1141
Znd. Eng. C h e m . Res. 1991, 30, 1141-1145
L
6
0
Secoctivity
A
Vield,C02
9
C8H80
V-Fe-0
LOOOC W/F :76gm.hr/g.mole T
I
- 80
> V-Ce-0 > V205 > V-P-0 > V-Mg-0 >
v-co-0
However, V-Ce-0 is more selective for p-tolualdehyde than is V-Fe-0. Acknowledgment
-60.:
r
.-
I
U
w w ul
-LO
z
We express our sincere gratitude to the Director, National Metallurgical Laboratory, for providing necessary facilities in the preparation of the manuscript. Literature Cited Bhattacharya, S. K.; Gulati, I. B. Catalytic oxidation of ortho-xylene: activity of fused vanadium catalysts. Chem. Ind. (London) 1954,
- 20
0
20 Wt.%
LO of
60
80
100
c e r i u m Oxide
Figure 4. Effect of weight percent of cerium oxide on yield and selectivity for p-tolualdehyde over V-Ce-0 catalyst.
corporation of phosphorous, magnesium, and cobalt to vanadium pentoxide decreases the activity, whereas cerium and iron promote the activity of Vz06 markedly. The activity of the catalyst is found to be in the following order:
NO.13, 1425-1426. Bhattacharya, S. K.; Gulati, I. B. Catalytic vapour phase oxidation of xylenes. Znd. Eng. Chem. 1958,50, 1719-1726. Mathur, B. C.; Viswanath, D. S. Catalytic vapour-phase oxidation of p-xylene over tin vanadate. J . Catal. 1974, 32, 1-9. Mathur, B. C.; Viswanath, D. S. Vapour Phase oxidation of p-xylene. Can. J. Chem. Eng. 1978,56, 224-229. Parks, W. G.; Allard, C. E. Vapour phase catalytic oxidation of organic compounds. Znd. Eng. Chem. 1939,31,1162-1167. Trimm, D. L.; Irshad, M. The influence of electron directing effects on the catalytic oxidation of toluenes and xylenes. J . Catal. 1970, 18, 142-153.
Received for review February 19, 1990 Revised manuscript received July 23, 1990 Accepted November 27,1990
Effect of Solvents on Thermal Cracking of Model Compounds Typical of Coal Koji Chiba, Hideyuki Tagaya,* T a k u Yamauchi, and Shimio Sat0 Faculty of Engineering, Yamagata University, Yonezawa, Yamagata 992, Japan
Conversions of bibenzyl and dibenzyl ether as coal models depend on the nature of the solvent. When solvents that were poor hydrogen donors, but their dehydrogenated radicals were more stable than donors, were used, bibenzyl and dibenzyl ether conversion were markedly enhanced. Positive effects were observed by the mixing of tetralin with nondonor solvents. However, negative effects by such mixing were observed in the case of dibenzyl ether. Through this study, the importance of radical-induced decomposition on cracking of coal model compounds was suggested. Introduction Coal is regarded as a highly cross-linked macromolecular network consisting of a number of stable cluster units connected by cross links (Gray and Shah,1981; Lucht and Peppas, 1981). The structure is highly complex; therefore, a convenient way to study the reactions of various types of bonds is to simulate the structure by using a variety of model compounds (Benjamin et al., 1978; Allen and Gavalas, 1984). Bibenzyl is the simplest model for dimethylene connecting units in coal (Poutama, 1980). The cracking of bibenzyl has been described by a free-radical mechanism (Stein et al., 1982; Buchanan et al., 1986). It has been accepted that the hydrogen donating ability of the solvent plays the most important role in coal degradation (Chiba et al., 1987b);therefore, interests in bibenzyl as a coal model were mainly focused on thermal cracking of bibenzyl in good hydrogen donor solvents. Cronauer et al. (1978) reported the mechanism and kinetics of reactions between bibenzyl and a few non-hydrogen donor or hydrogen donor solvents. They and Panvelker et al.
