Catalytic Preparation of Aniline - The Journal of Physical Chemistry

CATALYTIC PREPARATION O F ANILINE BY 0 . W , BROWN AND C. 0. HENKE

Introduction Sabatierl found that nitrobenzene was reduced by hydrogen in the presence of active nickel, yielding aniline, ammonia, benzene, cyclohexane, cyclohexylamine, dicyclohexylamine, cyclohexylaniline, and a little diphenylamine. With a more active nickel and at a higher temperature, the nitrobenzene was reduced even to ammonia and methane, while with a less active nickel only aniline was produced. He found copper to be a better catalyst than nickel for the production of aniline because its action did not extend to the aromatic ring, I n this paper we shall give the results of some studies of this reduction of nitrobenzene to aniline in the presence of nickel and copper catalysts. The object in view was to study quantitatively the behavior of these two catalysts and to determine the various factors affecting their activity, as measured by the yield of aniline secured.

Apparatus I n order to make these studies an apparatus was required which would permit: (1) an accurate method of measuring the temperature of the gaseous mixture while in contact with the catalyst; (2) an even flow of nitrobenzene which could be easily regulated and accurately known; (3) an even flow of hydrogen which could be easily regulated independently of the nitrobenzene; (4) a means of quantitatively receiving the product, so that it could be quantitatively determined. After many attempts an apparatus was finally devised which fulfilled all requirements, and since it has proved very satisfactory a full description follows. The apparatus is shown diagrammatically (to scale) in Fig. 1. A is a flowmeter used to measure the rate a t which the hydrogen was passed into the furnace. This was cali1

Sabatier: “La Catalyse,” 1st Ed., pp. 60, 91 and 107 (1913).

4

.

162

0. W. Brown and C. 0. Henke

Fig. 1

brated against an electrolytic hydrogen generator, the quantity of hydrogen liberated being calculated from the current used. The nitrobenzene is introduced a t A (after disconnecting the rubber tube) by means of a calibrated pipette. C consists of a glass tube, about 7 or 10 mm in diameter, sealed to a capillary tube T which is then bent as shown in the diagram. The capillary of a thermometer serves very well, especially if a thermometer with an unusually fine capillary is selected. This capillary is so fine that pressure is required t o force the nitrobenzene through it a t a fast enough rate. This pressure is secured by raising the mercury bulb shown a t B, the difference in the level of the mercury in the two bulbs, being a measure of the pressure on the nitrobenzene, and hence a measure of the rate of flow of the nitrobenzene. About 35 minutes are required for 2 cc of nitrobenzene to flow through the capillary when the pressure is 3.5 to 14 cm of mercury, the exact pressure depending on the size of the capillary. The rate of flow of nitrobenzene is quite constant since the amount of pressure due to the nitrobenzene is only 0.08 to 0.02 of that due to the mercury which is nearly constant. So that although the column of nitrobenzene decreases to nothing, the pressure remains nearly constant as it is practically all due to the mercury pressure which is changed but slightly. It was found that with a certain pressure of mercury the time required for 2 cc of nitrobenzene to flow through a capillary

Catalytic Preparation of Aniline

163

was very nearly constant, varying 2 minutes from the mean only occasionally, while usually the time was the same within a minute. Sometimes a small particle of dirt would get into C and then the capillary would need to be cleaned. Heating Element The heating element of the furnace was made in the following way. A 2 inch iron pipe, 29 inches long was first covered with a layer of asbestos paper which was cemented t o the pipe with sodium silicate solution. This was then covered with a layer of alundum cement. About 30 or 35 feet of chrome1 wire, size 16 B and S gauge, was then wound on top of this layer of alundum cement. In starting the winding the end of the wire is doubled for about 2 feet. This doubled end is brought around one end of the pipe and twisted tightly on the pipe, the end sticking out 6 to 10 inches for electrical connection. This first round of doubled wire does not do any of the heating but holds the wire in place and the winding is continued with the main strand of the wire, the rounds being about 3/4 inch apart. When the other end of the pipe is reached the wire is again doubled for about 2 feet and instead of going around the pipe in the same direction as before, for the last round the wire is bent back and brought around the pipe in the opposite direction. The bending back of the wire for the last round forms a loop and after.bringing the wire around the pipe it is brought through this loop, bent back and twisted tightly with one side of the loop, the'end again sticking out 6 to 10 inches for electrical connection. After winding on the wire another layer of alundum cement is applied so that the wire is entirely embedded in alundum cement which keeps i t from burning in two. Several layers of asbestos paper are then wrapped around the furnace for heat insulation. Catalyst Tube The tube containing the catalyst was a half inch wrought iron pipe 33 inches long and was held in the center of the heating jacket by the plugs of alundum cement E, E at each end. This provides an air space between the catalyst tube

