Constitution of Organic Acids Prepared from Colorado Oil Shale BASED ON n-BUTYL ESTERS W. E. ROBINSON, J. J. CUMMINS,
AND
K. E. STANFIELD
Petroleum and Oil-Shale Experiment Station, U. S. Bureau of Mines, laramie, Wyo.
A
KSOWLEDGE of the piincipal chemical structuies in the mixture of organic material, commonly called kerogen, in Colorado oil shale would be of value as a basis for the selection of favorable reactions and processing conditions for converting oil shale to liquid fuels. Only about 15y0 of the kerogen is soluble in common organic solvents. Consequently, to determine the constitution of the major portion of the kerogen, it is necessary to convert, or degrade, the material to soluble products which are more amenable to analysis. Heating in the presence or absence of hydrogen and oxidation has been used extensively t o degrade polymers, cellulosic materials, and the fossil fuelspetioleum, coal, and oil shale. This paper describes a study of the constitution of oiganic acids degraded from Colorado oil shale by oxidation x i t h alkaline potassium permanganate. The oxidation conditions were selected to obtain acids containing some of the structures present in the original kerogen. The acids Tvere then converted to n-butyl esters for characteiization. Based upon these esters, two general types of acids were derived from the kerogen-dicarboxylic acids of the alkane series ranging principally from oxalic to adipic acid, and a series of dicarboxylic acids of higher molecular weight which were not identified. The latter acids appear to consist predominantly of saturated cyclic structures rather than aromatic or paraffinic structures. Degradation of kei ogen by oxidation with alkaline potassium permanganate has several advantagrs. The method has been studied extensively as a means of drgrading compounds of known composition and type ( 2 , 6, 14, ZO), and for characterizing complex materials of unknown composition (1, 3-5, 7 , 8, 13, I S ) . Based upon this background of published information, the oxidation reactions are believed t o be predominantly degradative and are not complicated by secondary reactions, such as polymerization, which may occur in pyrolytic degradations. The degree of oxidation can be controlled to obtain products within different ranges of molecular weight. Thus, kerogen in Colorado oil Ehale is oxidized almost completelv to carbon dioxide and oxalic acid by treatment for 100 hours with a boiling solution of potassium permanganate, but yields variable amounts of organic acids by brief treatments xvith small amounts of the reagent 1134
( 1 5 ) . The organic acids formed by the oxidation are solul~lein the alkaline solution and can be separated from the oil-shale minerals. The organic materials in oil shales from different deposits may vary in composition, but only minor variations are observed for organic materials in various grades of oil shale within a given deposit. Thus, the kerogens in oil shales of the Green River formation in the vicinity of Rifle, Colo., exhibit minor physical varia.tions, but their over-all compositions are very uniform and average 76.1Tc carbon, 10.5y0hydrogen, 2.67, nitrogen, 1.35% sulfur, and 9.5% oxygen (19).
Experimental Procedure Oil Shale Sample. The oil shale mas obtained from a bed approximately T3 feet thick, known as the RIahogany zone, a t the Bureau of Mines Oil-Shale Experiment Station, near Rifle, Colo. The sample contained about 347, organic material (29 3 5 organic carbon) and assayed 66.3 gallons of oil per ton by the modified Fischer retort method (18). Prior to the oxidation treatment the oil shale was crushed and screened to pass a sieve of 100 mesh per inch. Preparation of Organic Acids. The oil shale was oxidized in two steps by a boiling solution of alkaline potassium permanganate similar to the procedure described in an earlier report ( 1 5 ) . Approximately 250-gram samples of the shale and 2500 ml. of xater were heated to approximately 70" C. in an open container. Solid potassium hydroxide was added in the ratio of 1.6 parts by weight t o each part of organic carbon in the sample. This was followed by the gradual addition of 6 parts of solid potassiuni permanganate to each part of organic carbon. Usually, SUEcient heat was evolved by the reaction to maintain the mixture a t the desired boiling temperature; othemise, some additional heat was applied. hfter reduction of the reagent, the mixture was allowed to cool. The insoluble manganese dioxide and unoxidized shale were filtered off and again oxidized in the manner described above, except that 2 parts of potassium permanganate to each part of organic carbon (in the original shale sample) n-ere used.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 7