(1982) concluded that the breakage of the carbon-carbon bond in bibenzyl occurs purely thermally and its rate is independent of the nature of the solvent present during the reaction. Their results imply that the bibenzyl free radical is very active and readily extracts hydrogen from any available solvents. However, a detailed discussion of the reactions taking place between bibenzyl and a variety of non-hydrogen donor solvents has not been published. In this paper we first briefly extend our previous communication (Chiba et al., 1985b) on the reactions between non-hydrogen donor solvents and bibenzyl. We also applied the results to the reactions between non-hydrogen donor solvents and dibenzyl ether. Experimental and Analytical Procedures The experiments were performed in a 100-mL magnetically stirred autoclave. The autoclave charged with bibenzyl(1.00 g) and solvent (or mixed solvents) (20.00 g) was filled with nitrogen at an initial pressure of 3 MPa. The autoclave was maintained at the reaction temperature
0888-5885/91/ 2630-1141$02.50/0 0 1991 American Chemical Society
1142 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 Table I. Thermal Cracking of BibenzsP bibenzyl yield conversolvent temp, "C sion toluene stilbene 1,2,3,5-tetramethyl450 22.0 17.9 0.0 benzene 1,4-diisopropylbenzene 450 25.5 25.6 0.0 tetralin 450 40.3 40.0 0.0 400 6.4 1.7 0.0 9,lO-dihydroanthracene 450 43.0 38.6 trace diphenylmethane 450 61.0 42.3 12.4 400 10.6 3.8 1.9 triphenylmethane 450 62.8 37.0 5.4 400 12.0 2.0 0.0 diphenylamine 450 57.3 42.4 1.2 indene 450 71.4 68.5 trace 1-methylnaphthalene 450 62.1 54.0 1.2 400 4.9 3.3 0.0 quinoline 450 58.3 55.8 1.3 fluorene 450 87.3 69.4 trace Conversion of bibenzyl and yields of toluene and stilbene are in mole percent.
(350,400, or 450 "C) for 1h. In the case of dibenzyl ether, the reaction temperature was 380 "C. It was already reported that secondary reactions of initially formed products occurred at 450 "C (Schlosberg et al., 1981). At 380 "C, we could prevent the secondary reactions. After cooling, the content of the reactor was taken out with acetone. The acetone solution was analyzed with GC using silicone grease DC550 (20%)/Cellite 545 (60-80 mesh) 4 mm X 2 m column. In the case of the two-step reaction, bibenzyl or dibenzyl ether (1.00 g) and solvent (10.00 g) were reacted at 450 or 380 "C for 1 h as the first step. Then, tetralin (10.00 g) was added and reacted further for 1 h as the second step. Evaluation of the solvent donor ability with anthracene was done as follows. The solvent (3 g) was mixed with anthracene (AN, 3 g) and was heated at 400 "C for 1h in a nitrogen atmosphere. The production of dihydroanthracene (DHA) and tetrahydroanthracene (THA) was confirmed by GC. The amount of THA was corrected to the amount of DHA. The value DHA/(DHA + AN) was defined as the hydrogen donating ability of the solvent.
Results and Discussion Conversion of Bibenzyl in Nondonor Solvent. Bibenzyl conversion obtained by using tetralin at 450 "C was 40.3%, as shown in Table I. It was known that 9,lO-dihydroanthracene was a more active donor than tetralin (Bockrath et al., 1984). However, the conversion using 9,lO-dihydroanthracene was 43.0% and close to that using tetralin, as Cronauer et al. (1978) noticed. Alkylbenzenes were not good solvents for bibenzyl conversion. When diphenylmethane, triphenylmethane, diphenylamine, indene, and fluorene were used, bibenzyl conversions were markedly enhanced, although the formation of stilbene was confirmed. These results are interesting, for those solvents are able to form stable radicals such as diphenylmethyl, triphenylmethyl, diphenylamino, indenyl, and fluorenyl radicals. Resonance stabilization energies of alkylbenzenes are smaller than those of diphenylmethane, etc. (Bockrath et al., 1984), indicating that hydrogens of diphenylmethane, etc., are more easily abstracted than those of akylbenzenes. These facta indicate that the effect of solvent on bibenzyl conversion depends on the nature of solvent. Buchanan et al. (1986) reported that, in the thermolysis of bibenzyl, bibenzyl is consumed by two distinct pathways, as shown in Scheme I. The one is homolysis in step 1,and the other one is induced decomposition by hydrogen atom transfer in step 2. In solvent, hydrogen abstraction
from solvent (RH) should occur to produce toluene in step 3.
