Hydrogen-transfer ability of extrographic fractions of coal-tar pitch

Fuel Processing Technology 69 Ž2001. 107–126 www.elsevier.comrlocaterfuproc

Hydrogen-transfer ability of extrographic fractions of coal-tar pitch J. Machnikowski a,) , H. Kaczmarska a , A. Leszczynska ´ a, b ´ P. Rutkowski a , M.A. Dıez , R. Garcıa ´ b, R. Alvarez ´ b a

Institute of Chemistry and Technology of Petroleum and Coal, Wrocław UniÕersity of Technology, Gdanska ´ 7r 9, 50-344 Wrocław, Poland b Instituto Nacional del Carbon ´ (INCAR), CSIC, Apartado 73, 33080 OÕiedo, Spain Received 21 June 2000; received in revised form 20 October 2000; accepted 27 October 2000

Abstract Coal-tar pitch was fractionated using extrography into classes of compounds of similar functionality and molecular weight. Hydrogen acceptor and donor abilities of the whole pitch and its extrographic fractions were evaluated by reaction at 3608C with tetralin and anthracene, respectively, and related to structural characteristics by elemental analysis, VPO, 1 H NMR and HPLC. Hydrogen acceptor ability ŽHAA. of the fractions increases markedly with the elution depth. There is a good correlation between HAA and the total oxygen content in the fraction. On the other hand, all extrographic fractions show comparable relatively low hydrogen donor abilities ŽHDA. in the initial period of reaction with anthracene due to a similar content of hydroaromatic rings and methylene bridges. On further treatment, F2 and F4 are differentiated by a stronger HDA than other fractions. Among the fractions, only in F2 the hydrogen donor ability outweighs over hydrogen acceptor ability. The opposite situation is observed for F5, F6 and F7. In the case of F3 and F4, a comparable amount of hydrogen is transferred from tetralin to fraction and from fraction to anthracene. There is no straight correlation between the observed hydrogen transfer behaviour of fraction and its ability to develop an anisotropic texture in carbonization product. Possible contribution of various fractions to the creation of optical texture of pitch coke is discussed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Coal-tar pitch; Extrography; Hydrogen transfer

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Corresponding author.

0378-3820r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 0 0 . 0 0 1 3 9 - 9

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1. Introduction Most of the current applications of commercial coal-tar and petroleum pitches include thermal processing leading to graphitizable carbons of developed optically anisotropic texture w1x. Dehydrogenative polymerization of pitch constituents occurring in the early stage of the treatment produces mesogenic molecules which are, in turn, able to rearrange into a liquid crystalline system of carbonaceous mesophase w2–4x. Hydrogen transfer is one of the main chemical reactions involved in the formation of the mesogens. Hence, the evaluation of pitches in terms of ability to donate and accept hydrogen atoms is a relevant characteristic. Pitches are extremely complex mixtures of organic compounds. Zander w5,6x as main constituents of typical coal-tar pitch lists polycyclic aromatic hydrocarbons ŽPAHs., structurally related heteroaromatic systems which are accompanied to a different extent by alkylated PAHs, PAHs with cyclopenteno moieties, partially hydrogenated PAHs, oligo-aryls and oligo-aryl methanes, hetero-substituted PAHs and carbonyl derivatives of PAHs. Due to its compositional complexity, pitch displays a dual nature in terms of hydrogen transfer ability. In the presence of hydrogen acceptor compounds it behaves as a hydrogen donor, when treated with hydrogen donor agents is a hydrogen acceptor. Hydrogen donor ability of pitch is primarily attributed to the presence of compounds with hydroaromatic and naphthenic rings w7x, but also methylene and ethylene bridges linking two aromatic rings can be a source of transferable hydrogen w8x. As primary hydrogen acceptor sites Yokono et al. w9x consider thermally induced reactive free radicals and oxygenated functional groups. Furthermore, transferable hydrogen reacts with aromatic carbon atoms with high energy density. Hydrogen-acceptor properties of heteroaromatic and hetero-substituted compounds depend on their reactivity under reaction conditions. Ketones, quinones and some ethers are readily hydrogenated in contrast to phenols, carboxylic acids and furan, pyrrol and pyridine derivatives which are inactive w10–12x. The unsubstituted aromatic compounds show different hydrogen acceptor ability. Acenaphthylene type compounds most readily accept hydrogen atoms due to the presence of an olefinic bond w13x. Kidena et al. w8x reported a distinct decrease in the amount of hydrogen transferred from coal to hydrocarbons in the range: naphthacene) anthracene) pyrene. Naphthacene was able to abstract hydrogen from methylene bridges with a lower activity than anthracene. Such a behaviour is predictable based on molecular orbital calculations. In the case of 9-methylanthracene, in addition to the expected production of 9,10-dihydro-9-methylanthracene, the formation of anthracene was observed suggesting that transferable hydrogen can accelerate the fission of aryl–alkyl bond w8x. The reaction with anthracene as a hydrogen acceptor and 9,10-dihydroanthracene or tetralin as donor agents carried out within the temperature range 380–4008C was able to clearly differentiate pitches in terms of hydrogen transfer ability w14–18x. The concept of the occurrence of hydrogen transfer reactions in carbonization and its relevance were advanced by Marsh and Neavel w19x. In this sense, the hydrogen transfer properties of pitch were studied extensively to evaluate its suitability as an additive modifying coking

