Energy & Fuels 1989, 3, 427-428 isogel" theory of coal plasticity and the radical stabilization theory of Grint et al. Michael Siskin,* Charles G. Scouten Corporate Research Laboratories of Exxon Research and Engineering Company Clinton Township, Route 22 East Annandale, New Jersey 08801 Received May 31, 1988 Revised Manuscript Received February 8,1989
427
Table I. Relative Abundances of the Functional Groups in Illinois No. 6 Coal and the Anticipated Products of Pyrolysisr re1 abundance, expected functional erouD mo1/100 mol of carbon Droducta alcohols 0.1 HzO phenols 5-6 CO, HZ0 ketones 10e co carboxylic acid and salta 0.8 COP carboxylic ester lo+ COP heterocyclic ethers 5 co The low designation signifies that the relative abundance of the structural element is less than 0.1 mo1/100 mol carbon.
Sources of Carbon Dioxide Formed during Coal Pyrolysis
Sir: Coal samples irrespective of their rank and nature evolve carbon dioxide when pyrolyzed. Indeed, Jungten reports that the rate of evolution of this gas from a German bituminous coal depends upon temperature and that the rate data exhibit distinct maxima at three different temperatures.' Generally, coal chemists have sought to relate results of this kind to the presence of different kinds of oxygen functional groups in the coal. In this regard it is well-known that carboxylic acids and their salts and esters evolve carbon dioxide during pyrolysis,2whereas phenols, ethers, and ketones mainly yield carbon m o n o ~ i d e .As ~~~ the information about coal accumulates: opportunities emerge for the establishment of relationships between the abundances of the functional groups and the pyrolytic products. The relevant information for Illinois No. 6 coal is summarized in Table I. Previous studies imply that carboxyl derivatives are uniquely responsible for the formation of carbon dioxide during coal pyrolysis.6 However, new studies performed in our laboratory strongly infer that the carbon dioxide yield is not uniquely dependent upon the carboxyl group concentration. This conclusion is based upon work with Wyodak and Illinois No. 6 coals obtained from the premium sample program of the Argonne National Laboratory and samples of these coals that were modified by removal of the carboxylic acids or by the incorporation of aliphatic esters, aromatic esters, and aromatic methoxy groups. The coals were selectively 0-alkylated by using the alkyl halides 1-5 (Chart I) by the method developed by Liotta' as modified by Ettinger and his co-workers8 (eq 1). The (coa1)OH + B (coa1)O-
-
+ RX
(coa1)O- + BH+ (coa1)OR + X-
B = KOH; RX = primary alkyl halide (1) Jungtan, H. Fuel 1984,63, 731. (2) Brown, R. F. C. Pyrolytic Methods in Organic Chemistry; Aca-
demic Press: New York, 1980. Clark, L. W. In The Chemistry of Carboxylic Acids and Esters; Patai, s.,Ed.; Interscience: New York, 1974; Chapter 12. (3) (a) Schubert, W. M.; Kintner, R. R. In The Chemistry of the Carbonyl Group; Patai, S., Ed.; Interscience: New York, 1966; Chapter 14. (b) Laidler, K. J.; McKenney, D. J. In The Chemistry of Ether Linkage; Patai, S., Ed.; Interscience: New York, 1967; Chapter 4. (c) Brown, R. F. C. Pyrolytic Methods in Organic Chemistry; Academic Press: New York, 1980. (4) (a) Cypres, R.; Bette-, B. Tetrahedron 1974,30, 1253; 1975,31, 359. (b) Bredael, P.; Vinh, T. H.; Braekman-Danheuv, C. Fuel 1983,62, 1193. (5) Stuck, L. M.; Willis, R. S. J. Org. Chem. 1985,50, 3566. (6) Schafer, H. N. S. f i e 1 1980,59, 302. (7) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277. (8) Ettinger, M.; Nardin, R.; Mahasay, S. R.; Stock, L. M. J. Org. Chem. 1986,51, 2840.
