US2765342A

US2765342A - Manufacture of aromatic parahydroxyamines

Info

Publication number: US2765342A
Application number: US316235A
Authority: US
Inventors: Spiegler Louis
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1952-10-22
Filing date: 1952-10-22
Publication date: 1956-10-02
Anticipated expiration: 1973-10-02

Description

Oct. 2, 1956 SPIEGLER 2,765,342

MANUFACTURE OF AROMATIC PARAHYDROXYAMINES Filed Oct. 22, 1952 2 Sheeds-Sheet 1 Re ulafed ad ifion of 8 o nirrobenzene 2 4 6 8 IO l2 l4 Tim e in hours %Complefion of reduction p Aminophenol Fiazg l l l I00 200 300 400 500 600 700 Hydrogen parfiq! pressure-mm of Hg Rate of Hydrogenation l l l l l I l I00 200 300 400 500 600 700 Hydrogen partial press re-mm ofHg INVENTOR LOUIS SPIEGLER BY 5', )WWALZZM ATTORNEY Oct, 2, 1956 Filed 001:. 22, 1952 L. SPIEGLER 2,765,342

MANUFACTURE OF AROMATIC PARAHYDROXYAMINES FIG. 4.

2 Sheets-Sheet 2 Q Q) (D A g e n r El None (Contra/1 I Q Quaternary ammonium compoun 7's I X Non-ouafernory compounds I l I Y I l I 0.4 0.5 0.6 0.7 0.8 0.9 L0

Rafe of hydrogeno/I'on INVENTOR LOUIS 'SPIEGLER BY o/MWM ATTORNEY United States atent 6 MANUFACTURE OF AROMATIC PARAHYDROXYAMINES Louis Spiegler, Woodbury, N. 5., assignor to E. I. du Pont de Nemours and Company, Wiimington, Del., a corporation of Delaware Application October 22, 1952, Serial No. 316,235

11 Claims. (Cl. 260575) This invention relates to an improved process for the preparation of aromatic parahydroxyamines, and more particularly to their preparation by the catalytic hydrogenation of nitro compounds.

A typical aromatic parahydroxyamine is p-aminophenol, which is a valuable intermediate in the manufacture of dyes and antioxidants. It is commonly made by the reduction of p-nitrophenol obtained by nitrating phenol, or from p-nitrochlorobenzene. It is obvious that a potentially much cheaper method involves the reduction of nitrobenzene to phenylhydroxylamine followed by rearrangement to p-aminophenol without isolation of the intermediate product. Such a process, in which the reduction of nitrobenzene is carried out with zinc dust, is described in U. S. Patent No. 2,132,454. The reduction may also be performed electrolytically. In U. S. Patent No. 2,198,249 there is disclosed a method by which mixtures of p-aminophenol and aniline are produced directly by the catalytic hydrogenation of nitrobenzene in mineral acid solution, using a hydrogen pressure of 200 to 750 p. s. i. Yields of p-aminophenol up to 57% are described in this patent. Although this process may be operated successfully, the reaction does not proceed as rapidly as is desirable and requires the use of high pressure equipment.

It is an object of this invention to provide a process for the preparation of aromatic parahydroxyamines by the catalytic hydrogenation of nitro compounds which proceeds rapidly, at low pressures and with the use of relatively mild agitation, to give higher yields than have heretofore been obtainable. A further object is to provide such a process in which the reaction may be controlled to give the parahydroxyamine and further reduction products in the proportions desired, e. g., in the reduction of nitrobenzene to produce p-aminophenol and aniline in whatever proportions are desired. Further objects will appear from the description which follows.

According to the present invention, an aromatic parahydroxyamine is prepared in admixture with the corresponding dehydroxylated amine by a process which comprises adding a nitro compound in which the nitro group is attached to an aromatic nucleus which is unsubstituted in the position para to the nitro group and hydrogen to a suspension of a hydrogenation catalyst in an aqueous solution containing from 1 to 25% by weight of sulfuric acid, at a temperature of from 50 to 145 C., the partial pressure of hydrogen being maintained below 760 mm. of mercury, and the rate of addition of the nitro compound being such that at no time does the amount of unreacted nitro compound in the suspension exceed its solubility therein.

It has been found that the process is further improved by having present in the acid suspension at least 0.01%

Patented Oct. 2, 1956 by weight, based on the weight of dilute acid solution, of a water-soluble quaternary ammonium compound.

