Separation and identification of nitrogen compounds in coal-tar pitch

Separation and identification of nitrogen compounds in coal-tar pitch

Separation compounds Jaroslav eern\i, and identification in coal-tar pitch Jii-i Mitera” of nitrogen and Petr VavreCka Czechoslovak Academy of Sc...

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Separation compounds Jaroslav


and identification in coal-tar pitch Jii-i Mitera”

of nitrogen

and Petr VavreCka

Czechoslovak Academy of Sciences, Institute of Geology and Geotechniques, V HoleSovikk$ch 41, 182 09 Prague 8, Czechoslovakia * institute of Chemical Technology, Department of Mass Spectrometry, Suchb$tarova 166 28 Prague 6, Czechoslovakia (Received 23 May 7988; revised 28 July 7988)


Chromatographic behaviour of 64 pure hydrocarbons and nitrogen compounds was verified by gel permeation chromatography. It was found that the elution volumes of almost all nitrogen compounds were higher than the elution volumes of aromatic hydrocarbons. This knowledge was used in the separation of

nitrogen compounds from coal-tar pitch extracts. The obtained concentrates with a high nitrogen content were analysed by mass spectrometry to determine the mass representation of the individual groups of nitrogen compounds. The analysed concentrates contained carbazoles and their benzo-derivatives as the main nitrogen compounds. (Keywords: pitch; nitrogen compounds; g.p.c.)

The presence of heteroatoms in hydrocarbon raw materials is often a source of deterioration of their utility properties. The sulphur and nitrogen compounds in fossil fuels cause air pollution after their combustion (SO,, NO,). In some petrochemical and carbochemical these compounds lead to technological processes, difficulties and deteriorate the properties of final The analysis of heterocompounds in products. hydrocarbon materials represents the fundamental prerequisite for the control of the undesirable properties, for the development of refining processes and for better understanding of reaction mechanisms of some processes. The majority of research papers in this field are concerned with nitrogen compounds of petroleum crudes. Usually, it is necessary to obtain a concentrate of polar compounds, the major part of which consists only of heterocompounds, and then to subject it to analysis using appropriate instrumental methods. The method most frequently used for the separation of polar concentrates from crude oil materials is adsorption and ion exchange chromatography1-6. For coal materials, adsorption chromatography is less suitable because of their high aromaticity and limited solubility in usual solvents. Successful separation was achieved only in the case of coal liquefaction products7,*. For this reason other separation processes are used, such as the precipitation of basic nitrogen compounds by gaseous hydrogen chloride7T9-“, or chromatography on deactivated silica gel modified by hydrochloric acid12-’ 4. Also the weak sorption ability of the dextran gel Sephadex LH-20 with tetrahydrofuran as elution solvent can be used’ 5. A concentrate of all nitrogen compounds (acid, basic and neutral) with other polar compounds (phenols) may be obtained by this separation. The attempts to characterize and identify the heterocompounds in unseparated samples afforded only qualitative infor001~2361/89/050596-05$3.00 4‘ 1989 Butterworth & Co.


(Publishers) Ltd.

FUEL, 1989, Vol 68, May

mation16,17. As regards coal materials, mainly the products of coal liquefaction7-10,14*16 or coal-tar distillates10,16,18,19 were analysed. This paper presents the possibility of the separation and identification of nitrogen compounds in coal-tar pitches. The knowledge of the types of these compounds is of particular interest for the solution of problems connected with the carbonization of coal-tar pitch used in the production of high quality coke, carbon libres and as a binder. EXPERIMENTAL The coal-tar pitch was obtained from the continuous distillation plant of Urxovy zavody, Valasske Mezii-ici as a sample of commercially available pitch with a softening point (KS) 82”C, carbon residue (Conradson) 57.7 wt %, toluene insolubles 3 1.8 wt % and quinoline insolubles 14.0 wt %. Exhaustive Soxhlet extractions of the pitch performed close to the boiling points of the solvents n-hexane and subsequently benzene, yielded the following extracts: HS, n-hexane solubles; BS-HI, benzene solubles and n-hexane insolubles; BI, benzene insolubles. The n-hexane and benzene were removed from the extracts in a rotary vacuum evaporator and a vacuum drier at 80°C. After this treatment the samples were used for further separation by gel permeation chromatography (g.p.c.). Benzene was used in place of toluene because of its lower boiling point and consequent better performance in the extraction process. The concentrates of nitrogen compounds were obtained from the coal-tar pitch extracts by g.p.c. on a glass column (13 mm i.d., 900 mm long) filled with Sephadex LH-20 (Pharmacia) with tetrahydrofuran as elution solvent. The 0.4-0.5g of the samples were

