Puri?ction of crude terephthalic acid by hydrogena
tion over a suitable catalyst is well-known. Hydrogena
tion offers the easiest route for removal of 4-carbox-'
ybenzaldehyde (4-CBA) impurity from the crude ter
ephthalic acid (TA). This invention is directed to an improved process for the hydrogenation of crude ter
ephthalic acid in the presence of a catalyst prepared by
utilizing palladium metal deposited upon an active car
bon support from soluble palladium complexes which
react with the carbon to produce a catalyst of improved
activity and/or selectivity in hydrogenating 4~carbox
ybenzaldehyde. '
Catalysts comprising a Group VIII metal upon an inert carrier are known for use in various hydrogena
tion reactions. They are usually prepared by impregnat
ing a support material with a solution of a compound of
a Group VIII metal and reducing the impregnated com
pound to the metal. Catalyst improvements typically
have been directed to obtaining increased hydrogena
tion activity rather than increased activity and/ or selec
tivity in hydrogenating speci?c compounds.
It is an object of the instant invention to provide an
improved method for preparing a supported catalyst of a Group VIII metal. A particular object is to provide a
method for preparing such catalysts having increased
catalytic activity and/or selectivity in the reduction of
4-carboxybenzaldehyde. Another object is to provide a
catalyst composition which comprises crystallites of
catalytically active palladium upon the surface of a
porous carbonaceous support material wherein a cata
lyst of improved activity and/or selectivity is obtained
for use in a process for reduction of 4-carboxybenzalde
hyde in puri?cation of crude terephthalic acid contain
ing up to 10,000 ppm of 4-carboxybenzyaldehyde. Still
further objects will be apparent from the following speci?cation.
The ?eld of this invention accordingly relates to
Group VIII metal catalysts for hydrogenation and puri
?cation of terephthalic acid suitable for polyester poly mers and copolymers useful in the manufacture of tex
tile ?bers. These polymers and copolymers have been
made by condensing terephthalic acid with ethylene
glycol and other dihydric alcohols.
As with other supported catalysts, the activity and
selectivity of a Group VIII metal catalyst upon a carrier
depends on numerous factors such as the amount of
Group VIII metal or metals present in the catalyst, the
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type of support, the method by which the Group VIII .
metal is deposited and the distribution of the metal on the support.
Such Group VIII catalysts are limited in their ability
to selectivity hydrogenate impurities in the terephthalic
acid, especially 4-carboxybenzaldehyde. Users of ter
ephthalic acid, such as textile ?ber manufacturers, often
put a rigorous limitation on the allowable concentration
of 4-carboxybenzaldehyde in terephthalic acid.
Typically, Group VIII metal catalysts, such as palla;
dium catalysts, are prepared by causing a palladium salt
to be adsorbed from a solution onto a carrier. In one
procedure as is taught in U.S. Pat. No. 2,857,337, the
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salt is then treated with a water-soluble metal hydroxide
or basic carbonate which is thereafter reduced to metal
lic palladium by reducing agents such as formaldehyde,
glucose, hydrazine, glycerine and the like. Other con
ventional methods of preparing palladium catalysts
have been taught. U.S. Pat. No. 2,802,794teaches im
pregnation of an activated alumina support material
with a solution of a compound of the platinum metal
group and reducing the impregnated compound to the
metal. The preconditioned activated alumina is ob
tained by heating a hydrated alumina to a temperature
of up to 800° C. whereby a microporous alumina is
obtained.