In a representative and preferred embodiment of this process para-aminophenol is made by the hydrogenation of nitrobenzene in the presence of from 0.1 to 3% of a supported platinum catalyst (including the support) and from 0.01 to 0.2% of octadecyl trimethyl ammonium chloride, in each case based on the weight of solution, at a temperature of from to C. and a partial pressure of hydrogen below 500 mm. of mercury, the rate of addition of nitrobenzene being as stated above.

The nitro compounds to which this invention is applicable are mononitro derivatives of aromatic compounds of the class consisting of hydrocarbons containing no more than two benzene rings and monochloro and dichloro derivatives thereof. It is necessary that the position in the aromatic nucleus which is para to the nitro group be unsubstituted, as this is the position to be occupied by the hydroxyl group. In referring to aromatic hydrocarbons containing no more than two benzene rings, it is intended to include compounds containing a single benzene ring, compounds containing two unfused rings as in the compound biphenyl, and compounds containing two fused benzene rings as in naphthalene. Representative of this class of nitro compounds are: nitrobenzene, o-nitrotoluene, o-nitrochlorobenzene, m-nitrochlorobenzene, 2,5 dichloronitrobenzene, 2 nitrobiphenyl and l-nitronaphthalene.

An essential feature of this invention is the combination of the regulated addition of the nitro compound to the reaction vessel and the use of subatmospheric partial pressure of hydrogen. In the process of U. S. 2,198,249 and in hydrogenations generally, the practice is to start with the full charge of the compound to be reduced present in the reaction vessel and to carry out the reaction at high pressure. It has now been found, however, that by adding the nitro compound at such a rate that no appreciable amount of it remains undissolved in the reaction mass, a very considerable increase in the rate of hydrogenation is obtained. The reaction proceeds smoothly and requires only mild agitation, whereas it has been found necessary to use high speed agitation in the process of U. S. 2,198,249. It is believed that a factor contributing to the improvement in the rate and smoothness of the reaction is the prevention of the formation of a separate phase of nitrobenzene in the reaction mixture, which tends to coat the catalyst and to decrease its activity.

Figure 1 in the accompanying drawing shows a graphical comparison of the rate of hydrogenation which is obtained when nitrobenzene is added slowly according to the process of this invention with the rate obtained when all the nitrobenzene is added to the reaction kettle at the start of the reaction. The graph shows that under the conditions of this test reduction of the nitrobenzene is complete in six hours when the rate of addition is such as to avoid appreciable amounts of undissolved nitrobenzene, whereas fourteen hours are required when all the nitrobenzene is present in the reaction kettle from the beginning. The reaction times shown in Figure 1 are illustrative but are strictly accurate only for the particular conditions of temperature, pressure, nature and amount of catalyst, acid concentration and degree of agitation employed in this particular test. Under many conditions, the differences in reaction rates between the two processes are even more striking.

A convenient method of adding the nitro compound 3 to the reaction mixture is by increments, each increment being added after its predecessor is substantially completely reduced. In order to satisfy the requirement that no more unreacted nitro compound be present in the suspension than is soluble therein, each incremental addition should be less than about 0.8% of the weight of dilute acid solution in the suspension.

It is often desirable to add the nitro compound in the form of a concentrated sulfuric acid solution. Commercial 96% acid is satisfactory for this purpose. Instead of adding the nitro compound in increments, satisfactory results are obtained by adding it continuously, provided that the rate of addition is suitably slow. The entire reaction may also be carried out continuously by mixing streams of dilute acid containing the catalyst, nitro compound either alone or in sulfuric acid solution, and hydrogen at the required rates and passing the mixture under suitable agitation through a reaction tube or vessel.

The rate and character of the reaction are strongly influenced by the partial pressure of hydrogen in the reaction vessel. The behavior of nitrobenzene in this regard is representative. In this reaction, it has been found that the ratio of p-aminophenol to aniline in the reaction products increases as the partial pressure of hydrogen decreases. Figure 2 shows this effect graphically. This figure shows that, under the conditions of this test, in order to obtain a yield of p-aminophenol greater than 80% with a corresponding yield of aniline of less than 20%, the hydrogen pressure should be about 250 mm. of mercury or less. If substantially pure p-aminophenol containing only small amounts of aniline is desired as the immediate reaction product, even lower partial pressures of hydrogen may be employed. The graph also shows that as the partial pressure of hydrogen approaches one atmosphere the reaction product becomes predominantly aniline. The actual relation between the amounts of paminophenol and aniline which are produced varies with changes in the other reaction conditions such as temperature, amount of catalyst, and particularly the presence of a quaternary ammonium compound. In any event, operationof the present process at pressures above one atmosphere produces too small amounts of p-aminophenol to be practical.