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properties of coal blends w9,13–15,20,21x. Recently, good correlations between hydrogen donor ability and development of fluidity during carbonization of coal or blends with additives have been reported w8,22–25x. However, in many cases, chemical characteristics of pitch as a whole, including hydrogen and heteroatoms contents and hydrogen atom distribution appear to be not fully consistent with the observed hydrogen transfer behaviour. Likely, the extreme compositional complexity of pitch can account for this fact. It has been, therefore, anticipated that evaluation of hydrogen donorracceptor abilities of pitch fractions and the correlation with their structural characteristics can be helpful in understanding the behaviour of the pitch as a whole. Yokono et al. w9x reported on essential differences in hydrogen donor and acceptor abilities of solvent fractions separated from coal-tar pitches. Pyridine insolubles showed the lowest values of both hydrogen donor and acceptor abilities. In contrast, hexane solubles freed from polar compounds showed the highest hydrogen donor ability. Recently, extrography has received a considerable attention as a method of fractionation pitches into classes of compounds of similar functionality and molecular weight w26–28x. Chemical constitution of the fractions produced using different modes of extrography has been well defined w26–31x. Substantial differences in the thermal behaviour including kinetics and mechanism of carbonisation and the optical texture of resultant carbons are reported among the various fractions as well as the corresponding fractions derived from different origin pitches w28,31–33x. The relevance of hydrogen donor properties and dilution effect of light extrographic fraction has been discussed w26x. The present work aims to widen the knowledge of hydrogen transfer reactions in pitches and their relationship with the pitch composition. A coal-tar pitch is fractionated by extrography. The hydrogen acceptor and donor abilities of the resultant fractions are assessed in reaction with tetralin and anthracene, respectively, and discussed in relation to their structural characteristics. Elemental analysis, vapor pressure osmometry, 1 H NMR spectroscopy and high-performance liquid chromatography ŽHPLC. are used for the characterization of the extrographic fractions. 2. Experimental section 2.1. Material used The coal-tar pitch used in this study was prepared by distillation of a commercial coal-tar in a continuous bench scale unit. The selected parent tar from Makoszowy coking plant in Poland has been commonly classified as heavy and aromatic one, due to its high density Ž1.19 grcm3 ., toluene and quinoline insoluble contents Ž11.8% and 7.1%, respectively. and its relatively low value of HrC atomic ratio. 2.2. Extrographic fractionation Pitch was fractionated into families of compounds of different functionality and molecular mass by extrography, according to the method described by Granda et al. w27x.

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Briefly, 4 g of pitch Ž- 0.2 mm in particle size., dispersed in dichloromethane, were mixed with 40 g of silica gel which was previously activated. After removing the solvent in a rotary evaporator, the residue was dried and placed in a glass column. At the bottom of the column, additional 20 g of unloaded silica gel was placed to avoid overlapping of fractions. The following sequence of solvents was used for the fractions elution: F1 eluted with n-hexane Ž150 ml.; F2 with n-hexanerbenzene Ž64:36 vrv; 220 ml.; F3 with chloroform Ž225 ml.; F4 with chloroformrdiethyl ether Ž95:5 vrv; 300 ml.; F5 with chloroformrethanol Ž93:7 vrv, 325 ml. and F6 with pyridine Ž325 ml.. An additional fraction F7 was obtained by Soxhlet extraction of the material retained on the silica gel with 300 ml of pyridine. Pitch and its extrographic fractions were characterized following the methodology summarized in Fig. 1. 2.3. Characterization of extrographic fractions Elemental composition was analyzed by a microanalyzer LECO CHNS 932 and oxygen directly using a LECO VTF 900 equipment. The number average molecular masses of fractions Ž Mn . were determined by vapor pressure osmometry ŽVPO. in a Knauer apparatus at 408C using CHCl 3 as solvent for fractions F2–F4 and at 608C using pyridine for fractions F6 and F7. Results of measurements for solutions with different concentrations were extrapolated to infinite dilution.

Fig. 1. Scheme of the methodology applied in the characterization of pitch and its extrographic fractions.

