Page 137 - 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 485

FIGURE 66 Formation of terephthalic acid.

acid, which is almost insoluble in acetic acid. The interme- oration/condensation of acetic acid. This heat can be used
diate product is 4-formylbenzoic acid. This cocrystallizes in the solvent distillation/purification part of the plant; see
with the final product. Since it is a mono-acid it is will Fig. 68.
cause a termination of the polymerizing chain. The oxidation proceeds in two steps, p-toluic acid is the
High molecular weights are needed to obtain strong intermediate. This mono-acid has a high solubility in this
materials. Thus, the presence of mono-acids is detrimen- medium and undergoes the second oxidation. The conver-
tal to the quality of terephthalic acid and they have to be sion is high, approaching 100% and the yield based on
carefully removed. One approach involves a second ox- p-xylene is ∼95%. The solubility of terephthalic acid in
idation carried out at higher temperatures and the other most solvents is extremely low, even at this high temper-
method involves reduction of the formylbenzoic acid to ature. The solubility in acetic acid at room temperature is
toluic acid. The latter in more soluble and stays behind in 0.1 g/l. At 200 C it is still only 15 g/l. Thus, most of the
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the liquid. The reduction is done with a palladium catalyst product crystallizes while it is being formed and, as men-
on carbon support. This method gives the highest quality tioned above, the intermediate formylbenzoic acid cocrys-
terephthalic acid. tallizes. The mixture of liquid and solid is led to a filter in
The polymerization with ethylene glycol can be carried which the crude product containing 0.5% of mono-acid is
out directly from the acid if this is very pure. The esterifi- collected. The liquid, containing traces of xylene, p-toluic
cation/polymerization is conducted at high temperatures acid, water, the catalyst, and some product is sent to a dis-
(200–280 C) using metal carboxylates as homogeneous tillation unit to remove water and to recover acetic acid
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catalysts. and the catalyst. The catalyst is relatively cheap, but the
heavy metals cannot be simply discarded and a recycle of
these robust catalysts would seem to be in place.
3. Process Description
As outlined above a purity of “only” 99.5% is a serious
Most of the terephthalic acid is produced with a catalyst problem in making a polymer feedstock and the mono-
system developed by Scientific Design. It was purchased acid intermediate has to be removed either by oxidation
by Amoco and Mitsui and is referred to as the Amoco Ox- under more forcing conditions (Mitsubishi) or hydrogena-
idation. The solvent is acetic acid. Compressed air is used tion to the more soluble 4-toluic acid (Amoco). The final
as the source for oxygen. The catalyst dissolved in acetic product contains amounts of 4-formylbenzoic acid as low
acid and the two reactants are continuously fed into the as 15 ppm. The proportion of toluic acid may be an order
reactor. The temperature is around 200 C and the pressure of magnitude higher. Aldehydes usually cause coloring of
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is approximately 25 bar. The reaction is very exothermic polymers and that is an additional reason to remove them
(1280 kJ/mol). This heat of reaction is removed by evap- carefully.

FIGURE 67 Structures involved in terephthalic acid synthesis.