Chemical nature of pre-asphaltenes from flash pyrolysis tars and supercritical gas extracts

Chemical nature of pre-asphaltenes flash pyrolysis tars and supercritical extracts John R. Kershaw and Andrew J. Koplick CSIRO Division of Applied Organic Chemistry, GPO Box 4337, Australia 300 7 (Received 70 August

1983; revised 29 February

from gas

Melbourne,

Victoria,

7984)

Pre-asphaltenes from flash pyrolysis tars of three Australian coals and a supercritical gas extract of one of these coals were studied by 13C and ‘H n.m.r. of the silylated pre-asphaltenes. Further information was obtained by hydrogenolysis of one of the pre-asphaltenes followed by g.c. analysis. 13C n.m.r. studies of the silylated derivatives and g.c. analysis of the hydrogenated pre-asphaltene showed the presence of long alkyl chains. The aromaticity of the pre-asphaltenes from the flash pyrolysis tars increased as the rank of the coal increased. The pre-asphaltene from the supercritical gas extract was less aromatic than that from the flash pyrolysis tar of the same coal. Average structural data for each of the pre-asphaltenes are repotted. (Keywords:

tar; pyrolysis;

asphaltenes)

are major constituents of some coal liquefaction products, for example, flash pyrolysis tars, supercritical gas extracts and solvent refined coal. It has been shown that the toluene (or benzene) insoluble material has a marked effect on the viscosity of coal liquids1p2 as well as causing processing difflculties3. However, little is known of the chemical composition of pre-asphaltenes in general and especially those from flash pyrolysis tars. In this Paper the chemical nature of pre-asphaltenes obtained from flash pyrolysis tars of three Australian coals and from a supercritical gas extract of one of these coals, was studied by elemental analysis, n.m.r. of the silylated derivatives and hydrogenolysis. The authors have previously studied the composition of resins and asphaltenes from a Millmerran flash pyrolysis tar by hydrogenolysis4*’ and the application of this technique is extended to pre-asphaltenes here. Pre-asphaltenes

EXPERIMENTAL Pre-asphaltenes (toluene-insoluble-pyridine-soluble product) from flash pyrolysis tars of Loy Yang, a Victorian brown coal, Millmerran, a Queensland subbituminous coal and Liddell, a New South Wales bituminous coal were obtained as previously described6. The supercritical gas extract of Liddell coal was obtained by extraction with toluene at 450°C and 10 MPa using a 1 dm3 semi-continuous reactor. The reactor was charged with coal (50 g) and .toluene (600 ml) and heated. When the temperature reached ~33oo”C, toluene (1 dm3 h-‘) was pumped via a dip tube, which acts as a preheater, into the bottom of the reactor and through the coal bed. On reaching the desired temperature and pressure the extraction was continued until the condensate was clear. The pre-asphaltene of the supercritical gas extract was obtained by filtering off the product which pre0016-2361/85/01002944%3.00 @ 1985 Butterworth & Co. (Publishers) Ltd

cipitated on cooling. This material was soluble in pyridine. Silylation of the pre-asphaltenes was carried out using the method of Snape and Bartle7. Analyses

Elemental analyses, were carried out by the Australian Microanalytical Service. Conradson carbon residue was determined by destructive distillation according to the ASTM D-189-76 method. Number-average molecular weights were determined by vapour pressure osmometry in chloroform solutions. 13C n m.r. spectra were measured at 62.9 MHz on a Brucker’WM250 or at 20 MHz on a Varian CFT-20 spectrometer with complete proton decoupling. Spectra were recorded in deuterochloroform containing Cr(AcAc), under conditions which give quantitative data8. J-modulation spectra, in which the primary (CH,) and tertiary (CH) carbons appear as positive signals and the secondary (CH,) and quaternary (C) carbons appear as negative signals were recorded as described elsewhere’. ‘H n.m.r. spectra were measured at 90 MHz in deuterochloroform on a Varian EM-390 spectrometer. Chemical shifts are reported relative to internal tetramethylsilane. G.c. analysis was carried out on a 50 m SCOT column with OV-101 silicone fluid stationary phase using a Varian 3700 chromatograph. G.c.-m.s. was carried out with a similar column coupled to a Finnigan 3300F mass spectrometer with Finnigan 6100 Data System. Hydrogenation of the pre-asphaltenefrom Millmerranflash pyrolysis tar

Hydrogenation was carried out in a small flow autoclave as described elsewhere4. The pre-asphaltene (1.7 g) and catalyst (sulphided cobalt molybdate on alumina, BASF M8-10, 2.8 g) were mixed and held within the reactor by steel wool plugs. The reactor was heated at a

