Energy & Fuels 1991,5,83-87 introduced and used for evaluating the steric requirement of polymeric substances. It is clear that Illinois No. 6 coal recognizes the bulkiness Of penetrant The rate Of penetracan be almost a by changing the alkyl substituent from normal to tertiary. This trend is true of several solvent systems. Illinois No. 6 coal is a molecular sieve which fractionated alkylbenzene derivatives. We thank Professor John Larsen for useful discussions and warm-hearted encouragement. We are also grateful for financial Support from the Japanese Ministry of Education through the Environmental Science Institute of Kinki University, and from
w*
83
the U.S. Department of Energy through the Ames Laboratory under Contract No. W-7405-ENG-82. Registry No. Benzene, 71-43-2;toluene, 108-88-3;o-xylene, 95-47-6; m-xylene, 108-38-3;p-xylene, 106-42-3; ethylbenzene, 100-41-4; propylbenzene, 103-66-1; i-propylbenzene, 98-82-8; 1,3,5-trimethylbenzene,108-67-8; methanol, 67-56-1; ethanol, 64-17-5; I-propanol,71-23-8; 1-butanol,71-36-3; n-amyl alcohol, 71-41-0; n-propylamine,107-10-8; butylamine, 109-73-9;hexylamine, 111-26-2;dipropylamine, 109-76-2; triethylamine, 121-44-8; aniline, 62-53-3;NImethylaniline, 100-61-8;NJV-dimethylaniline, 121-69-7; .pyridine, 110-86-1; 4-methylpyridine, 108-89-4; 2methylpplhe, 109-06-8;ko-butylamine, 7881-9;sec-butylamine, 13952-84-6; tert-butylamine, 75-64-9; n-butyl methyl ketone, 591-78-6: iso-butvl methvl ketone. 108-10-1: sec-butvl methvl ketone, 565-61-7-tert-b$yl methyl ketone, 75-97-8. "
Modeling Coal Liquefaction. 1. Decomposition of 4 41-Naphthylmethy1)bibenzylCatalyzed by Carbon Black? Malvina Farcasiu* and Charlene Smith* Pittsburgh Energy Technology Center, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236 Received July 9, 1990. Revised Manuscript Received September 10, 1990
The cleavage of methylene and ethylene bridges between aromatic moieties is a reaction relevant to coal liquefaction. To model this reaction, the decomposition of a polyfunctional organic molecule, 4-(l-naphthylmethyl)bibenzyl,I, was studied under thermal and catalytic reaction conditions. The thermal reaction of compound I proceeds only at temperatures above 400 "C. In the presence of high surface area carbon blacks, however, bond cleavage takes place at temperatures as low as 320 "C. The presence of carbon black results in not only higher overall conversion but extremely selective cleavage of a specific bond of compound I. Certain kinetic parameters for thermal and catalytic cleavage reactions of I were determined. Preliminary mechanistic studies indicate that an electron-transfer pathway may be operative in the carbon black catalyzed reactions of compound I.
Introduction The cleavage of covalent bonds is one of the essential reactions occurring during coal liquefaction. Thermal cleavage requires temperatures so high that other, mostly undesirable, reactions take place. This results in little, if any, selectivity in the cleavage process. However, specific bonds can be cleaved catalytically, at temperatures where thermal reactions are not important. Because of the complexity of coal structure, it is not possible to investigate directly and to optimize conditions for cleaving specific bonds, or to sort out desired processes from side reactions. In order to obtain meaningful data relevant to coal liquefaction and to be able to develop proper catalysts for the process, the study of model compounds containing the critical structural elements of coal in simpler structures is necessary. The study of appropriate model compounds allows determinations of kinetic parameters for reactions of specific bonds (rates, activation energies) that can be applicable to the cleavage of these 'Published in part in Prepr. Pap.-Am. Chem. SOC.Div. Fuel Chem. 1990,35,404. Oak Ridge Associated Universities Appointee, Postgraduate Research Training Program.
