A mechanistic numerical model for coal liquefaction involving

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Energy & Fuels 1990,4, 184-193

A Mechanistic Numerical Model for Coal Liquefaction Involving Hydrogenolysis of Strong Bonds. Rationalization of Interactive Effects of Solvent Aromaticity and Hydrogen Pressure Ripudaman Malhotra* and Donald F. McMillen Department of Chemical Kinetics, SRI International, Menlo Park, California 94025-3493 Received September 18, 1989. Revised Manuscript Received December 28, 1989

Experimental results have recently shown that bond scission of coal structures under liquefaction conditions is not limited to spontaneous thermal scission and that hydrogen transfer from a solvent or coal radical (radical hydrogen transfer, or RHT) must now be added to the H-transfer processes that can bring about hydrogenolysis of strong bonds. In this paper, we use thermochemical considerations to help delineate the factors that control this competition between (1)RHT, (2) a sequence of elimination of a free H atom followed by addition of the free H atom, and (3) the reverse of radical disproportionation (RRD). These various reactions have been included in a mechanistic numerical model for describing the cleavage of strong (i.e., nonthermolyzable) bonds using dinaphthylmethane as the substrate. The rate parameters used in the model were chosen either from the literature or by analogy with related structures in strict accord with thermochemical principles. The model successfully mimicked experimentally observed rates and selectivities for the cleavage of 1,2'-dinaphthylmethane (DNM) in anthracene/dihydroanthraceneand phenanthrene/dihydrophenanthrene solvent systems. The model shows that, a t constant donor level, addition of the nondonor results is only a minor change in the overall rate of DNM cleavage but that the contributions to the total cleavage resulting from free H atoms or RHT change markedly, RHT being more important a t higher Ar concentrations. The changes in the relative importance of the dominant H-transfer pathways profoundly impact the selectivity of cleavage and the efficiency with which the hydrogen is utilized for the cleavage of linkages. The model also provides explanations for several previously reported curious observations regarding the impact of solvent structure, hydrogen pressure, and the presence of methylnaphthalene on the conversion of coals to THF solubles. The greater beneficial effect of H2 pressure in anthracene systems appears to result from the greater concentration of the 9-AnH' radicals, notwithstanding their lower reactivity in abstracting H atoms from H2 vis-84s the 9-PhenH' radicals. In the anthracene system, methylnaphthalene serves as an additional initiator by RRD with anthracene, which is a good hydrogen acceptor. In phenanthrene, which is a poor hydrogen acceptor, the chief effect of added methylnaphthalene is to scavenge the H atoms, resulting in a net decrease in coal conversion.

Introduction In the conventional view of liquefaction, inherently weak bonds in coal undergo thermolysis upon heating, and the solvent merely serves to stabilize these thermally generated radicals. The solvent is not involved in engendering bond scission. We have previously shown' that this mechanism is not consistent with the observed order of liquefaction abilities of various solvents. This inconsistency has also been recognized by some other researcher^.^,^ We have propo~ed'*~*~ an alternative model for bond scission in coal liquefaction whereby solvents mediate hydrogenolysis of strong bonds with varying degrees of effectiveness. These (1) McMillen, D. F.; Malhotra, R.; Chang, S.-J.; Hum, G. P. Energy Fuels 1987. 1 . 193. (2) See, for example: (a) Finseth, D. H.; Bockrath, B. C.; Cillo, D. L.; Illig, E. G.; Sprecher, R. F.; Retcofsky, H. L.; Lett, R. G. Prepr. Pap.A m . Chem. Soc., Diu. Fuel Chem. 1983 28(5), 17. (b) Brower, K. R.; Pajak, J. J. Org. Chem. 1984, 49, 3970. (3) !a) Allen, D. T.; Gavalas, G. R. Fuel 1984, 63, 586. (b) Noor, N. S.; Games, A. F.; Abbot, J. M. Fuel 1985, 64, 1274. (4) McMillen, D. F.; Ogier, W. C.; Chang, S.-J.; Fleming, R. H.; Malhotra, R. Proceedings of the 1983 International Conference on Coal Science, Pittsburgh, PA, 15-19 August 1983 International Energy Agency: 1983; p 199. ( 5 ) McMillen, D. F.; Malhotra, R.; Chang, S.-J.;Nigenda, S. E.; Ogier, W. C.; Fleming, R. H. Fuel 1987, 66, 1611.

0887-0624/90/2504-0184$02.50/0

two views of coal liquefaction are depicted in Scheme I. We do not mean to imply that weak bonds do not undergo thermolysis during coal liquefaction. To the extent that there are weak bonds in the coal structure, they will undergo thermolysis; however, the critical factor that determines the efficacy of various solvent systems is evidently their ability in bringing about H transfers to cause cleavage of strong bonds. There are several pathways by which solvent systems can transfer hydrogen atoms to critical linkages: the system could generate free H atoms that add to the ipso positions, dihydroaromatics could transfer H atoms in a reverse radical-disproportionation reaction (RRD), or the recently documentedk8 radical hydrogen-transfer (RHT) reaction, in which solvent-derived radical species transfer H atoms to closed-shell species in a bimolecular step, could occur. Indeed, the thermochemistry of polycyclic aromatic hydrocarbons (PCAH) is such that the ArH' radicals will be present in a much greater (?lo5 times) abundance than (6) McMillen, D. F.; Chang, S.-J.;Nigenda, S. E.; Malhotra, R. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1985, 30(4), 297. (7) Billmers, R.; Griffith, L. L.; Stein, S. E. J.Phys. Chem. 1986, 90,

383.

(8) Metzger, J. 0. Angew. Chem., Int. Ed. Engl. 1986, 25, 80.

0 1990 American Chemical Society

Mechanistic Numerical Model for Coal Liquefaction

Energy & Fuels, Vol. 4, No. 2, 1990 185

Scheme I. Schematic Showing Conventional and Solvent-Mediated Hydrogenolysis Models of Coal Liquefaction

so

CONVENTIONAL MECHANISM: 50

Solvent

40

I pf

Solvent merely stabilizesthermally generated radlcals Is not involved in inducing bond cleavage

30

SOLVENT.MEDIATED HYDROGENOLYSIS:

