Note: Descriptions are shown in the official language in which they were submitted.
3~
PROCESS FOR PREPARI~IG TEREPIITHALIC ACID
-
BACKGROUND OF THE INVENTION
This invention relates to an oxidation process,
more particularly to a process for preparing terephthalic
acid by oxidation of p-toluic acid or mixtures of p-toluic
acid with p-xylene and/or with partially oxidized de-
rivatives thereof, such as p-tolualdehyde.
Terephthalic acid is of great commercial im-
portance as it is increasingly used as a starting material
~or the production of high molecular weight resins such as
fiber and film-forming polyesters.
The prior art teaches many processes for the
liquid-phase oxidation of alkyl-substituted aromatic com-
pounds to aromatic carboxylic acids. One of the first
patents in this field is U.S. Patent No. 2,245,528 in the
name of Loder, which discloses a one~step process for oxi-
dizing alkylaromatic compounds by molecular oxygen in the
presence of a metal catalyst, a solvent such as acetic acid
and optionally an oxidation initiator. However, even under
severe conditions, the yield in dicarboxylic acids is low.
For example, upon oxidizing a mixture of xylenes with air
in acetic acid containing cobalt- and manganese acetates
as catalysts at 185-200C under a pressure of 50 atmos-
pheres and in the presence of diethylketone as initiator,
,' ' ~
S7
-- 2 --
the yield in phthalic acids was only 2~, and the main re-
action products were toluic acids together with other inter-
mediate oxidation products.
A number of further patents disclose processes for
the oxidation of p-xylene in one step with improved yields
in terephthalic acid. These patents relate mainly to the
use of specific activators such as bromine-containing com-
pounds (U.S. Patent No. 2,833,816), ketones (U.S. Patent
No. 2,~53,514), or aldehydes (U.S. Patent No. 3,036,122).
Although some of these processes are applied commercially,
they nevertheless suffer from serious drawbacks. For in-
stance, severe corrosion problems arise when bromine-con-
taining activators are used. When a ketone or an alde-
hyde is employed, part of it is inevitably lost, and the
-~ 15 remaining portion is transformed mainly into acetic acid
which must be recovered, purified, and commercialized for
the process to be economically feasible. Nevertheless, de-
spite those drawbacks, the use of an activator is con-
sidered as being an essential requirement for efficiently
producing phthalic acids from xylenes.
In most cases, the use of a solvent is also
claimed as being necessary. Low-molecular-weight fatty
acids, more particularly acetic acid, are widely used for
this purpose. The added amount of solvent must be suffi-
cient in order to maintain the reactants and the reactionproducts in solution or at least in suspension without dif-
ficulty. In this way, the reaction mixture is easily agi-
tated, the dispersion of oxygen is improved, the formation
of by-products is minimized and the heat of reaction is
5~
- 3 ~
easily removed by solvent evaporation. However, under the
reaction conditions generally used, a signifi~ant a~ount
of solvent is lost by co-oxidation. Moreover, the solven~
must be separated from the other components of the reaction
mixture, and then be purified and recycled. Obviously,
this consumption of a part of the solvent and these opera-
tions to recover the remaining part result in additional
processing costs.
In order to avoid the above-mentioned important
problems, it has been suggested to carry out the oxidation
of p-xylene in the absence of a solventO In this case, it
is obviously necessary to work at a temperature which is
at least in the range of the melting point of p-toluic acid,
i.e., about 180C, in order to obtain a liquid reaction
mixture. The choice of temperature is therefore limited.
Moreover, in the absence of a solvent, the handling of the
reaction mixture as well as the separation and the puri-
fication of terephthalic acid are difficult. Generally,
the reaction is carried out to the point where the content
of terephthalic acid in the mixture does not exceed 60%,
preferably 45~, by weight. Beyond this point, "it becomes
difficult to handle the oxidation reaction mixture as a
slurry, and hence, the operation of the reaction is adversely
affected" (see U.S. Patent No. 3,883,584).
In the absence of a solvent, the removal of the
heat of reaction presents another difficult~ in indus~rial
scale production, as extensive fouling takes place in the
reactor, even when terephthalic acid is not present in large
amounts. Thus, in U.S. Patent No. 2,696,499, which relates
3,.~ 3s~ (
-. :
to the oxidation of xylene into toluic acids in the ab-
sence of a solvent, it is explained that "the essential -
design problem inherent in the cooling of the xylene oxi-
dation mixture is the prevention of the deposition of
s solids". -
. . .. .
U.S. Patent No. 3,406,196 describes a two-stage --
process, wherein water is used as a suspension agent for
tereDhthalic acid. In the first stage, an alkylaromatic
compound, more particularly p-xylene, is oxidized by means ~-
of air in the absence of any additional solvent into par-
tially oxidized compounds which, in the second stage, are -
" further oxidized at a higher temperature in the presence
of substantial amounts of water as a suspending medium.
Bromine or a bromine-containing compound must be present ~
15 to promote oxidation. Nevertheless, very high temperatures -
in the range of 200 to 275C and, more particularly, from
225C to 250C are required for achieving conversion of those par- -
tially oxidized compounds into terephthalic acid. Accord-
ingly, the same or even worse corrosion problems are neces-
sarily encountered than those which are present in proces-
sec wherein acetic acid is used as a solvent. Moreover, as
stateiin said patent, "appreciable losses of unreacted poly- -
alkylaromatic compound by degradation and other side re-
actio~s tend to occur when such compounds are exposed to
the higher temperatures found necessary for efficient con-
version o~ the partial oxidation products, produced in the
first stage of oxidation, to aromatic polycarboxylic acids".
Clearly, the teaching of this patent is that the use of
large amounts of water, even in the presence of a bromine
,,i .
~5~
-- 5 --
promoter, does not give satisfactory results for oxidizing
p-xylene into terephthalic acid in one step.
Actually, it has been known for a long time al-
ready that water is "a catalyst poison in oxidation re-
action" (U.S. Patent No. 2,696,499). According to the mosi
widespread opinion, water has an adverse effect upon the
reaction rate by interferring with initiation. As a general
rule, its presence is avoided as much as possible, regard-
less of whether a solvent and/or an activator are present.
Thus, U.S. Patent No. 3,064,044 describes an improved
technique for maintaining a final (bromine-promoted) oxi-
dation under substantially anhydrous conditions. In U.S.
Patent No. 3,519,684, which relates to an oxidation process
wherein peracetic acid is used as promoter, it is specified
that "preferably, nearly anhydrous conditions are employed,
although a water content of up to about 10% can-be tolerated
and a maximum water content of not greater than 5% is pre-
ferred". In a continuous process for the oxidation of
xylenes in the absence of any promoter and wherein the par-
tially oxidized intermediates are continuously recycled,
water is removed from the liquid effluent before recycling
the latter in order to retain "the water content in the
reaction mixture at less than 15% and preferably less than
5% of the total reaction mixture" (U.S. Patent No. 3,700,731).
More recently, it has been found that, unexpect-
edly, the oxidation of p-xylene into terephthalic acid can
be carried out in the presence of substantial amounts of
water as a solvent, although in the absence of any brominated
activator which earlier was considered as an essential
3~
requirement. This process is described in U.S. Patent ~o.
4,278,810 issued July 14, 1981 to Labofina, S.A. They
comprise oxidizing p-xylene in the liquid phase by an oxygen-
containing gas in the presence of p-toluic acid, water and
of a heavy metal salt as a catalyst at a temperature of
from about 14QC to about 22QC under a pressure sufficient
to maintain at least a part of the water in the liquid
phase. However, as the mutual solubility of p-xylene and
water is low at the workinq temperature, such mixtures of
water, p-xylene and p-toluic acid may separate into two
phases: an aqueous phase and an organic phase which is
rich in hydrocarbon and also contains an inportant portion
of the p-toluic acid which is present in the reaction mix-
ture. In this case, the oxidation reaction takes place
mainly in the organic phase where the concentration of
water is relatively low. Therefore, the desired solvent
effect of water is partly lost. Moreover, this phase separa-
tion causes important technical difficulties with regard
to homogen~zation, oxygen dispersion and mass transfer
effects.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
process for oxidizing p-xylene into terephthalic acid by
which terephthalic acid is obtained in high yield and good
purity, and by which the above-mentioned drawbacks of the
prior art processes are avoided.
