CATALYTIC ACTIVITY O F THALLICM BY 0 . W. BRO’KX, CHESTER BROTHERS AKD G. ETZEL
Introduction Thallium is an active hydrogenation catalyst1 and it is one of the three that give high yields of azobenzene. The activity of thallium decreases very rapidly with use. This decrease is probably caused by the melting and running together of the catalyst. Some catalysts, such as lead and bismuth, may be used at temperatures considerably above their melting points without undergoing any appreciable change in their physical condition. I n the case of lead, however, it was found that some lead catalysts ran together while others did not, depending, to a certain extent, upon the physical properties of the oxides from which the catalysts were made. I n yiew of the above, it was thought that a solid support might be found for a thallium catalyst that would adsorb the catalyst without reducing its a c t i d y , and thereby prevent it sintering. If sintering was prevented, the useful life of the catalyst would be prolonged. Experimental Work and Results The apparatus, method of work, as well as the purification of thallium, were the same as those previously described.? I n this investigation, homvci the catalyst tube used Tyas a one-half inch diameter glass combustion tube Three catalysts were prepared from precipitated thallic hydroxide; one was ignited in an electric muffle for two hours at 5oo°C., and the other a t 3ooOC , xyhile another portion was not ignited. All three were reduced a t 23oOC’. for 3 hours, with the rate of flow of hydrogen of z liters per hour. As may be seen in the work referred to above, the temperature a t the point where reduction takes place was much higher than at any other part of the catalyst column. I n order to keep the temperature as uniform as possible throughout the furnace, the flow of hydrogen was kept low. In spite of this precaution, the catalyst, after reduction, was found t o contain globules of massive thallium, showing that it had partially melted and run together. Thus, it may be concluded that an ignited thallium oxide will not give a catalyst which will not melt. I n an effort to find a catalyst which could be used satisfactorily without running together, solid supports for thallium were tried. The first of these was aluminum hydroxide, mixed with about 1 2 . 8 of ~ ~metallic thallium. This mixture was prepared as follows: sulfates of thallium and aluminum nere mixed and precipitated with ammonium hydroxide. The precipitated hydroxides were throughly mashed by decantation, filtered, dried, and ground in a mortar to a fine powder. A4bout2 5 g. of this powder, which had a light brown color was used to make a catalyst. It was reduced for 3 hours a t 23oOC.. the 1
Henke and Brown: J. Phys. Chem., 26, 631
(1922).
* Henke and Brown: J. Phys. Chem., 26, 631 (1922).
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CATALYTIC ACTIVITY O F THALLIUM
rate of flow of hydrogen being 2 liters per hr. The catalyst was then used a t 2 4 s O c ' . with a resulting yield of 1 . 7 7 ~aniline. At 3ooOC. t'he yield of aniline \vas s.s%,, and a t 400'C. was 1 4 . 9 7 ~ . At 40oOC. there was also a trace of azobenzene produced. As is evident from the data given, this catalyst was rery inact,ive. However, it appeared to be in good physical condition; it was black and without' lustre. The solid supports next tried were pumice and Nonpareil brick, which were crushed and passed through a Io-mesh sieve, and remained on a zo-mesh sieve. In each case the support was cleaned by boiling in nitric acid, washed with distilled water, and then ignited at 5ooOC. Each of these were immersed in a thallic sulfate solution, placed in a vacuum desiccator, and evacuated 7 times. The purpose of this was to replace the air in the pores of the support by the thallic sulfate solution. The supernatant thallic sulfate wa8 then poured off and ammonium hydroxide added to the support, which was soaked full of thallic sulfate solution. This precipitated thallic hydroxide in the pores and on the surface of the supports. Each impregnated support was then filtered, washed; and allon.ed t o dry a t room temperature. Thalliuni on pumice cat st was used in six experiments at temperatures ranging from 230'c'. to 300 The rate of nitrobenzene was 4 g. per hr. and of hydrogen 7 liters per hr. S o appreciable amount of azobenzene was secured in any experiment. The highest aniline yield was 6.2 j y c a t 3ooOC. This indicates that thallium on pumice is a poor catalyst. Thallium on Sonpareil brick was used at 2 4 260 ~ ~and 275OC. with the rate of flax of hydrogen a t 7 liters per hr. and the rate of flow of nitrobenzene at 4 g. per hr. S o appreciable amount of azobenzene was secured. The aniline yield was 6.6';; at 2 7 j"C., which was the highest yield secured. Asbestos n-as the last of the solid supports tried. It was boiled with nitric acid, washed, dried, soaked in thallic sulfate solution, desiccated, and t,reatetl with amnionim hydroxide. The catalyst prepared consisted of 2 4 g. of thal-
TABLE I Catalyst-Thallium on asbestos Rate of flow of nitrobenzene---qg. per hr. Temperature Flow of hydrogen Excess of hydrogen degrees C. in liters per hr. based on aaobenzene in per cent of theory
230
7
245
7 7
260 2 60 260 245
14
7
7
245
11
260 260 260
11
14 13
360 3 60 360 820 360 3 60 820 820 820 820
Material yield in per cent of theory .hiline hzobenaene Total
9.6 10.7 16. 12.6 14.I 8.4 4.3 9.5 11.8
7.8
I j . 2
70,9 83.7 83.9 79.0
78.2 81.2 82.0 75.5 74.8
24.8 81.6 99.7
96.5 93 1 86.6 85.4 91.5 87.3 82.6 '
0. W. BROWN, CHESTER BROTHERS AND G. ETZEL
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liuni on 9 g. of asbestos. In order to determine the best working conditions experiments were made a t different temperatures and at different rates of flow of hydrogen. The results of these experiments are given in Table I. The yields given are averages of from z to I Z experiments. From Table I it can be seen that the best yields of azobenzene and aniline were obtained at z6ooC. The rate of flow of hydrogen which gave the highest yield was 14 liters per hour for azobenzene, and 7 liters per hour for aniline. It may be concluded, therefore, that the best yields of azobenzene are obtained at a temperature of 26ooC., and with a rate of flow of hydrogen 14 liters per hour. The activity of the catalyst decreased slightly a t first, but after 56 rum it became constant. When temperatures as high as 2 7 5 to z80°C. were used the yields of azobenzene were 7 7 to 75'2. Experiments were also made in this laboratory by Dr. C. 0. Henke and E. D. Scudder using a I; inch vertical tube iron furnace 1 2 inches long. The catalyst used contained 60y0Tl2Os. The yields obtained were from 85 to 9 0 7 ~ theory azobenzene. The production of azobenzene amounted to more than 1 5 0 times the weight of thallium in the catalyst before rejuvenation was necessary. All catalysts tried on solid supports were in good physical condition after being used. The adsorption of thallium by the support was efficient in preventing it from melting and running together, although in the cwe of aluminum, pumice and Nonpareil brick their presence have reduced the adsorption of hydrogen and nitrobenzene to such an extent as to produce very poor catalysts. Asbestos not only adsorbs thallium preventing its running together, but also does not interfere with the adsorption by thallium of hydrogen and nitrobenzene, resulting in good yields of azobenzene and long life of the catalyst.
summary Finely divided metallic thallium melts and runs together when used as a catalyst, resulting in a loss of its activity. z . Thallium on asbestos is an excellent catalyst for the production of azobenzene, and the catalyst retains its activity for a comparatively long period of time. 3. A temperature of about 260' c'. gives the highest yields of azobenzene. I.
Laboratory of Physacal C h e m s t r y , Indaana University, Bloomzngton, I i d .
