Ethene trimerisation

Continuing research into this catalyst, Heinig et al recently developed multiple N- and P-modified ligands complexed with Cr" in order to monitor their activity towards ethene trimerisation at various temperatures... 

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. 

Figure 9.9. Mechanism for the trimerisation of ethene using chromium catalysts...

Figure 9.11. Ligand for use as a chromium catalyst for the trimerisation of ethene...

The role of metallacycles, as intermediates in chromium-catalysed ethylene trim-erisation, is highlighted in this article. It is also shown that when 37 is treated with ethene in a protonolysis reaction, hexane and butane are liberated (1 3 ratio hexane butane). This observation gives strength to the argument that metallacycles are intermediates in chromium-catalysed ethylene trimerisation. Scheme 13 provides a simplified representation for chromium-based ethylene trimerisation. 

Since the mechanism of ethene polymerisation commonly suggested for transition-metal catalysts involves the formation of metallacycle, development of this type of catalyst was performed in order to elucidate the type of intermediate involved in the reaction. Binuclear Cr(ii) metallacycles showed little production of 1-hexene when reacted with ethene at room temperature, even after reacting for over 24 h. However, mononuclear Cr(m) metallacycles were considered possible intermediates as these complexes are able to trimerise ethene. Thus, Monillas et al. concluded this to be the likely intermediate involved in the reaction. Nonetheless, the use of the Cr(i) dinitrogen complex yields the desired product, 1-hexene, in a mechanism... 

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