Nitrogen compounds in light cycle oils: identification and consequences of ageing

Nitrogen compounds in light cycle oils: identification and consequences of ageing Michel

Dorbon

and Christian

Bernasconi*

lnstitut FranCais du P&role, Centre d’Etude et de Dtkeloppement lndustriel, B.P. 3, 69390 Vernaison, France *ELF France, Centre de Recherche de Solaize, B.P. 22, 69360 Saint Symphorien D’Ozon, France (Received 3 March 1989)

Basic and non-basic nitrogen compounds were selectively extracted from different light cycle oils (LCOs). The isolated fractions were analysed by nitrogen selective detector gas chromatography and by gas chromatography/mass spectrometry. The basic compounds are aniline and alkyl derivatives from C,- to C,- anilines. The non-basic compounds are indole, C,- to C,- alkylindoles, carbazole and C,- to C,alkylcarbazoles. In the four different LCOs that we have investigated, the distributions of the nitrogen compounds are very similar and no other nitrogen compounds were found. It has been shown that after

ageing, the distribution of nitrogen compounds than the evolution of anilines and carbazoles.

is different: the evolution

of indoles is more important

(Keywords: diesel; petroleum; nitrogen compounds)

The upgrading of LCOs, middle distillates from the catalytic cracking process, as domestic fuel oil or diesel oil, is still a problem for many reasons. One of these is that LCOs are not steady: the colour changes and gums occur during storage; gum formation is the main shortcoming of LCOs because insolubles can plug filters or injectors in the use of blends of LCOs and straight run middle distillates as diesel fuel. It is generally believed that heterogeneous compounds such as nitrogen or sulphur ones are responsible for gum occurrence1-7. However, the exact mechanisms of gum formation and how such compounds are involved in them are still unknown. The information from total sulphur or nitrogen measurement is not sufficient for the understanding of these mechanisms. Moreover, it is believed that some heterogeneous compounds are more harmful than others but it is not known which4p6. Consequently, for a better understanding of the way the gums are formed and to know which compounds are really involved, it is necessary to identify all the heterogeneous compounds which are present and to analyse those compounds in LCO at different degrees of ageing. Previous papers describe the identification of nitrogen compounds in crude oils and in various petroleum or coal liquefaction productsE-21. Liquid chromatographic methods are used mainly for the extraction of these compounds, and gas chromatograpic methods for the separation of each compound after extraction; g.c.-m.s. is used for the characterization. Quinoline, benzoquinoline, carbazole, benzocarbazole and dibenzocarbazole derivatives were found in crude oils*p’5. In addition, aniline derivatives were found in some conversion products’ 5-18. This paper describes the identification of nitrogen compounds in different LCOs and the consequences of ageing on those nitrogen compounds. A basic nitrogen fraction and a non-basic nitrogen fraction were extracted 0016-236 l/89/08 IO67-08$3.00 Cj 1989 Butterworth & Co. (Publishers)

Ltd.

by adsorption on various adsorbants. The identification of the compounds was done by g.c.-m.s. and, when the standards were available, the identification was confirmed by coelution on gas chromatography. The analysis of the nitrogen compounds of LCO was performed on a gas chromatograph equipped with a thermoionic detector specific to nitrogen compounds. EXPERIMENTAL We investigated the nitrogen compounds of four fuels produced by fluid catalytic cracking. The properties of these fuels are given in Table I and the stabilities were studied previously22. The identification of nitrogen compounds was performed on LCO C3. The extraction of basic and non-basic nitrogen compounds was performed by low pressure liquid chromatography. For the extraction of basic nitrogen fraction, the preparation of the chloride acidic modified silica was carried out as described previously23. For the extraction of non-basic nitrogen fraction, the preparation of alumina was carried out as described previously”. Nitrogen selective detection gas chromatography was carried out using a Perkin-Elmer 8500 gas chromatograph equipped with a thermoionic detector and a packed column injector modified for wide bore columns. The column was a 15 m x 530pm i.d. fused silica capillary column coated with non-polar methyl silicone phase. G.c.-m.s. of basic and non-basic nitrogen fractions was carried out using a Hewlett Packard 5970 quadripolar mass spectrometer directly connected (without interface) to a Hewlett Packard 5890 gas chromatograph equipped with a split injector. The column was a 50 m x 220pm i.d. fused silica capillary column coated with a non-polar silicone phase. Acetylation of the basic nitrogen compounds” and permethylation of the non-basic nitrogen fraction” were performed as described previously. Ageing of LCO was