From the results of Cronauer et al. (19781, the pure thermal breakage of a carbon-carbon bond in bibenzyl, step 1 shown in Scheme I, is not affected by the nature of solvent. The higher conversion of bibenzyl in triphenylmethane, etc., than in tetralin must result from the radical-induced reaction, such as step 4. R' in step 4 includes benzyl radical and solvent-derived radicals such as diphenylmethyl radical. Bockrath et al. (1984) reported that the hydrogen donating ability of triphenylmethane, diphenylmethane, etc., are small compared to tetralin. We measured the hydrogen donating ability by using anthracene as a hydrogen accepter. The hydrogen donating ability, DHA/(DHA + AN), of tetralin was 0.247. The hydrogen donating abilities of diphenylmethane, triphenylmethane, and diphenylmethane were 0.032,0.030, and 0.040, respectively. The values were low, similar to the 0.037 value of 1-methylnaphthalene. The nondonor does not have enough hydrogen to stabilize intermediate radicals. Therefore, radical-induced carbon-carbon bond scission may produce adducts of solvent with bibenzyl and/or its derivatives. It is well-known that, even in hydrogen donor solvent, an adduct of the solvent occurred at relatively low temperatures around 350 "C (Cronauer et al., 1979b). Actually in this study toluene selectivities at 350 and 400 "C were less than 30%, even in tetralin. In non-hydrogen donor solvents, selectivitiesof toluene are smaller than those in donor solvents at 450 "C. In the case of triphenylmethane at 450 OC, 62.8% of bibenzyl was converted; however, the yield of toluene was 37.0%. The products obtained by using triphenylmethane at 450 O C were isolated by column chromatography on alumina. As a result, a large amount of toluene, a less amount of stilbene, small amounts of phenanthrene, 9phenylfluorene, and a very small amount of 1,1,1,2-tetraphenylethane were obtained. These were respectively identified by comparing the retention times in GC, 'H NMR, and IR spectra with those of the authentic samples. Phenanthrene and 9-phenylfluorene must be formed by dehydrogenation from stilbene and triphenylmethane, respectively, as shown in Figure 1. It indicated that not only triphenylmethane could donate a small amount of hydrogen atom but also bibenzyl itself could donate in non-donor conditions. We could not confirm, but it was proven, that the dimer of triphenylmethyl radical was a methylenecyclohexadiene structure (Lankamp et al., 1968), as shown in Figure 1. There was also a possibility of the presence of such a dehydrogenated dimer. 1,1,1,2-Tetraphenylethane was an addition product of triphenylmethyl radical with benzyl radical, as in step 5. High conversion near 90% was obtained when fluorene was used as a sol-
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1143 Table 11. Thermal Cracking of Bibenzyl in Binary Solvent of Tetralin and Other Solvent at 450 OC0 bibenzyl cosolvent of tetralin, (wt converyield % of solvent) sion toluene stilbene 40.3 40.0 0.0 none (0%) triphenylmethane (100%) 62.8 37.0 5.4 triphenylmethane (75%) 62.6 62.7 0.0 triphenylmethane*(75%) 10.9 2.5 triphenylmethane (50% ) 50.2 51.2 0.0 triphenylmethane (25%) 48.9 50.2 0.0 diphenylmethane (100% ) 61.0 42.3 12.4 diphenylmethane (75%) 52.2 53.3 0.0 diphenylmethaneb(75%) 5.3 2.5 43.1 43.4 0.0 diphenylmethane (50% ) 39.8 40.3 0.0 diphenylmethane (25%) ~
~~~~
~
~
Conversion of bibenzyl and yields of toluene and stilbene are in mole percent. bReaction at 400 "C.
IV Figure 1. Addition and dehydrogenated producta in the reaction of bibenzyl in triphenylmethane: (I) 1,1,1,2-tetraphenylethane;(11) 9-phenylfluorene; (111) phenanthrene; (IV) methylenecyclohexadiene structure.