164

0.W . Brow% and C. 0.Hevlke

and the heating element which leads to a more even temperature of the catalyst tube and the catalyst and prevents any spot becoming overheated. The capillary T sticks into the furnace 2 to 3 inches. When an experiment is carried out at a low temperature it should extend farther into the furnace than when a higher temperature is used. If it extends too far into the furnace a t the high temperature the nitrobenzene will char in the capillary and finally clog it. The capillary tube T and the hydrogen are led into the catalyst tube through a 1/4 inch iron pipe which extends about 8 inches into the furnace. The connection between the capillary tube and the quarter inch iron pipe a t H is made air tight by a short piece of rubber tube as is shown in the figure. To keep this rubber tube cool a piece of cloth is tied around the quarter inch iron pipe a t I, the ends of which dip into a beaker of water. Due to capillary action the cloth is kept wet and is a means of cooling the iron pipe around which it is wrapped. The quarter inch iron pipe (on the other end of the T) passes through the cap (which is screwed onto the catalyst tube) and is brazed to it a t D, in order to make it hydrogen tight. The column of catalyst was 10 inches long and was placed in the tube 3 inches from the end of the heating jacket. The catalyst was put a t this place by means of a strip of tin which was bent in the form of a long narrow trough. Thus a definite amount of catalyst, prepared in a certain way, was placed in a certain part of the tube. This left a space in the tube about 12 inches long where the nitrobenzene and hydrogen became thoroughly mixed and heated to the temperature of the catalyst before reaching the catalyst. Temperature Measurement The temperature was measured by a copper-constantan thermocouple, the voltage being read by a high resistance milli-voltmeter. The thermocouple was made by wrapping copper and constantan (advance) wire (gauge 16) separately with asbestos cord, after which they were wrapped together with the asbestos cord, and then the ends twisted and welded.

Catalytic Preparatiovl of Avliline

165

This was calibrated in position against tin, lead and zinc. The calibration curve is practically a straight line and does not change appreciably with use. The couple as made is flexible and may be bent. It is put in place through a hole drilled through the catalyst tube a t G. In order to make the tube air tight a t G it is necessary to put sodium silicate solution of 1.400 sp. gr. between the rounds of the asbestos cord and then a lute between the couple and the pipe. A lute consisting of 30 grams alundum cement, 15 grams ground pumice and 21 cc of a solution of sodium silicate of 1.200 sp. gr. was found to be very satisfactory by the writers. The lute must not be porous and at the same time it must not set so hard that it is difficult to break loose, since the thermocouple must be taken out every time a new catalyst is put in the furnace. As is shown in the diagram the thermocouple extends up the tube to the catalyst, so that the temperature measured is the temperature of the gaseous mixture as it leaves the column of catalyst. This temperature is quite different from that on the outside of the tube in the heating jacket. The temperature inside the catalyst tube was in all experiments kept within 3 O of the desired temperature. The condenser fits on a cork over the catalyst tube. In putting the cork on the catalyst tube, it is best to use a sodium silicate solution as an adhesive between the cork and the pipe in order to make it air tight. Materials Used The nitrobenzene used was purified by shaking it with sodium carbonate solution and then distilling it with steam. The distillate was dried with calcium chloride and then redistilled twice. The hydrogen used was commercial hydrogen put up in steel cylinders. This was purified by passing over red hot scrap copper, through concentrated sulphuric acid and then over stick caustic. Estimation of Aniline Produced The product (which had not already run into the flask) was washed from the condenser into the flask, and, after

166

0. W . Brown and C. 0. Henke

diluting to the mark, a 100 cc portion pipetted out, 25 cc concentrated HC1 added, and titrated with tenth molar sodium nitrite, starch iodide paper being used as an external indicator. The reaction involved in the titration is a diazotization. This reaction is rapid a t first but as the end point is approached it becomes slow. So that the end point is not reached until the solution will turn the starch iodide paper blue after standing several minutes. Any other amine besides aniline will give the same reaction. However the amount of other amines was not large (except in a few instances) so that this error is not great. The nitrite solution was standardized by titrating with a permanganate solution which had been acidified with sulphuric acid and heated to about 60 '.

Method of Procedure Before starting an experiment the furnace was heated to the proper temperature in a current of hydrogen (the current of hydrogen was continued during the entire time that the furnace was heatink or cooling). After securing the proper temperature and adjusting the flow of hydrogen, 2 cc of nitrobenzene was put into C from a calibrated pipette. The rubber tube was then put on C again and the mercury bulb raised to the desired height, the time being noted. During the experiment the temperature was kept within 3 degrees of the desired temperature. When the last of the nitrobenzene flowed through the capillary the time was again noted and from the time required for it t o flow through the capillary the rate of flow in grams per hour was calculated. This latter only occasionally varied as much as 6 yo. The pipette, under the conditions under which it was used, delivered 2.322 grams of nitrobenzene. After the last of the nitrobenzene had passed into the furnace the current of hydrogen was continued at least 20 minutes before disconnecting the condenser from the catalyst tube. This allowed all of the products of the reaction to be washed out of the furnace into the condenser.

Catalytic Preparation of Analint!

