Energy & Fuels 1993, 7, 1047-1053
1047
Carbonization of Anthracene and Phenanthrene. 2. Spectroscopy and Mechanisms Teruhiko Sasaki,? Robert G. Jenkins,$ Semih Eser*, and Harold H. Schobert' Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received March 29, 1993. Revised Manuscript Received August 17, 199P
The C, H analysis and FTIR and 'H NMR data on the carbonization products from anthracene and phenanthrene showed remarkable differences in reaction chemistry and mechanisms for these isomers. As different from phenanthrene, anthracene involves extensive hydrogenation reactions which lead to the formation of hydroaromatic compounds in the liquid phase during carbonization. Dreiding models of reaction products based on the C, H contents and spectroscopic data along with the minimization of the force field equation showed that anthracene produced planar oligomers more readily than phenanthrene. A mechanism is proposed for the two dimensional growth of discotic mesogens using Huckel molecular orbital calculations combined with structuralmodeling. Distinctly different chemistry and kinetics of carbonization in connection with geometrical arrangements of oligomers explain the different degrees of mesophase development obtained from anthracene and phenanthrene.
Introduction The formation of insoluble products from carbonization of anthracene and phenanthrene and their disappearance in initial reactions were found to proceed with very different kinetics, as presented in our companion paper,' and elsewherea2aThermal conversion of anthracene and phenanthrene has been studied extensively.24 However, the effects of the initial reactions on mesophase development are not well understood. A microscopic examination of the solid Carbonization products showed that anthracene produced a much higher degree of mesophase development than phenanthrene under comparable conditions.' Excluding some general considerations in recent publications,24 there are no proposed mechanisms to explain the formation of mesophase precursors (discotic mesogens)g from the two isomers as they relate to the different degrees of mesophase development. This paper presents the elemental analysis and spectroscopic data on the carbonization products of anthracene and phenanthrene in conjunction with structural modeling and simple quantum chemical calculations. Combined with the kinetic data presented in the companion paper, mesophase development is examined from the closely related aspects t Mitaubishi Oil Company Limited, 4-1,Ohigimachi, Kawasaki-Ku, Kawasaki, 210, Japan. 8 University of Cincinnati, College of Engineering, Cincinnati, OH,
45221-0018.
* Abstract published in Advance ACS Abstracts, October 1, 1993.
(1) Sasaki, T.; Jenkins, R. G.; Eser, S.; Schobert, H. H. Energy Fuels, preceding paper in this issue. (2) Scaroni, A. W.; Jenkins, R. G.; Walker, P. L., Jr. Carbon 1991,29,
969. (3) Peters, T. J.; Jenkins, R. G.; Scaroni, A. W.; Walker, P. L., Jr. Carbon 1991,29, 981. (4) Walker, P. L., Jr. Carbon 1990,28, 261. (5) Whang, P. W.; Dachille, F.; Walker, P. L., Jr. High Temps.-High Press. 1974, 6, 127. (6) Morita, M.; Hiroshawa, K.; Takeda, S.; Ouchi, K. Fuel 1979, 48, 269. (7) Stein, S. E. Carbon 1981, 19, 421. (8) Lewia,I. C.; Singer,L.S.,PolynuclearAromaticCompoun&,Ebert, L. B., Ed.; American Chemical Society: Washington, DC, 1988; Chapter 16. (9) Marsh,H.;Menendez,R. Inlntroduction to CarbonScience; Marsh, H., Ed.; Butterworths: London, 1989; Chapter 2.
