Energy & Fuels 1994,8, 219-226
219
Deuterium Incorporation during Coal Liquefaction in Donor and Nondonor Solvents Hossein A. Dabbagh, Buchang Shi, and Burtron H. David Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 40511
Charles G. Hughes Tobacco & Health Research Institute, University of Kentucky, 106 Tobacco & Health Research Building, Lexington, Kentucky 40506 Received December 29, 1992. Revised Manuscript Received October 26, 199P
Deuterium is selectively incorporated into the ring positions of 1-methylnaphthalene from gaseous deuterium, and the distribution of deuterium in the rings is essentially the same in the presence or absence of coal. Incorporation is under kinetic control. Ring exchange appears to be due to catalysis by the walls of the reactor. Incorporation in the methyl position does not occur to an appreciable extent at 385 or 450 "C in the absence of coal, but extensive incorporation does occur in the presence of coal. The pattern of ring substitution of deuterium precludes, or places severe restrictions on, the role of hydrogen shuttling by 1-methylnaphthalene as an explanation for the observation of higher coal conversion in this nondonor solvent than in a donor solvent, tetralin.
Introduction Critical features of a good recyclable solvent for coal liquefaction were identified by Whitehurst et a1.l as follows: (1) it must be a good physical solvent for the liquefaction products, (2) it must have the ability to shuttle hydrogen, (3) it must contain hydrogen donor components, and (4)it must have relatively few hydrogen consumers. This view that a desirable pasting solvent for coal liquefaction should have hydrogen donor properties was described by Neavek2he showed that the initial conversion in naphthalene (a nondonor) and in tetralin (a donor) was similar, but at longer times a higher conversion was obtained in the donor solvent. An idealized version of this situation was presented by Whitehurst et al.' and is illustrated in Figure 1. In this view, the initial solubilization of coal is depicted as being independent of the hydrogen donor potential of the solvent (region I, Figure 1). During the intermediate times, the initially solubilized coal fragments may follow one of two pathways: (1)in the poor hydrogen donor solvent, naphthalene or methylnaphthalene, retrogressive reactions become dominant so that insoluble material (mesoform) is formed, and (2) in a good hydrogen donor, tetralin or dihydrophenanthrene, the conversion of the initial coal fragments to lower weight materials is further assisted by hydrogen transfer. In region 3 of Figure 1,solvent rehydrogenation occurs as the demand for donatable hydrogen by conversion of coal fragments ceases. However, a measure of the extent of hydrogen transfer from tetralin and the attainment of a steady-state tetrslinhaphthalene ratio early (ca. 30 min at 385 "C) in the course of coal liquefaction cast some doubt upon this generalized mechanism.3 a Abstract published in
Advance ACS Abstracts, December 1,1993. (1) Whitehurst, D. D.; Farcasiu, M.; Mitchell, T. 0.; Dickert, Jr., J. J. The Nature and Origin of Asphaltenes in Processed Coals. EPRI AF480, July 1987. (2) Neavel, R. C. Fuel 1976,55,237.
0
Solu-
'O0 c
0
,
0
Hydrogen Transfer
rbilizationi
I
1
"ydrOWnrtlon
80
C
0)
0
5
60
a
i
-7
40
0
20
1 0
.
I
I d
I
I
1
3
6
9
12
1
I
15
Approximate Time, Minutes Figure 1. Idealized conversion-time curves for coal solubilization in a good and a non-hydrogen donor solvent during the solubilization, hydrogen transfer,and rehydrogenationperiods (redrawn with modifications, from ref 1).
The data shown in Figure 2 likewise cast doubt on the generality of the idealized scheme shown in Figure 1. It is noted that the conversion in Figure 2 is higher a t short reaction times in the nondonor solvent, 1-methylnaphthalene, than it is in the donor solvent, tetralin. Furthermore, when gaseous hydrogen is present the final conversion is the same in the two solvents. However, hydrogen must be present in the gas phase for the conversion to be as great in 1-methylnaphthalene as it is in tetralin; when nitrogen was substituted for hydrogen the maximum conversion was only about 35% (versus 90 5% in hydrogen) and the conversion began to decrease after about 15min when the added gas was nitrogen (Figure 3). The data shown in Figure 3 resemble those reported by NeaveL2 While Figure 1is frequently utilized, it must
-
(3) Chawla, B.;Keogh, R.; Davis, B. H. Energy Fuels 1989, 3, 236.