Solvent

CHZ H

I

i(Ar+Md+NX

i H

CH3

Solvent engenders bond scissmn

free H atoms. Of course, free H atoms are more reactive than ArH' radicals and one must consider both concentration and reactivity in determining their relative importance. Competition between cleavage resulting from RHT and that resulting from free hydrogen atoms or direct RRD depends on the reaction conditions and determines the efficiency of hydrogen utilization and product distribut i o n . ' ~ ~ This * ~ competition has been addressed experimentally in the last several years in this and other laboratories (primarily those of Stein' and Bockrathg). These studies have shown that (1)direct bimolecular transfer of hydrogen from a radical to an olefin or an aromatic, a reaction that had previously been without precedent in the chemical literature, does occur and can be competitivewith other hydrogen-transfer processes; and (2) RRD functions in some cases merely as a bimolecular radical initiation step and in some cases as a stoichiometric hydrogen-transfer step. With this work as background, our purpose here is to address the questions of the occurrence and the significance of such hydrogen-transfer reactions in coal conversion. To gain a better understanding of the competition between the various modes of H transfer, we have constructed a numerical model in which we have used cleavage of dinaphthylmethane as a surrogate for the cleavage of strong bonds during coal liquefaction. In this paper we describe this model in some detail, along with the relevant thermochemical kinetic background. We then compare the trends predicted by the model with some recent results from the literature on coal liquefaction. In particular, we show how some of the curious observations reported by Kwon'O on the effect of 1-methylnaphthaleneand hydrogen are successfully mimicked by the model. In addition, the model helps us to see the chemical reasons behind the observed effects. We wish to emphasize at the outset that this model is mechanistic, not merely parameter fitting, and that the (9) Bockrath, B. C.; Schroeder, K. T.; Smith, M. R. Prepr. Pap-Am. Chem. SOC.,Diu. Fuel Chem. 1988, 33(3), 325. (10) Kwon, K. C. Fuel 1985, 64, 747.

\_I

RRD

W+NX

RD GENERALIZED REACTION COORDINATE

Figure 1. Potential energy diagram for radical generation in various aromatic systems. (a) Difference between barrier for H elimination from 9-PhenH' and that for RHT to NX. (b) Difference between AHo for 9-AnH' and 9-PhenH' formation. (c) Difference between AHo* for ArH' formation and H transfer to NX in the phenanthrene and anthracene systems.

goal is not to predict liquefaction rates exactly but to help provide a general understanding of (and the ability to extrapolate) the trends that are observed experimentally and thereby to serve as a guide for further experimentation. Since the model is mechanistic, it is very detailed, but it is necessarily applied to a limited set of structures and reactions. At present it includes no provision for heteroatom removal, retrograde reactions, weak bond scission, or ionic reactions, nor does it have any provision for mass transport and swelling of coals. Since these factors undoubtedly contribute in varying degrees, the ability of the simplified model to explain liquefaction phenomena provides a rather severe test of the hypothesis that hydrogen-transfer-induced scission of strong bonds is a major factor in coal liquefaction.

Background The thermochemical requirement^^,',"-'^ for radical generation and hydrogen transfer by RRD and by RHT are shown in Figure 1, for a linkage to the 1-position of naphthalene in the phenanthrene and anthracene solvent systems. The position on the ordinate reflects the stoichiometry shown along the abscissa, that is, that for generation and consumption of one H atom or one H-atom carrier radical. Therefore the indicated AHo for the first step (RRD) is only half of that for the reaction of one ArHz molecule and one Ar molecule. This figure was drawn (11) McMillen, D. F.; Golden, D. M. Hydrocarbon Bond Dissociation Energies. Annu. Reu. Phys. Chem. 1982,33, 497. (12) (a) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic Press: New York, 1970. (b) Shaw, R.; Golden, D. M.; Benson, S. W. J.Phys. Chem. 1977, 81, 1716. (13) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley: New York, 1976. (14) Stein, S. E. A Fundamental Chemical Kinetics Approach to Coal Conversion. In Advances in Chemistry Series: American Chemical Society: Washington, DC, 1981; No. 169, p 97. (15) Herndon, W. C. J. Org. Chem. 1981, 46, 2119.

186 Energy & Fuels, Vol. 4 , No. 2, 1990

using 25.5 and 19.2 kcal/mol for the activation barriers for the RHT steps from 9-hydroanthryl and 9-hydrophenanthryl radicals, respectively. These values are arrived at by using the regimen discussed below. From Figure 1,we see that the overall enthalpy change for radical generation by RRD and reaction by RHT is more favorable in the phenanthrene system than in the anthracene system because phenanthrene is about 6 kcal/mol more stable than anthracene, while dihydrophenanthrene (PhenH,) is only slightly more stable (1 kcal/mol) than dihydroanthracene (AnHJ. By examining Figure 1, we can draw some semiquantitative conclusions regarding the competition between the various H-transfer processes. For the purpose of this exercise, when comparing the thermochemistry for cleavage by RHT and free hydrogen atoms, we will consider the phenanthrene system, because hydrogen atoms are more dominant in this solvent system. Likewise, when comparing the thermochemistry for cleavage by RHT and RRD, we will consider the anthracene system, because 9,lO-dihydroanthracene is a better H donor than 9,lOdihydrophenanthrene.11-15 Competition between RHT and Free H-Atom Addition. Since the elimination of X' from the intermediate NXH' will generally be far faster than any other reaction of that species, the rate-determining step in the hydrogen-transfer-induced bond scission is the formation of the intermediate. The enthalpy of activation for the formation of NXH' by RHT is represented by the height of the highest barrier along that route. This height will not be exactly proportional to the overall AH",because the intrinsic activation energy for the RHT step itself is expected to shift with the degree of endothermicity of that step. The curves drawn in Figure 1reflect the expectation that the more endothermic (or exothermic) the RHT step, the lower the intrinsic barrier (i.e., the activation energy in the exothermic direction). In other words, the more endothermic the RHT is, the closer the structure (and energy) of the transition state to that of the products is. [Put yet another way, with a more endothermic RHT, the intrinsic activation energy (E, in the exothermic direction) for the RHT step will be lower.] This trend, which evidently makes RHT in the anthracene system (solid line) more favorable than would be expected from a simple comparison of the overall AH" values,16 can be expressed in terms of the Evans-Polanyi factor, the fraction (of any increase in RHT endothermicity) by which the intrinsic activation energy is lowered. Rough expectations from the literature for other hydrogen-transfer reactions are that the Evans-Polanyi factor will amount to 0.2-0.4 of the increase in RHT endothermicity.17J8 From Figure 1we can see that the existence of the RHT process is reasonable. For the alternative reaction, unimolecular H elimination, even in the case where elimina(16) The overall AHo values for the reaction

(+)A.

1 + ;(ArH,) + NX

-

Ar

+ NX-H'

have been used previously by us1 as a figure of merit for solvent-mediated hydrogenolysis. As discussed here, this figure of merit takes into account only thermodynamic factors and not kinetic factors. It therefore gives the right order of hydrogenolysis activity in cases where kinetic factors do not bring about a crossover such as that seen for the anthracene and phenanthracene system. (17) Bockrath, D. C.;Bittner, E.; McCrew, J. J.Am. Chem. SOC. 1984, 106, 135. (18) (a) Kerr, J. A.; Moss, S. J. CRC Handbook of Bimolecular and Termolecular Gas Reactions; CRC Press: Boca Raton, FL,1981; Vol. 1. (b) Agmon, N. Int. J. Chem. Kinet. 1981, 13, 333.