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Pre-asphaltenes from flash pyrolysis tars: J. R. Kershaw and A. J. Koplick

band at 10-22.5 ppm was assigned to the methyl carbon while the aliphatic carbon at lower field was assumed to be due to methylene, methine and quaternary carbon. The r3C n.m.r. spectra of the silylated pre-asphaltene from the supercritical gas extract of Liddell coal is shown in Figure I, while the aliphatic region is shown in more detail in Figure 2 including the J-m~ulation spectrum. The major peak in the aliphatic region of all four r3C n.m.r. spectra was the E peak of long aliphatic chains at 29.5 ppm. Other sharp peaks at 14,23,32 and 29 ppm correspond to the u, fi, y and 6 of unsubstituted alkyl chains (> C,) (see Figure 2). J-modulation spectra of the pre-asphaltenes from the Millme~an flash pyrolysis tar and the supercritical gas extract support these assignments (see Figure2). From the n.m.r. data (see Table t) it seems that a significant fraction of the molecules contains a long alkyl chain in the preasphaltenes (see Table 2).

rate of z 100°C min-’ to a temperature of 400°C and maintained at that temperature for 1.5 h. Hydrogen (13.8 MPa, 1 dm3 min-‘) was passed through the reactor. The volatile products were collected in a high-pressure cold trap. After the more volatile products, Fraction l(O.16 g, 9%) and water (0.17 g), had been removed from the cold trap by distillation (room temperature, 13 Pa), the remaining product was washed from the trap with dichloromethane. The dichloromethane was removed under reduced pressure to give Fraction 2 (0.37 g). The reactor tube was extracted with dichloromethane/acetone (1 :l by volume). The solvent was removed under reduced pressure. This product was then fractionated by elution chromatography on silica gel. The 3040°C petroleum ether eluate (0.10 g) was added to Fraction 2 giving 0.47 g (28%). Fractions 1 and 2 were analysed by g.c. and g.c.-m.s. RESULTS AND DISCUSSION The silylated pre-asphaltenes were nearly completely soluble in chloroform (>95%) which allowed n.m.r. spectra to be obtained. Except for the inclusion of the noninterfering -OSi(CH,), bands, the n.m.r. spectra were representative of the toluene-insoluble products. Details of the spectroscopic and other analytical data obtained for the pre-asphaltenes are given in Table I. It is noticeable that the carbon aromaticity (f,), the fraction of the aliphatic carbon which is methyl and the percentage of aromatic hydrogen increase for the flash pyrolysis pre-asphaltenes as the rank of the coal increases, while the H/C atomic ratio decreases. The pre-asphaltene from the supercritical gas extract of Liddell coal was less aromatic than the pre-asphaltene from the flash pyrolysis tar of the same coal though the molecular weights were the same and considerably higher than for the other two samples (see Table I). In the aliphatic region of the r3C n.m.r. spectra, the Table 1 Analytical

Coal-derived

and spectroscopic

LoY Yang (brown)

Coal

H/C Atomic ratio Conradson carbon

From

silyktad

C H N S 0 (by diff)

(Wt%~

as a fraction

6 and ECH2 Q9-30

Flash pyrolysis tar Millmerren (sub-bituminous)

24 79.4 6.1 3.0 0.5 11.0

0.92 48 597

9.0 21

of aliphatic

carbon

Liddell (bituminous) 15 75.5 4.7 4.3 0.4 15.1 0.75 n.d.

Supercritical extract Liddell (bituminous)

20 77.6 5.7 2.4 0.8 13.5 0.88 68

0.57 0.31

5.9 30 0.68 0.39

924 6.4 36

925 6.3 33

0.71 0.41

0.66 0.37

groups of long aikyt chains ppm)

(36 C1 o alkyl chain (>Ca)

Number average chain length of alkyl groups

30

28 72.3 5.8 2.0 0.3 19.6 0.96 49 734

% Carbon as unsubstituted

e b c

Figure 1 62.9 MHz ’ 3Cn.m.r. spectrum of &ylated preasphaltene from a supercritical gas extract of Liddell coal

pre-asphaltenesa

Number ave. mol. wt % OH from ‘H n.m.r. Aromatic H (% total HI fa Methyl

(ppm)

data for pre-asphaltenes

liquid

Yield fwt 96 coal liquid} Elemental analysis (wt %t

6

Corrected for Si(CHs)3 content Includes 7 CH2 from aromatic ring n.d. = not determined

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1985,

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4

3

2

3

n.d.c

5

n.d.

5

n.d.

13

n.d .

12

gas

Pre-asphaltenes from flash pyrolysis tars: J. R. Kershaw and A. J. Koplick

Further evidence for the presence of long alkyl chains was obtained by hydrogenolysis of the Millmerran preasphaltene. The g.c. analysis (see Figure 2) of Fraction 2 showed n-alkanes up to C,, present with maxima at C 16117 and cn. Small amounts of the shorter (lower boiling) n-alkanes were found in the more volatile product, Fraction 1. The n-alkanes were identified by comparing their retention times with those of standards and b