Table 111. Two-step Treatment of BibenzylO bibenzyl yield conversolvent at first step sion toluene stilbene tetralin 59.7 58.8 0.0 73.2 74.0 0.0 triphenylmethane 75.3 71.7 1.2 diphenylmethane 67.5 67.9 0.0 indene fluorene 98.5 90.9 0.0 74.0 74.5 0.0 1-methylnaphthalene 79.5 79.5 0.0 quino1ine O Conversion of bibenzyl and yields of toluene and stilbene are in mole percent.
& 1
o o + & a & 40i & c0 g
olio
0
0
m
I1
0
0
0
0 IV
Figure 2. Addition and dehydrogenated producta in the reaction of bibenzyl in fluorene: (I) 9-benzylfluorene; (11) 9-benzyl.idenefluorene; (111) 9,Y-difluorenyl;(IV) 9,9'-difluorenylidene.
vent a t 450 "C. Reaction products obtained by using fluorene at 450 "C were also isolated by column chromatography on alumina. We confirmed the presence of 9,9'-difluorenyl and 9,9'-difluorenylidene, which were dehydrogenated dimers of fluorene, as shown in Figure 2. The presence of a benzylidenefluorene was also confirmed, which was the addition product of fluorenyl radical with benzyl radical. Conversion of Bibenzyl in Mixed Solvent. In the case of indene, conversion of bibenzyl was 71.4% and the selectivity of toluene was 95.9%. The hydrogen donating ability, DHA/(DHA + AN), of indene was 0.156. Indene itself is a nondonor, but indenyl radical was a stable radical and considered to accelerate the bibenzyl conversion. We already reported that indene oligomerized easily, and resulting oligomers had high hydrogen donating abilities (Chiba et al., 1985a; Tagaya et al., 1988). Therefore, it was considered that indene oligomer donated hydrogen to intermediate radicals to produce toluene, etc., in the reaction. Chiba et al. (1987a) and Tagaya et al. (1989) reported that, in the coal liquefaction using binary system solvents, positive and negative effects of mixing were observed. Effective solvents for coal liquefaction are available by clarifying the complex interactions of solvents during cod
liquefaction (Woodfine et al., 1989). When bibenzyl was reacted in the mixture of a nondonor and tetralin, as shown in Table 11, conversion of bibenzyl increased with an increase in the weight percent of the nondonor. Furthermore, almost all of bibenzyl was converted to toluene in the case where more than 25% tetralin was mixed with a nondonor. These facts indicate that bibenzyl conversion was accelerated by the radical-induced reaction of the nondonor, and intermediate radicals and/or products like stilbene were hydrogenated by tetralin to form bibenzyl that then went to form toluene. If bibenzyl is considered as a model of coal, toluene is like a liquefaction product and the yield of toluene is considered the same as the conversion of coal. In this study maximum toluene yields were obtained in 25% tetralin and 75% diphenylmethane or triphenylmethane. These results are similar to those of Malhotra and McMillen (1989) and Mochida et al. (1988). Mochida et al. (1988) reported coal liquefaction by using various mixtures of fluoranthene (FL) (4HFL). Maximum and 1,2,3,10b-tetrahydrofluoranthene oil yields were obtained by using the mixture of 75% 4HFL and 25% HFL. Combination of nondonor and donor solvents might be one of the ideal selections of liquefaction solvents. In this study selectivities of toluene at 400 OC were not 100%, even if tetralin was added with a nondonor. These results are consistent with that in Table I in which selectivity of toluene at 400 OC was only 22.6% even in 100% tetralin. To clarify the reactivity of addition products 1,1,1,2-tetraphenylethane (1.0 g) was reacted at 450 O C for 1 h in tetralin (20.00 g). All of 1,1,1,2-tetraphenylethane was converted into toluene and triphenylmethane. It indicated that the addition products might be reactive. On the same reaction conditions 94.5% of stilbene was hydrogenated by tetralin to give 37% toluene and 55.1% bibenzyl. Two-step Treatment of Bibenzyl. Products including adducts were reacted in tetralin, as shown in Table 111,
1144 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1
Table IV. Thermal Cracking of Dibenzyl Ether (DBE)' DBE yield converbenzsolvent, sion benzene toluene aldehyde 1,2,3,5-tetramethyl32.0 1.7 14.5 16.1 benzene 1,4-diisopropylbenzene 35.0 1.0 12.3 15.6 33.0 0.9 15.5 16.7 tetralin 63.9 3.1 26.4 34.3 diphenylmethane 58.1 2.4 27.0 26.3 triphenylmethane 41.8 0.9 19.8 21.1 1-methylnaphthalene 41.5 1.0 14.0 10.3 quinoline
0
t
90
0
a Conversion of dibenzyl ether and yields of benzene, toluene, and benzaldehyde are in mole percent.
