Energy & Fuels 1987, 1, 65-72
65
Pyrolysis of 0-Methylated Illinois No. 6 Coal. Reaction Pathways Sumit R. Mahasay, Rachel Nardin, and Leon M. Stock* Department of Chemistry, University of Chicago, Chicago, Illinois 60637
Robert F. Zabransky* Institute of Gas Technology, Chicago, Illinois 60616 Received March 27, 1986. Revised Manuscript Received September 11, 1986 Illinois No. 6 coal was modified by selective 0-methylation. The 0-methyl, 0-methyl-d,, and 0-methyl% coals were pyrolyzed in a wire-screen reactor at 600-850 O C . The char, tar, and gas yields were measured, and the distributions of the isotopic labels in the tar and gaseous products were determined. The product and isotope distributions provide insight concerning the thermal decomposition of the 0-methyl coal and the principal reaction pathways of the methyl radicals formed in these reactions. The results for the tar reveal that certain kinds of 0-methyl groups are selectively decomposed. Deuterium is incorporated into benzylic and aliphatic positions of the tar. The observations for the gaseous products imply that methyl radicals abstract hydrogen atoms selectively from the coal molecules and thereby initiate an array of other reactions. The work with labeled compounds indicates that the methyl radicals also undergo substitution and addition reactions that lead to low-molecular-weight hydrocarbons and other products. Methoxy radicals also participate in the reactions. Plausible reaction pathways are discussed.
Introduction The development of a thorough understanding of the microscopic chemistry of coal and the relationship between its structure and reactivity are essential for the successful creation of new conversion technologies. Recent advances in the field of coal pyrolysis have increased the understanding of thermal conversion reactions and provided empirical models for the examination of coal gasification reaction^.'-^ Still, discussions of the fundamental chemistry involved in these processes remain largely qualitative, primarily due to the limited knowledge of the structure of coal and the complexity of its reactions. New knowledge about functional groups in coal provides an opportunity to investigate its chemical behavior in a rational way. Recent work in this laboratory has quantitatively established the nature of the hydroxyl functional groups present in representative samples of Illinois No. 6 The results are summarized in Table I. It is possible to 0-alkylate this coal selectively by using mild bases to catalyze the reaction (eq 1and 2).536 Con(coa1)OH + B (coa1)O- BH+ (1) (coa1)O- RX (coa1)OR X(2) B = Bu,NOH or KOH; R = primary alkyl halides or tosylates sequently, many primary alkyl fragments can be introduced into known locations in this coal, and the chemistry of the fragments can be investigated in a rational way. The pyrolysis of coals modified by the introduction of 0-benzyl groups provided valuable information defining the course of the thermal decomposition reactions of phenylmethyl radicals and related chemistry.'~~The products obtained
-
+
+
+
+
(1)Howard, J. B.Chemistry of Coal Utilization; Elliott, M. A., Ed.; Wiley: New York, 1981; Second Supplementary Volume, Chapter 12. (2)Gavalas, G. R. Coal Pyrolysis; Elsevier: New York, 1982. (3)Solomon, P.R.; Hamblen, D. G. Chemistry of Coal Conuersion; Schlosberg, R. H., Ed.; Plenum: New york, 1985;pp 121-252. (4) Stock, L. M. Willis, R. S. J. Org. Chem. 1985, 50, 3566. (5)Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277. (6)Ettinger, M.; Nardin, R.; Mahasay, S. R.; Stock, L. M. J. Org. Chem. 1986,51, 2840.
0887-0624/87/2501-0065$01.50/0
Table I. Relative Abundances of the Hydroxy Compounds in Illinois No. 6 Coal Determined by Analysis of 0-Methylation Products re1 abundanceb of structural structural element" element/100 C 1. Region Centered at 61 ppm primary aliphatic alcohols hindered aromatic phenols
0.1 2.2
2. Region Centered at 55 ppm unhindered aromatic phenols dihydroxyaryl compounds simple aromatic phenols
0.6 3.3
3. Region Centered at 51 ppm aryl and alkyl carboxylic acids
0.8
"The resonances of the methyl ethers and esters of the 0methylated coal appear in three spectral regions. These groupings are shown in Figure 1. *There are approximately seven 0-methyl groups per 100 carbons in a typical fully methylated product.
from 0-benzyl-d, and O-benzyl-l-13Cfragments provided especially definitive information. The success realized in that series of experiments prompted this study of the pyrolysis of Illinois No. 6 coal modified with 0-methyl, 0-methyl-d,, and O-methyl-13Cgroups.
Results Synthesis of the Modified Coals. The selective introduction of 0-alkyl groups into Illinois No. 6 coal has been discussed by Liotta and his associates5 and by Ettinger and his associates.6 The alkylation af Illinois No. 6 coal occurs primarily on oxygen atoms when tetrabutylammonium hydroxide or potassium hydroxide is used to catalyze the reaction of methyl iodide or methyl tosylate with the coal in moist tetrahydrofuran. The initial me(7) Rose, G. R.; Zabransky,R. F.; Stock, L. M.; Huang, C. B.; Sriniv?, V. R.; Tse, K. T. Prepr. Pup.-Am. Chem. Soc., Diu. Fuel Chem. 1984, 29(2), 32. (8)Rose, G. R.; Zabransky,R. F.; Stock, L. M.; Huang, C. B.; Srinivas, V. R.; Tose, K. Fuel 1984,63, 1339.
0 1987 American Chemical Society
66 Energy &Fuels, Vol. 1, No. 1, 1987
Mahasay et al.
