Ring contraction and dehydrogenation in polycyclic hydroaromatics at coal liquefaction temperatures

Ring contraction and dehydrogenation polycyclic hydroaromatics at coal liquefaction temperatures Philip J. Collin, Trevor Michael A. Wilson

D. Gilbert,

Horst

Rottendorf

CSIRO Division of Fossil Fuels, P.O. Box 136, North R yde, NSW (Received 11 October 1984; revised 75 February 1985)

in

and 2113,

Australia

The thermal behaviour of 1,2,3,4,5,6,7,%octahydroanthracene and 1,,,,>‘,,, 2 3 4 5 6 7 8 9 10,11,12-dodecahydrotriphenylene has been observed over the temperature range 40&5Oo”C in the presence and absence of

bituminous coal. It is shown that the ratio of ring contracted product to dehydrogenated product increases with increasing reaction temperature, but the amount of ring dehydrogenated product is increased considerably by the presence of coal. The decomposition reactions are similar to those reported for tetralin, except that the polycyclic hydroaromatics react at much lower temperatures, and also further degrade by ring cracking reactions. (Keywords: coal; liquefaction; donor solvents; donor vehicles; pyrolysis)

Hydroaromatic compounds are excellent c.oal liquefaction reagents because of their ability to transfer hydrogen readily to free radicals generated during coal pyrolysis. Tetralin has frequently”’ been used as a model donor

vehicle due to its simple structure and availability, but little work has been published on the role of polycyclic hydroaromatics in coal liquefaction. These hydrocarbons (identified numerically in Figures 1-3) more closely model recycle vehicles produced in coal liquefaction plants. Although there are some reports of the hydrocracking of polycyclic aromatics12-14, only Cronauer and coworkers have studied the reactivity of a polycyclic hydroaromatic, namely, octahydrophenanthrene (1) (Figure 1) at coal liquefaction temperatures’. This paper reports studies of the thermal stability of the related hydroaromatics, 1,2,3,4,5,6,7,8-octahydroanthracene (2) 19293,4 95,6,7,8,9 910,11,12-dodecahydrotriphenylene and (3) in the presence or absence of coal. The parent

*a m

03 0

Schroeter”*‘*

121

Hal Reppelg

3

131 Hal

t

(393121 t

a9 0

Figure 1

Synthesis of donor vehicles. Hal, Br or Cl

001~2361/85/08128Cr06$3.00 :Q 1985 Butterworth & Co. (Publishers)

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FUEL,

1985,

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64,

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September

I1 1

hydrocarbons of these compounds, triphenylene and anthracene, are significant components of coal tar’ 5 and the hydroaromatic compounds are important components of recycle vehicles. The purpose of this study is to determine the effects of temperature on the availability of hydroaromatic entities for hydrogen donation in coal liquefaction. EXPERIMENTAL Materials

and reaction

procedure

Liddell bituminous coal (C, 83.0; H, 5.9; 0, 8.6; N, 2.1 wt%, dry, ash-free basis) was used in all experiments. Full analytical details of this coal have been given elsewhere16. To prepare sufficient amounts of vehicle, approximately 100 g of octahydroanthracene (2) and dodecahydrotriphenylene (3) were synthesized by the reactions outlined in Figure 1”~’ 9. To study the thermal stability of the hydroaromatics, the hydroaromatic (2 g), or hydroaromatic plus coal (1 g), was heated without shaking in stainless steel tubes in a preheated sandbath at a range of temperatures from 400 to 500°C for up to 1 h. After cooling, the products were exhaustively extracted with chloroform, the residue dried and the conversion yield calculated on a dry, ash-free material was subbasis16. The chloroform-soluble sequently extracted exhaustively with hexane. The hexane-soluble material was investigated by ‘H and ’ 3C n.m.r. spectroscopy and gas chromatography (g.c.) mass spectroscopy (m.s.). Conversion yields, which have been published elsewhere”, were determined from the dry, ashfree residue and found to increase with increasing reaction time and temperature, reaching a maximum of 85% at a reactor time of 60 min at 400°C 30min at 425°C and 1Omin at 450°C. Gas

chromatography-mass

spectrometry

The analyses were carried out on a Finnigan 4023 quadrupole instrument. The mass spectra were obtained