Figure 3 G.c. of Fraction 2 from hydrogenation of the preasphaltene from a flash pyrolysis tar of Millmerran coal. C, denotes n-alkane of chain length n

confirmed by g.c.-m.s. spectrometry. The n-alkanes are the major peaks above the large unresolved envelope. It appears that the aromatic/hydroaromatic/alicyclic compounds formed on hydrogenolysis of the preasphaltenes are a very complex mixture which is not resolved by the g.c. column. The n-alkanes account for about 15% of Fraction 2 (~4% of the total preasphaltene) which is in general agreement with the n.m.r. data (~5% of the pre-asphaltene as long alkyl chains). The fact that long alkyl chains, up to C,,, are present in pre-asphaltenes, though less than in the oil or asphaltene fractions, is a significant finding and indicates the importance of preserving these long alkyl chains in the further hydroprocessing stages if good quality diesel and jet fuel are to be made from coal. Long unsubstituted alkyl chains both attached to aromatic rings and as n-alkanes have been found in the oil fractions of many coal-derived liquids and there is also evidence of these alkyl substituents in asphaltenes from some liquefaction processess,6,10-13. Both flash pyrolysis and supercritical gas extraction are examples of lower yield processes and it may be considered that in these processes the preferential removal of the more aliphatic constituents of coal occurs. It would therefore be interesting to know whether the

a

I

.

.

.

.

.

1

.

.

.

.

20.0

30.0

I..

.

1

10.0

PP)I

Figure 2 Aliphatic carbon bands from 62.9 MHz 13C n.m.r. spectra of silylated pre-asphaltene from a supercritical gas extract of Liddell coal. a, Normal spectrum; b, CH,, CH +; CH,, C - spectrum

Table 2

Number of atoms and groups in the average molecule of the pre-asphaltenes Supercritical extract

Flash pyrolysis tar Atom

or Group

Loy Yang

Millmerran

Liddell

Liddell

44.2 42.6 9.0 1 .o 0.07

39.5 36.4 4.1 1.3 0.09

58.1 43.4 8.7 2.8 0.12

59.8 52.7 7.8 1.6 0.23

17.1 8.1 19.0 5.9

15.8 10.3 13.4 5.2

26.9 14.4 16.8 6.9

23.2 16.3 20.3 7.5

8.1 30.6

10.3 24.0

14.4 25.5

16.3 33.0

OH

3.9

2.1

3.5

3.4

Molecules with alkyl substituents, >ce f%)

n.d.

C H 0 N S Aromatic C-C Aromatic C-H Aliphatic C Methyl C Aromatic Aliphatic

a

or C-O a

H H

Assuming the number of aromatic

C-H

15

groups is the same as the number of aromatic

n.d.

gas

25

H groups

FUEL, 1985, Vol 64, January

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Pre-asphaltenes from flash pyrolysis tars: J. R. Kershaw and A. J. Koplick

preasphaltenes from higher yield processes also contain significant proportions of long alkyl chains. The number of different types of atoms and groups in the average molecule of each of the pre-asphaltenes based on the data in Table 1 are given in Table 2. This average property data (see Table 2) shows that the pre-asphaltenes from flash pyrolysis tars and supercritical gas extracts are relatively large and highly polar molecules. The H/C atomic ratio and the n.m.r. data (see Table I) together with the average property data (see Table 2) indicate that a considerable proportion of the aliphatic carbon probably occurs as bridging groups or in alicyclic rings but there are also long alkyl chains present. Coal-derived liquids with a high pre-asphaltene content, for example flash pyrolysis tars, have a tendency to coke on hydrotreatment3. This is possibly explained by the high Conradson carbon residues of the pre-asphaltenes (see Table 1).

nations, to Mr Ian Willing and Mrs Andrea Armstrong for recording n.m.r. spectra and to the CSIRO Division of Fossil Fuels for flash pyrolysis tars. REFERENCES

2

4

6 7 8 9 10 11

ACKNOWLEDGEMENTS The authors are grateful to Mrs Irina Salivin for molecular weight and Conradson carbon residue determi-

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Sternberg, H. W., Raymond, R. and Akhtar, S. Am. Chem. Sot. Div. Petrol. C/rem., Preprints 1975, 20(3), 711 Shiller. J. E.. Famum. B. W. and Sondreal. E. A. Am. Chem. Sot. Div. Fuel &em., Preprints 1977, 22(6), 33’ Wailcs, P. C. Fuel 1982,61, 1038 Koplick, A. J., Wailes, P. C., Galbraith, M. N. and Vit, I. Fuel 1983,62, 1167 Koplick, A. J., Wailcs, P. C., Salivin, I., Vit, I. and Galbraith, M. N. Fuel 1984,63,1570 Kershaw, J. R. and Kelly, B. A. Fuel Process. Technol. 1983,7,145 Snape, C. E. and Bartle, K. D. Fuel 1979,58,898 Shoblery, J. N. and Budde, W. L. Anal. Chem. 1976,48, 1458 Kershaw, J. R. and Willinn. I. Liauid Fuels Techno/. 1984. 2. 23 Yokoyama, S., Bodily,D. M. and Wiser, W. H. Fuel 1979, i&l62 Battle, K. D., Ladner, W. R., Martin, T. G., Snape, C. E. and Williams, D. F. Fuel 1979, 58, 413 Charlesworth, J. M. Fuel 1980, 59, 865 Grigson, S. J. W., Kemp, W., Ludgate, P. R. and Steedman, W. Fuel 1983,62,695