I
HANS FEILCHENFELD Research Council of Israel, P.O.B. 5192, Jerusalem, Israel
Separation of Aryl Sulfonic Acids I N T H E SULFONATION of akylated aromatic hydrocarbons with excess oleum or sulfuric acid according to the equation : ArH
+ HeSO4 e ArSOsH + HLO
(1)
it has long been established practice, both commercially and in the laboratory, to effect separation of most of the excess sulfuric acid by partial dilution of the reaction mixture with water (2,6). The desired sulfonic acid then separates from the spent acid, either as a crystalline solid or as a liquid layer, since these sulfonic acids are substantially insoluble in sulfuric acid of intermediate strength, even though they may be very soluble in either strong or weak arid. This approach is used commercially for separating the sulfonic acids of toluene, 1,2,4-trimethylbenzene, alkylated naphthalenes, petroleum lubricants (green acids), and dodecylbenzene (detergent alkylate). In the last case, usual commercial practice yields a spent acid in the concentration range 78 to 80%. The optimum degree of dilution has apparently been determined in the past empirically for each reaction, since no detailed data have been noted in the literature relative to the ternary systems involved. Accordingly, the author has made such studies for the cases of toluene, xylene, and a straight-run petroleum naphtha.
The solubility, S, of the sulfonic acid can therefore be separated into three effects
SI = solubility of the ArS03- ion SZ= solubility of the undissociated acid ArS03H S3 = solubility of the ArS03H2' ion The total solubility, S, will be the sum of the three solubilities
s = SI + sz + sx
(6)
T o explain the phenomenon of retrograde solubility of sulfonicacids in sulfuric acid qualitatively, the law of mass action is applied to Equations 2 and 5, approximating activities to concentrations. From Equation 2 it then follows that
S1 = [ArS03-] = KljArSOaH] [H2O1
[H30+] ( 7 )
Since the undissociated sulfonic acid in solution is in equilibrium with the solid sulfonic acid, one has $2
=
[ArSOaH] = Kz
(8)
For the solubility of the ion ArS03H: it follows from Equation 5 that
Inserting Equation 8 in Equations 7 and
9 and setting
Theory The principle on which the separation is based may be explained in the following way. In water and in dilute sulfuric acid the sulfonic acids dissolve easily and ionize according to the equation ArSOaH
+ H2O
ArS03-
+ Ha0
+
(2)
In strong sulfuric acid, on the other hand, the sulfonic acids behave as bases and ionize in either of the following two ways (5):
ArS03H
+ HzSOI~eHSO4ArS02* + + HzO
(4)
Let us assume Equation 3. Equation 4 gives qualitatively the same results. If the hydrolysis of concentrated sulfuric acid in water is taken into account, then Equation 3 can be transformed into ArSOsH+HaO+$ ArSOsH2++HzO ( 5 )
The following expressions are obtained:
S, = KtKe b and
+ +
S = SI SZ
5'3
=
The experiments are started with a saturated solution of sulfonic acid in water and sulfuric acid is added; b is small and it is possible to neglect the second and third terms of Equation 12; the solubility decreases as b, or in other words the hydrogen ion concentration, increases (by the addition of sulfuric acid). As the addition of sulfuric acid is continued the first term, SI,becomes so small that SZ can no longer be neglected and eventually a
point is reached on the solubility curve where both SIand S3 can be neglected when compared with SZ: the solubility of the sulfonic acid remains constant. I n the region of still higher concentrations of sulfuric acid most of the free water is taken up by hydration of the H + ion and 6 becomes large so that the third term $3 increases and becomes of paramount importance : the solubility S again increases. Therefore, as the concentration of sulfuric acid is increased, the solubility of sulfonic acid a t first decreases, remains then a t a fairly constant low value, and finally increases again. This theoretical prediction is substantiated by experimental data. Preparation of Sulfonic Acids