Polycyclic

by using the electron impact mode at an ionizing voltage of 70eV, filament emission current 250pA, and source temperature 260°C. Spectra were recorded every second over the range m/z 5M50. The components were separated on a silica WCOT column (SE30; 50 m x 0.2 mm i.d.) programmed from 10°C. The temperature programme was varied for different fractions to ensure maximum separation. The WCOT capillary column was coupled directly to the mass spectrometer ion source inlet. Data from the mass spectrometer were acquired and processed by an INCOS 2300 data system. Structural assignments were made by mass spectral comparison with the NBS/EPA/NIH data base, reference standards and individual mass spectral interpretation. N.m.r. spectroscopy

‘H and 13C n.m.r. spectra were recorded on a Jeol FX9OQ spectrometer. Quantitative 13C spectra were obtained using chromium trisacetylacetonate as relaxation reagent, a 45” pulse, inverse gated decoupling and a pulse delay of 4 s. Assignment of the degree of protonation of various carbons was made using INEPT21,22 and GASPE’ 3 sequences. Product identification and proportions

The majority of compounds were identified through their characteristic m.s. decomposition patterns and molecular ion, but the methylindans could not be identified by m.s. with any certainty. However, these

hydroaromatics:

P. J. Collin

et al.

compounds could be identified by their contribution to the n.m.r. spectra of the product mixtures. In particular the ‘H-n.m.r. spectra contained resonances from a doublet [J = 6.7 Hz at 1.37 ppm for (8)] characteristic of methyl groups adjacent to CH protons, and deshielded aromatic resonances expected for an indan structure were present in the r 3C n.m.r. spectra [at 143.1 and 147.9 ppm for (S)]. Carbon-13 assignment of resonances in product mixtures are shown in Table I. Product proportions were measured from the ‘H and r 3C n.m.r. spectra or from areas from the g.c.-m.s. traces assuming equal relative molar response and are listed in Table 2(for formulae see Figures 2 and 3).

RESULTS

AND DISCUSSION

Thermal cracking of hydroaromatics

At reaction times of 1 h octahydroanthracene (2) was found to be stable at 400°C. At 450°C only about 12 wt’$!< of the octahydroanthracene decomposed into two compounds, namely the methylindan (4) and the dehydrogenated derivative (5) (Table 2). This is about twice the amount of tetralin that decomposed at the same temperature 1,2,4 In contrast, about 30% of dodecahydrotriphenylene decomposed at 450°C for 1 h. Hence the relative order of thermal stability of the hydroaromatics is tetralin > octahydroanthracene > dodecahydrotriphenylene. Hooper and co-workers’ have shown that at low

(15) Pyrolysis

a

/

CPH”’

ax7 (11)

owH

t1

Pyrolysis

Figure

2

Reaction

pathways

(141

in the decomposition

‘cm3

of octahydroanthracene

FUEL, 1985, Vol 64, September

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Polycyclic

hydroaromatics:

Table 1 Assignments polycyclic aromatics

1

6iH 6’Y

2

6iH 6’Q.Z CS’H 6’% 6’H 6’V

3 4

5

6iH 6’sC

6

6’H 6i3C

7

6’H 6°C

8

6’H 6’V

9 18

for compounds

Observed

Compound”

6’H 6’V 6’H 6i3C

P. J. Collin detected

n.m.r. resonance

et al.

in thermal

reactions

in product

of

mixtureh

1.76 (Cp), 2.53 (Cp), 2.72 (Cp), 6.82 (s) 23.0 (CH,), 23.7 (CH,), 26.3 (CH,), 30.2 (CH,), 12.65 (CH), 134.1 (C), 135.0 (C) 1.76 (Cp), 2.69 (Cp), 6.78 (s) 23.5 (CH,), 29.0 (CH,), 129.5 (CH), 134.2 (C) 177 (Cp), 2.57 (Cp) 23.1 (CH,), 26.8 (CH,), 132.6 (C) 1.25 (d, J = 6.7 Hz) 19.9 (CH,), 31.0 (CH,), 35.1 (CH,), 39.1 (CH), 123.6 (CH), 124.8 (CH), 134.8 (C), 141.2 (C), 146.2 (C) 2.95 (Cp), 7.33 (e), 7.51 (s), 7.69 (e) 29.8 (CH,), 124.9 (CH), 126.7 (CH), 127.0 (CH), 132.1 (C), 136.1 (C) 1.15 (d, J=6.8Hz) 19.6 (CH,), 17.4 (CH,), 29.0 (CH,), 33.5 (CH,), 38.2 (CH), 130.0 (C), 130.4 (C), 133.4 (C), 133.8 (C), 138.7 (C), 144.0 (C) 3.12 (Cp), 7.44 (e), 7.98 (e) 123.0 (CH), 124.6 (CH), 129.2 (C), 130.9 (C), 133.8 (C) 1.37 (d. J = 6.7 Hz). 7.98 (e) 19.6 (CH,), 31.1 (CH,), 35:4 (CH,), 39.1 (CH), 126.1 (CH), 127.4 (CH), 127.6 (CH), 133.0 (C), 136.6 (C), 143.1 (C), 147.9 (C) 8.0 (e), 8.4 (s) 125.3 (CH), 126.2 (CH), 128.1 (CH), 131.6 (C)

