Formation of asphaltenes and carbenes during thermal conversion of petroleum maltenes in the presence of hydrogen

Formation of asphaltenes and carbenes during thermal conversion of petroleum maltenes in the presence of hydrogen Josef

Bla2ek and Gustav

Sebor

Department of Petroleum Technology and Petrochemistry, Technology, 766 28 Prague 6, Czechoslovakia (Received 17 April 1991; revised 14 May 1992)

Institute

of Chemical

Asphaltenes and carbenes separated from the high-boiling products of the thermal conversion of petroleum maltenes in the presence of hydrogen were analysed using elemental analysis, a.a.s., i.r. absorption spectroscopy, and vapour pressure osmometry. U.V. absorption spectroscopy and ‘H and 13C n.m.r. were also used for the characterization of asphaltenes. The reactions leading to the formation of asphaltenes

and carbenes during the thermal conversion of maltenes are discussed on the basis of the analytical data obtained. (Keywords: asphaltene; maltene; conversion)

Increases in the yield of light petroleum products by non-catalytic cracking and hydrocracking of petroleum residues represent one of the ways of effectively exploiting petroleum. In these thermal cracking processes, the main problem lies in the coking tendency of the resin and asphaltenic fractions of the residues processed. Thermal conversion of petroleum residues may be described in a simplified manner by means of the following series of partially reversible reactions between the relevant components : oilseresins+asphaltenes+carbenes+carboids The study of thermal conversion of petroleum resins in the presence of hydrogen may be essentially realized in two ways: (1) hydrocracking of resins in the natural mixture with asphaltenes and oil fractions (i.e. hydrocracking of residual oil or asphalt); (2) hydrocracking of separated resins. The disadvantage of the first method consists in the fact that it is not possible to study the changes of resins to asphaltenes, because the asphaltenes formed cannot be separated from those present originally in the petroleum residue, the latter in turn being also subjected to considerable changes. This disadvantage is eliminated in the second method, i.e. in the study of the conversion of separated resins. As it is relatively difficult to isolate a sufficient amount of the resins, the conversion has been studied for petroleum maltenes separated from propane asphalt, which contains a large amount of resins and a small amount of oils. There is a certain disadvantage of the study of the conversion of isolated maltenes, consisting in the fact that maltenes isolated from propane asphalt are more or less solid, making it necessary to dissolve these compounds in a suitable solvent, which of course represents a somewhat different reaction environment from a petroleum residue. 0016236l/93/02019946 J-‘: 1993 Butterworth-Heinemann

Ltd.

As the particular aim of the present work was to study conversion of petroleum resins or maltenes to asphaltenes and carbenes, the second method (i.e. the study of the conversion of maltenes isolated from the propane asphalt) was chosen in spite of its abovementioned drawbacks. Thermal cracking of isolated petroleum resins, but with a catalyst, was described by Sergienko’,“. This approach was also used for the study of thermocatalytic conversion of petroleum asphaltenes3-“. On the other hand, great attention has been paid to the study of non-catalytic thermal conversion of petroleum residues in the presence of hydrogen 12p18 However, the reactions leading to the formation of asphaltenes and carbenes during thermal conversion of petroleum maltenes have not been studied in detail. EXPERIMENTAL Maltenes were separated from the asphalt obtained from propane deasphalting of a broad vacuum residue of Romashkino petroleum by extraction with pentane. Maltene conversion was performed in decalin, in which maltenes are sufficiently soluble. Moreover, decalin has the character of a hydrogen donor’9-21 and thus it can replace hydroaromatic fractions used in industrial cracking petroleum residues in the presence of hydrogen. Thermal flow-through experiments were performed in a reactor containing 15 cm3 of glass beads (diameter 1 mm) or 15cm3 of crushed glass (SIMAX, particle size 0.25-0.50mm). For the experiments a lOwt% solution of maltenes in decalin was used, with a pressure of 10 MPa, a hydrogen flow rate of 10dm3 h- ‘, a liquid flow rate of 15 cm3 h-l and reaction temperatures of 400,425,450,475 and 500°C. After the removal of decalin and low-boiling reaction

FUEL, 1993, Vol 72, February

199

Formation Table 1

of asphaltenes

and carbenes: J. Bla2ek and G. Sebor

Mass balances of thermal conversion of mahenes Product yields (wt%)

Experiment no.

