Energy & Fuels 1987,1,193-198 thermolysis in the presence of 13C-labeledtetralin shows n-alkylnaphthalenes and n-alkyltetralins contain the 13C label and, therefore, are not an inherent part of the coal network that become soluble during thermolysis. In neither reaction do we mean to imply that all of these products are derived from fatty acid/solvent precursors. Calculations from solid-state 13CNMR data show that the amount of methylene (-CH,-) tied up in long chains is greater than that trapped in the coal as fatty acid. In addition, data from Ill. No. 6 coal, which contains no fatty acids, show small amounts of short chain (up to C-9) materials are present. We cannot exclude the possibility that part of the substituted naphthalenes and tetralins are bound to the coal polymer network or are trapped as noncross-linked components of the coal. We dd caution that it is possible to draw erroneous conclusions from the observation of these products without
193
recognition of their primary source in the reactions of coals. Acknowledgment. We thank the Division of Chemical Sciences/Office of Basic Energy Sciences, U.S.Department of Energy, who sponsored this research under Contract DE-AC05840R21400 with the Martin Marietta Energy Systems, Inc. Registry No. H3C(CH2)&02H, 57-11-4; H3C(CH2)&O2H, 334-48-5;H3C(CH2),50H,36653-82-4;C6H,CH3, 108-88-3; tetralin, 119-64-2.
Supplementary Material Available: Table 4, intensities of
P
+ 1 peaks in the mass spectra of substituted tetralins and
naphthalenes from liquefaction of Texas lignite; Figure 4, single ion chromatogram for the mjz 131 ion of oils from liquefaction of (a) Illinois No. 6 coal, (b) Illinois No. 6 coal mixed with capric and stearic acids, and (c) capric and stearic acids alone (2 pages). Ordering information is given on any current masthead page.
Hydrogen-Transfer-PromotedBond Scission Initiated by Coal Fragments Donald F. McMillen,* Ripudaman Malhotra, Georgina P. Hum,? and Sou-Jen Chang* Department of Chemical Kinetics, Chemical Physics Laboratory, SRI International, Menlo Park, California 94025 Received September 12, 1986. Revised Manuscript Received November 21, 1986
We report evidence to support a new mechanism for coal liquefaction in which strong linkages (i.e., nonthermolyzable linkages such as diarylmethanes, other alkylaromatics, and diary1 ethers) are cleaved a t 400 "C as a result of hydrogen transfer from solvent-derived cyclohexadienyl radicals in a direct bimolecular step (radical hydrogen transfer, RHT). Evidence has recently been presented for the operation of this pathway in model compounds. Addition of coal samples to these model compounds markedly accelerated the cleavage of the strong central bonds. Further, this cleavage exhibits the high selectivity typical of the RHT process, suggesting that radical hydrogen transfer, if anything, is more relevant to actual liquefaction than indicated by previous pure model compound studies. The relative liquefaction efficienciesof various donor solvents is shown to be much more readily rationalized by this mechanism than by the traditional bond.scission/radical-capping liquefaction mechanism.
Introduction It has been repeatedly observed' that at liquefaction temperatures coals are very effective promoters of hydrogen-exchange reactions between solvents and coal and among solvent structures themselves. This observation is consistent with the commonly held coal liquefaction mechanism: homolysis of weak linkages in the coal structure produces radicals that can abstract hydrogen (or deuterium) from solvent structures, producing solvent radicals that can,in turn, abstract hydrogen (or deuterium) from coal structures. Hydrogen exchange would then be a natural result of the spontaneous bond scission reactions generally considered to be responsible for coal liquefaction.
* To whom correspondence should be addressed.
Chemistry Laboratory, SRI International. *Postdoctoral Research Associate. Present address: Rohm and Haas Associates, Philadelphia, PA.