167

Preparation of Nickel Catalyst The nickel catalyst was prepared by igniting the nitrate and then reducing the oxide in hydrogen. The nickel nitrate used contained: 0.081 % Fe, 0.386 % Co, 0.003 yo SOa, 0.001 yo C1 and a trace of copper. The nitrate, with the addition of nitric acid, was ignited in a small porcelain evaporating dish in an electrically heated muffle. The temperature of ignition was measured by a copper-constantan thermocouple, the junction being in the dish just above the nickel oxide. The nickel oxide, after cooling, was powdered and put in the furnace as previously described. About 16 grams of the oxide were used each time. Experimental Results with Nickel Various investigators heretofore have paid but little attention to the temperature of ignition of the nitrate. Usually the temperature is reported as a “low red heat” or a “dull red temperature” which are rather indefinite and may admit of a variation of 100 degrees or even more. The effect of the temperature of ignition is given in Table I. The nickel oxide was not kept a t the indicated temperature for any length of time but was merely heated to that temperature and then allowed to cool. TABLE I Temperature of catalyst-192 ’. Rate of flow of hy-drogen--17 liters per hour. Rate of flow of nitrobenzene-3.9 grams per hour. Excess of hydrogen-710%.

-

- -_

Catalyst

Temperature of ignition of nitrate C

Material yield in % of theory

A5 AB A9 A8

353 412

84 87 87 08

475 535

168

\

0.

IV. Brown

and C.

0.Henke

experiments were required and the average of two or three is given in the table. From the table it will readily be seen that the temperature of ignition is very important and that ignition a t a dull red temperature means very little. A difference of 60" in temperature of ignitions means a difference of 19% in material yield. With different experimenters not accustomed to judging the temperature of a furnace from its color a dull red temperature may admit of a variation of 100". I n the first three of the above experiments the product was colorless, the material unaccounted for probably being reduced still further, I for Sabatier has shown that nickel is an excellent catalyst for taking nitrobenzene to cyclohexane and ammonia or even to methane and ammonia. In studying the effect of heating the reduced nickel in hydrogen, 450" was chosen as the best temperature of ignition of the nitrate. Hydrogen was passed through the tube containing the nickel oxide as it was heated so that reduction did not take place a t any particular temperature. The reduced nickel however was always heated to a definite temperature in the stream of hydrogen before any experiments were made. It seems to the writers that the highest temperature to which the catalyst was heated is the important factor and not the temperature a t which the reduction actually took place. The effect of heating the nickel in hydrogen to various temperatures is given in Table 11. In these ten experiments, each time after heating to the indicated temperature and cooling to the correct temperature experiments were made until constant results were obtained. The average of the constant results is given iv the table. The catalyst was not kept a t the indicated temperature for any length of time but was merely heated to that temperature and then allowed to cool. The results of Table I1 are plotted graphically in Curves A and B of Fig. 2. From the graph it is apparent that the activity of the catalyst is affected tremendously by the temperature to which it has been heated in hydrogen. Thus

Catalytic Preparation of Aniline .

After heating cat:lyst hydrogen t o C

335 390 435 475 322" 352 383 444 475

in


69.0 84.0 90.8 90.9 88.0 91 .o 92.7 89.9 69.8

169

170

0.W . Brown and C. 0.Henke

gives a low yield of aniline because it carries the reduction on farther than the aniline stage. Less active nickel prepared by heating to about 440" gives a high yield of aniline, while nickel still less active, prepared by heating to 535" gives a very low yield of aniline, the nitrobenzene being only partly reduced. The importance of the history of the catalyst will also be noted by comparing Curves A and B. The experiments represented by the two curves were carried out under the same conditions except that the catalyst had been oxidized after making the experiments shown in A and before making those shown in Curve B. Point d of Curve A is lower than point e of Curve B. The activity of the catalyst represented by point d was greater than that represented by point e , for in the experiment represented by d the low yield is due to the fact that the nickel carried the reduction farther than the aniline stage and is not due to incomplete reduction of the nitrobenzene. However the low yield of the experiment represented at f is due to incomplete reduction of the nitrobenzene and since f is lower than g, the activity of the catalyst in the experiment a t g was greater than in the experiment represented by f. This would indicate that the oxidation and reduction of the nickel catalyst decreased its activity since the activity of the catalyst in the experiments represented by the Curve A is greater than in those represented by Curve B. In using Table I1 it must be remembered that all the experiments were carried out a t the same temperature, the difference being that the nickel catalyst was heated in hydrogen to the indicated temperatures before use. The catalyst used was the same in all experiments. After heating to the indicated temperature it was cooled to 192" in each case, and the experiment carried out at this temperature. In the experiments of Table I1 results were constant al. most immediately except after heating to 475" and 535" The experiments, after heating to these two temperatures are given, in the order in which they were made in Table I11

171

Catalytic Preparation of Aniline TABLE I11 After heating nickel catalyst of Table I1 in hydrogen to 475". All conditions the same as in Table 11.