Reaction temperature (“C)

Reactor packing”

Secondary maltenes

Asphaltenes

Carbenes

Total

1

425

GB

92.1

7.4

0.07

99.6

2

450

GB

80.5

13.6

0.88

95.0

3

475

GB

60.0

12.3

3.0

75.3

4

500

GB

60.5

5.7

0.94

67.1

5

400

CG

93.0

6.6

0.04

99.6

6

425

CG

82.7

12.0

0.05

94.8

7

450

CG

67.2

11.4

3.1

81.7

8

475

CC

61.4

7.0

1.3

69.7

9

500

CG

57.3

6.2

0.84

64.3

’ GB, glass beads; CG, crushed glass Table 2 Elemental composition and relative average molecular weight of original maltenes and asphaltenes from their thermal conversion

Sample

Reaction temperature (“C)

Elemental composition (wt%) Reactor packing” C

Original mahenes

H

S

N

M,

84.9

10.2

3.5

0.5

970

Al A2 A3 A4

425 450 475 500

GB GB GB GB

83.3 84.8 85.2 83.9

8.1 1.2 6.6 6.3

3.7 3.7 3.6 3.4

0.5 0.8 1.0 1.3

1700 1390 860 620

A5 A6 A7 A8 A9

400 425 450 475 500

CG CG CG CG CG

82.9 84.6 85.5 85.2 83.9

8.1 7.5 6.4 6.4 6.3

3.9 3.8 3.8 3.3 3.3

0.6 0.8 1.0 1.3 1.5

2250 1510 1060 750 740

“See Table I

products, high-boiling reaction products were separated into maltenes, asphaltenes, carbenes and an insoluble fraction, using extraction with pentane, benzene and pyridine. The reactor packing was washed with benzene, transferred to an extraction thimble, and extracted with pyridine in a Soxhlet extractor. The extract and the benzene washings were combined and the solvents were removed. The resulting product was separated into maltenes, asphaltenes, and carbenes. Analytical

methods

Carbon, hydrogen and nitrogen contents were determined with a Perkin-Elmer analyser, model 240C. Sulphur contents were determined by the Schiiniger (oxygen-flask) method consisting in the combustion of a sample in oxygen and subsequent titration of sulphuric acidHwitahJa$~lO,), . C n.m.r. spectra of asphaltenes were determined with a Bruker model AM400 spectrometer; ‘H spectra at 400 MHz, 13C spectra at lOOMHz, using 0.25 g of sample in 2.5 cm3 of 0.05M solution of chromium acetylacetonate in CDCl, with the addition of 1 wt% of tetramethylsilane. Selected structural parameters describing the average molecule of the maltenes and asphaltenes were calculated according to Clutter et ~1.~~. Relative average molecular weights (M,) were deter-

200

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Vol 72, February

mined by vapour pressure osmometry with a Knauer 11.00 osmometer. For maltenes and asphaltenes, the measurement was performed in benzene at 37°C; carbenes were measured in pyridine at 60°C. The results were extrapolated to infinite dilution. Vanadium and nickel contents were determined by flame atomic absorption spectroscopy, after previous ashing of the sample. A Varian Techtron atomic absorption spectrometer, model AA-775, was used, with the calibration curve method. The content of metalloporphyrins was determined by the method described by Sugihara and Beanz3, which consists in graphic integration of the so-called Soret absorption band at 400nm. Measurements were performed on a Pye Unicam model SP800B absorption spectrophotometer. 1.r. spectroscopy was used for the characterization of selected asphaltenes and carbenes, by means of KBr pellets on a Perkin-Elmer model 325 spectrometer. RESULTS AND DISCUSSION Mass balances presented in Table 1 indicate that with increasing reaction temperature, the yield of conversion products after removal of decalin decreases. This is mainly due to the formation of gaseous products, which leave the reaction mixture with surplus hydrogen, and the formation of low-boiling liquid products, which escape during the elimination of decalin (by vacuum distillation and drying). With increasing reaction temperature, the yields of asphaltenes and carbenes increase to a maximum and then decrease again. Compared with the maximum yield of asphaltenes, the maximum yield of carbenes is obtained at a higher temperature and is about one-quarter of that of asphaltenes. The yield of secondary maltenes decreases with increasing temperature over the whole range. Asphaltenes Table 2 presents

elemental composition data for asphaltenes from thermal conversion of maltenes. These data indicate that during thermal conversion of maltenes, the hydrogen and sulphur contents of the asphaltenes formed decrease with increasing reaction temperature; under more severe conversion conditions, the decrease in hydrogen content is slower. Under milder conversion

Formation Table 3

Relative

of types of hydrogen .~

and carbon Relative

Reaction temperature (“C)

Sample Original

abundance

Reactor packing”

maltenes

atoms

abundance _

of asphaltenes

of the original of hydrogen ___~~~~

maltenes

and carbenes: J. Blaiek and asphaltenes

from their thermal

Relative

atom? ~~

and G. Sebor

abundance

conversion

of carbon

atoms’

C,,

C’” .