Malhotra and McMillen

tion is most favorable (phenanthrene), the barrier lies about 35 kcal/mol above the PhenH' radical. That means that an addition-elimination process for H transfer to a substituted naphthalene system involves overcoming a 35 kcal/mol barrier, only to fall down in the subsequent addition to the naphthalene nucleus, into another potential energy well that is just about as deep, -30 kcal/mol. Thus, an alternative involving a direct bimolecular transfer in which the full cost of producing a "free" H atom never has to be paid is eminently resonable; some benefit would be gained from the bond that is simultaneously being formed with the acceptor K system. This expectation has now been shown to be not only reasonable but ~ o r r e c t . ' ~ ~ ~ ~ ~ ~ Even in the case of phenanthrene, the barrier for RHT is roughly 16 kcal/mol lower than that for H elimination. A difference of 16 kcal/mol in activation energy translates into a rate factor of lo5.,at 400 "C. However, the A factor for the unimolecular H elimination is about 5 orders of magnitude greater than that for the bimolecular RHT, so the two processes are competitive. As discussed below, modeling shows that both processes make significant contributions to total cleavage and that a crossover in their relative importance is predicted with minor changes in reaction conditions. Regarding the competition between RHT and direct RRD from ArH,, the latter can happen if the cleavable substrate is a better (or more abundant and accessible) acceptor than the solvent PCAH (Ar). However, good solvents tend to be at least moderately good acceptors, and their concentration is usually substantial; hence, the conditions for substantial bond cleavage via direct RRD are generally not fulfilled in coal liquefaction. More specifically, Figure 1reflects the fact that, under conditions where there is a preequilibrium established between Ar, ArH,, and ArH' (by RRD and RD), two ArH' radicals are produced in each RRD step. This means that the enthalpy cost of a single carrier agent ArH' is only ' / , A H 0 for the RRD step. Thus, reaction via an RHT reaction of ArH' will tend to be preferred to the extent that the substrate is a poorer H-atom acceptor than the solvent and to the extent that l/zAHofor RRD is greater than the activation energy for the RHT step. The intrinsic activation energy for RHT in the thermoneutral case has been determined to be about 16 kcal/m01'~~~' and l/*AH" for RRD is about 17 kcal/mol in the anthracene system and larger in essentially all other Thus, we see that, with a 16 kcal/mol activation energy for thermoneutral transfer from AnH' &e., to a linkage on the 9-position of anthracene), transfer by RHT would be roughly competitive with direct transfer via RRD and would depend on the [An]/ [An-XI ratio.lg For all other conditions (nonthermoneutral RHT), the advantage to RHT should increase. In the specific case of coal liquefaction, significant bond cleavage via RRD from the hydroaromatic can be ruled out because H transfer via RRD would be approximately 100 times faster (at 400 "C) with 9,lO-dihydroanthracene than with 9,lO-dihydrophenanthrene. However, dihydrophenanthrene is typically equal to or better than dihydroanthracene as a liquefaction solvent. (19) Stein and co-workers have reported' that the kinetics of 2ethylanthracene reduction by 9,10-dihydroanthracene are consistent with RRD as the stoichiometric, rate-limiting step (except when the concentration of anthracene substantially exceeds that of 2-ethylanthracene). This reduction is an example of the limiting case referred to, in which the substrate is itself the best H-atom acceptor in the system and is present in higher concentrations than the solvent acceptor species. If these authors had monitored instead the (slower) cleavage of the 2-ethyl linkage (in which the 2-position of anthracene functions as a much poorer acceptor than the 9-position), they presumably would have found the cleavage-inducing transfer to occur principally by the RHT process.

Mechanistic Numerical Model for Coal Liquefaction

Energy & Fuels, Vol. 4, No. 2, 1990 187

Table I. Reaction Sequence for Dinaphthylmethane Bond Cleavage in Anthracene/DihydroanthraceneMixtures reactants products A factor,” M-’s-l activation energy: K reaction typeb

reaction no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

A13H, A0 A9, A9 MN, A0 MN-, A9 DNM, A0 DNM-, A9 A9, DNM H, DNM A9, MN MN, H H, AlOH H2, A9 DNM-, AlOH DNM, A9 A9 AO, H AlOH, DNM AlOH, DNM DNMH, A9 H, DNM DNMH H2, MNDNMH, AlOH DNM4, A9 DNM, A9 DNMH, A0 DNMH, DNM MN-, AlOH A9, MN CH3, AlOH DNMH, MN MN-, DNM4 MN2 MN-, MNH, MN HDNM

A9, A9 AlOH, A0 A9, MNMN, A0 DNM-, A9 DNM, A0 HDNM, A0 HDNM HNM, A0 MN-, H2 H2, A9 H, AlOH DNM, A9 DNM-A, 10H AO, H A9 A9, HDNM A9, DNMH DNM, AlOH DNMH DNM, H H, MN DNM4, A9 DNMH, AlOH DNMH, A0 DNM, A9 HDNM, DNM MN, A9 MN-, AlOH CH4, A9 DNM4, MNDNMH, MN MN-, MNMN2 N, CH3 MN-, N

0.3483+10 0.2643+10 0.261E+10 0.132E+10 0.174E+10 0.132E+10 0.140E+09 0.400E+ll 0.140E+09 0.600E+ 11 0.108E+12 0.5423+11 0.400E+09 0.200E+09 0.900E+14 0.400E+ 11 0.218E+10 0.104E+ 11 0.132E+10 0.2203+12 0.900E+14 0.5423+11 0.400E+09 0.400E+09 0.112E+10 0.2803+09 0.140E+09 0.400E+09 0.300E+09 0.400E+09 0.300E+09 0.400E+09 0.300E+16 0.300E+10 0.300E+ 15 0.300E+15

0.1873+05 0.1463+04 0.2213+05 0.6953+03 0.200E+05 0.116E+04 0.1283+05 0.1593+04 0.1283+05 0.2253+04 0.3333+04 0.1673+05 0.7473+04 0.9133+04 0.2203+05 0.8073+03 0.243E+05 0.2433+05 0.1853+03 0.1593+04 0.1583+05 0.1353+05 0.7823+04 0.8483+04 0.1283+05 0.5873+04 0.8303+04 0.6593+04 0.108E+05 0.3433+04 0.103E+05 0.6823+04 0.2873+05 0.000E+00 0.101E+05 0.101E+05

RRD RD RRD RD RRD RD RHT H-ADDN RHT H-ABST H-ABST H-ABST H-ABST H-ABST H-ELIM H-ADDN RRD RRD RD H-ADDN H-ELIM H-ABST H-ABST H-ABST RHT RHT RHT H-ABST H-ABST H-ABST H-ABST H-ABST THERMOL RECOMB R-ELIM R-ELIM

