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High-performance liquid chromatography in the study of high-boiling petroleum distillates
Petrol. Chem. U.S.S.R. Vol. 28, No. 3, pp. 197-201, 1988 Printed in Poland
0031-64J8/88 $10.00+.00 © 1990 Pergamon Press pie
HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY IN T H E S T U D Y O F H I G H - B O I L I N G P E T R O L E U M D I S T I L L A T E S ¥' T. S. NIKITINA,N. K. LYAPINA,YE. S. BRODSKII,A. D. ULENDEYEVAand A. A. BERG Institute of Chemistry of Bashkir Branch of U.S.S.R.Academy of Sciences (Received 9 November 1987)
INCREASEDattention is currently being paid to the problem of distillation of heavy petroleum feedstock in the light of reduction in crude oil resomces. Efficient refining of heavy petroleum feedstock necessitates a detailed analysis of the nature and structure of its constituents. High-boiling petroleum distillates are noted for a complex group composition and a variety of chemical structures. The production of information on their composition depends on the efficiency of separation methods ensuring sufficient simplification of fractions for subsequent analysis, and on the efficiency of the methods for identification of constituents and their quantitative analysis. The main method for identifying compounds (or groups of compounds) when studying high-boiling distillates should still be molecular mass spectrometry, about which a great deal of experimental data has accumulated in the field of study of the composition of hydrocarbons and heteroatomic compounds of lower and medium petroleum distillates. Separation of petroleum fractions into groups of compounds of more homogeneous composition enables mass spectrometry to be used successfully. The separation methods normally used to analyse medium fractions are rectification, liquid-adsorption chromatography, extraction, and complexing, but none are sufficiently efficient or selective with respect to heavy petroleum fractions [1]. High-performance liquid chromatography (HPLC) can be used as a most efficient method during group differentiation of high-boiling petroleum distillates [2, 3]. The aim of the present paper was to study the possibility of using HPLC as .a separatiort method when studying the composition of a high-boiling 450°-500°C fraction of industrial West Siberian crude oil. The present investigation was carried out with collaboration between the academies of science of the socialist countries. EXPERIMENTAL
The investigation was conducted on Sorbax Sil sorbent using a liquid chromatograph of the Du Pont company (U.S.A.). The detectors were a model 441 differential refractometer and a 254 nm UV detector. * Neftokhimiya 28, No. 5, 710-714, 1988. 197
198
T.S. NIKITINAet
al.
The 450°-500°C vacuum distillate investigated was obtained by distillation of industrial West Siberian crude oil at the experimental works of the BashNIINP Institute. The distillate yield amounts to 7.3 wt. %, the total sulphur content to 2.04 wt. ~o, and the sulphide sulphur content to 1.07 wt. %. The characteristics of the distillate are given in Table 1.
L
1 2
I
1
S
i
1
10
rain
Chromatogram of separation of a 450°-500°C distillate of industrial West Siberian crude oil; detectors - ]efractometer (a) and 245 nm U V detector (b).
