Supercritical fluid extraction of coal-tar pitch

FUEL PROCESSING TECHNOLOGY ELSEVIER

Fuel Processing Technology38 (1994) 1-15

Supercritical fluid extraction of coal-tar pitch Part 2. Extraction of the lower molecular weight species with aliphatic solvents John R. Kershaw*, Paul J. Smart CSIRO Division of Materials Science and Technology, Private Bag 33, Rosebank MDC, Clayton, Victoria 3169, Australia

(Received 18 May 1993;accepted in revised form 11 November 1993)

Abstract Extraction of coal-tar pitches has been carried out with a number of aliphatic hydrocarbons at temperatures between 210°C and 300 °C and pressure of 10 or 15 MPa resulting in extraction yields of up to 50%. The toluene-insoluble (TI) content, molecular weight and the rate of mesophase formation of the residual (refined) pitches increase as the extraction yield increases. The molecular weight of the extract also increases with extraction yield. The volatiles produced during mesophase formation from a supercritical hexane-extracted pitch were analysed by NMR and mass spectrometry and were similar to the hexane extract.

I. Introduction Carbon fibres are produced commercially from polyacrylonitrile (PAN) and from pitch. There are two main types of pitch fibres: (i) high-performance or mesophase fibres, which are produced from mesophase pitch resulting in a high degree of preferred orientation of the graphene layer planes along the fibre axis, and (ii) general purpose or pitch fibres, which are produced from isotropic pitch resulting in little, if any, molecular orientation during spinning and no extended graphitic structure in the final fibre 1-13. The pitch fibres have much lower tensile strengths and moduli than mesophase fibres with the tensile moduli of pitch fibres being about 4 - 2 0 % of those of mesophase fibres, depending on the final carbonisation temperature used.

* Corresponding author. Fax: 61-3-544-1128. Tel.: 61-3-542-2777. 0378-3820/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0378-3820(93)E01 10-Z

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J.R. Kershaw, P.J. Smart~Fuel Processing Technology 38 (1994) 1 15

Table 1 Analysis of pitches Pitch Softeningpoint (°C) Quinoline insolubles, QI (wt%) Toluene insolubles, TI (wt%) H/C atomic ratio Aromatic C (%C) Aromatic H (%H) Mol. wt. (VPO) Alcan coking value (wt%)

008 139 0.06 30.7 0.54 95 82 ND 62

011F 108 0.04 21,0 0.58 96 82 429 54.5

011ST 80 0.05 4.4 ND 96 83 398 ND

ND = not determined. The formation of mesophase from coal-tar or petroleum (e.g. Ashland A-240 ~R~) pitch is a slow process. It takes about 24 h to produce 80 + % mesophase at 400 °C from either coal-tar or A-240 pitch with stirring and nitrogen sparging. The lower molecular weight species ("lights") are known to inhibit mesophase formation [1 4]. Solvent extraction of petroleum pitch can leave a residue which readily forms mesophase [4-6]. For example, benzene or toluene extraction of Ashland A-240 petroleum pitch leaves a high molecular weight residue of about 10-20% of the pitch [4, 6], which is transformed into a high mesophase content pitch by heating between 230°C and 400°C for only 10 min [6]. Supercritical fluid extraction (SCFE) appears to be ideally suited to the extraction of the lower molecular weight component from pitch. Although there have been a limited number of reports of SCFE of pitch [7-12], the aim of these studies generally has been to extract the pitch from the undesirable particulate impurities present in pitch rather than to "strip" off the "lights" from pitch. In this paper we report on the SCFE of coal-tar pitch with aliphatic hydrocarbon solvents with the aim of removing approximately 30-50% of the pitch and leaving a residue that is a more suitable precursor for carbon fibre production than the starting pitch. Coal-tar pitches typically cont,qn 4 - 1 6 % insoluble particulate matter, normally characterised by its quinoline insolubility [13]. The quinoline insolubles (QI) were removed from the coal-tar pitches used in this study by filtration or supercritical toluene extraction prior to SCFE with aliphatic hydrocarbons.

