Metal carboxylate complexes relevant to the Fischer-Tropsch synthesis
Date
2005-03
Authors
Pienaar, Andrew
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
In a Fischer-Tropsch refinery environment carboxylate complexes are of
interest since the carboxylic acids present in product streams lead to
formation of carboxylate salts through leaching of process equipment and
catalysts. It is widely accepted that decomposition of organic (carboxylic)
acids catalysed by metals is controlled by the decomposition of metal salts or
complexes previously formed with such an acid. The determination of physical
and structural properties of caboxylate complexes could contribute to the
explanation of the mechanism involved in the decarboxylation of carboxylic
acids.
We have successfully determined the molecular structures of copper(II) allyl
acetate, zinc(II) formiate, zinc(II) isovaleroate, yttrium(III) acetate and
lanthanum(III) propionate. It has been established that zinc has a preferred
tetrahedral coordination in carboxylate complexes compared to the octahedral
coordination of copper, lanthanum and yttrium complexes considered. The
carboxylate O-C-O angle in these complexes range between 119° and 125°
and the conformation of the carbon chains is anti in all cases except for
copper(II) allyl acetate, where a gauche conformation is adopted.
Using structural methods such as TGA, infrared spectroscopy and X-Ray
powder diffraction and combining it with existing knowledge of yttrium
carboxylates and the effective use of computational chemistry – to calculate
favourable internal parameters, using DFT calculations and B-LYP level
theory - a likely structure for yttrium(III) propionate is proposed. The use of
infrared measurements were especially valuable towards predictions of
possible structures and the postulations of Nakamoto, on the relation between
carboxylate carbonyl stretching frequencies and the nature of the carboxylate
bond, could be used to accurately identify – except for the formiate salts of
zinc(II) and yttrium(III) – the bonding mode present in the relevant
compounds. We systematically tuned the non-cyclic organic part of the mono carboxylate
ligand by lengthening and branching of the alkyl chain and determined that
thermal decomposition and heat capacity of zinc complexes are a strong
function of the ligand, while the behaviour of analogous yttrium complexes is
hardly affected.
The thermal investigation of lanthanum(III) propionate yielded a result that is
in contrast with a previous study - where only CO2 was reported as byproduct
- and we report an alternative result which indicates formation of symmetric
ketones when the compound is heated to a high enough temperature. Earlier
general assumptions about the layer-like crystal structure of lanthanum
complexes coordinated by alkyl chain carboxylate are contradicted by the
crystallographic data we collected for this compound. The crystal packing of
lanthanum(III) propionate clearly shows a layered structure which is
unexpected for a carboxylate with such a short alkyl.
Description
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2005.
Keywords
Metal complexes, Fischer-Tropsch process, Carboxylic acids, Dissertations -- Chemistry, Theses -- Chemistry