It is a further object of the present invention to
provide such a process which does not require highly corro-
sion-resistant equipment and can be effected in conventional
stainless steel equipment.
- 6
~53.~
_ 7 _
It is a further object of the invention to pro-
vide such a process which can be performed in the presence
of substantial amounts of water without requiring the pre-
sence of any additional solvent.
It is a further object of the present invention
to provide a process wherein terephthalic acid is prepared
by oxidizing a reaction mixture which substantially is a
homogenous aqueous solution consisting essentially of the
terephthalic acid precursors which are to be oxidized,
water, and an oxidation catalyst dissolved therein.
It is a further object of the invention to pro-
vide such a process which does not require the use of a
promoter such as a bromine compound in addition to an
oxidation catalyst.
It is a further object of the present invention
to provide such a process wherein the terephthalic acid can
easily be recovered from the oxidized reaction mixture at
relatively moderate temperatures.
It is a further object of the present invention
to provide such a process wherein the catalyst and oxidation
intermediates can be recovered and re-used for oxidation.
It is a further object of the present invention to
provide such a process wherein terephthalic acid can be pro-
duced in an industrial process at relatively low costs.
It is a further object of the invention to provide
such a process which can be performed batchwise as well as
continuously.
It is a further object of the present invention to
provide such a process, wherein the amounts of catalyst,
3~
water, and p-toluic which are ~ecessary to secure e~ icient
oxidation of the starting material can easil~ be calcula~ed.
In order to accomplish the foregoing objects ac-
cording to the present invention, there is provided a pro-
cess for preparing terephthalic acid which comprises the
- steps of:
a) oxidizing a substantially homogenous liquid
reaction mixture comprising
- - at least one oxidizable terephthalic acid
precursor selected from the group consisting of p-toluic
acid, and mixtures of p-toluic acid and an oxidizable com-
pound selected from the group of p-xylene, partially oxi-
dized p-xylene derivatives, and mixtures thereof;
- an amount of at least 5% by weight of
water which is sufficient to obtain a workable slurry;
- an amount of an oxidation catalyst com-
prising at least one catalytically active metal compound
selected from the group consisting of manganese compounds,
cobalt compounds, and mixtures thereof, which is sufficient
to provide at least a minimum amount of M millimoles of
the catalytically active metal compound.per kg of the
liquid reaction mixture wherein M is defined by the follow-
ing equation (l)
M = Y (x f A) + B x (1)
Cx ~
wherein y represents the m~lar ratio of water to p-toluic acid
in the reaction mixture,
x represents the molar ratio of manganese/total
amount of manganese + cobalt in the catalyst
composition, i.e. Mn +-Co
11 1535~
A equals about 0.200,
B equals about 10.9,
C equals about 4.35, and
D equals about 0.0724 -
with a molecular oxygen-containing gas at a reaction tem-
perature of from about 140C to about 220C and at a pres-
sure sufficient to maintain at least part of the water in
the liquid phase at the reaction temperature; and b) recover- --
ing an oxidized mixture containing the terephthalic acid. -
By maintaining the catalyst concentration at at
least the above-defined minimum value, an efficient oxida-
tion of any of the above starting material mixtures into --
terephthalic acid in a homogenous liquid phase reaction
mixture is achieved.
- 15 Further objects, features, and advantages of the
invention will become apparent from the detailed descrip-
tion of the invention and its preferred embodiments which
follows when considered together with the accom?anying
drawing.
BRIEF DESCRIPTION OF THE DRAWING
The attached sheet of drawing represents a phase
diagram for mixtures of p-xylene, p-toluic acid and water
at a temperature of 185C.
DETAILED DESCRIPTION OF T~E INVENTION AND ITS PREFERRED
EMBODIMENTS
It is an important feature of the present in-
vention that the detrimental effect of water,which has been
widely recognized in the prior art,can be overcome when
the oxidation is performed under the above specific
- 10 -
conditions. It is a further feature o~ this invention
~hat it can be applied to the oxidation of various sub-
strates which may consist of p-toluic acid alone or in
admixture with p-xylene and/or partially oxidized deriva-
tives such as p-tolualdehyde. It is still another fea-
ture of this invention that such substrates can be oxi-
dized in a homogenous aqueous solution when the amount of
catalyst is chosen in correlation with the ratio between
the amounts of water and p-toluic acid which are present
in the system.
The process according to the present invention
may be carried out batchwise or in a continuous procedure.
The reactants are dissolved in water and the catalyst is
added to the resulting solution. Oxygen is then intro-
duced into this mixture and the oxidation is carried outat a temperature of between about 140C and about 220C
under pressure. The pressure is adjusted in such a manner
as to keep the reaction mixture in a substantially liquid
phase at the working temperature. Terephthalic acid se-
parates from the reaction mixture as a white crystallineprecipitate. In a continuous process, this precipitate
is continuously removed by any conventional solid-liquid
separation method, such as filtration, centrifugation, or
settling and decantation, and the remaining liquid con-
taining unconverted reactants and intermediate oxidation
- products is recycled into the oxidation zone. Fresh re-
actants are continuously added to compensate for tereph-
thalic acid thus withdrawn and for the formation of by-
products. Thus, no addition of extraneous chemicals is
;, ~ , . : ' '
5~57
required. Nevertheless, in carrying out the process o t~e -~
present invention, there may be added to the reaction mix- --
ture one or more organic solvents which do not interfere
with the reaction and which are relatively inert under the --
working conditions. Examples of such compounds that can
be used as a solvent in admixture with water are benzoic
acid and acetic acid. These compounds, which are actually
produced in small amounts during the reaction, may, there- -
fore, be allowed to accumulate to some extent in the re- -~-
10 action mixture. However, the advantages resulting from ~
this invention are obtained independently of the presence -
" of such compounds which, when employed, should not be
present in an amount exceeding the amount of water in the
system.
The amount of water which is suitably used in
the process according to the present invention-may vary
within a wide range, e.g., between about 5 and about 80~ -
by weight of the reaction mixture, depending on different ---
factors. As already emphasized, an éssential aspect of
the present invention is that the oxidation is carried
out in a homogenous aqueous system. Accordingly, the -
amount of water will primarily be chosen so that it is suf-
ficient to provide for substantially a homogenous aqueous
solution of the reactants, taking into account the other -
process variables such as temperature and the relative
amounts of the different compounds to be oxidized. For --
instance, when the process is applied to the oxidation of
p-toluic acid alone or in admixture with other o~ygenated
compounds which are relatively soluble in water such as
- 12 -
p-tolualdehyde, an amount of water will be chosen ~lhich i~
sufficient for completely dissolving the p-toluic acid at
the wor~ing temperature. As the solubility of p~tolui~
acid in water steeply increases at increased temperatures,
the amount of water to be used may be reduced as the tem-
~ perature is increased. As a general rule, however, the
amount of water will not be lower than 5~ and preferably
not lower than 10% by weight of reaction mixture.
When p-xylene is also a component of the re-
action mixture, the amount of water should not exceed an
amount beyond which separation of the mixture into two
liquid phases would take place. This amount obviously
depends on the amount of p-xylene in the mixture.
Fisure 1 is a triangular phase diagram for mixtur2s of
p-xylene, p-toluic acid and water at a temperature of
185C (in weight %). In this diagram, in Zone A the
system is a homogenous solution, and in ~one B it is
biphasic. As anyone skilled in the art can easily de-
termine, the boundary line between both of these zones -
does not vary greatly with temperature. It can be seen
that in order to avoid the presence of a substantial
organic phase, the amount of p-xylene must be limited.
