Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to an improvement in the
process for preparing terephthalic acid by -the oxydation of
paraxylene in solution in acetic acid accordiny to the equation;
C6H4(CH3)2 ~ 2 ~ C6H4(COOH)2 + 2H2O,
in the presence of a catalyst system comprising manganese,
bromine and cobalt.
Japanese patent publication 66643/1974, discloses
that manganese and cobalt acetates are suitable raw materials
for the preparation of a catalytic system for the preparation of
terephthalic acid by the oxidation of paraxylene in solution in
acetic acid, that bromine can be added as hydrogen bromide or
elemental bromine (but never as alkaline bromide), and that
acetaldehyde can b~ advantageously present in t.he oxidation zone.
The method is only partially satisfactory, however, i.e., only
when bromine is added as hydrogen bromide~ If, in prac-tice, the
catalytic system is prepared in acetic acid solution from manga-
nese acetate (or metallic manganese) and elemental bromine, which
preparation is not exemplified in the Japanese publication,
there is a strong danger of manganese dioxide precipitation
(MnO2), which leads to the clogging of manufacturing facilities
and, as a result, enormous losses of output capacity. This is
particularly true when the water amount is higher than 1% by
weight with respect to the acetic acid and when the ratio of
bromine to manganese is in excess of 1.65 by weight. Amounts of
water of as little as 0.2% by weight can sometimes cause very
troublesome cloggings, and, conse~uently, large output losses.
On the other hand, the use of elemental bromine (free
bromine) can have many advantages over the use of hydrobromic
acid. sromine, in fact, is readil~ available, from natural
3Q sources, while hydrogen bromidq is prepared from bromine through
a series of troublesome chemical pathways. Moreover, hydrogen
bromide is a gas which is typically available as a nearly 50%
~1- ~p, '
' 7
solu-tion, having a density of a.bout 1~5 g/cm3, while -the density
of bromine is about 3 g/cm3, double that o-f hydrogen bromide.
Therefroe, the storage volume for hydrogen bromide solutions
must be four times that of bromine.
An object of the invention is, thus, to provide an
improved process for the bromine-catalyzed synthesis of tere-
phthalic acid, in which the source of bromine is elemental
bromine.
This and other objects apparent from the following
disclosure are realized by the invention herein described.
In its broadest aspects, the invention comprises an
improvement in the process for the preparation of terephthalic
acid by the oxidation of paraxylene in acetic acid solution and
in the presence of a catalytic system based on manganese,
bromine and cobalt, wherein bromine is added as elemental (free)
bromine and wherein the resulting solid terephthalic acid is
separated from the mother liquor, at least a portion of said
: mother liquor being partially anhydrified.
The improvement comprises:
(a) preparing the catalytic system by bringing into
contact acetic acid, manganese or manganese compounds, elemental
bromine and a soluble cobalt compound, wherein the addition of
elemental bromine is carried out in the presence of a reducing
compound, pre~erably acetaldehyde, and
(b) feeding the catalytic system obtained from step
(a), containing manganese, bromine and cobalt in a dissolved
form, to an oxidation zone, optionally in admixture with the
paraxylene feed and/or with at least a portion of the partially
anhydrified mother liquor.
According to a preferred embodiment, -the manganese
amount should be from 50 to 1000 mg/Kg of acetic acid, the
manganese: cobalt ratio should ~e from 2:1 -to ~:1 and the
~3~ ~
bromine:manganese ratio should be from 0.5:1 to 1.65:1 by
weight. The amount of water should be preferably lower than
1%, or even 0.2% by weight, with respect to acetic acid, but
higher amounts can be present, provided the acetaldehyde:bromine
molar ratio is equal to or greater than 1, and preferably com-
prised between 1.1 and 1.~. The acetaldehyde reacts according
to the following equation:
CH3CHO + Br2 ~~ H20 ) 2HBr + CH3COOH
Minor amounts of bromo-organic compouds are also formed, for
instance monobromo-acetic acld and monobromo-acetaldehyde.
These compounds release the hydrobromic acid slowly, by way of
an hydrolysis.