Separation dissolved in 1.5 ml of tetrahydrofuran and were introduced to the column via a three-way cock. The eluent flow rate cf 40 ml h-l was ensured by a high pressure piston pump, the chromatographic fractions were collected by means of a fraction collector in portions of 5 ml. The tetrahydrofuran was purified by potassium hydroxide and metallic sodium (stabilizer elimination) and distilled just before use. Also the chromatographic fractions were evaporated in a rotary vacuum evaporator, dried in a vacuum drier and weighed immediately after elution. The g.p.c. column was tested and calibrated before pitch separation by means of standards -substituted and non-substituted aromatics, and nitrogen alkanes compounds. The weight of standard injection was about 5mg, the elution volumes were detected using a U.V. detector at 254 nm. The nitrogen concentrates were characterized by mass spectrometry using direct introduction of a sample to the ion source. The sample temperature was raised linearly from 25°C to 300°C at a rate of 64°C min - ‘. Ionization by low energy electrons (10 eV) was used, adjusted by ratioing the ion intensity m/z 107 to m/z 91 in an ethylbenzene spectrum to be equal to 2. Scanning time in the range from 40 to 800 m.u. was 4 s; the resolution 1500. The given sample was characterized by interpretation of a cumulative spectrum resulting from all recorded spectra in the sample evaporation profile. The individual groups of nitrogen compounds in the coal-tar pitch extracts were characterized by series of molecular ions with the number Z, determining the deficiency of hydrogen atoms from total saturation of the molecule according to the formula C,H 2n_ zN, assuming the nitrogen compounds contain only one nitrogen atom per molecule. The evaluation of spectra was based only on the molecular ions with intensities higher than 5 % of the intensity of the base peak. The mono-isotopic intensities of odd molecular ions were corrected for i3C isotope abundance of the preceding even ion. The sensitivities of detection of molecular ions were considered to be equal. The corrected molecular ion intensities were converted to a weight basis to determine the mass representation of nitrogen compounds of the individual groups Z in the samples. The average molecular weights of the groups Z were calculated from the equation

where A, is the average molecular weight of the compounds of the group Z; Mi is molecular weight of a molecular ion; and 1, is the corrected intensity of a molecular ion. The average molecular weight of analysed samples (li;l,) was obtained as a weighted average

where G, is the weight percentage in the sample. RESULTS


of the group Z


Gel chromatography of the standards The chromatographic behaviour of 64 hydrocarbons

of nitrogen


in coal-tar pitch: J. Cernq et al.

and nitrogen compounds on the Sephadex LH-20 with tetrahydrofuran as elution solvent is shown in Table 1 and the individual standards are arranged in accord with their elution volumes. This table also provides information about their formulae and molecular weights. The evaluation of the data given in Table 2 can result in Table 1


of g.p.c. column

Standard 4,CDiphenylstilbene Nonadecylbenzene Heptadecylbenzene . _ Pentacene Tridecylbenzene 9,10-Dibenzylanthracene n-Pentadecane p-Quaterphenyl Azulene 1,6-Diphenyl-1,3,5hexatriene 9,10-Diphenylanthracene 1,3$Triphenylbenzene Perhydropyrene p-Terphenyl o-Terphenyl Diphenylmethane 4,4_Dimethyldiphenyl Hexamethylbenzene Heptadecylcyclohexane Hexylcyclohexane 2-Methylanthracene 3,3_Dimethyldiphenyl 3-Methyldiphenyl Tetracosane Pyranthrene Retene 1-Phenylnaphthalene 4-Methyldiphenyl 1,2_Dimethylnaphthalene 1,3,5_Trimethylbenzene Triacontane Picene 2-Ethylnaphthalene Coronene 1,2,3,4_Dibenzoanthracene Anthracene Fluoranthene 1,4_Dimethylnaphthalene Cyclododecane Y-Methylanthracene Perylene Chrysene Naphthalene Biphenyl Cycloheptene Pyrene Indane 4,5_Methylenephenanthrene 1,2,7,8-Dibenzocarbazole Fluorene l-Methylindene Benzene Cyclohexane Phenanthrene 8-Methylquinoline 2-Methoxypyridine 2,3,5,6-Dibenzocarbazole 4-Benzylpyridine 2-Ethylpyridine 5,6_Naphthoquinoline 2,4_Dimethylquinoline Gramine 4-Ethylpyridine

