Catalysts constant

The hydrolysis of ester in presence of acid is first order reaction (keeping catalyst constant)... 

Contamination by heavy metals (V, Ni). To maintain the metal content on catalyst constant, usually a large increase in catalyst consumption (from an average of 0.15 lb/bbl up to and above 0.5 lb/bbl) is required. Alternatively special metal resistant catalyst can be applied in order to minimize catalyst consumption. Arbitrarily a metals content (Ni + V) of above 1500 ppm on catalyst is sometimes considered to be a metals contaminated resid operation. 

In separations limited by azeotrope formation under nonreactive conditions, the addition of a reaction (that is, usually adding catalyst) constantly changes the concentrations such that the separation continues beyond the azeotrope. In processes with coupled products (enantiomers, ortho-/meta-/para-substitution), the in-situ removal of the desired product will favor the reaction towards this product, and will therefore strongly increase the selectivity. 

Starting off from the initial dye (variable) and catalyst (constant) concentrations the absorption and scattering coefficients for these concentrations are evaluated. Then, these coefficients are used in a radiative transfer model to obtain for a large number of points within the reactor volume. 

If the extra degree of freedom is needed to keep the coefficient for heat transfer to the catalyst constant, the situation is not quite so bad. In order to fix fc7, dp must vary as something between the first and the three-halves power of G. Continuing with the same assumptions as in the case above, short tables have been calculated to show the relations when the activity is constant and when it is inversely proportional to the diameter of the particle. 

System II assnmes that the pore is filled with water vapor as shown. Given the details of the operating conditions and the water management scheme used, it is possible that some pores would be filled with liquid water, a case which is under study but not included in this report. Graphite is used as the carbon support in system III. System IV uses platinum ([100] Pt) as the catalyst. Constant density and constant temperature (NVT) MD simulations were used to study the systems as a function of humidity level. 

Disper- Surface Area mVg cat. Chemi- Catalyst Constant ... 

The rate of benzene production was also measured as a function of benzaldehyde concentration while keeping the concentration of the catalyst constant. A plot of the rate constant, kohs, vs. benzaldehyde concentration is shown in Figure 11.1. At low aldehyde concentrations the rate is first-order in (benzaldehyde), but as the aldehyde concentration is increased a saturation effect is observed. This behavior can be interpreted in terms of a rapid pre-equilibrium followed by a slow rate-limiting step as shown by Equation... 

With higher carbon monoxide concentrations and higher gas velocities, keeping the contact time of the gas with the catalyst constant, exactly identical curves to those shown were measured. As a higher linear gas flow rate did not result in an increase of the activity, the mass transfer limitation should be present only within the catalyst body. This was confirmed by an evaluation of fiie criteria of Mears [13] and Anderson [14] for mass and heat transfer limitation both within the catalyst body and from the bulk of the gas flow to the external surface of the body. 

As with any system, there are complications in the details. The CO sticking probability is high and constant until a 0 of about 0.5, but then drops rapidly [306a]. Practical catalysts often consist of nanometer size particles supported on an oxide such as alumina or silica. Different crystal facets behave differently and RAIRS spectroscopy reveals that CO may adsorb with various kinds of bonding and on various kinds of sites (three-fold hollow, bridging, linear) [307]. See Ref 309 for a discussion of some debates on the matter. In the case of Pd crystallites on a-Al203, it is proposed that CO impinging on the support... 

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. 

For example, the expansion of a gas requires the release of a pm holding a piston in place or the opening of a stopcock, while a chemical reaction can be initiated by mixing the reactants or by adding a catalyst. One often finds statements that at equilibrium in an isolated system (constant U, V, n), the entropy is maximized . Wliat does this mean ... 

An important point about kinetics of cyclic reactions is tliat if an overall reaction proceeds via a sequence of elementary steps in a cycle (e.g., figure C2.7.2), some of tliese steps may be equilibrium limited so tliat tliey can proceed at most to only minute conversions. Nevertlieless, if a step subsequent to one tliat is so limited is characterized by a large enough rate constant, tlien tire equilibrium-limited step may still be fast enough for tire overall cycle to proceed rapidly. Thus, tire step following an equilibrium-limited step in tire cycle pulls tire cycle along—it drains tire intennediate tliat can fonn in only a low concentration because of an equilibrium limitation and allows tire overall reaction (tire cycle) to proceed rapidly. A good catalyst accelerates tire steps tliat most need a boost. 

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

The complete assembly for carrying out the catalytic decomposition of acids into ketones is shown in Fig. Ill, 72, 1. The main part of the apparatus consists of a device for dropping the acid at constant rate into a combustion tube containing the catalyst (manganous oxide deposited upon pumice) and heated electrically to about 350° the reaction products are condensed by a double surface condenser and coUected in a flask (which may be cooled in ice, if necessary) a glass bubbler at the end of the apparatus indicates the rate of decomposition (evolution of carbon dioxide). The furnace may be a commercial cylindrical furnace, about 70 cm. in length, but it is excellent practice, and certainly very much cheaper, to construct it from simple materials. 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

In the kinetic runs always a large excess of catalyst was used. Under these conditions IQ does not influence the apparent rate of the Diels-Alder reaction. Kinetic studies by UV-vis spectroscopy require a low concentration of the dienophile( 10" M). The use of only a catalytic amount of Lewis-acid will seriously hamper complexation of the dienophile because of the very low concentrations of both reaction partners under these conditions. The contributions of and to the observed apparent rate constant have been determined by measuring k pp and Ka separately. ... 

The equilibrium constants obtained using the metal-ion induced shift in the UV-vis absorption spectrum are in excellent agreement with the results of the Lineweaver-Burke analysis of the rate constants at different catalyst concentrations. For the copper(II)ion catalysed reaction of 2.4a with 2.5 the latter method gives a value for of 432 versus 425 using the spectroscopic method. 

Herein ko is the second-order rate constant for the uncatalysed reaction and k. is the second-order rate constant for the reaction of the 2.4-catalyst complex. [2.4] is the concentration of free dienophile... 

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. 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

All measurements were performed at constant ionic strength (2.00 M using KNO3 as background electrolyte) and at pH 7-8. Ligand-catalyst ratio. 

Figure 4.1. The apparent rate constant of the Diels-Alder reaction of 4.8 with 4.6 versus the concentration of MeReO-j catalyst according to reference 7.

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