Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A PROCESS FOR THE PREPARATION OF P-TOLUIC ACID BY LIQUID
PHASE OXIDATION OF P-XYLENE IN WATER
FIELD OF THE INVENTION
The present invention relates to a process for preparation of p-toluic acid by
liquid
phase oxidation of p-xylene in water.
Particularly this invention relates to the process of preparation of p-toluic
acid by liquid
phase oxidation of p-xylene in water in the presence of p-toluic acid and
cobaltous
acetate or its combination with cerium(III)acetate as catalyst and a catalyst
consisting of
a cobalt salt or its combination with salt of cerium and or Manganese. More
particularly
this invention relates to a process for preparing p-toluic acid by oxidation
of p-xylene in
water as solvent.
BACKGROUND OF THE INVENTION
p-Toluic acid is an important chemical intermediate, which is widely used to
prepare
finished products in the manufacture of medicines, agro-chemicals, dyestuffs,
optical
brighteners etc. It is also used in organic synthesis of various chemical
compounds. p-
Toluic acid is produced as by- product during the production of tere-phthalic
acid/dimethyl tere-phthalate, which are mainly produced by liquid phase
oxidation of p-
xylene by oxygen/air in acetic acid medium.
Oxidation of p-xylene to tere-phthalic acid in acetic acid solvent in the
presence of
bromine or bromine containing compound as initiator and a catalyst containing
cobalt
and manganese components has been disclosed in US Pat. No. 2,833,816 and is
practiced world over. Homogeneous catalyst system consisting of cobalt,
manganese
and bromide is the heart of this process. Oxygen compressed in air is used an
oxidant
and acetic acid as solvent.
Although the use of bromine is advantageous for such liquid phase oxidation,
but its use
is associated with some drawbacks. The highly corrosive bromine acetic acid
water
environment during oxidation require the use of costly titanium lined
equipments in
some part of the process. Additionally, during reaction bromine also produce
methyl
bromide, which is hazardous gas. Moreover, in the bromine promoted process,
the
reaction mixture must be maintained "Substantially anhydrous" (US Pat.
No.3064044).
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Another patent (US Pat. No. 3046305) described a process for the preparation
of p-
toluic acid starting from toluene, wherein toluene is reacted with
chloroformamide under
the conditions of Friedel and Craft reaction, and the product thus obtained is
hydrolysed
to p-toluic acid. This process is not in practice.
There are other patents which report oxidation of p-xylene to terephthalic
acid in the
presence of water. Patent (US. Pat. No. 4,334,086) assigned to Labofina, S.
A.,
Belgium describes a two stage process for oxidation of p-xylene at the 170 C
in the
presence of Co-Mn catalyst and 10% (wt%) water in first stage. In second stage
partially oxidized compounds are further oxidized at 200 C in the presence of
20-70%
water. After the oxidation, water soluble compounds are separated from
insoluble
terephthalic acid at 180-200 C, which include p-toluic acid, catalyst and
other by
products, which are recycled for further oxidation. Crude terephthalic acid
crystals
obtained in sedimentation column still have about 4.5% p-toluic acid and 2.5%
4-
carboxybenzaidehyde as impurities. Labofina. S. A., (US Pat. No. 4,357,475)
also
described a correlation between temperature of sedimentation column and the
oxidation
temperature for similar process of oxidation of p-xylene in the presence of
water as
described above. Labofina. S. A., (US Pat. No. 4,259,522) also described
similar
process for iosphthalic acid by oxidizing m-xylene in the presence of water.
Most of the
patents, including above mentioned patents describe the processes for the
oxidation of
p-xylene to produce tere-phthalic acid as the main product. Little or scanty
information
is available for the processes to selectively produce p-toluic acid, by liquid
phase
oxidation of p-xylene in aqueous medium. Studies on oxidation of p-xylene to p-
toluic in
acetic acid medium in presence of bromide initiator has been described by S.
