Control of Isomer Ratio in the Dinitration of Toluene - ACS Symposium

21 Control of Isomer Ratio in the Dinitration of Toluene

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch021

G. F. P. HARRIS Research Department, Imperial Chemical Industries Limited, Organics Division, Hexagon House, Blackley, Manchester 9, England

Introduction Dinitrotoluenes are manufactured on a large scale to provide starting material for the production of isocyanates. As in the case of the mononitration of toluene a number of isomeric products are produced, viz.

313

Albright and Hanson; Industrial and Laboratory Nitrations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


314

INDUSTRIAL AND LABORATORY NITRATIONS

Thus most of the toluene i s converted to 2:4 and 2:6 dinitrotoluene. When the standard sulphuric/nitric acid nitrating mixture is used these two isomers are produced in the ratio 80:20 2.4:2.6 dinitrotoluene, which is the desired ratio for isocyanate production. Some of the so called "ortho"-isomers 3:4 and 2:3 dinitrotoluene - are also produced and a small amount of 2:5 dinitrotoluene i s present i n the reaction products. When the dinitrotoluenes are reduced to the corresponding amines and phosgenated for isocyanate production these "ortho" isomers do not react readily and give rise to quality problems in the final product. Hence there w i l l be considerable benefits if the 2:3 and 3:4 dinitrotoluene can be eliminated from the nitration product or at least reduced i n quantity. In previous work on the mononitration of toluene (1) with nitrating mixtures containing phosphoric acid the amount of metanitrotoluene (the precursor of 2:3 and 3:4 dinitrotoluenes) could not be reduced, but it was possible to alter the isomer ratio of the reaction products so that the amount of ortho nitrotoluene produced was decreased. As shown above, para nitrotoluene gives 100% 2:4 dinitrotoluene on further nitration, while the ortho isomer gives a mixture of 2:4 and 2:6 dinitrotoluenes. From the results of the work on mononitration of toluene it i s possible that the presence of phosphoric acid during nitration of o-nitrotoluene would decrease the amount of 2:6 dinitrotoluene (ortho nitration) produced. Thus mixtures of o-nitrotoluene and toluene could be converted to dinitrotoluenes and still give the desired 80/20 mixture of 2:4 and 2:6 dinitrotoluene. This would also reduce the amount of 2:3 and 3:4 dinitrotoluenes, since nitration of o-nitrotoluene gives only the 2:4 and 2:6 isomers. A series of experiments was carried out, therefore, on the dinitration of toluene/ortho nitrotoluene mixtures in the presence of phosphoric acid. Experimental The nitrations were carried out i n a 700 ml glass reactor fitted with a s t i r r e r and cooling c o i l and mounted i n a thermostated bath. The cooled nitrating mixture was added slowly to the toluene/ortho nitrotoluene mixture with vigorous s t i r r i n g and the temperature of the reactor contents maintained at 65 -3 0. After the addition of the nitrating mixture was complete the reactor contents were stirred for a further 2 hours at 65 C to complete the reaction. The reaction products were then poured on to ice, filtered and the solid product dried and weighed. In a few experiments some l i q u i d nitration products (mononitrotoluenes) were produced. These were recovered from the f i l t r a t e , washed, dried and weighed.


21.


1. 2. 3· 4.

HARRIS

Dinitration

of

Toluene

315

ρ

Strength o f phosphoric a c i d (% ° ς ^ Concentration o f P 0 (mois) Concentration o f s u l p h u r i c a c i d (mois) Ortho n i t r o t o l u e n e / t o l u e n e r a t i o 2