_

(211

I

09 0

00

I251

(211

( 23 I Figure 3

Additive

Products

from thermal

Temp. (“C)

decomposition

of hydroaromatic

of dodecahydrotriphenylene

compounds

Time (min)

Products”

Octahydroanthracene _ 400 425 _ 440 450 _ 458 _ 465 475 500 Coal 425 Coal 425 Coal 425 Coal 450 Coal 450 Coal 450

60 60 60 60 60 60 60 60 10 30 60 10 30 60

(2Y 1.00 0.94 0.89 0.88 0.76 0.72 0.60 o.13b 0.78 0.46 0.41 0.69 0.32 0.24

(4) trace 0.02 0.04 0.04 0.10 0.13 0.26 0.16 0.03 0.05 0.05 0.03 0.05 0.05

(5) trace 0.04 0.06 0.06 0.14 0.15 0.14 0.14 0.19 0.3 1 0.41 0.31 0.44 0.55

Dodecahydrotriphenylene _ 400 425 _ 440 _ 450 _ 455 465 _ 475 Coal 400 Coal 475

60 60 60 60 60 60 60 60 60

(3) 0.94b 0.80b 0.70b 0.70b 0.71* 0.39b 0.25’ 0.56’ 0.02b

(6) 0.04 0.15 0.25 0.25 0.24 0.53 0.46 0.12 0.04

(7) 0.02 0.05 0.05 0.05 0.04 0.08 0.07 0.27 0.01

(mole fraction)’

‘See Figures 2 and 3 for identification. Product proportions calculated b Measured from g.c.-m.s. data, assuming equal molar response ’ Based on identified products and starting material d Mole fraction of starting material left at end of reaction

1282

decomposition

temperatures (< 400°C) the major decomposition product of tetralin is naphthalene whereas at higher temperatures (450°C) methylindan is the major product. Similar behaviour is obtained with octahydroanthracene and dodecahydrotriphenylene. In the thermal decom-

‘See Fiaures 1-3 for identification * Only resonances not obscured by other products are shown. For ‘H data, Cp = complex multiplet, s = singlet, and d = doublet, e = AA’BB’ coupling pattern. For 13C data assignments from GASPE and INEPT experiments are shown in brackets

Table 2

Thermal

14.2 (C), 147.0 (C)

FUEL, 1985, Vol 64, September

(8)

(9)

(18)

0.13

0.07 trace 0.05 0.05 trace 0.05 0.05

0.05

0.05 0.05 trace 0.11 0.11 (2l)+(22)

(23)

(24)

(25)

0.22 trace trace

0.24

0.19

0.41

from n.m.r. data

Polycyclic

position of octahydroanthracene the ratio of ring contracted product (4) to dehydrogenation product (5) increases with increasing temperature from less than 1 at 425°C to almost 2 at 475°C (Figure 4). For dodecahydrotriphenylene the ratio of ring contracted product (6) (Figure 3) to dehydrogenation product (7) is much higher but, as for octahydroanthracene, it increases with pyrolysis temperature (Figure 4). At 45o”C, the ratio of ring contracted products to dehydrogenated product is about 0.7 for the octahydroanthracene reaction and 5.0 for the dodecahydrotriphenylene reaction. In contrast, the ratio for tetralin decomposition’ is 10 and for octahydrophenanthrene is about 0.6l. Thus the product proportions do not appear to reflect the thermal stability of the parent compounds. Considerable amounts of other products are also formed at 3475°C from all the starting compounds except tetralin. At 500°C over 86% of the octahydroanthracene has decomposed after 1 h. The major additional products appear to be the dehydrogenated methylindan (8) and anthracene (9) but further bond fracture occurs, leading to fragmentation of alicyclic rings to form substituted tetralins and naphthalenes (Figure 5). In the presence of coal, the decomposition of octahydroanthracene occurs much faster. At 450°C for 60 min, 75% of octahydroanthracene is converted to products, whereas only 12% is converted in the absence of coal. However, dehydrogenation rather than

P. J. Collin et al.

**!I I’O

2.0 b7l .