Therefore, when the oxidation of p-xylene according to
the process of the present invention is performed batch-
wise, p-xylene should be added progressively, either in-
termittentl~ or continuously, to the reaction mixture at
such a rate as to maintain the system in zone A. Accord-
ing to a preferred embodiment of the invention, the re-
action is performed in a continuous flow-process, whereby
~ ~5;3S~
i,
unreacted p-xylene together with the intermediate oxidation
products, i.e., mainly p toluic acid, are continuously re-
cycled into the reaction zone. In this case, the molar
ratio or p-toluic acid~p-xylene in the reaction mixture at
the steady state will be comprised in the range of between
about 3 and about 15, depending mainly on the temperature;
other wise stated, the reaction mixture will be comprised
in the grey area of the diagram in Figure l. It can be seen
that this area is almost entirely in Zone A, i.e., corres-
ponds for the main part to homogenous solutions. From theforegoing, it appears that it is always possible to adjust
temperature and~or the amount of water used as a solvent in
accordance with the present invention in such a manner that
a homogenous system is obtained. If the process according
to the present invention is carried out in a continuous
manner, it is preferable to use the continuous procedure
which is described ~n U.S. Patent No. 4,278,810.
But the other factors also have to be taken into account.
Thus, as terephthalic acid, the desired product, is sub-
stantially insoluble in the reaction mixture; an amount of
water must be added which is sufficient to obtain a work-
able slurry. However, there is no advantage in using such
a high amount of water that more than, e~g., 10% of tere-
phthalic acid is dissolved at the working temperature. Afurther factor which has to be taken into account is the
reaction rate: although it is possible within the process
according to the invention, to carry out the oxidation reac-
tion in a medium comprising as much as 80 wt. percent of
water
. . .
S;:~7
- 14 - -
or even more, the presence of an excessive amount of ~7ater ~-
may adversely affect the reaction rate. Moreover, on some
occasions, a part of the catalyst may be diverted from its
catalytic function by forming a black compound which pre-
cipitates from the oxidation medium. Furthermore, for
economic reasons, it is disadvantageous to lose a part
of the reactor capacity by using an excessive and need-
less amount of water. Generally, for these different
reasons, the amount of water present in the system will
not exceed 75 wt. % and preferably 60 wt. % of the re-
action mixture.
Suitably, the oxidation reaction is carried out
-at a temperature of at least 140C. Below this tempera-
ture, it is dif.icult to have the reaction mixture as a
homogenous solution. On the other hand, working above
220C would result in increased overoxidation, undesir-
able side reactions and corrosion problems. In most cases,
the reaction temperature will be between about 150C
- and about 190C.
The pressure is adjusted as a function of the
temperature. A pressure must be employed which is suffi-
ciently high above atmospheric pressure in order to main-
tain the reaction mixture in a liquid state at the working
temperature. A pressure in excess of this value is general-
ly useful for ensuring active oxidation. Generally, the
pressure will be comprised between about 5 and about
40 kg/cm2.
The oxidation catalyst used in the process of the
present invention may be a heavy metal compound composition
~ S3$~7
, ~
- 15 -
; comprising at least one metal compound selected fr~m the
group of manganese compounds, cobalt compounds, or mixtures
thereof, provided it is at least partially soluble in the
aqueous reaction mixture or is capable of forming a soluble
or at least partially soluble compound with one of the
reactants in this mixture. The metal compounds which can
be employed in the catalyst composition, suitably are salts.
In particular the salts of carboxylic acids are pre~erred,
e.g., acetates, naphtenates, toluates, and the like. Fur-
ther to cobalt and/or manganese compounds, the catalystcomposition may comprise compounds, in particular carboxylic
acid salts, of other metals which are conventionally used
in oxldation catalysts.
A fundamental aspect of the present invention is
that the minimum concentration of catalyst, which is neces-
sary for ensuring oxidation in a homogenous aqueous system,
depends upon the respective amounts of water and p-toluic
acid in said system. As a matter of fact, in a homogenous
aqueous solution of p-toluic acid and optionally p-xylene
and/or partially oxidized derivative(s) thereof, oxidation
cannot take place when the amount of active catalyst is
lower than a critical concentration M in millimolQs of the
metal compound per kg of reaction mixture given by the
following equation (1)
Cx + D (1)
where y is the molar ratio of water to p-toluic acid;
x is the amount of manganese in the metal catalyst
in parts by mole relative to the total amount of
Mn
manganese and cobalt therein, i.e., Mn + Co;
- 16 -
A = about 0. 200;
B = about 10.9; ç
C = about 4.35; and
D = about 0.0724.
. 5 The quantities A, B, C, and D, as given herein-
above, are the result of experimental determinations and
- therefore are subjected to measurement errors. It has
been computed that these quantities with their confidence
limits are respectively
A = 0.200 + 0.031; B = 10.9 + 1.3; C = 4.35 ~ 0.17; and
.. D = 0.0724 + 0.0117.
; ` Consequently, the value of M as calculated by means of
equation (1) for a given value of y and x has to be con-
; sidered as an estimate of the actual critical concentra-
. 15 tion which, in terms of statistics, is comprised within
a confidence interval centered on M. As those.skilled
in the art will realize, this confidence intervaldepends
on the values of y and x used in the calculation of ~,
but for each particular case, it can be calculated from
the above data by application of Taylor's formula. Thus,
for example, the critical concentration of catalyst will
be comprised within the confidence intervals
21.46 + 0.81 when y = 70 and x = 1.0
and 2.74 ~ 0.45 when y = 1 and x = 0.5.
The critical concentration of catalyst as de-
fined hereabove is the lowest concentration that can be
used under given conditions. At lower amounts, the pro-
cess will not operate. Obviously, it is preferred in prac-
tice to use an amount of catalyst higher than this minimum.
~ ~15~
- 17 -
As a matter of fact, the rate of oxidation increases as
the concentration of catalyst is increased. ~o~Jever, con-
centrations of catalyst higher than about 4~ millimoles of
metal compound per kg of reaction mixture are not advan-
tageous for economic considerations. Suitably, the cata-
lyst will be used in an amount which is comprised ~etween
a value slightly higher than the minimum M as calculated
from equation (1) and about 30 millimoles per kg of re-
action mixture.
As it can be seen from equation (1), the critical
concentration of catalyst increases if the concentration
of water is increased and the concentration of p-toluic
acid is decreased. Otherwise stated, oxidation cannot
take place in such an aqueous solution unless the concen- --
- 15 tration of p-toluic acid is above a critical value which
increases as the concentration of catalyst is decreased.
Thus, not only the catalyst but also p-toluic acid are
essential for allowing oxidation to take place in the pre-
sence of substantial amounts of water. Actually, due to
the combined action of the catalyst and of p-toluic acid,
both used in proper amounts in accordance with the pre-
sent invention, it is possible to oxidize p-xylene into
terephthalic acid without resorting to the use of costly
and/or corrosive activators, such as brominated compounds.
This effect of p-toluic acid is a quite unexpected aspect
of the present invention as other carboxylic acids, even
acids of similar structure, such as benzoic acid, do not
exhibit the same property.
