Energy & Fuels 1994,8, 932-936
932
Solvent Swelling Studies of Green River Kerogen John W. Lament and Shang Li Department of Chemistry, 6 East Packer Avenue, Lehigh University, Bethlehem, Pennsylvania 18015 Received December 8, 1993. Revised Manuscript Received February 15, 1994"
Kerogen has been isolated from Green River oil shale by dissolving the minerals in HF and HC1 and also by using aqueous ammonium sulfate. The resulting kerogen samples were swollen in 28 different solvents. The volumetric solvent swelling of the kerogen roughly follows the predictions of regular solution theory for almost all of the solvents including good hydrogen bond acceptors. The solubility parameter from the swelling studies is close to that calculated from Siskin's structure. Hydrogen bonding does not seem to be an important factor because good hydrogen bonders show swellings similar to non-hydrogen bonders of similar solubility parameter. The inorganic content of the isolated kerogen does not have a large effect on solvent swelling. Samples containing 42% inorganics generally swell somewhat more than samples containing 5 % inorganics. The cross-link density of this kerogen is high.
Introduction
extensively used in the work reported here, it is reprinted in Figure 1. Kerogens are the major organic constituents of sediTo apply solvent swelling to kerogen, one first needs to mentary rocks. Their chemical structures and geochemical isolate the kerogen. An ideal isolation method will recover alterations have been studied intensively.'v2 Kerogens are all organic components without alterting their structures. cross-linked macromolecular networks, but, except for Kerogens can be isolated by both physical and chemical coals, their network structure has not been investigated. separation methods.8 Chemical methods are more effecIn the hope of providing useful geochemical insight, we tive and most commonly used. Among them, HF/HC1 have begun to characterize the macromolecular structure demineralization is the classical and the most effective of Type I and I1 kerogens. This paper contains our initial one. Durand and Nicaise reviewed effects of HC1 and HF results from solvent swelling studies of kerogen isolated on kerogen." IR studies of Torbanite oil shale from from Green River oil shale. Australia revealed no obvious changes after HC1 and HF Our ultimate goal is to develop the techniques necessary treatment except the disappearance of silicate absorptions for characterizing the macromolecular structure of kerogen near 1100 cm-l. McCollum and Wolf recently suggested and to use that characterization to gain insight into the the use of aqueous caustic followed by acid demineralgeochemical process of kerogen maturation. Many techization to avoid the use of toxic HF.' The method was niques for characterizing macromolecules are available. demonstrated to be almost as effective as HF/HC1 One of the simplest methods is solvent swelling, which demineralization. The possibility that strong acid treathas had some success when applied to coals. Because ment may cause structural changes in kerogen during solvent swelling is experimentally easy and of demondemineralization is troubling. After determining that strated utility with coals, we elected to begin our work acid-base interactions between organics and minerals are with it. major forces binding Green River kerogen to the rock, Siskin developed a procedure which utilized ammonium We started our studies with kerogen isolated from Green sulfate to dissolve the carbonate rock and disrupt the River oil shale for two reasons: (1)a significant amount organic-clay interaction^.^ Their results demonstrated of structural work has been done on this shalela and (2) a mild procedure for its isolation has been d e ~ e l o p e d . ~ that aqueous ammonium sulfate (pH 5-6) treatment could remove 85 % of the shale minerals with 95 % recovery The early structure models were based on the results from of the organic matter. pyrolysis and mass spectrometry. Yen reviewed this work Solvent swelling has been used extensively to study coal and proposed a model.' Siskin recently published a macromolecular structure. W i l e quantitative interpretadetailed structure of Green River oil shale kerogen? tion of the results is often problematical, it is experimenBecause it is not widely available and because it is tally simple and has yielded significant insight into coal macromolecular structure and coal-solvent interAbstract published in Advance ACS Abstracts, April 1, 1994. (1) (a) Vandenbroucke, M. In Kerogen; Durand, B., Ed.; Editions actionsP12 Initially, the theory of solvent swelling of crossTechnip: Paris, 1980. (b) Yen,T. F. In Oil Shale; Yen,T. F.,Chilingarian, linked polymers developedby Flory and Rehner was used.13 G. V., Eds.; Elsevier Science: Amsterdam, 1976. Extensions of this theory by Kovac and by Peppas are (2) Tissot, B. P.; Welte, D.H. Petroleum Formation and Occurrence; Springer-Verlag: New York, 1978. better suited to the highly cross-linked C O ~ ~ S . It~ ~is J
-
~~
(3) Siskin, M.; Brons, G.; Payack, J. F. Energy Fuels 1987,1,248-256. (4) McCollum, J. D.; Wolff, W. F. Energy Fuels 1990,4, 11-14. ( 5 )Trewhella, M. J.; Poplett, 1.J. F.; Grint, A. Fuel 1986,65,541-546. (6) Siskin, M.; Scouten, C. G.; Rose, K. D.; Aczel, T.; Colgrove, S.G.; Pabst, R. E. Jr. Proceedings of the NATO ASZ Meeting: Composition, Geochemistry, and Conversion of Oil Shales; Kluwer Academic Publish-
ers: New York, in press.