0887-0624/87/2501-0193$01.50/0
We wish to report recent results that suggest the opposite: coal liquefaction is a result of hydrogen-exchange reactions, and furthermore, a substantial portion of these transfers do not involve free hydrogen atoms. We have recently suggested2that a significant portion of the molecular weight decrease that occurs during donor-solvent coal liquefaction results from a previously undocumented bimolecular transfer of hydrogen from cyclohexadienyl-type radicals (of either coal or solvent origin) to the ipso positions of linkages to aromatic systems (1) See, for example: (a) King, H.-H.;Stock, L. M. Fuel 1982,61,257. (b) Heredy, L. A.; Fugassi, P. In Coal Science; Gould, R. F., Ed.; Advances in Chemistry 55, American Chemical Society: Washington, DC, 1966; p 448. (2) (a) McMillen, D. F.; Chang, S.-J.; Nigenda, S. E.; Malhotra, R., American Chemical Society, Division of Fuel Chemistry, Preprints; 1985, 30(4),414. (b) McMillen, D. F.; Malhotra, R.; Chang, S.-J.; Nigenda, S. E.; Ogier, W. C.; Fleming, R. H., submitted for publication in F u d .
0 1987 American Chemical Society
McMillen et al.
194 Energy & Fuels, Vol. I , No. 2,1987
in coal structures, resulting in cleavage: coal’
I
(1
coal
koal’
I
J
H
coal
The same results as in reaction 1 could be achieved by H-atom elimination, followed by addition of free H-atoms to the substrate. This sequential mode of reaction has been shown by Franz and co-workers3 to a t least be a contributor in the case of the 1-hydronaphthyl radical and has been suggested by Gavalas4 to be dominant in that system. However, the selectivity observed in the cleavage of various substrates has suggested that in model compound studies under conditions typical of coal liquefaction the bulk of H-transfer does not take place via “free” hydrogen atom; instead, it is effected by cyclohexadienyl radicals in a single bimolecular step, radical hydrogen transfer (RHT).* Given the unknown character of the critical links in coal structures and the inability to directly “monitor” their cleavage during liquefaction, demonstrations that a substantial part of the molecular fragmentation responsible for liquefaction is indeed brought about by the RHT process can only be made by inference. In this paper, we first use data from the literature to show the inadequacy of the traditional mechanism and then report our results to support the role of RHT in coal liquefaction. Experimental Procedure Experiments were generally performed a t 400 “C in stainless-steel tubing bomb reactors and heated in a molten-salt bath controlled to within il “C. The solvent/coal/dinaphthylmethane ratio was 4/1/0.15. Blank experiments showed that a t this level the methylnaphthalenes from dinaphthylmethane scission exceeded the amounts of these products produced from the coal by a factor of more than 20. Products were analyzed by gas chromatography (GC) and, when desirable, by gas chromatographymass spectrometry (GC/MS). In most cases, substrate conversion was 10-30% in 2 h a t 400 “C. Rates are generally reported as defined first-order rate constants. T h e values reported as first-order rate constants are not meant to imply strict adherence to overall first-order kinetics; they should be considered as initial rates measured under a specific set of conditions which were maintained constant except for the parameter that was purposely varied. T h e qualitative conclusions drawn here are based on differences large enough to be unaffected by this simplified kinetic treatment. All chemicals except 1,2’-dinaphthylmethane (API Standard Reference Materials) were purchased from Aldrich Chemical Co. 1,2’-Dinaphthylmethane was shown by GC analysis t o be >99% pure and was used without further purification. Distillation of tetralin on a spinning band column provided tetralin with 98% pure and was used without further purification.