Furkhermore, it is apparent from equation (1),
that the critical minimum concentration of catalyst which
- 18 -
is necessary for ensuring oxidation to take place in a
homogenous aqueous system also depends on the relative pro-
portions of manganese and cobalt compounds within the e~-
ployed catalyst. It can be seen that manganese is marked-
ly more effective than cobalt. For instance, for allowing
. , .
oxidation to ta~e place in a 50 wt. ~ aqueous solution of
p-toluic acid (molar ratio water to p-toluic acid = 7.56),
the minimum concentration of catalyst,which is needed, is
4.5 or 20.9 millimoles/kg according to whether manganese
or cobalt is used as sole catalyst. For practical reasons,
-~ however, it is advantageous to employ a mixture of both
metals,as ccbalt generally exerts a beneficial effect upon
the reaction rate. Moreover, it has been observed that
when a mixture of manganese and cobalt compounds is em-
ployed as a catalyst, less overoxidation takes place and
higher yields in terephthalic acid are generally obtained
than when each metal is employed solely. For example, the
combustion ratio (the mole ratio of carbon dioxide evolved
per oxygen absorbed) generally has a value between 0.06
20 and 0.09 in the fixst case, whereas values of about 0.15
are observed in the second case. ~ctually, this beneficial
effect of using conjointly manganese and cobalt as catalyst
is a typical feature and a further advantage of the present
invention: when the reaction is carried out in a biphasic
system instead of a homogenous solution as in the present
process, combustion ratios of 0.12 or even more are regular-
ly observed regardless of whether manganese or cobalt or
mixtures thereof are used. In most cases, a value
of x in equation (1) of between about 0.1 and about 0.9
3S7
will be used with advantage to ensure active oxidation
in a homogenous aqueous medium and high yields in terepth-
thalic acid.
For the same practical reason as hereabove, it
may also be advantageous if in addition to cobalt and/or
manganese the catalyst comprises a further metal component
such as nickel, lead, or cerium. Although such other
metals are not essential for allowing oxidation to take
place in a homogenous aqueous medium, they may afford some
practical improvement with respect to, e.g., product pu-
rity and reaction rate.
A surprising feature of the present invention is
that the minimum concentration of catalyst to be used for
ensuring oxidation in aqueous medium can be calculated
from equation (1) for any composition of the reaction mix-
ture; and this equation is independent of such important
operating variables as temperature. This is illustrated
by the results shown in Table 1 wherein values of ~ ex-
perimentally obtained over a wide range of conditions are.
compared with the values calculated from equation (1).
Most of these results were obtained by the following ex-
perimental method.
Into a one-liter corrosion-resistant autoclave
equipped with a mechanical agitation device, a heatirg
jacket, a gas inlet tube, a vent, and a metering pump for
injecting a liquid, there are charged the different com-
ponents to be present in the oxidation reaction mixture,
i.e., p-toluic acid, water, and the catalyst. This mixture
is then heated while stirring and passing air therethrouah.
s~
- 20 -
Once oxidation has started, an aqueous solution of the ca-
talyst having the same catalyst concentration as the ini-
tial reaction mixture is progressively injected into the mix-
ture. As a result, the reaction mixture is progressively
diluted with water without any variation of the catalyst
- concentration. p-Xylene may also be present in the initial
mixture provided that its concentration is such that no
phase-separation takes place upon dilution of the reaction
mixture with the aqueous catalyst solution.
The degree of oxidation is observed by measur-
ing continuously the oxygen content of the exhaust gas by
means of an oxygen analyzer. As long as the concentration
of p-toluic acid in the system is sufficiently high in
order to ensure an efficient oxidation at the chosen cata-
lyst concentration, the reaction proceeds regularly at a
rate which decreases progressively as a result of dilution.
However, as soon as the critical concentration of p-toluic
- acid is reached, the reaction rate falls rapidly down to
almost nil. Then, injection is discontinued and the reaction
mixture is analyzed. The molar ratio of water to p-toluic
acid thus determined is o~viously the critical y value
corresponding to the given concentration M of catalyst or,
conversely, M is the minimum concentration of catalyst to
be used for ensuring oxidation in a reaction mixture where-
in the ratio between water and p-toluic acid is given
as y. Actually, M still depends on the composition of
the catalyst which may be expressed as the molar ratio
of manganese to the total amount of manganese and cobalt
- 21 -
in the used catalyst composition (x in equation 1).
Obviously, such determinations are subject to
experimental errors so that, for a given reaction mix~ure
(for a given y and x), different values of Mi are ex-
pected to be obtained which will statistically be dis-
tributed around the actual value with a standard de-
viation ~. Values of Mi thus determined for different
Yi and xi are shown in the following table. From these
data, it can be estimated that the standard deviation of
Mi from the value M as calculated in equation (1) is 0.70.
Accordingly, any experimental value Mi, which does not
differ from the calculated one by more than two standard
deviations, i.e., by more than 1.4, can be considered
as consistant with equation (1).
From the results given in Table I, it appears
clearly that:
1) Equatlon (1) is valid for any value of y and
x (compare for instance experiments ~o. 1 and 7 for y and
experiments no. 7 and 19 for x). -
2) Equation (1) is indépendent of the presence
; or absence of p-xylene (compare for instance experiments
no. 8 and 13).
3) Equation (1) is independent of temperature
(compare for instance experiments no. 3 and 9).
4) Equation (1) is independent of the presence
of metals other than manganese and cobalt (compare for in-
stance experiments no. 2 and 5).
3S7 ~
- 22 -
T A ~ L E
Reaction mixture (wt ~) M
~o. Temperature p_ p-toluic water Y .~ observed calc~-
_ _ xylene acid lated
1 1850.3 9.7 90.070.22 1.0020.121.5
2 1850.5 20.0 79.530.08 1.0010.110.6
3 1851.5 45.7 52.88.76 1.005.1 4.8
4 1700.4 10.6 89.063.54 1.0020.019.7
1700.6 23.5 75.924.38 1.009.0 9.1
(1) (1)
6 1701.4 43.5 55.19.60 1.005.1 5.1 -
7 1700.1 61.3 38.64.76 1.003.7 3.8
8 17011.6 76.6 11.81.17 1.003(2) 2.8
9 1601.3 42.6 56.19.95 1.005.1 5.2
1701.3 44.4 54.39.25 0.795.2 5.1
11 1700.3 23.0 76.725.21 0.5010.110.3
12 1700.2 10.8 89.062.34 0.4621.722.2
13 ~700.0 41.8 58.210.53 0.46 ~ 5.5 5.8
14 1600.5 43.9 55.59.57 0.465.5 5.5
1~ 1700.5 24.3 75.223.46 0.3310.110.6
16 1700.3 27.0 72.720.39 0.2510.110.2
17 1700.4 30.4 69.217.23 0.1710.110.1
18 1700.5 35.3 64.213.7-7 0.0910.110.7
19 1705.4 60.5 34.14.27 0.0011.711.8
1700.8 74.9 24.32.45 0.006.1 6.8
21 1700.0 81.4 18.61.73 0.003.9 4.8
(1) An equimolecular mixture of manganese and nickel acetates used
as catalyst. The calculation of x and M are made without taking
nicl~el into account.
(2) Value obtained ~7ithout dilution of the rPaction mixture. The
critical ratio of water to p-toluic acid was reached spon-
taneously, by consumption of the latter and production of
the former as a result of oxidation.
~SJ57
,
- 23 -
The teaching of the prior art concerning the se-
lection of the metal compound to be used for oxidizing
p-xylene and its partially oxidized derivatives to terepn-
thalic acid is somewhat'confusing. As a general rule how-
ever, cobalt is claimed to be the best catalyst, especiallyin the absence of a brominated activator, and manganese to
be less active if not inactive. For the oxidation of
p-toluic acid, manganese has been shown to be inactive
(N. Ohta,et al., Chem. Abstr. 56, 8620 g, 1962~ or even
10 inhibitory (V.N. Aleksandrov et al., Kinet. Katal. 15, 505,
1974). In other circumstances, manganese exhibits catalytic
activity but discoloration of terephthalic acid takes place
when the content of water is higher than 10% by weight of
the solvent or when the content o, manganese in the re-
action mixture is high. It is therefore highly unexpected
that in the process of the present invention, manganese has
an outstanding activity for allowing the oxidation reaction
to take place in the presence of large amounts of water
and that terephthalic acid is obtained in high yield as a
white cristalline precipitate which is especially suitable
for further purification up to the degree of purity which
is required for the production of polyester fibers (=fiber-
grade).
The invention will now be described by the follow-
ing examples which are given for the purpose of illustrationand are not intended to limit the scope of the invention.