It will be seen that the superior results obtainable by the process of this invention depend on the combination of the use of low hydrogen pressures and slow addition of the nitro compound. Thus if nitrobenzene is reduced using the slow addition technique but at the superatmospheric pressures of the prior art, the resulting product is almost entirely aniline and various by-products such as ammonia and cyclohexane, practically no p-aminophenol being formed. On the other hand, use of subatmospheric hydrogen pressures without regulated addition of the nitro compound results in undesirably low rates of reduction.

As the partial pressure of hydrogen in the reaction vessel decreases, the over-allrate of reduction of the nitro compound also decreases. This effect is shown graphically in Figure 3, where the rate of hydrogenation is expressed in terms of cubic feet of hydrogen consumed per hour. This efiect must be taken into account in selecting the optimum operating conditions. Instead of using a very low hydrogen partial pressure in order to obtain a high proportion of the parahydroxyamine in the reaction mixture, it is often preferable to use a somewhat higher pressure, thereby increasing the total amount of parahydroxyamine obtained in a given time. The optimum point will represent an economic balance of factors such as the cost of separating the amines from the parahydroxyamines as compared with the cost of the reduction, the availability of equipment, etc.

The partial pressure of hydrogen in the reaction vessel is equal to the total pressure in the vessel less the pressure of water vapor over the acid solution at the existing temperature. It is controlled by varying either the total pressure or the temperature. Partial pressures of 300 to 500 mm. are convenient to maintain when the temperature is between and C., since the reaction vessel may then be kept under a total pressure at or only slightly above atmospheric. Suitable reaction rates and yields are obtained at these temperatures and pressures.

The reaction is carried out at a temperature of from 50 to 145 C. and preferably at 75 to C. Below 50 C. the rate of reaction is impractically slow and above C. other reactions tend to occur. The rate of reaction is increased with increasing temperature, whereas the proportion of p-aminophenol in the reaction product appears to increase up to a maximum and then to decrease. At 500 to 540 mm. of hydrogen the yield of paminophenol from the reduction of nitrobenzene is a" maximum at 115 C., while at 240 to 300 mm. the maxi mum yield is obtained at 100 C. Use of temperatures in the range stated above as preferred results in convenience of operation and in suitably high reaction rates with satisfactory amounts of the desired parahydroxyamine in the reaction product.

The hydrogenation catalyst is preferably platinum supported on charcoal or other porous material. Other suitable hydrogenation catalysts include palladium, rhodium, and the sulfides of certain heavy metals such as molybdenum, cobalt and tungsten. The hydrogenation catalyst should be acid insensitive, i. e., it should not be rendered ineffective by the presence of the sulfuric acid. The concentration of catalyst in the reaction mixture atfects the process in much the same way as does the partial pressure of hydrogen. The rate of hydrogenation increases with increasing catalyst concentration, but there is a substantial accompanying decrease in the proportion of parahydroxyamine in the reaction products and also some increase in the amount of undesired decomposition products. The optimum amount of catalyst depends on the other operating conditions and on the results desired. When the catalyst consists of 1% platinum supported on charcoal, its amount (including the support) is preferably from 0.1 to 3% of the weight of the dilute acid solution.

The catalyst is suspended in a sulfuric acid solution containing from 1 to 25% acid. If the acid is too dilute, incomplete transformation of the intermediate aryl hydroxylamine to parahydroxyamine is obtained. Particularly in the lower part of the operating range of temperature, both over-all reduction rate and yield of parahydroxyamine appear to be favored by acid concentrations of at least 10%. At high temperatures, the acid should be more dilute.

It has been found that quaternary ammonium compounds exert a profound influence on the course of the hydrogenation, increasing the proportion of parahydroxyamine in the reaction product and at the same time increasing the rate of hydrogenation. The effect produced by these compounds is not merely a result of their dispersing action, since the improvement is obtained with quaternary ammonium compounds which are not dispersing agents and is not obtained with other dispersing agents which are not quaternaries.