I
401 0
0.1
0.2
D H A l 1DHA.ANl
I-)
Figure 3. Relationship between DHA/(DHA of bibenzyl in the two-step treatment.
0.3
+ AN) and conversion
- OCH, 2H.
+ HOCH,@
0
0
20
40
60
80
100
Tetralin in solvent ( w t % )
Figure 4. Conversion of dibenzyl ether in the mixture of tetralin and a nondonor solvent: (0) tetralin-triphenylmethane; (A)tetralin-diphenylmethane.
because there was a possibility that the adducts were hydrogenated easily by tetralin, as described above. A t the first step bibenzyl was reacted in a nondonor solvent at 450 OC. After the solution was cooled, tetralin was added and reacted further for 1 h. Toluene selectivities near 100% were obtained by such two-step treatments of bibenzyl. It indicated that almost all adducts were hydrogenated by tetralin to give toluene. Furthermore, conversions of bibenzyl in nondonor solvents were from 67.5% to 98.5% and fairly larger than 59.7% in tetralin. Conversion in indene was 67.5%. The value was larger than that in tetralin, but smaller than those in nondonor solvents. A large conversion of 98.5% was obtained when fluorene was used. The hydrogen donating ability, DHA/(DHA + AN), of fluorene was 0.05 and larger than 0.030-0.040 of nondonors. We already confirmed that fluorene and 9,9'-difluorenyl could donate hydrogen. A large conversion in fluorene showed that fluorenyl radical was more stable than diphenylmethyl, triphenylmethyl, and quinolyl radicals. The relationship between DHA/ (DHA + AN) and conversion of bibenzyl in the two-step treatment is shown in Figure 3. Conversion tended to decrease with an increase of DHA/(DHA + AN), except with fluorene. Conversion of Dibenzyl Ether in Nondonor Solvent. It is well-known that intermolecular (step 1 in Scheme 11) and intramolecular (step 2 in Scheme 11) reactions are involved in the dibenzyl ether thermolysis (Chiba et al., 1987c; Cronauer et al., 1979a). Panvelker et al. (1982) reported that, as with bibenzyl, the rate of conversion of dibenzyl ether was independent of the type of solvents. However, in this study, tetralin was not a good solvent for conversion of dibenzyl ether at 380 OC,as shown in Table IV. Higher conversions of dibenzyl ether were obtained in diphenylmethane and triphenylmethane than in tetralin.
Simmons and Klein (1985) reported that a free-radical chain reaction occurred in preference to a intramolecular reaction. They evaluated the high fraction of the freeradical chain reaction from thermolysis at 400 OC. Here we must pay attention to the selectivities of toluene and benzaldehyde that were near 100% even in nondonor solvents. And we could not confirm the production of benzyl alcohol. Certainly, Cronauer et al. (1979a) reported that benzyl alcohol reacted thermally to produce benzaldehyde even at 350 "C. However, the reaction rate was not large. Our results of thermolysis at 380 "C indicated that the fraction of the free-radical chain reaction was small at 380 "C compared to that at 400 "C. As shown in Scheme 11, radical-induced decomposition, as in step 3, was suggested. In this case, stable radicals acted as hydrogen shuttlers and they did not need hydrogen donors. Conversion of Dibenzyl Ether in the Mixture of Tetralin and Nondonor Solvent. The acceleration effects of a nondonor on dibenzyl ether conversion indicated the importance of the amounts of radicals in the course of reaction. If tetralin donated hydrogen atom to intermediate radicals, the amounts of radicals should be decreased. Actually tetralin addition to nondonor solvents was detrimental, as shown in Figure 