The preparation of the catalytic system according to
the invention is, at the beginning, slightly exothermical, but
in a short time supplemental heat is needed, and i-t is advisable
to operate between room temperature and the boiling point of
acetic acid, for instance from 20 to 120C, preferably from
40 to 90C. Cobal-t can be added, preferably as cobalt carbo-
nate or acetate (commercially available in the tetrahydrate form),
and manganese can be added as metal, finely subdivided, or inthe form of carbonate or acetate. Other elements, like for
instance chromium, can he optionally added. Another simple and
suitable source of manganese and cobalt is the recovery mixture
obtained from the treatment of the purge of the terephthalic
acid synthesis, and more particularly from the purge of the syn-
thesis mother liquor. This recovery mixture usually contains
acetates or carbonates and strongly oxidized compounds of man-
ganese which are not easily dissolved. The use of acetic acid-
acetaldehyde solutions permits the attack of every form of re-
covered cobalt and manganese, readily and quantitatively, andpromotes the conversion of brom1ne to hydrobromic ion.
Another way to carry out the process according to the -;
invention is to bring into contact first acetic acid, manganese
and elemental bromine, in the preserlce of a reducing compound,
preferably acetaldehyde, and then to add the soluble cobalt
compound. sesides acetaldehyde, other aldeh~des can be used,
such as for instance, paraldehyde, isobutyrraldehyde, parato-
luic aldehyde, para-carboxy-benzaldehyde and mixtures thereo-f.
Other reducing agents with satisfactory properties are the ali-
phatic alcohols, and particularly those having from 1 to 5
carbon atoms, for instance ethanol and isobutylic alcohol, as
well as diethylacetal and ethyliden-diacetate. Paraxylene it-
self could be considered a reducing agent in this sense.
The following examples illustrate the invention with-
out limiting, however, in any way its scope.
EXAMPLE 1
32.3 parts by weight of electrolytic manganese in the
form of flakes having a thickness of from 1 to 2 mm and a sur-
face of from 0.5 to 5 cm2; 150 parts by weight of acetic acid
containing 0.2% by weight of water, and 46 parts by weight of
liquid bromine at 99.8% (by weight), were loaded in the foregoing
order, and one at a time, into a stainless s-teel autoclave,
equipped with a rotating stirrer, a reflux cooler and a heating
jacket. m e resulting bromine:manganese ratio was 1.42 by
weight. A slightly exothermic reaction began, which brought
the temperature to 45C. Heat was then supplied to keep the
temperature at this value all through the reaction. After all
of the bromine had reac-ted, a suspension consisting of residual
manganese bromide and manganese acetate, in acetic acid, was
obtained. Under these conditions, the reaction of manganese
with bromine occurred more quickly then the reaction with acetic
acid. Then, 231 parts by weight of water were added, thus dis~
solving both bromide and acetate and promoting -the at-tack of
residual manganese by acetic acid. The resulting solution had
.
--4--
3~ ~
the following composi-tion:
Manganese (~n )7.03 % by weight
Bromine (Br~)10.00 % by weight
Free acetic acid24.82 % by weight
water50.20 % by weight
To this solution was added an amount of tetrahydrated cobal-t
acetate sufficient to obtain a Mn:Co ratio, by weight, of 4:1
and, as a consequence, a sr ~(Mnt+ + Co++) ratio; by weight, of
1.14. The resulting solution was used as catalyst in the oxida-
tion of paraxylene to terephthalic acid. ~he results obtained
were excellent.
EXAMPLES 2--4
m e procedure of Example 1 was repeated, varying only
the amount of cobalt in order to have Mn:Co ratios, respectively,
of 1:1, 2:1 and 3:1 by weight, corresponding to Br ~ Co+ )
ratios of 0.71, 0.95, 1.06, respectively. The results were
simiiar to those of Rxample 1.
EXAMPLES 5-8
The procedure of Examples 1-4 was repeated, except that
the ratio of Br to Mn was raised from 1.42 to 1.65 by weight.
The ratios of Br to (Mn + Co ) were, respectively 1.32, 0.82,
1.10, 1.24. Similar results were obtained.
EXAMPLE 9
Into the autoclave of Example 1, the following were
charged:
19 parts by weight of electrolytic manganese in flakes,
- 95 parts by weight of acetic acid at 99.8%, and
41.8 parts by weight of liquid bromine at 99.8%.
The Br:Mn ratio was 2.2 by weight. I'he temperature spontaneous-
ly reached 32C, and was raised to 45C by supplying heat.
: ,, ~ ~ f .