Results and Discussion Inadequacy of the Traditional Mechanism. One of the chief difficulties with the traditional mechanism of coal (3) Franz, J.; Camaioni, D., presented at the 185th National Meeting of the American Chemical Society, Seattle, WA, March 14-18, 1983. (4) Allen, D. T.; Gavalas, G. R. Int. J . Chem. Kinet. 1983, 15, 219.
liquefaction is the lack of correlation between a solvent‘s liquefaction effectiveness and its ability to scavenge radicals. Thus, 9,10-dihydroanthracene, which is, by a significant margin, the most effective radical scavenger among the commonly tested hydroaromatic model ~ o l v e n t s ,is~ ? ~ not a similarly effective solvent for coal liq~efaction.~+’ This is illustrated by the data in Figure 1 in which coal conversion effectiveness (data of Curtis et al.9) is plotted against relative rates of scavenging of benzyl radical (data , of Bockrath et a15). 9,lO-Dihydroanthracene is slightly less effective in coal conversion than 9,lO-dihydrophenanthene even though the former scavenges benzyl radicals -10 times more rapidly (at 400 “C). The lack of correlation is even more striking when one considers coal conversion in purely aromatic solvents. Here, the solvent makes no net contribution of hjldrogen to the coal and the traditional mechanism requires that there be in situ conversion of Ar to ArH2,which then acts as a scavenger. In Figure 2, coal conversion from the work of Davies et al.s is plotted against the radical scavenging rate observed5 for the respective dihydroaromatic derivative of the aromatic compound. Under “shuttling” conditions such as these, the traditional mechanism dictates that solvent effectiveness depends on two factors: (1)ease of conversion of Ar to ArH2 and (2) effectiveness of ArH, as a scavenger. However, Figure 2 shows the percent coal conversion (to quinoline solubles) is only half as much in anthracene as in pyrene! Similar resultslO have been reported by other workers as early as Orchin and S t ~ r c h . ~ (The recent results of Curtis, Guin, and Kwongshow little difference in liquefaction effectiveness between phenanthrene and anthracene, but this study was with a coal of low fluidity. Mochida and co-workers have shown Hshuttling by polycyclic aromatic hydrocarbon to be much less effective with coals that develop very little fluidity upon being heated to liquefaction temperatures.ll) A difference of 35-7070 on the scale of coal “conversion” is a very large difference. Moreover, if anything, the actual difference is even more striking than it appears in Figure 2, since that figure is based on equimolar ArH, formation in all cases. However, anthracene is well-known to be more easily reduced to 9,lO-dihydroanthracene than is pyrene to 4,5-dihydropyrene. Thus, there is more in situ production of the dihydroanthracene than of the dihydropyrene. That is, the most easily produced dihydro compound is also the most effective radical scavenger, but is not even close to being the most effective liquefaction agent. The ineffectiveness of anthracene is not easily attributed to its coupling reactions, since products of such retrogressive reactions are knownlb to be related primarily to the soluble fractions. Furthermore, in exploring the ineffectiveness of anthracene, Heredy and Fugassilb found substantially less tritium loss from ditritioanthracene than from ditritiophenanthrene. That is, the greater conversion in phenanthrene is associated with more H-transfer rather than with less coupling reactions. Bond Cleavage by RHT In contrast, with the conventional thermolysis/radical-cappingmechanism, a coal liquefaction mechanism incorporating RHT (reaction 1) (5) Bockrath, B. C.; Rittner, E.; McGrew, J. J. Am. Chem. SOC.1984, 106, 135. (6) Franz, J.; Camaioni, D., presented at the 185th National Meeting of the American Chemical Society, Seattle, WN, March 14-18 1983. (7) Orchin, M.; Columbic, C.; Anderson, J. E.; Storch, H. H. US. Bureau of Mines, Bulletin No. 505, 1951. (8) Davies, G. 0.;Derbyshire, F. J.; Price, R. J. J.Inst. Fuel 1977,50, 121. (9) Curtis, C. W.; Guin, J. A,; Kwon, K. C. Fuel 1984, 63, 1404. (10)McMillen, D. F., unpublished results. (11) Mochida, I.; Takanabe, A,; Takeshita, K. Fuel 1979,58, 17.