4. These results are in marked contrast to the results of dibenzyl conversion in which positive effects of mixing were observed. The difference between the results of bibenzyl and dibenzyl ether conversion might result from the difference of the primary reaction between them. In the case of bibenzyl, blenzyl radical was formed constantly by thermal decomposition and it could abstract hydrogen from the nondonor solvent to give a stable radical. On the other hand in the case of dibenzyl ether, radicals by free-radical mechanism might be fairly small compared to bibenzyl. Coal includes various types of bonds as its connecting units. A model compound study is a very convenient
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1145
method, and to combine model compound studies is im-
portant. Summary and Conclusions Important conclusions derived from this work include the following. 1. The observation that enhanced conversions of bibenzyl in poor donor solvents such as diphenylmethane, triphenylmethane, diphenylamine, etc., were higher than those in a donor solvent, tetralin, supports the conclusion that bibenzyl is converted t h r o u g h a radical-induced decomposition reaction b y solvent-derived radicals. 2. When tetralin was mixed with nondonor solvents, mentioned in 1 above, positive effects of mixing on the toluene yield were observed. The presence of solvent adducts with bibenzyl-derived radicals was suggested. It was considered that such adducts were hydrogenated b y tetralin to give toluene. 3. B y a two-step treatment, fairly large conversions of bibenzyl were obtained. In the first step, bibenzyl cracking was accelerated b y the action of stable radicals from a nondonor solvent. In the second step, addition products were hydrogenated b y tetralin to give toluene. 4. Higher conversions of dibenzyl ether were obtained in nondonor solvents than in tetralin. It was considered that nondonor solvents acted as radical sources to decompose dibenzyl ether and as hydrogen shuttlers. 5. Mixing of tetralin with a nondonor solvent was detrimental to conversion of dibenzyl ether. Tetralin might donate hydrogen to intermediate radicals to reduce radical concentrations, which caused the reduction of radical-induced decomposition. Acknowledgment We acknowledge H. Ando and T. Suzuki for helpful discussions. We are also grateful to J. Sugai, Y. Tsukahara, and M. Takeue for their technical collaboration. Registry No. Bibenzyl, 103-29-7; dibenzyl ether, 103-50-4; tetralin, 119-64-2; 1,2,3,5-tetramethiylbenzene,527-53-7; 1,4-diisopropylbenzene, 100-18-5; 9,10-dihydroanthracene, 613-31-0; diphenylmethane, 101-81-5; triphenylmethane, 519-73-3; diphenylamine, 122-39-4; indene, 95-13-6; 1-methylnaphthalene, 90-12-0; quinoline, 91-22-5; fluoreme, 86-73-7.
Literature Cited Allen, D. T.; Gavalas, G. R. Reactions of Methylene and Ether Bridges. Fuel 1984, 63, 586-592. Benjamin, B. M.; Raaen, V. F.; Maupin, P. H.; Brown, L. L.; Collins, C. J. Thermal Cleavage of Chemical Bonds in Selected Coal-Related Structures. Fuel 1978,57, 269-272. Bockrath, B.; Bittner, E.; McGrew, J. Relative Rate Constants of Hydrogen Transfer to Benzyl Radical. J. Am. Chem. SOC.1984, 106,135-138. Buchanan, A. C.; Dunstan, T. D. J.; Douglas, E. C.; Poutsma, M. L. Hydrogenolysis during Thermolysis of Surface-Immobilized Bibenzyl: Implications for Coal Chemistry. J. Am. Chem. SOC.1986, 108,7703-7715. Chiba, K.;Tagaya, H.; Sato, S.; Ito, K. Coal Liquefaction Using Indene-Tetralin and Indene-Decalin Mixtures as Solvent. Fuel 1986a, 64,6670.
