- Email: [email protected]
Comparison of the selectivity of extrographic and chromatographic fractionations
Comparison of the selectivity of extrographic and chromatographic f ractionations Jaroslav
Cernq,
Gustav
$ebor*
and Ji?i Mitera’
Institute of Geotechnics, Czech Academy of Sciences, V HoleSovitikktich 41, 182 09 Prague 8, Czechoslovakia *Department of Petroleum Technology and Petrochemistry, Institute of Chemical Technology, Technicki 5, 166 28 Prague 6, Czechoslovakia t Laboratory of Mass Spectrometry, Institute of Hygiene and Epidemiology, Srobdrova 48, 100 42 Prague 10, Czechoslovakia (Received 1 I February 1997)
In order to study fractionation
selectivity,
chromatography
of shale oil on silica gel was used as a reference
for comparison with the extrographic fractionation previously described. The overlapping effects between the adjacent fractions were evaluated using i.r., ‘H n.m.r. and mass spectrometry. It was found that extrographic fractionation produced better separation than chromatography. In both cases, subsequent separation of the n-hexane fractions on silica gel/silver nitrate columns into saturates and olefin/monoaromatic fractions was needed. (Keywords: fractionation;
chromatography;
separation)
The compound-class fractionations of coal-derived liquids by low pressure chromatography are often performed on silica gellp4. Using these fractionations a number of fractions can be obtained. However, the purity of the fractions is highly dependent on the activity of the silica gel. Silica gel is known to be a good sorbent for the separation of saturated hydrocarbons from aromatics, but some overlapping of aromatics with saturates occurs1’5-7. Despite the general use of silica gel for separation of saturates, in the recent work of Holmes and Raska’ separation was carried out using acidic alumina and silica gel with 20% silver nitrate. The use of silver ions has also been described by others9,10. Neutral oxygen, sulphur and pyrrolic-type heterocompounds are usually eluted from silica gel together with aromatic hydrocarbons. The efficiency of separation of aromatics from phenolic compounds depends on the elution solvent. Using toluene (or benzene), some phenolic compounds are eluted within the aromatic/ polyaromatic fraction’ “’ 2. On the other hand, the use of a weaker solvent leads to insufficient elution of polyaromatics2. In a previous paper ’ 3 the results of the compound-class fractionation of coal-derived liquids using an extrographic technique have been reported. The advantages of the extrographic technique are the possibility of high loading of the column by sample and a low consumption of both solvents and sorbents for the fractionation. Furthermore, the use of two filter sections in the fractionation column’3 allows a variety of sorbents to be used. Hence, developing the desired fractions and minimalizing the irreversible adsorption on sorbents can be optimized more easily. Thus, in the original work13, basic alumina in a one filter section was used to develop 001~2361/91;070857-04 C’ 1991 Butterworth-Heinemann
Ltd
the fractions of saturated and aromatic hydrocarbons, and compounds with pyrrolic nitrogen. The elution of polar compounds, namely those with acidic sites, was then directed through the second filter section filled with deactivated silica gel. The aim of this paper is to compare the separation efficiency of both fractionation techniques, i.e. chromatographic and extrographic, in relation to the overlapping effects between individual compound classes. Column chromatography of shale oil on fully active silica gel was chosen for comparison with the extrographic fractionation previously described’ 3. EXPERIMENTAL Both the extrographic technique and the fractionation procedure have already been described in detail’ 3. From the samples fractionated, shale oil (KoS&lov region, Czechoslovakia) was chosen for comparative fractionation using column chromatography. The chromatographic fractionation was performed on a glass column (21 mm i.d., 1000 mm long) filled with silica gel 60 (0.063-0.200 mm) that was activated in uucuo for 16 h at 130°C. Sea sand (30 g, 0.14.3 mm) was pre-coated by mixing with a solution of 3 g of shale oil in dichloromethane and subsequently evaporating the solvent. The elution solvents13 were pumped into the column using a plunger pump at a flow rate of The elution volumes collected were: lOmlmin_‘. n-hexane, 1000 ml; toluene, 700 ml; chloroform, 1500 ml; chloroform/lO% diethylether, 500 ml; tetrahydrofuran/ 15% methanol, 400 ml; methanol, 400 ml. The cut-off points between fractions were determined by the boundary between the elution solvents. The fractions
FUEL,
1991,
Vol 70, July
857
Extrographic and chromatographic fractionations: J. fern+ et al.