C ,z c>

c;

0.07

0.34

0.09

0.16

0.41

H,

H,

H.3

H,

0.13

0.62

0.18

-__.

Al

425

GB

0.18

0.53

0.16

0.12

0.52

0.08

0.09

0.31

A2

450

GB

0.24

0.42

0.13

0.21

0.64

0.08

0.07

0.2

A3

415

GB

0.30

0.25

0.09

0.36

0.74

0.06

0.04

0.17

A4

500

GB

0.30

0.24

0.08

0.38

0.74

0.05

0.03

0.17

I

A5

400

CC

0.17

0.55

0.16

0.11

0.50

0.08

0.10

0.32

A6

425

CG

0.22

0.44

0.15

0.18

0.59

0.08

0.08

0.26

Al

450

CG

0.30

0.29

0.08

0.32

0.74

0.05

0.04

0.17

A8

475

CC

0.31

0.23

0.06

0.39

0.79

0.04

0.03

0.13

A9

500

CC

0.29

0.22

0.07

0.42

0.78

0.05

0.03

0.15

‘See b H,, H,, H,, H,, ‘C,,, Ci4, Cf9, Ct,

Table I hydrogen atoms of CH, CH, and CH, in the a-position to hydrogen atoms of CH, in the j-position and CH and CH, hydrogen atoms of CH, groups in the y-position and more aromatic hydrogen atoms (6-9 ppm) carbon atoms in aromatic rings carbon atoms of various CH, groups in alkyl substituents carbon atoms of CH, groups in alkyl substituents (C, and carbon atoms of other saturated groips

Table 4 Selected structural parameters asphaltenes from their thermal conversion

of original

Structural

Sample

Reaction temperature (“0 ~ ~

(9.9-20.4ppm) higher alkyls)

and

parametersb

AS

Reactor packing”

n

W)

_._

maltenes

aromatic rings (2-4 ppm) groups in the B-position and more distant distant to aromatic rings (0.5-l ppm)

R,

Rs

7.2

48

6

6

Al A2 A3 A4

425 450 475 500

GB GB GB GB

4.8 3.3 2.2 2.1

41 33 21 27

17 16 10 7

12 11 I 6

A5 A6 A7 A8 A9

400 425 450 475 500

CC CC CC CC CC

5.1 3.7 2.3 1.9 2.0

42 31 29 23 23

21 16 13 10 9

15 12 9 6 6

Original

maltenes

“See Tabk I bn, average number of carbon atoms per saturated substituent of aromatic rings AS, percentage of non-bridge aromatic carbon with a saturated substituent R,, number of aromatic rings in an average molecule R,, number of saturated substituents of aromatic rings in an average molecule

conditions (samples Al-A3 and A5-A7), the carbon content increases with increasing temperature; under more severe reaction conditions (samples A4, A8 and A9), it decreases with increasing temperature. This behaviour is probably related to the total yield of asphaltenes, which is dependent on reaction temperature in a similar manner. The sum of the carbon, hydrogen, sulphur and nitrogen contents of the asphaltenes also has a similar dependence on reaction temperature, and lies in the range 94.9-96.8 wt%. This means that these asphaltenes contain about 3-5 wt% of oxygen, because their ash content lies in the range 0.05-0.10 wt%. Table 3 presents the relative abundances of the individual types of hydrogen and carbon atoms, and Table 4 summarizes the values of selected structural

(29.1-31.5

to aromatic

rings (l-2 ppm)

ppm)