This temperature w~s maintained for a period of 8 hours, -thus
achieving an almost total conversion of bromine (98.21%). The
addition of water can cause, in the presence of a strong oxi-
dizing agent such as unconverted bromine, the precipitation of
MnO2. Therefore, there is the risk of clogging the lines, which
risk can be eliminated only by removing the precipitate, what
requires complex operations and large apparatuses. By conse-
quence, 0.275 parts by weight of acetaldehyde were added for 1
part of unreacted residual bromine, which was immediately con-
verted to hydrobromic ion. Water was then added (304 parts by
weight), and the attck of manganese by the acetic acid was
brought to an end. Tetrahydrated cobalt acetate was then added
in such amounts as to provide a Mn:Co ratio 4:1 by weight, and
a Br :(Co+ + Mn++) ratio of:1.76. Neither cloggings nor other
troubles.of any kind occurred during the preparation of the
catalytic solution and during its utilization ln the oxidation
of paraxylene to terephthalic acid.
EXAMPLES 10-12
- The procedure of Example 9 was repeated, varying the
amount of cobalt acetate, so as to have Mn:Co ratios e~ual,
respectively, to 1:1, 2:1 and 3:1 by weight, corresponding to
ratios of Br to (Mn + Co ),respectively, equaI to 1.1, 1.46
and 1.65. The results were very satisfactory, especially when
the ratios 2:1 and 3:1 were used.
EXAMPLE 13
14.25 parts by weight of electroly-tic manyanese in flakes,
62.60 parts by weight of acetic acid at 99.8% by wei~lt, and
` 30 143.10 parts by weight of water
were charged in the foregoing order, and one time only, into
the autoclave of Exampl.e 1. The temperature was mai.n-tained at
. .
3~7
65OC for ~ hours, under stirring, and alll the manganese was
converted to ~he corresponding acetate. The released hydrogen
was diluted with nitrogen and vented to the atmosphere. To the
resulting solution, con-taining about 14.5% of free acetic acid,
6.91 parts by weight of acetaldehyde were added, whereupon, in
~5 minutes, 20.9 parts by weight of liquid bromine (Br:Mn = 1.46)
were gradually fed. The bromine was completely reduced without
any oxidation of manganese.
` EX~MPLE 13A
In the autoclave o~ Example 1 and by employing the
amount and the operating conditions of Example 13, the catalyst
system was prepared by feeding acetaldehyde in the form of
diethylacetal (18.53 parts by weight). Bromine was gradually
proportioned in 45 minutes and quantitatively-reduced without
precipitation of MnO2. Successively, 30.08 parts by weight of
tetrahydrated cobalt acetate were added, thus obtaining a con-
centrated catalytic solution suitable for being sent to the
paraxylene oxidation reactors. The Mn /Co ratio was equal to
3:1 by weight, and the Br /(Mn++ + Co++) ratio was equal to 1.
EXAMPLE 13B
Example 13 was repeated employing ethylidene diacetate
in an equivalent amount, instead of acetic aldehyde (22.92 parts
by weight of ethylidene diacetate). The reduction of elemental
bromine was quantitative. The final, perfectly limpid and con-
centrated catalytic solution, having a ratio Mn ~/Co++ = 3:1
and a ratio Br /(Mn++ + Co +) = 1, was then conveyed to the para-
xylene oxidation reactors.
EXAMPLE 1
Example 9 was repeated, but raising the tempera-ture
to 80C and obtaining a full conversion of the reagents.
EXAMPLE: 15
According to FIG. 1, the mo-ther liquor coming from
the centrifugation of terephthalic acid ~at 100-130C), contain-
ing about 90% by weight of acetic acid, was fed, tnrough line 2,
above the third tray from the bottom of a par-tially anhydrifying
column (stripper), to the head of which, through line 2, a
stream of aqueous acetic acid at about 70% by weight, free from
catalyst, formed by condensation of the vapours released during
the oxidation of paraxylene, was also fed. The acetic acid:
paraxylene ratio during the reaction was equal to 3:1 by weight,
and the water contained in said reaction mixture was comprised
between 3 and 4% by weight. In the fresh mixture that was fed
to the reactors 14, said ratio was about ~:1, the value of this
ratio decrea.sed successively, because of the extraction of a por-
tion of the condensed vapors at the synthesis temperature.