In this table, 0.3483+10 represents 0.348 X 1O1O, etc. RRD, reverse radical disproportionation; RD, radical disproportionation; RHT, radical hydrogen transfer; H-ADDN, hydrogen atom addition to an aromatic system; H-ABST, hydrogen abstraction by a radical or H atom; H-ELIM, j3-scission cleavage of ipso intermediates; THERMOL, thermolysis of an inherently weak linkage; RECOMB, radical recombination. (I

Figure 1also illustrates a “compensation” effect operating between ArH’ concentration and reactivity. In the case of anthracene, RHT proceeds through a high concentration of relatively unreactive radicals, whereas in the case of phenanthrene, RHT proceeds through a much smaller (about 200 times) concentration of more highly active radicals. The compensation between concentration and reactivity would be exact, were it not for the decreasing intrinsic activation energy that accompanies increasingly endothermic RHT. Other factors being equal, a larger number of less reactive radicals is more effective. Thus, by considering the thermochemistry illustrated in Figure 1, we can appreciate (1)the reasonableness of hydrogen transfer by RHT, (2) the partial “compensation” inherent in reaction via larger numbers of less reactive carriers or via smaller numbers of more reactive carriers, and (3) the favoring of RHT (relative to H elimination followed by addition of free H atoms) as the RHT step itself becomes more endothermic. Description of the Numerical Model. Having outlined the thermochemical factors controlling RHT, we used a mechanistic numerical model to quantitatively assess the interaction of these factors. Our goal in numerical modeling was first to use a chemically detailed picture to describe experimentally simplified model systems and then to use such a system as a surrogate for coal liquefaction. To the extent that such a simplified model of coal liquefaction (with absolutely no weak-bond thermolysis or heteroatom elimination) successfully mimics otherwise inexplicable coal liquefaction phenomenology, it would

provide strong support for the importance of hydrogentransfer-induced bond scission as a contributing process in coal liquefaction. The mechanistic numerical model we have constructed describes the cleavage of dinaphthylmethane (DNM) in Ar/ArH, systems. It explicitly includes radical intermediates, and forward and reverse reactions are written for each fundamental chemical step. The sequence of differential equations thus provided is then incorporated into a program that uses a numerical integration routine based on the Gear algorithm and is operated on a VAX 11/750 computer. The model addresses the competition between the four types of hydrogen-trnsfer reactions most likely to bring about the formation of cyclohexadienyl radical intermediates in the aromatic portions of the coal structures (reactions 7,8,17, and 27, where the number sequence corresponds to that in Table I). These reactions are shown in Scheme 11, together with the estimated enthalpy change for each transfer mode in the specific case of cleavage of DNM in anthraceneldihydroanthracene. The ipso-substituted intermediates, once formed, are known to undergo (at 400 “ C ) rapid and essentially irreversible loss of any alkyl linkage at the position to which hydrogen has been transferred. Table I shows the reactions and rate parameters included in the model that describe the reaction of dinaphthylmethane in starting mixtures consisting of anthracene, dihydroanthracene, and an inert diluent, biphenyl. The reactants and products are listed by the

Malhotra and McMillen

188 Energy & Fuels, Vol. 4, No. 2, 1990 Table 11. Structures Corresponding to the Identifiers Used in Table I

Scheme I1 -Hvdiwen-Transfer

\

A0

(kcallmol)

Reanion Number

+13

(7:

AlOH A9

N HDNM

MN

u u

m

w

MN-

MN2

(or isomers)

RT

DNM

DNM-

DNMH

DNM4

H, H2 CHa, CHI

Table 111. Rate Parameter Regimen for Reaction Types in Table I" A factor: activation energy,b reaction tvue M-1 s-l kcal/mol RRD 109.5(RPD/8)c E = (50 - AH0)0.18 + AHo RD 1Og,'(RPD/4) E = (50 + AH0)0.18e RHT 108.'(RPD/2) endothermic: E = AH' + (16.5 - 0.35AH') exothermic: E = 16.5 0.35(-AH0) H abstraction 1010,4(RPD) exothermic: E = 9.2 + (16.2 + (by H') AH0)0.25 H abstraction 108,5(RPD/3) endothermic: E = AHo + (16.0 - 0.35AH') (by R') exothermic: E = 16.0 0.35(-AHo) H' addition 1010,3(RPD) E = 2 + (40 + AH0)0.113 H' elimination 1013.9(RPD/2) E = AH" + 2(40 - AH0)0.113 -

(@

9 H', H2 CHB', CH4

identifiers used in the computer program. The structures associated with the identifiers in Table I are given in Table 11. In Table I, the A factors are given in units of M-'/s-' and the activation energies in kelvin (energy units/R). The last column indicates the type of reaction. In brief, the sequence of reactions is as follows. Initiation takes place by RRD between AnH2 and An or DNM. In the latter case, cleavage of DNM ensues directly if the

H abstraction (by CH,')

108.5(RPD/3)

R' elimination thermolysis

1014.0 1015.5(RPD)

endothermic: E = AH' (17.1 - AH0)0.25 exothermic: E = 9.4 (17.1 + AH0)0.25 E = 20 E = AH'

+ 9.4 +

+

Regimen is derived from data and correlations in refs 1 , 5 , 7, 9, 11-15, 17, 18 and 20. bBased on measured and estimated AH' values a t 298 K. In most cases adjustments to 400 "C will be less than the uncertainty of the estimates, and in many cases these adjustments are not expected to alter the trends reflected in the model. RPD = reaction path degeneracy. For AH' 1 50 kcal/ mol, E R R D = AH'RRD. eFor AH' I50 kcal/mol, ERD = 0.

initial transfer was to the ipso position. In the former case, two AnH' radicals are produced, which then undergo a variety of reactions including RHT to DNM, unimolecular loss of H atoms, and disproportionation. Once formed the H atoms either abstract hydrogen from benzylic positions to produce H2 or add to the aromatic systems. When H transfer occurs to the non-ipso positions of DNM, then of course no C-C bond cleavage can take place. These nonipso radicals can either lose that hydrogen back to the system (by H elimination or RHT, reactions 21 and 26) or acquire a second hydrogen, generally by H abstraction from AnH2. As will be seen, the efficiency of H utilization in the system is largely determined by which of these two reactions the non-ipso radicals undergo. The reactions in Table I are limited to those accounting for scission of the aryl-methylene and aryl-methyl linkages, reduction of the substrate (DNM DNMHJ in a simplified manner, and dehydrogenation of the dihydroaromatic solvent system; nevertheless, as many as 36 fundamental reactions are required. To minimize the chances that the availability of 72 parameters ( A factors

-

Mechanistic Numerical Model for Coal Liquefaction

Energy & Fuels, Vol. 4 , No. 2, 1990 189 a 1

[

.=.