Table 11. Results for the Pyrolysis of Illinois No. 6 Coal and Its Methylated Derivatives coal type temp, "C char, wt % tar, wt % gas, wt % by difference gas yields, mol/100 carbons CH,
coco2 C2H6
C2H4
C3Hs C3H6
raw
blank
blank
0-CH3 0-CH3 O-CD3 O-CD3 0J3CH3
0-13CH3 O - W H ~
723 59.2 32.9 7.9
697 64.0 24.0 12.0
834 51.4 21.6 27.0
681 62.7 34.3 3.0
848 54.7 23.4 21.9
717 58.6 18.1 23.3
839 58.3 22.6 19.1
690 61.2 17.0 21.8
770 50.0 23.1 26.9
798 50.5 24.8 24.7
1.25 0.89 0.84 0.25 0.27 0.05 0.08
2.76 2.68 3.59 0.48 1.00 0.12 0.13
3.88 3.46 2.18 0.60 1.35 0.09 0.46
0.98 0.37 0.21 0.07 0.06 0.02 0.02
6.55 3.61 1.10 0.32 0.72 0.05 0.51
3.53 2.08 2.87 0.53 0.24 0.13 0.24
0.0006 0.03 1.85 0.0004 0.002
0.13 0.32 2.40 0.001 0.0004
0.11 0.44 6.33 0.001 0.002
0.02 0.00 0.60 0.0002 0.0001
0.00 0.06 4.25 0.002 0.006
0.02 0.10 6.74 0.006 0.0002
5.36 3.87 1.60 0.32 0.42 0.09 0.22 0.003 0.02 0.09 1.28 0.11 0.001
4.91 1.56 1.30 0.43 0.31 0.12 0.19 0.004 0.060 0.19 1.39 0.002 0.0008
7.82 2.86 1.62 0.72 0.63 0.16 0.30 0.004 0.04 0.17 6.87 0.002 0.0008
12.74 2.03 1.36 0.45 0.59 0.10 0.24 0.002 0.004 0.12 3.25 0.004 0.0005
0.008 0.0003 0.001 0.007 0.002 0.0005 0.001 0.0003 0.0004 0.010
0.022 0.001 0.002 0.010 0.003 0.001 0.002
0.036 0.0005 0.002 0.005 0.004 0.0005 0.002 0.0004
0.0012 0.0003 0.0003 0.0020
0.032 0.0004 0.001 0.010 0.002
0.018 0.002 0.003 0.014 0.011
0.026
0.019 0.004 0.006 0.023 0.005
0.031 0.003 0.005 0.019 0.008
0.034 0.002 0.003 0.015 0.007
0.0001 0.0001 0.0002 0.001
0.002 0.0004 0.001 0.017
0.002 0.0009 0.002 0.032
0.0025 0.001 0.004 0.022
0.005
0.006
0.002 0.008
0.001 0.006
i-C4H10 n-C4H10
C4Hs H,O C6H6CH3 minor components, mo1/100 carbons butadienes i-CI n-C6 pentenes pentadienes cyclopentene cyclopentadiene n-C6 hexenes
cos
0.002 0.01
0.012
thylation reaction provides a coal with five 0-methyl groups per 100 carbons. A second methylation of this material produces a coal with 7.5 0-methyl groups per 100 carbons. Illinois No. 6 coal
B
B
(c0al)(0CH~)~
(coal)(OCH3),.5 (3) We elected to limit the degree of methylation so that several of the original hydroxyl groups would be retained in the product. It is pertinent that tetrabutylammonium ion, one of the catalytic agents for the etherification, is sometimes tenaciously retained in the reaction products even after repetitive washing of the material with aqueous methanol. Specifically, we found that traces of tetrabutylammonium ion derivatives appeared in pyrolysis mass spectral ana lyse^.^ While this contaminant can be removed by repeated washings with hot aqueous methanol, the problem can be wholly circumvented by the adoption of potassium hydroxide as the catalyst for the 0methylation reactionse6 In addition, potassium hydroxide is a particularly appropriate catalyst when modest levels of alkylation are desired. Pyrolysis Experiments. All pyrolysis experiments were performed in a wire-screen reactor of the type described by Anthony and his associates.10 This reactor was selected because it can be used over a broad range of experimental conditions. It also has the advantage of effectively eliminating secondary gas-phase reactions because the environment surrounding the screen and coal particles remains essentially at room temperature; hence, as the gaseous products form and diffuse away from the coal particle, they cool instantly. Char and tar yields were determined gravimetrically. The gaseous products were collected separately and analyzed mass spectrometrically. (9) We are indebted to R. E.
Winans of the Argonne National Laboratory for performing this experiment. (10)Anthony, D. B.; Howard, J. B.; Mehner, H. P.; Hottel, H. C. Reu. Sci. Instrum. 1974, 45, 992.
0.004 0.016 0.013 0.002 0.011 0.005
Table 111. Relative Abundances of Gaseous Products from the Pyrolysis of 0-Methyl-d, Coal re1 abundance, % re1 abundance, % product at 717 "C at 839 "C product at 717 OC at 839 O C 100 100 C3H.9 100 100 39 50 C3H7D 32 30 27 27 C3HeD2 19 16 59 69 C3HbD3 12 11 2 4 CsH4D4 1 3 ~~
100 37 16 6 1
100 32 18 10 3
a a
100 32 19 12
1
100 14 4 a
C3H3D6 C2H2D6
3 2
3 1
C3HD7 C3Ds
a
a
100 15 6 3 1
100 22 8 5 1
C3H6
C3H6D C3H4D2 C3H3D3 C3H2D4
a
1
100 4
100 2
1 100
100
C3HD5 C3D6 C6H6
CBHSD C6H4D2
C7Hs C7H7D
9
Trace; the quantity produced was 0.1-0.9%.
The data for the gaseous products obtained in the pyrolysis of different methylated coals are presented in Tables 11-IV. The intermediate molecular weight, volatile compounds produced during the pyrolysis of the 0-methyl-d3 and O-methylJ3C coals were collected as described in the Experimental Section and were examined by 13C and 2H nuclear magnetic resonance spectroscopy. The spectra of these products are contrasted with the spectra of the starting material in Figures 1 and 2.