2. Experimental 2.1. P i t c h e s

The QI-free coal-tar pitches were prepared as follows: 008 by filtration of a pitch/solvent mixture at 170°C, followed by vacuum distillation to remove the solvent; 011F by filtration of neat pitch at 200°C through 1.2 jam filter with filter aid; 011ST by supercritical toluene extraction of pitch at 350°C [12]. Analyses of the pitches are given in Table 1.

J.R. Kershaw, P.J. Smart/Fuel Processing Technology 38 (1994) 1 15

3

2.2. Supercritical fluid extraction The extractions were carried out in a 11 autoclave. The autoclave was charged with pitch and solvent (600 ml) and heated (ca. 7 °C min 1). When the extracting temperature was reached, solvent (1 l h -~) was pumped via a dip tube, which acts as a pre-heater, into the bottom of the autoclave and through the pitch. The pressure was controlled during the extraction by adjusting the throttling valves and the solvent and extracted pitch were condensed by a water-cooled condenser and collected. After the extraction had been carried out for the desired period, the pump was switched off and the pressure lowered in the autoclave. When the autoclave had cooled, any remaining solvent was removed by blowing compressed air over the residue. The residue was then scraped out of the autoclave, carefully dried in a vacuum oven and weighed. The solvent was removed from the extraction condensate under reduced pressure using a rotary evaporator. Samples of the extract for analysis were carefully dried in a vacuum oven prior to analysis.

2.3. Mesophase formation Method A Samples were placed in the heated probe of a proton magnetic resonance thermal analysis (PMRTA) spectrometer which was maintained at 400 °C. A stream of nitrogen was passed over the sample. The mesophase content was monitored with time. Method B Samples (3 g) were heated in a glass lined 80 ml stainless steel bomb to 400 °C or 425 °C and maintained at that temperature for 6 h, under a stream of nitrogen. Method C Pitch (250g) was heated in a reactor (500 ml) to 400°C and maintained at that temperature. The pitch was stirred (330 rpm) and nitrogen gas (0.5 l min -~) was sparged through the pitch. Samples were removed periodically with a dip tube to measure the mesophase content. Analyses

Quinoline insolubles (QI) were determined by filtration of quinoline solutions of the pitch through a 0.2 gm teflon filter using pressure filtration. Toluene insolubles (TI) were determined by the standard procedure (ISO 6376). Natural abundance 13C nuclear magnetic resonance (NMR) spectra were measured at 62.9 MHz on a Bruker WM 250 spectrometer with proton decoupling. Spectra were recorded in deuterochloroform containing Cr(AcAc)3 under conditions which gave quantitative data [14]. A 2 s delay between pulses was used, with the decoupler gated off during the delay. 1H N M R spectra were recorded at 250 MHz in deuterochloroform on the same instrument.

4

J.R. Kershaw, P.J. Smart~Fuel Processing Technology 38 (1994) 1 15

Table 2 Extraction of filtered pitch with aliphatic solvents Pitch

Quantity (g)

Extractant

Critical temperature, T~ (°C)

Temperature (°C)

Pressure (MPa)

Time (min)

Extract yield (wt%)

008 008 011F 011F 011F 011F 0tlF 011F 011F 011F

100 100 100 100 100 100 150 100 100 100

Pentane Hexane Hexane Hexane Hexane Hexane Hexane Heptane Cyclohexane Petroleum spirit (boiling range 80-110°C)

197 234 234 234 234 234 234 267 280

210 250 250 250 300 300 300 290 300

10 10 10 15 15 15 15 15 10

90 90 60 60 60 20 75 60 60

9 27 29 32 41 23 45 46 51

300

15

60

45

Electron impact mass spectrometry (EI/MS) was carried out on a HP5995A and chemical ionization mass spectrometry ( O / M S ) on a Finnegan 3300F mass spectrometer. The accumulated spectra were summed. Average molecular weights were determined by vapour pressure osmometry (VPO) in pyridine at 80 °C using a Knauer apparatus, calibrated with benzil. Mesophase content of heat-treated pitches was determined by P M R T A [153. Alcan coking values (STPTC PT 10-79) were determined by Koppers Coal Tar Products. Softening points were determined using a Mettler instrument.