__..._ _.~ Molecular formula

Molecular weight

C&m Cd,, C,‘AO C,,H,, C,,H,, C,,H,,

332.44 344.62 268.52 3 16.56 278.35 260.47 358.48 2 12.42 306.4 1 128.17 232.33 330.43 306.4 I 212.33 230.3 I 230.3 1 168.24 182.26 162.28 322.62 168.33 192.26 182.26 168.24 338.66 376.45 234.34 204.27 168.24 156.23 120.19 422.83 278.35 156.23 300.36 278.35 178.23 202.26 156.23 168.32 192.26 252.32 228.30 128.17 154.21 96.17 202.26 118.18 192.26 267.33 166.22 130.19 78.11 84.16 178.23 143.19 109.12 267.33 169.23 107.16 229.28 157.22 174.25 107.16

CxY,, C, sH,z C,,H,s C,oHs C,sH16 C&IS C&,s C,,Hzo C,sH,, C,sH,, C,,H,, C,,H,, C,zHls C,,H,, C,324 C,sH,, C,,H,, C,,H,, Cd,, C&,, C,sH,s C,,H,, C,P,2 C,,H,, C,H,, Cm% C,,H,, C,,H,, C&I, Cd14 C,,H,o C,,H,o C,,H,, C, 3~4 C,sH,, CmH,, C,sH,, C,oHs C,P,o C,H,,

C,,H,o &HI o C, 8’1, CmHI J C,,H,o C,oH,o C,H, C,H,, C,,H,o C,oH,N C,H,NO CzoH, aN C,,H,,N C,H,N C,,H,,N

C,,H,,N _ __ C,,H,,N, C,H,N

FUEL, 1989, Vol 68, May

Elution volume

(ml) 52 57 58 61 61 66 66 67 69 71 72 72 73 75 76 78 78 78 78 79 79 79 79 80 80 80 81 81 81 XI 81 81 81 82 82 83 83 83 83 84 84 85 85 85 85 85 86 86 86 87 87 87 88 88 89 92 92 92 91 97 98 100 101 107


Separation of nitrogen compounds in coal-tar pitch: J. Cern); et al LH-20conclusion that the Sephadex the tetrahydrofuran chromatographic system deviates from the general theory of gel chromatography. Some similar phenomena have been described previously2’-“. The principal rules of the elution behaviour of hydrocarbons in the Sephadex LH-20-tetrahydrofuran system can be defined as follows: 1. the long chain hydrocarbons have primarily the lowest elution volumes; 2. among n-alkanes, the elution volumes increase with increasing number of carbon atoms; 3. among alkylbenzenes the elution volumes decrease with increasing number of carbon atoms in the substituent ; with phenyl substituent have 4. aromatic hydrocarbons lower elution volumes than the corresponding condensed polyaromatics; aromatics elute earlier than their 5. the cata-condensed peri-condensed isomers; of hydrocarbons reduces their elution 6. the saturation volumes; on the aromatics ring 7. the position of the substituents influences elution behaviour. The majority of condensed polyaromatics eluted within a relatively narrow range (80 to 85 ml) so that no significant relation can be established. Generally, the chromatographic behaviour of hydrocarbons in the Sephadex LH-20-tetrahydrofuran system depends on the shape and saturation of molecules rather than on their molecular weight. The sorption properties of Sephadex LH-20 may play an important role in this chromatographic separation. This can be clearly seen in the case of nitrogen compounds, which possess higher elution volumes than those of polyaromatics. The influence of both the sorption and the polarity of the compounds on their elution behaviour is remarkable in the case of 1,2,7,8dibenzocarbazole and 2,3,5,6-dibenzocarbazole (Table 1). In the former case, the nitrogen atom is sterically hindered and, consequently, a lower ability to sorption than in the later can be expected. This corresponds with the elution volumes of both isomers. Similarly, the higher degree of steric hindrance in the case of 2-ethylpyridine can be observed in comparison with 4-ethylpyridine. Thus, it is possible to assume that, in a given chromatographic system, the behaviour of nitrogen compounds with a sterically hindered nitrogen atom will be similar to that of aromatic hydrocarbons. On the other hand, molecules with a higher polarity can be successfully separated from a hydrocarbon matrix, if adequate fractionation (in the present case at 90ml) is used. Coal-tar pitch separation Coal-tar pitch was extracted successively with nhexane and benzene. The yields of extracts and benzene insolubles as well as their elemental analyses are given in Table 2. Both the HS and the BS-HI extracts were further subjected to the g.p.c. separation. The mass distribution of both separations and the nitrogen content in the selectively combined fractions are given in Table 3. The main difference in the chromatographic behaviour of both extracts can be observed in the 40-70 ml range, i.e. the range in which cata-condensed polyaromatics and