Discussion Preparation of Modified Coals. The synthetic work proceeded very smoothly and all of the 0-methylated coals necessary for this investigation were prepared readily.
Pyrolysis of 0-Methylated Coal
Energy &Fuels, Vol. 1, No. I , 1987 67
L
Table IV. Relative Abundances of Gaseous Products from the Pyrolysis of O-Methyl-lsCCoal
I
product
a
CH4 13CH4
1420 2820
2600 3200
4250 3970
C2H6 l3CCH6 '3CZH6
300 55 15
430 83 20
240 43 10
C2H4
250 20
430 33 6
350 26
100 17 a
51 9 140 11
WCH4 13CzH4
b
C3H8 13CC2H8 '3C2CH8
,
1
1
58 56
62 60
1
1
54
52 50
90 13
a
a
a
140 13 5
200 19
a
a
C4HlO 13CC3Hlo
55
36
24
a
a
a
C4H8 WC3Ha
160 7
120 7
74 5
14
21
20
a
a
a
13co
1200 150
1900 220
1200 110
co2
1120
1200
880
16 70
12 90
27
1
C4H6
spectra at 100.1 MHz of extracts of (a) O-methyl-13C Illinois No. 6 coal and (b) the volatile pyrolysis products (tar) obtained from 0-methyl-l3C coal at 767 OC.
a
C3H6 '3CCzH6 13C&H6
CHEMICAL SHIFT, ppm
Figure 1.
re1 abundance: ppmv at 690 "c at 770 "c at 798 "c
13CC3H6
13C NMR
co I3CO2 CH3OH
WH30H
a
" A trace consituting a detectable amount between 1 and 5 PPm".
t
u I
I
I
,
4
2
0
6
1
4
2
CHEMICAL SHIFT, ppm
Figure 2. 2HNMR spectra at 61.4 MHz of extracts of (a) 0methyl-d3Illinois No. 6 coal and (b) the volatile pyrolysis products (tar) obtained from 0-methyl-d3coal at 878 O C . Several methods were used to determine the alkyl content of the coal. First, the differences in the carbon and hydrogen content between the reference sample and the methylated coal samples suggest that three 0-methyl groups per 100 carbons were introduced under the reaction conditions. Second, infrared spectra indicate the presence of the unreacted hydroxyl groups, the enhanced C-H stretching absorption due to the added methyl groups, and a weak ester carbonyl stretching band at 1740 cm-l. The infrared spectra of coal samples prepared with 0-methyl-d, groups exhibit additional carbon-deuterium stretching bands at 2250 and 2150 cm-l. Third, the 2HNMR spectrum of an extract of 0-methyl-d, coal (Figure 2a) shows a strong resonance centered at 3.7 ppm, which is indicative of the newly formed 0-methyl-d, linkages. The 13C NMR spectrum (Figure l a ) of the pyridine extract of coal methylated with methylJ3C iodide show three sets of resonances, which are designated 1-3 for convenient dis-
cussion, and each of which is characteristic of a particular class of methoxyl carbon atoms. This spectrum closely resembles the spectra of other methylation products reported earlier.*y5 However, the significant improvement in the resolution due to the higher magnetic field used in this study led to the resolution of the band in region 3 as described in the Experimental Section. The results obtained in the present study are in accord with the previous work and establish that 85% of the 0-methyl groups are bonded to aromatic structural elements. Pyrolysis, General Features. The pyrolytic reactions of Illinois No. 6 coal, the reference sample prepared by the treatment ef the coal with the basic catalyst in tetrahydrofuran but without the methylating agent, and the 0-methylated coals were accomplished without difficulty by using the wire screen reactor. Only a representative portion of the results obtained in this investigation are presented in Table 11. The mass balances sometimes exceeded 100% in this series of experiments. While the yield of char and tar obtained from the methylated coals are in the expected range and are within experimental error of the yields of the corresponding materials obtained with the reference, the measured yields of gaseous products tended to be high. The uncertainty arises in the experimental measurement of the pressure exerted by the gaseous compounds after they have been purged from the Tenax absorbant. This pressure is generally 20-50 Torr. Small leaks into the sampling bulb can, therefore, greatly effect the measurement, hence, the calculated gas yield. On the other hand, the mass of the char and tar can be readily assessed. Thus, the uncertainty introduced by the high mass balance is small, and the major trends in the course of the pyrolysis reaction can readily be discerned. The distribution of the gaseous products formed from the modified Illinois No. 6 coals used in this study are
68 Energy & Fuels, Vol. 1, No. 1, 1987
similar to the product distributions obtained with other The tar and char yields obtained from bituminous ~0als.l~ the modified coals are significantly lower than the yields of these materials realized with other bituminous coals under comparable conditions. It has been suggested that these differences arise because certain lower molecular weight substances are removed from the macromolecular network of the coal molecules during the alkylation reaction in tetrahydrofuran and during the workup procedure.8 Temperature has a significant effect on the yields of char, tar, and gas formed from the modified coals (Table 11). Generally, the char yields obtained from the bituminous coals decrease significantly as the temperature is raised from 600 to 800 C and continue to decrease, but less dramatically,as the temperature is raised to 850 OC.12 The available observations suggest that there is a significant change in the course of the pyrolysis reaction near 800 OC. A broader array of volatile products are obtained in the experiments carried out at temperatures above 800 "C. Presumably, an array of thermolytic reactions occur at these high temperatures and the product distributions become correspondingly more complex. Studies of the rate of gas evolution as a function of temperature have been carried out by several research groups.13J4 Although significantly