FUEL, 1989,

Vol 68, August

1067

Nitrogen Table 1

compounds

in light cycle oils: M. Dorbon and C. Bernasconi

Characteristics of the four middle distillates from catalytic cracking and nitrogen compound analysis Low s LCO B2

Distillate Sulphur (%)

0.84

Low s LCO c2 0.51

High S LCO B3

High S LCO c3

2.54

1.31

Total nitrogen (ppm)

630

670

440

590

Basic nitrogen (ppm)

100

75

90

99

Viscosity at 20°C (cSt)

3.79

4.56

3.53

3.59

Specific gravity 15°C kg I‘ 1

0.9355

0.9106

0.9382

0.9118

Distillation Initial point (“C) 50% point (“C) Final point (“C) Bromine index gBr,/lOO g

148

191

120

132

261 339

270 354

259 334

260 342

7.1

Mass spectrometry (%) Paraffins

14

18

8.9

9.8 11.7

12.2

10.8

9.2

16.8

7.8

9.8

Monoaromatics

10.9

10.6

11.0

12.1

Diaromatics

38.3

34.3

38.1

37.7

Triaromatics

18.0

17.5

13.4

14.7

Benzothiophenes

6.7

5.7

15.8

10.7

Dibenzothiophenes

2.9

2.9

3.1

3.3

Naphthenes

carried out using normalized method ASTM D4625: 400ml of sample was allowed to stand in contact with atmospheric air in glass bottles at 43°C for 12 weeks. 10 g of HCI modified silica gel

RESULTS AND DISCUSSION Extraction of the basic fraction The method we have used for the extraction of the basic material is derived from a method previously developed for the extraction of basic nitrogen compounds from crude oilsz3. The principle of the method is to trap the basic material on an acidic adsorbant, silica gel modified by hydrochloric acid. However, instead of using a large amount of adsorbant and a recycle column, we use only log of silica gel and a 1 cm i.d. glass column (Figure I). This simplification was made possible because LCOs are light cuts without resins and asphaltenes which interfere with bases and which are supposed to be trapped on the adsorbantz3. The non-basic compounds are eluted by CH,Cl, and the basic compounds are desorbed by methanol. The method for regeneration of free bases in a strong alkaline medium is as described by Schmitter et aI.23. The purification of the extract by reversed phase liquid chromatography (r.p.1.c.) on C,, chemically bonded silica gel is not necessary due to the absence of heavy compounds in LCO. By analysing the extract by gas chromatography with nitrogen specific detector and by g.c.-m.s., we checked that the extract contains only nitrogen compounds (Figure2). We compared the analysis by gas chromatography with nitrogen specific detector of the extract and of the LCO (Figure3). The same peaks are present in both chromatograms. The basic nitrogen compounds are in the lighter part of the total nitrogen compounds of LCO and are the minor nitrogen compounds of the LCO.

1068

FUEL, 1989, Vol68,

August

r

non basic compounds

basic

nitrogen

compounds

additionof NaOH in H20

by

1 Separation scheme of the extraction compounds from LCO

Figure

then extraction

toluene

of basic nitrogen

Extraction of the non-basic nitrogen compounds The scheme of the extraction is shown in Figure4. The method is derived from a previous one which was developed for the extraction of non-basic nitrogen compounds from crude oils. The principle of the extraction is based on the difference of affinity of nonbasic nitrogen compounds and non-nitrogen aromatic compounds for A1,03. However, this difference is slight’0*12. Consequently, due to the high level of aromatic compounds in LCO, it is necessary to do the