0887-0624/93/2507-lO47$04.00/0
of chemical mechanisms, reaction rates, and geometric factors. Experimental Section The experimental procedure for the liquid-phasecarbonization O C and phenanthrene at 540 O C under autogeneous pressure is given elsewhere.' The equipment and procedures used for obtaining the elemental analysis and spectroscopic data as well as the software used for structural modeling are described below. Elemental and Spectroscopic Analysis. Carbon and hydrogen contents of the reaction products were determined on 100-mg samples in a Leco CHN-600 elemental analyzer. The carbon and hydrogen contents of the reaction products were reproducible within 1and 0.1 wt % ,respectively of the reported values with an average closure of 99.2 % for the s u m of measured carbon and hydrogen contents. Diffuse reflectance Fourier transform infrared (DRIFT) spectra of the selected products were obtained on the mixtures of samples (5 w t %) with KBr in an Analect Instruments FX-6200 FTIR spectrometer. The spectra were transformed into Kubelka-Munk format for analysis.10 The ratio of the aliphatic to aromatic C-H was obtained by taking the ratio of the integrated intensitiesof aliphatic C-H stretch (2700-3000cm-l) and aromatic C-H stretch (3000-3100 cm-1) regions. lH NMR spectra were obtained on a Bruker WP200 pulse FT NMR spectrometer using 30-50-mg samples of anthracene products and 100-mg samples of phenanthrene products dissolved in 1mL of perdeuterated pyridine obtained from Aldrich. Aliphatic and aromatic hydrogen contents were determined by integrating the absorptions according to the following shifts from TMS: aromatic hydrogen, 9.0-6.5 ppm; H,, 4.0-2.0 ppm; Hg, 2.0-1.0 ppm; H,,1 . 0 . 5 ppm. Different aromatichydrogens in the carbonization products of anthracene were distinguished with respect to their positions on the anthracene molecule: H,(#l), 8.1-8.0 ppm; H,(#2), 7.5-7.4 ppm; H,(#9), 8.5-8.4 ppm. Structural Modeling. Structural modeling was performed using ALCHEMY I1 software (Tripos Associates, Inc.) on a Macintosh I1 microcomputer. This program allows the construction of graphicalforms of the Dreiding models. The atomic configurations in the molecule are then adjusted to minimize the of anthracene at 480
(10) Fuller, E. L., Jr.; Symryl, N. R. Fuel 1985,64,1143.
0 1993 American Chemical Society
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1048 Energy &Fuels, Vol. 7, No. 6, 1993
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Figure 1. Variation of atomic C/H ratio of the products from anthracene at 480 "C and phenanthrene at 540 "C with the corresponding heptane-insoluble contents of the products. force field equation, which includes terms for bond stretching, angle bending, torsional deformation, van der Waals interactions, and out-of-plane bending. For the modeling reported in this paper, the minimization was carried through 500 iterations.This program allows calculation of lengths of molecules. Molecular orbital calculationswere performed with Huckel molecular orbital (HMO) software (Trinity Software) also on a Macintosh 11. Results and Discussion Carbon/Hydrogen Analysis of Carbonization Products. Figure 1 shows the variation of the C/H ratio of the carbonization products from anthracene and phenanthrene as a function of the heptane-insolubles (HI)content of the products. Anthracene was carbonized at 480 "C for 1-5 h and phenanthrene was carbonized at 540 "C for 1-4 h (ref 1). Increasing C/H ratios with the increasing extent of conversion into insoluble materials result, obviously, from polymerization and condensation reactions that lead to loss of hydrogen. A clear distinction between the two compounds is that the anthracene curve shows plateaus where the C/H ratio remains almost unchanged with the increasing HI. The presence of this plateau is significant with respect to the reproducibility of the C and H contents and confirmed also by the NMR and FTIR data presented later. The phenanthrene products, on the other hand, show a monotonic increase in C/H ratio over the entire range of the HI content. This difference is especially pronounced in the range 50-60% HI, indicating a significant hydrogen loss in the phenanthrene products over a narrow range of HI content. Microscopic examination of the solid carbonization products showed that this range of HI marks the coalescence stage of mesophase development from both anthracene and phenanthrene. The almost unchanged C/H ratio of anthracene products in this range indicates extensive hydrogen transfer reactions accompanying the coalescence stage of mesophase development. This observation is confirmed by the NMR and FTIR data, presented below, indicating much higher concentrations of aliphatic hydrogen in anthracene products than in phenanthrene products at comparable conversion levels. Figure 2 shows the atomic C/H ratios of the three solvent insoluble fractions, heptane insolubles (HI), toluene insolubles (TI),and pyridine insolubles (PI) of anthracene products as a function of time at 480 "C. The fluctuations of the C/H ratios of all the solvent fractions within the first 2 h of carbonization indicate successive dehydrogenation and hydrogenation reactions, which are coincident with the nucleation and coalescence of mesophase spheres. These fluctuations suggest that hydrogen molecules which are produced by dehydrogenative oligomerization pro-
Fo
Sasaki et al.
