Energy & Fuels 1989,3, 557-561
557
Effect of Demineralization on the Macromolecular Structure of Coals John W. Larsen,* Cheng-Sheng Pan, and Susan Shawver Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received February 2, 1989. Revised Manuscript Received May 2, 1989
It appears that HCl/HF demineralization has little, if any, effect on the macromolecular structure of coals apart from those due to ion exchange and the removal of inorganics. Six coals varying in carbon content between 69% and 86% were demineralized in HCl/HF, dried, and extracted with pyridine, and the amount and molecular weight distribution of the pyridine extracts were determined. In addition, the pyridine extracts from three of the six, a lignite, a subbituminous coal, and Illinois No. 6 coal, were subjected to the demineralization procedure, which did not alter their molecular weight distributions. The observed alterations in the amounts and molecular weight distributions of the extracts from these three coals due to demineralization are probably due to solubility changes caused by converting acid salts to acids by ion exchange. For the two coals containing 82% and 84% carbon, demineralization did not alter extractability, and extract molecular weight changes were small and in opposite directions for the two coals. Demineralization of the 86% carbon coal slightly reduced extractability and the extract molecular weight. Solvent swelling studies of lignites and a subbituminous coal revealed no changes in organic structure due to demineralization. Selective ion-exchange and solvent-swellingexperiments were carried out in an attempt to determine whether ionic cross-links existed in low-rank coals. No conclusion could be reached due to the large changes in coal-solvent interactions caused by ion exchange.
Introduction Coals are often demineralized before use. It is important to know whether the demineralizationprocedure alters coal structure or reactivity. The demineralization procedure usually used is washing with aqueous hydrochloric and hydrofluoric a c i d ~ , ltreatment -~ which might be expected to cause structural changes due to acid-induced chemistry, such as ester hydrolysis and Friedel-Crafts reactions. This matter has become more important with the increasing use of Dyrkacz's maceral separation technique, which requires deminerali~ation.~Our concern here is with the effect of the demineralization procedure on the macromolecular structure. Specifically, does the usual demineralization procedure make or break network active bonds, thus altering the macromolecular structure? A related matter is whether ionic "cross-links" occur in low-rank coals. It is easy to visualize two carboxylate groups from different macromolecular segments interacting with the same inorganic counterion. In this situation, an ionic cross-link exists. Demineralization will destroy this ionic cross-link by converting the carboxylic acid groups to covalent acids. Both of these questions can be addressed by using two experimental approaches. The familiar solvent-swelling approach has been used to follow bond making and bond breaking in coals and can be applied in a straightforward A less familiar approach is to use the amount of (1) Radmacher, W.; Mohrhauer, P. Brennst. Chem. 1955,36(15/16), 236. (2) Bishop, M.; Ward, D. L. Fuel 1958,37, 191. (3) Karr, C. Jr., Ed. Analytical Methods for Coal and Coal Products; Academic Press: New York, 1978; Vol. 11. (4) Choi, C.; Dyrkacz, G. R.; Stock, L. M. Energy Fuels 1987, 1, 280-286. (5) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729-4735. (6) Peppas, N. A.; Lucht, L. M. Chem. Eng. Commun. 1984,30,291. (7) Suuberg, E. M.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987,1, 305-309.
(8)Bockrath, B. C.; Illig, E. G.; Wassell-Bridger, W. D. Energy Fuels 1987, 1, 226-227.
extractable material present in the coal and its molecular weight distribution as a probe of macromolecular structure alterations. If bonds are broken, more extractable material will be present since bond breaking will liberate it from the insoluble network. The molecular weight distribution of the extract may also change in complex ways.D This approach has been used to study the coalification process."" Both of these techniques were utilized in this investigation. A number of studies of the effects of demineralization on coal structure, properties, and reactivity have been carried out with conflicting results. Demineralization removes paramagnetic materials, giving a significant improvement in 13C NMR spectra.12 A French coal (St. Fontaine, 78.7% C) showed significant changes in the 13C-CP/MAS NMR spectrum after demineralization. There was a loss of aliphatic carbon signal between 30 and 80 ppm with an increase in signal intensity below 20 ppm.13 The spectra were also suggestive of a decrease in the number of alkylated aromatic carbons. In contrast to the work of Alemany,12 no signal enhancement due to demineralization was observed. The observed loss of signal could be due either to a decrease in the mobility of the aliphatic carbons or to chemical alteration. It is difficult to reach a conclusion about the origin of the changes observed. A differential IR study of a coal before and after demineralization showed no change in the aliphatic CH stretching frequency but did show an increase in the carbonyl stretch intensity.14 This increase has been (9) Larsen, J. W.; Wei, Y.4. Energy Fuels 1988,2, 344-350. (IO) Dormans, H. N. M.; van Krevelen, D. W. Fuel 1960,39,273-292. van Krevelen, D. W. Fuel 1965,44, 229-242.