FUEL, 1989, Vol 68, May

Table 2


Coal-tar pitch HS extract BSHI extract BI “Calculated

Table 3 pitch


of coal-tar

pitch and extracts Y

Yield (wt%)

C (wt%)


15.2 56.1 28.7

92.3 92.3 92.2 92.7

4.0 4.8 4.3 3.1


:t %)



1.1 0.7 1.0 1.4

2.6 2.2 2.5 2.8

1.92 1.59 1.79 2.48

by difference



of HS and BS-HI


HS extract


of coal-tar


Elution volume





twt %)

twt %)

(wt %)


4045 45-M 50-55 55-60 6065 65-70 70-75 75-80 80-85 85-90 90-95 95-100 100-200

0.1 0.2 0.3 0.6 1.2 3.6 10.7 25.1 32.0 16.3 4.3 2.4 3.8

0.7 3.0 5.0 6.3 7.2 8.4 12.2 19.3 20.2 8.4 3.4 2.3 3.7

0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.6 0.6 1.3 5.5 5.5 5.5



_ _ _ 0.1 0.1 0.3 0.3 0.6 5.4 5.4 5.4


.g j .I


40 -

.f z .









Figure 1 extract


Mass spectrum

250 m/z

of nitrogen



of coal-tar


pitch HS

highly condensed polyaromatic resins usually appear. Further, the bulk of the extracts is eluted between 70 and 90 ml, i.e. 84 wt % and 60 wt % for HS and BS-HI extracts respectively. The weight yields of fractions after an elution volume of 90 ml sharply decrease, but the elution of compounds continues to be approximately 200 ml. The drop in yields is accompanied by a rapid increase of nitrogen content in the eluate. This is in good agreement with column calibration. The elution volume of 90 ml was determined as a boundary between the elution of aromatic hydrocarbons and that of nitrogen compounds. The chromatographic fractions of 9&200ml were thus combined and analysed by mass spectrometry to provide a group analysis of nitrogen compounds. Analysis of nitrogen compounds of the HS extract The mass spectrum of the nitrogen compounds concentrate of the HS extract is shown in Figure I. It is

Separation of nitrogen compounds in coal-tar Table 4

Mass spectrometric


of nitrogen


of HS and BSHI




5 15 17 19 21 23 25 27 29 31 33 37

Types of compounds

carbazoles acridines phenanthro(bcd)pyrroles phenylnaphthylamines benzocarbazoles azapyrenes benzoacridines benzophenanthro(bcd)pyrroles dinaphthylamines dibenzocarbazoles azabenzopyrenes/azaperylenes dibenzoacridines dibenzophenanthro(bcd)pyrroles benzonaphthocarbazoles azadibenzopyrenes benzonaphthophenanthro(bcd)pyrroles azabenzoathanthrenes Average




et al.

extracts HS extract


J. Cernf


Content (wt%) _____~

(wt %)


1.2 19.4 4.3 9.2

149 178 184 207

7.5 1.7 7.7

189 184 219





8.5 4.3

238 246

7.1 8.1

239 259





1.4 3.4 _

279 284

6.0 8.9 4.9

286 288 310






apparent that the high intensities of molecular ions of odd m/z indicate a high degree of concentration of nitrogen compounds. The weight representation of the individual groups of nitrogen compounds according to their Z number is given in Table 4. The table also shows types of nitrogen compounds that can be assigned to the individual Z numbers, and which were identified in the mass spectrum according to their m/z. It should be noted here that different structural types, for instance, the phenanthro-bed-pyrrole and azacyclopentanoclefphenanthrene