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Figure 2. Atomic C/H ratios of heptane-,toluene-,and pyridineinsolubles(HI,TI, PI) and whole products (WP)from anthracene at 480 "C.
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Figure 3. Atomic C/H ratios of heptane-,toluene-,and pyridineinsolubles (HI, TI, PI) and whole products (WP) from phenanthrene at 540 O C . cesses are, in turn, actively involved in hydrogenation of the liquid-phase reaction products, giving rise to the cyclical patterns seen in Figure 2. The close correspondence between the C/H ratios of the solvent fractions in Figure 2 indicates a smooth sequence of molecular growth processes during carbonization of anthracene, as was also inferred from the kinetic data.' Figure 3 shows a corresponding plot of C/H ratios of the solvent fractions for the phenanthrene products. In contrast to Figure 2, the C/H ratios in Figure 3 do not show any significant fluctuations as a function of time. Another significant difference between Figures 2 and 3 is that the close correspondence between the C/H ratios of all the fractions of anthracene products is not seen for the case of phenanthrene. As evident from the large difference between the C/H ratios of HI and T I of phenanthrene products, the formation of T I (or PI) involves a significant dehydrogenation of HI during the relatively early stages of phenanthrene carbonization. This trend represents a clear contrast to the smooth conversion of HI to TI during anthracene carbonization, as was also noted in the kinetic parameters calculated for the different solvent insolubles from both c0mpounds.l FTIR and 'H NMR Analysis of Reaction Products. Figures 4 and 5 show, respectively, for the anthracene and phenanthrene products including the solvent-insoluble fractions, the FTIR parameters related to the ratio of the aliphatic (2800-3000cm-l) to aromatic (3000-3100cm-') C-H contents through the respective extinction coefficients. As evident from the formation of aliphatic C-H groups, Figure 4 shows that a significant chemical change starts to take place after half an hour at 480 "C during anthracene carbonization. There is a distinct maximum in the aliphatic to aromatic C-H ratio in Figure 4 for the whole products from anthracene after 2 hours at 480 "C. In comparison, the solvent-insoluble products show less pronounced maxima in the FTIR parameter. It was observed previously that the carbonization of A-240 pitch
Carbonization of Anthracene and Phenanthrene
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Figure 4. AliphatidaromaticC-H by FTIR in the whole reaction products (WP) and insoluble fractions (HI, TI, and PI) from anthracene at 480 OC.
0 1 .
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Figure 6. Aliphatic/aromaticC-H by FTIR in the wholereaction products (WP) and insoluble fractions (HI, TI, and PI) from phenanthrene at 540 O C .
