Metal catalysed acetylene oligomerisation
thesisposted on 2023-05-26, 05:38 authored by Karpiniec, SS
The oligomerisation of acetylene by metal catalysts has been investigated as a potential route to liquid products, in the context of Gas-to-Liquid generation of petrochemicals. The catalysts trialled are known for their high activities in the polymerisation and oligomerisation of ethylene. Group III, IV and V metallocenes `Cp_2MCl_n` `(M = Sc, Y, n = 1; M = Ti, Zr, Hf, V, n = 2)`, `Cp`\\(^*\\)\\( _2\\)`YCl*THF` and `[Cp`\\(^*\\)\\( _2CeCl]_n\\) were activated with a range of alkyl aluminium cocatalysts, `MAO` and `AlEt_xCl_(3-x)` `(x = 2,3)`, and exposed to acetylene. Diimine complexes of nickel and palladium were also trialled, as were a small range of chromium complexes, in the presence of MAO. Activities were extremely low for all of these complexes, except in the presence of AlEt3, where some light oligomers were produced (`C_4, C_6`). Further studies showed that growth occurred at `AlEt_3` itself, and that the transition metals were ineffective. Elevated temperatures and extended run times produced a complex range of oligomeric and polymeric products, some of which were identified with the use of GC-FID and GC-MS. Oligomer growth is slow, and branching is introduced at an early stage; several proposals as to the mechanism of growth were suggested. The use of hydrogen gas and high metallocene concentrations failed to provide effective chain transfer activity. This system was explored theoretically using DFT methods, which showed that dimeric aluminium species impede product growth beyond the first insertion; crystallographic evidence also supported this claim. The use of AlEtCl2 as an activator led to the copolymerisation of acetylene and aromatic solvents, and the nature of this process and the formed polymer were investigated in more detail. Bis(imino)pyridineiron(II) catalysts were trialled with acetylene, displaying high initial activity but quick deactivation. The catalyst containing 2,6-diisopropylphenyl substitution produces polyacetylene, as well as oligomers in the presence of the chain transfer agent `ZnEt_2`. The oligomer array is complex and was investigated by GC-FID and GC-MS; a mechanism is proposed for the formation of identified compounds. The use of more `ZnEt_2` generates a higher proportion of oligomer, but slows catalyst activity. Catalyst deactivation was investigated by SEM and ICP-MS, and found to be due to encapsulation within the insoluble polyacetylene. The catalyst was not able to effective co-polymerise acetylene and ethylene. The ortho-tolyl substituted catalyst primarily forms benzene from acetylene (cyclotrimer). Deuterium labelling studies suggest cyclotrimerisation via a metallocyclic mechanism, which is interrupted in the presence of the `ZnEt_2`. Hydrogen was not effective as a chain transfer agent for the iron catalysts.
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