-6.0

----I .

L

.

.

. 0.0

-

Total AnH.

f

-

,

3

DNMH-

1.0

1.5

2.0

2.5

[ANTHRACENE],M

, . ,

PhenH DNMH?

t;

OH. A AnH2

0.5

-8.01,

Total . 0o ,HPhenH. ., , ,

#

-9.0 0.0

- -

-

0.5

1.0

,

.,.., .,

1.5

,

,

,

,

]

2.0

2.5

[PHENAMHRENE], M

Figure 2. Computed effects of concentration of PCAH acceptors on the cleavage of dinaphthylmethanedue to different H-transfer agents at 400 "C. Base systems: 9,lO-dihydrophenanthreneor 9,10-dihydroanthracene,0.70 M; dinaphthylmethane,0.30 M. and activation energies) would reduce this modeling to a Table IV. Heats of Formation for Species Used in the parameter-fitting exercise, we have adhered to a fairly Numerical Model strict regimen in assigning these parameters. AHf0298,b AHf02981b Instead of discussing each reaction and the parameters speciesa kcal/mol suecies' kcal/mol used for it individually, we summarize in Table I11 the AlOH 38.2 DNM65.0 regimen used to choose the rate parameters of each reacA0 55.2 MN 27.7 tion type from thermochemical and kinetic data in the MNA9 63.8c 61.6 FO 50.0c HMN 50.1 literature.1~5-e~11-15~17~18~zo In essence, literature data for all H2 F9 68.3 0.0 of the reaction types except RHT allow only a very limited FlOH 37.1 H 52.1 range in which the parameters can be varied without DNM 36.1 N 36.1 pressing the limits of plausibility. For the RHT reactions DNMH MN2 66.2 58.5 (7,9,25,26, and 271, we adopted (on the basis of the results HDNM CH4 -17.9 58.5 in refs 6 and 7) an initial value of 15.5 kcal/mol for the DNM4d 31.6 CH3 35.1 activation energy in the thermoneutral case [Ea(0)]of Species identities are as shown in Table 11. *Measured heats of transfers between two naphthalene units and an Evansformation are from refs 7, 11, and 12; estimated heats or formation Polanyi factor (a)of 0.25 for the dependence of the change are from refs 12b, 13-15. In certain cases, such as HDNM, the in activation energy on the endothermicity of the reaction cyclohexadienyl radical produced by H-atom transfer to the 1position (ipso) of dinaphthylmethane, the heat of formation is for [i.e., Ma= Ea(0)- a A ( A H o ) ] . A separate program was the unsubstituted radical. This amounts to the zero-order aswritten to ensure that the regimen was applied uniformly sumption that group additivity holds for the reactions being conto all reactions and that any changes in the heats of forsidered, i.e., that substitution not at sites having significant odd mation of various species were incorporated systematically electron density upon H transfer does not affect the thermodyto all reactions involving that species. The heats of fornamics and kinetics of the H transfer significantly. Optimization mation used in the model are summarized in Table IV. resulted in AHf' values for A9 and FO that are 0.5 kcal/mol higher than the values obtained from refs 7a and lla. dTheformation of We varied the parameters Ea(0)and a for the five RHT DNM4 is, at this stage, treated in a simplified fashion: it is asreactions (7,9,25,26, and 27) to obtain the best agreement sumed that, once the reactive intermediate dihydronaphthalene between computed results and the experimentally observed structure is produced, subsequent reduction all the way to a tetrarates and selectivities of dinaphthylmethane cleavage as hydro species is rapid. Accordingly, the species DNM4 has a heat a function of aromatic and dihydroaromatic concentration. of formation corresponding to that for 1,2-dihydronaphthalene. The best agreement for the phenanthrene and anthracene system was obtained with Ea(0)= 16.5 and a = 0.35. The radical species (reactions 7 and 271, and RRD (reaction 17). ~ ~the '~~ ~~~ only modifications of our original, l i t e r a t ~ r e - b a s e d ~ ~ - l ~In computations resulting in this figure, the concenregimen were an increase of the A factor for H addition tration of aromatic was varied while the dihydroaromatic by 1.7 times and that for H elimination by 1.4 times and was held constant; that is, all computed rates in this figure increases of the heats of formation of phenanthrene and are for solvents of equal donor contents. the 9-hydrophenanthranyl radical by 0.5 kcal/mol each. In the anthracene system, the total computed cleavage rate changes little, increasing slightly as the anthracene Computer Model Compound Results concentration increases.21 In contrast to the relatively constant total rate, there are large changes in the contriFree H-Atom/RHT Competition. The single most butions made by RHT and free H atoms: at very low important trend delineated by the numerical model calanthracene concentrations, the free H-atom contribution culations is the "complementary" change in the relative is about twice the RHT contribution, whereas a t 2.1 M importance of transfer by free H atoms and by RHT as anthracene, the computed RHT contribution is about nine the degree of hydrogenation of the solvent changes. Figure 2 shows the computed rates of bond cleavage in DNM resulting from H transfer via the four most important (21) The rates computed here (Figure 2) are the 'direct" cleavage modes in anthracene/dihydroanthracene and phenanrates, that is, those resulting from transfer of a single H to the ipso position. Indirect cleavages resulting from the thermolysis of reduced threne jdihydrophenanthrene systems together with the products are not included. The experimental rates observed at very low total direct cleavage rates. The four modes are addition anthracene concentrations actually include a substantial amount of inof free H atoms (reaction 8), RHT from two different direct cleavage, the extent of which can be monitored by the amount of (20) (a) Tsang, W. J. Phys. Chen. 1986, 90,1152. (b) Tsang, W. Private communication, 1988.

methylnaphthalenes and tetralin (rather than methylnaphthalenes and naphthalene) produced. To facilitate comparison in the experimental values used for final adjustment of the model, we subtracted the contribution of these indirect cleavage products from the total observed rates.