I

Experiment number

Material yield in yo of theory

91.6 87.1 70.9 75.7 74.5 69.8 84.9 69.5

102A4 103A4 104A4 105A4 106A4" 107A4 108A4 109A4 100

90

80

io 60 50

40 '$0

z

4

6

8

.Fig. 3

A. After heating Ni t o 475'

B. After heating Ni t o 535" C. After heating Cu t o 535'

' 1

1

Experiment number


110A4 111A4* * 112A4. 113A4 114A4 135A4* 116A4 117h4

69.8 S7.1 61.6 48.6 38.2 46.4 32.8 22.1

The results of Table I11 are shown graphically in Fig. 3. The abscissa gives the order in which the experiments were carried out while the ordinate gives the percentage yield. Experiments 102A4 to llOA4 inclusive are plotted in Curve A and 111A4 to 117A4 inclusive in Curve B. The activity of the catalyst as measured by the material yield of aniline produced decreased continuously with use and finally became constant giving a material yield of 69.8% aniline. It will be IO noted that experiments 106A4 and 115A4 are higher than the preceding experiments would indicate. Both of these were the first experiments made in

* The first experiment after the catalyst had been idle overnight in an atmosphere of hydrogen. ** Prior to experiment l l l A 4 the catalyst was heated in hydrogen to 535'.

172

.O.

W.Brown and C. 0.Henke

the morning after the catalyst had been idle overnight in an atmosphere of hydrogen. Thus the catalyst seemed to recuperate when idle. Also experiment 110A4 gave a material yield of 69.8%, then after heating in hydrogen to 535", experiment l l l A 4 , the first experiment after heating to 535", gave a material yield of 87.1%, which would indicate thatt he catalyst is activated by merely heating in hydrogen. Onc omparing the curves of Fig. 3 it will be seen that the efficiency of the catalyst drops much more rapidly after heating to 535 " than when heated to 475". Also the slope of ab is practically the same as that of cd which would mean that the activity of the catalyst decreased linearly. The same thing holds for Curve B. The experiment represented by point a was the first experiment in the morning. The same thing is indicated by the other curve, the experiment represented by the point e being the first experiment in the morning. After experiment 117A4 this same catalyst was oxidized by heating in a current of air to 465". It was then reduced and heated in hydrogen to 260". Experiments carried out under the same conditions as 117A4 then gave 90% material yields of aniline. This indicates that oxidation restored the activity of the catalyst. The catalyst used to determine the best temperature for carrying out the reduction with nickel as catalyst was prepared by ignition of the nitrate a t 414"; this oxide was reduced at 322", used in a few experiments and then heated in hydrogen to 400". The results of experiments a t different temperatures are given in Table IV. It will be seen from Table IV that the best temperature for the reduction with nickel as catalyst is about 192". This is the temperature at which the effect of the rate of flow of the hydrogen was studied. The catalyst used for this was prepared by igniting the nitrate a t 475", heating the nickel, after reduction, to 352", using in a few experiments and then heating to 400". The results are given in Table V. From the table it will be seen that one may pass too large a current of hydrogen as well as too little hydrogen. At 69


173

Temperatye of catalyst, C


243 225 208 192 172 156

87.0 91.9 94.9 95.2 94.3 88.3

TABLE V

'.

Temperature of catal! it-192 Rate of flow of nitrc ienzene-3.8 Hydrogen per hour in liters

8.5 28 46 69 92

grams pt

Excess of hydrogen in

*

hour. Material yield in % of theory

yo

300 1000 2100 3290 4380

87.6 94.3 95.8 91.4 85.1

and 92 liters of hydrogen the product had a slight yellowish color while in all the other experiments the product was colorless. Thus at these rapid rates of hydrogen the time of contact of the nitrobenzene with the catalyst was evidently too short. At the lowest rate considerable of the nitrobenzene was probably reduced farther than aniline, for the product was practically colorless. A study of the effect of the rate of flow of nitrobenzene was then made. The same catalyst was used as in Table V. The results are given in Table VI. TABLE VI Temperature of catal st-192 O . Rate of flow of hydrc ,en--17 liters per hour. Nitrobenzene per hour in grams

11.6 5.34 3.1

Excess of hydrogen in %

170 490 900

I


.