35~
,,
EXA.~PL~
Into a corrosion-resistant autoclave of one-
!~ liter capacity equipped with a mechanical agitation device,
;~ a heating jacket, a gas inlet tube and a vent, there were
5 charged: --
. . .
p-xylene 45.0 g
p-toluic acid 187.5 g --
various oxygenated p-xylene -
derivatives 4.5 g --
water 63.0 g
manganese acetate 1.50 millimoles
cobalt acetate 1.74 millimoles.
The reactor was pressurized with air up to a pres- -
sure of 20 kg/cm2 and the above mixture washeated while --
stirring and admitting air at a flow rate of 92 liters per -
hour.
In the above charge, the molar ratio of water to --
p-toluic acid ~y)was ~63.0 x 136.15)/(18.02 x 187.5) = 2.54
and the molar ratio Mn/Mn+Co in thé catalyst (x) was:
- 1.50 / (1.50 + 1.74) = 0.46. By application of equation (1), --
the minimum concentration of catalyst which wasnecessary
in this case for ensuring oxidation is calculated as fol- -
lows:
M 2.54 (0.46 ~ 0.200) + 10.9 (0.46)
_ _ _ = 3.2 millimoles/kg.
4.35 (0.46) + 0.0724
Actually, in the present example, the concentra-
tion of catalyst was 10.8 millimoles/kg, i.e., about three
times the minimum amount. As a matter of fact, the re-
action startedspontaneously upon heating and took Flace
actively; as a result, the temperature increased rapidly,
L5~35~ ?
- 25 -
it wasmaintained at 170C by controlled cooling.
After a reaction period of 180 minutes, the oxy-
gen absorption amount~ to 40.9 liters (measured at room
temperature and a pressure of one atmosphere). The ad-
mission of airwaS then discontinued and the reactor wasprogressively depressurized in order to recover unreacted
p-xylene by stripping with water. The reactorwas finally
cooled and opened. The precipitate contained therein was
filtered, washed with water and dried under vacuum at
about 80C. Then itwas analyzed by a combination of methods
comprising acidimetry, polarography and vapor-phase chroma-
tography. It wasthus determined that 89.4~ of the p-xylene
fed and 23.1% of the p-toluic acid fed had been trans-
formed into the following products:
terephthalic acid90.2 g
4-carboxybenzaldehyde 8.8 g
other intermediates3.8 g
heavy by-products, 2.2 g.
Taking into account that 4-carboxybenzaldehyde
and other intermediate products would in fact be recycled
in a continuous process and the major portion thereof would
ultimately be transformed into terephthalic acid, it can be
estimated that the yield in the terephthalic acid in such
a continuous process would amount to more than 90 mole ~,
based on the amount of p-xylene consumed.
In another operation performed under the same
conditions, the hot reaction mixturewas submitted to
filtration: the resulting cakewas further washed on the
filter with hot water and dried under vacuum. Analysis of
the dried product shows that it consists of ~in ~ by weight~:
terephthalic acid 85.6 %
p-toluic acid 9.6 ~ -
4-carboxybenzaldehyde 3.6 %
The color of this sample wasdetermined by measur- -
ing the optical density of a solution thereof in diluted
ammonia, according to a method described in U.S. Patent
No. 3,354,802, except that a 5 cm cell is used instead of
a 4 cm cell. The values of optical density thus obtained
- 10 were given in the following Table II and werecompared with
the values o. optical density which are obtained with a
sample of co~ercial terephthalic acid of 99+% purity.
; - T A B L E II
,
; 15 CRUD~ S.~_IPLE ~ OPTICAL DE~L~.SI TY AT (mu):
. ,-
ORI GI~ PURITY (~It~) 340 380 400
Prepared by
the ~rocess
; according to 85.6 0.428 0.102 0.076
the present
invention
Commercial 99+ 0.726 0.183 0.111
product _
It can be seen that the crude sample obtained in
this process by simple filtration and superficial washing
has better color characteristics than a commercial sample
of terephthalic acid of much better purity. This crude
sample is therefore especially suitable for further puri-
fication up to the fiber quality.
57
- 27 -
E~ PLE 2:
The experiment as in the preceaing example ~Jas - -
repeated except that instead of 63.0 g of water there was -
used as solvent a mixture of:
water 33.0 g
acetic acid 30.0 g
After 180 minutes of reaction, the oxygen ab-
sorption amounted to 39.7 liters, i.e., about the same
amount as in the preceding example. The reaction mixture
10 was then treated and analyzed as described in the pre- ~
ceding example. It was thus determined that 88.0% of the
p-xylene fed and 23.5% of the p-toluic acid fed had been
transformed into the following products:
terephthalic acid 89.4 g
4-carboxybenzaldehyde 6.5 g
other intermediates 4.8 g
heavy by-products 3.2 g.
By the same method as in the preceding example it
can be estimated that in a continuous process wor~ing under
the conditions used in this example, the yield in tereph-
thalic acid would be about 90 mole % based on p-xylene
consumed.
As can be seen, these results are practically
identical to those of the preceding example where water
was used as sole component of the solvent.
EXh~PLE 3:
The experiment of Example 2 was repeated except
that formic acid was substituted for acetic acid. The sol-
vent used in this case was therefore a mixture of:
5~5~7
- 28 -
water 33.0 g
formic acid 30.0 g
After 180 minutes of reaction, the oxygen ab-
sorption amounted to 36.7 liters. By the same analytical
procedure as in the preceding examples, it ~as determined
that 88.0% of the p-xylene fed and 15.6% of the p-toluic
acid fed had been transformed into the following products:
terephthalic acid 66.7 g
4-carboxybenzaldehyde 6.8 g
other intermediates 7.0 g
heavy by-products 3.7 g.
By the same method as in the present example,
it can be estimated that in a continuous process, working
under the conditions of the present example, the yield in
terephthalic acid would be about 85 mole % based on p-xylene
consumed.
Although these results are not quite as good as
those of the preceding examples, they sho~1 that large
amounts of formic acid can be tolerated in the reaction
mixture ~lithout serious detrimental effects upon the yield
and the rate of the reaction. This is especially unexpected
as in the prior art formic acid has always been described
as a potent inhibitor of oxidation reactions. As formic
acid is al~ays produced in such reactions, elaborated and
expensive procedures have generally to be designed for
avoiding any accumulation thereof in the reaction mixture.
In the present process, formic acid is burned as attested
by the fact that carbon dioxide evolved in this experiment
was 6.0 liters instead of 3.8 liters as measured in the
S;~S~
- 29 _
experiment of example 1. Accoraingly, in the process of
the present invention, no special operation is needed for
removing formic acid from the reaction mixture.
EXA~LE 4: -
-
The experiment of example 1 was repeated except
that cobalt was used as sole catalyst. The actual charge
was the following:
p-xylene 45.0 g
-p-toluic acid 190.1 g
varlous oxygenated
derivatives 1.9 g -~
water 63.0 g
cobalt acetate 3.48 millimoles.
In this charge, y was 2.50 and obviously x was
0.00. Accordingly, the limiting amount of cobalt to en-
sure oxidation in this case was
M = 2.50o(oj224oo) = 6.9 millimoles/kg.
The concentration of cobalt acetate in the ini-
tlal mixture was 11.6 mmoles/kg, thus about twice the
limiting amount. After 180 minutes of reaction, the oxygen
absorption amounted to 41.5 liters. The reaction mixture
was then treated and analyzed as already described. It was
thus determined that 90.8% of the p-xylene fed and 19.5%
of the p-toluic acid fed had been transformed into the
following products:
terephthalic acid74.2 g
4-carboxybenzaldehyde 6.8 g
other intermediates 1.5 g
hea~y by-products 3.7 g.