(11) Larsen, J. W.; Mohammadi, M.; Yiginsu, I.; Kovac, J. Geochim. Cosmochim. Acta. 1984,48, 135-141. (12) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Stock, L. M. Fuel 1984,63, 513-521. (13) Tekely, P.; Nicole, D.; Delpuech, J. J. Fuel Process. Technol. 1987, 15, 225-231. (14) Yares'ko, T. D.; Skripchenko, G. B. Khim. Tuerd. Topl. 1977, 11(1), 72-74.
0887-0624/89125Q3-0557$Q1.5Q/O 0 1989 American Chemical Society
Larsen et al.
558 Energy & Fuels, Vol. 3, No. 5, 1989 Table I. Elemental Analysis of Coals Used % hydrogen % nitrogen 0.96 4.4 68.0 1.06 4.7 69.2 1.42 5.1 69.6 1.3 4.4. 69.9 1.4 4.9 69.9 1.90 5.0 80.6 1.90 5.5 82.5 1.30 5.2 84.3 1.57 5.2 86.3 5.1 1.4 69.7 1.3 4.8 69.2 1.3 4.8 69.0 1.4 5.1 70.3
% carbon (dmmf)
Rawhide Rawhide (DEM)* Texas lignite (Martin Lake) Texas lignite (CAW)' Texas lignite (DEM)b Illinois No. 6 (SIU) PSOC 1278' PSOC 1309' PSOC 1215' Big Brown lignite Big Brown lignite (CAW)' Big Brown lignite (DEM)* Texas Big Brown lignite
% oxygen (diff)
24.3 24.9 23.7 22.3 23.7 10.7 9.2 8.3 6.2 24.1 23.5 24.6 23.7
% ash -
7.5 0.14 14.06 8.55 0.49 13.48 15.7 10.1 7.3 8.9 4.39 0.75 13.0
'Citric acid washed. *Demineralized (HF/HCl). 'Data from Penn State Coal Data Base: Chemical Data 2.
confirmed, and a decrease in the intensity of the OH peak was simultaneously 0b~erved.l~ An IR study of coal extracts clearly revealed the increase in the carbonyl band after demineralization.16 Presumably, this increase results partially from the hydrolysis of esters and partially from the formation of carboxylic acid groups from carboxylate anions by ion exchange. A recent investigation showed an increase in carbonyl content and increased air oxidation due to demineralization." A very careful study of the effect of demineralization on the surface area of coals exists.18 The changes in surface area are often significant but are random: sometimes increasing and sometimes decreasing and seemingly uncorrelated with coal rank. The effect of mineral matter on the gasification of coals and chars has been investigated numerous times. Since the catalysis of char gasification by alkali metals and calcium is well-known, their removal should decrease char reactivity, and this has been obs e r ~ e d . ' By ~ ~adding ~ calcium back to the demineralized sample, Walker and co-workers were able to show that, at comparable calcium loadings, acid treatment reduced char reactivity.20 There are reports of increases in char reactivity after demineralization.21*n It is clear that mineral matter acts catalytically during the gasification of Demineralization also significantly affects the dilatometric properties of coals.24 While the temperatures at which various changes occur are modified slightly by acid treatment, there is an enormous loss of dilation. It drops from 68% to 0% on HF/HC1 treatment with an 84.5% C coal and from 155% to 3% with an 85.4% C coal.24 The authors suggest that the acid treatment causes substantial changes in the coal structure. It is quite possible that residual acid in the coal may induce polymerization on heating that results in a loss of plastic properties and a decrease in dilation. Mahajan has shown that little halogen (10.1% (wt) C1 and 0 . 1 4 2 % (wt) F) remains in the coal after demineralization.25 Also, dilation is known to be very (15)Urbanski, T.; Kuczynski, W.; Andrzejak, A.; Hofman, W.; Witanowski, M. Bull. Acad. Pol. Sci., Ser. Sei. Chim. 1960,8,19-22. (16)Andnejak, A.; Kurrynski, W.; Urbanski, T.; Witanowski, M. Bull. Acad. Pol. Sei., Ser. Sei. Chim. 1963,11, 201-204. (17)Kister, J.; Guiliano, M.; Mille, G.; Dou, H. Fuel 1988, 67, 1076-1082. (18)Mahajan, 0.P.; Walker, P. L., Jr. Fuel 1979,58,333-337. (19)Hengel, T.D.; Walker, P. L., Jr. Fuel 1984,62,1214-1220. (20)Radovic, L. R.;Stenko, K.; Walker, P. L., Jr.; Jenkins, R. G. Fuel Proc. Techn. 