1.5 -

zc .,” 1.0 CI a= 0.5 -

0

I

I

400

I

I

c20

I

110

Temperature

I

I

I

I

I

160 180 ( OC 1

0 500

on ratio of ring contracted to Figure 4 Effect of reaction temperature dehydrogenation products from thermal decomposition of octahydroanthracene ([4]/[5], 0) and dodecahydrotriphenylene ([6]/[7], 0)

isomerization is favoured by the presence of coal. For example at 425°C for a reaction time of 1 h dehydrogenation predominates over ring contraction by a factor of 8 when coal is present, but only by a factor of 2 when

cxa O

h ydroaromatics:

cd 00

\

/

2:30

7:30 Retention

Ftgure 5

Reconstructed

ion chromatogram

of the product

from thermal

1o:oo time treatment

12~30 (min

)

of octahydroanthracene

at 500°C for 1 h

FUEL, 1985, Vol 64, September

1283

Polycyclic

hydroaromatics:

P. J. Collin et al.

I 27:30

Retention Figure 6

Reconstructed

ion chromatogram

37:30 ( min)

32~30

of the product

time

from the reaction

of dodecahydrotriphenylene

12~30

with coal at 475°C for 1 h

coal is absent (Table 2). The data obtained for dodecahydrotriphenylene (Tuble 2) are similar to results obtained for octahydroanthracene except that, at 475°C in the presence of coal, decomposition is almost complete and further cracking and dehydrogenation has occurred to form phenanthrene (25) methylphenanthrene (24) and triphenylene (23) (Figure 6). The chemical transformations are summarized in Figure 2 which is based on the established mechanism for decomposition of tetralin 2,4,6,10,24-29. In effect, in the absence of coal thermal cleavage occurs to form the diradical (12). In rearranging hydrogen (12)-+ (13) the reaction proceeds through the same radical species that can be generated_ during coal hydrogenation, i.e. the /I radical (ll), and species in equilibrium with it (14). As temperature is increased these species are generated in greater concentrations, but are capped by hydrogen after rearrangement to (14). The alternative mechanism (2+ 15--* 16-+ lo+ 17+5) leading to loss of hydrogen from the tl radical (10) is not so temperature dependent, and hence the ratio of ring contracted product to dehydrogeneration product increases. In the presence of coal there is a greater concentration of the cr-tetralyl radical (10) because of hydrogen removal at the CIposition by coal radicals. Thus the ratio of ring contracted product to dehydrogenated product decreases.

It is also shown in this work that polycyclic hydroaromatics are less stable than tetralin. At reaction temperatures of 450°C ring contraction becomes important even in the presence of coal. Thus in a coal-derived donor solvent, higher boiling point fractions containing polycyclic hydroaromatics may lose their donor capacity during recycling. These results suggest that lower reaction temperatures (< 450°C) might be more suitable for liquefaction. However, at higher temperatures (> 450°C) aliphatic chain fission can also occur to reduce the hydroaromatic polycyclic material to naphthalene derivatives. Since one overall aim of the liquefaction process is to reduce the molecular weight of the coalderived molecules, this process is considered favourable. In all cases it is clear that during continuous operation of a liquefaction plant recycling material containing polycyclic hydroaromatics there will be an overall cracking process leading to naphthalene and tetralin derivatives. It is noteworthy that experiments in this laboratory36,37, in which oil generated from fresh coal has been continually recycled in batch autoclaves, have produced larger yields of gasoline fractions and also heavy oils which contain few polycyclic aromatics, partly hydrogenated or otherwise. These oils are rich in substituted naphthalenes and tetralins36*37.

Implications in coal liquefaction Conversion, particularly to hydrogen-rich volatile fractions, depends in part on the amount of transferable donor hydrogen in recycle solvents30-35. It is suspected that isomerization to indans is in part responsible for the deterioration of hydrogen donor capabilities of recycle solvents in coal liquefaction. The results presented here clearly show that octahydroanthracene and dodecahydrotriphenylene readily isomerize. However, the relative proportion of isomerized to dehydrogenated product can be decreased by the presence of coal. Thus for these components of recycle solvent, the coal to solvent ratio will be critical in determining the amount of aromatics in the solvent that can regenerate hydroaromatics with molecular hydrogenation.