H. Zaidi
[Applied Catalysis, 27, 99-106 (1986)]. Therefore, a process to produce p-
toluic acid as
main product by liquid phase oxidation of p-xylene in water under mild
operating
condition without using bromine compounds as initiator would not only be of a
great
economic and commercial potential, but also be an environment friendly
technology.
OBJECTIVES OF THE INVENTION
The main object of the present invention is to provide a process for
preparation of p-
toluic acid by liquid phase oxidation of p-xylene in water.
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Another object of the present invention is to provide a process for carrying
out oxidation
of p-xylene in the presence of ecofriendly solvents such as water and bromine
free
catalysts thereby avoiding the use of corrosive substances such as alkanoic
acid as
solvent and bromine compound as initiator.
Yet another object of the present invention is to provide a process wherein
the by-
products of the reaction such as terephthalic acid and carboxybenzaidehyde are
produced in minimum possible amount so that p-toluic acid is obtained in high
purity.
Yet another object of the present invention is to provide a process for
producing p-toluic
acid by oxidizing p-xylene with oxygen or air or N2 - 02 mixture in the
presence p-toluic
acid and water.
SUMMARY OF THE INVENTION
Accordingly the present invention provides a process for preparation of p-
toluic acid by
liquid phase oxidation of p-xylene in water which comprises oxidizing p-xylene
or a
mixture of p-xylene and p-toluic acid with oxygen, air or oxygen/ nitrogen
mixture, at a
pressure of 3-25 Kg/cm2 with an exit flow rate of 60-80 mi/min, in the
presence of
catalytically active salt of transition metal selected from the group
consisting of Co, Mn,
Ce and combination thereof in the range 1-200 mmol per mole of p-xylene in
water in
the range of 40-85% of total charge, at a temperature ranging between 130-190
C, for
a period of 5-10 hrs, cooling the above said reaction mixture and removing the
un
reacted p-xylene by washing with an organic solvent, followed by filtration to
obtain the
desired product.
In an embodiment of the present invention the amount of catalyst used is 5-160
mmol per mole of p-xylene.
In another embodiment the transition metal salt used is selected from the
group
consisting of cobaltous acetate, manganese acetate, cerium acetate and a
combination
thereof.
In yet another embodiment the cobaltous acetate used is 5-150 mmol per mole of
p-
xylene.
In yet another embodiment the concentration of cerium (III) acetate used is a
maximum
of 6 mmol per mole of p-xylene.
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In yet another embodiment the p-toluic acid used comprises of about 0.1 to
about 1.5
mmol per mole of p-xylene.
In yet another embodiment water comprises of about 50 to about 80 weight
percent of
the reaction mixture.
In yet another embodiment pressure is sufficient to keep water in liquid phase
preferably
in the range 5 to 20 kg/cm2.
In yet another embodiment, the reaction mixture is substantially free from
extraneous
organic solvent.
In yet another embodiment reaction products are recovered by filtration,
followed by
distillation of -filtrate and washings to recover unreacted p-xylene and a
part of
solvent(water).
In yet another embodiment the remaining water in the products consisting small
amount
of reaction products and catalyst is recycled in subsequent oxidation.
In still another embodiment of the present invention remaining water in the
products
consisting small amount of reaction products and catalyst is recycled in
subsequent
oxidation.
DETAIL DESCRIPTION OF THE INVENTION
The said process is carried out in a stirred reactor, in aqueous solution
containing 50-
80% weight percent of water at a temperature between 130-190 C and under 5-20
kg/cm2 pressure, sufficient to maintain water in liquid state, in the presence
of
catalytically active metal compound selected from Mn, Ce and its mixture with
cobalt
compound. After the reaction, product is separated out as solid product by
filtration.
Unreacted p-xylene is recovered from the filtrate by distillation and
recycled. Remaining
aqueous solution containing small amount of reaction products and catalyst is
recycled
in subsequent oxidation.