parameters, for asphaltenes from thermal conversion of maltenes. The contents of vanadium and nickel and the content of metalloporphyrins of both metals for the asphaltenes studied are shown in Table 5. The asphaltenes least changed by secondary reactions (samples Al and A5) are characterized--compared with the original maltenes-by nearly twice the relative average molecular weight, higher vanadium, nickel and oxygen contents, a higher relative abundance of aromatic carbon atoms, a lower hydrogen content, and a lower H/C atomic ratio. This indicates that these asphaltenes are probably formed by combination of two molecules of polar polyaromatic resins with high vanadium, nickel and oxygen contents. The combination probably proceeds via radical condensation of polyaromatic parts of these moleculesz4 or via their functional groups. Asphaltenes formed in this way are then transformed by a series of consecutive reactions. The main reaction is dealkylation, which results in a decrease in the degree of substitution of the aromatic structure of asphaltenes (a decrease in the AS value), a decrease in the relative abundance of H,, H, and H, hydrogen atoms, an increase in the relative abundance of aromatic hydrogen atoms (H,), an increase in the relative abundance of aromatic carbon atoms (C,,), and a decrease in the content of all types of saturated carbon atoms determined (Tables 3 and 4). There is also a decrease in the average number of carbon atoms in the saturated substituents (a decrease in the value of the parameter n) as a result of the shortening of the remaining alkyl substituents, which results in a further decrease in the relative abundance of H, and H, hydrogen atoms and in an increase in the relative abundance of H, hydrogen atoms, because especially CH, groups remain on the aromatic rings. This is why asphaltenes formed at higher temperatures have a highly aromatic character and have a low degree of substitution, with a low hydrogen content and consequently a low H/C atomic ratio. Under more severe reaction conditions, the yield of

FUEL,

1993,

Vol 72, February

201

Formation Table 5

of asphaltenes

Contents

of vanadium

and carbenes: J. Blaiek and G. Sebor

and

nickeland metalloporphyrins ofboth metalsin original

maltenes and asphaltenes

from their thermal conversion

V .~_.

Ni

(mgg-‘)

Fraction of V and Ni bonded in porphyrins (%)

0.14

0.05

0.35

18

Metal content Reaction temperature Sample ~ ~~ Original

(mgg-‘) Reactor packing”

(“C)

Porphyrins

maltenes

AI

425

GB

0.67

0.19

1.3

13

A2

450

GB

0.74

0.24

1.6

14

A3

475

GB

0.28

0.12

0.2

5

A4

500

GB

0.08

0.05

0.0

0

A5

400

CG

0.65

0.23

1.0

11

A6

425

CG

0.80

0.25

1.0

8

Al

450

CG

0.64

0.20

0.5

6

A8

475

CG

0.13

0.06

0.0

0

A9

500

CG

0.05

0.04

0.0

0

“See Table I

Table 6 Elemental of maltenes

composition,

relative

average

molecular

weight

nickel and ash contents

composition (WY%)

conversion

V

Ni

Ash (wt%)

_

_

_

0.68

0.51

0.42

1000

0.57

0.36

0.58

430

0.20

0.23

0.55

_

_

_

_

_

_

(“C)

Reactor packing”

C

H

S

N

Cl

425

GB

62.3

8.0

_

4.5

c2

450

GB

81.0

5.8

3.2

3.4

990

c3

475

GB

82.3

5.3

3.1

3.8

c4

500

GB

61.9

6.0

4.5

6.2

c5

400

CG

60.3

7.8

_

4.7

425

from thermal

Metal content (mgg-‘)

Sample

C6

of carbenes

_ Elemental

Reaction temperature

and vanadium,

CG

61.6

8.0

_

M,

4.3

_

c7

450

CG

82.9

5.2

3.5

3.5

1520

0.89

0.44

0.54

C8

475

CG

82.9

4.9

2.8

4.5

880

0.31

0.29

0.62

c9

500

CG

77.7

5.3

3.3

5.8

570

0.20

0.23

0.73

“See Table I

asphaltenes decreases with increasing temperature. This is probably due to the fact that the cracking of the asphaltene molecules formed is accelerated, which results on the one hand in increased oxygen and nitrogen contents in these substances and therefore in the transformation of asphaltenes to carbenes as a result of increasing polarity, and on the other hand in the formation of secondary maltenes and low-boiling cracking products. In addition, asphaltenes are formed from resins, especially under more severe reaction conditions, as a result of hydrogenation and dealkylation reactions. The aromatic character of the resins is increased in consequence of these reactions; the resins lose their solubility in pentane and are transformed to asphaltenes. Thus a considerable decrease in the relative average molecular weight of the asphaltenes is observed. At higher reaction temperatures, there is also intensive removal of vanadium and nickel (Tuble 5). The decrease in the content of metalloporphyrins of both metals with increasing severity of reaction conditions is more marked than that of non-porphyrinic complexes of vanadium and nickel; thus in asphaltenes obtained under the severest