The main point in this method of preparation is to obtain the free sulfonic acids without any sulfuric acid. There remain, for instance, 4 to 5% sulfuric acid in the p-toluenesulfonic acid manufactured by a process recently published (4), whereas the author's final product gave no precipitate with barium chloride. The method of preparation of ptoluenesulfonic acid was a combination of methods (3, 7, 70). Concentrated sulfuric acid and a large excess of toluene (British Drug Houses laboratory reagent) are introduced into a three-neck flask fitted with a mercury seal stirrer. The other necks are fitted with a thermometer and a reflux condenser, respectively. The reflux condenser is connected at its lower end to a water separation tube from which water can be removed periodically. The flask is heated so that the hydrocarbon boils briskly. It is very important that the mixture is efficiently stirred while sulfonation proceeds. The reaction is assumed to be completed when no further water collects in the separation tube. To prepare p-xylenesulfonic acid the same method is used with the following changes: p-xylene (B.D.H. laboratory reagent) is taken in small excess only over sulfuric acid; on the other hand, iso-octane (octane number grade) is added. This addition reduces the reaction temperature from 138' to less than 105' C.; above 110' C. sulfur dioxide evolution becomes appreciable, showing that undesirable side reactions take place to a considerable extent. For purification, the crude sulfonic VOL. 48, NO. 10
OCTOBER 1956
1935
acids are dissolved in as little water as possible, separated from the hydrocarbon layer, and filtered in order to remove the insoluble sulfone. The solution is then evaporated under vacuum on a boiling water bath; any remaining hydrocarbons and the greater part of the water are thus removed. The solid acid is filtered off and washed with cold concentrated hydrochloric acid. It is then dried in a vacuum desiccator over sodium hydroxide and calcium chloride. The case of the sulfonic acids obtained by the extraction of straight-run naphtha is somewhat different. Here, for the technical application, those acids are not specially purified as long as they are free from sulfuric acid. The straightrun naphtha used is a 130' to 170" C. cut from Qatar crude, containing 17yo aromatics with the average molecular weight of 114. The aromatics were first extracted with sulfuric acid and then liberated by hydrolysis. After washing and drying, this aromatic extract is sulfonated in the same way as p-xylene. Analytical Methods
T o determine the composition of mixtures of sulfonic acids with sulfuric acid the analytical method of Weiss and others (77) isfollowed. The method is based on the determination of the total acidity by direct titration and on the determination of sulfuric acid by precipitation with aniline in chloroform, hydrolysis, and titration. The concentration of the sulfonic acid is proportional to the difference of the two titrations. In the case of very low concentrations of sulfonic acids, a method of wet combustion of the sulfonic acid by chromic acid is used. This method when checked against solutions of pure sulfonic acids gives reproducible results. A 0.5-gram sample weighed accurately (by difference) in a Lunge pipet is introduced into a 250-ml. Erlenmeyer flask. Ten milliliters of 2N dichromate solution are pipetted into the same flask and about 10 ml. of concentrated sulfuric acid are added. The flask is heated for 10 minutes in a boiling water bath. Its contents are then transferred to a 250ml. volumetric flask and made up to 250-ml. voIume with water. .4 lOy0 potassium iodide solution is then added to aliquot parts (25 ml.) and the iodine determined by titration with 0.l.V thiosulfate. A blank is run in the same way and a t the same time. The empirical factor used for the calculation of the results is taken from experiments with solutions of known amounts of pure sulfonic acids. The equivalent weight of the sulfonic acid is determined by the method of Weiss and others (77) which is a modification of the ASTM method (7). Water is in general determined by the
1 936
Figure 1.
100
0
IO
Figure 2.
20
SULFURtC ACID Solubility o f p-toluenesulfonic acid at
30
40
50
60
fO
30" C.
80
90
0% 100
SULFURIC ACID Comparison o f solubility of p-toluenesulfonic acid
-.-.-
30' C .