SYNTHETIC FUELS AND CHEMICALS The combined alkaline filtrates from the above oxidations were acidified with 1 t o 1 sulfuric acid and evaporated to dryness, which removed the volatile acids. The dried product was then extracted with methyl ethyl ketone t o remove the nonvolatile organic acids. (If the latter acids were incompletely removed, the residue was treated with a small amount of water. This solution of dissolved organic acids together with some potassium sulfate was diluted with methanol to allow the salt to crystallize and be removed by filtration.) Finally, the solvent was removed from the extracts by distillation under reduced pressure and the residual organic acids were dried in a vacuum oven a t 60' C. Several samples of the oil shale were oxidized in the above manner. Preparation of n-BGtyl Esters. Eight hundred and eighty grams of the organic acids were esterified by boiling with 4500 ml. of n-butyl alcohol and 90 ml. of concentrated sulfuric acid for about 100 hours. This was done in a distillation flask, which was surmounted in turn by a 29-inch, vacuum-jacketed column and an enlarged Dean and Stark tube for the periodic removal of about 515 ml. of water formed by the reaction. After the esterification was completed, about one half of the excess n-butyl alcohol was removed by distillation. A liter of toluene was added to the residue and the distillation was continued until most of the solvent had been removed. The esters were dissolved in diethyl ether, then washed with several portions of a 3 7 , solution of sodium carbonate until free of acids, and with water until free of sodium carbonate. Finally, the crude esters (852 grams) nere recovered by drying the ether solution over anhydrous sodium sulfate and distilling off the solvent. All distillations were made under reduced pressure. Fractionation of %-Butyl Esters. The crude esters were fractionated in two steps. Five-milliliter fractions of the more volatile esters were removed a t temperatures up t o 110" C. by distillation in a still pot a t a n absolute pressure of 150 microns. These fractions were combined into nine larger fractions based upon their refractive indices. The residual esters were then fractionated a t 7- to 10-micron pressure in a centrifugal molecular still. Ten distillate fractions and a nondistillable residue were obtained, which corresponded to step increases in the temperature of the rotator up to the final temperature of 230' C. Properties of %-Butyl Esters. Refractive indices of the n-butyl esters were obtained by an Abbe-type refractometer, and densities were determined in Lipkin pycnometers of 0.5- or 1.0-ml. capacity. Saponification equivalents were determined by potassium hydroxide in diethylene glycol by a method similar t o that of Shaefer and Balling (16). The excess alkali was backtitrated with standard acid by means of a titrator. The ultimate compositions of the esters were determined on macro-sized samples by conventional methods of analysis: Carbon and hydrogen were determined by a combustion train, nitrogen by Kjeldahl digestion, sulfur by ignition in a Parr oxygen bomb, and oxygen was calculated as the difference between 100% and the sum of the percentages of other components. Molecular weights were determined by the rise-in-boilingpoint method, using benzene as the solvent and the apparatus described by Ketchum (9). I n these determinations, dibutyl sebacate and o-dibutyl phthalate were used as internal standards with the method of Kitson, Oemler, and Mitchell (10). Infrared and mass spectra were used t o determine the chemical structures of the esters. However, t o identify the esters by xray diffraction, it was first necessary t o convert them to crystalline derivatives. Crystalline benzylamides were prepared from several fractions by a method similar t o t h a t described by Stafford, Francel, and Shay ( 1 7 ) . A 1-ml. portion of the fraction was refluxed with 3 ml. of benzylamine and 0.1 gram of ammonium chloride for 1 hour. After slight cooling, the mixture was poured into 30 ml. of benzene and the precipitated amide was removed by filtration. The amine was then dissolved in hot ethanol and reprecipitated by pouring into 1N hydrochloric acid, July 1956
Table 1.
Step 1 2
Total a
Distribution of Organic Carbon in Products from Oxidation of Kerogen Total Carbon in Kerogen, % VolaNonvolaUnoxitile tile dized Cos acids acids carbona
Time, Min. 75
180 255
9.5
5.7 15.2
0.2
36.5
0.1
__ 21.3
0.3
57.8
53.8 26.7
Obtained by difference.
followed by filtering and washing with 507, ethanol. X-ray diffraction patterns of the dry, crystalline products were made, then compared with those of benzylamides obtained from known esters in the same manner.