different rates of gas evolution have been reported by the different research groups, it is evident that temperature has a significant influence on the product distribution, that the rates of evolution of the different gases achieve maximum values at different temperatures, that the rates of evolution of certain of these gases exhibit a complex dependence upon temperature with several distinct maxima, and that the chemical composition of the coal alters the rate of evolution of these gases. For example, methane is formed over a broad range of temperature and three distinct maxima can be distinguished in plots of the rate of its evolution as a function of temperature.13J4 The rate of carbon monoxide evolution is equally complex with three or more maxima discernible in the reaction ratetemperature profile.13J4 Clearly, these differences are rooted in the chemical nature of the coal molecules that must decompose to produce reactive intermediates that subsequently form one or another of the principal products. Solomon and Hamblen have proposed that these maxima occur because carbon monoxide originates from two different sources which are characterized by high and low activation energy processes, re~pectively.~ Solomon has also suggested that the carbon monoxide produced in the higher temperature regime, near 800 "C, originates from aryl ethers.12 Additional study of this aspect of the problem will be most rewarding. Pyrolysis, Volatile Liquid Products. Tar is a mixture of many compounds with molecular weights ranging from 100 to 2000 that is volatile at the pyrolysis temperatures.12 In related work, we observed that the pyrolysis of the modified coals in high vacuum from 350 to 450 O C produces a tar with a mass range from 100 to 1800 Da.15 In addition, several workers have pointed out that there is a striking similarity between the spectroscopic properties of the tar and the original coal.16-'* However, there are (11)Suuberg, E. M. Sc.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1977. (12)Solomon, P. R.;Hamblen, D. G. Chemistry of Coal Conuersion; Schlosberg, R. H., Ed.; Plenum: New York, 1985;pp 219-232. (13)Campbell, J. H. Fuel 1978,57, 217. (14)Juntgen, H. Fuel 1984,63, 731. (15)Stock, L. M.; Willis, R. S., unpublished results. (16)Brown, J. K.;Dryden, I. G. C.; Dunevein, D. H.; Joy, W. K.; Pankhurst, K. S. J.Znst. Fuel 1958, 31, 259. (17)Orning, A. A.; Greifer, B. Fuel. 1956, 35, 318. Solomon, P. R.; Colket, M.B. Fuel 1978, 57, 748.
Mahasay et al.
discernible differences in the aliphatic hydrogen content, and Solomon and his co-workers have noted that methyl groups appear to be more abundant in the tar than in the original coal.lg These observations prompted us to examine the 2H and 13CNMR spectra of the tars produced in the pyrolysis of 0-methyl-d, and O-methyl-13Ccoals. Three distinct resonance bands centered at 1.3, 2.5, and 3.8 ppm are observed in the 2H spectrum of the reaction product of 0-methyl-d, coal (Figure 2b). The resonance at 3.8 ppm, which is present in the starting material (Figure 2a) as well as in the tar, almost certainly arises from 0-methyl-d, groups. The new absorptions in the upfield region with broad maxima at 1.3 and 2.5 ppm establish that an array of new carbon-deuterium bonds are formed during the pyrolytic reactions. Chemical shift correlations indicate that the hydrogen atoms of methyl and methylene groups bonded to aromatic structures exhibit resonances between 2 and 3 ppm, whereas only hydrogen atoms of methylene groups in fully aliphatic environments could exhibit resonances near 1.3 ppm. Information concerning the pathways for hydrogen-transfer reactions during the thermal decomposition of coals in liquids at 350-450 OC provides a starting point for the discussion of these results.20 It is recognized that benzylic radicals can be formed readily either by carbon-hydrogen or carbon-carbon bond-cleavage reactions. Thus, deuterium can be introduced into benzylic positions via direct exchange at benzylic sites (eq 4 and 5) or by an abstraction reaction of a benzylic radical generated by the cleavage of an ethano bridge (eq 6 and 7). (coal)Ar(CH,),Ar(coal) (coal)ArCH(CHJ,Ar(coal) (4) (coal)ArOCD, + (coal)ArCH(CHz),Ar(coal) (coal)ArCHD(CHJ,Ar(coal) (coal)ArOCD,' (5)
-
--
-
+
( (coal)ArCHz)z
2(coal)ArCHz' (6) (coal)ArCH,' (coal)ArOCD3 (coal)ArCH,D + (coal)ArOCD,' (7) The new carbon-deuterium linkages generated in these processes would exhibit resonances near 2.5 ppm. The reactions outlined in eq 6 and 7 are similar to those proposed by Solomon and his co-w~rkers'~ to account for the abundance of methyl groups in tar; however, neither of these expected reaction sequences accounts for the appearance of deuterium atoms in aliphatic positions. A plausible reaction sequence that leads to aliphatic carbon-deuterium is shown in eq 8-11. (coal)CH,CH2CH2(coal)+ (coal)ArCH,' (coal)CHzCHzCH(coal)+ (coal)ArCH, (8)
+
(coal)CH,CH,CH(coal)
-
-
(coa1)CH;
--
+ CH,=CH(coal) (9)
(coal)CH=CH, + R' (coal)CHCH,R (10) (coal)CHCH,R + (coal)OCD, (coal)CHDCH,R + (coa1)ArOCD; (11) These reactions illustrate the formation of a vinyl group by @-scissionand the addition of another radical, R', to the double bond. Similar reaction sequences have been proposed to account for the addition of tetralin in coal liquidsz1and to account for the conversion of tetralin to (18)Poutsma, M. L.;Youngblood, E. L.; Oswald, G. E.; Cochran, H. D. Fuel 1982,61, 314. (19) Solomon, P. R.; Hamblen, D. G.Chemistry of Coal Conuersion; Schlosberg, R. H., Ed.; Plenum: New York, 1985;p. 167. (20)Stock, L. M.Chemistry of Coal Conuersion; Schlosberg, R. H., Ed.; Plenum: New York, 1985;Chapter 6.
