Energy & Fuels 2000, 14, 545-551
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Nonradical Reactions during Coal Conversion. A Search for Synchronous 1,4-Hydrogen Addition as a Precursor to Radical Reactions Anna Korda,† John W. Larsen,*,† Shona C. Martin,‡ Ajay K. Saini,‡ Harold H. Schobert,*,‡ and Chunshan Song‡ Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, and The Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received July 6, 1999
Previous studies on the relative reactivity of H2 and tetralin have shown greater hydrogen transfer from H2 to coal than from tetralin to coal at 350 °C. These results are not consistent with a radical pathway for hydrogen addition and require the existence of a nonradical route. We explored the possibility that this nonradical route is synchronous 1,4-H2 addition, which could occur either with acenes or with dihydroxybenzenes. Reactions of Illinois No. 6 coal with added phenanthrene, anthracene, or m-dihydroxybenzene provide no evidence in support of this addition reaction. The 1,4-addition of H2 to phenols produces carbonyl compounds. Reactions of Wyodak coal, which should have a higher population of phenolic groups than Illinois No. 6, show no evidence for a correlation of CO production with H2 utilization or for changes in carbonyl group population that would be consistent with 1,4-addition to phenols. In light of these negative findings, the likely nonradical pathway would then seem to be catalysis by mineral matter. This possibility was probed by comparing reactions of untreated and demineralized Wyodak. The untreated coal, in the absence of solvent, gives higher liquid yields and proportionately more oils and asphaltenes at the expense of preasphaltenes than does the demineralized coal. This indicates a role for the mineral matter in H2 utilization.
Introduction In 1990, Kabe and co-workers published some interesting and provocative data on the relative reactivity of Datong coal with H2 and tetralin.1 They clearly demonstrated greater hydrogen transfer to coal from H2 than from tetralin at 300 °C and 350 °C, and comparable transfer at 400 °C. Hydrogen exchange between coal and H2 is greater than between coal and tetralin at 350 °C and 400 °C. Huang has shown that, for a Texas subbituminous C coal, hydrogen consumption from H2 is greater than from tetralin at 350 °C (though both are low); however, in her system this was not the case at 400 °C.2 These results are fascinating and important because they are difficult to explain using radical reaction pathways. This behavior is not universal; for example, Wandoan coal behaves differently.3 The H-H bond strength is 104 kcal/mol, while the benzylic C-H bond strength in tetralin is about 82 kcal/ mol.4-6 It is inconceivable that a radical formed at 300 †
Department of Chemistry. The Energy Institute. (1) Kabe, T.; Yamamoto, K.; Ueda, K.; Horimatsu, T. Fuel Proc. Technol. 1990, 25, 45. (2) Huang, L. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, 1995. (3) Ishihara, A.; Morita, S.; Kabe, T. Fuel 1995, 74, 63. (4) Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98, 2744. (5) Kamiya, Y.; Futamura, S.; Mizuki, T.; Kajioka, M.; Koshi, K. Fuel Proc. Technol. 1986, 14, 79. (6) Franz, J. A.; Camaioni, D. M. J. Org. Chem. 1980, 45, 5247. ‡
°C would break a 104 kcal/mol bond in preference to an 82 kcal/mol bond. Yet this is required to explain these results using radical reactions. If all of the 22 kcal/mol difference in bond dissociation energy is preserved in the transition state for radical hydrogen abstraction, then tetralin will be favored over H2 as a hydrogen atom source by about 108. H2 competes with tetralin at 450 °C only because it participates in a radical chain reaction while tetralin does not.7 Two arguments can be used to show that H2 will be less competitive with tetralin in free radical reactions at lower temperatures. As the radicals from the coal get more stable, they become more selective and react more with tetralin and less with H2. In an experimental demonstration of this, lowering the bond strength of the molecule forming the initial radical from 56 kcal/mol to 50 kcal/mol lowers the amount of reaction with H2 in an H2-tetralin mixture from 36 to 30%.7 A bond having a half-life of 30 min at 350 °C has a strength of 46 kcal/mol, so its reaction with H2 would be less than 3% of its reaction with tetralin. Kabe’s data demand a nonradical pathway for the reaction of H2 with coals. There are several possible reaction pathways to consider. We shall address them, beginning with 1,4-addition of H2 to dienes. The synchronous 1,2-addition of H2 to alkenes (1) is an orbital symmetry forbidden reaction that must be catalyzed. The synchronous 1,4-addition to dienes (2) (7) Vernon, L. Fuel 1980, 59, 102.