The quaternary ammonium compound should be watersoluble and also should be stable toward dilute sulfuric acid. into two separate ions (a substituted ammonium cation and an anion such as a halide ion) or an inner salt such as a betaine. The compound preferably contains at least one alkyl group containing at least ten and no more than eighteen carbon atoms, although compounds such as tetramethyl ammonium chloride are also effective. Betaines having a normal alkyl radical of fourteen to sixteen carbon atoms substituted on the methylene carbon.

comprise another preferred group of compounds of this class. Representative quaternary ammonium compounds useful in the process of this invention include octadecyl trimethyl ammonium chloride, octadecyl dimethyl. ethyl ammonium bromide, dioctadecyl dimethyl ammonium It may be either of the kind which is ionizable.

chloride, dipentadecyl dimethyl ammonium chloride, dodecyl trimethyl ammonium chloride, C-cetyl betaine, C-dodecyl betaine and tetramethyl ammonium chloride. At least 0.01%, based on the weight of the dilute acid suspension, of the quaternary ammonium compound should be employed and ordinarily no further advantage is obtained by using more than 0.2%.

The mixture of reaction products which is present in solution at the end of the hydrogenation is separated by any convenient method. Thus the mixture of p-aminophenol and aniline which is obtained from the hydrogenation of nitrobenzene may be separated as described in U. S. Patent No. 2,198,249, by filtering off the catalyst, adding alkali to liberate the amines from their salts, removing the aniline by steam distillation and allowing the p-aminophenol to crystallize on cooling from the aqueous solution, after concentrating and adjusting to pH 6.0 to 6.3 if necessary. Alternatively, the sulfuric acid may be neutralized with lime, the aniline steam-distilled olf, the calcium sulfate removed from the hot solution by filtration and the p-aminophenol recovered as a powder from the aqueous solution by spray drying.

In the illustrative examples which follow, Examples 1 and 2 describe hydrogenations of nitrobenzene carried out at the same temperature but at different hydrogen partial pressures, and show the effect of this variation upon the rate of hydrogenation and the ratio of p-aminophenol to aniline. Example 3 shows the effect of lower temperature and higher partial pressure of hydrogen. Examples 4 and 5 illustrate the effect of decreasing the acid concentration. The efiect of decreasing the amount of catalyst is shown in Example 6. Example 7 describes a preferred set of conditions chosen to give a practical balance between yield and reaction rate. In Example 8 the efiect of various quaternary ammonium compounds and of non-quaternary dispersing agents are compared. The hydrogenation of nitro compounds other than nitrobenzene is shown in Example 9.

Example 1 A 10 gallon glass-lined reaction kettle equipped with a propeller agitator is charged with:

2,250 g. of 96% sulfuric acid 20,500 g. of copper-free water 10.5 g. of C-cetyl betaine 1.75 g. of metallic platinum dispersed on 175 g. of charcoal The kettle is flushed with hydrogen, and then heated to 100 Gil C. while sweeping through a vent with a slow stream of hydrogen. The kettle is then closed, and the pressure is built up to 200 mm. of Hg above the atmospheric pressure with hydrogen gas. Nitrobenzene is then pumped in in 25 gram portions, while hydrogen gas is fed simultaneously as rapidly as reduction occurs. The pressure is maintained at 200 mm.: mm. (gage). Pressure changes on a mercury-filled manometer are observed to determine when to add nitrobenzene and how to adjust the hydrogen flow. As long as the pressure tends to fall, indicating the presence of unreduced nitrobenzene, the i'eed of hydrogen is continued so as to keep the pressure substantially constant. When absorption stops, another increment of nitrobenzene is introduced. The actual average partial pressure of hydrogen in the reaction vessel averages about 240 mm. In this manner, 2500 g. of nitrobenzene are reduced in less than 5 hours giving a rate of 560 g. of nitrobenzene per hour.

The analyzed yield is 83.2% of p-aminophenol, and 16.8% of aniline, based on the nitrobenzene.

Example 2 Nitrobenzene is reacted by the procedure described in Example 1 except that the temperature is held at 98il C. and the pressure is maintained at 0 to 20 mm. (gage) with hydrogen, resulting in an average partial pressure of hydrogen of about mm. in the reaction vessel. Under these conditions, 1250 g. of nitrobenzene are reduced in 9 hours. The yield of p-aminophenol is 98.6% and of aniline 1.3%, in each case based on the amount of nitrobenzene reduced.