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1

H NMR spectra of extrographic fractions were recorded on a Bruker 300 spectrometer, using deutered chloroform as a solvent for fractions F2–F5 and deutered pyridine in the case of F6 and F7. Tetramethylsilane ŽTMS. was used as an internal standard. HPLC analyses were performed on the extrographic fractions F2, F3, F4 and F5, which were totally solubilized in dichloromethane. The analyses were carried out using a Hewlett-Packard HP1100 system incorporating two PLGel columns Ž300 mm length= 7.5 mm i.d.. packed with polyŽstyrenerdivinylbenzene. copolymer of different nominal ˚ respectively. and connected in series w34–36x. A diode array pore size Ž500 and 100 A, detector operating at a wavelength of 254 nm was used. The 254-nm wavelength showed to be appropriate for the different classes of compounds present in pitches w36x. The mobile phase was dichloromethanermethanol Ž9:1 vrv. at a flow rate of 1 mlrmin. Based on previous studies w34–36x, the HPLC method used allows a good separation of cata- and peri-condensed polynuclear aromatic compounds, which are present in pitches. The entire group of cata-condensed compounds can be further divided into three groups: cata1, cata2 and cata3. The chromatogram regions were defined as follows: cata1 Žrange of elution volume-Vr-12-17.9 ml., composed of heteroaromatics and compounds substituted with alkyl, aryl and heteroatomic ŽOH, N, Ar–O–Ar, etc.. groups; cata2 ŽVr s 17.9–19.1 ml., containing alkyl- and aryl-substituted PAHs and hydroaromatic and naphthenic compounds; cata3 ŽVr s 19.1–19.8 ml., consisting of unsubstituted and planar PAHs; peri ŽVr ) 19.8 ml., with two groups of peri-condensed compounds—peri1 ŽVr s 19.8–20.8 ml. and peri2 ŽVr ) 20.8 ml. —with the elution volume increasing with the degree of condensation and molecular weight. The elution volume intervals were established on the basis of a previous work using 80 standard PACs with molecular weight ranging from 78 to 533 atomic mass units and with different functionalities, commonly present in pitches w29x. For the study of the response factors, cata1, cata2, cata3 and peri fractions of a pitch, physically separated in the HPLC system were used. As no significant differences were found from one fraction to another w29x, for a semiquantitative analysis the integration data under the five major elution intervals of the chromatograms were considered. 2.4. Hydrogen acceptorr donor abilities of extrographic fractions To evaluate hydrogen acceptor and donor abilities of the pitch and its fractions, tetralin was used as a hydrogen donor and anthracene as a hydrogen acceptor. Heat treatments were performed at 3608C at a heating rate of 5 K miny1 with different soaking times Ž0.25–8 h.. The reagents, in a ratio 1:1 by weight Ž80 mg in total. were reacted in small sealed Pyrex-glass tubes Žvolume about 120 ml. providing liquid state at reaction temperature. After heating, the reaction products were extracted with n-hexane or n-hexanerdichloromethane. Qualitative and quantitative analyses of the solutions were performed by GC-MS. The hydrogen acceptor ability of pitchrfraction ŽHAA. was monitored by the conversion of tetralin into naphthalene. The parameter HAA was calculated as the amount of hydrogen in milligrams transferred from tetralin to 1 g of pitchrfraction, using Eq. Ž1. w37x: HAA Ž mg Hrg sample. s 4000 Te Xr132 Fr

Ž 1.

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The hydrogen donor ability of pitchrfraction ŽHDA. was monitored by the conversion of anthracene into 9,10-dihydroanthracene ŽDHA. and 1,2,3,4-tetrahydroanthracene ŽTHA.. These two major hydrogenated products, DHA and THA, were identified and evaluated by GC-MS analysis. Fig. 2 shows, as an example, the chromatogram corresponding to the reaction of anthracene and fraction F6 at 3608C and 4-h soak. Compounds with smaller retention time than anthracene Žpeak 3. were identified by GC-MS as 9,10-dihydroanthracene—DHA-Žpeak 1. with the main fragment ions at mrz 76, 89, 152, 165, 178, 179 Žrelative intensity 100. derived from the molecular ion mrz 180; and 1,2,3,4-tetrahydroanthracene—THA-Žpeak 2. with the main fragments at mrz 82, 115, 128, 141, 152, 154, 167, 165 derived from the molecular ion mrz 182. The

Fig. 2. GC-MS chromatogram of the extract from the reaction of fraction F6 with anthracene at 3608C and 4 h of soaking. Ž1. 9,10-dihydroanthracene ŽDHA.; Ž2. 1,2,3,4-tetrahydroanthracene ŽTHA.; Ž3. anthracene; Ž4. methylanthracenes.

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presence of methyl-substituted anthracenes Žpeak 4. with molecular ion at mrz 192 was observed. The parameter HDA was estimated as the amount of hydrogen in milligrams transferred from 1 g of pitchrfraction to anthracene, assuming that 1 mol% of the produced THA is equivalent to 2 mol% of DHA ŽEq. Ž2... HDA Ž mg Hrg sample. s 2000 An Xr178 Fr

Ž 2.