Malhotra and McMillen

Energy & Fuels, Vol. 4 , No. 2, 1990 I

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Dlnaphlhylmethane: 10 mot% Temperature: 400°C Tim: 5minto7hr

1.o

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I I I 8 10 12 An/AnHz RATIO

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Figure 3. Observed cleavage selectivity as a function of aromatic/dihydroaromatic ratio and the extent of dilution. Closed circles: undiluted. Open circles: 5 0 4 5 % dilution with biphenyl. times that of the free H-atom contribution. The consequences of this crossover can be quite profound, both in terms of the selectivity for DNM cleavage and for coal conversion itself. In previous model compound ~ t u d i e swe ' ~ have ~ ~ ~shown that the cleavage at the 1-position (leading to %methylnaphthalene) is favored over that at the 2-position (leading to 1-methylnaphthalene). In solvent systems containing roughly equal amounts of the aromatic and hydroaromatic, the selectivity of cleavage at 400 "C increased from roughly 2.0 in tetralin to 4.5 in anthracene/dihydroanthracene. Furthermore, within a given solvent system, the selectivity increased with increasing aromatic content. A t low anthracene concentrations, where free H atoms dominate, we expect a low selectivity because of the very high reactivity of H atoms. Figure 3 shows the observed selectivity in anthracene/dihydroanthracene systems. (The experimental selectivities and rates shown in Figures 3 and 4 were measured by reaction in sealed fused silica tubes in a molten salt bath, followed by analysis by GC-MS. The procedures used are described in ref 5.) Thus, the computed results are in accord with the experimental findings. The expected selectivity at low anthracene concentrations can be estimated as follows. Literature data for atom additions that are approximately 30 kcal/mol exothermic18,20suggest that only 10-20% of the 3 kcal/mol enthalpy difference for H addition to the 1-and 2-positions of DNM will be observed as a difference in the activation energies. At 400 "C, this attenuated difference corresponds (barring large steric effects) to a rate ratio of 1.3-1.6 favoring 2-methylnaphthalene production.*2 In Figure 3, we see that the lowest observed selectivity approaches these values, reaching about 2 at the lowest An/AnH2 ratios, with dilution of the solvent by biphenyl.23 Contribution by RRD, which is also highly selective, is relatively more important at high ArHz and may be preventing the observed selectivity from declining to less than 2. At high An/AnH2 ratios, where the model predicts that RHT ~

(22) A difference in AHo for H-atom addition of 3 kcal/mol, together with an Evans-Polanyi factor of 50.2 for a highly exothermic reaction, leads to an expected selectivity ratio of 51.6 for free H-atom addition. (23) In anthracene/dihydroanthracene,the 2-methylnaphthalene/lmethylnaphthalene (2-MeN/l-MeN) product ratio ranged from about 2 to about 7. In this solvent, where RHT tends to be more favored, it is difficult, even a t low anthracene concentrations, to disfavor RHT sufficiently to get the cleavage selectivity below about 3.5. Only with [An],, = 0, short reaction times, and an inert diluent could the bimolecular RHT process by sufficiently disfavored to approach the cleavage selectivity of approximately 1.6 expected for reaction solely via free H atoms.

dominates, a substantially higher selectivity is observed. For the anthracene case (at An/AnH, greater than 3), the predicted contribution from free H atoms is about a tenth or less than that of AnH'. We believe that the observed limiting selectivity of about 6.5 is the inherent selectivity of cleavage by RHT from AnH'. For the phenanthrene system, even at Phen:PhenH, ratio of 3 the cleavage by free H atoms is predicted to be equal to that by RHT. Thus, the computed trends in the H-atom/RHT competition are entirely consistent with the observed changes in cleavage selectivity. Figure 2 also shows the cleavage rates computed for the phenanthrene system, by exactly the same regimen as applied in the anthracene system. The net rate of cleavage is about 20% slower in the phenanthrene system. As expected, the individual rates display similar profiles. The chief difference is that cleavage via free H atoms is predicted to be relatively more important in this system than in the anthracene system. Experimentally, we have observed that the cleavage of DNM is about 30% slower and the selectivity of cleavage is lower in the phenanthrene system than in the anthracene system. As mentioned above, we used the relative rates of cleavage in anthracene and phenanthrene solvent systems as well as the changes in selectivity with the amount of aromatic in the solvent to arrive at the best values for E,(O) and a,namely, 16.5 kcal/mol and 0.35, respectively. The shift in the competition between cleavage by RHT and free H atoms that results from changes in aromatic concentration or on moving from the anthracene to the phenanthrene system is the natural result of the shifting population of carrier species. The relative steady-state concentrations of ArH' and H' are largely determined by equilibrium in reactions 15 and 16 (Table I). ArH'

15

5Ar + H'

For a given system (i.e., fixed thermochemistry), the larger the concentration of Ar is, the larger the proportion of the total H-transfer "activity" that resides as ArH' and, therefore, the greater the importance of bimolecular transfer from ArH' (RHT). This equilibrium also helps to explain the predicted greater importance of free H atoms in phenanthrene. Reaction 15 is 10 kcal/mol less endothermic in the phenanthrene system than in the anthracene system."J4J5 This lower endothermicity results in a substantially larger [H']/[ArH'] ratio in the phenanthrene system and, therefore, a more important role for free H atoms. Effect of Substrate Structure on Its Cleavage Susceptibility. Transfer of a hydrogen atom to an aromatic system by RHT, which is a near-thermoneutral process, will be much more selective than transfer via the highly exothermic addition of free H atoms. This selectivity results in large changes in the overall reactivity of aromatic clusters as the cluster size changes. For most of our modeling effort, we have used dinaphthylmethane as a surrogate structure. To assess the impact of the size of the aromatic cluster bearing a cleavage linkage, we systematically varied the thermicity of H transfer to our surrogate coal structure to correspond to that for transfer to benzene, naphthalene, phenanthrene, pyrene, and anthracene. No other changes in the model were made. (The zero-order assumption was that E,(O) is the same for all these systems.) For these systems, we computed the net rates of cleavage, as well as the contributions from RHT, RRD, and free H atoms, in a given solvent (anthracene/ dihydroanthracene). The results of this modeling, shown in Figure 4, bear out our expectation that the contribution

Mechanistic Numerical Model for Coal Liquefaction

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Energy & Fuels, Vol. 4, No. 2, 1990 191

and (3) improved hydrogen utilization efficiency. Solvent Dehydrogenation. Free H atoms can not only add to aromatic systems bearing linkages to induce bond scission but also participate in chain dehydrogenation processes by abstracting H to form H,:

I

,VI

H’

Net:

20

25 30 35 40 EXOTHERMICITY OF H-ADDITION TO THE AROMATIC SUBSTME (kcaVmol)

45

Figure 4. Computed pseudo-first-order rate constants for cleavage of Ar-X by various H-transfer agents in 50:50 anthracene:9,1O-dihydroanthracene.Also shown are experimental data ( X ) for cleavage of alkyl linkages to selected PCAHs.

