Catalyst equilibration effect

Product stereochemistry is a function of the specific catalyst used for hydro-genation. For example, palladium generally gives more of the thermodynamically stable product than other catalysts. This effect has been attributed to an increased rate of equilibration of the steps in the hydrogenation process. Consequently, palladium should not be used to hydrogenate readily isomerizable olefins such as A - and A -steroids. ... 

Effect of Catalyst Equilibration. Fig.7 shows the kinetic data of a steamed catalyst and of the equilibrated sample of the same catalyst, from the refinery FCCU. The equilibrium catalyst exhibits a significantly higher regenerability when compared with the steamed. The destruction of zeolite, consequently lower Sz/Sm, may be considered as the main cause for this behavior. 

Aluminum halide catalysts can effect such equilibrations via rearrangements involving intermediate carbonium ions. A saturated precursor with the... 

SO2 in absence of CO or Oj in the gas phase, the Pt surface is covered by sulfur little CO chemisorption is observed right after resuming the reactant s flow. As the temperature is increased, CO removes part of the sulfur from the Pt surface, and CO adsorption reaches levels similar to those previously observed on the equilibrated catalysts. This effect is very similar to the effect of Cl presented in Section 17.1, suggesting a common pathway to all these catalyst finishing effects. 

It has to be emphasized that all the above findings were obtained with systems completely free of any catalyst. The effect of an added catalyst (e.g. dibutyltin dilaurate) is first that the temperature at which the equilibration reaction takes place is much lower, and secondly that this process proceeds much faster than when the catalyst is excluded. This has been already found useful in the synthesis of the Diol-2 and Diol-3... 

Maximum conversion occurs by equilibration at the lowest possible temperature so the reaction is carried out sequentially on two beds of catalyst (a) iron oxide (400°C) which reduces the CO concentration from 11% to 3% (b) a copper catalyst (200°) which reduces the CO content to 0.3%. Removal of CO2 ( 18%) is effected in a scrubber containing either a concentrated alkaline solution of K2CO3 or an amine such as ethanolamine ... 

Figure 55.2. Effect of time-on-stream on -butane conversion (O ) and selectivity to MA ( ) of catalysts P/V 1.00 (open symbols) and PA 1.06 (full symbols) at 380°C after treatment of the equilibrated catdysts with air at 380°C for 1 h.

The method of preparation of the alumina has a marked effect on the product distribution as shown in Table VIII (47). Over the pure alumina (P) the olefinic products are nearly equilibrated. The alkali-containing catalysts, however, give kinetically controlled products. The very low activity of these catalysts for olefin isomerization had been ascertained independently. It may, therefore, be concluded that the compo.sition of the olefins produced at 350° is very nearly that of the primary dehydration products. 

The effect of the nature of ion pairs as nucleophiles in a metal-catalysed substitution reaction has been investigated by determining product ratios for the Pd-catalysed allylic alkylations of substrates (9)-(ll) under various conditions, particularly with respect to catalyst ligands, nucleophiles, and counterions. Each dienyl acetate ionizes to form initially the vinyl (7r-allyl)-Pd intermediate corresponding most closely to the leaving group, i.e. (12) from (9), (13) from (11), but (12) and (13) from (10). The initial intermediate can then either be trapped by the nucleophile or it can begin to equilibrate to some mixture of vinyl 7r-allyl intermediates. If nucleophilic addition occurs before full equilibration, the product ratio is different for each substrate if... 

Influence of catalyst load on the degradation of B02 solution. The profile for the color decay of 0.1 mM B02 solution by 0.01 g of C2-Ms at pH 3, in the absence of H2O2, is shown in figure 22. It is observed a 57% of color removal by adsorption after 5 h (but no equilibration was yet reached) whereas the solution was effectively colorless when 4 mM H2O2 was present under similar experimental conditions, as depicted in figure 23 (plot A). 

Most recently, we have investigated the use of iterative oxonium ylide [1,2]- or [2,31-shifts as a convenient approach to the polypyran domains often found in the marine polyether ladder toxins (Scheme 18.8) [21]. Initial studies indicated that [l,2]-shifts of O-benzyl oxonium ylides such as 19 a or 19 b were inefficient. Alternative metallocarbene processes including C-H insertion and dimerization were found to predominate in these cases, again suggesting that carbene-ylide equilibration may occur [21b]. On the rationale that concerted [2,3]-shifts of the corresponding O-allyl oxonium ylides might occur more readily, the allyl ethers 19 c, 19 d were then examined. These examples were much more effective, especially in conjunction with the optimized catalyst Cu(tfacac)2 [21a]. However, rhodium(II) triphenylacetate (Rh2(tpa)4) [22] was found to... 

Data reported in the present work demonstrate that the degree of crystallinity and the acid properties are related the amount of present at the surface of VPP. When the VPP is not fully equilibrated, and hence may contain discrete amounts of it is more selective to MA and less to PA. The reason is that in oxidized catalysts, the olefmic intermediate is preferentially oxidized to MA, rather then being subjected to the acid-catalyzed condensation with a second unsaturated molecule, to yield the precursor of PA. When instead the catalyst is more crystalline, and hence it does contain less oxidized V sites, its surface acid properties predominate over O-insertion properties, and the catalyst becomes more effective in PA formation. In this case, the selectivity to PA at 50% n-pentane conversion becomes comparable to that one of MA. 

Silica gel is an effective catalyst for the t-butylation of thiophene and benzo[/ ]thiophene using t-butyl bromide. 2,5-Di-r-butylthiophene and 3-f-butylbenzo[b]thiophene can be prepared easily by this procedure (84JOC4161). Alkylation of thiophene with f-butyl chloride, isopropyl chloride or ethyl chloride at - 70°C in the presence of A1C13 produced a-complexes under kinetic control. On thermal equilibration, migration of alkyl from position 3 to position 2, as well as disproportionation to dialkyl-and trialkyl thiophenes can occur (86T759). 

Polynuclear chromium(III) ethylenediamine complexes have been synthesized by methods similar to those applied for the ammine systems by using the combined catalytic effect of chromium(II) and charcoal on aqueous ethylenediamine buffer solutions (pH -8) of chromium(III) (40, 42, 60). As mentioned above, the use of catalysts is important when equilibration between the mononuclear species is required, but it is unnecessary when the aim is to produce polynuclear species. In fact, identical polynuclear species are formed in approximately the same ratio when buffered chromium(III) solutions ([Cr] =0.1 M, [en = 0., i M) without catalyst are kept for a few days at 40 50 C (40). 

A detailed study of the rearrangement of heptafluoro-2-phenylbut-1-ene (11) to but-2-ene 12 and the E,7. equilibration of 12 showed that lithium and sodium fluorides do not catalyze the rearrangement. Cesium, rubidium, and potassium fluorides are effective catalysts, in that order of decreasing reactivity.25... 

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