(7) Siskin, M.; Brons, G.; Payack, J. F. Energy Fuels 1989,3,10&109. (8) (a) Durand, B.; Nicaiae, G.In Kerogen, Durand, B., Ed.; Editions Technip: Paris, 1980. (b)Saxby,J. D. InOil Shale, Yen, T. F., Chiliiarian, G. V., Eds.; Elsevier Science: Amsterdam, 1976. (9) Sanada, Y.; Honda, H. Fuel 1966,45, 295.
0887-0624/94/2508-0932$04.50/00 1994 American Chemical Society
~
Energy & Fuels, Vol. 8, No. 4,1994 933
Solvent Swelling Studies of Green River Kerogen
C
w
Figure 1. Siskin’s model of organic material in Green River Oil Sale? impossible to tell a priori whether these models will also be applicable to Type I and I1 kerogens. That is one of the issues to be addressed as soon as the necessary data are in hand. A few words describing the physical chemistry of solvent swelling will provide a qualitative introduction to the technique. Rigorous and necessarily mathematical treatments can be found in the references.10Js16 When brought into contact with a solvent, a cross-linked macromolecular system will absorb the solvent and swell. At equilibrium, the free energy change for dissolution of the solvent in the macromolecule is exactly balanced by the elastic restoring force of the network. The elastic restoring force is entropic and depends on the average length of the macromolecular The free chains between cross-links (branch points), energy for dissolution of the solvent in the polymer is expressed by the Flory x parameter. The Flory treatment assumes that regular solution theory is followed (no specific interactions such as hydrogen bonding occur). The resulting Flory-Rehner equation is
aC.
(10) Larsen, J. W.;Green, T. K.; Kovac, J. J. Org. Chem. 1985,50, 4729-4735. (11) Barr-Howell, B. D.;Peppas, N. A.; Winslow, D. N. Chem. Eng. Commun. 1986,43,301-315. Lucht, L. M.;Peppaa, N. A.Fuell987,66, 803-809. (12) Quinga, E.M.Y.; Larsen, J. W. In New Trends in Coal Science; NATO AS1 Series;Yurum, Y., Ed.;Kluwer Academic Publiihere: New York, 1988. (13) Flory,P. J. Principle of Polymer Chemistry;Cornell University Press: Ithaca, NY, 1953. (14) Kovac, J. Macromolecules 1978, 11,362-365.
where up is volume fraction of polymer at equilibrium swelling, V I is the solvent molar volume, and p is the polymer density. is the number average molecular weight between cross links. This equation does not hold unless the chain lengths between cross-links are large, i.e. ,&is large. When the distance between cross-links no longer follows a Gaussian distribution, another theory must be used. Both Kovac and Peppas have developed such theories. The principal change of Kovac’s theory is to remove the assumption of a Gaussian distribution of chain lengths and to introduce an additional parameter N, the number of rotatable segments between branch points. Kovac’s equation is given below
A few data on solvent swelling of kerogens exist. Green River oil shale kerogen isolated by base extraction was swollenwith five solvents yielding a very rough bell-shaped curve and a solubility parameter for the kerogen of about 9.5 ( ~ a l / c m ~ ) lThe / ~ .swellings ~ of a Type I (Green River), a Type I1 (New Albany), and a Type I11 (Pittsburgh No. 8) kerogen in 6 different solvents were measured gravimetrically and compared.ls Both Types I and I11 show a bell-shaped dependence on solubility parameter with the peak for Type I appearing to come at a lower solubility parameter than Type 111. No pattern emerged for the (15) Barr-Howell, B. D.;Peppaa, N. A. Polym. Bull. 1985,13,91-96.