Table 11. Results of Pyrolysis Experiments yield, mo1/100 mol of carbon temp, carbon tot. sample OC CO CO, oxides H,O oxyeen Illinois No. 6 reference 717 4.9 2.2 9.3 1.0 10.4 O-(3-(4-methoxy872 10.4 2.1 14.5 2.3 16.8 pheny1)propyl) 043-methoxv847 7.8 2.4 12.7 2.1 14.8 benzyl) 0-(4-carbomethoxy)- 802 7.6 2.3 12.2 2.2 14.4 butyl
-
original reference LAH reduced 0-(4-(carboethoxy)butyl) 0-(3-(carboethoxy)benzyl)
Wyodak 10.1 1.7 9.2 3.6 8.2 2.6 9.5 3.1
784 826 748 737 748
11.2
2.2
13.6 16.3 15.6
1.6 1.3 2.6 3.8
15.2 17.7 16.0 19.5
15.7
4.3
19.9
13.4
Chart I. Alkyl Bromides Used for 0-Alkylation CH2Br
CH2Br
I
I
COOC2H5
2
OCH,
I BrCH2CH2CH2CH2COOCH3
3 BrCH2CH2CH2CH2COOC2H5
4
5
0-alkylated coals were washed carefully with 50% aqueous methanol and dried at 110 "C under vacuum. Previous experiments with labeled alkyl halides have shown that four to five alkyl groups are introduced per 100 mol of carbon, under our experimental condition^.^ The introduction of the esters are evidenced from the infrared frequencies, for example, the appearance of a new absorption near 1710cm-l together with a significant decrease in frequency at 3400 cm-l. The presence of the aromatic methoxy group is evidenced by the presence of strong carbon-oxygen stretching bands near 1250 cm-', initially absent in the original coal. For the Wyodak coal sample, solid-state C NMR spectroscopy and infrared spectroscopy establish that carboxyl groups are present. This coal was also altered by reducing these carboxyl groups with lithium aluminum hydride. The successful elimination of the carboxylic groups is assured by the C NMR spectrum of the product. Specifically, the resonance near 180 ppm disappears after reductioh. The pyrolysis experiments were performed in a wirescreen reactor of the type described by Anthony and his (9) Mahasay, S. R.; Nardin, R.; Stock,L. M.; Zabransky, R. F. Energy Fuels 1987, 1, 65.
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Energy & Fuels 1989, 3,428-430
428
associates.1° The pyrolysis was carried out in a helium atmosphere, and the pressure of the reactor was adjusted to 0.12 MPa. The sample was heated at lo00 "C s-l to the desired final temperature. Secondary gas-phase reactions were eliminated because the environment surrounding the screen and coal particle remain at room temperature; hence, as the gaseous products form and diffuse away from the coal particle, they cool instantly. The gaseous products were collected and analyzed mass spectroscopically. The gaseous products were identified by the individual retention times in the GC and their mass spectra. The data are presented in Table 11. For the Illinois No. 6 coal, the carbon monoxide and carbon dioxide yields are about 5.0 and 2.0 mo1/100 mol of carbon, respectively. If carboxyl groups were uniquely responsible for the formation of carbon dioxide, then the yield would be less than 1.0. This observation strongly suggests that other oxygen functional groups or other components of the mineral matter contribute to the formation of carbon dioxide. To address this issue in another way, we examined the Wyodak coal. The solid-state C NMR spectrum clearly indicates the presence of carboxyl groups in this coal. The pristine sample yields about 2.0 mol of carbon dioxide/100 mol of carbon, but a sample that was treated with base and acidified with dilute hydrochloric acid (reference sample) yields 3.5 mol/100 mol carbon. Presumably, acidification converts carboxylate acid salts to acids that decompose more readily to carbon dioxide." This observation is entirely in accord with the concept that there is a very simple relationship between the carboxyl group content and the carbon dioxide yield. But this interpretation was negated when the carboxyl groups in Wyodak coal were completely removed by reduction with lithium aluminum hydride and the carbon dioxide yield did not change. Indeed it remains 2.6 mol/ 100 mol of carbon. This observation unambigously reveals that the carbon dioxide yields are not directly proportional to the carboxyl content of the coal. We also probed this feature of the chemistry in another way by introducing other oxygen functional groups. The introduction of the aromatic methoxy groups leads to an increase in the carbon monoxide yield as expected (eq 2).