1 S!ale
oil
RESULTS AND DISCUSSION
1
Iextroqraphyor chromatography
n-hexanc
chromatoqra SiO,/AqN&
El.CY
tolusne
hy
n-horn.
E2,CZ
85% tetrahydrofuran 15% methanol
Figure 1 Separation fractionations
1E5,C5
1
scheme of the extrographic
and chromatographic
obtained were rotary evaporated and solvent residues were removed in a vacuum oven at 80°C. The saturated hydrocarbons were separated from the n-hexane fractions by chromatography on a column (13.5 mm id., 450 mm long) filled with silica gel 60 with 20% silver nitrate. The column was wetted with n-hexane, and nine 20 ml n-hexane subfractions of saturates were collected. Aromatics were back-flushed with 100 ml of toluene. The fractions were treated as described above. The separation scheme is shown in Figure 1.
The reagent grade solvents were distilled before use. The ethanol content in chloroform after distillation was < 0.1%. Silica gel 60 was pre-coated with silver nitrate from its solution in deionized water, and the sorbent was then dried and activated in vacua for 16 h at 130°C. The FT-i.r. spectra were recorded on a spectrometer in dichloromethane using a 0.6 mm NaCl cell at a concentration of 50 mg ml-l. The spectral regions 370&3300 and 180&l 500 cm - 1 were evaluated. The ‘H n.m.r. spectra were recorded on a spectrometer operating at 400.13 MHz in deuterated chloroform with tetramethylsilane as internal standard. Mass spectrometric analyses were carried out. The ionization energy was 70 eV in the case of saturated fraction analyses. For the aromatic fraction, ionization by low energy electrons (10 eV) was used, with the ion intensity ratio of m/z 107:91 in an ethylbenzene spectrum being adjusted to be equal to 2. Other experimental conditions were as follows: the temperature of the ion source was 200°C at a pressure of 10e4 Pa, and an acceleration potential of 3 kV. The benzene solutions (1 wt%) of analysed fractions were directly evaporated into the ion source of the mass spectromter and analysed at a probe temperature increasing from 50 to 300°C at a rate of 64°C min- ‘. Calculations were peformed on averaged mass spectra of evaporated material. The relative average molecular weight of saturated fractions was determined by vapour pressure osmometry in benzene at 37°C using an osmometer with an analogue meter. The instrument was calibrated with diphenylethane.
858
FUEL,
1991,
Vol 70, July
The yield and elemental analysis of the individual fractions derived from the extrographic (El-E6) and chromatographic (Cl-C6) fractionations of the shale oil are given in Table 1. The yields from the chromatographic fractionation were highest in the first three fractions. On the other hand, the extrographic method gave good yields in the fourth and fifth fractions. The yields and elemental compositions of the n-hexane fractions El and Cl (Table 1) were almost the same. Hence, the quality of these fractions can be considered to be similar. The n-hexane fractions are often referred to as being composed of saturated hydrocarbons only. However,the results of ‘H n.m.r. (Table 2) and mass spectrometry (Table 3) do not support this. A high amount of olefins and monoaromatics was found in both n-hexane