(16)Shadle,L.J.;Khan,M.R.;Zhang,G.Q.;Bajura,R.J.Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1989, 34 (l),55-61.
Larsen and Li
934 Energy & Fuekr, Vol. 8, No. 4, 1994 Table 1. Elemental Analyses of Kerogen, Amount of Kerogen Isolated, and Amount of Bitumen in Green River Oil Shale. sample
%C
(NH&SOd72h HF/HCL 2 h HF/HCL8 h HF/HCL48h
40.3 40.7 59.1 73.3
a
%H 5.2 5.8 7.7 9.4
% ash 41.9 38.2 17.0 5.2
7% bitumen 6.4 6.1
% kerogen* 27.9 28.2 26.1 26.5
Galbraith Laboratories. b By weight percent of starting shale.
2.00 1.80
I
--
B 1.60 -1.40 -1.20 -.
Table 2. Solvents Used for Swelling Kerogen. no. 1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
solvent n-pentane n-heptane methylcyclohexane cyc1ohexane o-xylene toluene benzene tetralin chlorobenzene 1-methylnaphthalene carbon dieulfide o-dichlorobenzene nitrobenzene biphenyl propionitrile nitroethane acetonitrile chloroacatonitrile nitromethane pyridine tetrahydrofuran carbon tetrachloride 1,1,2-trichloroethane 1,2-dibromoethane 2-propanol ethanol acetone dimethyl sulfoxide
(caUcm9112 7.0 7.4 7.8 8.2 8.8 8.9 9.2 9.5 9.5 9.9 10.0 10.0 10.0 10.6 10.8 11.1 11.9 12.6 12.7 10.7 9.1 8.6 9.6 10.4 11.5 12.7 9.9 12.0
a Brandrup, J.; Immergut, E.H.Polymer Handbook, 3rd ed.;John Wiley & Sons, Inc.: New York, 1989.
Type I1 shale in the six solvents used. Finally, Siskin reported the volumetric solvent swelling of Rundle oilshale kerogen in nine solvents.7
Results Kerogens were isolated from Green River oil shale by using both (NH&$~OIand HF/HCl demineralization method^.^^^ The HF/HC1 procedure was carried out for three different time periods: 2,8, and 48 h. Followingthe 8-and 48-h demineralizations,toluene extraction was used to remove the bitumen. The elemental analyses yield of kerogen and amount of bitumen extracted are given in Table 1. The volumetric expansion of the kerogen was determined by measuring the height of a column of kerogen in a 6-mm glass tube before and after immersion in a large excess of the swelling solvent. This simple technique is as effective with kerogen as it was with CO&.’~ The solvents used for the swelling measurements are listed in Table 2. Table 3 contains the results of some sets of repetitive measurements which demonstrate the reproducibility of the technique. These data are typical. All swellingvalues are the average of four measurements and the scatter is usually about 2-395 of the reported value. Figure 2 contains a plot of the swelling ratio (8”)for Green River kerogen isolated using Siskin’s ammonium (17) Green, T. K.;Kovac, J.;
h n , J. W.Fuel 1984,63,9361)38.