Nitrogen

compounds

in light cycle oils: M. Dorbon

and C. Bernasconi

40 g of A1203 + 2 % H20 I 50 ml of CH2C12hC5

m&hanol

40/60 100 ml of CHZCI2

non polar compounds

Polar compounds

+ aromatics

non basic nitrogen compounds + aromatics

CH2CI2hC5

100 ml of CH2Cl2hC5

40/60

40/60

non basic nitrogen aromatics

compounds

scheme Figure 4 Separation compounds from LCO

Figure 2 a, Total ion chromatogram of the basic fraction of LCO C3; b, NPD gas chromatogram of the basic fraction of LCO C3. Temperature programmed from 60°C tol60”C at 2.5”C min - 1and from 160°C to 200°C at 2”Cmin-’

of the extraction

of non-basic

nitrogen

separation on Al,O, adsorbant twice: the first extraction is a rough one and the first extract still contains non-nitrogen aromatic compounds. The second extraction eliminates the remaining non-nitrogen compounds in the non-basic nitrogen extract. As for the basic extract, the purification of the non-basic nitrogen fraction by h.p.1.c. is not necessary. We checked that the extract contained only nitrogen compounds by analysing it by g.c.-ms. and by chromatography with an NPD detector (Figure5). We checked that all the compounds of the non-basic nitrogen fraction were compounds of the LCO by analysing both LCO and the extract by gas chromatography with an NPD detector. Figure6 shows the similarity of the two chromatograms. The graph shows also that the non-basic nitrogen compounds are the major nitrogen compounds of the LCO. Characterization

of the basic extract

The extract was analysed by g.c.-m.s. The spectra of all the compounds show little fragmentation and a high molecular peak at an odd value, which means that these are nitrogen aromatic compounds. Figure 7 shows the total ion chromatogram of the basic extract and the selected ion chromatograms at m/z 93, 107, 121, 135 and 149. The peaks of these ion chromatograms could belong to aniline derivatives or pyridine derivatives. Both are basic nitrogen aromatic compounds16*23, and both belong to the same homologous series Z= - 5. Z is connected with the unsaturation number (cycles and double bonds or C+DB) by the formula: Figure 3 NPD gas chromatogram and b, total LCO C3. Temperature

of a, the basic fraction of LCO C3; programming as in Figure 26

C+DB=y

FUEL, 1989, Vol 68, August

1069

Nitrogen

compounds

in light cycle oils: M. Dorbon

and C. Bernasconi 2. The aniline

was identified by g.c.-m.s. and by coelution of the pure standard on gas chromatography. 3. It can be seen that at mass 107 there are two peaks and a shoulder can be seen on the first of these. By as the three g.c.-m.s., these peaks were identified isomers of methylaniline. Moreover, we checked that these compounds are not methyl pyridines which have very different retention times on gas chromatography. 4. Whenever the mass increase is 14, the number of peaks also increases, which means that all the basic compounds belong to the same series. 5. All the compounds were identified as alkylanilines by g.c.-m.s. Therefore, basic nitrogen compounds of LCO are aniline derivatives from aniline itself to C,-anilines. Compounds present at the most important level are C,-, C,- and C,- alkylanilines. We have previously seen that the three methylanilines are present. It is not possible to say if all C,_, C, and C,- alkylanilines are present; however, Fiyure 7 shows that, for each series,

Figure 5 a, Total ion chromatogram of the non-basic nitrogen of LCO C3; b, NPD gas chromatogram of the basic fraction C3. Temperature programming as in Figure 2h

fraction of LCO

I_____.______.

b

Ii

L

Figure 6 NPD gas chromatogram of: a, the non-basic nitrogen fraction of LCO C3; and b, total LCO C3. Temperature programming as in Figure 2b

However, compounds ones:

several factors show are aniline derivatives

that basic nitrogen rather than pyridine

1. After acetylation of the LCO, all the peaks due to basic nitrogen compounds appear at higher retention times, confirming the presence of an amino-functional group on the basic nitrogen compounds.

