Oligomerisation

Oligomerisation.—Much detailed discussion, based to a large extent on product characterisation and distribution observations, has appeared on catalysis of olefin oligomerisation by nickel complexes. Thus for the first example 7r-2-butenylnickel chloride, in contrast to the analogous 7r-allyl compound, is not a good catalyst for polymerisation of butadiene. But in the presence of trichloroacetate the 7r-2-butenyl complex does exhibit catalytic activity. This enhancement of activity is ascribed to the intermediacy of charge-transfer complexes. The mechanism of cyclotrimer-isation of butadiene has been considerably clarified by the characterisation of an intermediate (21) and determination of the distribution of isomers 

Herisson, Y. Chauvin, Nhu Hung Phung, and G. Lefebvre, Compt. rend., 1969, 269, C, 661. 

Palladium compounds are also useful catalysts for oligomerisation and co-oligomerisation reactions. Dimerisation or co-dimerisation of olefins catalysed by palladium(n) chloride or, better, PdCl2(C6H5CN)2 involves a palladium(ii)-hydride intermediate as the catalytically active species. A similar reaction to dimerisation is coupling, e.g. of benzene to give biphenyl, which is also catalysed by palladium(ii) chloride and proceeds via palladium-phenyl u-bonded species. The nature of intermediates in 

Onsager, H. Wang, and U. Blindheim, Helv. Chim. Acta, 1969, 52, 196. 

A besetting problem with the industrial process to remove traces of alkynes alkadienes from alkene streams using palladium catalysts has been the formation of higher hydrocarbons by oligomerisation. Although in this respect palladium is better than base metals such as nickel (which presumably explains why this cheaper metal is not used), and while the fraction of ethyne that reacts in this manner is small, nevertheless in a continuous operation these higher products accumulate, and cause problems. The carbonaceous deposits, so often mentioned, may be partly C2 species such as ethylidyne, but they also comprise adsorbed forms of oligomers in the steady state their formation is followed by release into the fluid phase. 

The ethene-based processes give only even-numbered alkenes (including the one making alcohols) while the other two processes produce both odd and even numbered alkenes. In the last two decades there has been a trend to close down wax cracking alkene plants, and the products are now increasingly derived from ethene. Note that ethene is also a cracker product. 

The catalyst is prepared in a pre-reactor from nickel salts with boron hydrides as the reductant under a pressure of ethene and then ligand is added. 

According to the specification by Shell the product contains only 3% of internal or branched alkenes. This is an important feature of the process. The ethene pressure has to be maintained at a high level, compared with the butene concentration, in order to obtain preferential insertion of ethene with respect to butene. Insertion of the latter leads to branched products. Another peculiar property of this catalyst is that it has very little activity for isomerisation under the conditions. Obviously this is also very important because the goal is making terminal alkenes. 

This means that the maximum amount of oligomers within a certain range can be calculated by this expression. For the synthesis of detergent alkenes this means that only half or less of the alkenes produced fall within the desirable range of useful materials, i.e. C6 through C2o compounds. 

In practice this range is somewhat smaller. Hence the selectivity to desired product is in the order of only 50 %  

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Properties. Liquid fuels derived from oxidative coupling/olefin oligomerisation processes would be expected to have properties similar to those derived from olefin oligomerisation pathways such as MTO/MOGD. 

Idemitsu Process. Idemitsu built a 50 t x 10 per year plant at Chiba, Japan, which was commissioned in Febmary of 1989. In the Idemitsu process, ethylene is oligomerised at 120°C and 3.3 MPa (33 atm) for about one hour in the presence of a large amount of cyclohexane and a three-component catalyst. The cyclohexane comprises about 120% of the product olefin. The catalyst includes sirconium tetrachloride, an aluminum alkyl such as a mixture of ethylalurninumsesquichloride and triethyl aluminum, and a Lewis base such as thiophene or an alcohol such as methanol (qv). This catalyst combination appears to produce more polymer (- 2%) than catalysts used in other a-olefin processes. The catalyst content of the cmde product is about 0.1 wt %. The catalyst is killed by using weak ammonium hydroxide followed by a water wash. Ethylene and cyclohexane are recycled. Idemitsu s basic a-olefin process patent (9) indicates that linear a-olefin levels are as high as 96% at C g and close to 100% at and Cg. This is somewhat higher than those produced by other processes. 

There are other commercial processes available for the production of butylenes. However, these are site or manufacturer specific, eg, the Oxirane process for the production of propylene oxide the disproportionation of higher olefins and the oligomerisation of ethylene. Any of these processes can become an important source in the future. More recentiy, the Coastal Isobutane process began commercialisation to produce isobutylene from butanes for meeting the expected demand for methyl-/ rZ-butyl ether (40). 

Oligomerization of Ethylene. 1-Butene is a small by-product in the production of linear alpha-olefins by oligomerisation of ethylene. Linear alpha-olefins have one double bond at the terminal position and comprise the homologous series of compounds with carbon atoms between 4 and 19. The primary use of alpha-olefins is in the detergent industry. About 245,000 t/yr of 1-butene was produced for chemical use in the Gulf Coast of the United States in 1988 (72). 

In Gegenwart von Tetrakis-[pyridin]-nickel-diperchlorat oder Nickel(II)-chlorid in Athanol mit Tetrabutylammonium-perchlorat als Leitsalz erhalt man unter partieller Hy-drierung und Oligomerisation aus Butadien-(1,3) all-trans-Hexadecatetraen-(1,6,10, J4) in Gegenwart von Bis-[triphenylphosphin]-nickel(II)-chlorid wird Octatrien-(l,3,7) er-halten4. 

A -Heterocyclic Carbene Complexes in Polymerisation, Oligomerisation and Telomerisation Reactions... 

Abstract Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in a-olefin polymerisation and oligomerisation, CO/olefm copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discnssed in subsequent sections. 

The Cr analogues 6 (Fig. 4.2) of the bis(carbene)pyridine systems were found to be exceptionally active for the oligomerisation of ethylene [10,11]. Activation with MAO led to optimal results, and complexes with Me, Pr and substitution... 

Keim W, Behr A, Roper M (1982) Alkene and alkyne oligomerisation, cooligomerisation and telomeiization reactions. In Comprehensive organometallic chemistry, vol 8. Pergamon Press, Oxford... 

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