Pyrolysis of 0-Methylated Coal methyltetralin and methylnaphthalene during liquefaction reactions.22 The role of unsaturated centers as substrates for radical addition reactions during coal pyrolysis is currently being investigated. While the very small amount of tar liberated in these pyrolysis experiments complicates the acquisition of 13C NMR data, it is clear that there are intense resonances in the 50-65 ppm region of the spectrum of the tar obtained from the O-methyl-13Ccoal (Figure lb). No other strong resonances are discernible in this spectra. It should, however, be pointed out that the detection limit of the NMR instrument is approximately 0.02 methyl group/lOO carbons in 2 ppm of the spectrum. This finding implies that few methyl-13Cgroups are incorporated into other structural fragments of the tar. We are continuing our study of other tars and chars to obtain more information on this significant point. There are small, but notable, differences in the 13CNMR spectra of the modified coal and the tar. Specifically, the relative intensities of the resonances in regions 1-3 (Figure 1) change from 0.25:1.00:0.15 in the starting material to 0.15:1.00:0.09 in the tar. Thus, according to the information summarized in Table I, the abundance of hindered aryl methyl ethers and methyl esters are selectively depleted during pyrolysis. The distribution of 2H and 13C in the tars establish several important points. First, many 0-methyl fragments survive the pyrolysis and appear in the tar, but the hindered methyl aryl ethers and the methyl carboxylates are selectively depleted. Second, there is also no evidence for the incorporation of methyl-13Cgroups into the volatile tar molecules. Thus, the methyl radicals produced in the homolysis reactions selectively undergo abstraction reactions rather than addition reactions to volatile aromatic molecules. This result is somewhat surprising because methyl radicals are expected to participate in aromatic substitution reactions under the conditions of these reactions. Additional experiments are planned to test this conclusion. In particular, it is pertinent that the arylmethyl-13C compounds may be more abundant in the nonvolatile char. Third, the highly aromatic volatile tar is also free of aryl deuterium atoms. This result is also somewhat surprising because aryl radicals generated during the pyrolysis should abstract deuterium from the 0methyl-d3groups. As in the previous case, aryl deuterium atoms may be selectively incorporated into the char and experiments are also under way to examine this issue. Fourth, both benzylic and aliphatic methyl, methylene, and methine resonances are detected in the spectra of the tar. Clearly an array of new carbon-deuterium bonds are formed via exchange and trapping reactions of the kind shown in eq 4-7. Pyrolysis, Gaseous Products. The 0-methyl groups that have been introduced into the coal can react in several ways. The information presented in Table I and the spectroscopic data shown in Figures 1and 2 indicate that the methyl groups are present predominantly as aryl ethers in the modified coals. Bond homolysis yields either methyl radicals (eq 12) or methoxy radicals (eq 13). The familiar (coal)ArOCH3 (coa1)ArO' + CH3' (69 kcal/mol) (12) (coal)ArOCH3 (coa1)Ar' + CH30' (101 kcal/mol) (13) Benson approach23was used to estimate the bond disso-
--
(21)Poutama,M. L.;Youngblood, E. L.; Oswald, G. E.; Cochran, H.
D.. -Fuel 1982.61. ... . -. - -1 - - I
314.
(22)King, H.H.; Stock, L. M. Fuel 1982,61, 257. (23)Benson, S.W.Thermochemical Kinetics; Wiley: New York, 1976.
Energy &Fuels, Vol. 1, No. 1, 1987 69 ciation energies for a phenyl methyl ether. This analysis indicates that methyl radicals should be formed predominantly. The methyl and methoxy radicals may also form from the methyl esters introduced into the coal during methylation (eq 14 and 15). Although such reactions
-
(coal)COzCH3
(coal)C02' (coal)C02CH3
(coal)CO,'
+ CH,'
(80 kcal/mol) t1 4 4
+ C02 (9 kcal/mol) (14b) (coa1)CO + CH,O' (100 kcal/mol) coal'
(15)
contribute to the chemistry of this coal, we shall defer a discussion of this aspect of the reaction until information is obtained for modified coals with ester fragments. Methyl radicals produce methane by the abstraction of hydrogen or deuterium atoms from the coal and low-molecular-weight hydrocarbons through coupling, substitution, and othe reactions. Methoxy radicals form methanol and the carbon monoxide. Approximately 92% of the 13C in the gaseous products obtained in the pyrolysis reaction at 770 "C is found in low-molecular-weight hydrocarbons, and 8% is found in methanol and carbon monoxide. This observation suggests that reactions 12 and 13 both occur but that the rate of formation of methyl radicals is about 10-fold greater than the rate of formation of methoxy radicals. Aryloxy and aryl radicals are formed in corresponding amounts in these reactions. Low-molecular-weight saturated and unsaturated hydrocarbons, carbon monoxide, carbon dioxide, and water vapor are the major gases formed in the pyrolysis reactions. The introduction of 0-methyl groups into this coal alters the absolute and relative yields of these gases. We have analyzed the changing product distributions and the fates of labeled methyl fragments to obtain a better understanding of the reactivity patterns of the methyl radicals, including, for example, information on the origin of ethane and the other volatile hydrocarbons. Generally, the yields of the unlabeled gaseous products increase when the temperature is increased. The yields of methane and methane-13C formed from the pyrolysis of O-methyl-13Ccoal illustrate this feature of the results (eq 16). The ratio of (coal)013CH3 690 "C 798 OC
-
13CH4 2820 ppmv 3970 ppmv
+
12CH4 1420 ppmv 4250 ppmv
(16)
unlabeled methane to methaneJ3C increased from 0.5 to 1.0 as the temperature increased. Temperature also alters the relative yields of the labeled and unlabeled methanes obtained from 0-methyl-d3coal. The changes in the ratios can be directly attributed to the increased yield of unlabeled methane from the coal at the higher temperature. Clearly, methane and presumably the other hydrocarbons are produced from the coal molecules in reactions with significantly larger energy requirements than exhibited by the homolysis of the 0-methyl groups in the modified coal. Thus, the reactions of the radicals within the decomposing coal particle exhibit significant selectively even at temperatures approaching 850 OC. Hydrocarbons other than methane are also formed, but in quantities much smaller than any of the one carbon compounds. The yields of ethane, ethene, propane, and propene and the relative abundances of the monodeuterio derivatives obtained from the pyrolysis of 0-methyl-d3coal at 717 and 839 "C are shown in Table V. Similar amounts of two and three carbon atom compounds are formed at these two temperatures. The presence of 13C(Table IV) in these substances suggests that
70 Energy & Fuels, Vol. 1, No. 1, 1987
Mahasay et al.