202

FUEL,

1993,

Vol 72, February

reaction conditions, no porphyrins were found (Table 5, samples A4, A8 and A9). Carbenes Table 6 presents elemental compositions, ash contents, relative average molecular weights and vanadium and nickel contents of carbenes separated from the reaction products. It is evident that the elemental composition of the carbenes is closely related to the relative abundance of these substances in the products of maltene conversion (Table I). Carbenes with very low yields (samples Cl, C5 and C6) have a low carbon content and it can be seen from the sum of the C, H, S and N contents and the ash content that they have a high oxygen content. The yield of carbenes increases with increasing reaction temperature, along with their carbon content; on the other hand, the hydrogen content and thus also the H/C atomic ratio decrease. Under severe reaction conditions, the content of carbenes in the reaction products decreases with increasing temperature (Table I). At the same time, the carbon content also decreases (samples C4 and C9), while the

Formation

of asphaltenes

and carbenes: J. Blaiek

and G. Sebor

Table 7 Yield. elemental composition, relative average molecular weight and vanadium and nickel contents of asphaltenes and carbenes obtained from extracts of reactor packings used for thermal conversion of maltenes Metal content

Elemental composition

(wt%)

WC’)

Sample”

Reactor packingb

Yield (wt%)

______~ C

~ H

S

N

M,

V

Ni

AI0

GB

28.2

82.2

1.6

3.3

0 44

O.IX

Cc;

16.1

83.9

8.0

3.4

I .9 I .9

2470

All

1960

0.33

0.12

Cl0

GB

3.6

73.1

5.6

3.1

0.68

0.95

Cl1

CC

80.9 __~

6.1 ~

2.9

I .09

0.53

~~

4.4 ~~~~ __.~

~~

“A. asphaltenes: C, carbenes hSee Tahk I ‘ Relative to the total amount of extract

hydrogen, nitrogen and probably also oxygen contents and the H/C atomic ratio of these carbenes increase. The increase in hydrogen content and thus also the increase in the H/C atomic ratio in carbenes obtained under the severest reaction conditions may be due to the fact that a certain number of the heteroatoms in these substances are bonded in functional groups (OH, COOH, NH, etc.). Carbenes formed in low yield at lower reaction temperatures (carbenes Cl, C5 and C6) probably also originate from highly polar compounds with heteroatoms partly bonded in functional groups. The relative average molecular weight of carbenes decreases with increasing reaction temperature and becomes very close to that of asphaltenes formed under identical reaction conditions (Table 2). Thus carbenes differ from asphaltenes particularly in their higher polarity, which is due to the higher content of heteroatoms, particularly nitrogen, oxygen and metals. The results (not presented here) of semiquantitative emission spectral analysis of the ash of selected carbenes confirm the fact that the ash also contains the products of corrosion of the hydrocracking apparatus. As carbenes are characterized by a high content of heteroatoms, samples C2-C4 were also analysed by infrared spectroscopy. The i.r. spectrum of selected asphaltenes (sample A2) was also measured for comparison. Strong absorption in the i.r. spectra of carbenes in the region 3100-3600 cm-’ is probably caused by hydrogen bridges of OH and/or NH groups. Strong absorption at 1690cm-’ and weak absorption at 1650cm-’ are probably due to the presence of C=O or C=N groups2’. Carbenes also have a relatively strong absorption of i.r. radiation in the region 1200-1300cm-‘, with weak maxima at 1070 and 1220 cm- ‘. These absorptions are probably due to C-0 and C-N bonds in various oxygen and nitrogen compounds; even the S=O bond cannot be excluded”. Asphaltenes (sample A2) differ from carbenes particularly in their lower absorption in the region 31OG 3600 cm- ’ and 1690 cm- ’ ; they also have a much weaker absorption in the region 1000-l 300 cm-l as a result of the lower content of heteroatoms (N, 0) in these asphaltenes than in the carbenes. By comparing the results of analyses of asphaltenes and carbenes, the conclusion may be drawn that carbenes are formed from asphaltenes with high nitrogen and oxygen contents mainly in consequence of dehydrogenation and dealkylation reactions by which the

aromatic character of these substances is increased, so that they become insoluble in benzene. Therefore the carbenes formed in greater quantity (C2--C4 and C7-C9) have a smaller H/C atomic ratio and a comparable relative molecular weight in comparison with asphaltenes obtained at the same reaction temperature. Carbenes are formed from asphaltenes also in consequence of the cracking of heterocycles present in asphaltenes. Part of these heteroatoms forms functional groups, increasing the polarity of these substances that become insoluble in benzene and pass into carbenes. Carbenes formed in both these ways are then modified by other reactions. Partial hydrogenation and cracking of functional groups can lead to the reconversion of carbenes into asphaltenes. Another result of the formation of functional groups from oxygen and nitrogen bonded in heterocycles leads to the conversion of carbenes into the pyridine-insoluble carboids that were found on reactor packings and filter crucibles used for the separation of reaction products. Estracts