94.0 94.8 95.8


174

Catalyst

B5 B9 B8 B11

Temperature of ignition of nitrate, ' C


353 414 472 535

95.8 97.5 90.7 90.2

Catalytic Pre~arationof Aniline

After heating catflyst in hydrogen to C

253 407 475 535

-


93.8 93.2 92.2 32.8

175

176

0.W . Brown and C. 0. Henke

reduced a t 250 ", used in several experiments and then oxidized by heating in a current of air to 475", after which it was reduced and the experiments indicated in the table carried out. The results of Table VI11 are plotted in Curve C of Fig. 2. The activity of the catalyst is not affected greatly until one gets above 475" when it is decreased tremendously. The difference between copper and nickel is again quite evident in this figure. The curve for copper does not have the first rise which the other two curves have, since the active copper does not attack the aromatic ring while the active nickel does. Also the activity of the copper is not killed so easily as that of the nickel. After repeatedly oxidizing and reducing the copper catalyst it gave 80% yields of aniline again. The oxidation did not bring its activity back as high as it was before heating to 535" in hydrogen while in the case of nickel 90% yields were obtained after oxidizing and reducing once. This indicates that the activity of the nickel catalyst is more easily restored than that of the copper catalyst. After heating the nickel catalyst in hydrogen to 535" the first experiment gave a high yield but this decreased after a few experiments to 22y0. With copper no high yields were secured and the drop in activity was almost immediate as is shown in Table IX. TABLE IX After heating copper catalyst of Table VI11 in hydrogen to 536'. Conditions same as in Table VI11 ~

Experiment number

93B4 94B4 95B4

~

~

Material yield in

51.2

33.4 32.3

70of

theory

177


immediately but sometimes as many as five or six experiments were necessary to bring it up to its full activity. This fact is brought out by the data in Table X. The experiments recorded in Table X are representative of this unexpected behavior which was observed many times. The catalyst used in Table X was prepared by ignition of the nitrate a t 414". The oxide was then reduced and heated in hydrogen to 314". TABLE X Temperature of catalyst-253 '. Rate of flow of hydrogen-17 liters per hour. Rate of flow of nitrobenzene-3.9 grams per hour. Excess of hydrogen--710%. Experiment number

198B9 199B9 200B9 201B9 202B9 220B9* 221B9 222B9 223B9


Experiment number


78.8 93.6 94.0 97.7 97.4 71.4 84.3 84.9 90.5

224B9 230B9** 231B9 2.32B9 233B9 235B9 * * * 23GB9 237B9

94.0 58.9 89.3 89.0 96.2 44.8 52.3 53.6

The results of Table X are shown graphically in the four curves of Fig. 5 . All four curves show a low first yield and a subsequent increase with use. If Curves C and B had been carried out one experiment farther it is probable that A, B and C would have reached about the same figure of 97y0. Although Curve D may have reached this same figure, not enough experiments were carried out to prove that it would. In each of the four curves the second experiment is consider* Prior to Experiment 220B9 the catalyst was oxidized by heating in a current of air t o 414', after which it was reduced and heated in hydrogen t o 314'. ** Prior to Experiment 230B9 the catalyst was oxidized by heating in a current of air to 414", after which it was reduced and heated t o 283'. *** Prior to Experiment 235B9 the catalyst was oxidized by heating in a current of air to 414".after which it was reduced and kept in a current of hydrogen, a t a temperature of 253' for 8 hours before use.

178


ably higher than the first one, while the third is nearly the l!A same as the second experi90 ment, after which it again increases. Curves A and B s 80 show a higher yield in the -z first experiment than do 5 Curves C and D. In the case W 70 P of Curves A and B the catalyst -I was heated in hydrogen to LU 60 314 O before use while in C and !? D the maximum temperature r was 253". It was thought a t first that probably the catalyst ORbfR OF fXPERlMfI1TS was not completely reduced in 2 3 4 5 A, B and C and so in D the Fig. 5 catalyst was heated in hydrogen for 8 hours a t 253 O before making the first experiment, while the others had been heated in hydrogen only about an hour. However Curve D shows lower results than the other three which proves conclusively that the low first yield was not due to incomplete reduction of the catalyst. In fact the three experiments of Curve D indicate that a longer time is necessary for the catalyst to attain its full activity when reduced in hydrogen for a long period than when reduced for a short period. On the adsorption theory of contact catalysis one might explain this by assuming that in order for the copper to act as a catalyst it must adsorb both the nitrobenzene and the hydrogen. Now if the copper has been kept in hydrogen for a long time it has covered its entire surface with a layer of adsorbed hydrogen. Then when the nitrobenzene vapors pass over it they do not come in contact with the copper but with this adsorbed layer of hydrogen. Consequently the nitrobenzene is not adsorbed by the copper and a low yield of aniline results. But some of the adsorbed hydrogen reduces some nitrobenzene and in these places little nuclei of 100

2

50u-..

179


adsorbed nitrobenzene are formed which gradually increase and result in a gradual increase in yield of aniline produced. This increase in activity was not observed with nickel except occasionally and then the increase was only slight. Also it did not increase after the second experiment. On the contrary heating in hydrogen seemed t o be beneficial to the nickel catalyst, as was brought out in the discussion under Table 111. With nickel the first experiment in the morning after lying idle in an atmosphere of hydrogen overnight was higher than the previous or succeeding experiments would indicate. On the other hand with copper it was observed many times that the first experiment in the morning was lower than the previous or succeeding experiments would indicate, the second experiment under duplicate conditions being 2 t o 4% higher than the first. When a copper catalyst is prepared under correct conditions it retains its activity for a long time. However if the nitrate is ignited at too low a temperature it loses its activity in a short tibme. This is shown in Table XI. The catalyst for these experiments was prepared by igniting the nitrate at 353" and reducing the oxide and heating t o 314" in hydrogen. TABLE XI Temperature of catalyst-253 '. Rate of flow of hydrogen-17 liters per hour. Rate of flow of nitrobenzene-3.7 grams per hour. Excess of hydrogen-750%. Experiment number