where, Te, An and Fr are the amount in milligrams of tetralin, anthracene and pitchrfraction, respectively, in the reaction mixture; X is the molar fraction of naphthalene and DHA q THA in the reaction products ŽEqs. Ž1. and Ž2., respectively.. Tetralin was selected as a hydrogen donor compound instead of 9,10-dihydroanthracene, commonly used in the study on pitches w9,10,12,16,18,21,23x due to its higher thermal stability. It has been well established that DHA starts to disproportionate to anthracene and THA andror dehydrogenate to anthracene at temperatures as low as 3608C and the reaction is sample-promoted w8,16x. On the contrary, tetralin is considered as practically stable up to 4008C w10–13x. GC-MS analysis of the reaction products of tetralin heated for 10 h under the conditions used in this study did not show any additional compound. The data are the average of at least two experiments. It was assumed that an experiment was reproducible if the data were within 10% of the previous value. At extended soaking times more complex reactions are expected to occur, leading finally to a state close to the equilibrium in hydrogen exchange between fraction constituents and hydrogen acceptorrdonor compound. It has been assumed that the amount of transferred hydrogen characterizes a total ability of a fraction to independent how complex reactions are involved in reaching the state.

3. Results and discussion 3.1. Characteristics of pitch and its extrographic fractions The analytical data of the pitch used in this study are similar to those reported for typical pitches produced by distillation of high temperature coke-oven tars w1x, except for the higher content of heavy constituents determined as insolubles in toluene and quinoline ŽTable 1.. Table 2 presents the distribution of the pitch components among the different extrographic fractions. As expected, the absence of compounds eluted with n-hexane as F1 indicates that no aliphatic compounds are present in the pitch. This is in agreement with previous results of several coal-tar pitches produced from different coal-tars and under different operating conditions w27,32x. When compared with commercial coal-tar binder pitches w27,32x this pitch contains a higher proportion of F2, F4 and F7 and a lower content of F5. Relatively high proportion of F4 Ž15.4 wt.%. is typical of coal-tar pitches distilled under reduced pressure and that of F7 Ž6.7 wt.%. is related to the heavy character of the parent tar. The unrecovered material during fractionation by extrography

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Table 1 Properties of the parent coal-tar pitch SP-Mettler Ž8C. a Ash Žwt.% db. b TI Žwt.%. c QI Žwt.%. d CY Žwt.%. e

76.8 0.24 28.5 10.7 41.6

Elemental analysis (wt.%, ash free basis) C H N S O diff ŽHrC.at

92.31 4.22 1.10 0.38 1.99 0.55

Hydrogen atom distribution (%)f H ar HF Ha HN H bq g

86.8 2.2 10.0 0.0 1.0

a

Softening point determined by Mettler method. Expressed as dry basis. c Toluene insolubles determined by ISO 6376-96 standard procedure. d Quinoline insolubles determined by ISO 6791-81 standard procedure. e Coking yield. f Har: Aromatic hydrogen Ž9-6 ppm.; H F : aliphatic hydrogen in methylene groups a to two aromatic rings –fluorene type-Ž4.5–3.3 ppm.; H a : aliphatic hydrogens in methyl or methylene groups in a-position to an aromatic ring Ž3.3–2.0 ppm.; H N : naphthenic hydrogen Ž2.0–1.4 ppm.; H bqg : aliphatic hydrogens in methyl or methylene groups in b- and g-position to an aromatic ring Ž1.4–1.0 and 1.0–0.5 ppm, respectively.. b

consists of insoluble compounds in organic solvents andror highly polar compounds irreversibly adsorbed on the silica gel. The number average molecular mass Ž Mn . and elemental composition of the extrographic fractions separated from the pitch are given in Table 3. The molecular masses strongly increase with the elution depth from 170 to about 900 Da for F2 and F7, respectively. The exception is higher Mn of F3 than F4. The explanation of the phenomenon is that extrographic separation depends essentially on the polarity of constituent. Therefore, it can happen that neutral pitch constituents eluted in F3 have a higher average molecular weight than strongly polar compounds concentrated in F4. In

Table 2 Distribution of the extrographic fractions of the pitch Fraction

F1

F2

F3

F4

F5

F6

F7

Unrecovered

Yield Žwt.%.

0.0

39.5

17.1

15.2

3.2

9.6

6.7

8.7

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Table 3 Molecular weight and elemental analysis of the extrographic fractions of the pitch Fraction

Mn

C Žwt.% db.

H Žwt.% db.

N Žwt.% db.

S Žwt.% db.

O Žwt.% db. a

ŽHrC.at

ŽNqSqO.r C at

F2 F3 F4 F5 F6 F7

170 520 410 n.d. 700 900

92.93 92.34 89.42 88.55 90.67 90.45

4.94 4.62 4.43 4.83 3.34 3.26

0.16 1.07 2.19 2.26 1.23 1.31

0.33 0.25 0.36 0.39 0.40 0.43

1.64 1.72 3.60 3.97 4.36 4.55

0.64 0.60 0.59 0.65 0.44 0.43

0.016 0.025 0.053 0.057 0.049 0.052

n.d. not determined. a Directly determined.