Table V. Yields of Two- and Three-CarbonCompounds in the Pyrolysis of O-Methyl-d3Coal total yield, mo1/100 C isotooic comDosition. mol % temp, "C 717 839
C2H4
0.24 0.42
CZH6 0.53 0.32
CaHs 0.23 0.22
C3H8 0.12 0.09
Table VI. Deuterium Distribution in Methane re1 yield, mol %
CH4 CH3D CHzDz CHD, CD4
64 30 6
44 17 12 26 1
40 20 11 27.5 1.5
a small proportion of these compounds arise from the coupling and addition reactions of methyl radicals, but the observations rather securely establish that the compounds arise predominantly by the decomposition of structural fragments present in the coal. Calkins and his co-workers have shown that there is a direct relationship between the amount of alkenes formed during pyrolysis and the amount of polymethylene fragments present in coal.x* Although no detailed reaction pathway was described, these workers proposed that the polymethylene fragments were the actual precursors of the unsaturated gases (C2H4,C,&, C4&) produced in the reaction. As discussed subsequently, the most probable pathways for their formation involve radical initiated @-scissionreactions that produce small unsaturated fragments from paraffinic chains. The methane formed during the pyrolysis of the 0benzyl-d, and 0-methyl-d, coals was also found to be labeled to a very significant degree (Table VI). The yield of methane-d3is high, about 27 % . In addition, significant quantities of methane-dz are formed from both 0-benzyl-d7and 0-methyl-d, coals. The occurrance of this compound among the pyrolysis products of both modified coals strongly suggests that the 0-methyl-d, coal undergoes extensive exchange during the pyrolytic reaction. These results are in accord with the idea, already expressed, that exchange occurs before the carbon-oxygen bond undergoes homolysis (eq 17 and 18). We estimate that about 40% (coal)ArOCD, (coal)ArCH, (coal)ArOCD,H (coal)ArCH,D (17)
+
(coal)ArOCD,H
-
-+
(coa1)ArO'
+ CDzH'
(18)
of the methane formed in the reactions of the 0-methyl-d, coal at 700-850 OC arises from the 0-methyl groups via such reactions. The methanelmethane-d ratios for the 0-benzyl-d, and 0-methyl-d, coals differ in a significant way. Specifically, the methylene group in the 0-benzyl-d, coal is about 1.5fold more effective as a hydrogen atom donor than the 0-methyl-d, group. This observation supports the conclusion that the reactions remain selective at high temperature. The ethane, ethene, propane, and propene produced from the 0-methyl-d3 coal are extensively labeled. It is striking that between 5% and 10% of these compounds contain two or more deuteriums. This feature is well illustrated by the difference in the results for the ethene produced from the 0-methyl-d, coal and the 0-benzyl-d, coal (Table VII). (24) Calkins, W. H.;Hagaman, E.; &Ides, H.Fuel 1984, 63, 113. (25) Calkins, W.H.;Tyler, R. J. Fuel 1984, 63, 1119.
C2H3D 20 11
C3H7D 19 18
C2H5D
23 20
12 16
Deuterium could, in principle, be incorporated into methane and ethane via abstraction reactions of the alkyl radicals. The ratios of methane to methane-d and ethane to ethane-d are between 2 and 2.5. Thus, the alkyl radicals formed from the original coal abstract hydrogen atoms from other coal molecules no more than 2.5 times more frequently than from the 0-methyl-d3groups. This finding and the deuterium NMR spectrum of the tar (Figure 2) support the view that the methoxy groups in the coal are efficient hydrogen atom donors. More striking is the fact that more ethene-d2is produced from the 0-methyl-d, coal than from the 0-benzyl-d, coal even though the donor in the benzyl-d, group is superior to the donor in the methyl-d, group. This observation implies that other more subtle processes are involved in the incorporation of deuterium into higher molecular weight gases. I t is also pertinent that 13Cappears in the two, three, and four carbon atom hydrocarbons obtained in the pyrolysis of O-methylJ3C coal. The relative abundances of 2H and 13Cin the gaseous hydrocarbons are summarized in Table VIII. Relatively large quantities of the two and three carbon atom compounds contain at least two 2H or one 13Catom. Indeed, 12% of the 13C atoms in the gaseous products obtained in the pyrolysis a t 690 "C appear in substances other than methane. The results summarized in Table VI11 establish that, within the limits of experimental error, similar amounts of compounds with two ,H or one 13Care obtained during the pyrolysis of the differently labeled coals. The appearance of ,H and 13C in these compounds can be most readily accounted for on the basis of dimerization, displacement, or addition-elimination reactions. While the observations of Calkins and his ~ o - w o r k e r s ~ ~ suggest that polymethylene fragments are responsible for the formation of low-molecular-weightalkenes, it is clear from our study that the reaction pathway also involves radicals originating elsewhere in the coal matrix. Although the evidence for radical displacement reactions (eq 19) at carbon atom centers is meager at low temperature, reactions of this kind may occur at high temperature. (coal)Ar0l3CH, + CH,' (coa1)ArO' + H3C13CH3 (19) However, a more conservative interpretation of the results stresses the involvement of multistep addition-elimination reactions as shown in eq 20-26. (coal)013CH3 (coa1)O' + 13CH3' (20)
-
+ - + + -
(coal)CH,CH=CHR
13CH3' (co~I)CH,CHCH(~~CH~)R (21)
(coal)CH,CHCH(13CH3)R
(coal)CHCH2CH(l3CH,)R (22)
(coal)CHCH2CH(13CH3)R (coal)CH=CH, R"
+ RC(13CH3)H
RC(13C3)H' R'H
RC(I3CH,)H' (23)
+ R'H RC(13CH3)H2+ R"
RCH=13CH2
R" other products R = hydrogen, alkyl, or aryl group
(24) (25) (26)
Energy & Fuels, Vol. 1, No. 1, 1987 71
Pyrolysis of 0-Methylated Coal Table VII. Deuterium Distribution in Ethene re1 yield, mol % from 0-benzyl-d, from 0-methyl-& ethene at 617 "C at 717 "C 85 72 21
15
6
The radical intermediates shown in these equations may undergo fragmentation, hydrogenabstraction, and coupling reactions to yield the variety of products observed in the pyrolysis. This general scheme also accounts for the labeled gases. Moreover, this reaction sequence, which can be formulated in a general way by replacing methyl-13C with a hydrogen atom, another alkyl radical, or, in appropriate circumstances, an aryl radical, provides a general scheme to account for the multiplicity of reaction products. The required alkene fragment noted in eq 21 may arise from a radical-induced or pericyclic dehydrogenation reaction of the complex coal molecule. The subsequent steps in the reaction (eq 22-26) are well-known processes for which there are ample precedents.