- 30 -
By comparing these results with those of example 1,
it can be seen that less terephthalic acid was produced al- -
though the oxygen absorption was slightly higher. As a
matter of fzct, by estimating as in the preceding example
the yield in terephthalic acid that would be achieved in a
~- continuous process, a value of 81~ is obtained instead of
90~. This difference clearly shows the yield advantage
resulting from using as ca~alyst a mixture of manganese and
cobalt instead of cobalt alone.
EXAMPLE 5:
The experiment of example 1 was repeated except
` that here it is manganese which was used as sole catalyst.
The actual composition of the charge was the following:
p-xylene 45.0 g
p-toluic acid 182.8 g
various oxygenated derivatives 9.2 g
water 63.0 g
mansanese acetate3.00 millimoles.
In this charge, y was 2.60 and obviously x was'
1.00. Accordingly, the limiting amount of manganese to en-
sure oxidation was
2.60 (l.00 + 0.200) + lO.9 = 3.2 millimoles/kg.
M 4.35 + 0.0724
In the present example, the concentration of
manganese was lO.0 millimoles/kg, i.e., about three times
the limiting amount as in example l.
After 180 minutes of reaction, the oxygen ab-
sorption was 25.3 liters, i.e., markedly lower than in the
preceding examples. The reaction mixture was then treated
5~
- - 31 -
and analyzed as already described. It was thus determir.e~
that 70.2~ of the p-xylene fed and 5.8% of the p-toluic
acid fed ha~ been transformed into the following products
terephthalic acid 31.2 g
4-carboxybenzaldehyde 5.9 g
other intermediates 6.3 g
heavy by-products 5.1 g.
By comparing these results with those of examples
1 and 2, it appears clearly that less reactants were con-
sumed here and that less terephthalic acid was produced.
Moreover, the yield in terephthalic acid of a continuous
process estimated from these data as in the preceding exam-
ples amounts to only 69%. Thus, although manganese is
markedly more efficient than cobalt to ensure oxidation in
the presence of water (as attested by the lower value cal-
culated for M by comparison with the value calculated for
cobalt alone), there is a definite advantage with respect
to both the rate and the yield of the reaction to use
manganese in association with cobalt as in example 1.
EXAMPLE 6:
The experiment of example l was repeated except
that in this case manganese was used in association with
nickel. The actual composition of the charge was the
following:
25 p-xylene 45.0 g
p-toluic acid 182.8 g
various oxygenated derivatives 9.2 g
ater 63.0 g
manganese acetate 1.50 millimoles
, ~ickel acetate 1.50 millimoles.
.
~^ .
~ S~ ~
- 32 -
In this charge, y was 2.60 and x was 1 00 as in
the preceding example, thus M was 3.2 millimoles/kg (nic~el
is not taken into account in the calculation of M). In --
the present example, the concentration of manganese was
5.0 millimoles per kg, thus 1.8 in excess over the limiting
amount. As a matter of fact, oxidation took place actively.
After 180 minutes, the oxygen absorption amounted
to 29.2 liters and the reaction mixture was analyzed as al- -
ready described. It was thus found that 79.1% of the p-xy-
,.
~- 10 lene fed and 3.3% of the p-toluic acid fed had been trans-
formed into the following products:
terephthalic acid 35.6 g
4-carboxybenzaldehyde 6.6 g
- other intermediates 2.6 g
heavy by-products 3.2 g.
By comparing these results with those of the
!~ preceding example, it can be seen that somewhat more oxygen
was absorbed and more terepthtahlic acid was produced than
when manganese was used alone at a concentration twice as'
high as in the present example. This shows that other
metals than cobalt can be used in association with manganese
to improve the rate of the reaction.
EX~PLE 7:
Into the same autoclave as in the preceding
25 examples, there was charged:
p-xylene 45.0 g
p-toluic acid 218.5 g
various oxygenated derivatives 6.5 9
water 30 0 g
. .
57
- 33 -
manganese acetate 0.52 millimoles
cobalt acetate 0.45 millimoles.
In this charge, the mole ratio of water to
p-toluic acid (y) was 1.04 and the mole fraction of manga-
nese in the catalyst (x) was 0.46. By application ofequation (1), the minimum concentration of catalyst to be
used in this case to ensure oxidation is therefore
M = 2.7 millimoles/kg. Actually, in the present example,
the concentration of catalyst was 0.97 millimoles for
300 g of initial mixture or 3.2 millimoles/kg, i.e. by
only 0.5 millimoles in excess over the limiting amount.
Nevertheless, oxygen absorption started spontaneously and
took place actively; as a result, the temperature in-
creased rapidly and was maintained at 170C by controlled
cooling.
After 395 minutes of reaction, 62.9 liters of
oxygen had been absorbed. The admission of air was then
discontinued and the same procedure as in Example l was ap-
plied for determining the composition of the reaction mix'
ture. It was thus established that 98.8% of the p-xylene
fed and Z9.2% of the p-toluic acid fed had been trans-
formed into the following products:
terephthalic acid 102.5 g
4-carboxybenzaldehyde 6.6 g
other intermediates 4.6 g
heavy by-products 3.6 g.
By estimating from these data, as in Example 1,
the yield in terephthalic acid that would be achieved in a
continuous process where the intermediates are recycled,
~ ,,
.... .
~S~5~
- 34 -
there is obtained a value of 80%, i.e. by 10~ lower tnan
the yield estimated in Example 1. The main difference
between both experiments is that in Example 1 the water
content of the charge was 21 wt % as compared with only
10% in the present case. The resulting difference in the
yield may thus be considered as a further illustration of
the advantage of carrying out the oxidation of p-xylene
in the presence of substantial amounts of water, in accord-
ance with the present invention.
COMPARATIVE E~lh~PLE
The same amounts of compounds as in the preceding
` example were charged into the autoclave except that only
0.30 mole of manganese acetate and 0.35 mole of cobalt
acetate were used. The total concentration of metal cata-
lyst was therefore 2,2 millimoles/kg, i.e. by 0.5 milli-
moles lower than the limiting amount as calculated in the
preceding example.
Upon heating the mixture in the presence of a
stream of air under the same conditions as in the preceding
example, oxygen absorption started similarly but after
about 70 minutes fell suddenly down to a negligible level.
Oxygen absorption amounted to only 6.7 liters.
EX~P_E 8:
Into the same autoclave as in Example 1, there
25 was charged
p-tolualdehyde 45.0 g
p-toluic acid 187.5 g
various oxygenated derivatives 4.5 g
water 63.0 g
- 35 -
manganese acetate 1.50 millimoles
cobalt acetate 1.74 millimoles.
As it can be seen, this mixture was very close to
those charged in Examples 1 to 5 except that p-tolualdehyde
5 was substituted for p-xylene. The molar ratio of water to
p-toluic acid was again 2.54. Thus, by application of
equation (1), the minimum concentration of catalyst to be
used for ensuring oxidation is M = 3.2 millimoles/kg.
Actually, in the present example, the concentration of ca-
10 talyst was 10.8 millimoles/kg, i.e. about three times as
high.
This mixture was heated in the presence of air as
in the preceding examples. Oxygen absorption started spon-
taneously at about 30C. Temperature was allowed to in-
15 crease and was maintained at 170C by controlled cooling.
After 180 minutes of reaction, the oxygen absorption amounted
to 25.6 liters. The admission of air was then discontinued,
180 ml of n-heptane was injected into the reactor for ex-
tracting unreacted p-tolualdehyde while continuing stirring
20 and heatir.g. The resulting mixture was then cooled and the
reactor was opened. The precipitate contained therein was
filtered, washed with n-heptane, dried under vacuum at 50C,
washed again with water and finally dried under vacuum at
about 70C. It was then analyzed by the same procedure as
Z5 in the preceding examples. The filtrate and washing were
combined and the heptane extract was separated by decanta-
v tion. An aliquot part of this extract was analyzed by vapor-
phase chromatography to determine unreacted p-tolualdehyde.
The remaining part was evaporated under vacuum up to dryness
- 36 ~
and the residue was analyzed by the same method as the first
precipitate.