REFERENCES

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FUEL, 1985, Vol 64, September

1 2 3 4 5 6 7 8 9 10 11

Cronauer, D. C., Jewel], D. M., Shah, Y. T., Modi, R. J. and Sheshadri, K. S. Ind. Eng. Chem. Fundam. 1979, 18, 368 Franz. J. A. and Camaioni. D. M. Fuel 1980, 59, 803 Penninger, J. M. L. Int. J. Chem. Kinet. 1982, 14, 761 Benjamin, B. M., Hagaman, E. W., Raaen, V. F. and Collins, C. J. Fuel 1979, 58, 386 Cronauer, D. C., Jewell, D. M., Shah, Y. T. and Kueser, K. A. Ind. Eng. Chem. Fundam. 1978, 17, 291 Franz, J. A. and Camaioni, D. M. J. Org. Chem. 1980,45,5247 Hooper, R. J., Battaerd, H. A. J. and Evans, D. G. Fuel 1979,58, 132 Poutsma, M. L., Youngblood, E. L., Oswald, G. E. and Cochran, H. D. Fuel 1982,61, 314 Yen,Y. K.,Furhani,D. E. and Weller, S. W. Ind. Eng. Chem. Prod. Res. Dev. 1976, 15, 24 Collin, P. J. and Wilson, M. A. Fuel 1983, 62, 1243 Vlieger, J. J., Kieboom, P. G. and van Bekkum, H. H. Fuel 1984,

Polycyclic 12

13 14 15 16 17 18 19 20 21 22 23 24

63, 334 Whitehurst, D. D., Mitchell, T. D. and Farcasiu, M. Coal Liquefaction. The Chemistry and Technology of Thermal Processes’, Academic Press, 1980, pp. 274346 Davis, G. O., Derbyshire, F. J. and Price, R. J. Inst. Fuel 1977.50, 121 Blom, P. W. E., Dekker, J., Fourie, L., Kruger, J. A. and Potgieter, H. G. J. J. S. Afr. Chem. Inst. 1975, 28, 130 Clar, E. ‘Aromatische Kohlenwasserstoffe’, Springer Verlag, Berlin, 1952, 2nd Edition Wilson, M. A., Rottendorf, H., Collin, P. J., Vassallo, A. M. and Barron, P. F. Fuel 1982, 61, 321 Schroeter, G. Ber. Dtsch. Chem. Ges. 1924, 57, 1997 Wilson, M. A., Rottendorf, H., Colhn, P. J. and Vassallo, A. M. Aust. J. Chem. 1984, 37, 2379 Reppe, W. Annalen 1955, 5%(l), 97 and 134 Rottendorf, H. in Abstracts and Papers 7th Australian Workshop on Coal Hydrogenation, 1982, p. 65 Thomas, D. M., Bendall, M. R., Doddrell, D. M. and Field, J. J. Magn. Reson. 1981, 42, 298 Bendall, M. R., Pegg, D. T. and Doddrell, D. M. J. Magn. Resort. 1981, 45, 8 Cookson, D. J. and Smith, B. E. Org. Magn. Reson. 1981,16,111 Wilson, T. P., Caflisch, E. G. and Hurley, G. F. J. Phys. Chem. 1958, 52, 1059

25 26 27 28 29 30 31 32 33 34 35 36 37

hydroaromatics:

P. J. Collin et al.

Collin, P. J. and Rottendorf, H. Proc. Int. Conf. Coal Sci.. Sydney. 1985, p. 710 Wilson, M. A., Collin, P. J., Barron, P. F. and Vassallo, A. M. Fuel Process. Technol. 1982, 5, 281 Wilson, M. A., Vassallo, A. M., Collin, P. J. and Batts, B. Fuel Process. Technol. 1984, 8, 213 Skowronski, R. P., Heredy, L. A. and Ratto, J. L. Am. Chem. Sot. Dir. Fuel Chem. Prepr. 1978, 23, 155 Skowronski, R. P., Heredy, L. A. and Ratto, J. L. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1979, 24, 155 Ruberto, R. G. Fuel Process. Technol. 1980, 3, 7 Seshadri, K. S., Ruberto, R. G., Jewell, D. M. and Malone, H. P. Fuel 1978,51, 549 Davies, G. O., Derbyshire, F. J. and Price, R. J. Inst. Fuel 1977, 50, 121 Derbyshire, F. J., Varghese, P. and Whitehurst, D. D. Am. Chem. Sot. Div. Fuel Chem. Preor. 1981. X$2). 84 Cronauer, D. C., McNeil,‘R. I., Young,‘D. C. and Ruberto, R. G. Fuel 1982,61, 610 McNeil, R. I., Cronauer, D. C. and Young, D. C. Fuel 1983,62, 401 Collin, P. J., Gilbert, T. D., Philp, R. P. and Wilson, M. A. Fuel 1983,62, 450 Rottendorf, H. Er&l Kohle 1983, 36, 35

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