10.1021/ef990146n CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
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is allowed and known. Its occurrence in aromatic systems was established by studying the reverse reaction, the pyrolysis of labeled 1,4-cyclohexadiene (3).8 Pyrolysis gives HD, requiring that the leaving atoms depart from the same face of the molecule in a synchronous process, and ruling out radical processes that would produce statistical mixtures of H2, D2, and HD. By the principle of microscopic reversibility, this is also the preferred addition mechanism.
The next question to address is whether there exist in coals 1,3-diene systems capable of adding H2. There are, and they fall into two main classes. The first is acenes. The addition of H2 to anthracene at room temperature is 17 kcal/mol exothermic.4,9
The same reaction for phenanthrene cannot occur by a synchronous process but must involve a radical reaction if it occurs. Anthracene and phenanthrene were independently heated to 400 °C with H2 in tubing bombs.10 In 1 h, 47% of the anthracene reacted to form 9,10-dihydroanthracene while only 2% of the phenan(8) Fleming, L.; Wildsmith, E. Chem. Commun. 1970, 223. (9) Shaw, R.; Golden, D. M.; Benson, S. W. J. Phys. Chem. 1977, 81, 1716. (10) Chiba, K.; Tagaya, H.; Suzuki, T.; Sato, S. Bull. Chem. Soc. Jpn. 1991, 64, 1034.
Korda et al.
threne reacted. The absence of the phenanthrene reaction demonstrates the nonoccurrence of radical addition reactions under these conditions. The most reasonable conclusion is that 1,4-addition of H2 to anthracene is occurring. Additionally, Stock has implicated the occurrence of 1,4-H2 addition reactions in coal conversion reactions carried out in the presence of naphthalenes.11 Anthracenes are not common in coals, but the thermodynamics of 1,4-H2 addition to β-naphthol and 1,3and 1,4-dihydroxybenzenes, as shown below as (5) and (6), are favorable.4 Such compounds are present in lowrank bituminous coals.12 Incorporation of H2 into coals by 1,4-synchornous processes may occur. We will present experimental evidence addressing this possibility.
Synchronous 1,4-H2 addition and catalysis by mineral matter are the only two processes for nonradical hydrogen incorporation that are worth considering. The other formal possibilities, such as carbonium ion or radical cation reactions, involve intermediates whose presence during uncatalyzed coal conversion are highly improbable. This paper presents some experiments designed to test synchronous 1,4-hydrogen addition as a nonradical pathway for H2 incorporation into coals during conversion. We also provide evidence to test catalysis by mineral matter as the alternative pathway. Perhaps the most obvious way of determining whether synchronous 1,4-H2 addition plays a role in coal conversion is to compare the effects of added anthracene and phenanthrene on coal conversion. Synchronous addition of H2 to anthracene is possible and yields the excellent radical hydrogen donor 9,10-dihydroanthracene. Phenanthrene cannot add H2 in a synchronous process. An anthracene-phenanthrene comparison is clouded by the much greater efficiency of phenanthrene as a coal extracting solvent.13 At their nearly identical boiling points (≈340 °C), phenanthrene dissolved 95% of the (11) King, H. H.; Stock, L. M. Fuel 1981, 60, 748. (12) Song, C.; Hou, L.; Saini, A. K.; Hatcher, P. G.; Schobert, H. H. Fuel Proc. Technol. 1993, 34, 249. (13) Davidson, R. M. Mineral Effects in Coal Conversion. IEA Coal Research Report ACTIS/TR22; IEA Coal Research: London, 1983.
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Energy & Fuels, Vol. 14, No. 3, 2000 547
Table 1. The Effect of Added Anthracene and H2 on Coal Conversiona
a
coal
reaction conditions
anthracene present
H2 present
conversion (%)
Wyodak Wyodak Wyodak
425°, 15 min, 425°, 15 min, 1-methylnaphtalene 425°, 15 min, 1-methylnaphthalene
no yes yes
no no yes
35% THF solution 51% THF solution 58% THF solution
Data from ref 8.