1985,10,311-326. (21)Linares-Solano, A.; Salinas-Martinez de Lecea, C.; RodriguezReinoso, F.; Almela-Alarcon, M. Fuel 1986,65,1345-1348. (22)Linares-Solano, A,; Mahajan, 0. P.; Walker, P. L. Fuel 1979,58, 327-332. (23)Oya, A.; Fukatsu, T.; Otani, S.; Marsh, H. Fuel 1983,62,502-507. (24)Wachowska, H.; Pawlak, W.; Andrzejak, A. Fuel 1983,62,85-88. (25)Mahajan, 0.P.Fuel 1985,64,973-980.
sensitive to surface properties, and the alterations of surface properties were not studied. Acid washing a subbituminous coal causes significant changes in its pyrolysis behavior.26 After demineralization, C02 was produced stoichiometricallyfrom the carboxylate groups in the coal, which apparently have to be in their acid form to undergo ready pyrolysis. Increases in tar and gaseous hydrocarbon yields were also noted. The effects of demineralization on a number of different chemical reactions of coals have been noted. In a wideranging study, demineralization was shown to decrease the solvent extraction yield caused by a variety of chemical reactions including acylation, reductive acylation, alkylation, reduction, and dep~lymerization.~' The authors concluded that mineral matter was acting as a promoter for all of these reactions, although this is a bit difficult to understand. Significant decreases after demineralization in the liquids formed on hydrogenation by an unspecified procedure have been reported.28 More convincing is a patent claiming significant increases in conversion and in the amounts of the more desirable products in SRC procm due to acid demineralization.B Finally, oxidation of coals at temperatures between 1and 300 "C is retarded by demineralization (see, however, ref 17).30 The data on the effects of demineralization on coal structure and reactivity are conflicting. It is clear that a very careful study of the effects of demineralization on a broad rank series of coals is called for. We do not contemplate such a study but do wish to present some data on the effect of demineralization on macromolecular structure. From the data in the literature, we can draw no conclusions about demineralization causing significant alterations in the organic chemical structure of coal. Experimental Section Samples. Coal samples were obtained from the Penn State Sample Bank and from Exxon Research and Engineering Co. They were dried to constant weight at 110 O C under vacuum and ground t o pass 100 mesh before use. Elemental analyses are contained in Table I. Demineralization. The Bishop and Ward modification of the procedure developed by Radmacher and Mohrhaurer was used.*3* (26)Franklin, H.D.; Cosway, R. G.; Peters, W. A.; Howard, J. B. Znd. Eng. Chem. Process Des. Dev. 1983,22,39-42. (27)Sharma, D. K.; Mirza, Z.B. Fuel 1983,62,916-917. (28)Larina, N.K.; Smutkina, Z. S.; Miesserova, 0. K.; Shulyakovskaya, L. V.; Titova, T. A. Khim. Tverd. Topl. 1978,12(5)46-47. (29)Dickert, J. J., Jr.; Mitchell, T. 0.;Whitehurst, D. D. U.S. Patent 4,257,869,March, 1981. (30)Sukhov, V. A.; Zamyslqv, V. B.; Sokolova, T. N.; Kovalenko, G. S.; Lukovnikov, A. F. Khim. Tverd. Topl. 1976,10(4),51-55. (31)van Bodegam, B.; van Veen, J. A. R.; van Kessel, G. M. M.; Sinnige-Nijssen, M. W. A.; Stuiver, H. C. M. Fuel 1984,63,346-354.