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FUEL, 1989, Vol 68, August

Figure 7 Total ion chromatogram, a, and ion chromatograms at b, 93; c, 107; d, 121; e, 135; f, 149 m/z of the basic extract of LCO C3

Nitrogen

compounds

in light

cycle

oils: M. Dorbon

and C. Bernasconi

many isomers are present, none of which are predominant. Other basic compounds which were previously found in crude oils or other petroleum products such as azaarenes (pyridine, quinoline, benzoquinoline etc.) are absent from LCO. A previous paper” describes anilines as the only basic nitrogen compounds of gasolines produced by catalytic cracking. This result is confirmed by the absence of other basic nitrogen compounds in LCO. Characterization

of the non-basic

nitrogen

fraction

This fraction was analysed by g.c.-m.s. As with the basic fraction, the spectra show little fragmentation and an important molecular peak at an odd value; it means that the compounds in this fraction are aromatic nitrogen compounds. Several factors show that these compounds are indole and carbazole derivatives : 1. Indole and carbazole derivatives are non-basic compounds because the free doublet of the nitrogen participates in the aromaticity of the live side cycle with the electrons of the double bonds24. 2. Indole, carbazole, l-methyl-, 2-methyl-, 3-methyland 4-methyl-carbazole were identified by g.c.-m.s. and by coelution of the pure standards on gas chromatography. 3. The analysis by gas chromatography of the fraction after methylation shows that all the compounds were N-methylated (the chromatograms before and after methylation are different). This shows that the compounds of the fractions have labile hydrogen. 4. All the compounds were identified as alkylindoles or alkylcarbazoles by g.c.-m.s. 5. In the first part of the chromatogram, the molecular masses of the compounds are 117, 131, 145, 159 and 173, showing that those compounds are indole, methylindoles and C,-, C,- and C,- indoles (Figure 8). 6. In the second part of the chromatogram, the molecular masses are 167,18 1,195 and 209, indicating that those compounds are carbazole, methylcarbazoles, C,- and C,- carbazoles (Figure 9). The non-basic nitrogen fraction contains indole and carbazole derivatives. The non-methylated indole and carbazole are present but there are no N-methylated compounds. The most numerous compounds present are methylindoles, C,-indoles, carbazoles, methylcarbazoles and C,-carbazoles. C,-indoles are the heaviest compounds of the indole derivatives and C,-carbazoles are the heaviest compounds of the carbazole derivatives. Investigation

of four

different

LCOs

The nitrogen compounds of four different LCOs were analysed (FigurelO). B2 and C2 are two low sulphur content LCOs and B3 and C3 are two high sulphur content LCOs (Table 1). The identification of the nitrogen compounds of the LCO C3 is shown in Figure 11. It can be seen that in the four samples B2, C2, B3 and C3 the same nitrogen compounds are present: anilines from aniline to C,-anilines, indoles from indole to C,-indoles and carbazoles from carbazole to C,-carbazoles can be seen on the four chromatograms which are very similar. The only difference is in the amount of each series: B2 has a low content of indoles, B2 and C2, which have the highest initial boiling points, have low contents of anilines.

c.

I

m/z

Indoles

=

145

E .iCdindoles

F

Figure 8 Total ion chromatogram, a, and ion chromatograms at b, 117; c, 131; d, 145; e, 159; f, 173 m/z of the non-basic nitrogen fraction of LCO c3

Nitrogen compounds that are present in the catalytic cracking feedstock are those of the heavy fraction of crude oils i.e. benzo- and dibenzocarbazoles and dibenzoquinolines8~‘0~1’~‘3. LCO indoles and carbazoles are probably produced by the incomplete cracking of benzo- and dibenzocarbazoles. Anilines can be produced by the cracking of the heterocycle of all heterocyclic nitrogen compounds. The large number of isomers means that the production of lighter aromatic nitrogen compounds from heavier aromatic nitrogen compounds by catalytic cracking is not selective. The absence of pyridine, quinolines and benzoquinolines and the low amount of anilines mean that azaarenes such as dibenzoquinolines are totally cracked or not cracked at all. Alternatively, they remain, as basic compounds, on the acidic site of the FCC catalyst, and are burnt during catalyst regeneration. We also investigated the nitrogen compounds of a LCO after ageing. No important differences can be seen in the distribution of anilines and carbazoles. On the other hand, the distribution of indoles is very different.