Experiment number


lllB5 112B5 113R5 114B5 115B5 llBB5* 117B5 118B5

95.0 9G.0 95.7 89.0 89.3 92.2 91.2 91.2

119B5 120B5 121B5 122B5 123B5 124B5 125B5

86.5 88.1 84.0 66.2 70.3 79.1 70.0

-

-

* This experiment was carried out with a rate of flow of hydrogen of 5.7 liters per hour and hence is not comparable t o the others.

180


The results of Table XI are plotted in Fig. 6. The full line shows the general tendency while the dotted line goes through all the points. Although the results are irregular they unmistakably point to a decrease in activity, which is evidently due to ignition a t too low a temperature for other catalysts ignited a t higher temperatures and used under the same conditions did not show this decrease in activity. The '5 irregularity of the results is Fig. 6 not experimental error but shows that the catalyst is on the border line between a good and a bad catalyst. Results are also irregular when using a copper catalyst that has been ignited a t 475 O or 535 O ; however, when using a copper catalyst that was ignited at 414", reduced and heated in hydrogen to 314" and used at about 260" results are not irregular. Since the data in Table X show that a copper catalyst does not reach its full activity until it has been used in five or six experiments and since this catalyst decreases in activity continuously one is led to believe that if this catalyst had not decreased in activity the highest yield would have been more than 96y0,probably near 1 0 0 ~ o . Then the point c of Curve B in Fig. 4 would have been higher than point d of the same curve. Then this curve would show a continual decrease in activity with increase in temperature of reduction. . The catalyst used to determine the best temperature for carrying out the reduction with copper as catalyst was prepared by ignition of the nitrate at 414". The resulting oxide was reduced and heated in hydrogen to 314") used in several experiments and oxidized and reduced twice a t the same tem-

'*


30 ".

2,

50

5

40 2

3o

J

~

7-

1

181

Local citation.

The results of Table IV with nickel as catalyst are plotted incurve A of Fig. 7. The best t e m p e r a t u r e for nickel is much lower than for copper, being about 192" for nickel. The optimum t e n perature range for nickel is likewise very narrow.

0. W . Brown azzd C. 0. Henke

182

Temperature of catalyst ' C


217 232 253 263 2,74 286

22.1 49.5 97.6 97.8 97.1 97.4 90.8 94.0

3OC5

322

Experiment number

13B2 14B2" 19B2" 24B2 29B2"* 33B2 3SB2 30B2 40B2 42B2 43B2

I

Material yield Hydrogen in Nitrobenzene in liters per hour grams per hour hydrogen Excess in Of% n % of theory

20 20 20 20 '17 17 17 17 17 17 17

4.1 4.1 4.1 4.1 3.3 3.3 3.3 3.3 3.3 3.3 3.3

800 800 800 800 840 840 840 840 840 840 840

90.6 90.6 63.5 27.3 85.5 83.9 74.1 72.7 68.0 63.3 56.2

* The experiments not listed were made under different conditions and so are not comparable with those listed in the table. * * Prior to Experiment 29B2 the catalyst was ozidized at 475" and reduced and heated in hydrogen to 400".


183

pared by ignition of the nitrate at 550" * 25 " and reduction of the -oxide at 440". The results of Table XI11 are plotted in Fig. 8. As abscissa are plotted the experiments in the order in which they were carried out. Curve A represents experiments 13B2 to 24B2, inclusive, while Curve B represents experiments 29B2 to 43B2, inclusive. The results indicate that the activity of the Catalyst decreases with use if used a t too high a t e m p e r a t u r e , 377" being then too high a temperature. In the first experiment the products condensed Fig. 8 nicely in the condenser, b u t after a few experiments white fumes (very difficult t o condense) appeared which increased in amount with each succeeding experiment and the yield in aniline decreased. With a slower rate of flow of the nitrobenzene the decrease in activity is not so rapid as with a higher rate as is shown by the fact that Curve B does not drop so rapidly as Curve A. Likewise the activity of a catalyst used at 286 O decreased from 96 to 76 yo in 26 experiments which indicated that 286" was also slightly too high. At 253" the catalyst showed no appreciable decrease in activity. Probably about 260" is the best temperature for carrying out the reduction with copper as catalyst. The effect of the rates of flow of hydrogen and nitrobenzene upon the yield of aniline was also studied. The results are given in Table XIV. The catalyst used in these experiments was prepared by igniting the nitrate a t 414" and reducing the oxide a t 314". This was used in a few experiments and was oxidized and reduced twice a t the same temperature before being used in the experiments given in the table.

184


TABLE XIV Temperature of catalyst-286

'.