Chiba, K.; Tagaya, H.; Yamauchi, T.; Tsukahara, Y. The Effect of Solvent on Thermal Cracking of Bibenzyl as a Coal Model. Chem. Lett. 1985b, 945-948. Chiba, K.; Tagaya, H.; Saito, N. Liquefaction of Yallourn Coal by Binary System Solvent. Energy Fuels 1987a, I , 338-343. Chiba, K.; Tagaya, H.; Kobayashi, T.; Shibuya, Y. Solvent Extract Liquefaction of Coal with Fractionated Anthracene Oil and Recycle Solvent. Znd. Eng. Chem. Res. 1987b, 26,1329-1335. Chiba, K.; Tagaya, H.; Suzuki, T.; Suzuki, T. Evaluation of the Hydrogen Shuttling Ability of Coal Liquefaction Solvents. Bull. Chem. SOC.Jpn. 1987c, 60, 2669-2670. Cronauer, D. C.; Jewell, D. M.; Shah, Y. T.; Kueser, K. A. Hydrogen Transfer Cracking of Dibenzyl in Tetralin and Related Solvents. Znd. Eng. Chem. Fundam. 1978, 17, 291-297. Cronauer, D. C.; Jewell, D. M.; Shah, Y. T.; Modi, R. J. Mechanism and Kinetics of Selected Hydrogen Transfer Reactions Typical of Coal Liquefaction. Znd. Eng. Chem. Fundam. 1979a, 18,153-162. Cronauer, D. C.; Jewell, D. M.; Shah, Y. T.; Modi, R. V.; Seshadrl, K. S. Isomerization and Adduction of Hydrogen Donor Solvents under Conditions of Coal Liquefaction. Ind. Eng. Chem. Fundam. 1979b,18, 368-376. Gray, J. A.; Shah, Y. T. Structure and Properties of Coal and the Mechanism of Coal Liquefaction. In Reaction Engineering in Direct Coal Liquefaction; Shah, Y. T., Ed.; Addison-Wesley: Reading, MA, 1981; Chapter 2. Lankamp, H.; Nauta, W. Th.; MaClean, C. A New Interpretation of the Monomer-Dimer Equilibrium of Triphenylmethyl and (Alkylsubstituted-diphenyl)Methyl Radicals in Solution. Tetrahedron Lett. 1968, 249-254. Lucht, L. M.; Peppas, N. A. Cross Linked Structures in Coals: Models and Preliminary Experimental Data. In New Approaches in Coal Chemistry; Blaustein, B. D., Bockrath, B. C., Friedman, S., Eds.; ACS Symposium Series No. 169: American Chemical Society: Washington, DC, 1981; Chapter 3. Malhotra, R.; McMillen, D. F. Factors Governing Efficiency of Hydrogen Utilization in Coal Liquefaction. Abstracts of Papers, International Conference on Coal Science; NEDO: Tokyo, 1989; p B5. Mochida, I.; Yufu, A.; Sakanishi, K.; Korai, Y. Influence of Donor Amount in the Hydrogen-Transfer Liquefaction of Australian Brown Coal. Fuel 1988,67, 114-118. Panvelker, S. V.; Shah, Y. T.; Cronauer, D. C. Hydrogen Transfer Reactions of Model Compounds Typical of Coal. Znd. Eng. Chem. Fundam. 1982,21, 236-242. Poutsma, M. L. Free-Radical Model for Coal Conversions. Effect of Conversion Level and Concentration on Thermolysis of Bibenzyl. Fuel 1980,59, 335-338. Schlosberg, R. H.; Ashe, T. R.; Pancirov, R. J.; Donaldson, M. Pyrolysis of Benzyl Ether under Hydrogen Starvation Conditions. Fuel 1981,60, 155-157. Simmons, M. B.; Klein, M. T. Free-Radical and Concerted Reaction Pathways in Dibenzyl Ether Thermolysis. Znd. Eng. Chem. Fundam. 1985,24,55-60. Stein, S. E.; Robaugh, D. A.; Alfieri, A. D.; Miller, R. E. Bond Homolysis in High-Temperature Fluids. J. Am. Chem. SOC.1982, 204, 6567-6570. Tagaya, H.; Katsuma, K.; Shibazaki, Y.; Chiba, K. Coal Liquefaction Using an Indene-Nondonor Mixture as Solvent. Fuel 1988, 67, 786-791. Tagaya, H.; Takahashi, K.; Hashimoto, K.; Chiba, K. Coal Liquefaction by Binary Solvent Systems Composed of Tetralin and Reducible Compounds. Energy Fuels 1989, 3, 345-350. Woodfine, B.; Steedman, W.; Kemp, W. Donor Solvent Interactions during Coal Liquefaction. Fuel 1989,68, 293-297.
Received for review February 12, 1990 Revised manuscript received October 22, 1990 Accepted November 20,1990