addition, it has already been reported w32x that F3 of coal-tar pitches is characterized by wide molecular size distribution. The increase in molecular masses is associated with a gradual decrease in hydrogen content from near 5 wt.% ŽF2. to about 3.3 wt.% ŽF7. and an increase in oxygen contribution from 1.6 wt.% ŽF2. to 4.5 wt.% ŽF7.. Nitrogen is concentrated mostly in fractions F4 and F5 Žmore than 2 wt.%. while sulfur is rather homogeneously distributed among fractions. Atomic ratio of heteroatoms ŽN q S q O. to carbon clearly discriminates between F2 and F3 by one side and F4 to F7 by the other, in terms of overall contribution of heteroatoms to the fraction structure. In general, the results are consistent with those reported earlier on the composition of coal-tar pitch fractions separated using the same extrography procedure w27,29–31x. The distribution of hydrogen atoms determined by 1 H NMR shows that F2 and F3 have the highest hydrogen aromaticity, being the proportion of aromatic hydrogen ŽH ar . about 85% of the total ŽTable 4.. In contrast, F5 and F7 are characterized by a lower hydrogen aromaticity Ž70–72% of aromatic hydrogen.. Some contribution of methylene-bridge hydrogen ŽH F . and naphthenic hydrogen ŽH N . to all the extrographic fractions can be observed. The former constitutes 3–4% and the latter, except for F5 and F7, about 1.5% of the total hydrogen. A considerable proportion of aliphatic hydrogen in b- and g-position to the aromatic ring is, in particular, surprising because it suggests the presence of a numerous long-chain ring substituents in F5 and F7. The data are not very

Table 4 Hydrogen atom distribution in the extrographic fractions of the pitcha Fraction

H ar

HF

Ha

HN

Hb

Hg

F2 F3 F4 F5 F6 F7

85.0 84.5 77.0 72.4 78.9 69.6

3.5 2.9 4.1 2.9 3.7 3.7

8.8 8.9 12.6 12.7 8.4 6.4

1.5 1.2 1.5 2.9 1.7 3.1

0.9 2.0 3.4 5.7 5.4 13.0

0.3 0.5 1.4 3.4 1.9 4.3

a

For symbols, see footnote f in Table 1.

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consistent with the analysis of the pitch as a whole ŽTable 1. and there is no reasonable explanation for the discrepancy. Fig. 3 presents HPLC chromatograms of the extrographic fractions F2–F5 of the pitch. The profiles differ from each other with a shift of the major peak to smaller elution volume as the most striking change with increasing number of fraction. Table 5 gives the proportion of the different classes of compounds in the fractions. The data clearly show that cata-condensed compounds prevail in all fractions, in particular in F4 and F5. Fractions F2 and F3 are similar in terms of the total relative proportion of cata and peri constituents, the latter being 35–40% of the fraction. However, they differ significantly when further classification is considered. The major peak in the elution profile of F2 is clearly centered into cata3 region ŽFig. 3., indicating a significant content of unsubstituted cata-condensed PAHs Ž36.2%.. When concentrations of other PACs classes are also considered, the data reveal that this fraction is particularly rich in PAHs eluted as cata2, cata3 and peri Ž94.7%. and it does not contain any significant amount of cata-condensed compounds bearing NH and OH groups Žcata1, 5.3%.. On the contrary, heteroaromatics and hetero-substituted compounds eluted as cata1 are the main cata-condensed components of F3 together with a considerable proportion of alkyl- and aryl-substituted PACs eluted as cata2. Fractions F2 and F3 also differ in the distribution of peri-condensed compounds. As expected, based on the HPLC profile, the majority of peri-condensed constituents of F3 are classified as peri2 corresponding to PACs of high degree of condensation. The presence of such compounds with relatively high molecular weight is in agreement with earlier works, using probe-MS, which revealed the presence in this fraction of a number of PAHs with molecular masses ranging from 302 to 450 w29,38x. F4, when analyzed by HPLC, appears to be the most homogeneous extrographic fraction with 77% of constituents falling into the region of cata1. Exceptionally low contribution of peri-condensed structures Ž- 8%. should be also noted. F5 shows a similar distribution of classes of compounds to F4. The only difference is a higher proportion of peri2 at the expense of cata1. A common feature of fractions F3, F4 and F5 is the low relative proportion of unsubstituted cata-condensed PAHs eluted as cata3 and peri-condensed PAHs with relatively low molecular weight. The HPLC analyses of the extrographic fractions are complementary to those obtained using elemental analysis, VPO and chromatographic and spectroscopic techniques. The above results in the characterization of the extrographic fractions and those reported in the previous studies w27,29–32,38x allow identification the following classes of PACs concentrated in the fractions isolated by extrography: F2—low molecular weight Ž- 300 Da. cata- and peri-condensed PAHs, cata-condensed PAHs substituted in part with alkyl- or aryl-groups and a limited contribution of compounds with furan or thiophene rings. F3—neutral nitrogen heteroaromatics Žcarbazole type. substituted or not and cataand peri-condensed PAHs of higher molecular weight than those of F2, in the case of cata-type a high degree of substitution with alkyl or aryl groups. F4—cata-condensed aza-compounds Žbasic., cata-condensed PAHs substituted with phenolic and different types of carbonyl groups Žketone, quinone. and some contribution of alkyl andror aryl substituted PAHs.