bituminous coals studied, while anthracene dissolved only 24%.14,15 The effects of added anthracene and phenanthrene on the hydrogenation of benzophenone by H2 have been studied as a model for coal conversion processes. The published results are conflicting, ranging between twice as much reduction with anthracene than with phenanthrene to similar effects of the two added compounds.10,16,17 The only study involving anthracene or phenanthrene, coal, and H2 that we found is summarized in Table 1. It does not provide support for a synchronous 1,4-H2 addition.18 We have studied the possible role of synchronous 1,4H2 addition by two independent methods. One is to compare the effects of added anthracene and phenanthrene on coal conversion with H2. The second is to search for enhanced carbonyl group formation during coal conversion. The thermodynamically favorable 1,4addition of hydrogen to phenols will generate carbonyl compounds. In addition, we have studied the possible role of mineral matter catalysis by comparing reactions of untreated and demineralized coal. To recapitulate this necessarily involved introduction: (1) some coals react faster with hydrogen than with tetralin, (2) this cannot be due to radical reactions, (3) 1,4-addition of H2 to dienes is a known reaction, (4) 1,4-addition of H2 to structures known to occur in coals is thermodynamically favorable, and (5) literature data are not sufficient to reach a decision on the existence of 1,4-H2 addition reactions during coal conversion. Results and Discussion Effect of Added Compounds on Conversion of Illinois No. 6 Coal. Our initial approach was to add phenanthrene (Phen), anthracene (An), and m-dihydroxybenzene to Illinois No. 6 coal reacting with tetralin with and without H2 (2000 psi) at 350 °C. It is easiest to begin with expectations, then compare them with the experimental observations reported in Table 2. If radical reactions are dominant, H2 will have little effect on the products at 350 °C. Bonds cleaved thermally at this temperature will have an overwhelming preference for abstracting H from tetralin (BDE 82 kcal/ mol) over H2 (BDE 104 kcal/mol). We observe (Table 2) that hydrogen at 2000 psi increases conversion to pyridine extractables from 56 to 65% and decreases the amount of hydrogen transferred from tetralin. The amount of hydrogen transferred from tetralin was determined by measuring by GC the amounts of naph(14) Golumbic, C.; Anderson, J. E.; Orchin, M.; Storch, H. H. U.S. Bur. Mines Rep. Invest. 4662, 1950. (15) Orchin, M.; Golumbic, C.; Anderson, J. E.; Storch, H. H. U. S. Bur. Mines Bull. 505, 1951. (16) Chiba, K.; Tagaya, H.; Suzuki, T. Bull. Chem. Soc. Jpn. 1987, 60, 2669. (17) Kamiya, Y.; Futamura, S.; Mizuki, T.; Kajioka, M.; Koshi, K. Fuel Proc. Technol. 1986, 14, 79. (18) Kwon, K. C. Fuel 1985, 64, 747.
Table 2. The Effect of Added Organics on the Conversion of Illinois No. 6 Coal to Pyridine Extractables on Reaction at 350 °C for 10 min coal + tetralin + fPyridine wt % H added H2 or N2 (2000 psi) + add 0.2 g Extract + wt % from tetralin N2 H2 N2 H2 N2 H2 N2 H2 a
none none Phen Phen An An C6H6O2a C6H6O2a
56 65 58 79 47 65 62 78
0.19 0.11 0.17 0.17 0.14 0.16 0.45 0.38
m-dihydroxybenzene.