:
1.004-
6.0
7.0
9.0
8.0
10.0
11.0
12.0
13.0
14.0
6, (cal/cm3)“2
Figure 2. Swelliig ratio of (N&)*SO, demineralized kerogen as a function of swelling solvent solubility parameter: (0)
+
nonpolar solvents, (B) polar solvents, ( ) H-bonding solvents. The solid line is the theoretical prediction by Kovac’s model when N = 1 and = 866. The d c h e d line is the theoretical prediction by Flory’s model when M, = 242. See Table 2 for solvent identification. I
I
t 20
1 .oo 6.0
7.0
8.0
9.0
10.0
1
\
’+
‘2
11.0
12.0
13.0
14.0
6, (cal/cm’)u2
Figure 3. Swelliig ratio of HF/HCl demineralized (2 h) kerogen as a function of swelling solvent solubility parameter: (A) nonpolar solvents, (A)polar solvents, (e) H-bonding solvents. The solid line is the theoretical prediction by Kovac’s model when N = 1 and Mc = 879. The dashed line is the theoretical prediction by Flory’s model when AXc = 238. See Table 2 for solvent identification. Table 3. Reproducibility of Swelling Measurements Using Ammonium Sulfate Demineralized Kerogen and Chlorobenzene date 11/20/92 01/31/93 03/26/93
run1 1.57 1.62 1.61
run2
run3
run4
average
1.56 1.62 1.64
1.60 1.61 1.57
1.53 1.67 1.62
1.57 0.03 1.63 f 0.03 1.61 f 0.03
*
sulfate procedure plotted against the swelling solvents’ solubility parameter. The swelling ratio is the volume of the solvent-swollenkerogen at equilibrium divided by the original volume of the dry kerogen. Swelling measurementa for each sample were continued until there was no further volume change over several days, demonstrating that equilibrium had been reached. Data for kerogens isolated by HF/HCl treatment for various times are shown in Figures 3-5.
Discussion The first striking feature of the data in Figure 2 is the very high swellings possible with this material which is 42 9% mineral matter. The presence of the mineral matter does not seem to hinder swelling at all. Figure 6 shows the swelling in 10 solvents of the kerogen demineralized by the 4 different methods. In 8 out of the 10 cases, the greatest swelling was observed with the sample having the
Energy & Fuels, Vol. 8, No. 4, 1994 936
Solvent Swelling Studies of Green River Kerogen 2.00 -. 1.80 -0 >
12
21
1.60 --
0
1.40 --
6.0
7:O
8:O
910
10.0 6,
11.0
12.0
13.0
14.0
(~allcm')''~
Figure 4. Swellingratio of HF/HC1demineralized (8h) kerogen as a function of swelling solvent solubility parameter: (0) nonpolar solvents, ( 0 )polar solvents, (e)H-bonding solvents. The solid line is the theoretical prediction by Kovac's model when N = 1 and f i C= 817. The dashed line is the theoretical prediction by Flory's model when Mc = 207. See Table 2 for solvent identification.
2.00
1.80
i+ i
1 .oo 6.0
I
I
7.0
9.0
8.0
10.0
11.0
12.0
13.0
14.0
6, ( c a l / ~ m ~ ) ~ ' ~
Figure 5. Swellingratio of HF/HC1demineralized (48h) kerogen as a function of swelling solvent solubility parameter: (v) nonpolar solvents, (v)polar solvents, (e)H-bonding solvents. The solid line is t_he theoretical prediction by Kovac's model when N = 1 and Mc = 808. The dashed line is the theoretical prediction by Flory's model when M, = 215. The curves are identical. See Table 2 for solvent identification. 2.1
*I
1.9 1
1.5-
$
A
m
+
1A 1
I
A
+ 1.
+ 8
9
10
6,
11
12
31 13
( c a 1 / m 3 ) 112
Figure 6. Swelling ratio of different demineralized kerogens as a function of swelling solvent solubility parameter: (H) (NH4)2SO4 demineralized kerogen, (+) 2 h HF/HC1 demineralized kerogen, (*) 8 h HF/HCl demineralized kerogen, (A)48 h HF/ HCL demineralized kerogen. See Table 2 for solvent identification.