Bond Scission Initiated by Coal Fragments
Energy &Fuels, Vol. 1, No. 2, 1987 195 loo,
is capable of accommodating the fact that 9,lO-dihydroanthracene is actually of similar effectiveness to 9,lO-dihydrophenanthrene or 4,5-dihydropyrene2and that anthracene is much less effective than pyrene or phenanthrene. The RHT mechanism for coal liquefaction in hydroaromatic solvents (nonshuttling circumstance) is shown in reactions 2 through 5. Consideration of net
,
I
I
I
2 4 6 B MeN RELATIVE RATE OF H-ABSTRACTION B Y BENZYL RADICAL AT 400°C
10
I
I
I
I
I
initiation/ termination H
H
H
H H ArH2
H
H
0
Figure 1. Conversion of coal in benzylic and hydroaromatic radical scavengers. At equal levels of available hydrogen in methylnaphthalene solution, 400 O C ; data of Curtis et al? Relative abstraction rates data of Bockrath et al.5
A
H Ar
ArH*
(2) propagation H
H
coal'
H
I
I
H
H Ar
ArH*
0
2 4 6 8 RELATIVE RATE OF H-ABSTRACTION FROM THE RESPECTIVE HYDROAROMATIC BY BENZYL RADICAL AT 400°C
10
Figure 2. Conversion of coal in aromatic solvents. A t 400 OC, data of Davies et al.8 Relative abstraction rates data of Bockrath e t aL5 (3b)
1
fast
H
h
coal
+
coal'. (3b)
by net reaction 5, the principal noncancelling factors that promote the net reaction are (i) decreasing stability of ArHz and (ii) increasing stability of Ar. An expression for the estimated relative cleavage rate of a given substrate, e&,
1
coal'
propagation H
coal'.
t
H
@2 H
t
coal'-H
in a series of ArH, solvents can be derived by the steady-state analysis shown in the Appendix. For this analysis, we assume [ArH'],, to be controlled by a preequilibrium (reaction l),and the change in the intrinsic activation energy for the RHT step to be a fixed fraction a of the change in the endothermicity of the step. The derivation gives
H H
H
net reaction H
H
kclv a
coa I '
H
([
+
1 1
;AHfo(ArH,-Ar) aAHfo(Ar-ArH*) /RT n where n = 2 or 4 for dihydro- or tetrahydroaromatics, respectively. The relative values of estimated Adv for some commonly used solvents are shown in Table I. While the exp
+ H H
b
t coal'-H
(5)
coal
reaction 5 for this chain sequence suggests (and steadystate analysis bears out2) that the stability of the intermediate radical ArH' largely cancels out: increasing stability of ArH' results in a higher steady-state concentration of ArH', but in a slower rate for reaction 3. As suggested
(12) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic: New York, 1970. (13)Stull, D. R.; Westrum, Jr., E. F.; Sinke, G. C. T h e Chemical Thermodynamics of Organic Compounds; Wiley: New York, 1969. (14) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley: New
York, 1976. (15) Shaw, R.;Golden, D. M.; Benson, S. W. J.Phys. Chem. 1977,81, 1716. (16) McMillen, D.F.,Trevor, P. L.; Golden, D. M. J. A m . Chem. SOC. 1980, 102, 7400. (17) Stein, E. Ado. Chem. Ser. 1982, No. 192, 97. (18) Herndon, W. C.J. Org. Chem. 1981,46, 2119.
McMillen et al.
196 Energy &Fuels, Vol. 1, No. 2, 1987
Table I. Estimated and Observed Rates of Solvent-Mediated Hydrogentilysis of Dinaphthylmethane, Coal Conversion, and Radical Scavenging obsd re1 obsd re1 Afff"298, figure of radical DNM kcal/mol" merit, Q obsd coal scavenging cleavage - . ArH, ArH, Ar a t 400 'Cbtc conversion, ratee ratef1,2,3,4-tetrahydronaphthalene 6.1 36.1 1.0 58 1.0 1.0 55.2 5.8 70 10 7.5 9,lO-dihydroanthracene 38.2 85 1.3 5.0 9,lO-dihydrophenanthrene 37.1 49.5 5.2 80-85 2.3 9.2 53.9 36 4,5-dihydropyrene (44.2)
OThermochemical values based on measurements given in, or estimates made on the basis of, ref 12-19. b Q = exp[[(l/n)AHf"(ArH,-Ar) in the present case, (Y = 0.25 is used. This value is in line with values generally ranging between 0.3 and 0.5 for 0 to 10 kcal/mol exothermic H-abstractions (metathesis) reactions (see ref 5 for an example). CCalculatedfor 50/50 ArH,/Ar. dFrom ref 9, T H F solubilities resulting from conversions in ArH, a t 400 "C. eFrom ref 5, relative rates adjusted from measurement T (170 "C) to 400 "C. fThis work, from integrated first-order rate constants obtained in -50/50 ArHJAr a t 400 OC.
