Chemistry Reference
In-Depth Information
Fig. 14.2
New process for
manufacturing 4-ADPA.
Through a textbook series of organometallic trans-
formations, the Rh catalyst oxidatively adds MeI,
inserts a molecule of CO and reductively eliminates
acetyl iodide, which is converted under the reaction
conditions to acetic acid and HI. The HI reacts with
a molecule of methanol to generate MeI for the next
cycle. The mechanism demonstrates the level of
detail to which homogeneously catalysed processes
can be understood and illustrates one of the differ-
ences between heterogeneous and homogeneous
catalysis. Homogeneously catalysed reactions gener-
ally proceed via discrete, structurally well-defined
intermediate complexes. Thus, it is generally easier
to control the reaction, and hence determine product
distributions, reaction rates, etc., using simple modi-
fications such as changing the nature of the ligand
around the metal. This control also could be used as
a tool to minimise the E value of an industrial
process.
The DuPont Ni-catalysed hydrocyanation of buta-
diene is also notable as a route for the production of
Fig. 14.3
Mechanism of Rh-catalysed methanol carbonylation.
nated a much longer route to the same compound
and has become Solutia's manufacturing method for
4-ADPA [31].
Early examples of more traditional transition-
metal-mediated homogeneous catalysis are still rel-
evant today with regard to their high efficiency. For
example, more than 50% of the world's acetic acid
is produced by [Rh(CO)
2
I
2
]
-
-catalysed methanol car-
bonylation, a route developed by Monsanto in the
late 1960s [32]. Third-generation processes have
been developed that replace the original Rh catalyst
with an Ir complex [33], and results from Japan
describe a new technique employing a Pd/het-
eropolyacid catalyst supported on silica [34]. The
mechanism of the carbonylation is shown in Fig.
14.3 [35].
A Rh(III) salt is converted to [Rh(CO)
2
I
2
]
-
by treat-
ment with an iodide promoter (MeI, HI, I
2
) and CO.