*
0887-0624/91/2505-0083$02.50/0
bonds in coal and/or coal liquids. To be relevant to coal liquefaction under heterogeneous catalysis, a study of model compounds should satisfy a number of requirements concerning the structure of the model compound, and the type of data to be gathered. In our opinion, some of the important criteria are the following: 1. The model compound should be liquid or solid under the reaction conditions to mimic the conditions prevalent during coal liquefaction. This usually means that the model compound should have a molecular weight of at least 300. 2. It is desirable to study model compounds containing several types of potentially reactive structural units. The presence of different reactive structures in the same molecule permits the study of competitive kinetics in the presence of intramolecular interactions. Because competitive reactions certainly occur in coal processing, a study of model compounds with only one type of bond that cleaves affords limited information. 3. The products of reactions should be unambiguously identified and their rate of formation determined. Determination of rates (both absolute and relative) and activation energies of different reactions is essential for the 0 1991 American Chemical Society
Farcasiu and Smith
84 Energy & Fuels, Vol. 5, No. 1, 1991
study of the mechanisms of reactions. Relative reaction rates and activation energies for cleavage of specific bonds is necessary information that cannot be obtained directly from the study of very complex systems such as coal. In the latter case only an overall conversion of a complex starting material to a complex product mixture can be determined. An attempt at calculating an activation energy for such a transformation gives a number with no physicochemical meaning. In these cases the use of appropriate model compounds is highly desirable. We have designed a family of hydrocarbon compounds that meets the above criteria as being appropriate for coal model compound studies. Each of the molecules contains both a methylene and an ethylene bridge as well as monocyclic and bicyclic aromatic units. We report the results of studies pertaining to one of our model compounds, 4(1-naphthylmethyl)bibenzyl,I. The chemistry of this compound was studied under thermal and catalytic reaction conditions. Because of the polyfunctional structure of I it was possible to determine relative reactivities of the different structural units in the presence of intramolecular interactions. A novel catalyst was employed (carbon black, Cabot Corp. Black Pearls 2000) for the catalytic reactions reported in this work. Carbon blacks are often used as supports for catalysts' and as molecular sieves.2 I t is perhaps less well-known that carbon black can have some intrinsic catalytic activity itself. For example, it has been reported that, after activation with ammonia, carbon black is a good oxidation catalysta3 Activated carbon also has been reported to catalyze the cracking of pure hydroc a r b o n ~ . ~ The mechanisms of these "carbon black catalyzed" reactions remain speculative.
Experimental Section Materials and Analytical Procedures. 9,lO-Dihydrophenanthrene (9,lO-DHP) was obtained from Aldrich Chemical Co.; Black Pearls 2000 carbon black was obtained from Cabot Corp. Elemental analysis of the carbon black, performed by Huffman Laboratories, Golden, CO, gave (%) C, 95.4; 0, 1.4; S, 1.8; ash, 1.3. Surface area (BET) quoted by Cabot is 1475 m2/g. 4(1-Naphthylmethyl)bibenzyl,I, was prepared in the laboratory of Prof. Paul Dowd a t the University of Pittsburgh. The compound was completely characterized by IR, 'H NMR, 13C NMR, elemental analysis and high- and low-resolution mass spectroscopy. The purity of the product was better than 99%. Spectral data of analytically pure compound I: mp 64.5-65 OC; 'H NMR (CDC13): chemical shift in ppm (s = singlet; d = doublet; t = triplet; q = quartet, m = multiplet; coupling in Hz) 7.98 (t,1 H, J = 5.32); 7.83 (t, 1 H, J = 4.2); 7.73 (d, 1 H, J = 8.20); 7.43 (m, 2 H); 7.38 (d, 1 H, J = 7.96); 7.24 (dt, 3 H, J = 7.47); 7.16 (dt, 3 H, J = 7.36); 7.08 (q, 4 H, J = 7.16);4.39 (s, 2H, Ar'CH2Ar); 2.85 (e, 4 H ArCH2CH2Ar). 13C NMR (CDC13): 140.9 (9); 139.53 (e); 138.22 (9); 136.93 (8); 134.16 (s); 132.12(9); 129.46 (d, J = 155.7); 129.38 (d, J = 155.5); 129.17 (d, J = 154.0);129.03 (d, J = 158.1); 128.97 (d, J = 158); 128.17 (d, J = 160.1); 127.93 (d, J = 161.2); 127.71 (d, J = 158.1);126.65 (d, J = 159.1);126.25 (d, J = 156.3); 126.20 (d, J = 158); 125.38 (d, J = 158); 123.77 (d, J = 157.2); 39.75 (t, J = 154.2);38.72 (t,J = 153.1) IR (KBr): u,, 3080, 3021, 2982,2963,1610,1432,1111,890,870,780. Low-resolution MS: m / z 322 M + (intensity) (33), 231 (loo),213 (12), 212 (22), 202 (81, 135 (9), 91 (13), 79 (9), 58 (8). The above substances were (1) Kaminsky, M.; Yoon, K. J.; Geoffroy, G. L.; Vannice, M. A . 2 SOC. 1989, I I I , 2377 and references therein. (2) Domingo-Garcia, M.; Fernandez-Morales, I.; Lopex-Garcia, F. J.; Moreno-Castilla, C.; Prados-Ramirez, M. J. J. Colloid Interface Sci. 1990, 136, 160 and references therein. (3) Boehm, H. P. Structure and Reactivity of Surfaces; Morterra, C., Zecchina, A., Costae, G. Eds; Elsevier Science: Amsterdam, 1989. (4) Greensfelder, B. S.; Voge, H. H.; Good, G. M. Ind. Eng. Chem. 1949, 41, 2573. CataZ. 1985, 91, 388. Venter, J. J.; Vannice, M. A. J. Am. Chem.