~~

Experiment number

157B6 153B6 154B6 155B6 156B6 158B6 159B6 160B6 161B6 162B6 163B6 164B6 165B6 166B6 167B6 168B6 169B6 170B6 171B6

Hydrogen in Nitrobenzene in Excess of Material yield Liters per hour grams per hour iydrogen in % in yo of theory

5.7 11.4 28.5 46 69 5.7 11.4 17 28.5 46 e9 92 5.7 11.4 17 28.5 46 69 92

4.0 4.0 4.0 4.0 4.0 5.6 5.6 5.6 5.6 5.6 5.6 5.6 7.3 7.3 7.3 7.3 7.3 7.3 7.3

1eo 420 1200 2000 3040 90 270 450 830 1400 2160 2920 40 180 320 610 1150 1620 2200

90.5 95.8 95.5 90.8 81.7 79.5 86.1 89.9 84.8 88.6 72.5 61.5 51.4 70.0 68.1 65.5 61.8 54.9 41.3

100 90

2 57

E 80

r; s

g 70

5

ul

F 60 -I

4

z

50

z 8

'

10

12

NITRODENZPNE FLOW IN CRAMS PER HR.

Fig. 9

A. 4 grams nitrobenzene B. 5.6 grams nitrobenzene C. 7.3 grams nitrobenzene D. 3.8 grams nitrobenzene

per per per per

hour hour hour hour

Fig. 10 A. 5.7 liters HZper hour B. 11.4 liters Hn per hour C. 46 liters Ha per hour D. 69 liters HZper hour E. 17 liters HZper hour (Ni)


185

The results of Table XIV are plotted in Figs. 9 and 10. In Fig. 9 the yield of aniline is plotted against the rate of flow of hydrogen expressed in liters per hour. The Curves A, B and C for the three different rates of flow of nitrobenzene are very similar. With each increase in rate of nitrobenzene the percentage yield of aniline increases. Curve D is drawn from the data of Table V with nickel as catalyst. A large excess of hydrogen does not decrease the yield with nickel as much as with copper. This illustrates the greater activity of the nickel catalyst as the time of contact of the nitrobenzene with the catalyst does not need to be so long with nickel as with copper. In Fig. 10 the yield of aniline in percent is plotted against the rate of flow of nitrobenzene expressed in grams per hour. The Curves A, B, C and D are all similar, the percentage yield of aniline being higher the lower the rate of the nitrobenzene. Curve E is drawn from the data of Table VI with nickel as catalyst. This is vastly different from the other curves the yield dropping less than 2% when the rate is increased from 3.1 to 11.6 grams per hour, while with copper the yield drops from 95.8 to 70%, a decrease of nearly 26y0 when the rate of nitrobenzene is increased from 4 t o 7.3 grams per hour. Hence the nickel can be used at a much greater rate than the copper. There was also a great difference in the physical appearance of the two catalysts after having been used. The nickel catalyst after use was a loose, noncoherent mass of black nickel, while the used copper catalyst was a compact red mass which was coherent. It could be shaken out of the furnace as a stick and was strong enough to be held in a horizontal position by holding one end. The older the copper catalyst the more firmly it was packed together. If by some means the copper catalyst could be kept from becoming packed, it might act a t as fast a rate as the nickel. The effects of length of time of heating in hydrogen has not been thoroughly studied although it appears that with copper merely keeping at an elevated temperature in hydrogen

.

0.W . Brown and C. 0. Henke

186

decreases its activity, while with nickel the reverse is true. This was brought out in the discussion under Tables 111, IX and X. The results of Table X do not show any decided effects of repeated oxidation and reduction of the copper catalyst. After using many other catalysts (the results of which will be given in a paper in a later number of this journal) it was suspected that the iron tube might have some catalytic effect upon the reduction of the nitrobenzene. A new iron tube just like the previous ones was put in the furnace. This had been cleaned with nitric acid (1: 1). After cleaning with the nitric acid it had been washed well with water in order to remove any nitrate that may have been formed by the action of the nitric acid upon the iron pipe. Before use the new tube was heated in a current of hydrogen a t 395" for one hour. It was then used in several experiments the results of which are given in Table XV. TABLE XV Rate of flow of hydrogen-17 liters per hour. Rate of flow of nitrobenzene-3.9 grams per hour. Excess of hydrogen-710%. Experiment number

12HO 13H0 14HO 15HO

Temperature

305 2G8 230 305

C


57.3 23.8 10.7 32.0

From the data of the table it is evident that the iron tube has considerable activity, although it decreases rapidly with use. Thus in the first experiment the yield of aniline was 57.3% while in the fourth experiment, which was carried out under the same conditions, the yield of aniline was 32.0%. The next experiment was with an iron tube that had not been cleaned with nitric acid, but was put in the furnace just as received without making any attempt t o remove any dirt or grease. This was heated in a current of hydrogen t o 410°, then allowed to cool to 300" and an experiment carried out

Catalytic Preparation of Aniliiae

187

under the same conditions as the first experiment of Table XV. The first experiment gave a yield of 28.2 yoaniline, as compared to a yield of 57.3y0 when cleaned with nitric acid. Three tubes lettered I,, M and N were then tried to see whether they would give similar results. In each case a new uncleaned tube was used. This was heated in a current of hydrogen to 565" before use. The results are given in Table XVI, several experiments being carried out with each tube, which are listed in the order in which they were made.