Experimental Results and Discussion The previous paper ( 1 6 ) showed that the organic material in Colorado oil shale was almost completely oxidized to carbon dioxide and oxalic acid by treatment with a n excess of boiling alkaline potassium permanganate ( 18 parts of potassium permanganate to 1 part of organic carbon) for 100 hours; the successive oxidations of the shale with one part of potassium permanganate to each part of organic carbon converted 81.1% of the kerogen t o organic acids; and approximately one half of these acids were of high molecular weight and were precipitated by acidifying the alkaline solution of the oxidation products. An equal portion of the organic acids was of lower molecular weight and remained soluble in the acidified solution. This paper is concerned principally with organic acids which were not precipitated by acidifying the alkaline solution of the oxidation products. T o increase the yields of these acids, larger quantities of potassium permanganate (6 parts of potassium permanganate to 1 part of organic carbon in the initial treatment and 2 parts of potassium permanganate to 1 part of organic carbon in the subsequent treatment) were used in the oxidations. The crude oxidation product was a brown mixture of amorphous and crystalline solids consisting of organic acids ranging from oxalic acids t o acids with molecular weights of approximately 800. The crude acids were partially soluble in water, diethyl ether, acetone, methyl ethyl ketone, methanol, and ethanol, but were almost completely insoluble in pentane, cyclohexane, benzene, chloroform, and carbon tetrachloride. Table I shows the results of a typical oxidation of the oil shale for a total of 4.25 hours. Thus, 57.8Y0 of the total organic carbon in the kerogen was converted to the crude, nonvolatile acids used in-this study, 15.270 was completely oxidized to carbon dioxide, 0.3% was converted t o volatile acids, principally acetic acid, and 26.77, of the initial carbon was unoxidized. The crude acids were converted to n-butyl esters t o facilitate their fractionation by distillation a t reduced pressure. The resulting 20 distillate fractions listed in Table I1 had densities of 0.9952 to 1.0865, refractive indices of 1.4230 to 1.4840, nuclear weights (the molecular weight of the ester fraction minus 100 times the number of ester groups) of 2 t o 241, and an average of two ester groups per molecule. As shown in Table 111, all of the distillate fractions contained nitrogen and sulfur, but only the sulfur contents (1.81 to 4.02YG) of fractions 1 t o 4 were unusual. The atomic hydrogen-carbon ratios increased from 1.86 to 1.93 for fractions 1 t o 4, but decreased from 1.93 to 1.68 for fractions 10 t o residue. Several n-butyl esters of the alkane series of dicarboxylic acids were identified in fractions 1 to 10 by infrared and mass spectral
INDUSTRIAL AND ENGINEERING CHEMISTRY
1135
Table II.
Fraction
Yield, %
1 2 3 4 5 6
40.6 2.6 2.7 2.1 2.2 2.6 2.1 2.2 2.2 1.5 1.0 1.3 1.8 1.4 2.1 1.6
7 8 9 10 11 12 13 14
15 16 17 18 19 20
1.7 1.5
1.7 2.3 22.8 ___ 100.0
Residue Total a
Physical Properties of n-Butyl Ester Fractions
Distillation Data Pressure, P Temp., 150 150 150 150 150 150 150 150 150 7-10 7-10 7-10 7-10 7-10 7-10 7-10 7-10 7-10 7-10 7-10 7-10
O
C.