thalene and methylindane in the products. H2 is clearly reacting, and must be doing so by nonradical pathways. The addition of phenanthrene should have little or no effect on the amount of hydrogen transferred from tetralin. It may lend to a slight increase in the amount of pyridine extract because it is itself a good dissociating solvent for bituminous coals. The first prediction is accurate (Table 2): there is little change in the amount of hydrogen transferred. The accuracy of the second is questionable. The increase in pyridine extractables from 58 to 79% is larger than anticipated, but may be due to the solubilizing properties of phenanthrene. Anthracene in the absence of H2 should give results similar to phenanthrene. With hydrogen and anthracene present, conversions greater than those obtained with phenanthrene and reduced H incorporation from tetralin are expected. Both of these predictions are wrong (see Table 2). With no H2, anthracene addition reduces conversion. However, in the presence of both H2 and anthracene, conversion is not enhanced. Furthermore, when anthracene is added to the system, the transfer of hydrogen from tetralin is about the same as that when only tetralin is used (i.e., without added anthracene). There is no evidence for the involvement of 9,10-dihydroanthracene formed in situ in the conversion. Finally, another molecule of different polarity capable of the synchronous addition of H2 was used. It is a phenol (m-dihydroxybenzene) and therefore a good H donor by radical abstractions from the hydroxyl groups. Because of this, we expect increased conversions in the absence of H2. Even greater conversion increases with H2 present are anticipated if 1,4-addition to the phenol occurs. We cannot predict the effect of the added phenol on the amount of H transferred from tetralin in H2, but it should be decreased with H2 present, due to competition of the dihydrophenol with the tetralin. The results reported in Table 2 are complex and equivocal, but more deny than support a significant role for synchronous 1,4H2 addition reactions in coal conversion. Probing 1,4-Addition to Phenols. To further probe the system, we looked for an enhanced population of carbonyl groups, the products of 1,4-H2 addition to phenols (eqs 5 and 6). Wyodak coal was chosen for this
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Table 3. Low-Temperature Noncatalytic Conversion of DECS-8 Wyodak Coal at 350 °C for 30 min under 1000 psi H2 or N2 (Cold) (All the Yield Data Are wt % Based on Dmmf Coal)
coal
solvent gas
gas (diff) wt %
hexane solubility wt %
toluene solubility wt %
THF solubility wt %
conversion dmmf wt %
CO dmmf wt %
CO2 dmmf wt %
CH4 dmmf wt %
C 2H 6
C 3H 8
C4H10
VD2h VD2h VD2h VD2h VD2h raw raw raw
no/H2 tetralin/H2 1-MN/H2 no/H2 no/N2 no/H2 tetralin/H2 1-MN/H2
3.3 4.2 4.0 FT-IR FT-IR 7.7 7.7 6.4
2.1 4.1 1.1
2.6 7.6 5.8
4.5 10.0 7.4
5.4 15.8 15.9
2.8 9.3 6.6
9.1 12.4 11.4
12.5 25.9 18.3 16.5a 14.7a 25.0 43.3 39.9
0.24 0.19 0.16 0.16 0.14 0.37 0.21 0.16
4.5 4.1 4.34 4.09 3.80 8.90 8.95 8.88
0.10 0.09 0.09 0.10 0.10 0.12 0.11 0.10
0.02 0.03 0.03 0.03 0.03 0.04 0.05 0.04
0.03 0.02 0.03 0.03 0.05 0.05 0.04 0.04
0.04 0.01 0.01 0.01 0.02 0.01 0.01 0.01
H2 consumed wt % 0.20 0.29 0.45 0.24 n/ab 0.61 0.63a 0.58
H from tetralin wt % 0.41
0.31a
a Three repetitions of this run gave the following values for hydrogen consumption from H and from tetralin respectively: 0.77, 0.62, 2 0.63 and 0.26, 0.27, 0.31. b n/a: not applicable.
Table 4. Results of Noncatalytic Liquefaction of Three Low-Rank Coals at 350 °C, with H2 at 1000 psi (Cold), without Solvent, and in 1-Methylnaphthalene and Tetralin: Results Expressed on dmmf Coal Basis, Compiled from Huang2 1a
coal used solvent conversion gas yield oil yield asphaltenes preasphaltenes a
(none) 19.1 6.5 7.4 1.2 4.1
1-MN 33.8 5.0 14.8 7.4 6.7
2 Tetralin 38.2 4.4 18.2 8.3 7.4
(none) 19.4 2.5 8.8 2.5 5.7
1-MN 29.6 4.7 7.0 10.3 7.7
3 Tetralin 42.8 3.6 15.7 12.1 11.4
(none) 15.4 6.5 8.4 0.3 0.2
1-MN 23.3 6.8 11.3 3.9 1.3
Tetralin 28.1 6.6 19.9 5.0 3.2
1, Texas Bottom seam subbituminous C; 2, Montana Dietz seam subbituminous B; 3, North Dakota Beulah seam lignite.