Summary and Conclusion An overview of the role of methyl radicals in coal gasification processes has been obtained by a study of the pyrolysis of 0-methyl coal. The pyrolysis products, both liquids and gases, originate in many selective, temperature-sensitive reactions. Thus, subtle changes in the product distribution occur as the temperature is changed. Nevertheless, the principal modes of reaction of the methyl radicals can be discerned by the use of isotopic labels. For example, the 0-methyl groups contribute to the production of volatile liquids through hydrogen-donation reactions. Methane, a significant product, arises from the 0-methyl groups and from the coal molecules via homolytic bond cleavage reactions and also via more complex processes involving addition-elimination reactions of other structural elements in the coal molecules. Several other hydrocarbons owe their origin to these methyl radicals. Notably, a significant proportion of alkenes formed in the reaction contain carbon atoms originally present in methyl radicals. These products apparently arise in multistep reaction sequences, which can be most readily accounted for as the basis of elimination and addition processes. Thus, coal gasification involves a multitude of complex reaction pathways, initiated and propagated by reactive intermediates generated by homolytic processes and probably by pericyclic reactions. This study clearly indicates that methyl radicals play a very important role in these processes.
Experimental Section Materials. A sample of unweathered, run of mine, Illinois No.
6 coal from the Peabody No. 10 mine in Pawnee, IL, was used in these experiments. Anal. Found: C, 67.65; H, 4.85; N, 1.16; Table VIII. 2Hand product CZH4 C2H6 C3H6
C3H8
lacContents of
2Ho
2Hl
72 65
21 24 12
80 61
20
C1, 0.05; S, 4.23, 0 (by difference), 9.58; ash, 12.38. The alkylating agents and most reagents were purchased from commercial sources and used without any further purification. However, tetrahydrofuran was freshly distilled from sodium benzophenone ketyl before use. Methyl-d3 iodide (99% isotopically pure) and methyl-13Ciodide (99% isotopically pure) were obtained from Cambridge Isotope Co. Preparation of Methylated Coals. The 0-methylated coal samples were prepared as described earlier by Liotta and cow o r k e r ~ .Typically, ~ the coal (1g) was stirred in freshly distilled tetrahydrofuran (25 mL) for 20 min. A solution of aqueous tetrabutylammonium hydroxide (1.54 M, 2 mL) was then added, and this solution was stirred for 2 h under nitrogen. The methylating agent (4.7 mmol) in tetrahydrofuran (5 mL) was added, and the reaction was allowed to proceed for 2 days. Tetrahydrofuran was removed with a rotary evaporator after acidification with dilute hydrochloric acid. The residue was collected on a Nucleopore Polycarbonate membrane filter (pore size 0.8 pm)and washed with 50% aqueous methanol until the washings were free of tetrabutylammonium salts. Tetrabutylammonium ion was detected with aqueous sodium tetraphenylborate and halide ions with aqueous silver nitrate. Finally, the samples were dried under vacuum at 60 "C for 2 days. Microanalytical data obtained established that the 0methylated coal contained (daf) 74.80% C and 5.73% H after a single methylation and 76.88% C and 6.48% H after two successive methylations. Comparison of the analytical results obtained by Huffman and those obtained by Commercial Testing and Engineering using the standard ASTM procedures for ultimate analyses supported the use of the microanalytical procedure. Control experiments revealed that the methylating agent used in this procedure is slowly hydrolyzed to methanol. When the 13C NMR spectra of the reaction mixture using methyl-C13iodide as the alkylating agent in tetrahydrofurand, was recorded, the intensity of the resonances due to 0-methyl groups did not change after the first day. However, the 13Cresonance of methyl iodide was still present. This resonance slowly decreased as the signal due to methanol grew. Pyrolysis Experiments. All pyrolysis experiments were performed in a wire-screen reactor of the type described by Anthony and his associates.1° In a typical experiment, 10-15 mg of coal (53-88 pm) was placed on a 325 mesh (44fim aperature), 316 stainless-steelscreen, which had been preconditioned by heating it to at least 1000 "C. The reactor was flushed with high-purity helium, which had been passed through a molecular sieve maintained a t 77 "C. The pressure in the reactor was adjusted to 0.12 MPa and the sample was heated at lo00 "C s-l to the desired final temperature. After the reaction system had cooled to ambient temperature, the gaseous products were collected by flushing several reactor volumes of helium through a Tenax-packed column (3 ft X 0.25 in.) maintained at 77 K. The reaction vessel was then opened, and the char and tar yields were determined gravimetrically by weighing the screen and the reactor internals and liners on which the tars had condensed. The gaseous products were transferred into a calibrated vessel by heating the Tenax column to 200 O C . The gas was analyzed by using a Finnigan Model 4510 GC-MS system. The gas, tar, and char yields and the composition of the gaseous products from representative experiments are summarized in Table 11. Products formed at a level less than 1ppmv are uncertain and not reported. The University of Chicago 500-MHz NMR spectrometer was used to obtain 'H NMR spectra and a Varian XL 4oo-hfHz NMR spectrometer was used for 13C NMR (100.3 MHz) and 2H NMR
the Two and Three Carbon Atom Hydrocarbons
re1 abundance, % 13CO 6 91 10 81 8 88 20 85
2H"22
=The average value was determined by (2H, + 13C1+ 13C2)/2.
