Of catalyst concentration

Endo-exo product mixtures were isolated using the following procedure. A solution of cyclopentadiene (concentration 2-10" M in water and 0.4 M in oiganic solvents) and the dienophile (concentration 1-5 mM) in the appropriate solvent, eventually containing a 0.01 M concentration of catalyst, was stirred at 25 C until the UV-absorption of the dienophile had disappeared. The reaction mixture (diluted with water in the case of the organic solvents) was extracted with ether. The ether layer was washed with water and dried over sodium sulfate. After the evaporation of the ether the... 

Since for the kinetic measurements a large excess of catalyst is used, the concentration of free catalyst [M" ]/ essentially equals the total concentration of catalyst. Moreover, since ko k jt and under the conditions of the measurements the concentration of free 2.4 is small, the contribution of the uncatalysed reaction can be neglected and equation 7 simplifies to ... 

Numerous attempts to determine the equilibrium constants using titration microcalorimetry failed, due to solubility problems encountered at the higher concentrations of catalyst and dienophile that are required for this technique. 

Sulphuric acid catalysed nitration in concentrated nitric acid, but the effect was much weaker than that observed in nitration in organic solvents ( 3.2.3). The concentration of sulphuric acid required to double the rate of nitration of i-nitroanthraquinone was about 0-23 mol 1, whereas typically, a concentration of io mol 1 will effect the same change in nitration in mixtures of nitric acid and organic solvents. The acceleration in the rate was not linear in the concentration of catalyst, for the sensitivity to catalysis was small with low concentrations of sulphuric acid, but increased with the progressive addition of more catalyst and eventually approached a linear acceleration. 

Unlike the effect of sulphuric acid upon nitration in nitric acid ( 2.2.3 where zeroth-order reactions are unknown), the form of the catalysis of zeroth-order nitration in nitromethane by added sulphuric acid does not deviate from a first-order dependence with low concentrations of catalyst. ... 

One important application of the variable-time integral method is the quantitative analysis of catalysts, which is based on the catalyst s ability to increase the rate of a reaction. As the initial concentration of catalyst is increased, the time needed to reach the desired extent of reaction decreases. For many catalytic systems the relationship between the elapsed time, Af, and the initial concentration of analyte is... 

Until now we have been discussing the kinetics of catalyzed reactions. Losses due to volatility and side reactions also raise questions as to the validity of assuming a constant concentration of catalyst. Of course, one way of avoiding this issue is to omit an outside catalyst reactions involving carboxylic acids can be catalyzed by these compounds themselves. Experiments conducted under these conditions are informative in their own right and not merely as means of eliminating errors in the catalyzed case. As noted in connection with the discussion of reaction (5.G), the intermediate is stabilized by coordination with a proton from the catalyst. In the case of autoprotolysis by the carboxylic acid reactant, the rate-determining step is probably the slow reaction of intermediate [1] ... 

An interesting situation is obtained when the catalyst-solvent system is such that the initiator is essentially 100% dissociated before monomer is added and no termination or transfer reactions occur. In this case all chain initiation occurs rapidly when monomer is added, since no time-dependent initiator breakdown is required. If the initial concentration of catalyst is [AB]o,then chain growth starts simultaneously at [B"]q centers per unit volume. The rate of polymerization is given by the analog of Eq. (6.24) ... 

Recovered catalyst and blowdown gas (- 3% of the flue gas) exit from the bottom of the separator to an electrostatic precipitator or to a small, fourth-stage cyclone for further concentration of catalyst fines. The flue gas, with 70—90% of the catalyst particles removed, passes from the separator into the power expander. 

Implement procedural controls on the amount or concentration of catalyst or initiator to be added... 

The catalytic efficiency increases, under comparable conditions (pH, concentration of catalyst, etc.) in the sequence Cl < Br - S(CH3)2 < SCN < SC(NH2)2 < I . Titration with a calibrated solution of NaN02 (usually 0.05 to 0.10 m) is used for the analytical determination of aromatic amines, dissolved in aqueous H2S04 or HC1. Here nucleophilic catalysis is achieved by adding KBr. This allows a titration to be completed much faster than without that addition. 

The effect of catalysis by trifluoroacetic acid on chlorination in carbon tetrachloride has also been determined272. For 1,2,4,5-tetramethylbenzene, with low concentrations of catalyst, the order in catalyst is three-halves, but for toluene (which requires a higher concentration) the order is mixed three- and five-halves the indication is, therefore, that a minimum of three catalyst monomers (or one monomer and one dimer) are necessary. Since trifluoroacetic acid is very likely to be dimeric in carbon tetrachloride, the concentration of monomer is pro-... 

Since there is inherent in reactions which give low selectivities, the possibility that non-competitive conditions are responsible, Olah and Overchuck359 have measured directly the rates of benzylation, isopropylation, and fer/.-butylation of benzene and toluene with aluminium and stannic chlorides in nitromethane at 25 °C. Apparent second-order rate coefficients were obtained (assuming that the concentration of catalyst remains constant), but it must be admitted that the kinetic plots showed considerable departure from second-order behaviour. The observed rate coefficients and kreh values determined by the competition method are given in Table 88, which seems to clearly indicate that the competitive ex-... 

Crosslinking resoles in the presence of sodium carbonate or potassium carbonate lead to preferential formation of ortho-ortho methylene linkages.63 Resole networks crosslinked under basic conditions showed that crosslink density depends on the degree of hydroxymethyl substitution, which is affected by the formaldehyde-to-phenol ratio, the reaction time, and the type and concentration of catalyst (uncatalyzed, with 2% NaOH, with 5% NaOH).64 As expected, NaOH accelerated the rates of both hydroxymethyl substitution and methylene ether formation. Significant rate increases were observed for ortho substitutions as die amount of NaOH increased. The para substitution, which does not occur in the absence of the catalyst, formed only in small amounts in the presence of NaOH. 

