Metal carboxylate complexes relevant to the Fischer-Tropsch synthesis

Pienaar, Andrew (2005-03)

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2005.


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.

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