b
a
C
d
e
Figure 1. 4-(1-Naphthylmethyl)bibenzyl (I). Letters a-e denote potential sites of bond cleavage. Table I. Products of Bond Cleavage Reactions of ComDound I bond cleaved observed Droducts
e
w w
used as received. Dichloromethane was stored over 4A molecular sieves. Glass reaction tubes were made from Pyrex tubing, 5 X 7 mm (i.d. X 0.d.). Sealed sample tubes were approximately 75 mm in length. Gas chromatographic analyses were carried out on a Hewlett Packard Model 5730A gas chromatograph equipped with an SE30 60-m column. Gas chromatography/mass spectra (GC/MS) were obtained on a Hewlett Packard GC/MS Model 5985 instrument operated using 70 eV impact voltage and equipped with a 30-m SE-52 column. Identification of reaction products was accomplished by GC/MS analysis and, when possible, by GC comparison with an authentic chemical sample. Reported product yields and overall conversion of I are based on capillary GC measurements using an external standard. The most volatile material, toluene, could not be determined accurately, but the quantity was always found to be close to that of (naphthyltoly1)methane. The amount of toluene reported was set equal to the amount of (naphthyltoly1)methane found (molar basis). General Experimental Procedure. The reaction components (9,10-DHP, ca. 100 mg; I, ca. 25 mg; and Black Pearls 2000, 0, 2,5, 10 wt %, based on I) were weighed into open-ended glass reaction tubes. The tubes were flame .sealed, taking no precaution to exclude air. Warm water was used to melt the hydrogen donor and effect the mixing of the reactants. The samples were placed upright in a temperature equilibrated Lundberg muffle furnace and heated at the indicated temperatures for the given times. The samples were removed from the oven, cooled to room temperature, and diluted with ca. 0.5 mL of dichloromethane. The samples were filtered through a plug of MgSO., and glass wool. An additional 0.5 mL of dichloromethane was used to wash the filter and, in the catalytic reaction, the carbon black catalyst. An aliquot of the resulting solution was analyzed by gas chromatography.
Results For convenience of discussion, the potential sites of bond cleavage of I are labeled by the letters a-e (Figure 1). The products that can be derived from the cleavage of these
Energy & Fuels, Vol. 5, No. 1, 1991 85
Modeling Coal Liquefaction Table 11. Influence of Temperature on Overall Conversion of ComDound In conversion at X% catalyst 2 5 10 tema O C 0 320 375 400 429
0 0 3.0 28.7
0.8 9.1 17.6 53.8
16.2 33.6 75.9
3.9 27.0 43.6 87.2
Reaction conditions: sealed tube; 1 h; weight ratio 9,lO-DHP:I 4 1 ; catalyst weight based on I. Table 111. Selectivity of Bond Breaking of Compound I at X % Black Pearls 2000 Catalysta % of total product at X% catalyst bond broken 0 2 5 10 82 87 90 a 40 b 4.2 5 5 5 d 56 13 8 5
.
nReaction conditions: sealed tube; 1 h; 419 "C;weight ratio 9,lO-DHP:I 4:l; catalyst wt % based on I.