of a more selective H-transfer agent (RRD and RHT pathways) will be greater for the cleavage of linkages to larger aromatics. The results illustrate that the susceptibility to cleavage by RHT is a strong function of ring size: as the cluster bearing the linkage changes from a benzene unit to a 1-substituted pyrene, the computed rate of RHT-induced cleavage increases by more than 3 orders of magnitude, while cleavage resulting from free H atoms increases by only about a factor of 3. Figure 4 also shows experimental points corresponding to the total cleavage rates for linkages to phenyl, 1-naphthyl, 1-pyrenyl, and 9-anthryl units. As can be seen, the model correctly predicts the trends and gives values in reasonable agreement with the experimental results for a very wide range of substrate reactivity. Thus, the reactivity ratios of various clusters can be quite different in two solvents if the ratio of the active H-transfer agents in them is different. This result could account for the different relative effectiveness of some solvent pairs with different coals. For example, the well-known effectiveness of unhydrogenated PCAHs such as pyrene for solubilizing midrank bituminous coals (e.g., Pittsburgh No. 8) probably reflects both the shuttling effectiveness of pyrene and the susceptibility of moderate-sized PCAH in the coal to cleavage by RHT. In contrast, the relatively greater need for a “good” hydrogen donor (e.g., 9,lO-dihydrophenanthrene)for conversion of low-rank coals could reflect the dominance of smaller PCAHs in these coals and, therefore, a greater dependence on highly active transfer agents such as free H atoms, which are relatively more prevalent in dihydrophenanthrene. Added significance for efficient coal conversion comes from the fact that not only are free H atoms unnecessary for cleavage of linkages to large PCAHs, they also tend to lead to wasteful consumption of H’ to make H, and light hydrocarbon gases.

Relevance of the Free H-Atom/RHT Competition to Coal Conversion In this section we discuss the possible impact on coal conversion of shifts in the competition between free H atoms and RHT, the two principal hydrogen-transfer modes that lead to strong bond scission. Three major consequences of hydrogen transfer by RHT rather than free H atoms are (1)absence of H2 formation, (2) increased selectivity of H transfer to more reactive aromatic systems,

ArH,

-

- + - +

+ ArH, ArH’

H,

+ Ar

Hz

H’

ArH’

Ar

(11)24

(15)24

(dehydrogenation)

Because ArH’ cannot produce H2 by abstraction, Hz formation is minimized when transfer by RHT is maximized. The consequences of decreased production of Hz achieved by thermal dehydrogenation of hydroaromatic solvent components are clear: The cost (in terms of high hydrogen pressure capability and catalyst) of regenerating or maintaining a given donor level is generally a very significant part of liquefaction process costs; to the extent that a process is operated under conditions that allow hydroaromatic hydrogen to revert to H,, this expenditure is simply squandered. Hydrogen Utilization Efficiency. The net effect of minimized solvent dehydrogenation and increased selectivity is the improved efficiency with which hydrogen is utilized to produce liquids. We have discussed the question of efficiency and factors controlling it elsewhere.25 Briefly, an efficiency of 1.0 is attained when there is a net transfer of hydrogen only to aromatic positions bearing linkages; i.e., all hydrogen is used to bring about hydrogenolysis. However, when there is H, formation or multiple hydrogen transfers to positions not bearing linkages, reduction without cleavage will result and the hydrogen utilization efficiency will be decreased. What determines the extent of reduction is the fate of the cyclohexadienyl radicals produced when the initial hydrogen transfer is to a non-ipso position. When the concentration of aromatic in the solvent is high (i.e., conditions that favor RHT), the chances are increased that the non-ipso radical, which cannot undergo /3-scission to eliminate a linkage as a radical fragment, will simply transfer its “extra” hydrogen back to the aromatic solvent component to regenerate the “pool” of ArH’ carrier radicals. However, when the Ar concentration is low and that of the ArH, is high (i.e., conditions that favor free H atoms), the non-ipso radical will instead obtain a second hydrogen by abstraction from the ArH, (reaction 23). The dihydroaromatic structure thus produced can very easily accept a third and a fourth hydrogen (the third by RHT or free H and the fourth by abstraction) to produce a tetrahydro-coal structure, which could then undergo cracking to give light gases. Thus, under conditions where hydrogen transfer is predominantly by RHT (substantial concentrations of Ar), hydrogen is consumed only by transfer to positions bearing linkages. Transfer does take place to other positions as well, but in the presence of substantial concentrations of good acceptors, this hydrogen will simply be returned to the solvent system to be used elsewhere. We have used a similar line of reasoning to rationalize the recent observations of Gorbaty and Maa on the hydropyrolysis of coals.26 They reported that when the (24) See Table I. (25) McMillen, F.; Malhotra, R.; Tse, D. S. Interactive Effects between

Solvent Components: Possible Chemical Origin of Synergy in Liquefaction and Coprocessing. Submitted for publication in Energy Fuels. (26) McMillen, D. F.; Malhotra, R.; Nigenda, S.E. P r e p . Pap-Am. Chem. SOC.,Diu.Fuel Chem. 1987, 32(3), 180.

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192 Energy & Fuels, Vol. 4, No. 2, 1990 too

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Anthracene Phenanthrene

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WyolNpNeN Conditions

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KyN2

Ky/Np/MeN Conditions

KyIHzIMeN

Figure 5. Effects of methylnaphthalene and hydrogen on the conversion of Wyodak and Kentucky coals. Data from ref 10.

reaction temperature exceeded a certain threshold during temperature-programmed hydropyrolysis, a pronounced exotherm re~ulted.~’Furthermore, the incremental volatiles resulting from the presence of hydrogen (over those observed in the base case of straight pyrolysis in nitrogen) were alsmost exclusively gases if the threshold was exceeded but oils if the temperature was kept below the threshold. At the higher temperatures encountered during the exotherm, the indigenous PCAH structures in coal are less effective in retrieving hydrogen transferred to the non-ipso positions. Thus, the non-ipso radicals can go on to produce hydroaromatic structures, which, in turn, could lead to light gases.

*0°

f

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0

+ MeN = H2

+ MeN

Baseline

CASE CONDITIONS:

Effect of Reaction Conditions on Conversion in Phenanthrene and Anthracene Figure 5a shows some of the results of Kwon that illustrate the effect of hydrogen pressure and addition of “inert solvent” on the conversion of a Wyodak coal in anthracene and phenanthrene.1° Figure 5b shows a similar set of results for Ky-9 coal. The trends are similar for the two coals; the discussion will focus on the Wyodak coal, where they are more pronounced. Three trends of interest can be noted: (1) Coal conversion to T H F solubles in aromatic solvent (no donor) in the absence of H2 is greater in phenanthrene than in anthracene. (2) Addition of methylnaphthalene increased the coal conversion in the case of anthracene and decreased it in the case of phenanthrene. (3) Addition of H2, in the case of the low-rank coal, benefits the anthracene system much more than the phenanthrene system. Because the trends reported by Kwon seemed so curious, conversion of several coals under these same sets of conditions has been reexamined by Baldwin and Shin.28 They essentially reproduced trends 1 and 3. They also observed a detrimental effect of 1methylnaphthalene in the case of the phenanthrene system, although no marked improvements were found for the antracene case. The effects of 1-methylnaphthalene addition may seem small on the compressed scale of Figure 5, but the differences are clearly significant. The trends are the same for both coals, and the contrasting effects are totally inexplicable in terms of the traditional radical-capping liquefaction mechanism. However, these trends can be rationalized in terms of solvent-mediated hydrogenolysis and are mimicked by the numerical model. Figure 6 shows the predicted effect of l-methylnaphthalene and hydrogen pressure on strong bond scission under conditions intended to be similar to Kwon’s (27) Gorbaty, M. L.;Maa, P.S. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986, 3I(4), 5. (28) Shin, S.G.; Baldwin, R. M.; Miller, R. L. Prepr. P a p - A m . Chem. SOC.,Diu. Fuel. Chem. 1988, 33(3), 265.

