the hydrogenation of sulfur containing heterocycles is not easily accomplished unless there are no unshared electron pairs on the sulfur atom or the catalyst used is not affected by the poison. The hydrogenation of the cyelie sulfone, 58, takes palace over an excess of palladium in acetic acid at room temperature and atmospheric pressure (Eqn. 17.57). Thiophene, itself, can be hydrogenated to tetrahydrothiophene over rhenium heptasulfide at 250°C and 300 atmospheres of hydrogen or over a large excess of palladium in methanolic sulfuric acid at room temperature and 3-4 atmospheres. No hydrogenolysis of the carbon-sulfiir bond was observed in these reactions. 

Hydroxyethyl)-pyridine was dehydrated to 2-vinyl-pyridine in liquid phase over solid acid catalysts, with very high selectivity and fairly good reaction rate at relatively low reaction temperature (160°C). The catalytic activity is well correlated with the presence on the catalyst surface of medium to weak Bronsted acid sites. The analysis of coke left behind onto the catalyst and the effect of partial poisoning of catalytic activity by CO2 indicate that the reaction takes place through two mechanisms, involving either a Bronsted acid site or a couple of acid-base sites. 

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