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Fig. 3. HPLC profiles of the extrographic fractions of the coal-tar pitch.

F5—cata-condensed aza-compounds, different types of aza-carboniles and some contribution of peri structures of relatively high degree of condensation; generally, composition similar to that of F4 but higher condensation degree.

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Table 5 Integration data of HPLC regions attributed to different classes of compounds in the extrographic fractions of the pitch Fraction

cata1

cata2

cata3

Scata

peri1

peri2

Speri

F2 F3 F4 F5

5.3 38.6 77.1 64.7

18.4 23.2 13.1 12.3

36.2 3.3 2.1 4.4

59.9 65.1 92.3 81.4

23.1 5.8 3.5 5.4

17.0 29.1 4.2 13.2

40.1 34.9 7.7 18.6

F6rF7—high molecular weight heteroaromatic compounds with different types of substituents Žmostly polar oxygen functionalities.. Compounds containing methylene bridges and hydroaromatic rings are present to different but limited extent in all the extrographic fractions. 3.2. EÕaluation of hydrogen acceptorr donor abilities The reactions of pitch and extrographic fractions with tetralin as a hydrogen donor compound and anthracene as a hydrogen acceptor compound were performed at a temperature as low as 3608C to avoid contributions related to thermal decomposition of reactants. Tetralin and anthracene singly heated at these reaction conditions are thermally stable. It has been also assumed that under mild conditions reactive species are produced to a very limited extent by the treatment of pitch itself. The common feature of all the reactions is a continuous transfer of hydrogen between reactants over the whole period of soaking. Fig. 4 shows the variation in the amount of hydrogen transferred from tetralin to the pitch ŽHAA. and from the pitch to anthracene ŽHDA. with soaking time up to 8 h at 3608C. The reaction between pitch and donorracceptor agents occurs continuously but with decreasing kinetics over the whole period of soaking. About half of the total hydrogen is transferred within the first 1 h of soaking and initial 15 min seem to be crucial for evaluating transfer ability. After 6-h soaking the exchange is practically completed and the state near equilibrium established. The state corresponds to about 5.5 mg of hydrogen donated to 1 g of pitch from tetralin and about 4.4 mg of hydrogen abstracted from the pitch constituents by anthracene. Therefore, using these specific reagents and reaction conditions the pitch is stronger acceptor than donor of hydrogen. The extrographic fractions show different ability to accept hydrogen in the reaction with tetralin at 3608C. As for the whole pitch, a considerable part of hydrogen is transferred within initial period of soaking and significant differences between fractions are seen already after 15 min soaking at 3608C, without any exception ŽFig. 5.. Generated already in the initial period of treatment, the differences among fractions in the hydrogen acceptor ability enlarge markedly with soaking time. For all the fractions a continuous but decreasing kinetic with time transfer of hydrogen is observed over the whole period of soaking. Expressed as milligrams of hydrogen abstracted from tetralin

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Fig. 4. Variation in the amount of hydrogen transferred from tetralin to the pitch ŽHAA. and from the pitch to anthracene ŽHDA. with soaking time at 3608C.

by 1 g of sample after 4 h of soaking, the HAA amounts to only 2.2 mg H for F2 while about 10 mg H for F6 or F7.

Fig. 5. Variation in the amount of hydrogen transferred from tetralin to the extrographic fractions of the pitch ŽHAA. with soaking time at 3608C.