FUEL, 1989, Vol 68, August

1071

Nitrogen

compounds

in light cycle oils: M. Dorbon and C. Bernasconi

Indole

,

IL m/z Cl

=

181

Carbazoles

I

B

:

LCCJ c2

il_.,.,,,

indole

1

indole

II I

carbazole

Figure 9 Total ion chromatogram, a, and ion chromatograms at b, 167; c, 181; d, 195; and e 209 m/z of the non-basic nitrogen fraction of LCO c3

The amount of indoles relative to the amount of anilines and carbazoles decreases after ageing (Figure IO), indicating that the evolution of indoles during ageing is more important than the evolution of the other nitrogen compounds. Moreover Figure I2 shows that the amounts of some alkylindoles have decreased more than others; for instance, methyl-indoles 15, 16 and 17, dimethyl-indoles 19, 21 and 22 and trimethyl-indoles 26 and 28 have decreased more than the others. However, due to the absence of standards, it was not possible to identify those compounds exactly. A previous publication describes non-basic nitrogen heterocyclic compounds with alkyl groups and particularly alkylpyrroles and alkylindoles, as those most influencing sediment formation4. More recent papers described a mechanism of gum formation which involves alkylindoles 5~6,25 . The analysis of nitrogen compounds from four different LCOs confirms the presence of indoles and alkylindoles. The greater decrease in concentration of alkylindoles than other nitrogen compounds in a LCO after ageing, confirms the involvement of alkylindoles in sediment formation. CONCLUSIONS All the nitrogen compounds contained in four different LCOs were extracted as basic and non-basic ones. They were characterized by g.c.-m.s. and, when the standard

1072

FUEL, 1989, Vol 68, Awust

Figure 10 NPD chromatograms e, C3 after ageing. Temperature

\I

I

of LCO a, B2; b, C2; c, B3; d, C3; programming as in Figure 2h

was available, they were positively identified. The only nitrogen compounds found were basic aniline derivatives and non-basic indole and carbazole derivatives. No other nitrogen compounds were found. Indoles and carbazoles are probably produced by the cracking of benzo- and dibenzocarbazoles. The same nitrogen compounds were found in the four LCOs and their distributions were very similar. The analytical method employed allows the evolution of the nitrogen compounds during ageing to be followed. The presence of indoles which are supposed to have the most deleterious effect on the stability of LCOs was confirmed. Moreover, it has been shown that the evolution of alkylindoles during ageing is more important than the evolution of other nitrogen compounds such as anilines and carbazoles, particularly the evolution of some alkylindoles. The complete identification of these nitrogen compounds has to be performed for a better comprehension of the mechanism of gum formation during ageing. However, the involvement of indoles in gum formation was confirmed.

Nitrogen

1

:

aniline

2

-

3

:

7 10

: C2 aniline : C3 anillnes

4 II II 12 15

: -

Cl

indole I.4 : 17

18

-

23

24 32

-

31 38

C4

-61

:

C3

C3

L-

indoles

11

indoles indoles

I

15

I

carbazoles

I

lk%tes Figure 11

and C. Bernasconi

anllines indoles

: C4 39 : carbazole 40 : 1 M carbazole 41 : 3 M carbazole 42 : 2 M carbazole 43 :4 M carbazole 44 - 51 : C2 carbazoles