bonds are easily predicted. Thus, cleavage of the bond between the naphthyl unit and methylene linkage (bond a) would produce naphthalene and 4-methylbibenzyl. In practice, along with the expected products from the direct cleavage reactions of bonds a-e, we also observe formation of some secondary reaction products (Table I). Tetrahydronaphthalene is the secondary product resulting from a subsequent hydrogen transfer to naphthalene; p-xylene is the product of further cleavage of bond a from (naphthyltoly1)methane. The reaction products indicated in Table I are formed in varying amounts related to the overall conversion of compound I. Under the reaction conditions described in this paper, 9,lCbdihydrophenanthrene also undergoes reactions to give, by cleavage of the 9,lO carbon-carbon bond, o,o'-dimethylbiphenyl, and by aromatization, phenanthrene. These products were identified as described in the Experimental Section. Mass balances were in excess of 94%. Results of elemental analyses on recovered carbon black catalysts indicate that there is no detectable adsorption on carbon black of I or its reaction products. Reactivity and Specific Cleavage of the CarbonCarbon Bonds of I in the Presence of OJO-DHP, Catalyzed by Black Pearls 2000. Reactions of I, 9,lODHP, and Black Pearls 2000 (0-10 w t %) were conducted over a temperature range of 320-430 "C in order to assess the influence of reaction temperature on conversion of I. As indicated in Table 11, 4-(l-naphthylmethyl)bibenzyl undergoes no thermal reaction (0% catalyst) at temperatures lower than 400 "C. This is in contrast to the reactivity that is observed in the presence of Black Pearls 2000 where there is low but significant conversion even at 320 "C (3.9% conversion, 1 h, 10% Black Pearls 2000). The overall conversion of I increases with increasing reaction temperature. The overall conversion of I was found also to increase with higher catalyst loading. However, it was determined that the reaction system is overloaded at 10% catalyst and that addition of only 8% Black Pearls 2000 catalyst gives comparable conversions. Aside from the increased overall conversion of I in the presence of Black Pearls 2000, there is a dramatic change in the selectivity of bond cleavage, relative to the thermal reaction. Representative results obtained from reactions performed at 419 "C are collected in Table 111. The ihermal reaction displays only slight selectivity, with the cleavage of the bibenzylic linkage, bond d, being marginally
Table IV. Reaction Rate Constants (k)and Activation Energies - (E.) . _. for Thermal Cleavage of Bonds a and d of Compound 1; temp, O C k, X lo-', min-' kd X lo4, min-' 400 -2.4 -2.7 408 5.2 6.1 419 12.5 16.3 429 18.3 26.7 &(bond a) (kcal/mol) -60; En(bond d) (kcal/mol) -70
Reaction conditions: sealed tube; 1 h; weight ratio 9,lO-DHP:I 4:l.
favored over that of bond a. The catalytic reactions show a strong preference for cleaving the bond between the carbon atom of the polycyclic naphthyl unit and the methylene carbon atom adjacent to it (bond a). The results reported in Table I11 include both the catalytic and the thermal contributions to the reaction. When the product compositions are determined after omitting the thermal component, the catalytic reaction is found to be practically 100% selective for bond a cleavage. The cleavage of the bond between the naphthyl and methylene units of model compound I is essentially the only reaction taking place under catalytic conditions at all temperatures studied. Appropriate control experiments show that the cleavage reaction is specific for the carbon-carbon bond between a polycyclic condensed aromatic unit and the adjacent aliphatic carbon. In contrast, the reactions of the monocyclic compounds bibenzyl and diphenylmethane with BP2000 under the reaction conditions described here do not result in the cleavage of the bond between the phenyl ring and the adjacent aliphatic carbon. However, the reaction of 1,2-dinaphthylethane, under the same catalytic reaction conditions yields naphthalene and ethylnaphthalene, indicating specific cleavage of the bond between the polycyclic naphthyl unit and adjacent ethylene substituent. Results with other substituted naphthalenes and benzene compounds further confirm these finding^.^ Although the details are beyond the scope of this text, it is important to mention that the use of other high surface area carbon blacks also results in selective cleavage of bond a. Under the same reaction conditions, graphite does not exhibit any catalytic activity. Determination of Kinetic Parameters Associated with the Reactivity of I under Thermal and Catalytic Reaction Conditions. Reaction rate constants for the thermal and catalytic reactions of I were calculated by plotting the kinetic data, which fit first-order behavior. Apparent activation energies for the cleavage of bonds a and d were also determined. Table IV presents the calculated kinetic parameters for the thermal reaction of I. The reaction rate constants for both cleavage of bonds a and d increase as the reaction temperature is increased, with the rate of cleavage of bond d being faster than that of a. Further, the thermal cleavage of bond d is more sensitive to temperature than is the thermal cleavage of a, and it is this reaction that predominates at higher temperatures. Table V gives the calculated kinetic parameters for the only reaction of I under catalytic conditions, the cleavage of bond a. Reaction rate constants are generally increasing with increasing reaction temperature and catalyst concentration. An apparent anomaly is seen where the calculated reaction rate constants for the 10% catalyst reactions reach a maximum at 419 "C and then fall slightly ( 5 ) Farcasiu, M.;Smith, C.; to
be submitted for publication.