Energy & Fuels, Vol. 3, No. 5, 1989 559
Macromolecular Structure of Coals
Table 11. Effect of Demineralization on Pyridine Extractability and Extract Molecular Weight Distribution coal demineralized coal % mass mass % extract balance, extract balance, coal % C (dmmf) solublesa % Mn Mw solubles residueb % Mn ~~
Rawhide
69
7
123
Rawhide
69
8
116
Lignite (Martin Lake)
70
12
125
111. No. 6
81
17
113
PSOC 1278 PSOC 1309 PSOC 1215
82.5 84.3 86.3
14 27 32 31
105 102 105 101
9i extract = w t dried extract/wt starting coal.
a
5000 5300 4900 4600 5000 5200 3900 4300 3500 5300 4800 5100
Mw
15
11
104
1500
3600
17
13
104
1600
3700
16 13 15 27
14 9 5 21
102 104 110 105
2100 2400 1800 2200
5000 6100 4100 4700
25
20
104
2000
4400
% extract = (wt starting coal - wt dried residue)/wt starting coal.
Table 111. Molecular Weight Distributions of Pyridine Extracts before and after Treatment with HF/HC before acid after acid treatment treatment coal % extract M. M, M, M, 4640 2110 4860 6 2000 lignite 2220 9 2210 5180 5150 rawhide 4550 2020 4470 111. No. 6 17 2020 ~~
2200 2200 2000 1800 2100 2200 1800 2000 1500 2400 2200 2300
~
HF and HCl treatments were carried out in an N2 atmosphere, and all exposure of the coal to air was minimized. Extraction and HPLC Molecular Weights. The procedures used have been described in
Results and Discussion First, consider the effect of demineralization on the extractability of the two low-rank coals: Rawhide subbituminous and Martin Lake lignite (Table 11). The pyridine extract is 7% of the Rawhide coal, and the weight of the solid residue has increased sharply due to pyridine retention, giving a large mass increase on extraction. After demineralization, the mass balance on extraction is much improved and the amount of material extracted increases significantly. Coal molecules present as salts would be insoluble in pyridine, and the conversion of these salts to their carboxylic acid form is the most straightforward explanation for the large increase in extractability. The most reasonable explanation for the decrease in pyridine retention caused by demineralization is the solvation of ions by pyridine or the formation of ion-pyridine complexes in the raw coals. Removal of the ions by ion exchange then leads to less pyridine uptake. With both coals, acid washing results in increased amounts of extracts of significantly lower molecular weight. The molecular weight decrease could be due to bond cleavage or the additional extract having a lower molecular weight than the original extract. To decide between these possibilities, the extracts were treated under the standard demineralization conditions and their molecular weights determined before and after that treatment. With both lignite and Rawhide extracts, treatment with HF/HC1 does not alter the molecular weight distribution (Table 111). The fact that these extracts contain less than 10% of the coal and may not be representative introduces some uncertainty. However, it seems highly likely most of the species present in the coals are represented in the extract, and the constancy in molecular weight indicates that little bond cleavage is occurring during the demineralization procedure. The decrease in molecular weight of the extract on demineralization is ascribed to the extraction of additional low molecular weight material as the result of
ion-exchange processes. A caveat: rapid acid-catalyzed hydrolysis breaking all possible bonds during the first acid treatment (demineralization) is another (improbable) explanation for these observations. Cleavage of esters has been claimed to be important in the extraction of low-rank coals by amines.31 Moving up in rank, the demineralization of Illinois No. 6 coal is described in Table 11. Demineralization causes at most a very small change in the amount extracted by pyridine. The molecular weight of the extract from the demineralized coal is larger than that of the untreated coal. The most obvious conclusion is that the acid causes condensation reactions, but there are data that controvert this explanation. Treatment of the coal extract with HF/HC1 does not change its molecular weight (Table 111). It is possible that condensation reactions occur in the solid coal but are not possible in the extract. Alternatively, the composition of the extract has changed while its quantity remained constant. We are unhappy with both of these rationalizations. Demineralization does decrease the amount of pyridine irreversibly absorbed by the coal during extraction. PSOC 1238 behaves similarly to the Illinois No 6 coal except that the mass balance is somewhat harmed by demineralization. Since this particular data point is not consistent with that for other coals of similar rank, we regard it as suspect. The increase in the extract average molecular weights after demineralization lies just outside of the normal experimental reproducibility of the molecular weight apparatus. These data are generally similar to those obtained for the Illinois No. 6 coal. The two highest ranking coals (PSOC 1309 and PSOC 1215) show small decreases in the molecular weight of the extract after demineralization, decreases that are within experimental error. We conclude that the demineralization procedure has no effect on the macromolecular structure of these coals. For the low-rank coals studied and for the two high-rank coals, HF/HC1 demineralization does not make or break bonds. For some mid-rank bituminous coals, either acidcaused chemistry occurs or the demineralization process alters the composition of the extract. In order to distinguish between chemical alteration and the extraction of different material before and after demineralization, coal extracts were subjected to the demineralization procedure. Extracts from Illinois No. 6, Rawhide subbituminous, and lignite coals were treated with HF and HC1 under the demineralization conditions. The data are contained in Table I11 and show remarkable constancy. It is very clear that the extracts are stable under demineralization conditions, and by extension, these coals must also be stable
Larsen et al.