process are more probably


rather than to the substituted phenanthro-bed-pyrrole. On the basis of a comparison of corrected intensities of molecular ions m/z=203 and 217 (Z=21), which are in the ratio of 1:4, it is possible to estimate azapyrenes and benzocarbazoles contents, respectively. Similarly, the ratio of corrected intensities of molecular ions for m/z=253 and 267 (Z=27), which are 1:l, makes it possible to estimate the ratio of azabenzopyrenes/azaperylenes and dibenzocarbazoles. These estimates are based on the assumption of low degree of alkylation in the analysed samples. The degree of substitution of nitrogen compounds is low. It has been found that compounds with the pyridine and/or pyrrole rings have a maximum of 4-5 carbon atoms in alkyl substituents and the content of these substituted nitrogen compounds drops speedily with the increasing degree of substitution. The contamination of the nitrogen compounds concentrate by aromatics was low. Anthracenes/phenanthrenes m/z= 178, pyrenes/fluoranthenes m/z= 202, and benzopyrenes/perylenes m/z = 252 have been found as major polyaromatic impurities. According to the

azacyclopentanodef-phenanthrene may have identical molecular masses (m/z= 191). It has been shown by gas chromatography16 that the content of azacyclopentanodef-phenanthrene in coal-tar pitch is negligible in comparison with the content of phenanthrobed-pyrrole. For this reason, as a similar pitch was being used, it was assumed that the molecular ions of nonsubstituted nitrogen compounds with an even number of carbon atoms in the molecule can most probably be assigned to nitrogen compounds with a pyrrole ring. On the other hand, the odd number of carbon atoms characterize the compounds with the pyridine ring. On this basis it is possible to conclude that the analysed nitrogen concentrate contains carbazoles and their benzo-derivatives as the main nitrogen compounds (Table 4). The molecular ion m/z= 149 (Z= 5) can be simply assigned to alkylated pyridine with five carbon atoms in alkyl substituents. However, the compounds that may be the source of the carbazole formation in the carbonization

Similarly, among substituted compounds with number Z= 19, it is possible to observe relatively high intensity of the molecular ion m/z= 219, pertaining to N,Nphenylnaphthylamine



Vol 68, May



of nitrogen


in coal-tar


J. Cern); et al. CONCLUSIONS







Figure 2 Mass spectrum HI extract

of nitrogen


of coal-tar

pitch BS-

average molecular weight of the nitrogen compounds (M,=218, Table 4), C,,H,,N can be considered as the average molecule. The nitrogen content determined in the analysed concentrate, 5.4 wt % (Table 3), can be compared with the theoretical nitrogen content of C,,H, ,N, 6.45 wt %, and is more than seven times as high as the nitrogen content of the initial n-hexane pitch extract.

The described method of preparation of nitrogencompound concentrates by means of g.p.c. in the Sephadex LH-20-tetrahydrofuran system is effective. It is particularly suitable for condensed aromatic materials, where other methods usually fail. An adequate chromatographic fractionation enables the isolation of not only the basic, but also slightly acid and/or neutral nitrogen compounds. The separation method was applied to samples of coaltar pitch extracts. The elemental analysis of the g.p.c. fractions has confirmed the relatively sharp division nitrogen between compounds and aromatic hydrocarbons. REFERENCES 1 2 3

Anulysis of nitrogen compounds of the BS-HI extract The mass spectrum of the nitrogen compounds concentrate of the BS-HI coal-tar pitch extract is shown in Figure 2, and the calculated weight representation of the individual Z groups of nitrogen compounds is in Table 4. Everything that was said about the HS extract can also be applied to the BS-HI extract, but a slightly elevated average molecular weight and a shift in the distribution of nitrogen compounds towards higher molecular weights can be observed. In comparison with HS extract, the content of carbazoles and benzocarbazoles (Z = 15 and 2 1 respectively) is lower, and the content of dibenzocarbazoles/azabenzopyrenes (Z =27) is higher. The ratio of intensities of molecular ions of the compounds with pyridine and/or pyrrole rings has remained the same. It was previously reported i’ that in the case of aromatic hydrocarbons, the content of the benzopyrenes (Z = 28) is dominant in the BS-HI extract, while the pyrenes (Z = 22) dominate in the HS extract. In the case of nitrogen compounds this shift is not so marked, and benzocarbazoles (Z = 2 1) remain dominant compounds in the nitrogen concentrate of both the HS and the BS-HI extracts. The nitrogen content in this concentrate was 5.5 wt % (Tab/e 3), and is in good agreement with the theoretical value of 5.62 wt %.


FUEL, 1989, Vol 68, May

8 9 10 11 12 13 14 15

16 17 18 19 20 21 22

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