Various processes have been described in the literature for production of
terephthalic
acid by liquid phase oxidation of p-xylene in acetic acid solvent in the
presence Co/Mn
catalysts and bromine containing compound as initiator. Although, effect of
water as
diluent has also been reported by Hanotier et. al. (US Pat. No. 4,334,086 &
4,357,475)
in such processes, but little information is available on oxidation of p-
xylene by
molecular oxygen in aqueous medium to produce p-toluic acid as major project.
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The oxidation reaction is carried out wherein the catalyst is selected from
compounds of
Co, Mn in combination or without cerium; the concentration of catalyst being
in the
range 1-200 mmol per mole of p-xylene. The compounds of cobalt and manganese
are
preferably cobaltous acetate and manganous acetate; the concentration of
cobaltous
acetate being in the range 5-150 mmol per mole of p-xylene. The compound of
cerium
is cerium (III) acetate.
In the said process presence of p-toluic acid is crucial and plays a
significant role during
oxidation of p-xylene in water. During the investigations it was found that at
130 C,
16.6% p-toluic acid of p-xylene resulted in maximum conversion of xylene
(76.87%) with
80.03%, 16-79% selectivities to p-toluic acid and terephthalic acid
respectively.
Presence of p-toluic acid is essential to oxidize p-xylene in water and
minimum of
16.6% of p-xylene is required to get maximum conversion of p-xylene into the
oxygenated products. In the process of the present invention, the
concentration of p-
toluic acid is in the range of about 0.1 to about 1.5 mmol per mole of p-
xylene.
In the present invention the oxidation is carried out in presence of water as
solvent.
Presence of water as solvents makes a three phase system for the oxidation
making the
reaction more difficult. The present invention reveals that 55-80% water as
solvent can
be conveniently used for the oxidation at 130-150 C. When lower amount of
water was
used i.e. less than 55% of reaction mixture, the reaction mixture became a
thick slurry,
difficult to stir and proper mixing of reactants could not be achieved even
with
mechanical stirring. Poor mixing of reactants led to lower conversion of 'p-
xylene
probabiy due to increased resistance to diffusion of 02 into liquid phase.
Although,
conversion of p-xylene, as well as selectivity to terephthalic acid increases
to some
extent with increase in temperature from 130 C to 150 C, but selectivity to p-
toluic acid
remains almost the same.
In the present invention Cerium (III) acetate when used as co-catalyst in
combination
with cobaltous acetate in the concentration upto 6 mmol per mole of p-xylene
increased
the selectivity of p-toluic acid. The presence of cerium(III)acetate with
cobalt(II)acetate
plays a significant role during oxidation. Cerium(III)acetate when used with
cobalt(li)acetate [- 5% (mol %) of Co(II)] as catalyst and p-xylene was
oxidized in water
(77% of the charge) at 130 C temperature and 20 kg/cm2 pressure of oxygen in
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presence of this catalyst [6.7% (mol%)] of p-xylene, the selectivity of p-
toluic acid was
increased from 77.53% to 84.68%.
In present invention the reactor contents are cooled, filtered and washed with
water.
The filtrate and washings are distilled, wherein, unreacted p-xylene and same
part of
water are distilled off as azeotropic mixture of p-xylene and water. p-Xylene
thus
obtained is recycled. Residual aqueous solution containing catalyst and small
amount
of oxidation products is also recycled in subsequent oxidation.
The present invention will be described in more detail with reference to the
following
examples, wherein different process conditions/reaction parameters are given
for the
sake of illustration only and should not be considered as limiting the scope
of the
invention.
Example -1
p-Xylene, (30 g), p-toluic acid (5.0 g), cobaltous acetate (5.0 g) and water
(150 g) were
charged in an autoclave. This reactor was then pressurized with oxygen to
about 15
kg/cm' pressure and heated to 130 C. The pressure of the reactor was
maintained at
20 kg/cm2 with continuous stirring and exit oxygen flow rate at 75 ml/min.
After 6 hr of
reaction, the reactor was cooled, the contents were filtered and washed with
toluene.