5

2

As i n the work on mononitration ( l ) the phosphoric a c i d s were prepared by adding e x t r a to commercial phosphoric a c i d . The strengths o f the a c i d s produced were expressed i n terms o f (% Ρ 0 ) s i n c e concentrated phosphoric a c i d i s a complex mixture o f the ortho, pyro and meta a c i d s . The amount o f n i t r i c a c i d d i d not appear to be important as l o n g as there was s u f f i c i e n t to convert a l l the toluene and ortho n i t r o t o l u e n e t o d i n i t r o t o l u e n e . A s l i g h t excess (1.04 mois HNO /mol o r t h o n i t r o t o l u e n e + 2.08 mois HNO /mol toluene) was, t h e r e f o r e , used i n a l l subsequent experiments. I t also appeared important to reduce the amount o f water present t o a low l e v e l so the n i t r i c a c i d s t r e n g t h was f i x e d a t 95% ^ Ο - ζ · The process was optimised i n terms o f the four v a r i a b l e s l i s t e d above. A s t a t i s t i c a l l y designed s e r i e s o f experiments was c a r r i e d out i n which the four v a r i a b l e s were each i n v e s t i g a t e d a t three l e v e l s , v i z . 1. Phosphoric a c i d s t r e n g t h 68, 72 and 78% °s 2. Phosphoric a c i d c o n c e n t r a t i o n 1.0, 2.5 ana 3.0 mois ^^pc* 3· S u l p h u r i c a c i d c o n c e n t r a t i o n 2.0, 4.25 and 6.0 mois ^ k 98% s u l p h u r i c a c i d . 4. O r t h o n i t r o t o l u e n e / t o l u e n e r a t i o 0.2, 0.6 and 0.8. p

e

2

8 1 8

27 Experiments - a r e p l i c a t e o f the f u l l f a c t o r i a l o f experiments with 4 v a r i a b l e s - were c a r r i e d out and the r e s u l t s analysed s t a t i s t i c a l l y . The o r t h o n i t r o t o l u e n e / t o l u e n e r a t i o was used as the response and each o f the independent v a r i a b l e s as l i n e a r and squared terms. A l l the square terms were r e j e c t e d and the equation f i t t e d was ONT/Toluene Ratio = Ο.69 + 0.08

+ 0.11

(y -

73)* + 0.13

(z

(x -

4)^

- 4)^ - 0.70

(R

- 4)^

where χ = H^SO^ cone, (mois) y ss phosphoric a c i d s t r e n g t h (%

ζ ss 2 ° 5 P

c o n c

*

(

m o l

^^5^

s)