Separation of the distillate was preceded by preliminary removal of resinous compounds using an MPS-700 sorbent in a Soxhlet apparatus by the procedure given in [4]. The sorbent particle size was 0.16-0.07 mm, and the average pore size ranged from 600 to 800 A. Porous glasses exhibiting high selectivity to resinous compounds hardly adsorb hydrocarbons and sulphur compounds. The characteristics of deresinated distillate are given in Table 1. Analytical separation of the distillate investigated was carried out on a 4.6 m m x 25 cm column; Sorbax Sil was adopted as the sorbent, and the particle size was 5/tm. The efficiency of the columns amounted to 10,000 theoretical plates. n-Hexane was used as the mobile phase and the solvent of the samples. The specimen in hexane was filtered through a 0.5/~m filter and introduced via a tap into a chromatographic column prewashed with n-hexane with a flow rate of 1 ml/min. First of all, saturated compounds recorded from the signal of the refractometer peak were washed out. Then the sulphur and aromatic compounds recorded by the UV detector were eluted. Semipreparative soparation of the given HPLC distillate was carried out on a 21"2 mm x 25 cm column with a particle size of the Sorbax Sil sorbent of 15/tin. A quantity of 100 mg distillate was dissolved in 2 ml of n-hexane, filtered, and introduced into the chromatographic column in a similar way. The rate of the mobile phase (n-hexane) was 4 ml/min. First, fractions of saturated compounds were collected (Figure, zones 1 and 2). Then the sulphur and aromatic compounds (zones 3-7) were eluted. After fractions of these compounds had been sampled, the mobile phase was changed for a polar eluent (methanol) to remove polar compounds (frac-
High-boiling petroleum distillates
199
tion 8) from the column. Then the column was again washed with n-hexane and used for repeated analysis. As can be seen from the chromatogram (Figure), efficient separation of the distillate did not occur. However, fractions samples in different parts of the general broad peak had different properties: refractive index, elemental composition, spectral characteristics. For mass-spectrometric analysis, a 450°-500°C distillate from industrial West Siberian crude oil was separated into 8 fractions according to yield time. Such separation was optimum according to data of preliminary investigations. The fractions obtained by adsorption separation were analysed on a Varian MAT 311A mass spectrometer with electron impact ionization at 70 eV, a scanning rate of 5 see/10 days, and a mass range of 31-1000 Da. The sample was introduced directly into the ion source and heated at a constant r~te to 350°(21 in 3 rain. Mass spectra were recorded constantly on magnetic disc. Some of the mass spectra recorded (normally 8-10) were printed out. These were chosen according to the curve of change in the complete ion current and covered the entile range of its variation. Calculation and processing of spectra were carried out by the scheme given in [5]. RESULTS AND DISCUSSION
The 450°-500°C distillate of industrial West Siberian crude oil (Stot=2.04~) is characterized by a significant content of organosulphur compounds (29.3~), about half of which are thiophene derivatives. The average molecular weight of the distillate is 460, and its resin content amounts to 5.9 ~ (,Table 1). The distribution of the main groups of compounds between fractions obtained by HPLC adsorption separation f~om 450°-500°C distillate of industrial West Siberian crude oil is presented in Table 2. An analysis of the mass spectra of fractions obtained by preparative separation of distillate showed the following. TABLE 1. CHARACTERISTICS OF A
Product Initial distillate Distillate after removal of resins Resins
450°-500°C DISTILLATE OF INDUSTRIAL WEST SIBERIAN CRUDE
Yield, wt. ~
Molecular weight
95'18
460 453
5.9
-
OIL
Elemental composition, wt. °/ /o i i H S [ 85.10 i~ 12.32 2'04 86.25] 11.37 7'05 1 81.48 9 . 4 5 1.99 C
O (by difference) Traces 0.54 ! 0"33 N
1.81
5.27
Fraction 1 contains paraffinic and naphthenic hydrocarbons. The molecular weight distribution peak is roughly C 3 6 , and the upper limit of the masses of ions in mass spectra reaches C5o. In the mass spectra there are pronounced individual peaks of characteristic ions of isoprenoid-type polycyclic naphthenic h y d r o c a r b o n s - s t e ranes and triterpanes (rn/z 217, 218, 259, 191, 121, 135, 149).
200
T . S . N1KITINA et al.