under similar conditions also displayed a marked maximum in the ratio of aliphatic to aromatic C-H.ll The differences between the FTIR parameters of the solventinsolubles and the whole product suggest that the corresponding solvent-soluble materials are more aliphatic. For the linear condensed ring aromatic compounds (e.g., naphthalene, anthracene, tetracene) there is a reduction in solubility parameter of the dihydro derivative compared to the parent aromatic compound. By use of the standard definition12of solubilityparameter as 6 = (AHv/V)o.6, where AHv is the enthalpy of vaporization and V the molar volume, 6 drops from 9.81 for anthracene to 7.75 for 9,10-dihydroanthracene. (The solubility parameters of the three solvents used in this work range from 7.19 for heptane to 10.19 for pyridine.) Thus, it is expected that partial hydrogenation of anthracene oligomerswould reduce their solubility parameters, and cause much less variation in the aliphatic/aromatic C-H ratio of the solvent-insolubles than that of the soluble materials. A striking difference between the FTIR parameters of the anthracene and phenanthrene products is that the solvent-insoluble fractions of phenanthrene products contain a larger proportion of aliphatic functionalities than does the whole product over the entire range of conversion (Figure 5). Also, in addition to relatively lower concentration of aliphatic C-H, there is no distinct maximum in the ratio of aliphatic to aromatic C-H for the phenanthrene products, except for the HI fraction. In contrast to the anthracene case, Figure 5 indicates that the larger molecules (insoluble fractions, especially HI) in the phenanthrene products are more aliphatic in character than small (soluble) molecules. It has been shown that for reductions of anthracene and other fused-ring aromatics (in this case, (11) Eser, S. ;Jenkins, R. G. Elctended Abstracts, Carbon 88; Society of Chemical Industry: London, 1988, p 407. (12) Isaacs, N. S. Physical Organic Chemistry; Longman: Essex, UK, 1987; p 188.
Figure 6. Aliphatic/aromatichydrogen by lH NMRof pyridine4 soluble products from anthracene at 480 C and phenanthreneat 540 "C.
in sodium and liquid ammonia) the coefficients of the HOMO may be more important in predicting reaction products than overall electron densities.13 Our HMO calculations for anthracene and phenanthrene show indeed a higher HOMO coefficient for the 9-position in anthracene, and a lower ?r electron energy for the molecule, than for phenanthrene. This suggests again that in the early stage of reaction, with abundant monomers present in the liquid, hydrogenation and formation of aliphatic C-H is easier for anthracene than for phenanthrene. However, this relative ranking changes once condensation reactions begin to become important. HMO calculations for 1,9'-bianthryl and for 9,9'-biphenanthryl show that it is the latter compound which now has the lower total T energy and a higher highest occupied molecular orbital (HOMO) coefficient for the reactive positions (e.g., for the 9-position in l,g'-bianthryl and the 10-position in 9,9'biphenanthryl). This relative change in T energies and HOMO coefficients at the reactive positions accounts for the lower aliphatic hydrogen content of phenanthrene products during initial carbonization but the greater aliphatic character of the larger phenanthrene products. The lH NMR data for the anthracene and phenanthrene products shown in Figure 6 show a much lower aliphatic hydrogen content of the phenanthrene products. As was discussed above, it has long been established that phenanthrene is more difficult to reduce than anthracene.14 Comparison of many reaction schemes shows that somewhat more severe conditions are required for conversion of phenanthrene to the 9,lO-dihydro derivative than for the similar reaction with anthracene.15J6 This situation pertains also to catalytic hydrogenation of the two c o m p o ~ n d s . ~Phenanthrene ~J~ has a higher resonance stabilization energy than anthracene, 92 us 84 kcal/mol, respe~tive1y.l~ In addition, the loss of resonance energy accompanying reduction to the 9,lO- derivatives is greater for phenanthrene, 20 us 1 2 kcal/mol, respectively.16 The distribution of aromatic hydrogen and total aliphatic hydrogen atoms is shown in Figure 7 as a function of reaction time during the early stages of anthracene carbonization. In agreement with the FTIR data, there appears to be a significant chemical activity after half an (13) Demus, D. Mol. Cryst. Li9. Cryst. 1988, 165, 45. (14) Whitmore, F. C. Organic Chemistry; Dover: New York, 1951; p 747. (15) Streitwieser, A., Jr.; Heathcock, C. H. Introduction to Organic Chemistry; Macmillan: New York, 1985; p 988. (16) Hudlicky, M. Reductions in Organic Chemistry; Ellis Horwood: Chichester, UK, 1984; pp 46-53. (17) Bass,K. C. Organic Syntheses;Wiley: New York, 1973;Collect. Vol. 5, p 398. (18) Phillips, D. D. Organic Syntheses;Wiley: New York, 1963; Coll. v 0 1 . 4 , ~313. (19) March, 3. Advanced Organic Chemistry; Wiley: New York, 1985.
