Hydroformylation olefin isomers

In the hydroformylation of an olefin not only are aldehydes formed that directly correspond with the used olefin isomer but also all the other theoretically possible isomeric 2-alkyl-branched aldehydes [49] ... 

Consider as a prototype the network 11.13 of n-heptene hydroformylation, keeping in mind that die arrows represent multistep pathways and that the reactions of higher straight-chain olefins involve still more parallel pathways of internal olefin isomers to aldehyde isomers and on to alcohol isomers. In such networks, all but one of the aldehyde-to-alcohol conversions involve the reaction of an aldehyde group on a secondary carbon atom, so that all these pathways can be assumed to involve essentially the same rate coefficients of their steps. Only the conversion of the straight-chain aldehyde (n-octanal to n-octanol in network 11.13) must be expected to occur with somewhat different rate coefficients. Likewise, all con-... 

Example 12.1. Hydroformylation of long-chain 1-olefins with phosphine-substituted cobalt hydrocarbonyl catalysts. Hydroformylation of long-chain 1-olefins with phosphine-substituted cobalt hydrocarbonyl catalysts provides a striking example of coupled parallel steps and the potential of an uncommon heat-transfer problem. The network is of the type 12.5 below, with the A, as the olefin isomers and the P, as the isomeric alcohol products (arrows represent multistep pathways see also Example 5.3, Figure 5.9, and network 5.43 in Section 5.3 and network 7.40 in Section 7.4). 

When used as a catalyst, cobalt does not compare favorably to rhodium, particularly in terms of its activity and selectivity. Nevertheless, cobalt-based processes remain competitive for hydroformylating highly branched olefinic isomer mixtures containing internal double bonds. In addition to the problem... 

It was repeatedly reported that olefin isomers with non-terminal and terminal double bonds give the same ratio of aldehyde isomers on hydroformylation I. Goldfarb and M. Orchin [131] and V. L. Hughes and I. Kirshenbaum [126] showed that this generalization is incorrect. 

Three significant, commercial processes for the production of amyl alcohols include separation from fusel oils, chlorination of C-5 alkanes with subsequent hydrolysis to produce a mixture of seven of the eight isomers (Pennsalt) (91), and a low pressure 0x0 process, or hydroformylation, of C-4 olefins followed by hydrogenation of the resultant C-5 aldehydes. 

Butylene isomers also can be expected to show significant differences in reaction rates for metaHation reactions such as hydroboration and hydroformylation (addition of HCo(CO). For example, the rate of addition of di(j -isoamyl)borane to cis-2-huX.en.e is about six times that for addition to trans-2-huX.en.e (15). For hydroformylation of typical 1-olefins, 2-olefins, and 2-methyl-l-olefins, specific rate constants are in the ratio 100 31 1, respectively. 

The formation of isomeric aldehydes is caused by cobalt organic intermediates, which are formed by the reaction of the olefin with the cobalt carbonyl catalyst. These cobalt organic compounds isomerize rapidly into a mixture of isomer position cobalt organic compounds. The primary cobalt organic compound, carrying a terminal fixed metal atom, is thermodynamically more stable than the isomeric internal secondary cobalt organic compounds. Due to the less steric hindrance of the terminal isomers their further reaction in the catalytic cycle is favored. Therefore in the hydroformylation of an olefin the unbranched aldehyde is the main reaction product, independent of the position of the double bond in the olefinic educt ( contrathermodynamic olefin isomerization) [49]. 

The ligands synthesized were also apphed to the isomerizing hydroformylation of more reactive 2-pentene. At 120 °C/ 20 bar quantitative conversion of olefin to aldehydes was achieved within 40 min. Trends similar to those described for internal octene hydroformylation were found. The regioselectivity obtained for the individual ligands tends to be 5% higher compared to that for the octenes. Thus, in the presence of 10 75% of n-hexanal were determined, compare Table 3. Obviously, 2-pentene is able to react more smoothly to the terminal isomer compared to olefins having the double bond in an more internal position. Illustrative for this effect are also literature results obtained for 2- and 4-octene.4,5... 

Because of the extreme industrial importance of simple hydrocarbons such as propylene in hydroformylation, the reaction of a-olefins has been studied in much detail. As noted before, the formyl group can be attached to either of the carbon atoms which constitute the original double bond. For olefins of greater than C3 chain length, the formyl group may, under certain conditions, also be attached to a carbon atom which was originally saturated. But for propylene only two isomers are possible, as shown in Eq. (25). 

Further progress in providing linear aldehydes from olefinic substrates has been provided by modified rhodium catalysts. Without modifiers, the product from the hydroformylation has very low normal iso isomer ratios 1-octene gave only 31% of the linear isomers in one example (28). 

The catalyst containing 2.0% Rh, insoluble in organic solvent, was used for hydroformylation of 1-hexene at 80°C and 43 atm of 1/1 H2/CO. The catalyst concentration was 1 mmole Rh per mole of olefin. After 4 hours a 41% yield of aldehyde was obtained, with a 2.5 1 isomer ratio. Some isomerization to internal olefins also occurred. A significant feature was the rhodium concentration of 2 ppm in the product. 