dao"
n2O"
0.9952 1.0050 1.0040 1.0036 1.0012 0.9931 0.9912 0.9897 0.9843 0.9877 0.9924 0.9963 1.0061 1.0121 1.0182 1.0215 1.0267 1.0277 1.0287 1.0117 1.0865
1.4230 1.4260 1.4278 1.4295 1.4315 1,4350 1.4380 1,4405 1.4450 1.4545 1.4570 1,4600 1,4640 1,4670 1.4720 1.4745 1.4765 1.4785 1.4805 1.4840 1.520
40
55- 60 65- 75 75- 85 75- 85 75- 85 85-100 100-1 05 100-105 105-110 110 120 130 140 150 160 170 180 190 200 230 230
Molecular Weight Calcd. Deterfor mined nucleus 207 216 234 238 239 236 248 254 271 307 304 308 333 367 395 410 431 462 457 480 1041
2 25 41 41 48 47 51 57 76 115 119 121 137 151 165 173 202 229 225 241 560
No. of Ester Groups" 2.05 1.91 1.93 1.97 1.91 1.89 1.97 1.97 1.95 1.92 1.85 1.87 1.96 2.16 2.30 2.37 2.29 2.33 2.32 2.39 4.79
Calculated from saponification equivalents and molecular weights of fractions. ~~
~~~
analyses of the esters, and by x-ray diffraction analyses of their crystalline benzylamine derivatives. A s shown in Table Is', the n-butyl esters of oxalic, succinic, glutaric, and adipic acids were identified by all three methods, while esters of malonic, pimelic, and suberic acids a-ere identified only by the mass or infrared spectra. I n addition, mass spectra indicated the presence of a trace of n-butyl azelate. Based only on mass spectra analyses, the crude esters contained approximately 42.4% butyl oxalate, 7.1CCbutyl succinate, 6.0Yc butyl glutarate, 1.9% butyl adipate, 1.1% butyl pimelate, 0.5% butyl suberate, 18.2YC unidentified distillate, and 2223Yc nondistillable residue. The experimental data indicate that a t least two general types
Table 111. Fraction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Residue
Ultimate Composition of n-Butyl Ester Fractions Atomic Weight % of Fraction H/C C H N S 0" Ratio 57.34 57.01 59.20 57.50 61.44 63.72 63.38 62.97 64.79 66.86 66.87 67.30 67.54 68.05 67.43 67.22 67.66 68.22 68.78 69.08 69.19
8.91 9.08 9.49 9.25 9.77 9.76 9.71 9.62 9.79 9.77 9.68 9.62 9.66 9.64 9.44 9.33 9.36 9.55 9.64 9.68 9.66
0.01 0.02 0.02 0.04 0.02 0.02 0.03 0.04 0.10 0.25 0.35 0.41 0.46 0.56 0.65 0.67 0.67 0.64 0.63 0.65 1.20
1.81 3.64 3.78 4.02 2.05 0.45 0.38 0.30 0.13 0.08 0.17 0.20 0.34 0.28 0.29 0.22 0.15 0.22 0.22 0.24
1.55
31.93 30.25 27.51 29.19 26.72 26.05 26.50 27.07 25.19 23.04 22.93 22.47 22.00 21.47 22.19 22.56 22.16 21.37 20.73 20.35 18.40
1.86 1.91 1.92 1.93 1.91 1.84 1.84 1.83 1.81 1.75 1.74 1.71 1.72 1.70 1.68 1.67 1.66 1.68 1.68 1.68 1.68
Calculated as difference between 100% and sum of percentages of other elements. a
1136
of esters were prepared from the kerogen acids. The more volatile fractions x-hich resulted from extensive degradation of the kerogen consisted, in part, of n-butyl esters of the lower members of the alkane series of dicarboxylic acids. These were identified by the spectra and x-ray diffraction analysis mentioned above and were further indicated by the properties of the fractions. Thus, fractions 1 t o 10 contained an average of two ester groups per molecule; their refractive indices and atomic hydrogencarbon ratios were within the ranges expected for esters of the above type. Kone of the properties of the fractions, nor analyses for unsaturated structures by infrared spectra, indicated the presence of aromatic structures. However, the fractions contained constituents other than esters of the alkane dicarboxylic acids. I n particular, fractions 1 to 5 contained 1.81 to 4.0270 sulfur from some unknown source and fractions 6 to 20 contained increasing amounts of esters of a different type. The typical structures or components in the ester of higher molecular weight were not determined. Individual esters could not be identified by mass and infrared spectra; neither could
Table
Fraction 1 2 3 4 5 6 7 8 9 10
IV.