The reactor packing extract was separated into maltenes, asphaltenes and carbenes. Tab/e 7 summarizes the data on elemental composition, the vanadium and nickel contents of asphaltenes and carbenes and the relative average molecular weight of asphaltenes obtained from the extracts. The asphaltenes from the extracts have a high relative average molecular weight, which indicates that these asphaltenes are modified only a little by consecutive reactions. They are characterized by high contents of nitrogen, vanadium, nickel and probably also oxygen (as may be found by calculation) compared with the original maltenes (Tuhlr 2). This confirms the above discussion indicating that during thermal conversion of maltenes, asphaltenes are formed mainly by condensation of two molecules of highly polar resins. Higher yields of asphaltenes in the extracts compared with maltene conversion products (Table I) are probably due to the fact that asphaltenes formed in large amounts as a result of the condensation of maltenes (resins) are partly reconverted to maltenes; in this way, the amount of asphaltenes in the hydrogenation products decreases during passage through the whole reactor. Carbenes from the extracts have a similar composition to carbenes from the products of maltene conversion obtained at higher temperatures (Tuhk 4). The presence of large amounts of carbenes in the extracts may be

FUEL,

1993,

Vol 72, February

203

Formation

of asphaltenes

and carbenes: J. Blaiek

and G. Sebor

attributed to the reverse transformation of carbenes to asphaltenes during passage through the whole reactor and to the low solubility of carbenes in the decalin solution of maltene conversion products, which results in the accumulation of carbenes in the reactor.

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3 4

Sergienko, S. R. Trudy Inst. Nefti 1958, 12, 168 Sergienko, S. R., Taimova, B. A. and Talalaev, E. I. in ‘Vysokomolekularnye Neuglevodorodnye Soedineniya Nefti’, Izd. Nauka, Moscow, 1979, p. 122 Ignasiak, T. J. and Strausz, 0. P. Fuel 1978, 57, 617 Hernandez, J. 0. and Choren, E. Am. Chem. Sot. Div. Pet.

CONCLUSIONS

5

Schucker, k. C. and Kewesha, C. F. Am. Chem. Sot. Div. Fuel

During thermal conversion of petroleum maltenes in the presence of hydrogen, asphaltenes and carbenes are also formed besides gaseous and low-boiling liquid products and secondary maltenes. The asphaltenes are formed mainly by condensation of two molecules of aromatic maltenes with higher nitrogen, oxygen and metals contents (polar highly aromatic resins). Under more severe conditions, asphaltenes are formed from resins also as a result of dehydrogenation and dealkylation reactions. The ‘primary’ asphaltenes formed in this manner are then modified particularly as a result of cracking and hydrogenation/ dehydrogenation reactions. Carbenes are formed probably as a result of dehydrogenation and dealkylation of highly polar asphaltenes and cracking of heterocycles present in asphaltenes, leading to the formation of functional groups containing oxygen or nitrogen, so that the carbenes formed are highly aromatic compounds with high oxygen, nitrogen and metals contents.

6

Sebor, G.1 Reynoso, S. B., Hajek, M., Weisser, 0. and Mostecky, J. Coil. Czech. Chem. Commun. 1981,46,409 Sebor, G., Weisser, 0. and Hajek, M. Chem. Wchnik 1981,33,

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Nakajima, T., Ishikura, T., Asaha, H., Yamakawa, I. and Tanobe, C. Nikon Daigaku Seisankogakubu Hokoku 1981, 14, 119 (auoted from Chem. Abstr. 96. 71482) Speight, J. G. and Pancirov, R. J. Liq. Fuels’Technol. 1984,2,287 Al-Samarraie, M. F. and Steedman, W. Fuel 1985, 64, 941 Savage,P. E., Klein, M. T. and Kukes, S. G. Am. Chem. SOC. Div. Fuel Chem. Preprints

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14 15 16 17 18 19 20

ACKNOWLEDGEMENTS

22

The authors wish to thank Dr M. Hajek, laboratory of n.m.r. spectrometry, and colleagues from the laboratory of atomic absorption spectroscopy and the laboratory Prague Institute of Chemical of i.r. spectroscopy, Technology, for the measurements performed.

23 24

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1980, 25(l),

361

8

21

204

1979, 24(4), 1009

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