0887-0624/94/2508-0219$04.50/00 1994 American Chemical Society
Dabbagh et al.
Energy &Fuels, Vol. 8, No. 1, 1994
Table 1. Properties of Western Kentucky #9 Pyro Coal property weight % PropeW weight %
ash (dry) VM (daf)' FC (daf) C (daf)
0 '
' 0
10.31 43.72 56.28 82.15
H (dan N (daf) S (daf) 0 (daf)
5.72 1.71 3.93 6.49
Dry ash free. 60
50 44
1-Methylnaphthalene A Tetralln
t/ 10
I
I
I
I
I
20
30
40
50
60
Time, Minutes
Figure 2. Conversion of an Alston Western Kentucky #9 coal in a nondonor and donor solvent (redrawn from ref 4) a t 385 "C and about 2000 psig.
40
t
1
* O t
0
0
10
20
30
40
50
KO
c
70
TIME (MIX.)
Figure 3. Conversion of an Alston Western Kentucky #9 coal in 1-methylnaphthalene in a hydrogen or nitrogen atmosphere (ref 4).
be emphasized that it is an idealized representation that includes the implied assumption that the initial thermolysis leading to coal solubilization does not have a hydrogen demand and that the same high (>80%) conversion is effected in tetralin and naphthalene. Few data are available at the short times to substantiate the plot in Figure 1since it is difficult to obtain reliable shorttime liquefaction data. Thus, most of the reported data are as shown in Figure 3; the maximum in the curve implies that Figure 1 is an oversimplification and needs to be verified. In any event, the data in Figure 2 illustrate clearly that there is a demand for hydrogen during coal solubilization for short reaction times. The data shown in Figure 2 are not restricted to the Alston coal; about one-third of both eastern and western Kentucky coals exhibits a total conversion after 15 min of reaction at 385 "C in l-methylnaphthalene that is approximately the same, or higher, than is obtained in t e t ~ d i n . ~ Kershaw and Barrass5reported that although deuterium studies of coal reactions6,' and coal liquids8 were not new, the use of deuterium to study the mechanism of coal (4) Keogh,R.A.;Chawla,B.;Tsai,K. J.;Davis,B.H.Prepr.Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1988, 33 (3), 333. (5) Kershaw, J. R.; Barrass, G. Fuel 1977,56,455. (6) Fu, Y. C.; Blaustein, B. D. Chem. Ind. 1967,1257. (7) Kessler, T.; Sharkey, A. G . Spectrosc. Lett. 1968, 1 , 177. (8) Schweighardt, F. K.; Bockrath, B. C.; Friedel, R. A.; Retcofsky, H. L. Anal. Chem. 1976,48,1254.
hydrogenation had not been reported to that time (1977). Some of the early coal liquefaction studies using deuterium as a tracer indicated that the rapid exchange would limit the usefulness of deuterium in coal liquefaction studies (e.g., refs 9-13). Franz and Camaioni14found differences in the rate of equilibration of D in 1,l-dideuteronaphthalene and into two coals. With an Illinois #6 coal, complete scrambling occurred in 35 min at 427 "C. King and Stockl5JGandWilsonet a1.13utilized dueterium tracer studies and obtained data to show that coal molecules selectively and reversibly extract deuterium from the benzylic position of tetralin. Cronauer et al." made the observation that the initial phase of the conversion of a Powhatan coal takes place in a very short time whereas hydrogen transfer from the solvent continues for a longer time. Kabels reviewed his results for the use of tritium and 1% in mechanism studies of coal liquefaction. Apart from any kinetic isotope effect and the means of detection of the isotope, the studies with tritium should be directly comparable to the present study. Kabe18 reported that hydrogen exchange between solvent and gaseoushydrogen during coal liquefaction is low even in the presence of a catalyst. He reported that in the process of the primary liquefaction coal was thermally decomposed and hyrogenated by hydrogen atoms released from a hydrogen donor solvent. He considered the catalyst function as effecting the cyclic conversion naphthalene tetralin naphthalene which therefore provides a donor molecule to transfer hydrogen atoms to coal. Heredy, Skowronski and co-workers19-21reported that both hydrogen donation and hydrogen exchange involving the a-positions of tetralin have a significant role in stabilizing the fragments that result from the thermal decomposition of coal. They also reported that there was a direct route for deuterium incorporation into coal products from the gas phase without the participation of the solvent.