largest mineral matter content. This makes it unlikely that the mineral matter is cross-linking the organic matter in the demineralized kerogen. If it were, surely the sample containing 5.2% mineral matter should swell more than
the sample containing 42 % mineral matter. Because the opposite is observed, it is unlikely that the mineral matter in the demineralized kerogen serves to cross-link the kerogen. It is possible that the strong acids cause reactions which cross-link the kerogen network resulting in reduced solvent swelling. If this were so, longer acid exposure should lead to larger reductions in swelling. It does not. The swelling results of the kerogens demineralized in HF/HC1 for three different times are scattered and provide no evidence for strong acid-induced cross-links. Hydrogen bonds play a small role in the self-association of this kerogen and in its interactions with solvents. Several good hydrogen-bonding solvents are very poor swelling solvents. In general, solvents capable of forming strong hydrogen bonds (e.g., THF and pyridine) behave similarly to nonpolar solvents having similar solubility parameters. Polar solvents behave similar to nonpolar solvents. Aromatic and nonaromatic compounds behave similarly. Chlorine-containing solvents show anomalously high swellings when compared to allof the other solvents. Many of these solvents are dense and the kerogen floats on them. This makes measurement of the swollen volume difficult and Qvmeasured with these solvents is subject to more uncertainty than with the other solvents. This kerogen isolated by any of the methods used seems to roughly follow regular solution theory. Regular solution theory predicts that there should be a maximum in the plot of swelling vs solubility parameter and that maximum should occur at the solubility parameter of the polymer, in this case the kerogen. Figures 2-5 all show rough agreement with this prediction. Drawing the curves by l/~. eye, the maxima fell between 9.5 and 10 ( ~ a l / c m ~ )The solubility parameter of this kerogen was calculated to be 9.5 (cal/cm3)'l2 using Siskin's structure and van Krevelen's group equivalents.18 The lines in Figure 2-5 were not drawn by eye but were calculated from the Flory-Rehner (- - -) and Kovac (-) treatments using the MCvalues given in the figure legends, a solubility parameter for the kerogen of 9.75 ( ~ a l / c m ~ )and l / ~ the , average molar volume of solvents used 95 cm3/mol. The actual range of solvent molar volume is from 53 to 154 cm3/mol with 36% being within 15cm3/mol of 95 cm3/mol. The fits are reasonable. This and the agreement of the calculated and measured solubility parameter for the kerogen induce us to calculate the number average molecular weight between cross-links. Both the Flory-Rehner and Kovac treatments were used to calculate the number average molecular weights between cross-links which are reported in Table 4. The FloryRehner equation gave MCbetween 207 and 242 for the four cases, and Kovac's equation resulted in M cbetween 808 and 879 for the limit of low value N = 1. While the Flory-Rehner equation adequately fits the swelling data (see Figures 2-5), the number average molecular weight between cross-links (MC) calculated from it is unrealistically low. If the kerogen is tightly crosslinked, application of the Flory-Rehner equation is questionable. The Kovac equation must give greater values for M cand presents us with the task of estimating N. Since our main interest is in comparing Mcvalues for kerogen of different type and maturation, we will avoid assigning any quantitative estimate of Mc.We do point out that this kerogen is highly cross-linked. (18)van Krevelen,D.W.Properties ofPoZymers;Elsevier: Amsterdam, 1972.
Larsen and Li
936 Energy & Fuels, Vol. 8, No. 4, 1994