I*rl

w] 0.OSY

Tme W o o 1 T W . 4W°C

[acrc]O.lM

H2

0.8M

S.IY(ZWplW

Figure 6. Computed effects of methylnaphthalene and hydrogen on the rate of DNM cleavage in anthracene and phenanthrene

with very low donor content.

(very low donor level). A small level of donor solvent was included in the computation because the coal structure itself provides some donor content. (In the figure, the computed bond cleavage rates in anthracene and phenanthrene have been normalized to the base case for both systems.) As can be seen by comparing Figures 5 and 6, the contrasting response of the anthracene and phenanthrene solvent systems to the addition of l-methylnaphthalene and H2 pressure is mimicked to a surprising degree. The differences between phenanthrene and anthracene systems can be understood by recalling that in the phenanthrene system free H atoms are more important, whereas in the anthracene system cleavage by RHT is more important. The differing response of the two solvent systems to 1-methylnaphthalene results from the differing impact on the two carrier species: free H atoms and ArH’ radicals. In systems where radical generation is by RRD, benzylic hydrocarbons can function both as radical scavengers and as initiators. In aromatics that are very good H-atom acceptors (such as anthracene), benzylic hydrocarbons like methylnaphthalene can serve as initiators by transferring a hydrogen to the PCAH acceptor.

@&

+&

(3)24

This reaction serves to increase the steady-state concentration of the H-atom carrier species ArH’, and thereby

Mechanistic Numerical Model for Coal Liquefaction

increases the rate of bond scission induced by transfer from ArH'. In phenanthrene, which is a poorer H acceptor, the addition of a given quantity of the same benzylic compound, methylnaphthalene, will result in a much smaller number of RRD transfers. Furthermore, since a substantial portion of the H-atom activity in this solvent is as free H atoms, the methylnaphthalene will tend to act as a scavenger.

Thus, the result is that an active carrier (the H atom itself) has been scavenged to produce a nonhydroaromaticradical (naphthylmethyl radical) that is useless as an H-atom carrier. The net result of methylnaphthalene addition to the phenanthrene system is a decrease in the H-transfer activity of the system, exactly opposite to the effect in the anthracene system.29 An examination of the computed rates of individual reactions leads also to an understanding of the effects of H2 in the two systems. The addition of H2 results in the formation of H atoms, by the sequence of reactions shown by Vernon30 to be accessible under some liquefaction-related conditions: AnH' H2 zAnH, H' (12424 PhenH'

+ + + H2 * PhenH2 + H'

(12b)24

The relative rates of reactions 12a and 12b depend on the steady-state concentrations of the respective ArH' species and the values of the abstraction rate constants. The much higher radical stability in the anthracene system that results in a much higher [ArH'] will also result in a lower abstraction rate constant. These two factors will partially, but not completely, compensate: their product is significantly higher in anthracene than it is in phenanthrene. Thus, the effect of added H2will be greater in anthracene, even though a smaller fraction of the H transfer occurring in that system is the result of free H atoms. This prediction, however paradoxical, is strikingly parallel to the results observed by Kwon. Kwon suggested that the increased benefit from H2 pressure in the anthracene system is a result of the greater buildup of dihydroanthracene, which, being a better scavenger of radicals, then leads to a greater conversion. However, according to our model, the greater buildup of dihydroanthracene is a result of the increased steady-state concentration of ArH', not the original cause of greater coal conversion. (29) The reasons offered here for the effect of methylnaphthalene will not explain the astounding difference between 1-methylnaphthalene and 2-methylnaphthalene reported by Davis and co-workers for the conversion of a certain Kentucky No. 9 coal [Keogh, R. A.; Chawla, B.; Tsai, K.-J.:Davis, B. H. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1988, 33(3), 3331. (30) Vernon, L. W. Fuel 1980, 59, 102.

Energy & Fuels, Vol. 4 , No. 2, 1990 193

Conclusions We have described here a mechanistic numerical model for coal conversion that focuses exclusively on cleavage of strong bonds induced by a combination of competing hydrogen-transfer processes, chief among which are radical hydrogen transfer and free H-atom addition. The main conclusion that we can draw from this model is that the different H-transfer pathways are competitive. The utility of the model lies not in predicting the rates exactly but in its ability to predict how the competition changes with reaction conditions. The predicted trends coincide extremely well with the otherwise inexplicable trends observed by Kwon for the conversion of real coals. Given the obvious simplifications in the model as currently used, such as the omission of weak-bond scission, retrograde reactions, and mass transport effects, we take the success described here as additional convincing evidence that hydrogenolysis of strong bonds plays an important role in coal liquefaction and that, despite the obvious complexity of coal, the use of a mechanistic model can lead to an understanding of liquefaction phenomenology not otherwise accessible. Analysis of competing H transfers via RHT and free H atoms with the help of a mechanistic numerical model leads to an explanation of otherwise puzzling phenomena: In all PCAH solvent systems, there is a competition between radical hydrogen transfer and free H-atom addition, Cleavage by RHT is favored with higher aromatic content of the solvent and lower temperatures. A greater proportion of cleavage is induced by radical hydrogen transfer in anthracene; in phenanthrene, cleavage by free H atoms is substantially more important. A t very low donor contents, additions of a scavenger (e.g., methylnaphthalene) to solvents that are very good H-atom acceptors (Le., solvents where RHT tends to be dominant) increases the H-transfer capability of the system by increasing the steady-state concentration of the carrier species ArH'. At very low donor contents the addition of a scavenger to solvents that are only modest H-atom acceptors (where free H-atom addition tends to be the dominant transfer mode) decreases the H-transfer capability of the system by scavenging H atoms. Application of hydrogen pressure enhances cleavage rates more in anthracene than in phenanthrene because thermochemical and kinetic factors are more favorable for H-atom production by the abstraction of H from H2 by AnH'. Although we find these explanations of intriguing liquefaction phenomena to be convincing, we present them as only speculative explanations. They are to be subjected to other tests and improved upon as necessary. We hope that technologically useful suggestions will arise out of that testing procedure. Acknowledgment. Financial support for this work provided by the US.Department of Energy under Contract No. DE-FG22-84PC70810 and Contract No. DEFG22-86PC90908 is gratefully acknowledged.