172
INDUSTRIAL AND ENGlNEERING CHEMISTRY
pounds tried were: p-amidophenol, m-toluylenediamine, p-hydroxydiphenyl, hydroquinene, phenyl-@-naphthylamine, triacetin, nitrobenzene, and symmetrical di-b-naphthyl-pphenylenediamine. Although some of the materials, such as p-amidophenol, m-toluylenediamine and triacetin, caused the cements to thickell \%,henstored at 250 C,, they did not retard the degradation brought about by storing at 50" C. Di-pnaphthyl-pphenylenediamine showed possibilities of retarding thinning caused by elevated temperatures, but precautions had to be taken to exclude light a t all times, since otherwise a photochemical reaction took place which caused the cement to thin rapidly and turn a dark brown color. This material is usable in cements which are put up in metal cans or tubes.
Vol. 26, Xo. 2
mission and exceedingly helpful suggestions made this paper possible. LITERATURE CITED Asano, K., India Rubber J . , 703 307, 337, 389 (1925). Bary, P., and Fleurent, E., Rubber Chem. Tech., 4, 505 (1931). Ibid,, 6 , 111 (1933). Bernstein, G . , K o u o ~ ~ z12, . , 183 (1913). ( 5 ) Dufraisse, C., Rubber Chem. Tech., 6, 157 (1933). (1) (2) (3) (3)
k;
(1832j,
(8) G
~T, L,,~lnst. ~~ ~~Ind, T b~ ~ b~,4, 413 ~ ~ ~(1927). .~ , (9) Porritt, B. D., India Rubber J . , 60, 1159 (1920). (10) Porritt, B. D., and Frye, J. D . , Inst. Rubber I n d . Trans., 3, 203 (1E)23).
ACKNOWLEDGMENT The author wishes to express his sincere appreciation to N. A. Shepard, J. N. Street, and C. R. Park, whose per-
RECEIVED September 20, 1933. Presented before the Division of Rubber Chemistry a t the 86th Meeting of the American Chemical Society, Chicago, 111.. September 10 t o 15, 1933.
Removal of Thiophene from Benzene HARRY N. HOLMES AND NORVILBEEMAN, Oberlin College, Oberlin, Ohio
E
VER since Meyer discovered thiophene in 1882 (IS), there has been a demand for thiophene-free benzene which is needed, as pointed out by Ardagh and Furber ( 2 ) ,chiefly for the manufacture of dyes and for the preparation of certain c. P. chemicals. The first mention made of sulfuric acid as a means of removing thiophene from benzene appeared in the report of Meyer in 1882 ( I S ) . He called attention to the fact that common coal-tar benzene, after boiling for 10 hours with concentrated sulfuric acid on a water bath with a reflux condenser, failed to give the familiar Baeyer indophenin reaction with isatin. Since that time the sulfuric acid treatment has been used extensively. Deniges ( 5 ) , Dimroth (6), Paolini and Silberman (14), and Ardagh and Furber ( 2 ) have made use of such salts as mercuric acetate and mercuric stearate in thiophene removal. Dutt and Hamer (7) claim that chlorination will free benzene from thiophene. A light treatment with sulfuric acid, followed by chlorination, is said to be used now in the United States. ANHYDROUS ALUMINUM CHLORIDE TREATUENT In 1894 the Soci6t6 Anonyme des MatiBres Colorantes de St. Denis (Paris) took out patents for the process of removing thiophene from commercial benzene by warming it with 5 to 20 per cent anhydrous aluminum chloride (15). Two years later Haller and Michel (9) reported the results of refluxing, fur a half-hour, benzene containing thiophene with from 0.1 to 5 per cent of aluminum chloride. They stated that refluxing is not essential, for they agitated benzene, as free as possible from water, with aluminum chloride in the cold and obtained a red substance which deposited as a viscous liquid. Since they preferred refluxing, it would seem that their method of agitating in the cold must have been less effective. Heusler ( 1 1 ) described a process of heating benzene containing thiophene for approximately 9 hours in a reflux apparatus with about 5 per cent aluminum chloride. He called attention t o the reddish mass which separated and to the fact that the reaction appeared to start even before heating. At the end of his report, announcement was made of a British patent ( I O ) covering the use of anhydrous aluminum chloride and other metal chlorides for the removal of thiophene from benzene, with concentrations between 5 and 20 per cent over a temperature range of 100" to 600" C.