1060 Energy & Fuels, Vol. 7, No.6,1993
Scrsaki et al.
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Figure 7. Distribution of aliphatic and aromatic hydrogens in pyridine-d soluble products from anthracene at 480 "C. 0
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hour at 480 OC. Figure 7 shows a continuous decrease in the number of hydrogen atoms a t the 1-and 9-positions without much change in the 2-position (except a t 2 h) with the increasing extent of reaction. The decrease in aromatic hydrogen concentration can be attributed to oligomerization and hydrogenation reactions (producing aliphatic hydrogens). A sharp increase in the position-2 hydrogens from 1.75 to 2 h, which is again coincident with the initiation of mesophase coalescence, is due to a steep decline in the position 1 and 9 hydrogens taking active parts in oligomerization and hydrogenation reactions. The higher reactivity of the 1- and 9-positions (toward hydrogenation and/or oligomerization) can be explained by the relatively high resonance stabilization energies of 9(22.3 kcal/mol) and 1-positions (18.5 kcal/mol) compared to position 2 (15.3 kcal/mol).20 It should be noted that the order of reactivity of these positions as suggested by the respective resonance stabilization energies is parallel to that indicated by the free valence indices.' Figure 8 shows the fraction of aliphatic hydrogen and pyridine-insolubles contents of anthracene products. The data for the fraction of aliphatic hydrogen after 1 h of reaction time represent only the pyridine-soluble materials in the products. It was observed that the mesophase spheres started to appear after 1h, while coalescence of the spheres was observed after 1.5 h at 480 "C.' Figure 8 shows that the aliphatic hydrogen content stays almost unchanged during the nucleation of mesophase spheres after a rapid initial increase prior to the formation of the spheres and pyridine insolubles. The coalescence stage is coincident with another period of rapidly increasing aliphatic hydrogen concentration which goes almost parallel with the formation of pyridine insolubles. Clearly, the formation of aliphatic hydrogen in the soluble fraction is closely tied to the formation of insolubles (or, to some extent, to mesophase development) from anthracene. (20) Stein, S.E.: Golden, D.M.J. Org. Chem. 1977,42,839.
Figure 9. A proposed structure for the carbonizationproducts from anthraceneat 480 "C for 1.75 h (a);an edge-on view of the proposed structure(b);an edge-onview of the structureproposed by Marsh and Menendezs for the lamellar molecules resulting from pitch pyrolysis (c).
Combined with the FTIR data on the different solvent insolubles fractions, this observation indicates that most of the hydrogen produced by aromatic polymerization reactions is used to hydrogenate the unreacted anthracene and/or relatively small product molecules. FTIR and NMR data show that, for both anthracene and phenanthrene, significant extents of chemical change take place both prior to and during the mesophase formation. A clear distinction between anthracene and phenanthrene reactions is the Occurrence of extensive hydrogen transfer reactions during carbonization of anthracene, which lead to the retention of substantial quantities of hydrogen in the liquid phase, as was also evident in a previous study.2 High concentrations of hydroaromatic compounds and readily available donor hydrogen should increase the fluidity and the fluid range during anthracene carbonization. These conditions can explain the higher degree of mesophase development from anthracene as well as the differences in the kinetics parameters for the carbonization of the two compounds.' Structural modeling based on the elemental analysis and spectroscopic data on the reaction products suggests that geometric considerations may also play a significant role in the observed difference between the mesophase development from anthracene and phenanthrene. Some reaction mechanismsrelating chemical changes to certain geometrical arrangementsare discussed below in the light of structural modeling. S t r u c t u r a l Modeling a n d Reaction Mechanisms. Anthracene Products. A proposed average structure for the carbonization products from anthracene a t 480 "C for 1.75 h is shown in Figure 9a. Although Figure 9a does not represent an unequivocal assignment of the structure of this material, there is generally a good agreement between the experimental structural parameters for the anthracene products and the proposed structure, as
Energy & Fuels, Vol. 7, No. 6,1993 1OS1
Carbonization of Anthracene and Phenanthrene
a