560 Energy & Fuels, Vol. 3, No. 5, 1989
Table IV. Nonpolar Solvent-Swelling Ratios of Three Coals Demineralized by Citric Acid Wash (CAW) and by Treatment with HR/HCl IDEM) Rawhide Big Brown lignite Big Brown lignite 2 solvent n-pentane cyclohexane o-xylene toluene benzene chlorobenzene tetralin 1,2-dichloroethane carbon disulfide acetonitrile
dry 1.0 1.0 1.1 1.2 1.1 1.3 1.2 1.3 1.3 1.4
CAW
1.0 1.2 1.2 1.2 1.3 1.2 1.4 1.3 1.3
DEM 1.0 1.1 1.2 1.3 1.3 1.3 1.2 1.4 1.3 1.4
under the demineralization conditions. Therefore, the differences observed in the molecular weight distributions of material extracted from these coals before and after demineralization are due to either changes in the nature of the extract induced by demineralization or experimental error. There are changes in the extractability of potentially soluble material resulting from chemical or physical changes caused by demineralization. All observed changes in the amount of extract and its molecular weight distribution can be rationalized on the basis of changes in molecular solubility due to the ion exchange of protons for cations producing carboxylic acids. We find no evidence for extensive bond cleavages or condensation chemistry, even in coals of low rank. Because of the small amount of extract used from the low-rank coals, these conclusions must be regarded as uncertain until confirmed by other techniques or studies of a more representative sample of the coal. The conclusions about the higher rank coals are much more certain. The experimental probe used is neither a delicate nor a precise one, and we cannot say what may be happening to macerals present in small amounts. We can say that the demineralization procedure does not have significant effects on the size of the macromolecules present in these coals. It is worth comparing the reproducibility of molecular weight distributions obtained when an extract is run repeatedly (Table 111) and when a coal is extracted several times and the several extracts are subjected to analysis (Table 11). The average molecular weights for a single extract run repeatedly usually agree to f l O O amu. The divergence is many times this for a coal that is extracted several times and the extracts compared. It is the extraction itself that is the source of the divergence. Coal extraction processes and results are very sensitive to sample history and treatment details. We will be publishing on this topic in the future. For the present, we note that the origin of the scatter of the data in Table I1 is not in the instrumental techniques used but in the irreproducibility of the extraction process. Demineralization effects on the insoluble portion of the coals can be evaluated by using solvent swelling. For this work, we used ion exchange with citric acid to remove a portion of the inorganics present as well as complete demineralization using HF/HCl. It is easy to visualize ionic cross-linking existing in low-rank coals. If two carboxylate anions from different macromolecular segments are both ionically bonded to a single calcium ion, an ionic cross-link would exist. There are obviously many variations on this theme. We attempted to use solvent swelling techniques to detect ionic cross-links in two different samples of Big Brown lignite from Texas and in Rawhide subbituminous coal. The procedure used was to swell three samples: the coal, the same coal having ion-exchangeable cations removed by
dry
CAW
DEM
dry
CAW
DEM
1.1 1.2 1.2 1.2 1.2 1.2 1.3 1.3
1.0 1.2 1.3 1.2 1.3 1.2 1.3 1.2
1.1 1.2 1.3 1.3 1.4 1.2 1.4 1.3
1.1 1.1 1.2 1.2 1.2 1.1 1.3 1.2 1.4
1.0 1.1 1.3 1.2 1.3 1.2 1.3 1.2 1.3
1.1 1.3 1.3 1.3 1.4 1.2 1.4 1.3 1.4