Table 4. Number Average Molecular Weiaht mr Cross-Link of the Demineralized Kerogen 42% mineral 38% mineral 17% mineral 5.2% mineral MC(Kovac) &fc(Kovac) Mc (Kovac) Mc (Kovac) MC MC MC MC solvent Flory) N = l N - 2 N = 3 (Flory) N = l N = 2 N = 3 (Flory) N = l N = 2 N = 3 (Flory) N = l N a 2 N - 3 402 n-pentane 487 methylcyclohexane 978 711 715 761 555 487 260 621 235 887 645 565 201 301 1130 819 cyclohexane 189 223 527 515 450 710 920 663 577 838 605 246 311 1162 831 721 o-xylene 695 501 437 148 353 169 634 458 400 185 557 404 221 825 593 515 toluene 121 329 144 453 394 540 389 339 288 133 497 359 313 455 169 632 benzene 227 962 258 850 611 531 689 258 589 689 598 962 689 598 258 962 tetralin 192 179 718 514 446 416 165 619 445 668 479 221 825 588 509 387 chlorobenzene 223 522 1137 810 834 600 575 923 662 285 1063 759 657 305 701 1-methylnaphthalene 247 241 92 89 343 247 215 210 107 400 286 334 548 389 336 147 249 carbon disulfide 291 256 689 218 814 583 505 663 955 680 588 1084 769 o-dichlorobenzene 302 1129 799 149 198 559 404 352 459 740 529 189 776 554 480 208 706 505 439 nitrobenzene 421 1614 1153 999 1571 1123 974 344 432 1288 926 586 805 biphenyl nitroethane 106 80 539 2045 1496 1313 chloroacetonitrile 893 653 574 235 nitromethane 577 505 438 288 1077 760 655 189 706 199 253 743 530 460 943 669 pyridine 783 557 481 512 210 783 557 481 210 783 557 481 210 224 836 593 tetrahydrofuran 1415 1003 866 148 558 405 354 230 537 460 860 618 1720 1213 1044 379 carbon tetrachloride 1087 765 657 141 528 380 331 226 558 291 843 598 517 915 647 1,1,2-trichloroethane 245 548 395 343 325 190 707 505 438 119 448 283 146 269 1006 711 612 1,2-dibromoethane 124 233 469 342 300 266 266 130 492 359 315 95 362 233 95 362 2-propanol 1024 751 660 262 881 647 569 231 431 269 153 585 379 105 403 297 ethanol 59 acetone 1119 813 711 191 725 123 470 345 304 530 460 345 1299 942 823 297 dimethyl sulfoxide 537 590 514 215 808 581 505 238 879 630 547 207 817 866 242 620 average ~~
Table 5. Reversible Swelling of HF/HCl (8 h) Demineralized Kerogen in Chlorobenzene sample first swelling second swellinp HF/HCl8 h w/o extraction 1.45 1.53 HF/HCl8 h w/toluene extraction 1.49 1.54 Kerogen was swollen in chlorobenzene at 60-70 OC under dry N2 atmosphere overnight, and chlorobenzene was removed under vacuum. a
Native coals are strained and upon swelling can rearrange to a lower energy conformation.1B-21This phenomenon leads to an initial irreversible swelling after which repetitive swellings are reversible. Kerogen isolated by 8-h HF/HCl treatment swelled reversibly as shown by the data in Table 5. Kerogen isolated using Siskin's ammonium sulfate procedure behaved differently. It was swollen in chlorobenzene at 60-70 "C overnight and the chlorobenzene was removed. The chlorine content was 0.14% before the chlorobenzene treatment and 0.17% after. All of the chlorobenzene was removed. Table 6 shows the result of swelling the kerogen in three solvents before and after the chlorobenzene treatment. Exposure to the chlorobenzene has significantly reduced the solvent swelling. This may be due to a conformational rearrangement analogous to that observed with coals. This logical supposition will require further testing before it can be accepted. ~
Table 6. Irreversible Swelling of (NHl);SO4 Demineralized Kerogen in Three Solvents before and after Chlorobenzene Treatment QV
solvent benzene chlorobenzene carbon disulfide
before 1.50 1.63 1.60
after 1.31 1.43 1.42
Experimental Section Green River oil shale (20-100 mesh) from Colorado was used for this work. Kerogens were isolated from oil shale by HF/HCl and aqueous ammonium sulfate demineralization procedures. The details of these methods have been described previously.83 Soxhlet extraction with toluene was carried out after the 8-h and 48-h HF/HC1 demineralizations. Demineralizations and Soxhlet extraction were preformed under dry N2 atmospheres. Solvent swelling techniques are described in refs 17 and 22 All the solvents were high-purity grades purchased from Aldrich. THF was distilled to remove the inhibitors, and kerogen swelling with T H F was performed under a dry N2 atmosphere23
.
.
Acknowledgment. We are grateful to Dr. Michael Siskin for very helpful information and the supply of Green River Oil Shale. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
~~
(19) Brenner, D. Fuel 1985,64 167-173. (20) Larsen, J. W.;Azik, M.; Korda, A. Energy Fuels 1992,6,109-l10. (21) Cody,G. D. Jr.; Larsen, J. W.; Siskin, M. Energy Fuels 1990,2, 340-344.
(22) Larsen, J. W.; Shawver, S. Energy Fuels 1990,4, 74-77. (23) Perrin, D. D.; Amarego, W. L. F. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1980.