Fractiort 2 consists mainly of saturated cyclic hydrocarbons containing up to 7 condensed rings. It also contains aromatic compounds: alkylbenzenes and naphthenebenzenes with 1-6 naphthenic rings in the condensed system. There are no organosulphur compounds in fractions 1 and 2 (Table 2). TABLE 2. COMPOSITION (WT. ~oo) OF FRACTIONS OBTAINED BY ADSORPTION SEPARATION BY HIGHPERFORMANCE LIQUID CHROMATOGRAPHYFROM A 450°-500°C DISTILLATE OF INDUSTRIAL WEST SIBERIAN CRUDE OIL (YIELD OF FRACTIONS GIVEN IN BRACKETS) Type of compound
Hydrocarbons paraflinic naphthenic aromatic alkylbenzenes naphthenob~nzenes naphtbalenes naphthenonaphthalenes polycyclic Organosulphur compounds thiacycloalkanes thiophene derivatives benzothiophenes naphthenobenzothiophenes dibenzothiophenes naphthenodibenzothiophenes benzthiapyrene naphthobenzothiophenes Organonitrogen compounds
11241
( )
(__)
25 75
81 7 12
6
(?7)
(?2)
(5)
(8)
15 37 11 15 2
8 33 23
5 45
36
29
58
11 4 2 1
7 11 9 3
1
1
[
(71)
58
) 7 8 1 6
42
Mainly benzene structures and their derivatives with 1-5 condensed naphthenic rings in the molecule and naphthalene and naphthenonaphthalene hydrocarbons were isolated in fraction 3. Polycyclic hydrocarbons and also benzo- and dibenzothiophenes and their naphthenic derivatives are present in this fraction. The main components of fraction 4 are naphthalenes, acenaphthenes, fluorenes, and benzo- and dibenzothiophenes. The benzothiophene derivatives contain 1-4 condensed rings. Fraction 5 consists mainly of polynuclear aromatic hydrocarbons: phenanthrenes, naphthenophenanthrenes, pyrenes, chrysenes, and also dibenzo- and naphthobenzothiophenes. This fraction also contains condensed saturated cyclic structures with 1-5 naphthenic tings. Saturated cyclic sulphides containing up to 6 condensed naphthenic rings and polynuclear aromatic hydrocarbons were concentrated in fraction 6. Fraction 7 comprises thiacycloalkanes with a degree of cyclicity of ~ 8, and also polycyclic nitrogen-containing aromatic compounds.
High-boiling petroleum distillates
201
The fraction of polar compounds 8 eluted with methanol comprises a complex mixture of sulphur-, oxygen-, and nitrogen-containing condensed compounds, which is not readily identified. Qualitative analysis of mass spectra made it possible to assume that they contain high-molecular aromatic hydrocarbons. Thus HPLC made it possible efficiently to separate saturated hydrocarbons, aromatic hydrocarbons, and sulphur-containing compounds with different numbers of aromatic rings and to concentrate saturated sulphides and neutral nitrogen-containing compounds in the latter fractions. Such a distribution between fractions is most favourable for mass-spectral analysis, since it makes it possible to eliminate a number of significant superimposition conflicts in the mass spectra. Furthermore, although the fractions obtained have a complex composition, the groups of compounds concentrated in them have characteristic peaks in different regions of the mass spectra, and they differ in the characteristic forms of distribution of the intensities of peaks in the characteristic groups of ions. Consequently, HPLC on Sorbax Sil sorbent can be used as a convenient separation method for the general analysis of high-boiling petroleum distillates. CONCLUSIONS
1. High-performance liquid chromatography on Sorbax Sil sorbent has been used to separate a 450°-500°C distillate of industrial West Siberian crude oil. Sorbax Sil made it possible to isolate saturated hydrocarbons and to separate aromatic hydrocarbons according to the degree of cyclicity. 2. It has been established that thiacycloalkanes are distributed between fractions cf polycyclic aromatic hydrocarbons and organooitrogen compounds, while thiophene compounds are concentrated with aromatic hydrocarbons. 3. The use of high-performance liquid chromatography makes it possible to eliminate major mutual superimpositions of the mass spectra of the different compounds. REFERENCES