Example 3 Nitrobenzene is reacted by the procedure described in Example 1 except that the temperature is held at 65 +1 C. and the pressure is maintained at 0 to 20 mm. (gage) with hydrogen, resulting in an average partial pressure of hydrogen of about 600 mm. in the reaction vessel. Under these conditions, 2500 g. of nitrobenzene are reduced in 6 hours. The yield of p-aminophenol is 27.2%and of aniline 72.4%.

Example 4 The equipment used in the preceding examples is charged with:

19,480 g. of copper-free water 1.75 g. of platinum dispersed on 17 5 g. of activated carbon 1070 g. of 96% sulfuric acid Operating as in Example 1, but at 500 mm. gage pressure and 100 C., resulting in a hydrogen partial pressure of about 540 min., and feeding a solution consisting of 1845 g. of nitrobenzene and 767 g. of 96% sulfuric acid in 25 g. portions, 1845 g. of nitrobenzene are reduced in 4 hours. The analyzed yield in the reduction liquors is 47.3% of p-aminophenol and 52.7% of ani line, based on the nitrobenzene.

Example 5 The equipment used in the preceding examples is charged with:

19,300 g. of water 214 g. of 96% sulfuric acid 1.75 g. of platinum dispersed on g. of activated carbon Operation under the same conditions of temperature, pressure and method of nitrobenzene addition as in Example 4 results in a 22.9% yield of p-aminophenol and 77.1% yield of aniline. The rate of reduction is 420 g. of nitrobenzene per hour.

Example 6 The equipment used in the preceding examples is charged with:

18,450 g. of water 2150 g. of 96% sulfuric acid 0.35 g. of platinum dispersed on 35 g. of activated carbon The temperature is maintained at 100 C. and the pressure at 500 mm. gage, giving a hydrogen partial pressure of about 540 mm. A solution of nitrobenzene in concentrated 96% sulfuric acid containing 2 moles nitrobenzene per mole of sulfuric acid is added in 25 g. portions. The rate of reduction is 330 g. of nitrobenzene per hour, and the yield in the reduction liquors is 65.3% of paminophenol and 34.7% of aniline, based on the nitrobenzene.

Operating as above but with 0.17 g. of platinum, the rate of reduction is 193 g. of nitrobenzene per hour, with yields of 70% of p-aminophenol and 30% of aniline.

Example 7 Nitrobenzene is hydrogenated in a 5 liter creased flask having four vertical creases to act as baffles and equipped with a stirrer having a vertical 5.5 inch blade, a circular lower edge and a maximum height of 1.5 inches. The stirrer is operated at 700 to 800 R. P. M. The flask is provided with a heating jacket, inlet and outlet tubes for hydrogen, a manometer and an inlet for the solution of nitro compound. Into the flask are placed 1000 g. of water, 66.2 g. of 96% sulfuric acid, 0.75 g. of C- cetyl betaine and 0.016 g. of a platinum catalyst supported on 1.6 g. of charcoal. A mixture of 68.2 g. of nitrobenzene and 28.2 g. of 96% sulfuric acid is added to the flask at a rate of between 0.5 and 0.6 cc. per minute per liter of solution, while at the same time hydrogen is added at such rate that the pressure in the flask remains essentially constant. The temperature is maintained at 8788 C. and the total pressure at 760 mm., the partial pressure of hydrogen being about 300 mm. The sulfuric acid added with the nitrobenzene is equivalent to the basic reduction products formed from the nitrobenzene, so that addition of the mixture to the flask produces no change in the overall acidity. Hydrogen is absorbed at a rate of between 0.70 and 0.42 cu. ft. per hour. The nitrobenzene is added in about 120 minutes. The solution then contains 52.4 g. of p-aminophenol and 7.0 g. of aniline, equivalent to yields of 87% and 13% respectively.

Example 8 Nitrobenzene is hydrogenated as in Example 7, using the quaternary amonium compounds and dispersing agents shown below. The reaction temperature in each case is 78 C. and the total pressure one atmosphere, giving a partial pressure of hydrogen of 460 mm. The amount of platinum catalyst used is 0.02 g., supported on 2 g. of charcoal. Between 0.5 and 0.7 g. of the quaternary or other agent is added. The rate of reduction shown below is expressed as cc. of nitrobenzene hydrogenated per minute per liter of charge. The results of these reductions are as follows:

Rate of Yield of Agent reduction p-aminophenol Percent None (control) 0. 525 55 Quaternary compounds:

O-eetyl betaine 0.830 78 O-dodecyl betaine 0.672 73 Dodecyl triruethyl ammonium chloride- 0. 837 77 Octadecyl trimethyl ammonium chloride.. 1. 005 80 Octadecyl dimethyl ethyl ammonium bromide 0. 570 80 Dioctadecyl dimethyl ammonium chloride 0. 500 80 Dipentadecyl dimethyl ammonium chlori e 0. 564 80 Tetramethyl ammonium chloride 0. 084 64 Non-quaternary compounds:

Triethylamine sulfate 0. 483 57 Tributylamine sulfate 0. 472 58 Dioctadscyl propyleneamine dioleate 0. 404 68 Ethylcne-diamine tetraacetic acid 0. 70 55 Condensation product from ethylene oxide and oleic alcohol 040 71 Example 9 Each of the nitro compounds shown in the table below is hydrogenated by a procedure similar to that described in the two preceding examples. Octadecyl trimethyl ammonium chloride is used in each case. The nitro compound, fused when necessary, is added without sulfuric acid (other than that present initially in the flask) except where noted. The hydrogenation of o-nitrotoluene is. carried out at 78 C. and 460 mm. partial pressure.

of hydrogen, while all of the other compounds listed are hydrogenated at C. and 300 mm.

Catalyst Nitro compound cone, g. Products Formed Yield,

Pt/1,000 cc. Percent solution 3-methyl-4-amino- 80. 1 o-nitrotoluene 0. 0215 phenol.

' o-toluidine 19. 9 l-nitronaph thalene 0. 0215 {iggfigggg%g figtfi" g 3-chloro-4-amino- 81. 4 o-nitrochlorobenzenc 0. 0200 phenol.

o-chloroaniline 18. 1 3-chloro-4-amino- 86. 4 0. 0100 phenol.

o-chloroaniline l3. 6 IllPbkCillOfOXllttO- 2-chloro-4-ami11o- 32. 0

benzene 0. 0200 phenol.

m-chloroaniline 68. 0 2,5-dlchloro-4- 26. 0 2,5-diohloronitroaminophenol.

benzene 0. 0200 o-and m-chlorol9. 2

1 aniline.

2,5-dichloroaniline 54. 8 2-aInil1o-5-hydroxy- 73. 2 2-nitrobiph enyl 0. 0200 biphenyl.

2-an1inobipheny1 26. 8

1 Added as a 33% solution in 96% H S 0 In Examples 1-6 above, including those in which no dispersing agent is present, there is used only a moderate amount of agitation, furnished by a propeller stirrer at 300 R. P. M., while in Examples 7-9 a flat bladed stirrer operating at 700800 R. P. M. is employed. For larger quantities in larger equipment, the requirements are still easily met by the type of agitation ordinarily available. On the other hand, the hydrogenation described in U. S. 2,198,249 requires speeds of 2500 R. P. M. and peripheral velocities of 1500 ft; per minute for best results. By the process of the present invention, the ratio of paminophenol to aniline may be widely varied at will and may be increased until p-aminophenol is almost the only product. Compared again with the process of, U. S. 2,198,249, the process of the present invention operates faster, at lower temperatures and pressures and with smaller amounts of catalyst, and produces substantially 100% combined yield of p-aminophenol and aniline. For these reasons, it lends itself readily to continuous operation as described above.

This application is a continuation-in-part of my U. S. application Serial No. 259,988, filed December 5, 1951.

I claim:

1. A process of preparing an aromatic parahydroxyamine by the reduction of a nitro compound which is a mononitro derivative of an aromatic compound of the class consisting of hydrocarbons free from non-aromatic hydrocarbon substituents other than lower alkyl radicals and containing no more than two benzene rings and monochloro and dichloro derivatives thereof, said nitro compound having its nitro group attached to an aromatic nucleus which is unsubstituted in the position para to the nitro group, which comprises adding the said nitro compound and hydrogen to a suspension of an acid-insensitive hydrogenation catalyst in an aqueous solution containing from 1 to 25% by Weight of sulfuric acid, at a temperature of from 50 to C., the partial pressure of hydrogen being maintained below 760 mm. of mercury and the rate of addition of the nitro compound being such that at no time does the amount of unreacted nitro compound in the suspension exceed its solubility therein.

2. A process according to claim 1 in which the acid suspension contains from 0.01% to 0.2% by weight, based on the weight of dilute acid solution, of a water-soluble quaternary ammonium compound of the group consisting of tetraalkyl ammonium halides containing at least 1 and not more than 2 alkyl groups of at least 10 carbon atoms and not more than 18 carbon atoms, and betaines containing a C-normal-alkyl group of from 12 to 16 carbon atoms.