- - - 460'5 O c. C. difference between 100% and the sum of the acids. However, this is occasionally checked by direct determination of water with Karl Fischer reagent in a solvent of dioxane and pyridine (8). The direct determinations are in excellent agreement with the results obtained by the method of differences.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Procedure
The Phase sulfonic acidsulfuric acid-water is investigated in the following way. The sulfonk acid together with sulfuric acid and water is shaken in a thermostat for a few hours and then left undisturbed, generally
100% 0
IO
26
30
Figure 3.
40
60
60
70
80
90
100%
SULFURIC ACID Solubility of p-xylenesulfonic acid - - - 3600'' C. C.
toluenesulfonic acid at 30" C. with tie lines converging on the solid of the composition C7H&03H.HgO. It follows that p-toluenesulfonic acid crystallizes with 1 molecule of water. Figure 2 compares the solubility of p-toluenesulfonic acid at 45" and 60" C . with that of 30" C. I t can be seen that the solubility increases with rise of temperature. Figure 3 shows the solubility of pxylenesulfonic acid at 30" C. and at 60" C. Figure 4 shows the solubility of a mixture of sulfonic acids from naphtha extraction. Although this mixture is liquid a t ordinary temperatures a considerable separation of sulfuric acid is possible. The lower layer can be obtained practically free from sulfonic acids while the upper layer may contain as much as 25YG of sulfuric acid. The curves define the limit within which the composition of the two liquid layers vary. The straight lines connect compositions of the upper and lower layer in equilibrium with each other.
Conclusions Ternary phase diagrams have been determined for the systems sulfonic acidsulfuric acid-water for the sulfonic acids of toluene (at 30°, 45", and 60" C . ) , p-xylene (at 30" and 60" C.), and a straight-run petroleum naphtha boiling from 130" to 170" C. (at 30" (2.). With each of the three compounds, maximum separation occurred at a spent acid composition of about 65YG sulfuric acid and 3570 water. Optimum conditions for effecting separation can be established by use of the diagrams presented.
20
Literature Cited (1) American Society for Testing Ma-
0
IO
20
30
40
50
60
70
80
90
100%
SULFURIC ACID
Figure 4. Solubility at 30" C. of sulfonic acids prepared from an aromatic naphtha extract Curves define limits within which the composition of the two liquids vary. positions of upper and lower layer in equilibrium with each other
overnight. Samples are taken for analysis from the two phases which are in equilibrium with each other. In pure sulfonic acids one of the phases is solid; in this case a sample of the wet solid is occasionally taken. The composition of the pure solid is extrapolated from
Straight lines connect com-
the known compositions of the liquid and of the wet solid by the method of wet residues (9).
Resut's The results are shown in the diagrams. Figure 1 shows the solubility of p-
terials, Philadelphia 3, Pa., "ASTM Standards on Petroleum Products," D-855-52T p. 345, November 1953. (2) Crouch, W. W. (to Philipps Petroleum Co.), U. S. Patent 2,628,200 (Feb. 10, 1953). (3) Englund, S. W., Aries, R. S., Othmer, D. F., IND. END. CHEM.45, 189 (1953). (4) Gilbert, E. G., Jones, E. P., Zbid., 43, 2022 (1951). (5) Gillespie, R. J., Leisten, J. A . . Quart. Revs. (London) 8, 40 (1954). (6) Kirk, R. E., Othmer, D. F., "EncycloDedia of Chemical Technolow." bol. 13, p. 327, Interscience, s e w York, 1954. Meyer, Hans, Ann. 433, 327 (1923). Mitchell, John, Smith, D. M., "Aquametry," p. 245, Interscience, New York, 1948. Schreinemakers, F. A. H., Z. physik. Chem. 11,75 (1893). Vogel, A. I., "Textbook of Practical Organic Chemistry," 2nd ed., p. 532, Longmans, Green and Co. Ltd., London, 1951. Weiss, F. T., Jungnickel, J. S., Peters, E. D., Heath, F. D., Anal. Chem. 25, 277 (1953). RECENEDfor review June 6 , 1955 ACCEPTED February 27,1956 VOL. 48, NO. 10
OCTOBER 1956
1937
