Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE PRODUCTION OF DINITROTOLUENE
OR MONONITROBENZENE
Field of the Invention
s This invention relates generally to aromatic
nitration reactions and, more specifically, to a process
for nitrating toluene to dinitrotoluene or benzene to
mononitrobenzene.
Background of the Invention
Nitration reactions of aromatic hydrocarbons
are generally conducted in mixed acid systems, such as
mixed nitric and sulfuric acids. However, these mixed
acid systems usually involve reconcentration of the spent
sulfuric acid after the nitration reaction. This
reconcentration step is time consuming, energy intensive
and requires the use of expensive materials of
construction. In a~dition, the use of sulfuric acid
tends to result in significant nitrocreosol and cyanide
by-product formation which requires expensive waste-water
treatment to remove.
In view of these disadvantages associated with
mixed nitric/sulfuric acid systems, there have been
recent attempts to perform gas phase or liquid phase
nitrations in concentrated nitric acid in the absence of
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sulfuric acid. By way of illustration, U.S. Patent No.
4,064,147 discloses the preparation of aromatic mononitro
compounds (such as mononitrobenzene) by a liquid phase
reaction with nitric acid having an acid concentration of
between 70 percent and 100 percent by weight using a
reaction temperature of between 0~C and 80~C. When
employing a relatively reactive compound such as benzene
or toluene as a starting material, this patent teaches
that a nitric acid concentration of between 70 and 90
percent by weight is preferred. The disclosure of this
patent requires a ratio of nitric acid plus water to
organic components of not below 3 when using 70 percent
nitric acid, and not below 8 when using 100 percent
nitric acid. However, it has now been found that such a
high acid ratio using 100 percent nitric acid tends to
- favor dinitro-compound production, not desired by the
patentee in the '147 patent.
Since mononitrobenzene is useful in producing
MDI and since dinitrotoluene is useful as an intermediate
in producing TDI, new proce~ses for the selective
manufacture of these intermediates would be highly
desirable to the polyisocyanate manufacturing community.
Summary of the Invention
The present invention relates to a process for
nitrating benzene or toluene by a liquid phase nitration
reaction of anhydrous nitric acid with benzene or toluene
in a reactor at a reaction temperature not exceeding
80~C in the absence of sulfuric acid to produce
mononitrobenzene or dinitrotoluene in a product mixture,
followed by vacuum distillation of the product mixture to
remove unreacted nitric acid.
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This and other aspects of the present
invention will become apparent upon reading the following
detailed description of the invention.
Detailed Description of the Invention
In accordance with the process of the present
invention, the nitration reaction is conducted using
anhydrous nitric acid in the absence of sulfuric acid.
As used herein, the term ~anhydrous nitric acid~ is
intended to designate nitric acid having an acid
concentration of between 95 and 100 weight percent,
preferably at least 98 weight percent, the remainder
being water. It is desirable to minimize the amount of
water in the reaction mixture since water (a) causes the
nitration reaction to stop at the mononitration stage in
toluene reaction and (b) prevents the nitration of
benzene to mononitrobenzene.
The process of the present invention utilizes
a one-step reaction in a single phase liquid medium and
does not involve the formation of the two phase emulsions
observed in conventional, mixed sulfuric/nitric acid
nitration processes. Another surprising aspect of this
invention is that the reaction can be conducted under
moderate reaction conditions to provide an excellent
yield of the desired mononitrobenzene or dinitrotoluene
product. Thus, the reaction is suitably conducted at a
reaction temperature not exceeding 80~C, preferably
between 0~C and 60~C, more preferably between 10~C
and 60~C, most preferably between 20~C and 30~C.
The reaction is suitably conducted at atmospheric
pressure, although superatmospheric pressure can be
employed if desired. The reaction time is typically less
than one-half hour, preferably less than 15 minutes, and
more preferably les~ than 5 minutes.
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For the reaction of toluene to dinitrotoluene,
the molar ratio of nitric acid plus water to toluene
employed is generally between 10:1 and 15:1, preferably
between 11:1 and 12:1.
For the reaction of benzene to mononitro-
benzene, the molar ratio of nitric acid plus water to
benzene employed is generally between 2:1 and about 4:1,
preferably between 2.5:1 and 3.5:1.
Operating within the above-recited broad
ranges of molar ratios (and particularly within the
preferred ranges) maximizes the production of the desired
product and minimizes by-product formation.
After reaction and product formation, it is
desired that excess (unreacted) nitric acid be removed
from the reactor, preferably by vacuum distillation, thus
providing a low temperature, low pressure distillation.
Suitable distillation temperatures range from 30~C to
60~C. Suitable distillation pressures range from 50 mm
of Hg to 300 mm of Hg.
Following removal of the excess anhydrous
nitric acid, DNT separation from the distillation still
bottoms can be effected by phase separation, brought
about by the addition of a small quantity of water or
dilute nitric acid. Washing with water and a basic
solution produces a purified DNT product. These wash
waters are free of the nitrocresol impurities observed in
the wastewater produced in a conventional, mixed
sulfuric/nitric acid DNT process. The aqueous nitric
acid from the phase separation step can be purified~by
toluene extraction, the toluene phase being recycled to
the reaction step and the 60-70% aqueous nitric acid
phase reconcentrated, sold or used in other product
manufacture. Analogous phase separation procedures can
be employed for nitrobenzene separation and recovery.