Ease of product separation - Since the organic layer, substantially free from water-soluble contaminants, can simply be decanted off, product separation is often simple, resulting in less waste. It is important to remember that the concentration of X in the organic phase cannot exceed the concentration of catalyst (unless it is soluble in the absence of a catalyst). 

Mass transfer-limited processes favour slurry reactors over monoliths as far as the overall process rates are concerned. Moreover, slurry reactors are more versatile and less sensitive to gas flow rates. However, the productivity per unit volume is not necessarily higher for slurry reactors because of the low concentration of catalyst in such reactors. There also is no simple answer to the selectivity problem, and again, each process should be compared in detail for both reactors. 

In Equations 6.61, U denotes the concentration of catalyst present in the reactor (Ck) and u2 the hydrogen pressure (P). As far as the estimation problem is concerned, both these variables are assumed to be known precisely. Actually, as it will be discussed later on experimental design (Chapter 12), the value of such variables is chosen by the experimentalist and can have a paramount effect on the quality of the parameter estimates. Equations 6.61 are rewritten as following... 

Pseudo-first-order rate constants (k0t,s) determined from the linear relationship of In (Am-At) with time, were calculated for different concentrations of catalyst. A pseudo-first-order rate constant was also calculated for the uncatalyzed hydrolysis at pH 8.5, and was substracted from the values found for the catalyzed hydrolysis. 

Alternatively, esterification of carboxylic acid can be carried out in aqueous media by reacting carboxylic acid salts with alkyl halides through nucleophilic substitutions (Eq. 9.10).20 The reaction rate of alkyl halides with alkali metal salts of carboxylic acids to give esters increases with the increasing concentration of catalyst, halide, and solvent polarity and is reduced by water. Various thymyl ethers and esters can be synthesized by the reactions of thymol with alkyl halides and acid chlorides, respectively, in aqueous medium under microwave irradiation (Eq. 9.11).21 Such an esterification reaction of poly(methacrylic acid) can be performed readily with alkyl halides using DBU in aqueous solutions, although the rate of the reaction decreases with increasing water content.22... 

Some of the vinyl monomers polymerized by transition metal benzyl compounds are listed in Table IX. In this table R represents the rate of polymerization in moles per liter per second M sec-1), [M]0 the initial monomer concentration in moles per liter (M) and [C]0 the initial concentration of catalyst in the same units. The ratio i2/[M]0[C]0 gives a measure of the reactivity of the system which is approximately independent of the concentration of catalyst and monomer. It will be observed that the substitution in the benzyl group is able to affect the polymerization rate significantly, but the groups that increase the polymerization rate toward ethylene have the opposite effect where styrene is concerned. It would also appear that titanium complexes are more active than zirconium. The results with styrene and p-bromostyrene suggests that substituents in the monomer, which increase the electronegative character of the double bond, reduces the polymerization rate. The order of reactivity of various olefinically unsaturated compounds is approximately as follows ... 

Whichever way we choose to describe the design, it (and the others of this type) has some attractive features. We will illustrate these features with a numerical example. For our example, we will imagine an experiment where the scientist is interested in determining the influence of temperature and of catalyst on the yield of a chemical reaction. The questions to be answered are does the concentration of catalyst make a difference, and does the type of catalyst make a difference The experiment is to consist of trying each of the four available catalysts and three solvents, and determining the yield. The experiment can be described by Table 10-3. 

Somewhat better results were obtained by the use of (Ph3P)2PtI2 in a polar solvent such as dimethylformamide (122). At 180°C and 250 atm H2/CO, an 89% conversion to aldehydes with an isomer ratio of 4.3 1 was obtained in 1 hour. Relatively high concentrations of catalyst (2500 ppm as Pt metal) were required. Palladium, as (Ph3P)2PdI2, was less effective and also produced considerable amounts of lactones. 

Andrianov et al. (52) studied the rate of disappearance of Si—H as a function of time, temperature, and concentration of chloroplatinic acid in an equimolar solution of isoprene and dodecamethylhexasiloxane, H(Me2SiO)5SiMe2H. With a constant concentration of catalyst a plot of % conversion of SiH up to at least 60% vs. time at 20°, 50°, 70° and 110°C gave a family of straight lines. The slopes of the lines increased by a factor of 2.5-3 between 50° and 110°C. 

A linear relationship between % SiH and time suggests pseudo-zero-order kinetics, in which the rate of reaction appears to be independent of the concentrations of isoprene and siloxane. A plot of % conversion of SiH vs. concentration of catalyst at 110°C for 5 hours also gave a straight line, indicating that the rate of reaction is directly proportional to concentration of catalyst, i.e., first-order in catalyst. 

Benkeser et al. (55) showed that a mole ratio of 2Cl3SiH to 1-hexyne would react completely in a long time. In 24 hours, a 98% yield of monoadducts had formed. After 111 hours, the same mixture yielded 20% 1,6-and 18% 1,2-bistrichlorosilylhexane. By increasing the concentration of catalyst, the yield of these two diadducts was increased to 44% and 40%, respectively. In this work, 1-trichlorosilyl-1-hexene behaved very much like 1-phenyl-l-hexene, shown in Table VI to form 1,6-diadducts. 

Here, L is a mobile ligand which can leave the metal site (M) open briefly for reaction with A in the initial step of the catalytic cycle. The transformation of the M A complex into products completes the cycle. The equilibrium in step (1) lies far to the left in most cases, because the ligands protect the metal centers from agglomeration. Thus, the concentration of M is very small, and the total concentration of catalyst is cMr = cm a + cm l- The rate law which arises from this mechanism is... 

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