Farcasiu and Smith
86 Energy & Fuels, Vol.5, No. 1, 1991 Table V. Reaction Rate Constants (k)and Activation Enemies (E.) for Catalytic Cleavage of Bond a of Compound i at X% Black Pearls 2000 Catalyst' k x IO-', min-1 at X% catalyst* temp, O C 2 5 10 360 7.9 27.1 43.6 400 26.3 60.9 86.8 92.2 408 419 47.7 100.2 149.4 429 48.2 116.2 146.6
E,, kcal/mol
-25
-
19
-
17
OReaction conditions: sealed tube; 1 h; weight ration 9,lODHP:I 41;w t % catalyst based on I. bReported for catalytic contribution only. Thermal background has been subtracted.
at 429 "C. This deviation is most likely related to the high catalyst loading and high reaction temperature and not the real kinetics of the system. The apparent activation energy calculated for the catalytic cleavage of bond a is reduced by a factor of at least 2.4 relative to the thermal case. The deviation in E, with respect to catalyst concentration may indicate that diffusion is a factor under the heterogeneous reaction conditions used.6 Reactivity and Specific Cleavage of I Catalyzed by Black Pearls 2000 in the Absence of 9,lO-DHP. The catalytic reaction, in the absence of 9,10-DHP, shows the same remarkable selectivity toward bond a cleavage. In the absence of a hydrogen donor, compounds heavier than I are formed. This indicates that I itself is acting as the hydrogen source by dehydrogenation and condensation reactions. Besides the heavier compounds, we also observe the formation of some methyldihydrophenanthrene, formed presumably by cyclization of 4-methylbibenzyl.
Discussion A high surface area carbon black (Black Pearls 2000) was found to be an active and exceptionally selective catalyst for specific bond cleavage of the polyfunctional hydrocarbon compound, I. The catalytic reaction is extremely selective for breaking the bond between the polycyclic naphthyl unit and the methylene linkage of I. This selectivity is displayed over the temperature range from 320 to 430 "C and loading concentration range of 2-10 wt YO. Preliminary Investigations of the Mechanism of Action of Black Pearls 2000 with I. Under thermal conditions, in the presence and the absence of an H donor, bonds a and d of compound I, cleave to a similar extent. The thermal reaction of I most likely involves free-radical intermediates,' formed by the initial homolytic cleavage of bond d, which can then abstract hydrogen from reactant, products, or added H donors.8 The free radicals formed in the second step can also donate hydrogen atoms from 0-positions to other free radicals (disproportionation), or to aromatic rings to form cyclohexadienyl free radicals. The homolytic mechanism results in cleavage of bonds a and d to similar extent. By contrast, a mechanism through which bond a is selectively cleaved operates in the presence of carbon black. This mechanism involves a one-elec€ron-transfer reaction as the initial step. The rationale for (6) Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis;MI" Press: Cambridge, MA, 1970. (7) Whitehurst, D.D.; Mitchell, T. 0.; Farcasiu, M. Coal Liquefaction-The Chemistry and Technology of Thermal Processes; Academic Press: New York, 1980. (8)McMillen, D.F.; Malhotra, R.; Chang, S. J.; Ogler, S.; Nigenda, S. E.; Fleming, R. H. Fuel 1987,66,1611.McMillen, D.F.;Malhotra, R.; Hum, G. P.; Chang, S. J. Energy Fuels 1987,I , 193.