3. Results and discussion

3.1. Extraction of filtered pitch The majority of this study was carried out on 011F pitch, which had been filtered at 200°C to r e m o v e the QI. Two preliminary runs were carried out using the higher softening point, 008, pitch. Supercritical fluid extraction of filtered pitch was investigated using various aliphatic hydrocarbons. The results are given in Table 2 together with the critical temperature of the solvents. Extraction yields were for duplicate runs with the yields for individual runs being within + 2% of the cited value in Table 2. It is apparent from the data in Table 2 that a number of solvents is suitable for extraction of 30-50% of 01 I F pitch at pressures of 10-15 M P a and temperatures of 250-300°C. In a commercial process, an inexpensive mixed hydrocarbon solvent such as a petroleum distillation cut would be preferable to a more expensive single solvent. Extraction with a petroleum spirit shows this approach to be feasible.

J.R. Kershaw, P.J. Smart/Fuel Processing Technology 38 (1994) 1-15 Table 3 Properties of residues and extracts from 011F pitch Extractant

None Hexane Hexane Hexane Hexane Heptane Cyclohexane Petroleum spirit

Residue

Extract MW"

Yield (wt%)

Toluene insolubles

Softening point (°C)

Alcan coking value (wt%)

MW a

100 71 68 59 77 54 49 55

21.0 30.6 34.2 43.3 26.0 44.7 49.7 46.4

108 171 186 178 147 ND 190 ND

54.5 ND b ND ND ND ND ND 74.8

429 496 ND ND ND ND 646 604

-261 ND ND ND ND 301 ND

a VPO in pyridine. b N D = not determined.

For supercritical fluid extraction, the maximum extraction should occur close to the critical temperature of the solvent where the fluid density is the greatest. We have previously reported a slight decrease in extraction of coal-tar pitch with supercritical toluene (To = 319°C) at 400°C than at 350°C 1-12]. However, for hexane (To = 234 °C) extraction, higher yields are obtained at 300 °C than at 250 °C (see Table 2). The increased volatility of pitch constituents at 300 °C compared to 250 °C may partly explain this. It also seems possible that contact between the solvent and pitch will be less efficient at lower temperature. Extraction of the "lights" will result in increasing pitch viscosity as the extraction proceeds. The pitch viscosity will be considerably higher at 250 °C than at 300 °C and hence mixing between the solvent and pitch will be more difficult at the lower temperature. The extractions were carried out in an unstirred reactor, where contact between the pitch and solvent is potentially relatively inefficient, especially if the pitch has a high viscosity. Low volatility of the pitch components and poor contact between the pitch and solvent molecules may also be contributing factors in the low extraction with pentane at 210°C. The properties of the residual (refined) pitches together with molecular weight data for the extracts are given in Table 3. It is apparent from these data that, as more of the pitch is extracted, the toluene-insoluble (TI) content, softening point and molecular weight of the refined pitches increases. The molecular weight of the extract also increases with the amount extracted. (The molecular weights were determined by vapour phase osmometry in pyridine. The filtered and refined pitches were not totally soluble in pyridine and it seems probable that the small pyridine insoluble portion is of high molecular weight.) The TI and number-average molecular weights (K/,) correlate well with the weight% of pitch extracted (see Fig. 1). Electron impact mass spectra of the pitch extracts were similar with major ions at 202, 228, 252, 276, 302, 326, 350, 352 and 376. However, as the amount extracted increases, there is a shift of the ion profile to higher molecular weight. This is illustrated in Fig. 2.

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J.R. Kershaw, P.J. Smart/Fuel Processing Technology 38 (1994) 1 15

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Time (hrs) Fig. 3. Mesophase formation in PMRTA at 400°C: (A) filtered pitch; (B) hexane extracted (29%); (C) cyclohexane extracted (51%); (D) heptane extracted (46%).

The formation of mesophase from the refined pitches (extraction residues) has been studied using three procedures. Small-scale maturation studies were carried out by the following methods: (a) Placing samples in the heated probe of a PMRTA spectrometer which was maintained at 400°C. A stream of nitrogen was passed over the sample. The mesophase content was monitored with time. Fig. 3 shows the change in mesophase content with time for three refined pitches and the precursor filtered pitch, while Fig. 4 relates the amount of mesophase formed after 6 h with the amount of pitch extracted prior to mesophase formation. (b) Heating 3 g samples at 400°C or 425°C in a small bomb under a nitrogen atmosphere. The results are shown in Fig. 4. Fig. 4 shows that the amount of mesophase formed after 6 h increases with the amount of "lights" extracted from the pitch. The rate of formation of mesophase is considerably higher for the refined pitches compared to the precursor pitch in the PMRTA studies (see Fig. 3). At about 70% mesophase, the rate decreases for all the samples (see Fig. 3), and this may indicate the point of inversion with the mesophase becoming the continuous phase. The onset of mesophase is much slower in the filtered pitch than for the refined pitches (see Fig. 3). The much higher content of lower

