Catalyst dependence isomerization products

In some cases the tendency of double bond to migration rather than saturation is so large that even platinum does not prevent it. In these cases homogeneous catalysts may be useful. A case in point is coronopilin (7), which affords either dihydrocoronopilin (8) or the isomerized product isocoronopilin (9), depending on the catalysts. 

We showed (14) that formation of the isomerization products is kinetically controlled and that it depends on the catalyst system employed, the principal conjugated diene isomer being either the trans-2-trans-4-hexadiene, cis-2-trans-4-hexadiene, or 1,3-hexadiene. 

Microwave activation of alkane transformations was studied in detail by Roussy et al., who summarized their results in several papers [2, 28, 29, 79]. Isomerization of hexane, 2-methylpentane, 2-methyl-2-pentene, and hydrogenolysis of methylcydo-pentane have been investigated, and the diversity of possible effects has been specified [2]. The course of 2-methylpentane isomerization on a 0.3% Pt/Al203 catalyst depended on the mode of heating - the distribution of hexane products was different... 

Instead of having the olefin insertion reactions, the calculations indicate that M2b and M2c can only proceed uphill with the reductive elimination of HB(OH)2, leading to the formation of M3, an olefin complex which could be in principle obtained directly from the addition of olefin to the catalyst Rh (PH3)2C1. The olefin complex M3 then could undergo a-bond metathesis processes with HB(OH)2, giving two isomeric products M4 and M5 depending on the orientation of the HB(OH)2 borane. The a-bond metathesis processes are however found to be unfavorable because of the very high reaction barriers (Figure 4). 

Under the operating conditions, the reaction intermediates (w-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy. In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function. 

Industrially relevant consecutive-competitive reaction schemes on metal catalysts were considered hydrogenation of citral, xylose and lactose. The first case study is relevant for perfumery industry, while the latter ones are used for the production of sweeteners. The catalysts deactivate during the process. The yields of the desired products are steered by mass transfer conditions and the concentration fronts move inside the particles due to catalyst deactivation. The reaction-deactivation-diffusion model was solved and the model was used to predict the behaviours of semi-batch reactors. Depending on the hydrogen concentration level on the catalyst surface, the product distribution can be steered towards isomerization or hydrogenation products. The tool developed in this work can be used for simulation and optimization of stirred tanks in laboratory and industrial scale. 

Figure 2 shows the results of isohexane cracking on MCM-41 and HZSM-5 as examples of mesoporous silica and acidic catalysts. On all the catalysts, products mainly composed of C2 to C4 components as cracking products and C6 components as isomerization products, and the products of possible secondary reactions were not appreciably observed probably because of low conversion level. Since the amount of C2 component was very close to that of C4 components, it is considered that isohexane is cracked in two modes giving C2+C4 and two C3 molecules. In the case of MCM-41, cracking of isohexane proceeded above 598 K, and temperature dependence was not so large below 723 K, but very large above it, as shown in Fig. 2a. On the other hand, HZSM-5 gave smooth temperature dependence as shown in Fig. 2b. Another significant difference between MCM-41 and HZSM-5 was the distribution of cracking products The ratio of C3/C4 was much larger on HZSM-5 than on MCM-41.

As might be anticipated, isomeric products resulting from cationic rearrangements have been observed. The yield of ether is dependent on the amount of the catalyst used, the strength of the acid, and the reaction temperature.392 Optimum conditions generally involve trace amounts of acid catalyst and room temperature reaction. Acids with weakly nucleophilic conjugate bases are recommended so as not to compete with the alcohols for the cationic intermediate.393... 

There are two broad categories of xylene isomerization catalysts EB isomerization catalysts, which convert ethylbenzene into additional xylenes and EB dealkylation catalysts, which convert ethylbenzene to valuable benzene coproduct. The selection of the isomerization catalyst depends on the configuration of the UOP aromatics complex, the composition of the feedstocks and the desired product slate. 

Chromatograms obtained while heating the three PE-catalyst samples show catalyst-dependent differences in volatile product distributions. Figure 2.3 shows the gas chromatograms obtained at the temperatures corresponding to the maximum volatile product evolution rates for each PE-catalyst sample. Figure 2.3 clearly shows that relative hydrocarbon product yields depended on which catalyst was employed. For the PE-HZSM-5 sample, many isomeric hydrocarbons were detected, most of which were low molecular weight substances with short retention times. Volatile product diversity is less evident... 

Isomerization. With Pd catalysts, and, to a lesser extent, with Pt catalysts, a mixture of isomeric products may be obtained due to positional isomerization of double bonds during hydrogenation. As illustrated below, 5yn-addition of H2 to either face of the double bond in alkene A furnishes cw-decalin C. However, 5yn-addition of Hj to the isornerized alkene B can produce the cw-decalin C and/or trans-decalin D, depending on which face of the double bond undergoes addition by H2. In fact, hydrogenation of A in the presence of Pt furnishes 80% of the thermodynamically more stable trans-decalin and only 20% of cw-decalin. 

The tetramers 18 are stereoisomers of 3,3,6,6,9,9,12,12-octamethylpentacyclo-[9.1.0.0. 0 . 0 °]dodecane, as shown by extensive spectroscopic studies. The product distribution is only slightly dependent on the substituents in the 3-position of the cyclopropene ring. With bis(dibenzylideneacetone)palladium as catalyst, about 6% of the hexamethyl-tron -tris-cr-homobenzene 7 is additionally produced. When R R, a large number of isomeric products is formed and a complete analysis of all the tetrameric products has not yet been possible." ... 

Callot [14,15] and Noels [16,17] examined the reactions of rhodium porphyrins [14, 15] and carboxylates [16, 17] with various alkanes. When n-alkanes 17 were utilized, complex mixtures of isomeric products were always obtained (Scheme 4). Functionalization at C2 (as in 18) was nearly always the major product, and, depending on the catalyst used, Cl (19) or C3 (21) could be the site of the second most common attack. Branched alkanes also led to multiple products, but C-H insertion at tertiary sites was generally favored. 

While it is evident that aUyhc acetoxylation and related reactions proceed via two different mechanisms, mainly depending on the structure of the alkenes, it is less clear how to choose reaction conditions in order to favor one route or the other. There is some evidence from early smdies that the use of polar solvents such as DMF will promote aUyl acetate formation. It also appears that excess acetate promotes the formation of products compatible with the rr-allyl route. This is also suggested by recent factorial experiments with variation of carboxylate concentration. Since trimeric palladium acetate will induce rr-allyl formation from a series of monoolefins, it might be assumed that high concentration of palladium acetate could be used for creating conditions that favor a 77-aUyl route. " Another possibility is adding strong acids, which can increase the electrophilicity of the catalyst, but this can drive the reaction toward homoallylic acetates and other isomerized products. "... 

In an unsymmetrical system, in which the two alkene arms are not identical, it is critical to ensure that the desired ene component reacts with the catalyst to start the sequence. If the other ene component reacts, then an isomeric product will be formed (Scheme 8.116). This can usually be achieved by ensuring that the intended first ene component is the least substituted. In a comparison of closely related substrates, the alkene with the least substitution was the one where the reaction initiated, giving isomeric products depending on whether or was a hydrogen or a methyl group. ... 

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