Total unreacted p-xylene was determined from organic phase and content of a
toluene
trap (kept after the reactor where through exit gases passed during the
reaction) by
analyzing these by gas chromatography. Solid product was also analysed by G.C.
Analysis _of reaction products/ stream indicated 65.41% conversion of p-xylene
into
oxygenated products (mole%) as p-toluic acid (p-TA), 82.39%, 4-
carboxybenzaldehyde
(4-CBA) 1.82%, terephthalic acid (TPA), 13.50% and others 1.35%.
Example - 2
p-Xylene (30.0g), was oxidized with oxygen at 150 C in the presence of 5.0 g p-
TA, 5.0
g cobaltous acetate and 150 g water. The reaction was carried out as in
Example-1.
78.23% Of p-xylene was converted into p-TA, 90.56%; 4-CBA, 2.75%; TPA, 6.32%
and
others 0.30%.
Example - 3
p-Xylene (30.0 g) was oxidized with oxygen at 150 C in the presence of 5.0 g
cobaltous
acetate as catalyst and 150 g of water. The reaction was carried out and
processed as
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in Example-1. In the absence of p-TA, oxidation did not take place and almost
all of p-
xylene was recovered unreacted.
Example-4
p-Xylene (30.01 g) was oxidized with oxygen at 150 C in the presence of 4.4 g
cobaltous acetate and 0.3 g cerium(III)acetate as catalyst in 150 g of water.
The
reaction was carried out and worked up as in example-1. In the absence of p-TA
but in
the presence of cerium with cobalt. 20.05% Of p-xylene was converted into
oxygenated
compounds. White solid product was found consisting of p-TA 96.62%
terephthalic acid
0.42% and others 2.96%.
Example -5
p-Xylene (30.0g) was oxidized with molecular oxygen at 130 C in the presence
of 10.04
g p-TA, 5.0 g of cobaltous acetate as catalyst and 150 g of water as solvent.
The
reaction was carried out and worked up as in example-1. 72.2% Of p-xylene was
converted into oxygenated compounds as p-TA, 77.53%, 4CBA, 4.75%, TPA, 16.98%
and others 0.73%.
Example -6
p-Xylene (30.0g) was oxidized with molecular oxygen at 130 C, under the
experimental
conditions mentioned in example-5 except that a mixture of 0.31 g of cerous
acetate
and 4.48 g of cobaltous acetate was used as catalyst. 67.34% Of p-xylene got
converted into oxygenated compounds as p-TA, 84.68%, 4-CBA, 2.04% and TPA,
13.22%.
Example -7
p-Xylene (30.0g) was oxidized with oxygen at 130 C in the presence of 10.0 g p-
TA, 5.0
g of cobaltous acetate as catalyst and 150 g water as solvent. Reaction was
carried out
at 10 kg/cma pressure and worked up as in example-1. 73.38% Of p-xylene was
converted into oxygenated compounds as p-TA, 82.88%, 4-CBA, 2.87% and TPA,
14.25%.
Example -8
p-Xylene (30.0g) was oxidized with oxygen at 130 C in the presence of 5.Og of
p-TA,
5.02 g of cobaltous acetate and 50 g of water as solvent. Reaction was carried
out at
20 kg/cm2 pressure and worked up as in example-1. 67.46% Of p-xylene was
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converted into oxygenated compounds as p-TA, 78.99%; 4-CBA, 1.46%; TPA, 16.24%
and others 3.3%.
The main advantages of the present invention are
The followings are the advantages of the process for the preparation of p-
toluic acid by
liquid phase oxidation of p-xylene in aqueous medium.
1. The most important advantage of the process is the use of water as solvent,
which is very much safe, non flammable, non toxic, easily available and cheap.
2. Quick heat transfer from the reactor during exothermic oxidation reaction
in
aqueous medium make the process safe.
3. Bromine free catalyst and exclusion of acetic acid solvLnt make the process
non-
corrosive. Thus no special or costly equipment is required.
4. Higher conversion of p-xylene and higher yield of p-toluic acid are
comparable
with those using acetic acid as solvent.
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