-
-
(9) Cronauer, D.C.; McNeil, R. I.; Young, D. C.; Ruberto, R. G. Fuel 1982.61. 610. (10) Wilson, M. A.; Collin, P. J.; Barron, P. F.; Vassallo, A. M. Fuel Process Technol. 1982, 5, 281. (11) Wilson, M. A.; Vassallo, A. M.; Collin, P. J.; Batta, B. D. Fuel Process Technol. 1984, 8, 213. (12) Wilson, M. A.; Collin, P. J.; Barron, P. F.; Vassallo, A. M. Fuel Process Technol. 1982,5, 281. (13) Wilson, M. A.; Vassallo, A. M.; Collin, P. J.; Batts, B. D. Fuel Process Technol. 1984, 8, 213. ,Diu. (14) Franz,J. A,;Camaioni,D.M. Prepr. Pap.-Am. Chem. SOC. Fuel Chem. 1981,26,105. (15) King, H. H.; Stock, L. M. Fuel 1980,59,447. (16) King, H. H.; Stock, L. M. Fuel 1982, 61, 257. (17) Cronauer, D.C.; McNeil, R. I.; Young, D. C.;Ruberto, R. G . Fuel 1982, 61, 610. (18) Kabe, T. J. Jpn. Pet. Znst. 1986,29,346. (19)Skowronski, R. P.; Ratto, J. J.; Heredy, L. A. At. Nucl. Methods Fossil Energy Res.; Filby, R. H., Ed.; Plenum: New York, 1980; pp 207nn
LO.
(20) Skowronski, R. P.; et al. Tech. Progress Reports for US.DOE, FE-2781, FE-2328 and FE-11418. (21) Heredy,L. A.;Skowrinski,R.P.;Ratto,J. J.;Goldberg,I. B.Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1981,26,114.
Energy &Fuels, Vol. 8, No.1, 1994 221
Deuterium Zncorporation during Coal Liquefaction
Table 2. Deuteration of 1-Methylnaphthalene at 2000 psi Dz, 385 OC, 1 h of Reaction Time deuterium content ( % ) reactor reactor used for coal conversion new reactor used reactor with Ni graphite flakes stirred autoclave glass liner at 425 O c a
condensed inside glass-liner condensed outside glass liner
reactor with glass liner a
Do 69 68 49 27 38 92
Di
D2
Ds
D4
Ds
25 25 33 32 35 7.0
5.0
-
-
-
6.0 13 23 17
1.0
1.0
4.0 11
8.5
-
1.0 5.0
2.5
-
2.0 1.0
-
A glass liner waa fitted into a 75-cm3stirred stainless steel autoclave.
Pajak and Brower22-26 obtained kinetic isotope data to indicate a concerted transfer of a pair of hydrogen atoms from the 1- and 2-positions of tetralin during coal liquefaction. Because of the similarity of the conversion in l-methylnaphthalene and tetralin4as well as the observation that gaseous hydrogen must be present in order to obtain the high conversion with 1-methylnaphthalene,it is desirable to define the extent that hydrogen is transferred through the solvent rather than directly to the coal. For this study, the incorporation of deuterium in the solvent from gasphase deuterium in the presence and absence of coal as well as determining the extent and nature of deuterium incorporation in the coal solubility classeswas undertaken.