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The reaction with tetralin clearly differentiates the fractions in terms of hydrogen acceptor ability ŽHAA. that increases in the order: F2 - F3 - Pitch, F4, F5 - F6, F7 The variation in the rate of hydrogen acceptor ability with time can be discussed as related to the type of active sites that are able to abstract hydrogen from tetralin. It has been assumed that strong active sites are responsible for Afast reactionB occurring within the initial 15 min of soaking. Hence, the amount of hydrogen transferred within this period should correspond to a number of the strong active sites. In addition to free radicals, according to the present state of knowledge on fraction composition, oxygen groups, in particular of carbonyl and ether type, and basic nitrogen heteroaromatics are good candidates to be dehydrogenation agents. Further increase in HAA with soaking time can be considered as an effect of hydrogenation of aromatic structures occurring selectively in dependence on PAH topology and the heteroatoms and substituents present on aromatic rings. On the other hand, it has been established that peri-condensed PAHs are practically unreactive and among cata-condensed structures, those with a linear arrangement are more readily hydrogenated than with an angular one w13x. Hydroxyl substituents, in particular, were reported as accelerators of the hydrogenation reaction w10x. Therefore to the low HAA of F2 contribute a low proportion of reactive oxygen groups Žshort reaction time. and a considerable content of peri-condensed PAHs and unsubstituted cata-condensed PAHs, when prolonged treatment is used. In contrast, a high degree of ring substitution, including reactive carbonyl and hydroxyl groups, and a limited proportion of peri-condensed compounds are responsible for the high initial value of HAA for fractions F4 and F5 and its faster increase with soaking time. Similar explanation seems to be applicable to the behavior of F6 and F7. In general, the measured hydrogen acceptor abilities can be correlated to the total content of oxygen in fractions. Fig. 6 shows the variation in the amount of hydrogen transferred to anthracene from extrographic fractions of the pitch with soaking at 3608C ŽHDA.. Within the initial period of soaking all the fractions show rather similar HDA values ranging from 0.8 to 1.4 mg Hrg sample. However, essential differences can be observed in variation of the hydrogen transfer rate with soaking time. For F2 and F4 the amount of transferred hydrogen increases rather monotonously with time. F6 and F7 practically loose their donor abilities after 1 h of treatment. According to the HDA values at 4 h soaking the following order from higher to lower HDA can be established: F2 ) F4 ) F3, F5, F6 ) Pitch ) F7 The analysis of the material produced in the reaction with anthracene as a hydrogen acceptor suggests that all the fractions contain similar and relatively low amount of hydrogen readily transferable in the initial stage Ž15 min. of the treatment. F3 and F7 show slightly lower values of HDA Ž0.8 mg H per g fraction as compared to about 1.4 mg H per 1 g for other fractions.. Assuming that hydroaromatic rings and methylene bridges are the primary source of the labile hydrogen the results are rather consistent with the low proportion of this type of hydrogen evaluated by 1 H NMR. A detailed inspection of 1 H NMR spectra reveals interesting differences among fractions in the H a

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Fig. 6. Variation in amount of hydrogen transferred from the extrographic fractions of the pitch to anthracene ŽHDA. with soaking time at 3608C.

region Ž2.0–3.3 ppm.. F2, F6 and to a lower extent F4 only give the band at 2.5–3.0 ppm attributed to the hydroaromatic hydrogens w16x, and therefore responsible in part for the hydrogen donor properties. It is believed that despite the low treatment temperature the additional labile hydrogen is liberated due to the rearrangement of extrographic fraction compounds. The behavior on further soaking can be considered as a result of competition in consuming this donating hydrogen between anthracene and fraction components. Indeed, reaction with anthracene creates a more complex system than in the case of tetralin due to presence of two major hydrogenation products—DHA and THA. The results confirm the earlier reports on the material of higher hydrogen donor ability promoting the formation of THA w16,18x. It seems that, in addition to the generally accepted reaction of disproportionation, a regressive reaction of donating hydrogen to fraction constituents of acceptor properties and a further hydrogenation of DHA by the donor constituents can contribute to the observed increase in the THArDHA ratio with reaction time ŽTable 6.. In the case of F2, anthracene is hydrogenated more readily than the fraction constituents leading to a continuous increase in DHA and THA. When the fraction constituents contain strong acceptor sites most of the labile hydrogen is consumed by intramolecular transfer. This is in particular the case of F7, but also of F5 and F6 ŽTable 6.. Fig. 7 shows, as an example, the kinetics of the DHA and THA formation during reaction with some extrographic fractions. As a rule, the proportion of THA in the reaction products increases at a higher rate than that of DHA and after about 1 h soaking

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Table 6 Proportion of DHA and THA in the reaction products of the extrographic fractions with anthracene at 3608C and different soaking time F2