52

in light cycle oils: M. Dorbon

anillnes

: Cl : C2 :

compounds

I

NPD

I

I

7.50

I

I

chromatogram

I

22.50

15.00

of LCO C3. Temperature

I

programming

I

I

I

37.50

30.00

I 45.00

I

I

I

I

52.50

as in Figure2b

REFERENCES 1 2

3 4 A

5 6

7 8

>

I

I

I

I

I

9 10 11 12 13 R

14 15

1

I

I

Figure 12 NPD chromatograms LCO C3; and b, C3 after ageing

I

focussed

I

I

on the indole zone of: a,

16 17

Pedley, J. F., Hiley, R. W. and Hancock, R. A. Fuel 1987, 66, 1646 Suttertield, F. D., Steele, W., Archer, D. G., Chirico, R. D. and Strube, M. M. 3rd International Conference on stability and handling of liquid fuels, London, 13-16 September 1988 Frankenfeld, J. W., Taylor, W. F. and Brinkman, D. W. Ind. Eng. Chem. Prod. Res. Div. 1983, 22, 608 Frankenfeld, J. W., Taylor, W. F. and Brinkman, D. W. lnd. Eng. Chem. Prod. Res. Div. 1983, 33, 615 Pedley, J. F., Hiley, R. W. and Hancock, R. A. Fuel 1988, 67, 1124 Hiley, R. W. and Pedley, J. F. 3rd International Conference on stability and handling of liquid fuels, London, 13-16 September, 1988 Frankenfeld, J. W. and Taylor, W. F. Am. Chem. Sot. Div. Fuel Chem., Prepr. 1978, 23, 205 Schmitter, J. M., Vajta, S., Arpino, P. in ‘Advanced in organic geochemistry 1979’ (Eds. A. G. Douglas and J. R. Maxwell), Pergamon Press, Oxford, 1980, 67 Schmitter, J. M. and Arpino, P. Mass spectrometry reviews 1985, 4, 87 Dorbon, M., Schmitter, J. M., Arpino, P. and Guiochon, G. J. Chromatogr. 1982, 246, 514 Ignatiadis, I., Arpino, P. and Dorbon, M. Revue de I’lnstitut Franqais du P&role 1986, 41, 551 Dorbon, M., Ignatiadis, I., Schmitter, J. M., Arpino, P., Guiochon, G., Toulhouat, H. and Hut, A. Fuel 1984,63, 565 Dorbon, M., Schmitter, J. M., Guarrigues, P., Ignatiadis, I., Ewald, M., Arpino, P. and Guiochon, G. Org. Geochem. 1985, 7,111 Ignatiadis, I., Dorbon, M. and Arpino, P. Analysis 1985,13,406 Guarrigues, P., Dorbon, M., Schmitter, J. M. and Ewald, M. in ‘PAH: formation, metabolism and measurement’ (Eds. M. Cooke and A. J. Jones), Batelle Press, Columbus, Ohio, USA, 1983, 451 Schmitter, J. M., Ignatiadis, I., Dorbon, M., Arpino, P., Guiochon, G., Toulhouat, H. and Hut. A. Fuel 1984. 63. 557 Ignatiadis, I., Schmitter, J. M. and Arpino, P. J. Chromkogr. 1985, 324, 87

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Nitrogen 18 19 20 21 22

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compounds

in light cycle oils: M. Dorbon

and C. Bernasconi

Di Sanzo, F. P. J. ofHRC and CC 1981, 4, 649 Latter, J. C., Valle Hernandez, D. H. and Cagniant, D. Fuel 1988, 67, 1446 Dzidic, I., Balicki, M. D., Rhodes, I. A. L. and Hart, H. V. J. Chromatogr. Sci. 1988, 26, 236 &tman, C. E. and Colsjo, A. L. Fuel 1988,67, 396 Bernasconi, C., Chaffardon, A., Charleux, P., Denis, J., Gaillard,

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23 24 25

.I. and Durand, J. P. 3rd International Conference on stability and handling of liquid fuels, London, 13-16 September, 1988 Schmitter, J. M., Ignatiadis, I., Arpino, P. and Guiochon, G. Anal. Chem. 1983, 55, 1685 Roberts, J. D. and Caserio, M. C. in ‘Basic principles of organic chemistry’, W. A. Benjamin, New York, 1965 Pedley, J. F., Hiley, R. W. and Hancock, R. A. Fuel 1989,68,27