1. N.K. LYAPINA, Khimiya i fizikokhimiya seraorganicheskikh soyedinenii neftyanykh distillyatov (Chemistry and Physical Chemistry of Organosilicon Compounds of Petroleum Distillates), p. 48, Nauka, Moscow, 1984 2. S. C. SAUTONI and R. E. SWAB, J. Chromatogr. Sci. 13, 361, 1975 3. W. A. DARK, J. Liquid Chromatogr. 6, 2, 325, 1983 4. M. A. PARFENOVA, A. A. VOL'TSOV, N. K. LYAPINA and F. M. LATYPOVA, Auth. Cert. 1281587 (U.S.S.R.), Byull. izobr., 1, 10, 1987 5. Ye. S. BRODSKII, Metody issledovaniya sostava organicheskikh soyedinenii nefti i bitumoidov (Methods for Investigating Composition of Organic Compound.s of Petroleum and Bitumoids), p. 57, Nauka, Moscow, 1985
0031-64J8/88 $10.00+.00 © 1990 Pergamon Press pie
HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY IN T H E S T U D Y O F H I G H - B O I L I N G P E T R O L E U M D I S T I L L A T E S ¥' T. S. NIKITINA,N. K. LYAPINA,YE. S. BRODSKII,A. D. ULENDEYEVAand A. A. BERG Institute of Chemistry of Bashkir Branch of U.S.S.R.Academy of Sciences (Received 9 November 1987)
INCREASEDattention is currently being paid to the problem of distillation of heavy petroleum feedstock in the light of reduction in crude oil resomces. Efficient refining of heavy petroleum feedstock necessitates a detailed analysis of the nature and structure of its constituents. High-boiling petroleum distillates are noted for a complex group composition and a variety of chemical structures. The production of information on their composition depends on the efficiency of separation methods ensuring sufficient simplification of fractions for subsequent analysis, and on the efficiency of the methods for identification of constituents and their quantitative analysis. The main method for identifying compounds (or groups of compounds) when studying high-boiling distillates should still be molecular mass spectrometry, about which a great deal of experimental data has accumulated in the field of study of the composition of hydrocarbons and heteroatomic compounds of lower and medium petroleum distillates. Separation of petroleum fractions into groups of compounds of more homogeneous composition enables mass spectrometry to be used successfully. The separation methods normally used to analyse medium fractions are rectification, liquid-adsorption chromatography, extraction, and complexing, but none are sufficiently efficient or selective with respect to heavy petroleum fractions [1]. High-performance liquid chromatography (HPLC) can be used as a most efficient method during group differentiation of high-boiling petroleum distillates [2, 3]. The aim of the present paper was to study the possibility of using HPLC as .a separatiort method when studying the composition of a high-boiling 450°-500°C fraction of industrial West Siberian crude oil. The present investigation was carried out with collaboration between the academies of science of the socialist countries. EXPERIMENTAL
The investigation was conducted on Sorbax Sil sorbent using a liquid chromatograph of the Du Pont company (U.S.A.). The detectors were a model 441 differential refractometer and a 254 nm UV detector. * Neftokhimiya 28, No. 5, 710-714, 1988. 197
198
T.S. NIKITINAet
al.
The 450°-500°C vacuum distillate investigated was obtained by distillation of industrial West Siberian crude oil at the experimental works of the BashNIINP Institute. The distillate yield amounts to 7.3 wt. %, the total sulphur content to 2.04 wt. ~o, and the sulphide sulphur content to 1.07 wt. %. The characteristics of the distillate are given in Table 1.
L
1 2
I
1
S
i
1
10
rain
Chromatogram of separation of a 450°-500°C distillate of industrial West Siberian crude oil; detectors - ]efractometer (a) and 245 nm U V detector (b).