NATUREOF
THE
REACTION BETWEEN ALUXINUM CHLORIDE AND
THIOPHENE
Heusler suggests that an intermediate compound is formed (probably red in color) between the thiophene and the aluminum chloride, a product somewhat soluble in benzene and decomposed by water with regeneration of thiophene. Boedtker (4) observed, in the Friedel-Crafts reaction with benzene, the liberation of hydrogen sulfide and hydrogen chloride, and attributed these products to the reaction between the thiophene in the benzene and the aluminum chloride. I n the present work it was observed that, when benzene containing thiophene, after drying several days over fused calcium chloride, was treated with anhydrous aluminum chloride, there was an evolution of hydrogen sulfide arid hydrogen chloride a t the time when the mixtures were distilled after decanting from the reddish sludge a t the bottom of the reaction flasks. This was observed following contact over a range of 6" to 65" C.
ESTIMATION OF THIOPHENE IN BENZENE The first test for thiophene was the indophenin test discovered by Baeyer, depending upon the blue color obtained by reaction between thiophene and the isatin dissolved in sulfuric acid. It was applied erroneously as a test for benzene until Meyer isolated thiophene and demonstrated that it was this compound, present as a n impurity in coal-tar benzene,' that was responsible for the color reaction. This test was later modified and improved until it can now be used to detect concentrations as low as 0.0005 per cent ( I , 3, 16). The sensitiveness of the test depends upon the presence of some oxidizing agent, such as ferric chloride or nitric acid. It has been the authors' experience that isatin solution shaken with air, but without nitric acid, produces a satisfactory color reaction within an hour. I n testing for the last traces of thiophene it was found advisable to shake thoroughly 25 cc. of the benzene to be tested with 2 cc. of c. P. concentrated sulfuric acid. When the layers separated, 1 cc. of the sulfuric acid layer mas drawn off, and to this was added by pipst 0.1 cc. of a sdution of isatin 1
Very impure bensene may contain as much as 0.6 per cent by weight
of thiophene (Richter, V. von, "Organic Chemistry," Vol. 111, p. 21, Blakiston, 1923), but the usual water-white benzene purchased by this laboratory was seldom found to run higher than 0.05 per cent.
February, 1934
I Y D U S T H I A L A4N D
E NG I N E E R I N G C H E M I ST R Y
113
in sulfuric acid (0.4 gram per 100 cc.). This was shaken and this product from the first treatment was again shaken with allowed to stand. -4faint greenish yellow color develops with aluminum chloride, but in the ratio of 1.0 gram per 100 cc. (just double the first), the cumulative effect was that of reducing the a thiophene concentration as low as o , o o o 3 ~per cent,, T~~~~~ thiophene content to a little less than 2 per cent. When this are difficult t o detect if the benzene is of the quality which twice-treated product was Once more shake11 with aluminum darkens when shaken with concentrated sulfuric acid. "Thio- chloride, again doubled (cumulative amount equivalent to 3.5 grams per 100 cc.), all the thiophene was removed. Thus by doubling the concentration of aluminum chloride in three successive treatments, all the thiophene was removed from a benzene containing 0.447 per cent thiophene by weight far more efficiently than by a single aluminum chloride treatment of 5.6 grams per 100 cc. Following each treatment, the product was decanted and distilled.
TEMPERATURE EFFECT Work a t different temperatures was carried on in the agitator, a large water bath in which the reaction flasks were placed and rocked. Vigorous agitation was essential to successful removal, as shown by comparison between poor stirring and good agitation. Difficulty was experienced in preventing the entrance of moist u r e i n t o the reaction flasks. A sufficient n u m b e r of satisfactory determinations were made, however, to assure a good degree of reli1 2 3 4 5 gims del. por loo CC. ability and to warrant reporting in Figure 2. The four curves selected for Figure 2 repBETWEEN EFFECTS OF SINGLE AND SUCCESSIVELY FIGURE1. DIFFEHEXCE resent the two t e m p e r a t u r e e x t r e m e s DOUBLED TREATMENTS dictated by boiling point and freezing point phene-free" is usually interpreted to mean less than 0.0005 of benzene, and the more effective temperatures of 35" and 25' C. The benzene used for this series contained 0.353 to per cent of thiophene. 0.355 per cent thiophene. IMPROVED METHODFOR REMOVAL OF THIOPHEUE FROM It is evident that approximately 35" C. is the optimum temBENZEXE perature for the process and that two additions of the chloride with two distillations would be adequate for preparation of The problem of removing thiophene from benzene is a t h i o p h e n e - free natural extension of the work which this laboratory has purbenzene. sued on the more general problem of sulfur removal from gasoline, kerosene, and other hydrocarbons ( l a ) . Attention was EFFECTOF centered rather early upon the use of anhydrous aluminum MOISTURE chloride in converting thiophene into a material more readily ..