Fractionation of Water-Soluble Acids Obtained
by Alkali-Oxygen Oxidation of Coal R. S. MONTGOMERY
AND
P. J. SIENBNECHT
The Dow Chemical C o . , M i d l a n d , M i c h .
Here's how these water-soluble organic acids
. . . can be easily and inexpensively fractionated . . . can be sublimed under vacuum to produce anhydride and acid mixtures
C
OAL can be oxidized in alkaline media t o yield a mixture of
water-soluble, predominantly aromatic acids. This mixture of acids can be produced cheaply, but commercial exploitation has been retarded because of t h e lack of applications for such a complex mixture. Commercial applications could be more readily developed if this complex mixture could be broken up into simpler mixtures, or, more desirably, separated into pure
compounds. The work reported in this paper a as carried out in a n effort t o develop a commercially practical fractionation of these coal acids. T h e coal acids have been fractionated by conversion t o their esters and subsequent crystallization ( 3 ) or molecular distillation (6) of these esters. A solvent fractionation method (4,6 ) has also been used, which entails t h e addition of a nonsolvent t o a solution of t h e acids, thereby causing t h e precipitation of a portion of t h e mixture. These methods are not suitable for t h e large scale commercial fractionation of t h e coal-acid mixture. Two fractionation methods of possible commercial importance were investigated in the present study. T h e first mas a solvent fractionation more convenient than t h a t used b y the previous investigators. T h e second was a p H separation which made use of t h e differences in acid strength of t h e components of t h e mixture. T h e acids used in this study mere obtained b y t h e oxidation of a n aqueous alkaline suspension of a Pocahontas b o . 3 coal by gaseous oxygen a t 270' C. and total pressures of about 900 pounds per square inch gage (1, 2 ) . I n the reaction, approximately 500/, of the carbon of the coal was converted t o carbon dioxide and t h e balance t o water-soluble, organic acids which were recovered, after acidification of the solution with sulfuric acid, by extraction with methyl ethyl ketone. The mixed acids, after vacuum drying, were a yellow, friable solid. T h e mixture is nearly completely soluble in oxygenated solvents such as water and the lower boiling alcohols, ketones, ethers, and esters, and insoluble in low boiling aliphatic and aromatic hydrocarbons. SOLVENT FRACTIONATION
For the solvent fractionation technique used in t h e present study, a n aqueous solution of the coal acids was extracted with a sequence of mixtures of a solvent and nonsolvent. T h e extracting solvent mixture was made progressively richer in t h e solvent component. T h e amount of t h e coal acids extracted as a function of t h e proportion of the solvent (methyl ethyl ketone) in the extracting mixture is plotted in Figure 1. A maximum a t about 82% methyl ethyl ketone was obtained when both carbon tetrachloride and petroleum ether were used as the nonsolvents. T h e molecular weight, equivalent weight, and functionality of t h e fractions are plotted as functions of t h e proportion of t h e solvent in t h e extracting mixture in Figures 2, 3, and 4, respectively.
Figure 1
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Figure 2 1274
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Figure 5
Figure 4
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p H O F S O L U T I O N BEFORE E,XTRACTlON
Both molecular weight and functionality tend t o increase as the proportion of t h e solvent is increased. T h e equivalent weight, however, tends to decrease as the proportion of t h e solvent increases. It seems logical t h a t t h e more polar components, of lower equivalent weight, should be more difficult t o extract from a n aqueous solution with a mixture of organic solvents than the less polar components, of higher equivalent weight. pH FRACTIONATION
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T h e fractionations based on t h e differences in acid strength of t h e components were carried out b y making a n aqueous solution of t h e coal acids basic and then acidifying in steps and extracting with methyl ethyl ketone between t h e additions of acid. As can be seen from Figure 5, t h e largest fractions were obtained at a p H of approximately 3.0. T h e molecular weights of t h e fractions are plotted in Figure 6. They decrease from a value of about 310 a t a p H of 2.2 t o a minimum of about 210 a t a p H of 4.7 and then increase again t o a value of 500 a t p H 8.2. The equivalent weights of t h e fractions are plotted in Figure 7 and are in sigmoidal form, with t h e higher equivalent weights obtained a t the less acidic p H values. T h e functionality of t h e fractions (Figure 8) increases as t h e p H values decrease. T h e curve shows plateaus or inflection points at functionalities of approximately 1, 2, 3, and 4. T h e carbon-hydrogen ratios of t h e fractions are plotted in Figure 9. T h e carbon-hydrogen ratio shows a maximum of about 1.4 at p H 2.7, while the noncarboxylic oxygen is relatively constant a t about 1.5 atoms of oxygen per molecule from a p H of 2.0 t o a p H of 4 6 (see Figure 10). At p H values higher t h a n 4.6, t h e noncarboxylic oxygen increases with p H t o a value of 9 atoms of noncarboxylic oxygen per molecule at p H 8.0. These high values of noncarboxylic oxygen for the fractions obtained under essentially neutral conditions, however, may well be due, a t least t o some extent, to extraction of partial sodium salts by t h e methyl ethyl ketone. Both t h e solvent fractionation and t h e p H fractionation techniques are methods b y which primary fractionations can be achieved on the coal acids inexpensively and easily. T h e mixture of coal acids is obtained from the reactor in t h e form of a basic solution. It could either be acidified and extracted in steps or acidified completely and extracted with a sequence of solvent mixtures very easily without adding significantly t o t h e cost of the process. B y comparing the curves obtained using the solvent fractionation technique with those obtained using the p H method, i t can readily be seen t h a t the p H method is greatly superior. From the functionality curve (Figure 8 ) , it can be seen t h a t the coal-acid mixture could b e readily fractionated b y this technique into simpler mixtures of approximately one, two, three, and four acid groups per molecule. This would be very important for applications such as polyesters, where monobasic acids are undesirable.