Separation of the distillate was preceded by preliminary removal of resinous compounds using an MPS-700 sorbent in a Soxhlet apparatus by the procedure given in [4]. The sorbent particle size was 0.16-0.07 mm, and the average pore size ranged from 600 to 800 A. Porous glasses exhibiting high selectivity to resinous compounds hardly adsorb hydrocarbons and sulphur compounds. The characteristics of deresinated distillate are given in Table 1. Analytical separation of the distillate investigated was carried out on a 4.6 m m x 25 cm column; Sorbax Sil was adopted as the sorbent, and the particle size was 5/tm. The efficiency of the columns amounted to 10,000 theoretical plates. n-Hexane was used as the mobile phase and the solvent of the samples. The specimen in hexane was filtered through a 0.5/~m filter and introduced via a tap into a chromatographic column prewashed with n-hexane with a flow rate of 1 ml/min. First of all, saturated compounds recorded from the signal of the refractometer peak were washed out. Then the sulphur and aromatic compounds recorded by the UV detector were eluted. Semipreparative soparation of the given HPLC distillate was carried out on a 21"2 mm x 25 cm column with a particle size of the Sorbax Sil sorbent of 15/tin. A quantity of 100 mg distillate was dissolved in 2 ml of n-hexane, filtered, and introduced into the chromatographic column in a similar way. The rate of the mobile phase (n-hexane) was 4 ml/min. First, fractions of saturated compounds were collected (Figure, zones 1 and 2). Then the sulphur and aromatic compounds (zones 3-7) were eluted. After fractions of these compounds had been sampled, the mobile phase was changed for a polar eluent (methanol) to remove polar compounds (frac-
High-boiling petroleum distillates
199
tion 8) from the column. Then the column was again washed with n-hexane and used for repeated analysis. As can be seen from the chromatogram (Figure), efficient separation of the distillate did not occur. However, fractions samples in different parts of the general broad peak had different properties: refractive index, elemental composition, spectral characteristics. For mass-spectrometric analysis, a 450°-500°C distillate from industrial West Siberian crude oil was separated into 8 fractions according to yield time. Such separation was optimum according to data of preliminary investigations. The fractions obtained by adsorption separation were analysed on a Varian MAT 311A mass spectrometer with electron impact ionization at 70 eV, a scanning rate of 5 see/10 days, and a mass range of 31-1000 Da. The sample was introduced directly into the ion source and heated at a constant r~te to 350°(21 in 3 rain. Mass spectra were recorded constantly on magnetic disc. Some of the mass spectra recorded (normally 8-10) were printed out. These were chosen according to the curve of change in the complete ion current and covered the entile range of its variation. Calculation and processing of spectra were carried out by the scheme given in [5]. RESULTS AND DISCUSSION
The 450°-500°C distillate of industrial West Siberian crude oil (Stot=2.04~) is characterized by a significant content of organosulphur compounds (29.3~), about half of which are thiophene derivatives. The average molecular weight of the distillate is 460, and its resin content amounts to 5.9 ~ (,Table 1). The distribution of the main groups of compounds between fractions obtained by HPLC adsorption separation f~om 450°-500°C distillate of industrial West Siberian crude oil is presented in Table 2. An analysis of the mass spectra of fractions obtained by preparative separation of distillate showed the following. TABLE 1. CHARACTERISTICS OF A
Product Initial distillate Distillate after removal of resins Resins
450°-500°C DISTILLATE OF INDUSTRIAL WEST SIBERIAN CRUDE
Yield, wt. ~
Molecular weight
95'18
460 453
5.9
-
OIL
Elemental composition, wt. °/ /o i i H S [ 85.10 i~ 12.32 2'04 86.25] 11.37 7'05 1 81.48 9 . 4 5 1.99 C
O (by difference) Traces 0.54 ! 0"33 N
1.81
5.27
Fraction 1 contains paraffinic and naphthenic hydrocarbons. The molecular weight distribution peak is roughly C 3 6 , and the upper limit of the masses of ions in mass spectra reaches C5o. In the mass spectra there are pronounced individual peaks of characteristic ions of isoprenoid-type polycyclic naphthenic h y d r o c a r b o n s - s t e ranes and triterpanes (rn/z 217, 218, 259, 191, 121, 135, 149).
200
T . S . N1KITINA et al.