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The following examples are intended to
illustrate, but in no way limit the scope of, the present
invention.
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EXAMPLE 1
Synthesis of Dinitrotoluene
A four milliliter glass vial, equipped with a
magnetic stir bar and a silicone septum, was immersed in
a water bath. The reaction vial was flushed with
nitrogen at a rate of 20 cc/min, purging to a 100 ml
glass receiving flask immersed in an ice water bath. To
the reaction vial was fed 50 ml of 98 percent HNO3,
75.0 g, 1.13 mole of HNO3 and 10 ml, 8.67 g, 0.094 mole
of toluene. Feed rates were 0.60 ml HNO3/min and 0.12
ml toluene/min, controlled by Sage Instrument Syringe
pumps, Models 351 and 3S5. Reactor content was adju~ted
to 2 ml, by height adjustment of the reactor exit line in
the reaction vial, for a mean reaction residence time of
2.8 minutes. The reactor water bath was maintained at 15
+ 5~C by the periodic addition of ice during reactant
addition. Upon completion of the reactant addition, the
reactor contents were stirred for 3 minutes, then purged
to the receiver. A total of 83.31 g of pale yellow
product solution was obtained. Distillation of this
solution (53~C, 75-160 mm Hg) gave 38.75 g of pale
yellow acid which analyzed, by titration with
standardized NaOH, as 100 percent HNO3. The pot
contained 44.21 g of pale yellow solution; 0.42 g of
HNO3 was lost to the walls of the glassware, leaving an
estimated 0.43 g of product lost to NO2 vapors during
the distillation. The pot solution was diluted with
21.72 g of water and extracted with 33.30 g of toluene.
Separation of the layers furnished 48.74 g of weak,
30 l aqueous acid and 48.11 g of toluene/DNT solution The
organic layer was washed once with 20 ml of water, then
dried over MgSO4 and filtered. DNT recovery was
calculated at 86 percent, with a.normalized GC analysis
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of 0.02 weight percent 4-nitrotoluene, 17.36 percent
2,6-DNT, 0.48 percent 2,5-DNT, 78.47 percent 2,4-DNT,
1.65 percent 2,3-DNT, 1.92 percent 3,4-DNT and 0.09
percent TNT. HNO3 accountability, as recovered weak
acid and DNT equivalent, was 99 percent.
EXAMPLE 2
Additional Syntheses of Dinitrotoluene
In the manner described in EXAMPLE 1, 100 ml
of 98 percent HNO3, 150.0 g, 2.38 mole HNO3 and 21
ml, 18.2 g, 0.20 mole of toluene were fed at 0.80 ml/min
and 0.17 ml/min, respectively, to the reaction vial. A
total of 166.26 g of pale yellow product solution was
obtained. The product was heated for two hours at 55~
to 60~C, then cooled and diluted with 46.5 g of ice
water. The resulting suspension was extracted once with
41.5 g of toluene and then a second time with 46.3 g of
toluene. The combined toluene extract was extracted with
3 x 15 ml of 5 percent sodium hydroxide solution. The
combined, yellow caustic extract was cooled, acidified
with dilute sulfuric acid, and extracted with 3 x 10 ml
of methylene chloride. After evaporation of the bulk of
the methylene chloride, the methylene chloride extract,
containing the acidic organic specie~ from the original
DNT product, was characterized by gas chromatography/mass
spectrometry analy~is. No mononitro- or dinitro-cresol
species were detected (minimum detectability calculated
at 2 ppm, based on original weight of DNT produced).
Additional experiments were performed to
~ define the reactant ratio suitable for selective DNT
synthesis. These products are characterized in TABLE I
below for various molar ratios of HNO3 to toluene.
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TABLE I
Toluene Nitration
HNO3/Toluene Product in Wt. %
Mole Wt.
SSample Ratio Ratio o-NT m-NT p-NT DNT
3.4 2.3 53.17 4.04 39.29 3.49
2 5.6 3.8 2B.80 2.70 29.57 38.93
3 7.8 5.3 8.21 1.72 14.66 75.40
4 11.6 7.9 0.94 ---- 0.22 98.84
10 Reaction at 54~ to 57~C
NT - mononitrotoluene, ortho, meta and para isomers
EXAMPLE 3
Synthesis of Nitrobenzene
In the manner described in EXAMPLE 1, 7.0 ml of
98 percent HNO3, 10.5 g, 0.163 mole of HNO3 and 5.0
ml, 4.39 g, 0.056 mole of benzene were fed at 0.22 ml/min
and 0.135 ml/m$n, respectively, to the reaction vial.
The 14.65 g of pale yellow product solution was diluted
with 42.49 g of ice water and extracted with 2 x 15 ml of
20 methylene chloride. Dilute acid recovery was 51.39 g,
for an organic recovery of 5.75 g, by difference. Gas
chromatographic analysis of the organic product showed
only nitrobenzene, exclusive of the methylene chloride
solvent peak, for a recovery of 0.047 mole (83 percent)
25 of nitrobenzene. HNO3 accountability, as recovered
weak acid and nitrobenzene equivalent, was 96 percent.
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