Abstract
Ionic liquids (ILs), consisting of ions that are liquid at ambient temperatures, can
act as solvents for a broad spectrum of chemical processes. These ionic liquids
are attracting increasing attention from industry because they promise significant
environmental as well as product and process benefits. ILs were used as
solvents for two industrially important homogeneous reactions namely metathesis
of 1-octene and the hydroformylation of vinyl acetate.
In the metathesis of 1-octene, several reaction parameters were investigated,
including temperature, catalyst (type and concentration) and influence of ionic
solvent and conventional solvents. Temperature and catalyst concentration were
found to be rate-determining factors, but played smaller roles in determining the
outcomes of the reactions compared to the influence of individual ILs. It was
discovered that more polar ILs were favourable in producing high rates and
selectivities. Imidazolium-based cations and tetrafluoroborate anions were
superior in activity when compared to other combinations of cations and anions.
The addition of catalyst promoters such as phenol and tin(II) chloride were also
investigated and found to enhance metathesis rates in “neat” reactions. These
catalyst promoters inhibited metathesis rates when used in combination with ILs.
In the hydroformylation of vinyl acetate, several reaction parameters were
investigated, including temperature, catalyst concentration, vinyl acetate
concentration, ligand concentration, syngas pressure and influence of ionic
solvent and conventional solvents. It was shown that high n : i ratios of aldehyde
products were formed with specific IL systems. Also, low ligand concentrations
and low vinyl acetate concentrations increased selectivities, although rates of
reactions were somewhat compromised. Lower syngas pressure and lower
temperatures afforded enhanced selectivities, again at the expense of reaction
rates. Depending on whether fast reaction rates or high regioselectivity is
required, the IL and general reaction conditions can be tailored to fit the needs of
the reaction. It was discovered that aromatic-containing ammonium-based ILs
v
afforded high rates at low selectivity. Bulkier ammonium cations tended to give
lower rates but the selectivity was significantly enhanced. Impurities present in
ILs have also been shown to have a marked effect on hydroformylation rates and
selectivity.
The reader will be accompanied along a path designed to discover an optimised
set of reaction conditions, the path of which will take the reader from reactions
providing low selectivities, low turnover numbers and low yields to a much
brighter picture, namely extremely high selectivities, turnover numbers and
yields.
Prof. D.B.G. Williams