The bottom liquid of the stripper, operating at tem-
peratures equal to or. slightly lower than 130C, contained about
0.03% by weight of suspended solids, consisting predominantly
of terephthalic acid that should be preferably recovered. A
portion of the bottom liquid, containing about 97% by weight of
acetic acid and about 50% of the catalyst fed to the column, be-
sides oxidation intermediates, was recycled, through l.ine 3, to
the tank containing the feed mixture for the oxidation reactors.
A second portion of the bottom liquid passed to the column re-
boiler, consisting of a still pot, equipped with a stirrer, and
of a heat exchanger, arranged in series and steam heated. A
portion of the concentrate leaving the pot formed the process
:~ purye and was sent, through line 4, to a thin layer evaporator
(not shown) and then to an incinerator. I'he remaining portion
of the liquid leaving the tan]s pas.sed through the exchanger
and flowed back -to -the still pot, where the vapours were released
and flowed back to -the column bot-tom. The residence time on the
column bottom, equal to about 6 minutes, was not so long as to
alter the organic substances o~ the recycled solution 3.
The vapours 15 passed from the colurnn head to a
second (azeotropic) column for the recovery of acetic acid,
equipped with a reboiler, a reflux condenser and a mixing tank.
Into this tank, through line 5, an amount of azeotropic agent
(isobutyl acetate), sufficient to make up for the losses, was
added. rrhe organic phase -that separated flowed back to the
column, while the aqueous phase that collec-ted in a trap passed,
through line 6, to a stripping column for the recovery of the
azeotropin~ agent and of the methyl acetate therein. rrhe acetic
acid flowing out from the bottom contained 3% by weight of H20
and was sent, through line 7, to ~he tank containing the feed
mixture, where it was admixed with the recycle coming from line
3 and with fresh paraxylene coming from line 8. rrhe catalyst
system was prepared in- a stainless steel autoclave, equipped
with a rotating stirrer, a reflux condenser and a heating
jacket, to which were added in the following order:
14.25 parts by weight of electrolytic manganese in flakes,
through pipe 10;
62.60 parts of 99.8% ace-tic acid, by weight, through line .
9, and
1~3.10 parts by weight of water through line 12.
The temperature was kept at 65C, for 8 hours, under stirring,
and all the manganese was converted to the corresponding acetate.
The released hydrogen was diluted with nitrogen and vented to
the atmosphere. To the resulting solution, containing about
14.5% of free acetic acid, 6.91 parts by weight of acetaldehyde
were added, and 20.9 parts by weight of elemental bromine (Br:
Mn = 1.~6 by weight) were gradually fed in ~S minutes. r~he
bromine was thoroughly converted to hydrobromic ion, without any
.
. .
3~ ~
precipitation of MnO2. 30.08 parts by weight of -tetrahydrated
cobalt acetate were then added, thus obtaining a concentrated
solution that was proportioned in-to the tank where the feed mix-
ture was prepared. The Mn:Co weight ration was 3:1 and the
sr : (Mn~+ + Co++) weight ratio was 1:1. This ratio of the fresh
concentrated catalyst took into account also the bromine losses
occurring during the reaction.
EXAMPLE 16
Through line 12 in FIG. 1, 89.3 parts by weight of a
mixture of manganese carbonate and of cobalt carbonate (equal to
32.3 parts by weight of manganese and 10.76 parts by weight of
cobalt), mixture recovered from the ashes of the synthesis
purges coming from the mother liquor treatments, were charged
into the autoclave. Through line 9, 212 parts by weight of
acetic acid (containing 12.9 parts by weight of acetic aldehyde)
and 367 parts by weight of water were added thereto. The tem-
perature was brought to 72C, and 46 parts by weight of liquid
bromine were fed through line 11 by means of a submersed
plunger, in 40 minutes, under stirring. The mixture was further
stirred for a few hours at 72C, and a perfectly clear solution
.ontaining acetates and bromides of manganese and cobalt, in the
most suitable ratios for the terephthalic acid synthesis, was
obtained.
EXAMPLES 17-26
Example 15 was repeated, but varying the percentage of
the reagents and raising the temperature up to 78C. At the
conclusion of the reac-tion, residual bromine could not be found,
with the exception of Example 26, where the presence of residual
bromine, in an amount of 0.01% by weigh-t, was ascer-tained. The
results obtained are recorded in Table I.
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EX~PLES 27-37 :
Using the procedure of Examples 17-26, tests were
carried out using other aldehydes instead of acetaldehydes.
The results are recorded in Table II.
