Modification of Phenolic Oximes for Copper Extraction
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Date
2008Author
Forgan, Ross Stewart
Metadata
Abstract
The thesis deals with the modification of salicylaldoxime-based reagents used in
hydrometallurgical extraction, addressing rational ligand design to tune copper(II)
extractant strengths and also the development of reagents which are capable of
transporting transition metal salts.
Chapter 1 reviews current solvent extractant technology for metal recovery,
including the limited knowledge of the effect of substituents on extractive efficacy.
Advances in leaching technology have led to systems wherein increases in process
efficiency could be obtained using reagents which can transport both a transition
metal cation and its attendant anion(s), and the potential advantages of metal salt
extractants are discussed. The problems encountered when trying to extract
hydrophilic anions selectively into organic media are also considered.
Chapter 2 discusses techniques used in industry to tune reagent properties, many of
which depend on the importance of H-bonding in non-polar solvents. Synthesis of a
series of 5-alkyl-3-X-2-hydroxybenzaldehyde oximes (X = a range of substituents) is
described and copper extraction experiments are reported. 3-Substitution is found to
alter reagent strength by two orders of magnitude, with 3-bromo-5-tert-butyl-2-
hydroxybenzaldehyde oxime the strongest extractant. An analysis of X-ray
structures of several ligands and copper(II) complexes is given in an attempt to
establish whether trends in the solid state structures can account for variations in
extractant strength. A more detailed analysis of the hydrogen bonding in
salicylaldoximato copper(II) complexes and ligand dimers is carried out in Chapter
3, with the aim of defining how substituent effects could be used to design reagents
with appropriate extractive behaviour. 3-X-2-Hydroxybenzaldehyde oximes with no
5-alkyl substituent are synthesised and subjected to a detailed study by X-ray
crystallography and computational techniques, which, alongside evidence provided
by CID-MS experiments, suggest that the dominant substituent effect in determining
extractant strength is the ability to “buttress” the pseudomacrocyclic hydrogen
bonding motif involving the oximic hydrogen and phenolic oxygen. Ligands with 3-substituents capable of accepting H-bonds were found to be stronger extractants than
those which could not, and the steric hindrance afforded by bulky substituents made
3,5-di-tert-butyl-2-hydroxybenzaldehyde oxime the weakest extractant. Ligand
acidity is also noted to have a significant effect on reagent strength, with electronwithdrawing
substituents lowering the pKa of the phenolic proton and increasing
extractive efficacy.
Chapter 4 focuses on metal salt extraction, and the development of selective, robust
and hydrolytically stable reagents. Six novel extractants, based on a salicylaldoxime
scaffold with a pendant dialkylaminomethyl arm, are described. Only 5-tert-butyl-3-
dihexylaminomethyl-2-hydroxybenzaldehyde oxime and 3-tert-butyl-5-
dihexylaminomethyl-2-hydroxybenzaldehyde oxime have sufficient solubility to be
effective reagents. The former extracts CuCl2 and ZnCl2 in a highly efficient
manner, with one mole of metal salt extracted per mole of ligand, twice the expected
capacity. X-ray structure determination of complexes of the related ligand 5-tertbutyl-
2-hydroxy-3-piperidin-1-ylmethylbenzaldehyde oxime defines the binding
mode, with the chloride anions bound to the inner sphere of the metal cations.
Loading and stripping experiments show it to be an extractant with potential
commercial application. Cation and anion selectivity of the two extractants defined
above is the focus of Chapter 5, which begins with an overview of techniques and
attempts to attenuate the Hofmeister bias, the main factor in the selective extraction
of hydrophilic anions into organic media. pH loading profiles show the 3-
dihexylaminomethyl isomer to be an effective CuCl2 and CuSO4 extractant, but the
cation extractive efficacy of the 5-isomer is hampered by the 3-tert-butyl group.
Both ligands are found to be selective for Cl- > SO4
2-, following the Hofmeister bias.
Further information on anion binding is provided by solid state structures of copper
salt complexes, showing that in all cases the copper(II) cation interacts in some way
with the anion. Cation extraction is affected significantly by the anion present, with
FeIII selectively extracted against CuII in the presence of SO4
2- which is consistent
with cation-anion interactions having great influence on the overall stability of the
ligand-metal salt assembly.