8
l3C2 2 4 4
12
3
13c1
I 16
mult substitution av value, % 8 15 10 17
72
Energy & Fuels 1987,1, 72-75
(61 MHz). The 2H NMR spectra were recorded in methylene chloride (Aldrich, Gold Label), and the solvent signal (5.32 ppm) due t o 2H in natural abundance was used t o calibrate the field. The infrared spectra of the coals (2-4%) in dry potassium bromide were recorded with a Nicolet 20 spectrometer. The pellets
were dried in vacuo at 65 "C for 20 h before the spectra were recorded.
Acknowledgment. We are pleased to acknowledge the support of this work by the Gas Research Institute.
Effect of Pressure on Carbon Dioxide Induced Coal Swelling P. J. Reucroft* and A. R. Sethuraman Department of Metallurgical Engineering and Materials Science, University of Kentucky, Lexington, Kentucky 40506 Received September 9, 1986. Revised Manuscript Received October 22, 1986
Dilatometric studies have been carried out on coal samples exposed to carbon dioxide at 5 , 1 0 , and 15 atm. The sample dimensions increase on exposure to carbon dioxide, the effect increasing as pressure increases. The time to reach equilibrium swelling is shorter at the higher pressure. The swelling effect increases as the coal carbon content decreases. It is estimated that the swelling effect can account for 20-50% of the surface area determined by C02 adsorption methods.
Introduction BET surface areas determined by COPadsorption are generally much higher than N2surface area values.14 This has usually been attributed to the micropore system in coal samples not being completely accessible to N2 molecules at 77 K because of an activated diffusion process and/or shrinkage of pore^.^,^ Adsorption studies on coal employing organic vapor adsorbates yielded BET surface areas ranging from 9 to 200 m2/g depending upon the adsorbate vapor used.' It was also observed that the maximum surface area was obtained when the solubility parameter (6) of the adsorbate vapor was similar in value ~m-'.~). to the solubility parameter of the coal (-10 The results suggested that swelling may play an important role in determining coal surface area values obtained by gas adsorption methods. Maximum swelling (and surface area) occurs when the solubility parameter of the adsorbate is close to that of the coal. The studies further indicated that COz adsorption results in higher surface area values compared to N2 adsorption because the solubility parameter of C 0 2 (6 = 6 . 1 caP5 ~ m - ' . ~is) closer to the solubility parameter of many coal samples than that of N2 (6 = 2.6 cal ~ m - l . ~ The ) . ~ macromolecular nature of coal and the effect of solvent vapors and liquids in determining coal swelling characteristics has been well established in recent studies.+12 (1) Mahajan, 0. P. Powder Technol. 1984, 40, 1. (2) Mahajan, 0. P. In Coal Structure; Meyers, R. A., Ed.; Academic: New York, 1982; p 51. (3) Mahajan, 0. P.; Walker, P. L. Jr. Anal. Methods Coal Coal Prod. 1978, I, 125. (4) Mahajan, 0.P. In Sample Selection, Aging and Reactivity of Coal; Klein, R., Wellek, R., Eds.; Wiley: New York, in press. (5) Anderson, R. B.; Bayer, J.; Hofer, L. J. E. Fuel 1965, 44, 443. (6) Walker, P. L., Jr.; Geller, I.; Nature (London) 1956, 178, 1001. (7) Reucroft, P. J.; Patel, K. B. Fuel 1983, 62, 279. (8) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983.
0887-0624/87/2501-0072$01.50/0
Table I. Representative Analyses of the Three Types of Coals KCER KCER 7463 7259 KCER 7122 (sample sample (sample 1) (sample 2) 3) county Webster Hopkins Carlisle seam thickness 57.0 in. 6.0 ft 52.0 in. coal rank bituminous subbituminous lignite anal. as received (wt % ) volatile matter 40.7 34.9 33.0 fixed carbon 49.5 38.5 13.7 7.0 21.6 19.2 ash moisture 1.9 5.3 34.2 total sulfur 2.9 4.6 2.3 ultimate anal. (wt % daf)" 83.8 18.3 65.8 carbon hydrogen 5.7 5.5 7.8 nitrogen 1.0 1.0 0.1 sulfur 3.2 6.3 4.9 oxygen (by difference) 6.3 8.9 21.4 "daf = dry, ash free.
Direct confirmation of the swellling effect of CO, on coal has also been obtained by dilatometric studies.13 Volume increases of up to 1.3% were obtained on exposure to COP at 1 atm. negligible effects were obtained on exposure to N2, He, and Xe. It was estimated that swelling of this magnitude could account for up to 1 4 . 5 % of the reported C 0 2 BET surface areas. (9) Gorbaty, M. L.; Mraw, S. C.; Gethner, J. S.; Brenner, D. Fuel Process. Technol. 1986,12, 31. (10) Lucht, L. M.; Peppas, N. A. AIP Conf. Proc. 1980, No. 70, 28. (11) Larsen, J. W.; Kovac, J. In Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symposium Series 71; American Chemical Society: Washington, DC, 1978. (12) Brenner, D. Fuel 1985, 64, 167. (13) Reucroft, P. J.; Patel, H. Fuel 1986, 65, 816.
0 1987 American Chemical Society