b a Figure10. Structureof 1,Vdimer of anthracene(a);an edge-on view of the structureof the 1,Y dimer after energy minimization (b). Table I. Comparilron of Calculated and Observed Structural Parametem for Carbonization Products Obtained from Anthracene at 480 “C for 1.75 h experimental calculated from data Figure 9a 1.66 1.68 C/Hratio no. of nonprotonated C 20 20 0.32 0.32 HdHU
summarized in Table I. The HJH, ratio in Table I which is for the pyridine-d soluble fraction comprising approximately 70 w t 96 of the reaction product should approximate the same ratio for the whole product. An important feature of the proposed structure for the reaction products shown in Figure 9a is revealed in the edge-on view shown as Figure 9b. It is clear from Figure 9b that the structure is nearly planar. It has been well established that the structural requirements for the formation of a mesogenic discotic liquid crystal phase are flat, disk-shaped molec~les.~*21-~ For comparison, Figure 9c is an edge-on view of the structure proposed by Marsh and Menendezg for the lamellar molecules resulting from pitch pyrolysis. The structure in Figure 9c was treated in the same iterative minimization procedure in ALCHEMY I1 as used to generate Figure 9b. The similarity between our proposed structure for anthracene products and the Marsh-Menendez structure for pitch pyrolysis products is striking. We consider the genesis of the proposed structure from the condensation of anthracene molecules. Although the 9,lO-positions in anthracene are the most reactive, the dimerization of anthracene actually leads to a variety of possible isomers.‘ We consider the l,Y dimer as an example of the type of structure that initiates condensation. As customarily drawn (e.g., Figure loa), the molecule could mistakenly be thought to be planar, but, in fact, after minimization the structure is distinctly nonplanar, as shown in the edge-on view in Figure lob. The development of planarity in the anthracene dimer is achieved by the formation of a second intramolecular C-C bond between the two anthracene moieties. A Hiickel molecular orbital calculation for 1,Y-bianthryl identified seven carbon atomsas having particularly high coefficients for the highest occupied molecular orbital (HOMO): the 2-, 5-, 1’-,2’-, 7‘-, 8‘-, and 1W-psitions. The two atoms (21) Gray, G.W.In Polymer Liquid Crystals; Cifem, A, Krigbaum, W. R, Meyer, R B., Eds.; Academic Prese: New York, 1982;Chapter 1. (22) Hendrikx, Y.; Levelut, A. M. Mol. Cryst. Li9. Cryst. 1988,265, 233. (23) Ostnwskii, B. I. Sou. Sci. Rev., Sect. A. 1989,12,85.
b
Figure 11. Structure formed by the formation of the aecond C-C bond in the anthracene dimer (a); an edge-on view of the structureformed by the formation of the second C-C bond in the anthracene dimer (b).
Figure 12. Nonplanar structure of the anthracene trimer.
meeting the criteria of (a) existing on different anthracene moieties, (b) having high HOMO coefficientsand (c) being in geometric proximity adequate to form a new C-C bond are the 2- and 8’-carbons. The resulting structure is shown in Figure 11; the edge-on view in Figure l l b , when compared with Figure lob, shows the significant increase in planarity achieved by formation of the second C-C bond. A Hiickel molecular orbital calculation for the new dimeric structure indicates that likely points of reaction for formation of a trimer are the 3-, 4-, 5, 6-, and 4’-positions. The formation of a trimer by condensation of a third anthracene moiety by formation of a bond between the 9”-position of the third moiety with the &position of the existingdimer will again lead to astructure which is distinctly nonplanar Figure 12. By use of the same argument as for formation of the second intramolecular bond between anthracene moieties in the dimer, the nonplanarity of the trimeric structure (Figure 12) is removed by the formation of another intramolecular bond, a process which leads directly to the trimeric structure of Figure 9, which constitutes the formation of a discotic mesogen having structure and composition in good agreement with the experimental data for 50-60% heptaneinsolubles. Thus in the oligomerization of anthracene to form a liquid crystalline mesophase which subsequently undergoes coalescence, both chemical and geometric factors are a t work. The condensation of an additional anthracene moiety with an existing structure through a single C-C bond will lead to nonplanar, and hence non-liquidcrystalline, structures. Achievement of planarity requires reactive (high HOMO coefficient) carbon atoms on the rings which are also in reasonable proximity for further reaction. The formation of the second C-C bond between
Smaki et al.