+ cuAfff"(Ar-ArH')]/RT];
predicted relative rates in column 4 do not exactly parallel the relative extents of coal conversion in column 5,9they are far closer than the relative rates measured for radical scavenging (column 6)5 and match rather closely the measured hydrogenolysis rates for dinaphthylmethane in the respective solvent systems (column 7). Steinz1has recently -reported results illustrating conditions where H-transfer to the critical linkage by reverse radical disproportionation, shown as the initiation step (reaction 2), leads directly to bond cleavage. Although this situation holds for very weakly bonded ArH, and when the ArH2/Ar ratio is high, such transfer does not appear to be responsible for the bulk of bond cleavage reactions during coal liquefaction. If it were, then, 9,10-dihydroanthracene, being far and away the most weakly bonded ArH, commonly used, would be the best liquefaction solvent. As discussed above, it clearly is not. The detailed kinetics of reactions 2 through 5 aside, we speculated that if bond scission does result from hydrogen transfer as in reaction 1, then there could be a parallelism between hydrogen scrambling during liquefaction and scission of Ar-X bonds. If coals promote hydrogen exchange in model compounds, they could also promote scission of suitably structured models and also of the coal structures themselves. We have previously used 1,2'-dinaphthylmethane as a surrogate for strongly bonded coal structures not subject to thermal cleavage (half-life for thermolysis at 400 "C is 1108h), but which visibly (albeit slowly) undergo cleavage resulting from H-atom transfer in donor solvents at 400 OC.z In order to determine the reactivity of strongly bonded structures, such as diarylmethanes, under conditions closer to actual coal liquefaction, we conducted a series of "hybrid" experiments in which model compounds were added to coal liquefaction experiments. The results in Table I1 show that this suspected correlation between ability to promote H-exchange and promotion of bond scission is indeed observed. The addition of two different bituminous coals to the tetralin/dinaphthylmethane system increases the rate of dinaphthylmethane cleavage by factors of about 40, decreasing the half-life for scission at 400 "C from 240 h to about 6 h. This moves dinaphthylmethane (or any alkylnaphthalenestructure) from the apparent position of a linkage that is refractory under coal liquefaction conditions to one whose scission may be of real importance in donor solvent liquefaction. Similarly, (19) McMillen, D. F.; Golden, D. M. Annu. Reu. Phys. Chem. 1982,33, 497. (20) McMillen, D. F., Fleming, R. H.; Laine, R. M.; Malhotra, R.; Nigenda, S. E.; Ogier, W. C. 'Effects of Amine Solvents and Oxygen Functionalities on Coal Liquefaction"; Interim report, Electric Power Research Institute Project 2147-5, Report No. AP-4176, Aug. 1985, p 5-9. (21) Billmers, R. L.; Griffiths, L. L.; Stein, S. E. American Chemical Society, Division of Fuel Chemistry, Preprints; 1985, 30(4) 283.
Table 11. Promotion of Alkyl-Aryl Bond Scission Rates in 1,2'-Dinaphthylmethane and p -Benzylphenol by Bituminous Coals at 400 O C extrapolated cleavage cleavage 2-Me-N/ rateb half-life, 1-Me-N coal added" solvent (X105) h' ratio 1,2'-Dinaphthylmethane none tetralin 0.08 240 2.5 pyrene