n-Butyl Esters Identified by Mass and Infrared Spectra and X-Ray Diffraction" %-Butyl Ester of Following Acids MaSUC- GluPiOxalic lonic cinic taric Adipic melic Suberic MXI MXI MX M MXI I MX M MXI I MXI MX MI I MXI MX MXI MXI MX MXI MXI MX M MXI MXI M M M MI MXI M M M M M M
a I. Infrared spectra. M. Mass spectra. X. X-ray diffraction.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 7
SYNTHETIC crystalline benzylamide derivatives be prepared from fractions 10 t o 20 for identification by x-ray diffraction. Nevertheless, some information concerning the nature of the esters can be deduced from the properties of these fractions. I n general, the molecular weights ranged from 304 to 480, but the compositions of the individual fractions were very similar and contained an average of 67.73% carbon by weight, 9.58% hydrogen, 0.54% nitrogen, 0.22% sulfur, and 21.93% oxygen. _. 'lhe atomic hydrogen-carbon ratios decreased with increase in molecular weight and averaged about 1.7,
.e4 100
July 1956
I
I 300
A N D CHEMICALS
I 400
I 500
MOLECULAR WEIGHT
Figure 1.
ular weight. (The opposite trend would be shown by the alkane series.) Esters of monoor dibasic acids of the alkane series were not detected in fractions 10 t o residue. These points are further illustrated in Figure 2, which shows the type formulas (based only on carbon and hydrogen) of several known materials plotted against the kerogen ester fractions. The normal alkane series has a type formula of CnH2n+2, cyclohexane C,H2,, and benzene C,Hz,-a. At least three factors can alter the type formula of a given series-namely, the presence of carbonyl groups, unsaturated groups, and ring structures. Each causes a decrease of 2 in the hvdrogen value-for examde. one double bond changes the alkane series from CaHzn+z t o CnHzn. Type formulas were calculated for each of the kerogen ester fractions on a nuclear basis. Nitrogen and sulfur were not con"
I e00
FUELS
&
I
Specific refraction-molecular
- Cn Hzn-lo - Cn "2n-8 4 3
8
weight relation of n-butyl esters
-
- CnH2n-6 Q - Cn H 2 ~ - 4
w 'CnHpn-2 L
> +
-CnH2n
c-c- c-c- c
- 'nH2n+2
-
I
1
l
l
l
l
I 2 3 4 5
l
l
l
l
.
l
l
l
l
I
l
l
l
i
l
l
l
6 7 8 9 IO II 12 13 14 15 16 17 18 1920 RES ESTER FRACTIONS
Figure 2.
Type formulas of various materials
INDUSTRIAL AND ENGINEERING CHEMISTRY
1I31
Table V.
Fraction 10 11
12 13 14 15 16 17 18
19 20 Residue Kerogen
Empirical Formulas and Atomic Ratios of Nuclei of Kerogen n-Butyl Esters Empirical Formula of Nuclei
H/C 1.95 1.92 1.85 1.85 1.81
1.77 1.74
1.72 1.75 1.75 1.76 1.73 1.67
Atomic Ratios O/C N/C 0.080
0.092 0.076 0.079 0.060 0.084 0.099 0.109 0.103
0.089 0.083 0.067 0.094
0.008 0.011
S/C
0.001
0.002 0.011 0.003 0.012 0.003 0.015 0.003 0.018 0.003 0.017 0.003 0.016 0.002 0 . 0 1 4 0.002 0.014 0.002 0.014 0 , 0 0 3 0.025 0.002 0.030 0.006
those present in the original keiogen, as the atomic ratios of the elements in the ester nuclei were similar to those in the original lie1ogen. The production of essentially only dicarboxylic acids indicates that Colorado oil-shale kerogen does not have a highly condensed structure, as multifunctional acids containing more than two carboxyl groups are also produced by oxidation of this t)-pe structure. The kerogen appears to be substances of high molecular weight, composed predominantly of saturated i ings, hich may be partially condensed or connected by short aliphatic chains. Oxygen q a y occur as carbonyl groups, hydroxyl gioups, or ether linkages. Presumably, substances of this tvpe would be oxidized progressively by alkaline permanganate into series of saturated dicarboxylic acids of Ion-er molecular weight until, finally, the ring structures x-ould be ruptured t o yield aliphatic dicarboxylic acids of six caibon atoms, or less, plus carbon dioxide.