Experimental Section Materials. The coal sample was collectedat the mine, ground to -100 mesh, stored in a nitrogen-filled sealed bottle in glovebox, and dried prior to a liquefaction run. Since the Alston mine was closed and sealed when this study was initiated, it was necessary to use another western Kentucky coal. The properties of the coal are summarized in Table 1. The compounds used as solvents were obtained from Aldrich and were used without further purification. Deuterium was obtained from Cambridge Isotope Laboratories (99.8%). Liquefaction Runs. The coal liquefaction experiments were carried out in 50-cm3stainless steel microautoclave reactors. In a typical run, 5.0 g of coal, 7.0 g of 1-methylnaphthalene (or other solvent),and deuterium (800psig a t ambient temperature, about 2000 psig a t reaction temperature) were added to a reaction vessel fitted with two stainless steel balls to effect mixing; the reactor was then immersed into a fluidized sand bath. Heat-up time to 385 OC was less than 2 min. The reactor was vertically shaken at 400 cpm. At the end of the experiment, the reaction was quenched by reducing the temperature of the reactor to less than 100 OC in less than 2 min by immersing it in a sand bath held a t ambient temperature. After the reactor had cooled to room temperature it was connectedto an evacuated manifoldof known volume;the residual gas was expanded into the manifold and the total pressure obtained. Gas composition was obtained using a gas analyzer. Analysis for conversion was by solubility class using pyridine, benzene, and pentane as the solvents.2s In several of the experiments with D2, a Pyrex long-neck glass reactor was constructed and used to minimize contact with the metal reactor. The microautoclave was also modified so that the glassreactor bulb was in the main reactor vessel, but larger tubing was used so that the long glass neck could extent out of the fluidized sand bath. During the run, dry ice was placed in a container around the reactor neck to effect reflux and to prevent the solvent from distilling from the glass reactor. Glass beads were placed inside the glass reactor to effect mixing. The glass (22) Pajak, J. Fuel Process Technol. 1989,23, 39. (23) Pajak, J. Fuel Process Technol. 1991, 27, 203. (24) Brower, K. D.;Pajak, J. J. Org. Chem. 1984, 49, 3970. (25) Pajak, J.; Brower, K. D. Energy Fuels 1987,1, 363. Brower, K. D. J. Org. Chem. 1982,47,1189. (26) Keogh, R. A.; Davis, B. H. J. Coal Quality 1988, 7, 27.
Table 3. Positions of Deuteration of Model Compounds (at 386 OC. Dz 2000 p B i , 1 h, and a Stainless Steel Reactor) deuterium content ( % ) model comDd D" D, Dz DS Dd DK naphthalenea 78 20 2.5 1-methylnaphthalene 65 28 6.0 1.0 1-ethylnaphthalene 82 15 2.5 0.7 0.3 85 13 1.5 0.5 1-phenylnaphthalene I-methoxynaphthalene 51 38 9.0 1.0 5.0 18 1-hydroxynaphthalene 36 42 Reaction time, 30 min. reactor was prevented from contacting the steel reactor wall by surrounding it in glass wool. This reactor prevented the liquid from contacting the steel reactor and at the same time contacting the D2 a t high pressure a t reaction temperature. The utility of this glass reactor was demonstrated by the deuterium of phenanthrene using a 5 % Pt-charcoal catalyst. Even perdeuterophenanthrene was formed that contains C12Dm. In someruns with both coal and solvent,the solventwas isolated from the oil sample using a microdistillation technique. For example, the middle cut of the 1-methylnaphthalene solvent isolated from the oil fraction had a purity of greater than 97% as determined by GC analysis. The distillation was continued to remove residual solventfrom the oil fraction to provide a sample of the high boiling oils produced during the run. The deuterium content of solventmolecules was obtained using a GC/MS. Spectra of pure compounds were obtained to make correctionsfor the M + 1and higher peaks. Spectra were obtained using 10 eV of ionization energy. The deuterium NMR determinations were performed on a 300-MHZ or 400-MHZ Varian instrument.
Results The initial studies with 1-methylnaphthalene at 385 "C indicated that either the autoclave sealant or the autoclave walls could catalyze deuterium exchange (Table 2). Using a glass reactor, very little (if any) H/D exchange occurred during the same time period. It is evident that the walls of the steel autoclave catalyze exchange during 1 h of reaction so that about 40% of the 1-methylnaphthalene had exchanged deuterium for one or more hydrogens. No evidence was obtained for methyl migration to produce 2-methylnaphthalene. The deuterium distributions in six model (naphthalene and naphthalene substituted at the 1-positionwith methyl, ethyl, phenyl, methoxy, or hydroxyl groups) compounds after 1h in D2 at about 2000 psig and 385 O C in a stainless steel microautoclave are shown in Table 3. While the absolute values for a particular compound will vary from run to run, it is apparent that the substituent in the 1-position of naphthalene has an impact upon D exchange. Substitution of either ethyl or phenyl for H in the ring-1 position has little impact upon the extent of exchange. However, the exchange is increased when methyl is substituted for H in the 1 position, and the exchange is
Dabbagh et al.