work because it has a high population of phenols. The data are shown in Table 3. With vacuum-dried Wyodak coal (WD), conversion is highest in tetralin/H2, intermediate in 1-methylnaphthalene/H2, and lowest in no solvent/H2, all at 350 °C for 30 min. Tetralin is a more effective hydrogen donor than H2, as expected if radical processes dominate. We offer no explanation for the enhanced H2 consumption in 1-methylnaphthalene. However, it is noteworthy that quite similar results were obtained by Huang for three different coals, all reacted at 350° in tetralin/H2, 1-methylnaphthalene/H2, and no solvent/ H2.2 Her relevant results are summarized in Table 4. The results for the undried coal (in the last three rows of Table 3) are intriguing. Conversions are enhanced relative to the dried coal. Hydrogen reactivity is nearly constant, independent of solvent. Most interesting is that more hydrogen is transferred to the coal than from tetralin. This is consistent with nonradical pathways for hydrogen incorporation. These results also differ from the results obtained with the same coal dried. The reproducibility of our results is acceptable, as shown by the data in the footnote to the table. We turn now to the population of carbonyl groups in the reacted coal. The occurrence of 1,4-addition reactions to phenols produces ketones, so their population, or the amount of CO formed, will be increased if the 1,4addition of H2 is occurring. We must rely on the vacuumdried samples to avoid complications due to water, and so compare the dried coal heated in N2 and H2 for 30 min at 350 °C and 100 psi (cold) gas. The difference spectrum (Figure 1) shows the loss of carbonyl, not its creation. It appears as if there is a slightly greater loss for the H2 product. These samples show comparable amounts of decarbonylation (Table 3). A comparison of all the data for H2 incorporation with the amounts of CO produced reveals no correlation between these two. These observations are not consistent with 1,4-H2 addition to phenols to produce hydrogen donors. There is no evidence either for enhanced decarbonylation or
Figure 1. IR Spectra of Wyodak coal treated with H2 at 350 °C (top spectrum), of Untreated Wyodak Coal (middle spectrum), and of their difference (top-middle).
for carbonyl group population. It does not seem likely that such carbonyl groups would form CO2 on decomposition. These data provide evidence that nonradical pathways for H2 incorporation into coals exist and that such incorporation does not occur by the synchronous addition of H2 to diene systems. This leaves the question of the identity of the nonradical pathway. We speculate that it is catalysis by mineral matter. Such catalysis is well established in some systems.13 Whether the catalyst might act by bringing about the addition of hydrogen to groups in the coal to produce hydrogen donors or by producing dissociated or activated H2, we do not know. Probing Mineral Matter Effects. To assess the possibility that mineral matter could have a role in catalyzing a nonradical pathway, we examined the behavior of untreated and demineralized Wyodak subbituminous coal at 350 °C.
Nonradical Reactions during Coal Conversion
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Table 5. Results of Noncatalytic Liquefaction of Untreated and Demineralized Wyodak Coal at 350 °C in H2 at 1000 psi (Cold), % dmmf Basis pretreatment
solvent
gas yield
oil yield
asphaltenes
preasphaltenes
conversion
(none) demineralized (none) demineralized (none) demineralized
(none) (none) 1-MN 1-MN tetralin tetralin
3.3 10.0 4.0 7.7 4.2 12.0
2.1 0.8 1.1 5.6 4.1 7.1
2.6 0.4 5.8 3.3 7.6 6.3
4.5 5.3 7.4 10.3 10.0 14.4
12.5 16.6 18.3 26.9 25.9 40.3
Table 6. Comparative Results of Noncatalytic Liquefaction of Demineralized Wyodak Coal at 350 °C in 1000 psi (Cold) N2 and H2, % dmmf Basis atmosphere
solvent
gas yield
oil yield
asphaltenes
preasphaltenes
conversion
hydrogen nitrogen hydrogen nitrogen hydrogen nitrogen
(none) (none) 1-MN 1-MN tetralin tetralin
10.0 10.7 7.7 8.7 12.0 10.4
0.8 1.2 5.6 5.8 7.1 10.3
0.4 0.7 3.3 1.4 6.3 3.6
5.3 5.2 10.3 8.4 14.4 17.8
16.6 17.8 26.9 24.4 40.3 42.1