1052 Energy & Fuels, Vol. 7, No.6,1993
two close, high-HOMO carbons on adjacent anthracene moieties is responsible for "pulling" the structure into the planar configuration desired for a discotic mesogen. As presented above, the NMR and FTIR data on the anthracene products showed high concentrations of aliphatic hydrogen. The origin of the aliphatic hydrogen is a result of the mechanism of free radical attack. The oligomerization process begins with generation of an anthryl radical,which can then attack another anthracene molecule. However, free radical substitution a t aromatic carbon does not ordinarily proceed via hydrogen abstraction.lg The reaction may proceed instead via formation of a resonance-stabilized radical, which then combineswith a second radical. A generic mechanism given by Marchlg is
An alternative mechanism proposed by Whang6 is the fir& order bimolecular process leading to the diradical and 9,10-dihydroanthracene
The formation of the second intramolecular C-C bond (Le., the bond responsible for pulling the molecule into a relatively flat configuration) is very likely a free radical analog of the intramolecular Scholl which, as normally performed in the laboratory, proceeds via an arenium ion intermediate. The intramolecular Scholl reaction is particularly important for large fused-ring systems,19 and would proceed with an increase in C/H ratio. An important aspect of mesophase formation from aromatic molecules is two-dimensional polymerization processes to build large and planar molecules.25 There is no mechanism proposed in the literature to explain the formationof large disk-shaped moleculesfrom anthracene. Based on the reactivity considerations alone, one can envisage that the polymerization reactions will involve the most reactive 9- and 10-positionsof anthracene which would produce one-dimensional growth of ribbon-shaped molecules as shown below
/n\
(24)Clowea, G.A. J. Chem. Soc. 1968,2519. (25) Lewis, I. C. Carbon 1982,20,519.
This pentamer of anthracene with the indicated onedimensional preferential growth would not produce discotic mesophase. A plausible two-dimensional growth scheme can be built on the trimer shown in Figure 9 by adding two more anthracene molecules (indicated with shaded rings) as illustrated below
In contrast to the ribbon-shaped pentamer, the pentamer shown above is a discotic mesogene which can grow in two dimensions to maintain its disk shape for more favorable mesophase development. The essenceof two-dimensional growth is the formation of five-member rings between anthracene molecules, as also suggested by energy minimization and molecular orbital calculations. The formation of anthracene dimers containing five-member rings during pyrolysis of anthracene has been r e p o r t d B It can be considered that most of the five-member rings in the oligomers are eliminated through condensation reactions in the later stages of mesophase development. The linear configuration of anthracene molecules should allow the filling of the voids formed as a result of Condensation, producing well aligned pregraphitic structures. PhenanthreneProducts. As shown in Figure 1,there is a significant change in C/H ratio as the products of phenanthrene carbonizationpass through the range of 5060 % heptane-insolubles, coincident with mesophase coalescence. In comparison, the C/H ratio of the anthracene products is essentially unchanged in this range. The distinction in the two cases arises from differences in the molecular shapes and the consequent ability to form a liquid crystalline phase. A t 50% heptane-insolubles, a proposed structure for phenanthrene products is shown in Figure 13a. This structure is compatible with the observed atomic C/H ratio of 1.63 for the 50% heptane insolubles produced a t 540 "C for 2.5 h. As can be seen from Figure 13b, at 50% heptane-insolubles, the phenanthrene trimer is not planar and is thus unlikely to form a liquid crystallinemesophase. The sharp increase in C/H ratio coincident with the mesophase nucleation and coalescence results in a structure having atomic C/H of 1.75. The increased C/H ratio is achieved by formation of additional C-C bonds between the phenanthrene moieties. As in the case of anthracene, the additional intramolecular bonds are responsible for pulling the molecule into a much more planar structure, as shown in Figure 13c. The initiation of mesophase nucleation from phenanthrene thus takes place at significantly higher conversion levels to insoluble products compared to the (26) Badger, G.M.h g . Phy8. Org. Chem. 1965,3,1.