222 Energy & Fuels, Vol. 8, No. 1, 1994
Table 4. Deuterium Distribution in 1-Methylnaphthalene with or without Coal (W. Ky. #9 Pyro) in a 50-cma S.S. Reactor, 2000 psi Dz, with Deuteroacid deuterium content" ( % ) without coal
with coal position (ppm)'
8 (8.10)d 5 (7.94) 4 (7.80) 6 , 7 (7.61) 3 (7.47) 2 (7.41) M (2.83)e Dimolecule reaction time (h) e
385 "C
450 O C 22 22 39 6 4 7
23 23 28 12 7 8
27 26 45 12 7 10
31 0.23 0.17
43 1.37 0.5
0.06 0.17
27 34 8 5 7
22 22 39 6 4 7
35 0.15
31 0.74
17 16 41 9 7 10 5 0.51
0.25
1.0
0.17
19
35
100 OC
450 "C
385 OC
17 17 40 9 7
18 18 34 11 8 11 3 1.51
10 4 0.93 0.25
16
D20/HClb
14 11 35 6 3 30
23 17 26 9 4 20
2 0.65
0
0.5
0.0
48
24
17 17 47
16 51
5 4 8
6
5 9
3 0.18 0.25
0.5
CH3C02Db
a Experimental error & 3 % . At 1 atm. 2H NMR signal, see Figure 8 for assignments. Letters refer to positions shown in Figure 8. Distribution of deuterium ( % ) in the methyl group. The remaining ( % ) deuterium is distributed in the ring positions 2-8.
Table 5. Deuterium and Product Distribution of t h e Decomposition Products of 1-Methoxynaphthalene in a 50-cma S.S. Reactor at 2000 Dsi Dz. 385 OC, and 1 h deuterium content ( % ) product naphthalene 1-methylnaphthalene 1-methoxynaphthalene 1-naphthol othersa a
coal present no Yes no Yes no Ye5 no Yes no Yes
w t % by GC
do 52 49 60 52 45 45 76 33 -
0.6 0.6 7.1 3.0 74 57 8.5 25 9.5 15
di 39 42 32 35 41 41 20 49 -
d2 8 7 5 11 13 13 3 17 -
d3
d4
ds
0
4 2 2 1 1.6 1.6 -
-
Decalin and tetralin.
essentially in the ring only (Table 4). The more electronreleasing groups (OH and OCH3) cause the exchange rate to increase over that of the methyl substituent. Thus, the order of exchange rate for the substituents is as follows: -OH > -0CH3 > -CH3 > -H
0
= -C2HS
8 I"
C Q
->
A
0
\
\
cn 1-Methoxynaphthaleneundergoes conversionunder the 1-h reaction conditions to produce other products, predominantly 1-naphthol and 1-methylnaphthalene (Table 5 ) ; similar data are obtained for the conversion of 1-methoxynaphthalene in the presence of coal. However, the extent of chemical exchange is greater in the presence of coal, and the products appear to shift to favor the formation of 1-naphthol. As suggested by a reviewer, the additional conversion of the 1-methoxynaphthalene in the presence of coal may be due to abstraction of a hydrogen from the methylgroup, thereby increasing the double bond character of the CO bond to allow the elimination of the phenyl radical and the formation of formaldehyde. Deuterium incorporation in the solvents and the products formed from the solvents appear to follow a common pattern in the presence or absence of coal, except possibly 1-naphthol. Exchange studies were attempted with 1-naphthoicacid, however, this compound underwent complete conversion with extensive D incorporation during the 1-h reaction period to produce essentially naphthalene and l-methylnapthalene, with naphthalene the dominant product. The extent of deuterium incorporation with time into 1-methylnapthalene, based on the percentage of do remaining, is shown in Figure 4 for conversionin the presence
\
U
Q c
\
80
EQ
\
I
3
a,
U S 3
I
0 I 10
I
20
I 30
I 40
I 50
I 60
Time, Min. F i g u r e 4. Percentage of undeuterated methylnaphthalene following conversion at 385 "Cand about 2000 psi Dz for various times in the absence (A) and presence (0) of coal (multiple data points for a particular time and condition represent repeat runs).
or absence of coal. There is little difference in the extent of exchange whether coal is present or absent. Furthermore, the extent of deuterium incorporation does not show the initial rapid increase with time as does the extent of coal solubilization (Figures 4 and 5 ) . Thus, coal conversion after 10 min is about 75 % of the conversion at 60 min, but deuterium incorporation at 10 min is only about 20% of that incorporated after 60 min.