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EXAMPLE_38
According to FIG. 2, the mother liquor coming from the
centrifugation of terephthalic acid and containing about 85% by
weight of acetic acid, was fed, through line 1, above the third
tray from the bottom of a partially anhydrifying column, -to the
head of which a stream of aqueous acetic acid at abou-t 70% by
weight, free from catalyst, that had formed in other process
steps, was fed through line 2. The liquid on the column bottom
contained about 0.03% by weight of suspended solids, predominant-
ly consisting of terephthalic acid, which should be preferably
recovered.
A portion of the bottom liquid, containing 94% by weight
of acetic acid, about 80% of the catalyst fed to the column and
oxidation intermediates, was recycled to the synthesis feed
tank throug pipe 3. A second portion of the bottom liquid passed
to the column reboiler, subdivided into a still pot, equipped
with a stirrer, and into a heat exchanger, arranged in series
` and steam-heated. A liquid and concentrated synthesis purge was
sent, through pipe 4, to a thin layer evaporator ~not shown) and
then to an incinerator. The remaining portion of the liquid
~ leaving the pot passed through the exchanger and flowed back to
;` the still pot, where the vapours were released and flowed back
to the column bottom. The residence time in the lower part of
the column, about 6 minutes, was not so long as to alter the
organic substances of recycled solution 3. The vapours flowing
out from the column head, passed to a second (azeotropic) column
for the acetic acid recovery, equipped with a reboiler, a reflux
condenser and a demixing tank. Into this tank, through pipe 5,
an amount of azeotroping agent (isobutyl acetate) sufficient to
make up for the losses, was introduced. A baffle easily separated
he organic phase, that returned to the column, from the aqueous
~14
phase, that collec-ted in a trap and passed, -through line 6, to
the treatments for recovering the azeotroping agent. Ihe acetic
acid flowing out from the bottom contained 3.5% by weight of
H20 and was sent, through ].ine 7, to the synthesis feed tank,
where it was mixed with the recycled liquid coming from line 3
and with fresh paraxylene coming from line 8.
Through pipe 10 coming from a proportioning hopper,
32.3 parts by weight of electrolytic manganese were fed to an
autoclave equipped with a heating jacket. Through pipe 9
150 parts by weight of acetic acid were fed, thus converting, at
650C and in about 8 hours, metal manganese to the corresponding
acetate. Through line 12, also 231 parts of H20 containing
`~ 15.2 parts by weight o~ acetaldehyde were added. Finally, al- r
ways at 65C, 46 parts by weight of liquid bromine at 99.8% were
fed in 45 minutes. Bromine was thoroughly converted to hydro-
bromic ion without the oxidation of manganese and precipitation
of MnO2. At the conclusion of the elemental bromine reaction,
tetrahydrated cobalt acetate was added in a Mn/Co ratio by weight
e~ual to 3/1. The resulting catalytic solution, having the
:: 20 following ratio:
Br~/(Mn++ + Co++) = 1.09, entered, through line 13, the feed
tank of the terephthalic acid synthesis reactors.
E~AMPLE 39
According to FIG. 2, a solution of manganese acetate
and of cobalt acetate, in which the Mn:Co ratio was 3:1 by
weight and in which bromine was absent, was fed, through line
13, to a storage tank for the feed of terephthallc acid to -the
synthesis. Into this tank, through line 3, also a portion of
3Q the synthesis mother liquor, partially anhydrified in the proper
column and containi.ng about 50% of the catalytic system contained
in the original mother liquor, was introduced. Elemental
-15-
~ ~J
bromine was added, through line 17, to the portion of partially
anhydrified mother liquor flowing in line 3 and containing al-
dehydes (in particular carboxy-benzaldehyde) in amounts sufficient
to prevent MnO2 from precipitating within the underlying tank.
The sr :(Mn + Co ) ratio calculated in respect of
cations Mn++ and Co++ coming from line 13, was 1.09 by weigh-t,
and the temperature of -the liquid 3 was 90C. The corresponding
amount of bromine had to be increased by an amount sufficient
to make up for the loss of bromine in the mother liquor (about
0.07 parts by weight of bromine for 1 part by weight of manganese
- and cobalt contained in the liquid recycled through line 3. All
-~ ~
`` of the bromine was found quantitatively as hydrobromic ion in ~`
synthesis feed line 14.
:`` ' :
::
.
`.
-16-
.. . . . ..