F3

F4

F5

F6

F7

Soaking time 0.25 h DHA Žwt.%. THA Žwt.%. THArDHA ratio

7.1 1.8 0.25

7.4 0.0 0.0

7.5 1.7 0.23

7.6 1.7 0.22

5.7 3.2 0.56

8.2 0.0 0.0

Soaking time 0.5 h DHA Žwt.%. THA Žwt.%. THArDHA ratio

6.7 2.5 0.37

6.0 1.4 0.23

8.4 2.9 0.35

8.3 1.5 0.18

8.5 3.8 0.45

6.6 1.0 0.15

Soaking time 1 h DHA Žwt.%. THA Žwt.%. THArDHA ratio

10.6 7.0 0.66

8.0 2.2 0.28

12.3 4.5 0.37

10.2 4.3 0.42

11.8 6.4 0.54

9.2 3.9 0.42

Soaking time 4 h DHA Žwt.%. THA Žwt.%. THArDHA ratio

18.0 36.0 2.00

17.1 13.5 0.79

17.3 20.2 1.17

13.8 12.8 0.93

12.8 12.5 0.98

7.8 6.0 0.77

practically only THA contributes to the observed increase of the transferred hydrogen amount. In consequence, after some reaction time THA becomes a major product of anthracene hydrogenation. Fig. 7 shows, however, that the ratio of hydrogen transferred to DHA and THA is characteristic of donor material. The reaction products of F2 with anthracene contain the highest proportion of THA, those from F3 and F7 the lowest ŽTable 6.. Comparison of HAA and HDA values determined for the extrographic fractions in reaction with the selected donor and acceptor compounds and under the conditions used in this study leads to the conclusion that only F2 hydrogen donor ability outweighs over hydrogen acceptor ability. The opposite situation is observed for F7, F6 and F5. Fractions F3 and F4 show, however, comparable donor and acceptor abilities, being higher in the case of F4. Experimental values of HDA and HAA of pitch soaked for different times at 3608C are rather consistent with estimations based on the corresponding data Žfraction contribution and HDA or HAA value. for extrographic fractions ŽTable 7., except for HDA at 4 h. It should be noted that all the calculated values are underestimated, because the unrecovered part of pitch Ž8.7%. is not taken into considerations. This fully justifies slightly higher experimental values of HDA. Opposite trend observed in the case of HAA indicates that new hydrogen acceptor sites are created during extrographic fractionation. It has been well established that coal-tar pitch is a very complex system with constituting molecules forming aggregates by non-covalent bonds. It is believed that during fractionation this kind of bonds can be destroyed and new hydrogen acceptor sites created. There is no reasonable explanation of the apparent inconsistency between

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Fig. 7. Kinetics of DHA and THA formation during reaction of the extrographic fractions F2, F4 and F7 with anthracene at 3608C.

experimental and estimated values of HDA at 4 h of soaking. F2 contributes in more than 50% to the excessive estimation.

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Table 7 Experimental and estimated from extrographic fractions values of HDA and HAA of pitch Soaking time Žh.

0.25 0.50 1.0 4.0 a

DHDAa

HDA Experimental

Estimated

1.23 1.63 2.22 3.41

1.03 1.21 2.17 6.52

0.20 0.42 0.05 y3.11

DHAAa

HAA Experimental

Estimated

1.75 2.10 2.62 4.94

1.86 2.32 2.84 4.79

y0.11 y0.22 y0.22 0.15

Difference between experimental and estimated values.

The determined in the work hydrogen transfer behaviour of extrographic fractions seems to suggest a decreasing ability of the fractions to produce anisotropic coke with elution depth. In particular, fractions F5, F6 and F7 of dominating hydrogen acceptor properties could be considered as poor quality components of carbonizing system. However, the study on carbonization of extrographic fractions separated from a similar coal-tar pitch using the same fractionation procedure, reveals well-developed anisotropy in all products with most oriented texture in the case of F4 coke w32x. This apparent inconsistency can be explained in terms of thermal stability of compounds constituting the fractions. For F2, in contrast to the hydrogen transfer properties, the carbonization behaviour is representative of narrow class of constituents, which are reacting but not distilling under the treatment conditions. At the same time, the F2 constituents pay not questionable role as solvent and solvation agent in the complex pitch system. On the other hand, the high molecular-weight compounds present in fractions F6 and F7, due to suitable size and structure of molecules, seem to be capable of instant transformation into mesophase. Such a transformation is believed to occur without considerable structural rearrangement and hydrogen transfer reactions are not involved at this stage in the process. Finally, F4 and perhaps F3 appear to be the most valuable fractions in terms of carbonization behaviour. The balanced hydrogen transfer properties and suitable size and configuration of structural units are responsible for creation of carbonizing system of optimum properties. It seems that the hydrogen donor and acceptor abilities should be considered rather as a general characteristic of extrographic fraction, one of several factors that control its carbonization behaviour. When considering the fraction contribution to the creation of optical texture of pitch coke its carbonization yield and interactions between components on the treatment should be also considered. In addition, the proportion of QI and highly polar compounds irreversibly adsorbed on the silica can be of primary importance.

4. Conclusions Hydrogen acceptor ability of fractions, measured in reaction with tetralin, increases markedly with the elution depth of the extrographic separation and a good correlation between HAA and the total oxygen content in the fractions can be found. All

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extrographic fractions show comparable hydrogen donor abilities in the initial period of the reaction with anthracene. This is likely due to a similar content of aromatic systems containing hydroaromatic rings and methylene bridges. On further treatment, F2 Žlow molecular-weight unsubstituted PAHs. and F4 Žheteroaromatics andror heterosubstituted cata-condensed compounds. are characterized by a stronger ability to donate hydrogen to the system than other fractions. In the reaction with the selected donor and acceptor compounds under the conditions used in the study only for F2 hydrogen donor ability outweighs over hydrogen acceptor ability. The opposite situation is observed for F5, F6 and F7 mainly composed of heteroaromatics andror heterosubstituted compounds. In the case of F3 and F4, a comparable amount of hydrogen is transferred from tetralin to the extrographic fraction and from the extrographic fraction to anthracene. From the results of this work, no straight relation would be expected to occur between hydrogen donor and acceptor abilities of a fraction and its carbonisation behaviour. For a prediction of the optical texture of a pitch coke the contribution of QI and highly polar compounds together with the interaction occurring during the treatment needs also to be considered.

Acknowledgements The authors thank the State Committee for Scientific Research ŽPoland. and the Spanish Council for Scientific Research-CSIC-ŽSpain. for a Joint Research Action Ž99PL0030..

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