treatment with aqueous citric acid, and finally the coal demineralized by using the standard HF/HCl procedure. These samples were swollen in different solvents. The destruction of ionic cross-links may result in an increase in the swelling of the citric acid washed or demineralized coals. In an equilibrium swelling experiment, the swelling force due to penetration of the coal by the swelling solvent is exactly balanced by the elastic restoring force of the network when the system is at equilibrium. The elastic restoring force is a sensitive function of the cross-link density of the macromolecular network. The driving force for penetration of the solvent into the coal is due to the concentration gradient between the neat solvent and the interior of the coal as well as any favorable interactions between the solvent and the coal. The two factors controlling the ultimate amount of swelling are the degree to which the solvent interacts with the coal and the cross-link density of the coal. Previously, we argued for the use of nonpolar solvents since random mixing of the coal network and the solvent is assumed in quantitative treatments of this In Table IV is recorded the swelling of three coals, before and after demineralization,in 10 solvents selected for their inability to form hydrogen bonds. The swelling of the coals is quite small in all of these solvents. There is a tendency toward greater swelling of the demineralized coal, but the differences are small. The only system for which they are clearly outside of experimental error is Texas lignite. These data do not allow a conclusion on the existence of ionic cross-links to be reached. The changes are too small and irregular to interpret reliably. Unfortunately, demineralization may not disrupt the cross-link. A pair of carboxylate anions that once were associated by ionic bonding to a cation are converted by ion exchange to carboxylic acids. If these remain close to each other, they can now hydrogen bond to each other. It is quite possible that ionic cross-links are being replaced with high efficiency by hydrogen-bond crosslinks. The existence of these would not be detected by nonpolar solvents that do not interfere with hydrogen-bond crosslinks. The way to test for this new association is to carry out coal swellings in hydrogen-bond-acceptingsolvents that will disrupt hydrogen bonds but not ionic bonds. Polar solvents were used, and the data are reported in Table V. Demineralized coals swell more than do untreated coals. Interpretation of this result is complicated by the fact that demineralization may significantly alter the coal-solvent interactions. A carboxylate anion is not a hydrogen-bond donor, while a carboxylic acid is. This will increase the interaction of the demineralized coals with hydrogen-bond-accepting solvents. This experiment thus has two variables: the magnitude of the coal-solvent interaction and the potential presence of ionic cross-links (cross-link density). It is impossible to separate these
Macromolecular Structure of Coals
Energy & Fuels, Vol. 3, No. 5, 1989 561
Table V. Polar Solvent-Swelling Ratios for Three Coals Demineralized by Citric Acid Washing (CAW) and by Treatment with HF/HCl (DEM) solvent pyridine NMPa tetrahydrofuran nitrobenzene ethanol (I
drv 2.4 2.3 1.7 1.2 1.6
Rawhide CAW 2.7 2.6 2.1 1.6 1.6
DEM 2.5 3.1 2.3 1.7 1.7
drv 2.0 1.8 1.7 1.2 1.6
Big Brown lignite CAW DEM 2.8 2.9 3.0 2.9 2.3 2.3 1.5 1.6 1.6 1.7
drv 2.2 2.1 1.6 1.2 1.6
Big Brown lignite CAW 2.8 2.9 2.1 1.5 1.5
2 DEM 3.1 3.0 2.2 1.5 1.7 ~~
~
N-Methyl-2-pyrrolidone.
Table VI. Solvent-Swelling Ratios for Acetylated and Methylated Coals and the Ratio of the Solvent-Swelling Ratios of Citric Acid Washed and Acetylated Coal (Q& Rawhide solvent Dvridine NMPb tetrahydrofuran nitrobenzene ethanol a Qml =
QcM/Qacr.