From those different analyses, it was determined -
that 99.9% of the p-tolualdehyde fed and 10.7% of the ~-toluic -
S acid fed had been transformed into the following products: --
terephthalic acid 58.4 g
4-carboxybenzaledhyde 7.1 g
other intermediates 6.1 g
heavy by-products 5.4 g
10 CO~ARATIVE EXAMPLE 1
The same amounts of compounds as in the preceding
example were charged into the autoclave except that only
0.3 millimoles of both manganese and cobalt acetates were
used. The total concentration of catalyst was therefore
2.0 millimoles/kg, i.e. by 1.2 millimoles lower than the
limiting amount as calculated by equation (1). --
This mixture was heated in the presence of air
under the same conditions as in the preceding example.
After 180 minutes of reaction, the oxygen absorption amounted
to only 9.0 liters. The reaction mixture was then treated
and analyzed as in the preceding example. It was thus de-
termined that the p-tolualdehyde fed had been completely
transformed into the following products:
terephthalic acid 2.3 g
p-toluic acid 31.7 g
4-carboxybenzaldehyde 3.8 g
other intermediates 0.9 g
heavy by-products 2.0 g
It can be seen that in the present case p-tolu-
aldehyde has been mainly trans~ormed into p-toluic acid
~ - 37 ~
without significant formation of terephthalic acid.
COMPARATIVE EXAMPLE 2
In this example, p-tolualdehyde was used as sole
substrate to be oxidized. The actual charge was the follow-
ing:
p-tolualdehyde 225.0 g
water 75.0 g
manganese acetate 1.50 millimoles
cobalt acetate 1.50 millimoles.
Upon heating this mixture in the presence of air
under the same conditions as in Example 8, oxygen absorp-
tion started similarly but then slowed down markedly. ~fter
about 240 minutes of reaction at 170C, the oxygen absorp-
tion amounted to 29.6 liters. By treating and analyzing
the resulting reaction mixture as described above, it was
aetermined that 99.7% of the p-tolualdehyde fed had been
transformed into the following products:
terephthalic acid 4.6 g
p-toluic acid 171.4 g
4-carboxybenzaldehyde 4.6 g
other intermediates 1.3 g
heavy by-products 1.3 g.
It can be seen that, here again, p-toluic acid was
the main product from the oxidation of p-tolualdehyde and
that only small amounts of terephthalic acid has been formed.
This result strikingly illustrates that to oxidize p-tolu-
aldehyde into terephthalic acid in accordance with the pre-
sent invention, p toluic acid must be present in sufficient
amount from the beginning of the reaction.
, , - ,
5~35~ ~
- 38 -
E~AMPLE 9:
The experiment of Example 8 was repeated except
that 3.48 millimoles of cobalt acetate were used as sole
catalyst. By application of equation (1), it can be cal-
culated that the minimum concentration of cobalt to be used
..
in this case is M = 7.0 millimoles/kg. Actually, the con-
centration of cobalt catalyst in the present example was
11.6 millimoles/kg.
-Here again, oxygen absorption started at low tem-
perature. Nevertheless, heating was so applied as to main-
tain a temperature of 170C. After about 210 minutes of re-
action, the oxygen absorption amounted to 20.8 liters. The
reaction mixture was then treated and analyzed as in the
preceding example. It was thus determined that 99.9% of the
p-tolualdehyde fed and 6.0% of the p-toluic acid fed had
been transformed into the following products: --
terephthalic acid 49.1 g
4-carboxybenzaldehyde 6.9 g
other intermediates 6.5 g
heavy by-products 6.3 g.
CO2~ARATI~E EX~PLE
Into the same autoclave as in the preceding exam-
ples there was charged:
p-tolualdehyde 69.6 g
p-toluic acid 123.0 g
various oxygenated derivatives 3.0 g
water 105.0 g
cohalt acetate 3.48 millimoles.
Thus, the same amount of cobalt catalyst was used
57
- 39 -
as hereabove (11 6 millimoles/kg) but here the molar ratio
of water to p-toluic acid was 6.45 instead of 2.5~. The
minimum concentration of cobalt catalyst to be used in this
case is therefore M = 17.8, i.e. by 6.2 millimoles more than
actually present in the charge.
This mixture ~as heated in the presence of air.
Here again, oxygen absorption took place immediately. How-
ever, after 45 minutes it fell down to a negligible level:
nevertheless, heating at 170C was continued. After 300 min-
utes of reaction, the oxygen absorption amounted to onlylO.Oliters. The reaction mixture was then treated and
` analyzed as described in Example 8. It was thus determined
that 90.1% of the p-tolualdehyde fed has been transformed -
into the following products:
terephthalic acid 3.7 g
; p-tbluic acid 53.5 g
4-carboxybenzaldehyde 3.3 g
other intermediates 0.7 g
; heavy by-products 6.6 g.
It can be seen that p-tolualdehyde has been trans-
formed mainly into p-toluic acid without significant for-
mation of terephthalic acid.
EXAMPLE 10:
Into the same autoclave as in the preceding exam-
ples there was charged:
p-toluic acid 214.3 g
various oxvgenated derivatives 10.7 g
water 75.0 g
cobalt acetate 1.74 millimoles
manganese acetate1.50 millimoles.
- 40 -
Thus, in this example no p-xylene nor p-tolualde-
hyde were used. The mole ratio of water to p-toluic acid
(y) was 2.64 and the mole fraction of manganese in the ca-
talyst (x) was 0.46 as in Example 1. The minimum concentra-
tion of catalyst to be used as calculated from equation (1)is M = 3.3 millimoles/kg. Actually, in the present example,
the concentrations of catalyst was 3.24 millimoles for 300 g
of reaction mixture or 10.8 millimoles/kg, i.e. about three
times the limitin~ amount.
This mixture was heated in the presence of air
under the same conditions as in Example 1. In this case
again, the reaction started spontaneously. After 180 min-
utes of reaction 19.4 liters of oxygen had been absorbed and
the reaction ~las discontinued by cooling. The autoclave was
15 then opened and the precipitate contained therein was treated
and analyzed as described in Example l. It was-thus de-
termined that 33. 26 of the p-toluic acid fed had been trans-
formed into the ~ollowing products:
terephthalic acid 75.5 g
4-carboxybenzaldehyde 6.1 g
other intermediates 1.8 g
heavy by-products 0.1 g.
The net yield of terephthalic acid based on p-toluic
acid trans~ormed is 87 mole %. However, taking the inter-
2i mediate oxidation products as 4-carboxybenzaldehyde into
account, the actual yield in terephthalic acid, as would be
obtained in a continuous process where intermediates are re-
cycled, can be estimated to be 97 mole ~6.
This example clearly demonstrates that by the
~ ~5~
process of the present invention p-toluic acid can be oxidi~e~
efficiently and in high yield into terephthalic acid e~en in th~
absence of p-xylene or any other easily oxidi~able compound as
,promoter.
EXAMPLE 11:
Into the same autoclave as in the preceding examples,
there was charged:
p-xylene 3.0 g
p-toluic acid 142.6 g
various oxygonated derivatives 4.4 g
water 150.0 g
manganese acetate1.05 millimoles
cobalt acetate1.05 millimoles
Thus, in this example water amounted to 50 wt. % of
the initial mixture: y was 7.95 and x was obviously 0.50. By
application of equation (1), the minimum concentration of cata-
lyst to be used is M = 4.9. Actually in the present example, the
concentration of catalyst was 7.0 millimoles/kg, i.e. by 2.1
millimoles in excess over the limiting amount.
Air was admitted into the reactor at a flow rate of
110 liters per hour under a pressure of 20 kg/cm2 and the mixture
was heated while stirring~ When the temperature was about 170C,
a minute amount of t-butylhydroperoxide was added to help the
reaction to start. As~,a,result, oxygen absorption took place
immediately. The temperature then increased rapidly and was main-
tained at 185C by controlled cooling. After 300 minutes of reac-
tion, the oxygen absorption amounted to 23.5 liters. The reaction
was then
-- 42 --
discontinued and the resulting mixture was treated and ana-
lyzed as in Example 1. It was thus determined that 95.0%
of the p-xylene fed and 40.3% of the p-toluic acid fed had
been transformed into the following products:
terephthalic acid 53.5 g
.