J.R. Kershaw, P.J. Smart~Fuel Processing Technology 38 (1994) 1 15

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molecular weight components ("lights") in the filtered pitch initially prevents mesophase formation, possibly as they cause high mobility in the system. It is only after the amount of lights, and hence mobility, is reduced that mesophase formation proceeds at a satisfactory rate. Larger batches of refined pitches were prepared, and these were matured in a 500 ml reactor which allowed efficient sparging with nitrogen and stirring. The rate of formation of mesophase for a hexane-extracted pitch is compared to filtered pitch in Fig. 5. The hexane-extracted pitch formed

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mesophase in approximately half of the time of its precursor pitch (see Fig. 5). However, the formation of mesophase is still a relatively slow process, taking about 12 h to form a pitch with 85% mesophase, and there is a significant weight loss during mesophase formation: 33% from a hexane-extracted pitch and 51% from filtered pitch.

3.2. Extraction of supercritical toluene extracts In addition to the particulate matter, supercritical toluene does not extract some of the higher molecular components from coal-tar pitch [12], as indicated by the lower average molecular weight of extracted (011ST) compared to filtered pitch (see Table 1). Thus, extraction of toluene extracts with supercritical hexane produced a pitch with both high and low molecular weight components removed. Coal-tar pitches contain significant amounts ( ~ 10-20%) of high molecular weight components with molecular weights in excess of about 1000 [16, 17]. It might be expected that these high molecular weight components may undergo further reactions making them more intractable and resulting in a mesophase pitch with poor spinnability. Removal of the "lights" would increase the rate of mesophase formation. Lewis et al. [18] have found that a lower molecular weight and a more uniform molecular weight distribution improve the spinnability of mesophase pitch. Narrower molecular weight distribution in the starting isotropic pitch should result in a narrower molecular weight distribution in the resultant mesophase pitch and thus improved spinnability of the

10

J.R. Kershaw, P.J. Smart~Fuel Processing Technology 38 (1994) 1 15

mesophase pitch together, possibly, with improved fibre properties. Stevens and Diefendorf [19] have noted that the molecular weight distribution affects the morphology of the resultant mesophase. Although with the present equipment, two extractions were necessary to produce pitch with both the "heavies" and "lights" removed, this would be achieved in a single commercial SCFE process by controlling the temperature and pressure of the let down vessels. Data for the supercritical toluene/supercritical hexane extraction are given in Table 4. This strategy results in pitch from which the quinoline insolubles, "heavies" and "lights" have been removed. Hexane extraction of the toluene-refined pitch gives higher extraction yields than for the filtered pitch, but the amount of "lights" removed based on the starting pitch appears similar (see Tables 2 and 4). The extracts from hexane extraction of filtered and supercritical toluene-extracted pitches had very similar average molecular weights of 261 and 267, respectively. Comparing the supercritical toluene/hexane pitches with filtered pitch (011F), it is noticeable that the former have lower TI content but higher molecular weights (see Tables 1 and 4). The rate of formation ofmesophase from supercritical toluene/supercritical hexanerefined pitch was significantly faster than for filtered pitch (see Fig. 5). It appears that removal of 32% "lights" outweighs the disadvantages of the loss of 16% "heavies" as far as the rate of mesophase formation is concerned. The weight loss during mesophase formation was 38% for the SCFE pitch compared to 51% for filtered pitch. Another batch of supercritical toluene/hexane-extracted pitch was matured in a similar reactor, but with improved sparging, with the aim of ascertaining the composition of the condensate formed during mesophase formation. The results are given in Table 5. The carbon and hydrogen distribution in the condensate is similar to the lower molecular weight fraction of coal-tar pitch (011F) [17]. In addition to the analysis given in Table 5, mass spectrometry was carried out on the condensate. Methane chemical ionization mass spectrometry showed the major peaks (M + 1)+ and the less intense (M + 29) + [20] correspond to unsubstituted polycyclic aromatic hydrocarbons (PAHs) with molecular weights of 202, 228, 252, 276 and 302. Other less-intense peaks correspond to the other PACs listed in Table 6. Generally, it is not possible by CI/MS to differentiate between PAHs containing a C H 2 group and methyl substituted PAHs. However, NMR data (see Table 5) indicate that both are present. The El/MS ion profile of the condensate from maturation of the supercritical toluene/hexane extracted pitch is compared with the hexane extract from preparation of the pitch in Fig. 6. The similarity of these ion profiles is apparent and indicates that similar molecular weights and structures are removed in the supercritical hexane extraction and in the subsequent mesophase formation of the extraction residue. The ion profiles are supported by the similar average molecular weights measured by vapour phase osmometry of the hexane extract (267) and maturation condensate (283). Two possible explanations are: (i) Supercritieal hexane only extracts part of the lower molecular weight components. Those remaining after extraction are very similar to those extracted. (ii) Similar polycyclic aromatic compounds (PACs) to those extracted by supercritical hexane are formed during heat-treatment to mesophase. It has been suggested by Zander [21] that much of the higher molecular weight fraction