Results are provided in Table 5. In the absence of solvent, the conversion of the demineralized coal is greater than that of the treated coal, 17 vs 13%. This difference is actually the resultant of two countervailing changes: demineralization decreases gas make, from 10.0 to 3.3%, which is partially offset by an increase in total yield of liquid products, from 6.5 to 9.2%. Furthermore, the liquid product slate from the untreated coal is shifted toward lighter products, relative to the liquids from the demineralized coal. For example, preasphaltenes account for 82% of the liquids from the demineralized coal, but only 49% of the liquids from the untreated coal. Presuming that the reduction in iron concentration on demineralization is a result of removal of pyrite, this could account for the change in liquid yield and product slate. Catalysis of reaction of the untreated coal by pyrite could both increase liquid yield and favor lighter products. If the greater gas make from the demineralized coal is due to higher yields of CO2, this would indicate that removal of inorganic cations during demineralization would promote the more facile pyrolysis of -COOH rather than -COO-M+ or (-COO-)2M2+. Table 6 summarizes the results of liquefaction of the demineralized coal conducted in N2 and in H2. No significant differences exist between the results in these two atmospheres for any of the three sets of conditions used. In fact, in two of the three cases, the conversion in H2 is slightly lower than that in N2. That the results are so similar in the absence of solvent indicates that there is no effective utilization of H2 by the demineralized coal and that the results in either atmosphere are simply the result of pyrolysis. The reaction of H2 with the demineralized coal at 350 °C therefore represents a situation where there is essentially no utilization of hydrogen. If that result is contrasted with the effect of not demineralizing the coal, i.e., of reacting the untreated coal, several effects are evident. Reacting the untreated coal results in the following changes, relative to the results obtained with the demineralized coal: (a) an increase in total liquid yield by ≈40%, from 6.5 to 9.2%; (b) a decrease in the yield of preasphaltenes, from 5.3 to 4.5%, which translates into a marked decrease in the preasphaltenes when expressed as a percentage of the total liquid product, from 82 to 49%; (c) an increase in oil yield by a factor of ≈2.5, from 0.8 to 2.1%; and (d) an increase
in asphaltene yield by a factor of 6, from 0.4 to 2.6%. These comparisons provide strong evidence of a role for the mineral matter in hydrogen utilization. Presuming that demineralization removes a significant portion of pyrite originally present in the coal, then these results are comparable to other work by Tomic,19 Artok,20 and Huang,2 all of whom observed that utilization of H2 is ineffective in the absence of an active hydrogenation catalyst. In contrast, demineralization has no detrimental effect when reactions are conducted in tetralin. This is exactly in keeping with Huang, who found that an active catalyst is not necessary for effective utilization of hydrogen from a donor solvent.2 Hence it is reasonable to expect that similar effects might be observed in reactions of untreated coal (i.e., mineral-matter catalyst present) with H2 and demineralized coal with tetralin. In the latter case there should be no effective way of utilizing the gaseous H2. Results from these experiments are provided in Table 7. Though the magnitudes of the conversions and yields differ, it is noteworthy that when the product yields are expressed as a percentage of the total liquid, or the liquid yield as percentage of total conversion, the results are in fact quite similar. It appears that mineral-matter-catalyzed reaction of H2 is comparable, chemically, to reaction of hydrogen from a donor solvent, though in terms of magnitudes of yields, the latter is the more effective process at these conditions. Demineralization also increases the liquid yield obtained in tetralin, relative to reaction of the untreated coal in tetralin. Joseph suggested that cations (in the coal mineral matter) inhibit hydrogen transfer from donor solvents to free radicals, in effect promoting retrogressive reactions.21 The enhancement may also be due simply to increased solvent access into the interior of the coal from the physical removal of the mineral particles.22 Our data allow no insight into the means by which the minerals exert their effect, and we do not wish to speculate. (19) Tomic, J.; Schobert, H. H. Energy Fuels 1996, 10, 709. (20) Artok, L.; Schobert, H. H.; Erbatur, O. Fuel Proc. Technol. 1994, 37, 211. (21) Joseph, J. T.; Forrai, T. R. Fuel 1992, 71, 75. (22) Martin, S. C.; Schobert, H. H. Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem. 1996, 41, 967.