+E ec adsorbed upon a suitable porous solid. Fortunately it turned When one out that a small excess of the powdered aluminum chloride drop of water is i \ served as an excellent adsorbent for the addition product added t o 1OOcc. :6c first formed. Two aspects of the problem presented themof drj7 benzeneselves a t once: the effect of successive treatments, using 3 thiophene mixsmall quantities of aluminum chloride, and the effect of treatt u r e , t h e re2 ment a t different temperatures. m o v a l of thio54c Dry benzene containing 0.447 per cent thiophene by weight p h e n e b y 0.6 was treated in 100-cc. portions with varying quantities of alumi- gram anhydrous num chbride. The results are shown graphically by curve 1, aluminum chlo2c Figure 1. When 100 cc. of the benzene-thiophene mixture were shaken vigorously at room temperature for a half-hour with ride per 100 cc. is by 0.5 gram anhydrous aluminum chloride, decanted, and distilled, r e d u c e d the distillate contained a little more than 38 per cent of the about one-third. original concentration of thiophene. When still another fresh T h o u g h t h i s 0 1 2 100-cc. portion was treated similarly, but with about 1.9 grams, BP-S klC1, p e r IOO'Cc. the thiophene content was reduced t o slightly more than 8.5 amount of water FIGURE 2. EFFECT OF TEMPERATURE ON per cent of the original concentration. I t is evident that even i s m o r e t h a n a single treatment of about 5.6 grams does not remove all the enough to satuREMOVAL OF THIOPHENE thiophene. Suspecting that this reaction of thiophene with rate the benzene, anhydrous aluminum chloride was a combination of chemical reaction and adsorption, it was reasoned that the red, viscous far less will appreciably cut down the efficiency of the reacliquid formed by reaction was covering up portions of the alumi- tion. num chloride, thus preventing thorough contact bvtween the After a little experience one is able, in a rough way, to detect aluminum chloride and the rest of the mixture. A procedure the amount of water present by the manner in which the acwas then adopted for treating the same sample of the benzene containing thiophene with successive portions of fresh aluminum tion proceeds, If the benzene is very dry and contains about chloride and portions which, at the same time, were doubled with 0.2 to 0.3 per cent thiophene, the first shaking with alumieach successive treatment. Thus curve 2, Figure 1, shows that, num chloride produces almost a t once a deep red liquid which when 100 cc. of the same mixture were first treated with 0.5 adheres t o the surface of the aluminum chloride. Any gram aluminum chloride, the product contained slightly more than 38 per cent as much thiophene as before treatment. This aluminum chloride not covered by this viscous material reis exactly like a single treatment with aluminum chloride. When mains finely granular. If only slightly moist the red colored
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4
1
I
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INDUSTRIAL AND ENGINEERING CHEMISTRY
material forms much as when dry, but a fine floc, evidently aluminum hydroxide, separates as a suspension. Should the benzene be quite wet, and the aluminum chloride insufficient in amount to take up the moisture, in place of the red coloration a somewhat sticky yellow suspension forms, considerably flocculated.
PREPARATION OF PURE BENZESE I n the preparation of pure benzene the treatment naturally depends upon the quality of the benzene with which one starts. If the thiophene content of the initial product is not more than 0.02 or 0.03 per cent by weight, a single treatment of about 10 to 12 grams of aluminum chloride per liter of benzene may be sufficient. If the benzene contains relatively very much moisture, this amount will be more efficiently used in two or more successive treatments, managed as proposed above. If time is of no great consequence, the benzene may first be dried over a little fused calcium chloride. A small quantity of concentrated sulfuric acid shaken with the benzene is, of course, effective both as a preliminary drier and as a thiophene remover, but i t is subject to the same objection as the sulfuric process in general-i. e., loss through sulfonation. One of the criteria of benzene purity is that c. P. concentrated sulfuric acid, after agitation with the benzene, should not turn dark upon standing. This darkening of the sulfuric acid layer is probably due to the presence of sulfur compounds. The sulfur compounds formed during aluminum chloride treatment are removed by washing the final distillate with sodium hydroxide solution, or merely by shaking for a longer period with solid sodium carbonate or sodium hydroxide. Where such washing is followed by distillation, a thiophene-free product is obtained which does not darken sulfuric acid even after long standing. When the initial product is a crude benzene, containing 0.2 per cent thiophene or more, a single treatment with aluminum chloride is definitely ineffective, and one must resort to the plan of successive treatments which has been described above. After each vigorous agitation (one-half hour), the benzene should be decanted from the red liquid formed and be distilled. It has been objected that three distillations are too many for industry. If so there is no doubt that two, or even one for purer raw material, will meet most standards of thiophene content. Filtration through a long column of aluminum chloride has been tried with poor results. Time of contact is inadequate. COMP.4RATIVE LOSSES O F BENZENE The loss of benzene in the sulfuric acid process has been variously reported. Ellerton (8) stated that it ranged from 8 to 12 per cent. The real meaning of these losses is given in the second paragraph below. While working with relatively small quantities of an intermediate grade of benzene in this laboratory, it was observed that the losses in the aluminum chloride treatment are due (I) to slight reaction between aluminum chloride and benzene and (2) to washing and to the films retained on container walls. At the temperature of optimum removal the first is almost negligible, and on an industrial scale the total loss would undoubtedly be less than 1 per cent of the benzene treated. Of course a crude benzene must receive a light preliminary treatment with sulfuric acid in order to remove unsaturated paraffin hydrocarbons. It is not claimed here that anhydrous aluminum chloride could usurp that particular function of the acid. However, there are real advantages in following this preliminary acid treatment with aluminum chloride in order to complete effectively the removal of thiophene.