Carbonization of Anthracene and Phemnthrene
Energy & Fuels, Vol. 7, No. 6,1993 1053 anthracene and phenanthrene structures at coalescence of mesophase were measured using the scale modeling subroutine of ALCHEMY 11. Calculated values of X are 1.55 for the anthracene product and 1.86 for the phenanthrene case. This reasonable agreement suggests that the coalescenceof mesophase is being driven by the formation of geometrically similar structures. The comparatively low degree of mesophase development from phenanthrene can, thus, be related to differences in the chemistry and kinetics of carbonization which are closely related to molecular geometry involved in the formation of oligomers from the two isomers.
Conclusions b
C
Figure 13. A proposed structure for phenanthrene products obtained at 540 "C for 2.5 h (a);an edge-on view of the proposed structure (b); planar structure of the phenanthrene trimer achieved by the formation of additional intramolecular bonds (C).
case of anthracene.' This delay in the formation of mesophase can be explained by the nonplanarity of oligomerizationproducts obtained from phenanthrene at lower conversion levels. A t high conversion levels, the viscosity of the carbonizing medium will be too high and the planar molecules will be too large to move about freely to allow a high degree of mesophase development. A rapid increase in the molecular size of phenanthrene products during nucleation and coalescence stages' also inhibits mesophase development. Further, it can be seen by comparing Figure 13cand 9b that the voids that would be created upon further growth of the phenanthrene trimers would be much more difficult to fill than those in anthracene products because of the angular structure of phenanthrene. Stabilization of a nematic liquid crystal state depends upon a critical packing fraction of the m01ecules.l~This packing fraction is in t u m dependent in the length-tobreadth ratio X of the molecules, where X has been definedl3 by
x = ( L / 2 w )+ 1 The widths W and lengths L of the proposed trimeric
Carbonization of anthracene and phenanthrene involves remarkably different chemistry, reaction mechanisms, and molecular configurations. The main chemical difference appears to be the liquid-phase retention of the hydrogen formed by the polymerization reactions during carbonization of anthracene. FTIR analysis of the whole reaction products and the different solvent-insoluble fractions (HI, TI, and PI) from anthracene showed that smaller molecules are hydrogenated more readily than the larger molecules. Extensive hydrogenation reactions, which increase the fluidity and extend the range of fluidity of the carbonizing materials, coincide with the nucleation and coalescenceof mesophase spheres during anthracene carbonization. In contrast, the initial stages of mesophase formation from phenanthrene are marked by the rapid loss of hydrogen from the phenanthrene products with insignificant extents of hydrogenation reactions. The kinetics of insolubles formation from anthracene and phenanthrene showed that, despite a lower initial reactivity of phenanthrene, phenanthrene moieties can grow rapidly in size after a relatively high activation energy barrier is overcome.' From a chemical viewpoint, therefore, a lack of donor hydrogen in the liquid phase combined with, and partially responsible for, a rapid growth of molecular size restricts mesophase development from phenanthrene. Structural modeling of the carbonization products showed that anthracene produces planar moieties (mesogens) more readily than phenanthrene. The use of Dreiding models with energy minimization along with simple molecular orbital calculations provided an explanation for the different molecular mechanisms involved during carbonization of the two isomers. A proposed mechanism for two-dimensional growth of mesogens from anthracene involves the initial formation oligomers including five-member rings.
Acknowledgment. The authors are grateful for the financial support for T. Sasaki and for supplies and analytical work provided by the Mitsubishi Oil Co. Ltd.