Energy & Fuels, Vol. 8, No.1, 1994 223
Deuterium Incorporation during Coal Liquefaction
S
I
\
-_ 0
\
\
1
0
10
20
30
40
50
60
70
'..
80
Time (Min.)
Figure 5. Coal conversion (pyridine soluble) versus time for the liquefaction of Western Kentucky #9 Pyro coal in a 50-cm3SS reactor a t 2000 psi deuterium a t 385 O C for 1h (substituent in 1 position of naphthalene is: H, 0;CH,, A; OCH3 0).
0
'770
The extent of exchange with naphthalene for any comparative time is less than that with l-methylnaphthalene; likewise,the extent of exchange with naphthalene is the same whether coal is present or not (Figure 6). Methoxynaphthalene undergoes more extensive exchange than 1-methylnaphthalene, but the extent of exchange is the same whether coal is present or not (Figure 6). The deuterium distribution at various ring and methyl positions can be learned from deuterium NMR data for the exchanged compound. The spectrum in Figure 7 shows that the dominant ring substitution positions of l-methylnaphthalene are the a positions of the unsubstituted ring or the positions ortho and para to the substituent on the substituted ring. The deuterium exchange therefore occurs by a mechanism that is under kinetic control rather than by a mechanism that would produce a statistical distribution. Exchange also occurs with the methyl group hydrogens when coal is present, but little exchange occurs in the absence of coal (Table 4). Deuterium exchange with 1-methylnaphthalenewas also effected using either CH3COzD or DCl in Dz0; the deuterium distribution for the acid-catalyzed exchange was very similar to that obtained with gaseous Dz at 385 "C (Table 4). Note that the percent deuterium in the methyl position is based on the total deuterium in the 1-methylnaphthalene, and the ring distribution is based on percentages which are normalized for ring substitution only. The distribution of deuterium in the l-methylnaphthalene recovered following exchange at 385 or 450 "C, either in the absence or in the presence of coal, is given in Table 4. The total incorporation of deuterium is similar whether coal is present or not; however, the distribution between the methyl position and the ring is quite different. I n the absence of coal less than 5 % of the deuterium is incorporated into the methyl position, but 30-40% of the deuterium is incorporated into the methyl position when coal is present. Thus, the coal, or more likely the reactive species formed during coal thermolysis, causes the incorporation of deuterium into the methyl position. The relative distribution of deuterium in the ring positions is
0 65,
I
I
I
I
I
I
Time, Min.
Figure 6. Percentage of undeuterated methoxynaphthalene (top curve) and naphthalene (bottom curve) following conversion a t 385 O C and about 2000 psig Dz in the absence (A)and presence (0) of coal.
Figure 7. Deuterium NMR spectrum of 1-methylnaphthalene following exchange at 385 "C,2000 psi Dz, and 60 min in the absence (top spectrum) and presence (bottom spectrum) of coal.
essentially independent of the reaction temperature or whether coal is present or not.
Dabbagh et al.
224 Energy & Fuels, Vol. 8, No. 1, 1994
Table 6. Deuterium Incorporation (Relative %) in Fractions of the Solubility Fractions from the Conversion of Western Kentucky #9 Pyro Coal at 385 "C,2000 psi Dz deuterium distribution (% of total) aromatics (5.5-9.0) ppm pyridine" 16 39 (46)' 0 33 20.0 44 (55)' 0 46
coal fractionf HI HSd 1-methylnaphthalene HI HSd 1-methylnaphthalene HI HSd
solvent tetralin
a (2.2-4.1) ppm
30 (35.7) 47 30 (37.5) 42
e
34
aliphatics 6 (1.3-2.2 ppm) 13 (15.5) 17 6 (7.5)
(-.7-1.3 ppm) 2.0 (2.4) 3 0 2 e
10 e
e
36
0
y
17
3
deuterium inb soluble fractions 0.45 1.76 0.37 2.29 0.870 3.40
Percent of aromatic deuterium in the incorporated pyridine (7.10,7.50, and 8.80ppm). Normalized based on CD3N02. Percentage based on pyridine free. d Fraction boiling above 1-methylnaphthalene.e Amount of deuterium too small to measure. f HI: hexane insolubles. HS: hexane solubles. 0
"
.-.'2.