R = 2:4/2:6 d i n i t r o t o l u e n e isomer r a t i o



316

INDUSTRIAL AND

LABORATORY

NITRATIONS

S o l i d and l i q u i d r e a c t i o n products were analysed s e p a r a t e l y by gas l i q u i d chromatography f o r unchanged s t a r t i n g m a t e r i a l , mononitrotoluenes and the f i v e d i n i t r o t o l u e n e s , and the percentage conversion to each r e a c t i o n product and the 2:4/2:6 d i n i t r o t o l u e n e r a t i o c a l c u l a t e d . A few samples were analysed by i n f r a red methods to check the accuracy o f the GLC method o f a n a l y s i s . The 2:4/2:6 isomer r a t i o s determined by the two methods agreed w e l l f o r samples which contained l e s s than about 3% " o r t h o " isomers (2:3 and 3^4 dinitrotoluenes). I f more than t h i s amount o f "ortho" isomers was present i n f r a red measurements gave too high a value o f the isomer r a t i o . Results 1. Blank Experiments on Toluene N i t r a t i o n . Under manufacturing c o n d i t i o n s the n i t r a t i o n o f toluene with a mixture c o n t a i n i n g 64.0 wt.% H SO, , 27.2% HNO and 8.8% water g i v e s a r e a c t i o n product with 2:4/2:6 d i n i t r o t o l u e n e isomer r a t i o s o f 4.0 - the d e s i r e d mixture f o r isocyanate p r o d u c t i o n . Laboratory s c a l e experiments on the n i t r a t i o n o f toluene with t h i s mixture gave a product with an isomer r a t i o o f 4.37 and " o r t h o " isomer content o f 3·6%. T h i s i n c r e a s e i n the amount o f 2:6 d i n i t r o t o l u e n e i n the r e a c t i o n product i s probably a s c a l e e f f e c t due to d i f f e r e n c e s i n a g i t a t i o n and method o f adding the n i t r a t i n g mixture between the l a b o r a t o r y and manufacturing s c a l e equipment. 2. N i t r a t i o n o f O r t h o n i t r o t o l u e n e . I n i t i a l experiments were c a r r i e d out on the n i t r a t i o n o f o r t h o n i t r o t o l u e n e alone. The standard s u l p h u r i c / n i t r i c a c i d mixture was used with the a d d i t i o n o f phosphoric a c i d . I t was found that a 2:4/2:6 isomer r a t i o o f about 3·0 could be obtained by u s i n g l a r g e q u a n t i t i e s o f polyphosphoric a c i d . Though t h i s was an improvement on the r a t i o o f 1.86 obtained when no phosphoric a c i d was present i n the n i t r a t i n g mixture, i t was l e s s than the 4.0 isomer r a t i o necessary f o r isocyanate manufacture. A l l further work was, t h e r e f o r e , c a r r i e d out with t o l u e n e / o r t h o n i t r o t o l u e n e mixtures. 3· Toluene/Orthonitrotoluene Mixtures. Preliminary experiments showed that the presence o f some s u l p h u r i c a c i d (as w e l l as phosphoric a c i d ) i n the n i t r a t i n g mixture was d e s i r a b l e to give r a p i d conversion to d i n i t r o t o l u e n e . These experiments i n d i c a t e d that the f o l l o w i n g four v a r i a b l e s a f f e c t e d the isomer r a t i o produced i n the n i t r a t i o n o f toluene/ortho n i t r o t o l u e n e mixtures.



21.

HARRIS

Dinitration

of

317

Toluene

The percentage f i t o f t h i s equation was 87% and the r e s i d u a l standard d e v i a t i o n was 0.10 with an F value s i g n i f i c a n t a t the 0.1% l e v e l . Thus the major causes o f v a r i a t i o n have been considered and the l i n e a r r e l a t i o n s h i p i s o f the c o r r e c t order. T h i s equation enables the r e l a t i o n s h i p between o r t h o n i t r o t o l u e n e / t o l u e n e r a t i o i n the n i t r a t i o n feedstock and the 2:4/2:6 d i n i t r o t o l u e n e isomer r a t i o o f the product to be c a l c u l a t e d f o r any given n i t r a t i n g mixture with a composition w i t h i n the experimental range. The amount o f "ortho" isomers i n the r e a c t i o n product v a r i e d c o n s i d e r a b l y with r e a c t i o n c o n d i t i o n s . Table 1 shows the r e s u l t s o f a s e r i e s o f experiments with a range o f phosphoric a c i d s t r e n g t h s , mol. concentrations o f p 2 0,- 0 1 1 ( 1 **2^4 8 1 1 1 ( 1 orthonitrotoluene/toluene r a t i o s . Table I Orthonitrotoluene/ toluene ratio