100 150 150

Feed (g)

250 300 250 300

Temperature (°C)

Hexane extraction

10 15 10 15

Pressure (MPa)

60 60 60 60

Time (min)

43 55 43

Extraction yield (wt%)

" Based on starting pitch. b Pitch heated with a mixture of tetralin/toluene prior to toluene extraction. Pitch residue heated at 425°C, 6 h under N2 in 80ml bomb. d ND = not determined.

A 15 B 21 C 21 D 12b

Toluene extraction, "heavies" removal (wt%)"

Data for toluene-hexane extractions of 011 pitch

Table 4

33 39 30

"Lights" removed (wt%) a

Mesophase (%F 40 ND ND 59

TI

ND 16.3 10.3 29.6

MW

468 489 ND d 627

Residue

267 ND ND ND

Extract MW

2'

12

J.R. Kershaw, P.J. Smart/Fuel Processing Technology 38 (1994) 1-15

Table 5 Data for maturation of a toluene/hexane SCFE pitch Pitch Average MW TI (wt%) AIcan coking value (wt%)

512 20.4 69.5

Maturation conditions

Temperature Sparge rate Time

400 °C 230ml/min with N2 7.1 h

Mesophase pitch Yield (wt%) Mesophase (%) Softening point

58 81 339 °C

Condensate

Yield (wt%) Average MW

30 283

N M R data Aromatic carbon (%C) Hydrogen type (%H) Aromatic Ring-joining methylene/methine Benzylic Aliphatic

94 85 4 9 2

Table 6 CI-MS data on the condensate from maturation of toluene/hexane pitch Compound type

Molecular weights

PAH CHz containing PAH CH3 substituted PAH N-PAC S-PAC

202,228,252, 276, 278, 302 (216)",240,(266),(290),(316) (216), 242,(266),(290),(316) 217,241,243, 265,267, 291, 293, 317 234,284

a Figures in parentheses could equally belong to two compound types.

of coal-tar pitch has an oligomeric structure, consisting of medium-sized aromatic units connected by C-C single bonds (oligo-aryl), methylene groups, ether bridges, etc. Rearrangement/polymerization of many of these oligomers to more planar structures will be required before they can form mesophase. Some cleavage of the oligomeric structures will occur during the long heat treatment, resulting in mediumsized aromatic units (4-8 condensed aromatic rings) similar to those present in unrefined coal-tar pitch being produced and removed during mesophase formation. We are unable to differentiate at present between the two possibilities. However, greater differences in the average molecular weights and ion profiles would have been