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Table 7. A Comparison of Results of Reaction of Untreated Wyodak Coal in Hydrogen and No Solvent, with Demineralized Wyodak Coal in Tetralin at 350 °C, % dmmf Basis
oil yield asphaltene yield preasphaltene yield total liquid yield oils as a percent of liquids asphaltenes as a percent of liquids preasphaltenes as a percent of liquids total liquids as a percent of conversion
After reaction under H2 at 350 °C, the most notable feature of the whole product FTIR spectrum is a decrease in hydroxyl concentration. FTIR examination of the THF-insoluble residue from this experiment confirms the loss of -OH functionality coupled with the disappearance of an ester shoulder on the CdO band. The FTIR spectrum of the THF-insoluble residue from the reaction conducted with tetralin displays somewhat contrasting features. The carboxyl stretching region is much broader; moreover, the ester shoulder is more pronounced and the carbonyl region is much more defined. The FTIR spectra of the THF-extracted untreated coal and the THF-insoluble residues from reactions conducted at 350 °C under N2 demonstrated that under all conditions, i.e., both in the presence and absence of solvent, no marked changes were observed. Summary and Conclusions On the basis comparative BDE values, the presence of H2 should have little effect on products from liquefaction in tetralin at 350 °C, because it should be much easier to abstract H• from tetralin than from H2. However, the liquefaction of Illinois No. 6 coal shows that H2 enhances conversion to pyridine-extractables and decreases the amount of hydrogen consumed from tetralin. H2 must certainly be reacting, and doing so by a nonradical pathway. One possible nonradical pathway would involve synchronous 1,4-H2 addition. If this were the case, enhanced conversions, combined with reduced hydrogen utilization from tetralin, should be obtained by adding compounds known to be capable of synchronous 1,4-H2 addition. Reactions in the presence of anthracene do not support this. Results for reaction with added m-dihydroxybenzene are more equivocal, but tend also to refute the possibility of 1,4-H2 addition. If 1,4-H2 addition to phenols occurs, the result should be an enhanced population of carbonyl groups in the reaction residue. This was tested using Wyodak subbituminous coal, which is likely to have a higher population of phenol groups than Illinois No. 6. The reaction of dried Wyodak coal shows that tetralin is a more effective hydrogen donor than H2, which is exactly what would be expected if radical processes dominate. However, intriguing results are obtained if the coal is reacted without drying. H2 reactivity is independent of solvent, and more hydrogen is utilized from H2 than from tetralin, consistent with nonradical pathways. FTIR analyses of the products from heating dried Wyodak coal in N2 and H2 provide no evidence for 1,4-H2 addition to phenols, viz., no evidence for enhanced decarbonylation or enhanced carbonyl group population, nor any correlation between H2 consumption and CO production.
untreated coal, H2 atmosphere, 1000 psi (cold)
demineralized coal in tetralin
2.1 2.6 4.5 9.2 22.8% 28.3% 48.9% 73.6%
7.1 6.3 14.4 28.3 25.1% 22.3% 50.9% 70.2%
Table 8. Characteristics of Coals Used in This Worka proximate, % as received volatile matter fixed carbon ash moisture ultimate, % dmmf carbon hydrogen nitrogen sulfur oxygen
Illinois No. 6
Wyodak
36.9 40.9 14.2 8.0
32.4 29.3 9.9 28.4
80.8 5.2 1.4 2.5 10.1
75.8 5.2 1.0 0.5 17.5
a Data for Illinois No. 6 from Argonne Premium Coal Sample Program.24,25
In light of these negative tests for synchronous 1,4H2 addition, we considered mineral matter catalysis as an alternative pathway for nonradical hydrogen consumption. This possibility was examined by comparing reactions of untreated and demineralized Wyodak coal. Reaction of demineralized Wyodak in N2 and H2 (and without solvent) produces nearly identical conversions and liquid product slates, indicating that, in the absence of mineral matter, there is no effective use of H2. Comparing reactions of demineralized and untreated Wyodak in H2, again without solvent, shows that with the mineral matter present greater liquid yields are obtained and the lighter products (oils and asphaltenes) are enhanced relative to preasphaltenes. Clearly H2 is reacting with the untreated coal, and since the only difference is the presence or absence of mineral matter, it seems that mineral matter catalysis is a route to nonradical hydrogen incorporation. Experimental Section Materials. Two coals were used for this work, Illinois No. 6 and Wyodak. The Wyodak coal was provided by the DOE/ Penn State Coal Sample Bank and Data Base (sample DECS8). The proximate and ultimate analyses are summarized in Table 8. The vacuum-drying of Wyodak was performed at 100 °C for 2 h, and subsequent cooling was done in the vacuum oven. All solvents and reagents were obtained from Aldrich in the highest purity and were used without further purification. Demineralization of the Wyodak DECS-8 was carried out by successive acid treatments. The first stage was an HCl wash (10 mL per gram of coal) to remove alkaline earths that would form insoluble fluorides in the HF wash stage. The HCl wash was done at 60 °C for 1 h. In the second stage, a 40% HF solution (10 mL per gram of coal) was also used at 60 °C for 1 h. The final stage was a thorough wash with distilled water to remove any residual HF. Both the untreated and demineralized samples were analyzed by ICP. Samples were ashed at 950 °C and the ash was then fused with lithium metaborate.
