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Metal catalysed acetylene oligomerisation

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Karpiniec, SS (2010) Metal catalysed acetylene oligomerisation. PhD thesis, University of Tasmania.

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Abstract

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
Cp2MCln (M = Sc, Y, n = 1; M = Ti, Zr, Hf, V, n = 2), Cp*
2YCl×THF and [Cp*
2CeCl]n
were activated with a range of alkyl aluminium cocatalysts, MAO and AlEtxCl3-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 (C4, C6). Further studies showed
that growth occurred at AlEt3 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 ZnEt2. 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 ZnEt2 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 ZnEt2.
Hydrogen was not effective as a chain transfer agent for the iron catalysts.

Item Type: Thesis (PhD)
Keywords: catalysis, acetylene, homogeneous, oligomerisation, synfuels, transition metal
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Date Deposited: 04 May 2011 03:13
Last Modified: 11 Mar 2016 05:53
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