Fractiort 2 consists mainly of saturated cyclic hydrocarbons containing up to 7 condensed rings. It also contains aromatic compounds: alkylbenzenes and naphthenebenzenes with 1-6 naphthenic rings in the condensed system. There are no organosulphur compounds in fractions 1 and 2 (Table 2). TABLE 2. COMPOSITION (WT. ~oo) OF FRACTIONS OBTAINED BY ADSORPTION SEPARATION BY HIGHPERFORMANCE LIQUID CHROMATOGRAPHYFROM A 450°-500°C DISTILLATE OF INDUSTRIAL WEST SIBERIAN CRUDE OIL (YIELD OF FRACTIONS GIVEN IN BRACKETS) Type of compound
Hydrocarbons paraflinic naphthenic aromatic alkylbenzenes naphthenob~nzenes naphtbalenes naphthenonaphthalenes polycyclic Organosulphur compounds thiacycloalkanes thiophene derivatives benzothiophenes naphthenobenzothiophenes dibenzothiophenes naphthenodibenzothiophenes benzthiapyrene naphthobenzothiophenes Organonitrogen compounds
11241
( )
(__)
25 75
81 7 12
6
(?7)
(?2)
(5)
(8)
15 37 11 15 2
8 33 23
5 45
36
29
58
11 4 2 1
7 11 9 3
1
1
[
(71)
58
) 7 8 1 6
42
Mainly benzene structures and their derivatives with 1-5 condensed naphthenic rings in the molecule and naphthalene and naphthenonaphthalene hydrocarbons were isolated in fraction 3. Polycyclic hydrocarbons and also benzo- and dibenzothiophenes and their naphthenic derivatives are present in this fraction. The main components of fraction 4 are naphthalenes, acenaphthenes, fluorenes, and benzo- and dibenzothiophenes. The benzothiophene derivatives contain 1-4 condensed rings. Fraction 5 consists mainly of polynuclear aromatic hydrocarbons: phenanthrenes, naphthenophenanthrenes, pyrenes, chrysenes, and also dibenzo- and naphthobenzothiophenes. This fraction also contains condensed saturated cyclic structures with 1-5 naphthenic tings. Saturated cyclic sulphides containing up to 6 condensed naphthenic rings and polynuclear aromatic hydrocarbons were concentrated in fraction 6. Fraction 7 comprises thiacycloalkanes with a degree of cyclicity of ~ 8, and also polycyclic nitrogen-containing aromatic compounds.
High-boiling petroleum distillates
201
The fraction of polar compounds 8 eluted with methanol comprises a complex mixture of sulphur-, oxygen-, and nitrogen-containing condensed compounds, which is not readily identified. Qualitative analysis of mass spectra made it possible to assume that they contain high-molecular aromatic hydrocarbons. Thus HPLC made it possible efficiently to separate saturated hydrocarbons, aromatic hydrocarbons, and sulphur-containing compounds with different numbers of aromatic rings and to concentrate saturated sulphides and neutral nitrogen-containing compounds in the latter fractions. Such a distribution between fractions is most favourable for mass-spectral analysis, since it makes it possible to eliminate a number of significant superimposition conflicts in the mass spectra. Furthermore, although the fractions obtained have a complex composition, the groups of compounds concentrated in them have characteristic peaks in different regions of the mass spectra, and they differ in the characteristic forms of distribution of the intensities of peaks in the characteristic groups of ions. Consequently, HPLC on Sorbax Sil sorbent can be used as a convenient separation method for the general analysis of high-boiling petroleum distillates. CONCLUSIONS
1. High-performance liquid chromatography on Sorbax Sil sorbent has been used to separate a 450°-500°C distillate of industrial West Siberian crude oil. Sorbax Sil made it possible to isolate saturated hydrocarbons and to separate aromatic hydrocarbons according to the degree of cyclicity. 2. It has been established that thiacycloalkanes are distributed between fractions cf polycyclic aromatic hydrocarbons and organooitrogen compounds, while thiophene compounds are concentrated with aromatic hydrocarbons. 3. The use of high-performance liquid chromatography makes it possible to eliminate major mutual superimpositions of the mass spectra of the different compounds. REFERENCES
1. N.K. LYAPINA, Khimiya i fizikokhimiya seraorganicheskikh soyedinenii neftyanykh distillyatov (Chemistry and Physical Chemistry of Organosilicon Compounds of Petroleum Distillates), p. 48, Nauka, Moscow, 1984 2. S. C. SAUTONI and R. E. SWAB, J. Chromatogr. Sci. 13, 361, 1975 3. W. A. DARK, J. Liquid Chromatogr. 6, 2, 325, 1983 4. M. A. PARFENOVA, A. A. VOL'TSOV, N. K. LYAPINA and F. M. LATYPOVA, Auth. Cert. 1281587 (U.S.S.R.), Byull. izobr., 1, 10, 1987 5. Ye. S. BRODSKII, Metody issledovaniya sostava organicheskikh soyedinenii nefti i bitumoidov (Methods for Investigating Composition of Organic Compound.s of Petroleum and Bitumoids), p. 57, Nauka, Moscow, 1985