13

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Fig. 6. Mass ion profiles: (A) supercritical hexane extract of supercritical toluene-extracted pitch; (B) volatilesfrom maturation of supercritical toluene/hexane-extractedpitch. expected for a two-stage removal of lights based on our previous studies of supercritical toluene extraction [127 and the chemical composition [17] of coal-tar pitch. The latter explanation therefore appears to be the most likely. It has been reported by Yudate et al. [22, 23] that tetralin hydrogenation of coal-tar pitch produces, on subsequent maturation of the hydrogenated pitch, a mesophase pitch with a relatively low softening point and good spinning properties. We have confirmed these findings. Preliminary studies to ascertain whether hydrogenation can be combined with SCFE were carried out. The pitch was heated with a mixture of tetralin and toluene (1: 1) at 400°C for 1 h prior to extraction with supercritical toluene (see ref. [12] for full details). The tetralin/toluene extract was subsequently extracted with supercritical hexane (see Table 4). It was not possible to obtain accurate extraction yield figures for the hexane extraction of the tetralin/toluene extracts due to incomplete removal of tetralin from the tetralin/toluene extract. It is noticeable that the molecular weight and TI are significantly higher for the tetralin/toluene/hexane pitch compared to the toluene/hexane pitch (see Table 4), as is the softening point, 172 °C, for the tetralin/toluene/hexane pitch compared to 137 °C for the toluene/hexane pitch. The tetralin/toluene/hexane pitch forms mesophase more readily than the toluene/hexane pitch (see Table 4) as was expected from its higher molecular weight and TI. 13C and 1HNMR spectra of the tetralin/toluene/ hexane-treated pitch showed that ring hydrogenation had occurred, as is indicated from the decrease in aromatic carbon and hydrogen contents compared to the untreated pitch (see Table 7). Further support is obtained from additional peaks at ca. 62.8 and 1.8 ppm, which can be ascribed to ~-CH2 and [~-CH2 groups in hydroaromatic structures [24] in the 1H N M R spectrum of the tetralin/toluene/hexane pitch compared to that of toluene/hexane pitch. Any residual tetralin remaining after toluene extraction was removed by the subsequent hexane extraction.

14

J.R. Kershaw, P.J. Smart~Fuel Processing Technology 38 (1994) 1 15

Table 7 NMR data for tetralin/toluene/hexane and toluene/hexane-extracted pitches Toluen¢/hexane pitch Aromatic C (%C) Hydrogen type (%H) Aromatic Ring-joining methylene/methine Benzylic Aliphatic

Tetralin/toluene/hexane pitch

97

93

88 2 8 2

78 2 11 9

4. Conclusions

SCFE with aliphatic hydrocarbon solvents of the "lights" from coal-tar pitch produce residual (refined) pitches which form mesophase more rapidly than the unrefined pitch. Extraction with supercritical hexane of supercritical toluene extracts of coal-tar pitch indicates that QI removal and pitch fractionation could be achieved in a single process. This could easily be combined with hydrogenation using a hydrogen-donor solvent. The extract from supercritical hexane extraction and the condensate formed in the subsequent maturation have similar molecular weight profiles, indicating that similar compounds are formed during mesophase formation as are extracted from the starting pitch.

5. Acknowledgements

We are grateful to lan Willing (CSIRO Division of Chemicals and Polymers) for NMR spectra, Tim Parks (CSIRO Division of Coal and Energy Technology) for PMRTA studies, Bob Western for mass spectra and Krista Black for QI, TI and average molecular weight determinations.

6. References [1] Edie, D.D., 1990. Pitch and mesophase fibers. In: J.L. Figueiredo, C.A. Bernardo, R.T.K. Baker and K.J. Huttinger (Eds.), Carbon Fibers Filaments and Composites. Kluwer Academic, Dordrecht, pp. 43 72. [2] Singer, L.S., 1985. The mesophase in carbonaceous pitches. Faraday Discuss. Chem. Soc., 79: 265-272. [3] Lewis, I.C., 1987. Chemistry of pitch carbonization. Fuel, 66:1527 1531. [4] Venner, J.G. and Diefendorf, R.J., 1984. Pitch-solvent interactions and their effects on mesophase formation. Am. Chem, Soc. Symp. Ser., 260:219 232. [5] Diefendorf, R.J., 1984. Mesophase formation in polycyclic aromatic compounds; a route to low cost carbon fibres. Am. Chem. Soc. Symp. Ser, 260:209 218. ~6] Diefendorf, R.J. and Riggs, D.M., 1980. Forming optically anisotropic pitches. US Patent 4,208,267.

J.R. Kershaw, P.J. Smart~Fuel Processing Technology 38 (1994) 1 15

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