Summary Oxidation with alkaline permanganate is one means of degr ading the complex organic material, or kerogen, in Colorado oil shale t o series of organic acids which are more amenable to analysis and may be indicative of the structures in kerogen. This study of the constitution of the kerogen acids is based upon the properties of their n-butyl esters. The kerogen acids were principally dicarboxj-lie, of t x o types: (1) Fifty-nine per cent of the oxidation product consisted of dicarboxylic acids of the alkane series-namely, oxalic, succinic, glutaric, and adipic acids. These acids resulted from extensive degradation of kerogen and its oxidation products of higher molecular weight. (2) Forty-one per cent of the oxidation product consisted of dicarboxylic acids with higher molecular weights up to about 800. None of the acids in this series was
1138
identified, but based upon the properties of their n-butyl esters, the acids appeared to consist predominantly of saturated cyclic structures rather than aromatic or paraffinic structures. These acids contained approximately one atom of oxygen per molecule in excess of that present in acid groups. Identification of additional components of the acids of higher molecular weighr would contribute t o a better understanding of the structure of kerogen, as the nuclear portion of the acids appeared to be similar to kerogen itself.
Acknowledgment This project was conducted under a cooperative agreement between the Bureau of Mines, United States Department of Interior, and the University of Wyoming. Special acknowledgment is made to G. L. Cook, R. A. Meyer, F. M. Church, D. 6 . Earnshaw, H. N. Smith, H. H. Heady, J. A. Lanum, Jr., and R. R. VanDeventer for their assistance in the analyses presented in the report.
Literature Cited Bone, W. A., Horton, L., Ward, S.C., PPOC. R o y . SOC.(London) 127A, 480-510 (1930).
Bone, W. A., Parsons, L. G. B., Sapiro, R. H., Groocock, G. lI., Ibid., 148A, 492-522 (1935).
Dancy, T. E., Giedroyc, V,, J.Inst. Petroleum 36, 607-23 (1950). Down, A. L., Himus, G. W., Ibid., 27, 426-45 (1941). Dunstan, A. E., “Oil Shale and Cannel Coal.” vol. 1, pp. 115-23. Institute of Petroleum, London, 1938. Gilman, H., “Organic Chemistry,” vol. IV, p p . 1122, 1211-14, Wiley, New York, 1953. Juettner, B., Smith, R. C., Howard, H. C., J. Am. Chem. Soc. 59, 236-41 (1937).
Kent, C. R., Australian Chein. Inst. J. a n d Proc. 6, 203-23 (1939).
Ketchum, D., Anal, Chem. 19, 504-5 (1947). Kitson, R. E., Oemler, A. N., Mitchell, J., Ibid., 21, 404-7 (1949).
Lange, N . A,, “Handbook of Chemistry,” 8th ed., p. 1421, Handbook Publishers, Sandusky, Ohio, 1952. Lanin, V. A., Pronina, &I. V., Bull. Acad. Sci. C9.R.S.S., Classe Sci. Tech. 1944, 745-51. Mariani, E., Spinelli, F., Ann. chim. (Rome)40, 512-26 (1950). Randall, R. B., Benger, M., Groocook, G. M., PTOC.Roy. SOC. (London) 165A, 432-52 (1938). Robinson, W. E., Heady, H. H., Hubbard, A. B., IXD. Esc,. CHEM.45, 788-91 (1953). Shaefer, W. E., Balling, W.J., Anal. Chem. 23, 1126-8 (19.51). Stnfford, R. W., Francel, R. J., Shay, J. F., Ibid.. 21, 1454-7 (1949).
Stanfield, K. E., Frost, I. C., U. S.Bur. Mines, Rept. Invest. 4477 (1949).
Stanfield, K. E., Frost, I. C., lIcAuley, W. S., Smith, H.
S..
Ibid., 4825 (1951). Ward, J. J., Kirner, W.R.. Howard, H. C.. J . Am. Chem. Soc. 67, 246-53 (1945). RECEIVED for review October 24, 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTED February 6 , 1956
Vol. 48, No. 7