---'t. 1.1
- ._- -' -_ _
-!-
I,
__ - L---O--2
-1
*I
I
I
42.0%
-
+
1
13.0%
45%
Figure 8. Deuterium NMR spectrum of asphaltene plust preasphaltene fraction of coal liquefaction products following conversion at 450 O C and 2000 psig D2 for 30 min.
Data are presented in Table 6 for the incorporation of deuterium into the coal-derived products for conversion using tetralin, 1-methylnaphthalene, or l-methoxynaphthalene solvent. The oils are for the residue of the hexanesoluble fraction after removing the solvent by distillation. More deuterium is incorporated into the aliphatic position of the coal-derived oils than in the ring positions. Furthermore, the dominant aliphatic exchange in the oil is in the a-position to the ring. The hexane-soluble materials contain a significantly larger percentage of deuterium than the hexane-insoluble fractions (80% and 86% of total for tetralin and methylnaphthalene, respectively). This result implies that the oil formation results in aliphatic bond breaking with subsequent incorporation of D from the gas phase. The fraction of exchange in the aromatic position in the asphaltene plus preasphaltene fraction is as great or slightly greater than that in the corresponding oil fraction. The extent of exchange in 1-methylnaphthalene is presented together with coal solubilization data in Table 7 . It is apparent that deuterium exchange is not very extensive in 10 min whereas the coal has undergone solubilization to approach it maximum conversion. If the coal products add 2 wt % Dz during solubilization, this could correspond to 20% of the H in the solvent. The NMR spectrum of the pyridine-soluble, pentaneinsoluble fraction from coal liquefaction at 450 "C is presented in Figure 8. It is observed that deuterium is incorporated into aromatic, aliphatic, and active hydrogen
Table 7. Deuterogenation-Coal Liquefaction of Western Kentucky #9 Pyro at 385 "C, 2000 psi DZin a 50-cmSS.S. Reactor at Various Reaction Times in the Presence of 1-Methylnaphthalene deuterium content ( 7 % ) of reaction coal 1-methylnaphthalene time (min) conversion (%) & dl dz d3 10 76.0 93 6 1 0 30 80.5 75 21 4 0 60 80.5 69 26 5 0
positions of the coal-derived products. The spectrum also shows deuterium peaks for pyridine superimposed upon the spectrum for deuterium attached to aromatic carbons. The pyridine used as solvent for solubility class separations did not contain deuterated molecules;hence, the deuterium in the pyridine was incorporated into the coal products during the analysis. Presumably, the H-D exchange in pyridine is a result of exchange with the active hydrogen present in the coal-derived species during the analysis; because of the amount of deuterated pyridine in the solid it is more likely that the exchange occurred during drying rather than with the larger volume of pyridine involved in the Soxhlet extraction, but we do not have data to demonstrate this. Deuterated pyridine was always observed in the coal-derived sample although in most cases it was not nearly as prominent as shown in Figure 8. The deuterium NMR spectrum of tetralin following conversion at 385 "C for 1h showed that the exchange on
Energy & Fuels, Vol. 8, No. 1, 1994 225
Deuterium Incorporation during Coal Liquefaction the aromatic ring corresponded to 68% of the total exchange. The exchange that occurred in the hydrogenated ring favored the a position by nearly 2:l (=a:8) (Table 6). The exchange with 1-methoxynaphthalene was very selective. As shown in Figure 9 top (top spectrum), the ring protons are sufficiently separated to permit analytical determinations for all rings positions. However,deuterium exchange was detected in 1h at 385 "C only on the ring containing the methoxy substituent; furthermore, the exchange occured only at the positions ortho and para to the methoxy group, with 70% of the exchange occurring in the para position (Figure 9, bottom spectrum).