our conclusion is outlined in the following discussion. We believe that a simple carbonium ion mechanism is unlikely to be responsible for the activity and selectivity that is associated with Black Pearls 2000. Under welldefined carbonium ion reaction conditions using a strong acid catalyst (trifluoromethanesulfonicacid), we found that compound I does undergo cleavage of bond a but the soformed 4-methylbibenzyl product entrs very quickly into the transalkylation reaction, donating the methyl group to phenanthrene. Conversely,the use of a weaker acid such as trifluoroacetic acid does not result in any reaction of compound I.9 It appears that a substance with acidic functionality that is strong enough to cleave bond a of I must also be acidic enough to induce transalkylation. As no evidence of transalkylation reactions is observed in the case of reactions of compound I with Black Pearls 2000, we conclude that the carbon black catalyst is not acting as an acid catalyst in this reaction. As was mentioned earlier, there is no thermal reaction of compound I at temperatures lower than 400 "C. However, in the presence of carbon black, the catalytic cleavage of bond a is observed at temperatures as low as 320 "C. A well-known, yet poorly understood, phenomenon is that many solid catalysts become active at a certain characteristic temperature and are inactive below this temperature.1° A new technique, charge distribution analysis (CDA), developed by Freund" may help to explain the catalyst-temperature relationship. Charge distribution analysis measures mobile charges in solid materials and allows determination of the mobility of these charges and their sign. It is also possible to estimate the surface charge carrier densities. When Black Pearls 2000 alone is analyzed by CDA it is found that the surface of the catalyst becomes positively charged at 320 "C. This finding is interesting in that we observe measurable conversion of compound I at 320 "C (3.9% conversion, 1h reaction time) only in the presence of catalyst Black Pearls 2000 (Table 11). There is no thermal reaction of I at this relatively low reaction temperature. The CDA results for Black Pearls 2000 may have some mechanistic implication and could help to explain both the increased conversion of I in the presence of Black Pearls 2000 and the selectivity of the process. A possible explanation for the action of Black Pearls 2000 could be that, with the surface of Black Pearls 2000 being positively charged at the reaction temperature, the aromatic compound I in contact with it gives up an electron, forming a cation radical species. This now one electron deficient derivative of compound I then undergoes specific bond cleavage reactions that lead to the observed product distribution. It is reasonable to assume, based on oxidation half-wave potentials,12 that the naphthyl moiety of I would be the structural unit most susceptible to one-electron oxidation. The ion radical of compound I, with the charge localized on the naphthyl unit, is then sufficiently activated so that selective cleavage of the adjacent bond takes place. The removal of an electron from a molecule to form a cation radical species is known to lead to accelerated rates of bond fragmentati~n.'~ (9)Farcasiu, M.; Smith, C., unpublished results. (IO) Samorjai, G. A. Diu.Colloid Surf. Chem., 199th Natl. Mtg. Am. Chem. SOC.,Boston, M A 1990,Abstract No. 52. (11)(a) Freund, M. M.; Freund, F.; Batllo, F. Phys. Reo. Lett. 1989, 63, 2096. (b) Freund, F. Personal communication. (12)Pysh, E.S.;Yang, E. C. J.Am. Chem. SOC.1963,85, 2124. (13) Maslak, P.; Asel, S. L. J . Am. Chem. SOC. 1988,110, 8260 and
references therein.
Energy & Fuels 1991,5, 87-92 Preliminary theoretical calculations applying extended Huckel molecular orbital calculations (EHMO) performed by Subbaswamy" indicate that cation radical species are plausible intermediates in the selective cleavage of bond a in I under carbon black catalyzed reaction conditions. EHMO calculations for compound I indicate that, as expected, bond d has the lowest strength.14 In a thermal process without an initiator, breaking of this bond should, most probably, initiate further reactions. Using EHMO, it is possible to mimic the oxidation described above and to remove "mathematically" one electron from I. In keeping with the known values for oxidation potentials of naphthalenic and benzene compounds,12removal of the electron would be done preferentially from the naphthyl unit. Relative bond strengths of the ion radical of I with the charge centered on the naphthyl moiety can be computed. When the calculations are performed in this way, it is seen that the bond between the naphthyl unit and the methylene linkage is the weakest in the ion radical and so (14) Subbaswamy, K. R. University of Kentucky, personal communication.