Nonradical Reactions during Coal Conversion Table 9. Analyses of Untreated and Demineralized Wyodak Samples by ICP-AAS, wt %a ash iron calcium magnesium sodium potassium a
untreated
demineralized
8.94 5.53 13.2 3.02 1.12 0.78
0.37 56.8 9.23 1.51 0.20 0.24
Metal concentrations are expressed as percent of the ash.
Solutions were then analyzed using a Leeman Laboratories PS3000UV inductively coupled plasma spectrophotometer. A comparison of the ash yields and principal elements in the ash is provided in Table 9. The demineralized coal samples were dried in a vacuum oven at 110 °C for 2 h prior to use. Reactions. All coal conversion reactions of Illinois No. 6 were carried out in stainless steel tubing bombs agitated vertically at more than 200 cycles/min, a rate shown in control experiments to be fast enough to overcome mass transport effects present at low agitation rates. Heating was done in a fluidized sand bath, and 75% of the desired temperature was reached in 20 s. Reactions of Wyodak were carried out in 25 mL horizontal microautoclaves with 4 g of coal at 350 °C for 30 min (plus 3 min for reactor heat-up to the reaction temperature in a fluidized sand bath). When a solvent (tetralin or 1-methylnaphthalene) was used, the ratio of solvent to coal was 1:1 by weight. The reactor was purged with H2 at least three times, and finally pressurized with 6.9 MPa (cold) H2 or ultrahigh purity N2. Product Workup and Analyses. After reactions of Illinois No. 6, pyridine extractibilities were measured using Soxhlet extractors in which the coal-pyridine mixture in the filter was at about 85 °C. Reported extractibilities are based on the weight of the vacuum-dried reside to constant-weight-extraction residue. Elemental analyses were performed by Galbraith Laboratories (Knoxville, TN). Hydrogen transfer from tetralin was determined by capillary column gas chromatography utilizing added standard. (23) Song, C.; Saini, A. K.; Schobert, H. H. Energy Fuels 1994, 8, 301. (24) Vorres, K. S. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1988, 33(3), 1. (25) Vorres, K. S. Users Handbook for the Argonne Premium Coal Sample Program. U.S. Department of Energy Report W-31-109-ENG238; U.S. Department of Energy: Washington, DC, 1989.
Energy & Fuels, Vol. 14, No. 3, 2000 551 The gaseous products were analyzed by GC. The liquid and solid products from reactions of Wyodak were separated by sequential Soxhlet extraction with hexane to obtain oils; with toluene to obtain asphaltenes; and with tetrahydrofuran (THF) to obtain preasphaltenes. The THF-insoluble residue was washed with acetone followed by pentane to remove any residual THF. All recovered products were dried under vacuum at 110 °C for ≈10 h. Conversion was calculated on the basis of recovered THF-insoluble residue. Fourier transform infrared (FTIR) spectra of vacuum-dried coal and liquefaction residues were recorded on a Digilab FTS60 spectrometer by co-adding at least 200 scans at a resolution of 2 cm-1. The samples were prepared as KBr pellets. The KBr was dried at 110 °C for over 10 h in a vacuum oven prior to sample preparation. An accurately weighed amount of the predried coal or residue sample (2 ( 0.01 mg) was mixed with a preweighed amount of KBr (250-300 ( 0.01 mg) in a PerkinElmer Wig-L-Bug Grinder-Mixer for 120 s. Further details can be found elsewhere.23 Due to the need to do FTIR analysis of the whole (unfractionated) product and the need to check the reproducibility of the coal conversion data, we designed a special procedure for some tests. In such cases, the liquefaction reactions were conducted as described above. After the reaction, the gaseous products were collected and analyzed by GC. However, the liquid + solid products, which have a solidlike appearance, were divided into two parts. The first part was used for the preparation of the KBr pellet sample. The other part was subjected to THF extraction, with the intent to obtain both coal conversion data and the THF-soluble and -insoluble products for later characterization. We have also tried to analyze the THF-extracts, but it was difficult to make the KBr pellet, because the extract was sticky. The results presented in Table 3 show that the reproducibility of coal conversion data and gas analyses were satisfactory.
Acknowledgment. J.W.L. and A.K. are grateful to the Division of Fossil Energy of the U.S. Department of Energy for their support of this work. H.H.S. and C.S. gratefully acknowledge the Federal Energy Technology Center of the U.S. Department of Energy for financial support. EF990146N