it is in tetralin, a good hydrogen donor solvent. This enhanced conversion could be viewed as being due to hydrogen shuttling by the formation of di- or tetrahydromethylnaphthalene. If the methylnaphthalene is to be effective in adding hydrogen, we could visualize 1,2 deuterium addition as
Discussion Any molecule containing hydrogen is a potential hydrogen donor. However, a donor solvent is usually considered to be one that transfers hydrogen and that stable molecules are involved in the donor reaction. Thus, the aromatic system of methylnaphthalene can function in this role, e.g., in the scheme shown below (for brevity not all possible structures are shown): CH3
I
The methylnaphthalene system can shuttle (transfer) hydrogen in other &actions such as the ones shown below:
(2)
(3)
CH3
CH3
I
I
(4)
CH3
C
I (5)
Some coals utilize gaseous hydrogen very efficiently so that conversion during the early period of liquefaction is greater in 1-methylnaphthalene, a nondonor solvent, than
\H
If deuterium shuttles by a 1,2-dihydro-l-methylnaphthalene intermediate (reaction 61, exchange should occur predominantly in the ring 2 position since the 1-position contains a methyl group. If the shuttling occurred through reaction 7, exchange a t the ring 3 and 4 positions should occur in equal amounts since there should be equal probability for subsequent removal of H or D from each of the positions during removal of these atoms to re-form the 1-methylnaphthalene. Likewise, if shuttling occurred through reaction 8, the exchange at ring positions 2 and 3 should be equal. The data clearly eliminate the participation of reaction 7 since the 4-(para) position exchanges to a much greater extent than the 3- (metal position. Likewise the ring a position in the unsubstituted ring exchanges more rapidly than the fl position. Reaction 8 is also eliminated because of the difference in exchange in the substituted ring positions 2 and 3. Obviously, the addition of hydrogen atoms (reactions 2-5) to the aromatic ring cannot be eliminated from these data. The extent of deuterium incorporation into solvent molecules appears to be about the same in the presence or the absence of coal. Even though the ratio of methyl-to-ring exchange is different when coal is present, the results of the deuterium studies appear to eliminate the shuttling of hydrogen by solvent molecules as being responsible for the superior conversion in the nondonor 1-methylnaphthalene. Furthermore, incorporation of deuterium in the ring is very similar in the presence or absence of coal; the ring incorporation appears to be due primarily to exchange catalyzed by the reactor. Exchange in the methyl position, on the other hand, appears to be due to coal-induced reactions. On the basis of the relative rate constants obtained by Bockrath et aL2' for hydrogen transfer to benzyl radicals, tetralin should be a much better donor of hydrogen atoms than 1-methylnaphthalene. However, the difference in the extent of deuterium exchange in the methyl position of 1-methylnaphthalene in the presence and absence of coal shows that such hydrogen transfer does occur with 1-methylnaphthalene. T h e present data do not permit a (27) Bockrath, B.; Bittner, E.; McGrew, J. J. Am. Chem. SOC.1984, 106,136.
Dabbagh et al.
226 Energy & Fuels, Vol. 8,No. 1, 1994
5
4
m
7
5
4
m
----
,
- 7 - 1 - 1 - 1 7
6
I
I
I
j
5
1
1
I
l - - ’ V - -
4
3.20
1.32
Figure9. Proton NMR spectrum (top spectrum)and deuterium NMR spectrum (bottom spectrum)of 1-methoxynaphthalenefollowing conversion at 385 O C and 2000 psig Dz for 1h. quantitative evaluation of the degree of coal liquefaction that corresponds to the hydrogen transfer of the methyl group of 1-methylnaphthalene, but they do demonstrate that the reaction does occur. The extent of deuterium incorporation is much greater in the oil fraction (80-85 % ) than in the asphaltene plus preasphaltene fraction. Furthermore, the oil fraction comprises only about 20-25 96 of the pyridine-soluble products (oils, asphaltenes, and preasphaltenes). This requires the oils to undergo at least 25 times more incorporation and/or exchange than the asphaltene and/
or preasphaltene fractions. This observation implies that an average “oil molecule” is the result of multiple bond breaking events or that intermediates that lead to oils undergo multiple exchanges prior to stabilization.
Acknowledgment. This work was supported by the Commonwealth of Kentucky and DOE Contract No. DEFC22-88PC8806 as part of the Consortium for Fossil Fuel Liquefaction Science (managed by the University of Kentucky).
