Ethene oligomerisation

A second process that came on stream in the fifties is the oligomerisation of ethene using cobalt complexes, but the number of homogeneously catalysed... 

The higher alkene feed (C10-14) for the production of detergent alcohols is either a product from the wax-cracker (terminal and internal alkenes) or the byproduct of the ethene oligomerisation process (internal alkenes). In the near future a feed from high-temperature Fischer-Tropsch may be added to this. The desired aldehyde (or alcohol) product is the linear one and the cobalt catalyst must therefore perform several functions ... 

Alcohol dehydration. Other routes to linear higher alcohols are available such as the reduction of fatty acids and an aluminium based oligomerisation of ethene followed by oxidation. The higher alcohols are used as such or they are dehydrated to make 1-alkenes. 

Ethene oligomerisation. In view of the above limitations there is a demand for a process that selectively makes linear 1-alkenes. Three processes are available, two based on aluminium alkyl compounds or catalysts and one on nickel catalysts. The aluminium processes use aluminium in a stoichiometric fashion and they produce a narrow molecular weight distribution (a Poisson distribution, vide infra). 

Figure 9.7. Example of a new catalyst for ethene oligomerisation and polymerisation...

Another, highly selective oligomerisation reaction of ethene should be mentioned here, namely the trimerisation of ethene to give 1-hexene. Worldwide it is produced in a 0.5 Mt/y quantity and used as a comonomer for ethene polymerisation. The largest producer is BP with 40 % market share utilizing the Amoco process, formerly the Albemarle (Ethyl Corporation) process. About 25 % is made by Sasol in South Africa where it is distilled from the broad mixture of hydrocarbons obtained via the Fischer-Tropsch process, the conversion of syn-gas to fuel. The third important process has been developed by Phillips. 

Tridentate ligands for cobalt and iron catalysts. The catalysts discussed earlier in the section on ethene oligomerisation can also be used for making polymers, provided that they are suitably substituted. In Figure 10.30 we have depicted such a catalyst, substituted with isopropyl groups at the aryl substituents on the imine group, as in Brookhart s catalysts [49], The initiation is now carried out by the addition of MAO to a salt of the cobalt or iron complexes. The catalysts obtained are extremely active, but they cannot be used for polar substrates. 

When chain transfer is very fast, the reaction observed is the alkoxycarbonylation of ethene, which is nothing but a perfect chain transfer after the insertion of just two monomers. In recent years several very fast catalysts for this reaction have been reported as we will see in the next section. The first effective catalysts reported by Sen [8] (Pd(BF4)2 and PPh3) also give relatively fast chain transfer as the Flory-Schulz constant for their product was only 0.3-0.4 (see Chapter 9.2.1 on oligomerisation) and the product is an oligomer rather than a polymer. 

The first large-scale application was the Phillips Triolefin Process (1966) in which propene was converted into ethene and 2-butene. Due to market changes the reverse process, in which propene is produced, became more attractive later. This process has been in operation since 1985. Another process is the Shell Higher Olefin Process (SHOP) in which ethene is oligomerised and the products are metathesised into detergent range olefins. The same company developed a process in speciality chemicals in which alpha-, omega-dienes are formed from cyclic alkenes. 

Figures 1 and 2 show the product distribution as a function of time of the reaction of propene over fully oxidised (Mo +) and fully reduced (Mo +) MoH-mordenite (6.1 wt % Mo, 1.95 Mo/unit cell) respectively. The products are not those of clean metathesis, with only traces of ethene produced, the predominant products being cis-2-butene, pentenes and propane. The products probably arise from cracking, oligomerisation and self-hydrogenation reactions of primary metathesis products, catalysed by residual acid sites. The plain H-mordenite catalyst also exhibits the ability to oligomerise and hydrogenate propene at these temperatures (producing cis-2-butene and propane).

Naked [Fe4] ions in the gas phase were shown by mass spectrometry to oligomerise ethyne to benzene via the cationic intermediate [Fe(C2H2)m] (m = 1-4)256. r was reported that the ethene ligand in [Fe(CO)2 P(OMe)3 2(Tl2-C2H4)] could be substituted by diphenylacetylene to form [Fe(CX))2 P(OMe)3 2(il -C2Ph2)]. The X-ray crystal structures of the alkene and alkyne complexes were determined257. Reaction of [CpFe P(OMe)3)2(NCMe)]+[PF6] with [PhCXH]... 

Treatment of [CpTiCl(dmpe)2] with one equivalent of MeLi affords [CpTi(Me)(dmpe)2]. This complex is an active catalyst for the oligomerisation of ethene to but-l-ene, 2-ethylbut-l-ene and 3-methylpent-l-ene. The mechanism of the catalysis is proposed to involve titanacyclopentane intermediates. The complex [Cp TiMe2][MeB(C6Fj)3] acts as both a Ziegler-Natta catalyst for the polymerisation of alkenes, such as ethene and propene, and as a carbocationic initiator for the polymerisation of election rich alkenes such as A -vinylcarbazole and vinyl ethers. It also polymerises styrene to syndiotactic polystyrene, probably via a carbocationic initiated polymerisation. 50... 

The Nickel Triad. - The square-planar nickel(II) complex [Ni(Me)(PMe3)(2-phosphanylphenolato)] and the five-coordinate species [Ni(Me)(PMe3)2(2-phosphanylphenolato)] have been prepared and found to be effective one-component catalysts for the oligomerisation of ethene. The aryl-nickel(II) complexes 38 have been found to be remarkably efficient catalysts for... 

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