Phosphoric acid strength (% P O ) 2

68

0.2

5

Phosphoric 'Sulphuric acid acid cone, cone. (mole 0^) (mois H ^ ) p

2

1.0

6.0

% "Ortho" 2:4/2:6 isomers in d i n i t r o r e a c t i o n toluene ratio product

0.2

72

4.0

6.0

1.9 2.0

0.6

68

4.0

6.0

1.5

4.01

0.6

78

4.0

2.0

4.16

0.8

78

4.0

6.0

1.7 1.4

0.8

68

1.0

1.4

3.37

0.8

72

1.0

4.25 6.0

1.3

3.51

4.48 5.20

4.30

As a n t i c i p a t e d the amounts o f " o r t h o " isomers decrease with increase i n the amount o f o r t h o n i t r o t o l u e n e i n the mixture being nitrated. The minimum q u a n t i t y o f ortho isomers i n the r e a c t i o n product was 1.3% compared with 3·5% f o r the n i t r a t i o n o f toluene alone with a s u l p h u r i c / n i t r i c a c i d mixture. Discussion Examination o f equation ( l ) shows that there i s no optimum, w i t h i n the range o f the experiments, i n the amount o f phosphoric a c i d used. The s t r o n g e r the a c i d and the g r e a t e r the number o f mois ° c t the l a r g e r the o r t h o n i t r o t o l u e n e / t o l u e n e r a t i o which can be used i n the r e a c t i o n mixture to give a p a r t i c u l a r 2:4/2:6 isomer r a t i o . p

2


INDUSTRIAL A N D LABORATORY

318

NITRATIONS

There are, however, p r a c t i c a l l i m i t a t i o n s to the r e a c t i o n c o n d i t i o n s which can be employed. Phosphoric a c i d c o n t a i n i n g more than about 78% P 0 - * too viscous to handle, while more than about 4 . 0 mois ^ 2 ° 5 * n i t r a t i n g a c i d gives very l a r g e volumes o f r e a c t i n g mixture. Since the l a b o r a t o r y s c a l e blank experiments with toluene alone and a s u l p h u r i c / n i t r i c a c i d n i t r a t i n g mixture gave a value o f 4.37 f o r the 2:4/2:6 isomer r a t i o t h i s f i g u r e was s u b s t i t u t e d i n equation ( l ) and the o r t h o n i t r o t o l u e n e / t o l u e n e r a t i o c a l c u l a t e d f o r a range o f phosphoric a c i d s t r e n g t h s , number o f molecules P 0 and s u l p h u r i c a c i d . The r e s u l t s o f these c a l c u l a t i o n s are shown i n Table I I . s

p


nt

h

e

p

Table I I C a l c u l a t e d Values o f ONT/Toluene Ratio to give an Isomer Ratio o f 4.37 A H S0^ Cone, (mois) 2

Phosphoric A c i d Strength

P_0 2

5

cone. (mols)

ONT/Toluene t o g i v e Isomer Ratio o f 4.37 Wt. % Orthonitrotoluene i n feedstock

6.0 78

6.0 78

4.25 78

4.25

4.25

4.25

72

72

78

4.0

2.5

4.0

2.5

4.0

2.5

0.59

0.42

0.52

0.35

0.26

0.09

46.5

38.5

43.6

34.2

11.8

27.9

Thus with a n i t r a t i n g mixture c o n t a i n i n g 4 . 0 mois P 0 _ 78% Ρ 0 phosphoric a c i d together with 6.0 mois k 98% s u l p h u r i c a c i d (column A - Table 2) i t i s p o s s i b l e t o use a feedstock c o n t a i n i n g up to 46.5% by weight o r t h o n i t r o t o l u e n e and s t i l l produce a product with the d e s i r e d 2:4/2:6 d i n i t r o t o l u e n e isomer r a t i o . Such a product would contain about 1.7% "ortho" isomers· 2

H

S 0

0

8

2


0

8

21. HARRIS

319

Dinitration of Toluene

Nitration in the presence of phosphoric acid, therefore, serves the purpose of providing an outlet for a waste product (orthonitrotoluene) from the mononitration of toluene, and reduces the amount of undesirable "ortho isomers in the reaction product. It has a further advantage in reducing the amount of nitric acid required to produce dinitrotoluene, since it is only necessary to introduce one nitro group into the ortho nitrotoluene in the toluene/orthonitrotoluene mixture. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch021

11

Literature Cited 1.

Harris, G.F.P.

Isomer Control in the Mononitration of Toluene - American Chem. Soc. Symposium, Philadelphia, April 1975 (This volume).


Control of Isomer Ratio in the Dinitration of Toluene - ACS Symposium

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