Propene- and butene-oligomers are complex mixtures. A typical isomer distribution is shown in Fig. 24. According to the thermodynamical stability the double bonds are distributed along the chain, terminal double bonds are present only in traces. To get predominant terminal products, a catalyst must provide extremely fast terminal hydroformylation activity for the traces of terminal olefins, a high isomerization activity to supply the terminal double bonds as fast as they are consumed, and low hydroformylation activity for internal double bonds. 

Raffinate-II typically consists of40 % 1-butene, 40 % 2-butene and 20 % butane isomers. [RhH(CO)(TPPTS)3] does not catalyze the hydroformylation of internal olefins, neither their isomerization to terminal alkenes. It follows, that in addition to the 20 % butane in the feed, the 2-butene content will not react either. Following separation of the aqueous catalyts phase and the organic phase of aldehydes, the latter is freed from dissolved 2-butene and butane with a counter flow of synthesis gas. The crude aldehyde mixture is fractionated to yield n-valeraldehyde (95 %) and isovaleraldehyde (5 %) which are then oxidized to valeric add. Esters of n-valeric acid are used as lubricants. Unreacted butenes (mostly 2-butene) are hydroformylated and hydrogenated in a high pressure cobalt-catalyzed process to a mixture of isomeric amyl alcohols, while the remaining unreactive components (mostly butane) are used for power generation. Production of valeraldehydes was 12.000 t in 1995 [8] and was expected to increase later. 

Rhodium-phosphine catalysts are unable to hydroformylate internal olefins, so much that in a mixture of butenes only the terminal isomer is transformed into valeraldehydes (see 4.1.1.2). This is a field still for using cobalt-based catalysts. Indeed, [Co2(CO)6(TPPTS)2] -i-lO TPPTS catalyzed the hydroformylation of 2-pentenes in a two-phase reaction with good yields (up to 70%, but typically between 10 and 20 %). The major products were 1-hexanal and 2-methylpentanal, and n/i selectivity up to 75/25 was observed (Scheme 4.12). The catalyst was recycled in four mns with an increase in activity (from 13 to 19 %), while the selectivity remained constant (n/i = 64/36). 

C4 Alkenes. Several industrial processes have been developed for olefin production through catalytic dehydrogenation138 166 167 of C4 alkenes. All four butenes are valuable industrial intermediates used mostly for octane enhancement. Isobutylene, the most important isomer, and its dimer are used to alkylate isobutane to produce polymer and alkylate gasoline (see Section 5.5.1). Other important utilizations include oxidation to manufacture maleic anhydride (see Section 9.5.4) and hydroformylation (see Section 7.1.3). 

Table VI shows that regardless of the starting isomeric olefin, selectivity is lower with Co2(CO)8 than with rhodium this is especially true when the starting isomer is conjugated. Moreover, when hydroformylation is done with Co2(CO)8, two aromatic ring effects are observed ...

With Rh, the phenomena are strikingly different since the aromatic ring does not influence the selectivity which is always very high, no matter what olefinic structure (except for 1,1-diphenylpropene). Isomer selectivity depends on several factors, mainly the structure and the stereochemistry of the alkenylbenzene. With conjugated alkenylbenzenes, addition of the CHO group occurs preferentially on the a carbon when the a and / carbons are monosubstituted. When the ft carbon is disubsti-tuted, because of steric requirements, hydroformylation is less selective and occurs on both the a and y carbon ... 

The other feature of hydroformylation of ( )- and (Z)-2-pheny 1-2-butenes concerns the distribution of aldehydic isomers 26 and 27. With Co2(CO)8, whatever the stereochemistry of the olefin, the distribution of these two aldehydes is exactly the same. On the other hand, with Rh/Al203 the reaction is more or less oriented towards the formation of one of the aldehydic isomers. Moreover, the phenomenon is more striking with (Z) than with (E) stereoisomer. 

The plot of data for the carbonyl, Rh6(CO)i6, (Figure 1) shows that the rate of 1-hexene isomerization exceeds that of hydroformylation. At olefin conversion in excess of 50%, little 1 isomer remains. An increase in branched aldehyde relative to linear aldehyde accompanies the change in isomer distribution. The absence of aldehyde hydrogenation is complete even at very high conversion levels using the conditions cited. 

Hydroformylation of linear olefins in a conventional cobalt oxo process (see Section 5.3) produces increasing linear-to-branched aldehyde ratios as the carbon monoxide ratio in the gas stream is increased up to 5 MPa (50 atm), but there is little further effect if the reaction mixture is saturated with carbon monoxide. An increasing partial pressure of hydrogen also increases this ratio up to a hydrogen pressure of 10 MPa. As the reaction temperature is increased, the linear-to-branched aldehyde ratios decreases. Solvents in conventional cobalt-catalyzed hydroformylation affect the isomer distribution. In propylene... 

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