87
the most susceptible to breaking. Work is in progress aimed at elucidating the mechanism of action of this carbon black as well as other carbon blacks or carbon-derived materiah6 We are also studying the reactivity of other related model compounds within the same family of I.
Acknowledgment. We thank Prof. Paul Dowd and his group at the University of Pittsburgh for the synthesis and characterization of compound I, Louise Douglas for GC/ MS work, Prof. K. R. Subbaswamy for EHMO calculations, Prof. F. Freund for CDA measurements, and Prof. Dan Farcasiu for helpful discussions. This work was supported in part by an appointment to the Postgraduate Research Training Program under Contract NO. DEAC05-760R00033 between the US. Department of Energy and Oak Ridge Associated Universities. Reference in the paper to any specific commercial project, process, or service is to facilitate understanding and does not necessarily imply ita endorsement or favoring by the United States Department of Energy. Registry No. I, 127833-53-8.
129XeNMR Investigation of Coal Micropores Chihji Tsiao and Robert E. Botto* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received June 22, 1990. Revised Manuscript Received July 30, 1990
The microporous structures of three Argonne Premium coals, and a weathered sample and three oxidized samples of Illinois No. 6 coal (APCS No. 31, have been investigated by "Xe NMR spectroscopy. An analytical model has been developed which approximates the average pore sizes and pore swelling characteristics for the coals according to changes in lBXe chemical shifts as a function of xenon pressure. Pore regions can be described which differ in size (-6 and -10 A in diameter) or chemical composition. The effects of weathering and oxidation at elevated temperatures on the pore size and swelling of Illinois No. 6 coal have also been examined.
Introduction Coals are generally thought to be cross-linked, threedimensional macromolecular networks whose main building blocks are aromatic and hydroaromatic units connected and by predominately methylene (-CHz-), oxygen (-0-), sulfur (-S-) cross-links.' The irregular arrangement of these building units in three dimensions is responsible for the extensive pore structures. In general, three pore-size regimes exist in coals: macropores with diameters of more than 500 A, mesopores with diameters in the range of 20-500 A, and micropores with diameters less than 20 A. Porosity in coal is of great significance because of its influence on coal behavior during mining, preparation, and utilization pr0cesses.V For these reasons, many analytical means have been employed during the past few decades in an attempt to characterize coal porosity, pore volume, surface area, and pore-size distribution. The subject has been reviewed extensively by Mahajan4 and Marsh.6 *Author to whom correspondence should be addressed.
Recently, Larsen and Wernett6 have studied the adsorption (BET) of COz and a series of aliphatic hydrocarbons on Illinois No. 6 coal. These authors suggested that many pores are closed to the external surface and to reach them an adsorbate must diffuse through solid coal, rather than through the pore network. Bartholomew et al.' determined surface areas of three Argonne Premium (1) Larsen, J. W.; Kovac, J. In Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symposium Series No. 71; American Chemical Society: Washington, DC, 1978; pp 34-49. (2) Mahajan, 0. P.; Walker, P. L., Jr. In Analytical Methods for Coal and Coal Products; Karr, Jr., C., Ed.:Academic Press: New York. 1978 Vol. 2, pp 125-162. (3) Mahajan, 0. P.; Walker, P. L., Jr. In Analytical Methods for Coal and Coal Products; Karr, Jr., C., Ed.; Academic Prees: New York, 1978; Vol. 2, pp 465-494. (4) Mahaian. 0. P. In Coal Structure: Mevers. R. A... Ed.: . Academic Press: New-York, 1982; pp 51-87. (5) Marsh, H. Carbon 1987, 25, 49-68. (6)Larsen, J. W.; Wernett, P. Energy Fuels 1988, 2, 719-720. (7) Bartholomew, C. H.; White, W. E.; Thornock, D.; Wells, W. F.; Hecker, W. C.; Smwt, L. D.; Smith, D. M.; Williams, F. L. Prepr. Pap. Am. Chem. Soc., Diu. Fuel Chem. 1988, 33, 24-31.
0 1991 American Chemical Society Q88~-Q62~/91/25Q5-QQ87$02.5Q~Q