3. A, process of preparing para-aminophe'nol which comprises adding nitrobenzene and hydrogen to a suspension of an acid-insensitive hydrogenation catalyst in an aqueous solution containing from 1 to 25% by weight of sulfuric acid, at a temperature of from 50 to 145 C., the partial pressure of hydrogen being maintained below 760 mm. of mercury and the rate of addition of nitrobenzene being such that at no time does the amount of unreacted nitrobenzene in the suspension exceed its solubility therein.

4. A process according to claim 3 in which the acid suspension contains from 0.01% to 0.2% by weight, based on the Weight of dilute acid solution, of a water-soluble quaternary ammonium compound of the group consisting of tetraalkyl ammonium halides containing at least 1 and not more than 2 alkyl groups of at least carbon atoms and not more than 18 carbon atoms, and betaines containing a C-normal-alkyl group of from 12 to 16 carbon atoms.

5. A process according to claim 4 in which the quaternary ammonium compound is octadecyl trimethyl ammonium chloride.

6. A process according to claim 4 in which the quaternary ammonium compound is a betaine having a normal alkyl radical of fourteen to sixteen carbon atoms substituted on its methylene carbon.

7. A process according to claim 4 in which the quaternary ammonium compound is C-cetyl betaine.

8. A process according to claim 3 in which the nitrobenzene is added in the form of a concentrated sulfuric acid solution.

9. A process according to claim 3 in which the hydrogenation catalyst is a supported platinum catalyst.

10. A process according to claim 3 in which the nitrobenzene is added incrementally, each increment being less than 0.8% of the weight of dilute acid solution in the suspension and being added after the preceding increment is substantially completely reduced.

11. A process of preparing para-aminophenol which comprises adding nitrobenzene and hydrogen to a suspension of a supported platinum catalyst in an aqueous solution containing from 1 to 25% by Weight of sulfuric acid and from 0.01 to 0.2% by weight of octadecyl trimethyl ammonium chloride, at a temperature of from to C., the amount of catalyst (including its support) being from 0.1 to 3% of the weight of solution, the partial pressure of hydrogen being maintained below 500 mm. of mercury and the rate of addition of nitrobenzene being such that at no time does the amount of unreacted nitrobenzene in the suspension exceed its solubility therein.

References Cited in the file of this patent UNITED STATES PATENTS 2,198,249 Henke et a1 Apr. 23, 1940 2,292,879 Kise Aug. 11, 1942 2,587,572 Tyron Feb. 26, 1952 2,619,503 Benner et al Nov. 25, 1952 FOREIGN PATENTS 559,730 France Sept. 30, 1923

Claims

1. A PROCESS OF PREPARING AN AROMATIC PARAHYDROXYAMINE BY THE REDUCTION OF A NITRO COMPOUND WHICH IS A MONONITRO DERIVATIVE OF AN AROMATIC COMPOUND OF THE CLASS CONSISTING OF HYDROCARBONS FREE FROM NON-AROMATIC HYDROCARBON SUBSTITUENTS OTHER THAN LOWER ALKYL RADICALS AND CONTAINING NO MORE THAN TWO BENZENE RINGS AND MONOCHLORO AND DICHLORO DERIVATIVES THEREOF, SAID NITRO COMPOUND HAVING ITS NITRO GROUP ATTACHED TO AN AROMATIC NUCLEUS WHICH UNSUBSTITUTED IN THE POSITION PARA TO THE NITRO GROUP, WHICH COMPRISES ADDING THE SAID NITRO COMPOUND AND HYDROGEN TO A SUSPENSION OF AN ACID-INSENSITIVE HYDROGENATION CATALYST IN AN AQUEOUS SOLUTION CONTAINING FROM 1 TO 25% BY WEIGHT OF SULFURIC ACID, AT A TEMPERATURE OF FROM 50 TO 145* C., THE PARTIAL PRESSURE OF HYDROGEN BEING MAINTAINED BELOW 760 MM. OF MERCURY AND THE RATE OF ADDITION OF THE NITRO COMPOUND BEING SUCH THAT AT NO TIME DOES THE AMOUNT OF UNREACTED NITRO COMPOUND IN THE SUSPENSION EXCEED ITS SOLUBILITY THEREIN.