Catalyst concentration effect

The indicated first-order rate dependence on [HCo(CO)4] is observed in 1,4-dioxane solvent, as shown by the constant value of k over a range of HCo(CO)4 concentrations. A slightly higher concentration dependence may be observed in 2,2,2-trifluoroethanol (e.g., Expt. 9). It is proposed that such behavior, if significant, could be the result of greater ionic dissociation of HCo(CO)4 at the lower concentrations in this more polar solvent. A catalyst concentration effect on product selectivity was reported by Keim et al. (59), but rate effects are not reported and possible secondary reactions are not taken into account. 

The Knoevenagel condensation reaction of benzaldehyde with ethylcyanoacetate (Scheme 1) was first studied on CsNaY 7Cs in order to check the better conditions to control the different reaction parameters (solvent effect on the rate and on the selectivity of the uncatalyzed and catalyzed reactions, mass effect of the catalyst, concentration effect of both reactants) [21],... 

In many cases, however, well-designed catalysts provide intrinsically different reaction paths, and the specific nature of the catalyst surface can be quite important. This is clearly the case with unimolecular reactions for which the surface concentration effect is not applicable. 

It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

The relative effectiveness of nucleating agents in a polymer can be determined by measuring recrystallization exotherms of samples molded at different temperatures (105). The effect of catalyst concentration and filler content has been determined on unsaturated polyesters by using dynamic thermal techniques (124). Effects of formulation change on the heat of mbber vulcanization can be determined by dsc pressurized cells may be needed to reduce volatilization during the cure process (125). 

Primary Variables. The most obvious variables are those whose effects on performance are to be evaluated directiy these ate the variables that, most likely, created the need for the investigation in the first place. Such variables may be quantitative, such as catalyst concentration, temperature, or pressure, or they may be quaUtative, such as method of preparation, catalyst type, or batch of material. 

Concentration Effects. The reactivity of ethyl alcohol—water mixtures has been correlated with three distinct alcohol concentration ranges (35,36). For example, the chromium trioxide oxidation of ethyl alcohol (37), the catalytic decomposition of hydrogen peroxide (38), and the sensitivities of coUoidal particles to coagulation (39) are characteristic for ethyl alcohol concentrations of 25—30%, 40—60%, and above 60% alcohol, respectively. The effect of various catalysts also differs for different alcohol concentrations (35). 

Distribution of Catalyst in Pores Because of the prac tical reqmrements of manufacturing, commercial impregnated catalysts usually have a higher concentration of ac tive ingredient near the outside than near the tip of the pores. This may not be harmful, because it seems that effectiveness sometimes is better with some kind of nonuni-form distribution of a given mass of catalyst. Such effects may be present in cases where the rate exhibits a maximum as a function of... 

Saturation of the oil with hydrogen is maintained by agitation. The rate of reaction depends on agitation and catalyst concentration. Beyond a certain agitation rate, resistance to mass transfer is eliminated and the rate oecomes independent of pressure. The effect of catalyst concentration also reaches hmiting values. The effects of pressure and temperature on the rate are indicated by Fig. 23-34 and of catalyst concentration by Fig. 23-35. Reaction time is related to temperature, catalyst concentration, and IV in Table 23-13. 

FIG. 23-35 Effect of catalyst concentration and stirring rate on hydrogenation of soybean oil. (Swern, ed., Baileys Industrial Fat and Oil Products, vol. 2, Wiley, 1979.)... 

The rate of hydrolysis in the presence of resins increases with the number of catalytically active ions. In some reactions, the reaction rate is a linear function of the quantity of catalyst added [26,34]. Figure 1 shows the effect of varying catalyst concentration on the rate of hydrolysis of ethyl acetate. Higher values of q are shown with the larger amount of catalyst. 

Table 3 Effect of Varying Catalyst Concentration on the Hydrolysis of Ethyl Acetate (0.2 M) at 45°C...

Equilibrium (continued) calculations, 192 constant, 151, table, 154 crystallization and, 144 dynamic nature of, 144, 165 effect of catalyst, 148 effect of concentration, 148 of energy, 167 of randomness, 166 of temperature, 67. 148, 167 factors determining, 155, 158 law of chemical, 152, 173 liquid-gas, 66 qualitative aspects of, 142 quantitative aspects of, 151 recognizing, 143 slate of, 142, 147 sugars, 425 thermal, 56... 

Jacobsen developed a method employing (pybox)YbCl3 for TMSCN addition to meso-epoxides (Scheme 7.22) [46] with enantioselectivities as high as 92%. Unfortunately, the practical utility of this method is limited because low temperatures must be maintained for very long reaction times (up to seven days). This reaction displayed a second-order dependence on catalyst concentration and a positive nonlinear effect, suggesting a cooperative bimetallic mechanism analogous to that proposed for (salen)Cr-catalyzed ARO reactions (Scheme 7.5). 

The effects of manganese on the cobalt/bromide-catalyzed autoxidation of alkylaromatics are summarized in Figure 17. The use of the Mn/Co/Br system allows for higher reaction temperatures and lower catalyst concentrations than the bromide-free processes. The only disavantage is the corrosive nature of the bromide-containing system which necessitates the use of titanium-lined reactors. 

In this work, LDPE and HDPE were used as the waste plastics and ZSM-5 and RFCC were used as the waste catalyst. The effects reaction temperature and catalyst concentration on the production of liquid products were investigated in a semi-batch reactor. 

Reaction progress kinetic analysis offers a reliable alternative method to assess the stability of the active catalyst concentration, again based on our concept of excess [e]. In contrast to our different excess experiments described above, now we carry out a set of experiments at the same value of excess [ej. We consider again the proline-mediated aldol reaction shown in Scheme 50.1. Under reaction conditions, the proline catalyst can undergo side reactions with aldehydes to form inactive cyclic species called oxazolidinones, effectively decreasing the active catalyst concentration. It has recently been shown that addition of small amounts of water to the reaction mixture can eliminate this catalyst deactivation. Reaction progress kinetic analysis of experiments carried out at the same excess [e] can be used to confirm the deactivation of proline in the absence of added water as well to demonstrate that the proline concentration remains constant when water is present. 

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