Abstract
Sasol endeavors to expand its current business through the beneficiation of commodity
feedstreams having marginal value into high-value chemicals via cost-effective
processes. In this regard Merisol, a division of Sasol has access to phenolic and cresylic
feedstreams, which have the potential to be converted to fine chemicals.
Targeted products include para-anisaldehyde, ortho- and para-hydroxybenzaldehyde
which are important intermediates for the manufacture of chemicals used in the flavor
and fragrance market and various other chemicals. The use of CO technology (HF/BF3)
to produce these aldehydes in a two-step process from phenol as reagent is
economically attractive due to the relative low cost and other benefits associated with
syngas as reagent. The aim of the study was to evaluate and understand this relatively
unexplored approach to the formylation of aromatic compounds.
The reactivity of both phenol and anisole proved to be much lower than that of toluene.
The main aldehyde isomer (para) produced in these HF/BF3/CO formylations as well as
of other ortho-para directing mono-substituted benzenes tested in our laboratories were
in accordance with published results. Efforts to increase substrate conversion resulted in
substantial secondary product formation and mechanistic investigations showed this to
be a consequence of the inherent high acidity of the reaction environment.
The effect of different substituents on the relative formylation rates of benzene
derivatives was investigated. These results showed that methyl groups are activating
while halogens are deactivating relative to benzene as substrate. The decrease in
reactivity from fluorobenzene> chlorobenzene> bromobenzene is in accordance with
formylation trends observed in other acidic systems.
Deuterium labeling experiments were applied to gain additional information on the
formylation reaction mechanism. This study provided interesting but inconclusive results
in support of the so-called intra-complex mechanism.
All reported studies as well as our own work suggested that HF and BF3 in (at least)
stoichiometric amounts are required for effective formylation with CO. Under these
conditions this methodology for effecting aromatic formylation is not economically viable.
Industrial application of formylation using CO will require the development of new
catalysts or methodology to allow the use of HF/BF3 in a catalytic way. In this regard
ionic liquids as a new and ecological-friendly field was explored. Chloro-aluminate ionic
liquids promote the carbonylation of alkylated aromatic compounds, but fails in the case
of oxygenated aromatics. Aldehyde yields of formylation in the acidified neutral ionic
liquids were generally similar compared to reactions conducted in HF as solvent/catalyst.
Formylation of anisole and toluene, but not of phenol in the neutral ionic liquids resulted
in increased secondary product formation in comparison with hydrogen fluoride used as
solvent/catalyst. This difference in behavior is not understood at present, but suggests
that phenol is a good substrate for formylation in this medium, particularly with the
development of a system catalytic with respect to HF/BF3 in mind.
Prof. C.W. Holzapfel