Page 123 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

Catalysis, Homogeneous 471

technical literature say “oxo” reaction) is employed for Thus the reaction is highly exothermic and favored by
the large-scale preparation of butanal and butanol, 2-ethyl thermodynamics at temperatures roughly below 200 C.
◦
hexanol, and detergent alcohols. Butanal and butanol Hydrogenation of the alkene to alkane is thermodynami-
are used in many applications as a solvent, in esters, cally even more attractive. Often this reaction is observed
in polymers, etc. The main use of 2-ethylhexanol is in as a side reaction.
phthalate esters, which are softeners (plasticizers) in In industrial practice the older cobalt catalyst is still
PVC. The catalysts applied are based, again, on cobalt used today for the conversion of higher alkenes to deter-
and rhodium. gent aldehydes or alcohols (>C 12 ). The cobalt process
requires high pressures (70–100 bar) and temperatures
◦
2. Cobalt-Based Oxo-Process (140–170 C). Aldol condensation and hydrogenation of
the alkene to alkane (∼10%) are undesirable side reac-
Roelen accidentally discovered the hydroformylation of tions for the detergent alcohols. Interestingly, the cheaper
alkenes in the late 1930s while he was studying the con- internal alkenes can be used for this process and yet the
version of synthesis gas to liquid fuels (Fischer-Tropsch outcome is mainly a terminally hydroformylated, linear
reaction) using a heterogeneous cobalt catalyst. It took aldehyde. Often the separation of catalyst, product, by-
more than a decade before the reaction was taken further, product, and starting material is tedious. For propene
but now it was the conversion of petrochemical hydrocar- the most economic processes are rhodium-based catalysts
bons into oxygenates that was the driving force. It was commercialized in the 1970s.
discovered that the reaction was not catalyzed by the sup-
ported cobalt but, in fact, by HCo(CO) 4 formed in the
3. Rhodium-Based Hydroformylation
liquid state.
A key issue in the hydroformylation reaction is the ra- Fundamental work by Nobel laureate Wilkinson demon-
tio of “normal” (linear) and “iso” (branched) product be- strated that rhodium triphenylphosphine catalysts allowed
ing produced. Figure 37 explains this colloquial expres- the operation of the hydroformylation reaction at much
sion. The linear (“normal”) product is the desired product; lower pressure (1 bar was reported by Wilkinson) and
the value of butanal is higher because this is the product temperature than the cobalt process. The selectivity was
which can be converted to 2-ethyl hexanol via a base- also reported to be considerably higher, virtually no hy-
catalyzedaldolcondensationandahydrogenation.Thede- drogenation was observed and the linearity was in some
tergent alcohols should be preferably linear because their cases as high as 95%. The rhodium catalysts were re-
biodegradability was reported to be better than that of the ported to be three orders of magnitude faster in rate. The
branched product. The linearity obtained in the cobalt- resulting milder reaction conditions would give much less
catalyzed process is 60–80%. The reaction mechanism condensation products. In 1971 Union Carbide Corpo-
for cobalt is similar to that of rhodium, which will be dis- ration, Johnson and Matthey, and Davy Powergas (now
cussed in the next section. Kvearner) joined forces to develop a process based on
Thermodynamics of hydroformylation and hydrogena- this new finding. As yet it is only applied for propene.
tion at standard conditions are as follows: Hydroformylation of propene is of prime importance and
worldwide probably more than 7 million tons of butanal
H 2 + CH 3 CH=CH 2 + CO → CH 3 CH 2 CH 2 C(O)H
are produced this way annually.
G15 −33 −28(l) =−10 kcal/mol A convenient catalyst precursor is RhH(CO)(PPh 3 ) 3 .
Under ambient conditions it will slowly convert 1-alkenes
H 5 −26 −57 =−36 kcal/mol
into the expected aldehydes. Internal alkenes exhibit
◦
hardly any reaction. At higher temperatures (100–120 C)
H 2 + CH 3 CH=CH 2 → CH 3 CH 2 CH 3
pressures of 10–30 bar are required. Unless a large excess
G15 −6 =−21 kcal/mol
of ligand is present the catalyst will also show some iso-
H5 −25 =−30 kcal/mol merization activity, but the internal alkenes thus formed

FIGURE 37 The hydroformylation reaction.