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SUBLIM4TION
As shown in Table I, t h e coal acids can be vacuum sublimed a t about 1 mm. of mercury t o yield a nearly white mixture of phthalic anhydride, methylphthalic anhydride, benzoic acid, and toluic acid. A yield of sublimate of only about 13y0,however, was obtained under t h e conditions used. T h e most important component of t h e mixture Mas phthalic anhydride, which amounted t o about three quarters of the sublimate. From t h e data, it is evident t h a t at least t h e main portion of t h e sublimate was not present in t h e original coal acid mixture, but resulted from a thermal decomposition of some of the components of t h e original mixture. If this were not the case, materials as volatile as phthalic anhydride and benzoic acid would be sublimed rapidly and not require the length of time and the high temperature necessary t o obtain the maximum yield of sublimate. Subli nation is not very useful when applied t o the entire coalacid mixture because of the low yields of sublimate. However, it may be applicable t o fractions of t h e original mixture which have been obtained by some other fractionation method. This
Table I.
Time, Hours 0.5 2 4
0.5 2 4
0.5 2
4
Yield and Analysis of Sublimation Products of Coal Acids
Temp.,
C. 76
75 75 100 100 100 125 125 125
Portion Sublimed,
%
4 8 8 9
10
13 10 13 12
Phthalic anhydride,
%
67 76 76 77 78 77 76 78 72
Analysis of Sublimate Methyl phthalic Benzoic anhydride, acid,
%
%
24 15 15 15 14 14 15 14
17
Toluic acid,
%
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S o m e d a t a of p o s s i b l e theoretical interest were also obtained. The existence in the sublimate of materials containing a methyl group on the aromatic ring is especially interesting, as this type of compound has not previously been i d e n t i f i e d among the coal o x i d a t i o n products. Their presence indicates the presence of structures containing methyl groups not only in the oxidation products of coal but also in the coal polymer itself.
Table 11. Yields and Analysis of Sublimation Products of Coal Acid Fractions Analysis of Sublimate
PH Range
Portion of Orig. Mixture, %
8.0-G,3 6.3-5.5 8.0-6.1
2 6 2
GO 70
9
Portion Sublimed,
Mass
120,
Unaccounted for,
%
%
%
pH Fractionation, Sublimed 1 Hour a t 90’ C. 49 9 1 77 13 40 49 1 45 5 GO 14 2 68 16
.. ..
%
Phthalic anhydride,
Methylphthalic anhydride,
%
%
Benzoic acid,
%
Toluic acid,
..
.. ..
..
Mixed Solvent Fractionation, Sublimed 2 Hours a t 130” C. 80
18 24
90
22 10
95
17 23
65 39 GO 34 5
5
4 .
4
..
5 4 2 1
.. .. .. ..
..
possibility was investigated by the sublimation of fractions obtained using the pH technique. The weakly acidic, monobasic fractions which were obtained a t near neutral values of p H were selected because it was felt that they would be more volatile than the polybasic acids. As expected, substantially larger yields of sublimate were obtained from these fractions (see Table 11). Also as expected, the sublimate contained much more of the monobasic acids. I n the p H range of 8.0 to 6.3, where Figure 8 indicates the fractions to be essentially monobasic, 90% of the sublimate consisted of benzoic and toluic acids and only 10% consisted of dibasic acids. I n the pH range of 6.3 t o 5 . 5 , where Figure 8 indicates roughly equal amounts of monobasic and dibasic acids, 50% monobasic and 50% dibasic acids were obtained in the sublimate. These results show not only that sublimation may be advantageously applied to fractions of the coal-acid mixture, but also thbt the functionality data plotted in Figure 8 are substantially correct. 600
EQUIVALENT WEIGHT OF RECOVERED FRACTIONS I O F C O A L A C I D S A S A F U N C T I O N O F E X T R A C T I O N PH
29 55 36 62 89
1 2 2 3
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ACKNOWLEDGMENT
T h e authors wish t o express their appreciation to C. H. Ruof, Coal Research Laboratory, Carnegie Institute of Technology, for the sample of the coal acids and t o R. S. Gohlke and Denzel Deline for the mass spectrographic and molecular weight determinations, respectively. LITERATURE CITED
Franke, N. W., and Kiebler, M. W., Chem. Inds., 58, 580 (1946). Franke, iY.W., and others, IXD.ENG.CHEM.,44, 2784 (1952). Howard, H. C., “Chemistry of Coal Utilization,” H. H. Lowry, ed., pp. 363-74, Wiley, New York, 1945. (4) Roy, 4 . N., and Howard, H. C . , J . Am. Chem. Soc., 74, 3239 (1952). (5) Ruof, C. H., Savich, T. R., and Howard, H. C., Ibid., 74, 3239 (1952). RECEIVED for review November 26, 1954. ACCEPTEDFebruary 18, 1955. Presented before the Division of Gas and Fuel Chemistry a t the 126th Meeting O f the A U E R I C A N CHEMICAL SOCIETY, New Y o r k , N. Y . , 1954.
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