'TABLE XVI Rate of flow of hydrogen-17 liters per hour. Rate of flow of nitrobenzene-3.9 grams per hour. Excess of hydrogen-710%. Experiment number

1L SL 3L

4L 51, 1M 2M 3M 4M IN 2N 3N 4N" 5N

Temperature of catalyst C

322 322 322 305 305 335 335 335 320 305 305 305 305 305


26.9 33.2 31.3 14.7 12.2 41.4 25.7 17.5 13.5 20.4 19.4 14.7 18.2 15 . O

The results do not show exactly the same behavior with each tube, which is as one would expect. However it will be noted that with use the yields at 300" approach a value of from 12 t o 15%. The yield is highest at first and decreases with use. Experiment 1L however is lower than the succeeding one, which we are unable to explain, except that it be an error. * This was the first experiment after the catalyst had been idle for about two hours and indicates that the catalyst (iron pipe) recuperates when idle in an atmosphere of hydrogen.

188

0. W . Brown and C. 0. He?zke Summary of Results

1. A suitable apparatus for the systematic study of catalysis in the vapor phase has been described. 2. The most favorable temperature of ignition of nickel nitrate for the reduction of nitrobenzene to aniline has been found to be about 450". 3 . It has been shown that merely heating the reduced nickel in hydrogen suffices to decrease the activity of the nickel. The most favorable temperature for heating in hydrogen was found to be about 380" since a lower temperature produced a more active nickel which carries the reduction too far. 4. After heating the reduced nickel catalyst in hydrogen to a high temperature, it did not lose its activity immediately, but lost it with use, its activity decreasing almost linearly. 5 . The best temperature for carrying out the reduction with nickel as catalyst was found to be about 192". 6. From a study of the rates of flow of hydrogen and nitrobenzene it appears that the rate of flow of the gaseous mixture through the tube is of more importance than 'the percentage excess of hydrogen present. 7. The best temperature for the ignition of copper nitrate for the reduction of nitrobenzene to aniline was found t o be about 415". At lower temperatures the copper loses its activity with use and does not give as high yields as when ignited a t about 415 ". 8. Heating the copper catalyst in hydrogen reduced its activity but little until it was heated above 475", its activity being 60% lower (as measured by aniline yield) when heated to 535 " than when heated to 475". 9. The copper catalyst lost its activity almost immediately, when heated to a high temperature (535 ") in hydrogen, while the nickel catalyst lost its activity with use. 10. A new copper catalyst or one that has been oxidized and reduced gains in activity with use for four to six experiments before it gives constant results. Long reduction in

Catalytic Preparation of Aiziline

189

hydrogen increases the length of time required for it to attain its maximum activity. 11. The best temperature for carrying out the reduction of nitrobenzene with copper as catalyst was found to be about 260'. It was pointed out that probably the best temperature for copper when prepared in one way would not be the best temperature for copper when prepared in another way. 12. The activity of a copper catalyst decreases when used a t too high a temperature (377 ') . The decrease is more rapid with a more rapid rate of flow of nitrobenzene. 13. A study of the rates of flow of hydrogen and nitrobenzene showed : (1) with a constant rate of flow of hydrogen the lower the rate of flow of nitrobenzene the greater the yield of aniline; (2) with a constant rate of flow of htrobenzene, an increase in the rate of flow of hydrogen first increases the yield of aniline and then decreases it, the increase and decrease being much more marked with copper than with nickel; (3) it seems that the time of contact of the gaseous mixture with the catalyst is of more importance than the percentage excess of hydrogen present. 14. It has been shown that the activity of an ordinary wrought iron pipe is considerable. Its activity decreases with use. Its activity is greater when cleaned with nitric acid before being put in the furnace than when put in the furnace and used without being cleaned with nitric acid. 15. The activity of nickel and copper catalysts, for reducing nitrobenzene to aniline, is restored by oxidation and reduction although not to so great an extent with copper as with nickel.

Conelusions Although a higher material yield of aniline can be obtained with copper as catalyst than with nickel as catalyst, yet the writers are of the opinion that nickel would be better and cheaper in commercial practice than copper because it can be used a t a much greater rate. The best temperature for igniting the nickel nitrate is about 450°, the best tempera-

190

0. W . Brown a i d C. 0. Hevlke

ture to which t o heat the nickel in hydrogen after reduction is about 380" and the best temperature for carrying out the reduction is about 190". Laboratory of Physical Chemistry Indiana University Bloomington

Catalytic Preparation of Aniline - The Journal of Physical Chemistry

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