Vol. 26, No. 2
DISCUSSION OF RESULTS Although the use of anhydrous aluminum chloride as a thiophene remover was suggested as early as 1894, and foreign patents were taken out t o cover the process, it is evident that there were serious flaws in the procedure advocated, for t h e process did not become commercial. However, the commercial demand for thiophene-free benzene is small. The chief flaw in the earlier work was the incomplete removal of thiophene by any but excessive amounts of aluminum chloride. At the time this earlier work was carried out, the colloid point of view had not been developed and i t was natural, as a study of the older patent literature discloses, to think wholly in terms of chemical reaction and to attach significance to the higher temperatures. The development of colloid chemistry has emphasized the necessity of taking account, especially in industrial processes, both of chemical reaction and of colloidal adsorption. It is unquestionably a new idea to explore for a temperature neither too high for effective adsorption nor so low as to inhibit the primary chemical reaction. For most processes involving both chemical reaction and adsorption, there is a temperature of optimum effect, a temperature sufficiently high for rapid reaction and sufficiently low for adequate adsorption. There is strong evidence for believing that aluminum chloride functions in the double role of reaction and adsorption agent. Therefore, successive treatments at 25" t o 35" C., using small quantities of aluminum chloride, have proved most efficient for the reaction between aluminum chloride and thiophene, and for the adsorption and subsequent removal of the products of this reaction. A crude but cheap anhydrous aluminum chloride such a s that prepared by the McAfee process for oil cracking has been tested and found to be satisfactory in thiophene removal. If, in an attempt to simplify the operation, slow filtration through layers of aluminum chloride is tried, it will be found advisable to keep the benzene and the filters a t about 35" C. Acid released by contact with the aluminum chloride could be removed by a basic layer (lime, etc.), in the filtration system. The red sludge previously mentioned could be eliminated by simple drainage and the operations probably reduced to one final distillation. LITERATURE CITED (1) Am. Chem. Soc., Com. on Analytical Reagents, IND.ENG. CHEM.,Anal. E d . , 4,347-50 (1932). (2) Ardagh, E. G . R., a n d Furber, C . M., J . SOC.Chem. Ind., 48. 74-76T (1929). (3) Bauer, F. W., Ber., 37, 1244-45 (1904). (4) Boedtker, E., Compt. rend., 123, 310-11 (1896). (5) Denigks, G., Bull. SOC. chim., 13, 537 (1895); 15, 862, 1064 (1896); J . SOC.Chem. Ind., 14, 893 (1895); 15, 746 (1896): Compt. rend., 120,628, 781 (1895). (6) Dimroth, O., Ber., 32,758-65 (1899); 35,2035 (1902). (7) D u t t , P. K., and Hamer, J. D., British P a t e n t 117,683 (July 27, 1917); J. SOC.Chem. Ind., 37,540A (1918). (8) Ellerton, J. G., Ibid.. 31, 10-12 (1912). (9) Haller and Michel, Bull. SOC. chim., 15, 390, 1065-70 (1896). (10) Heusler, F., British P a t e n t 4769 (1897). (11) Heusler, F., 2. angew. Chem., 9, 288, 318, 750 (1896); J . SOC. Chem. I n d . , 26, 131 (1897). (12) Holmes, H. N., Elder, A L., Beeman, Norvil, J . Phw. Chem., 36, 2981-93 (1932). (13) Meyer, Victor, Ber., 15,2893-95 (1882) ; 16,1465-78 (1883) (14) Paolini and Silberman, Rend. accad. Lincei, 24, 209 (1915). (15) SOC. Anon. des Matihres Colorantes de St. Denis (Paris), 2. angew. Chem., 8 , 114 (1895); French P a t e n t 240,111 (1894); German P a t e n t 79,505 (1894). (16) Wray, E., J . SOC.Chem. Ind., 38,83-84T (1919). I
RECEIYED May 29, 1933. Presented before the Division of Industrial and Engineering Chemistry a t the 85th Meeting of the American Chemical Society, Washington, D. C., March 26 t o 31, 1933.