4-carboxybenzaldehyde 5.0 g
other intermediates 1.6 g
heavy by-products 1.6 g.
COMPARAT:liVE EXAMPLE 1
The same experiment as in the preceding example
was exactly repeated except that the half amount of each
metal catalyst was used. The total concentration of catalyst
was thererore 3.5 millimoles/kg, i.e. by 1.4 millimoles
lower than the limiting amount.
This mixture was heated in the presence of air
under exactly the same conditions as in the above example.
- No oxygen absorption took place at all by maintaining this
mixture at 185C for 160 minutes, despite two successive
additions of t-hutylhydroperoxide to help the reaction to
start.
; By considering the results obtained in the above
example where active and substantial oxidation took place
in the presence of large amounts of water, it appears clear-
ly that such an oxidation is only feasible when the proper
amount of catalyst is used as specified in the present in-
vention.
COMPAR~TIVE EXAMPLE 2
The same experiment as in Example 11 was repeated
except that benzoic acid was substituted for a part of
'
J
- 43 -
p-toluic acid. The actual composition of the charge was
the following:
p-xylene 3.0 g
p-toluic acid 44.5 g
benzoic acid 93.9 g
various oxygenated compounds 2.0 g
water 156.6 g
manganese acetate 1.05 millimoles
cobalt acetate 1.05 millimoles.
In this example y was 26.59; thus, by application
of equation ~1), the minimum concentration of catalyst to
be used for ensuring oxidation of the above mixture is
10.7 millimoles/kg, i.e. more, by 3.7 millimoles, than the
amount actually present. By contrast, if benzoic acid had
the same effect as p-toluic acid for ensuring Gxidation in
aqueous medium, i L should be taken into account in the cal-
culation of y and M. In this case, we would have
y = (156.6/1~.02)/(44.5/136O15 + 93.9~122.12) = 7.93, i.e.
the same value as calculated for y in Example 11, where
0 active oxidation took place.
The above mixture was heated at 185C under the
same conditions as in Example 11 and here again some t-butyl-
hydroperoxide was added to help initiation. However, no
significant oxidation took place even upon continuing
heating for 180 minutes. This demonstrates that, at least,
benzoic acid has not the same activity as p-toluic acid to
promote oxidation in the presence of water as carried out
in the process of the present invention.
5~
- 44 -
CO~lPARATIVE E~IPLE 3
The same experiment as in Example 11 was repeated
except that acetic acid was substituted for a part of
p-toluic acid. The actual composition of the cnarge was -
5 the following: --
p-xylene 3.0 g
p-toluic acid 52.7 g
acetic acid 55.2 g
various oxygenated compounds 1.3 g
water 187.8 g
manganese acetate 1.05 millimoles
cobalt acetate 1.05 millimoles.
-In this example y was 26.92 so that ~ was 10.8, i.e.
about the same as in the preceding comparative example. Here
again, if acetic acid had the same promoting effect as p-
toluic acid for oxidation in aqueous medium, it should be ta-
ken into account for the calculation of y and M. In this case
we would have y = (1~7.2/18.02)/(52.7/136.15 + 55.2/60.0;)
= 7.95, i.2. exactly the same value as in Example 11.
The above mixture was heated at 185C under the
same conditions as in Example 11 and here again some t-butyl-
h~droperoxide was added to help initiation. After 30G min-
utes of reaction the oxygen absorption amounted to only
3.8 liters. This result demonstrates once again that the
promoting effect of p-toluic acid as it appears in the pro-
cess of the invention is not displayed by other carbo~ylic
acids, i.e. is specific of p-toluic acid.
EX~_iPLE 12:
Into the same autoclave as in the preceding exam-
ples, there ~,Jas charged:
S7
- 45 -
p-xylene 6.0 g -
p-toluic acid 183.3 g
various oxygenated derivatives 5.7 g -
water 105.0 g -
manganese acetate 1.80 millimoles.
Here, y = 4.33 and, as manganese was used as sole
catalyst, x = 1.00. Therefore, by application of equation (1),
the minimum concentration of manganese to be used in this
case is M = 3.6 millimoles/kg. Actually, in the present
example, the concentration of manganese was 6.0 millimoles/kg,
thus well in excess over the limiting amount.
This mixture was heated in the presence of air
exactly under the same conditions as in Example 11 except
that temperature was maintained at 170C instead of 185C.
After 305 minutes of reaction, the oxygen absorption amounted
to 21.2 liters. Upon treating and analyzing the resulting
reaction mixture as in the preceding examples, it was de-
termined that about 96~ of the p-xylene fed and 29.9% of the
p-toluic acid fed had been transformed into ihe following
products:
terephthalic acid 44.8 g
4-carboxybenzaldehyde14.2 g
other intermediates 0.9 g
heavy by-products 1.2 g.
25 COMPARATIVE EXAMPLE
The same amounts of compounds as in the preceding
example were charged into the reacto- except that only 0.60
millimoles of manganese acetate was used as catalyst. The
concentration thereof in the mixture was therefore
5~f -
- 46 -
2.0 millimoles/kg, i.~. by 1.6 millimole lower than ~he
limiting amount. As a result, no oxygen absorption tooX
place upon heating the mixture for 60 minutes in the pre-
sence of air under the same conditions as in the preceding
example, even after addition of t-butylhydroperoxide to
help initiation~
EX~PLE 13:
Into the same autoclave as in the preceding
examplesj there was charged:
p-xylene 6.0 g
p-toluic acid 218.3 g
various oxygenated derivatives 2.2 g
water 73-5 g
cobalt acetate 3.48 millimoles.
Thus, in this example it is cobalt which was used
as sole catalyst, i.e. x = 0; On the other handj y was 2.54.
By application of equation (1), the minimum concentration
of cobalt to be used in this case is ~1 = 7.0 millimoles/kg.
Actually, in the present example, the concentration of
0 cobalt was 11.6 millimoles/kg.
Air was admitted into the reactor at a flow rate
of 90 liters per hour under a pressure of 20 kg/cm and the
mixture ~las heated up to a temperature of 170C. While
heating, the reaction started spontaneously and was con-
tinued for 240 minutes. At the end of the reaction, theoxygen absorbed amounted to 29.8 liters. Upon treating and
analyzing the reaction mixture as described in Example 1,
it was determined that 89.3% of the p-xylene fed and 37.0
of the p-toluic acid fed had been transformed into the
following products:
s~
- 47 -
terephthalic acid 81.5 g
4-carboxybenzaldehyde 6.8 g
other intermediates 1.2 g
heavy ~y-products 3.0 g.
COMPAR~TIVE E~l~lPLE
The same amoun'sof compounds as in the preceding
example werecharged into the reactor except that only 0.90
millimole of cobalt acetate was used as catalyst. The con-
centration thereof in the mixture was therefore 3.0 milli-
moles/kg, i.e. by 4.0 millimoles lower than the limiting
amount. As a result, no oxygen absorption took place upon
heatins this mixture for 240 minutes in the presence of
air under the same conditlons as in the preceding example,
even after repeated additions of t-butylhydroperoxide.
It is to be understood that the foregoing des-
cription of the fundamental novel features of the present
invention is merely illustrative of preferred embodiments.
Those skilled in the art will appreciate that many variations
may be made. For example, in carrying out the process of
the present invention, there may be added to the reaction
mixture one or another extraneous compound used as activa-
tor in other processes, e.g. an aldehyde or a ketone or
still a bromine-containing compound. Such additives may
even be advantageous with respect to e.g. the reaction rate.
These variations, modifications or changes may be made with-
in the scope of the following claims without departing from
the spirit thereof.