ACT 1.8 1.8 1.8 1.3 1.3
MET 2.1 2.3 1.8 1.9 1.1
Big Brown lignite Qmi
1.5 1.4 1.2 0.9 1.2
ACT 1.7 2.1 1.9 2.3 1.5
MET 2.0 2.2 1.9 2.0
Big Brown lignite 2 Q ~ I
1.6 1.4 1.2 0.7 1.1
ACT 2.0 3.2
Qnl
1.7 1.4
0.9 1.1
1.4 0.9
N-Methyl-2-pyrrolidone.
variables by using only these data. It is clear from the data in Table V that demineralization, either by ion exchange or HF/HC1 treatment, strongly enhances coal swelling. It is not revealed whether this enhanced swelling is due to increased coal-solvent interactions or due to the removal of ionic cross-links. This explanation must be contrasted with the effects of demineralization on pyridine retention by the coals. Demineralization decreases pyridine retention, probably by removing some materials that strongly bind pyridine. It increases coal-pyridine swelling. Pyridine retention is not due to a general coal-pyridine interaction but to the presence in the coal of a small amount of material that interacts strongly with pyridine. Clays are one possible contributor. The solvent-swelling increase caused by demineralization involves more pyridine and may originate in increased coal-pyridine hydrogen bonding. It is possible to increase coal-pyridine hydrogen-bond interactions while decreasing the amount of pyridine retained because the species responsible for pyridine retention and hydrogen bonding are different. In order to remove the complications due to hydrogen bonding to the solvent, the three coals were 0-acetylated by using acetic anhydride and pyridine or 0-methylated by using Liotta's procedure, and the swelling of these derivatives was measured in polar solvents." These data appear in Table VI and demonstrate that hydrogen bonding makes a major contribution to coal swelling. Compare the swelling ratios of the citric acid washed and the acetylated coals (Qrel = Qcit,icaCid/Qacetyla~). In the acetylated coal, there can be no hydrogen bonds to the solvent, while these are present in the citric acid washed coal. If the ratio of these two swellings is calculated, it will equal 1if hydrogen bonding to polar solvents in the citric acid washed coal does not enhance swelling. To the extent that hydrogen bonding to the solvent enhances coal swelling, Qd will excced 1. These ratios are shown in Table VI and exceed 1,except when the solvent is nitrobenzene. Nitrobenzene differs from the other solvents in that it is (32) Blom, L.; Edelhauser, L.; van Krevelen, D. W. Fuel 1959, 18, 537-538. (33) Liotta, R.; Rose, K.; Hippo, E. J . Org. Chem. 1981,46,277-283.
not a good hydrogen-bond acceptor and is therefore not expected to have Qrel> l.34 Our conclusion is that demineralization strongly enhances the interactions between hydrogen-bond-accepting solvents and coals. We have failed in our attempt to reach a conclusion about the existence of ionic cross-links in coals. The changes observed with nonpolar solvents are too small to permit any firm conclusion. In polar solvents, any effects of ionic cross-links are overwhelmed by increased interactions between the solvent and the coal due to the formation of new strong hydrogen-bond donors in the coal. It is clear that the demineralization of coals does have significant effects. The data considered in this paper show that the origin of these effects is the formation of new carboxylic acid groups replacing carboxylate anions, an ion-exchange phenomenon. We have found no evidence for the occurrence of significant bond cleavages or condensation reactions due to the normal demineralization procedures. The solvent-swelling technique and the molecular weight determinations are not extraordinarily sensitive probes capable of detecting small structural alterations. They are, however, very reliable indicators of significant structural change and they reveal no such significant macromolecular structural change induced by HF/HC1 demineralization. For many uses of the coal scientist, this procedure appears to be quite safe. We do not contemplate further studies in this area. We do not regard the work we have done as a definitive resolution of the question addressed and hope that other probes will be used and more careful examinations of the affect of acid demineralizationon broad rank series of coals will be carried out. It is a set of control experiments that needs to be done.
Acknowledgment. We thank the US.Department of Energy and the Exxon Education Foundation for financial support of this work. The perceptive comments of Gary Dyrkacz are gratefully acknowledged. Registry No. HF,7664-39-3; HCI, 7647-01-0; pyridine, 11086-1. (34) Barton,A. F. M. CRC Handbook of SolubiZity Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983; p 106.