-
(coal)OCH2C6H40CH3 (coai)OCH2C6H40'+ CH3' (24 (COal)OCHzC6H@'
- co
(2b)
In contrast, when an ester group is introduced into Illinois No. 6 coal, the carbon dioxide yields do not increase appreciably, even though the total oxygen yield increases significantly. It should be noted that methyl esters were introduced on the basis of the idea that they form carboxy radicals that undergo facile decarboxylation (eq 3). Al-
-
(coal)OCHzCHzCHzCHzC02CH3 (coal)OCHzCH2CHzCHzCOz* + CH,' (3a)
-
(coal)OCH2CHzCH2CH2COz* (coal)OCHzCHzCHzCHz'+ COP (3b) ternately, ethyl esters were covalently incorporated into the Wyodak coals. These esters decompose thermally to form acids initially,12 which further decompose during (10) Anthony, D. B.;Howard,J. B.; Meissner, H. P.; Hottel, H. C.Reu. Sci. Instrum. 1974, 45, 992. (11) Schenkel, H. Helu. Chim. Acta 1946,29,936. Schenkel, H.; Klein, A. Helu. Chim. Acta 1945, 28, 1211. (12) (a) DePuy, C. H.; King, R. W. Chem. Reu. 1960, 60, 431. (b) Smith, G.G.;Kelly, F. W. h o g . Phys. Org. Chem. 1971, 8, 75.
0887-0624/89/2503-0428$01.50/0
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pyrolysis (eq 4). The yield of the carbon dioxide was (coal)OCHzCHzCHzCHzC02CzH6 (coal)OCHzCHzCH2CH2COzH + C2H4 (4a)
-
(coal)OCHzCH2CHzCHzCOzH (coal)OCHzCH2CH2CH3+ C02 (4b) similar to the yields obtained with the reference coal sample even though the total oxygen yield was higher. Thus, our results for the modified coals indicate that the introduction of an aromatic methoxy group leads to a specific enhancement of the carbon monoxide yield but that the introduction of aromatic and aliphatic carboxyl groups does not produce additional quantities of carbon dioxide. As already mentioned, previous workers have proposed that the carbon dioxide formed during coal gasification is derived from the carboxyl groups present? Our results, in contrast, show that the carbon dioxide yield is not closely related to carboxylic group abundance; neither the introduction nor the removal of carboxyl groups alters the yield appreciably. These new results imply that the carboxyl groups present in coal are not uniquely responsible for the formation of carbon dioxide. To determine whether the carbon monoxide was converted to carbon dioxide in the gas phase or during the workup of the products, we introduced 018-labeledcarbon monoxide into the reaction vessel together with unlabeled carbon dioxide. The reaction system was subjected to a thermal pulse in the usual way, and the reaction products were isolated. No labeled carbon dioxide was detected by mass spectroscopy. Thus, it seems reasonable to conclude that the carbon monoxide and carbon dioxide interconvert in the coal particle prior to convergence into the gas phase. Acknowledgment. We wish to thank Dr. Bruce Solka and Gerald Koncar for performing the detailed mass spectral analysis of the pyrolysates. We gratefully acknowledge the support of this research by a grant from the Gas Research Institute. Registry No. 1, 57293-19-3; 2, 874-98-6; 3, 62290-17-9; 4, 5454-83-1;5, 14460-52-7; COP, 124-38-9; CO, 630-08-0. Kuntal Chatterjee, Balkrishna Bal, Leon M. Stock* Department of Chemistry The University of Chicago, Chicago, Illinois 60637 Robert F. Zabransky Gas Research Institute, Chicago, Illinois 60616 Received November 30, 1988 Revised Manuscript Received February 8, 1989
Capillary Zone Electrophoresis of Fuel Materials
Sir: This communication demonstrates the potential of capillary zone electrophoresis (CZE) for the high-resolution separation of polar and high molecular weight fuel-related materials. A wide range of compounds can be induced to exhibit varying electrophoretic mobilities by varying the pH and composition (e.g., using organic modifiers) of the buffer solution. CZE is a form of free electrophoresis conducted in small-diameter capillaries (generally with