fractions, and the subsequent efficient separation of these fractions is discussed below. The yields of the fractions E2 and C2 differ; the differences can be seen in the elemental composition, namely the heteroatom contents, as well as in the structural parameters derived from ‘H n.m.r. spectroscopy (Tab/e 2). The significant qualitative differences between these two fractions are also apparent from their i.r. spectra in the region 370&3300 cm-’ (Figures 2 and 3). In contrast to fraction E2, the spectrum of fraction C2 shows absorption bands at 3600 and 3460 cm-’ which correspond to the hydroxylic and N-H groups, respectively. These analyses demonstrate that the aromatic fraction developed by the chromatographic techique is highly contaminated by hydroxyl- and/or pyrrolic-type compounds whereas the extrographic technique provides an aromatic fraction almost free of contamination. The extrographic fraction E3 does not have a counterpart in chromatographic fractionation. In this fraction the pyrrolic-type nitrogen compounds are highly concentrated, and the hydroxyl compounds are still not eluted. This is due to the use of the basic alumina filter13. The main portion of the chloroform-eluted phenolic fraction C3 was not obtained at the solvent front; elution started approximately at a volume of 1100 ml and continued up to 1300 ml. Fraction C3 seems to be similar
Table 1
Yield and elemental
analysis
of fractions ~-
Fraction El E2 E3 E4 E5 &S El/AR Cl c2 c3 c4 c5 :7/s Cl/AR
Yield (wt%)
C (wt%)
H (wt%)
N (wt%)
41.7 14.4 1.9 17.4 10.1 I.0 16.5 24.6
86.3 88.3 82.5 79.0 74.9 _b
0.2 4.2 2.0 3.4 _h
85.7 86.5
13.7 8.9 8.8 8.8 8.2 _b 14.3 12.8
39.8 23.6 17.7 1.1 3.3 0.9 16.0 23.4
86.6 86.5 79.2 75.9 75.5 _b
13.4 9.0 8.8 8.2 8.1 -b
0.6 2.3 3.7 4.1 _b
85.9 86.8
14.1 12.8
-
a By difference *Not determined
_
(O+S)” (wt%) _ 2.6 4.5 10.2 13.5 -b _ 0.7 _ 3.9 9.7 12.2 12.3 _b _ 0.4
C/H 0.52 0.83 0.78 0.15 0.76 0.50 0.56 0.54 0.80 0.75 0.77 0.78 0.51 0.57
Extrographic Table 2 _ ___
Structural ~_
parameters
derived
from ‘H n.m.r.
and chromatographic
fractionations:
J. Cerng et al.
spectroscopy C”” PT
Cil0Jh ar
0.52
0.25 _
0.14
0.42
0.14
0.44
0.58
0.21
0.17
0.07
0.65
0.24
0.10
0.32
0.38
0.13
0.48
0.21
0.20
0.07
0.49
0.88
0.13
0.28
0.45
0.14
0.42
0.17
0.19
0.06
0.53
0.86
c4
0.10
0.30
0.45
0.15
0.42
0.13
0.19
0.10
0.59
0.76
C5
0.12
0.33
0.43
0.12
0.44 ~~
0.15 _~
0.21
0.08
0.58
0.82
Fraction
H,,
H*
H,
H,
El
0.04
0.08
0.63
0.25
0.08
E2
0.21
0.33
0.34 _
0.12 _
E4
0.13
0.27
0.46
E5
0.15
0.31
0.40
El/AR
0.07
0.14
Cl
0.04
c2
0.17
C3
E3
.L
’ CFr = H,, x H/C, fraction of protonated aromatic carbon ’ Cp” = HJ2 x H/C, fraction of alkyl substituted aromatic carbon or condensed aromatic carbon r c:, =j: - C;:b, fraction of aryl substituted ” H,,,/C,, = (Cyr+Ci:b)/f,, atomic H/C ration of the hypothetical unsubstituted 0 = C~“/(C~‘, + Czb). degree of substitution of the aromatic rings
-.
CC< dr
d
H,,,/C,,
0.20
0.07
0.44
0.87
0.17
0.18
0.07
0.51
0.83
0.19
0.21
0.04
0.53
0.91
~~
c::
Table 3 (vol%) Compound
Mass spectrometric
type
n- and lsoalkanes Cycloalkanes monobitritetrapentahexaMonoaromatics
type analysis
of the n-hexane
fractions“’
Cl
El
El/S
27.9
26.2
26.2
20.4 16.9 8.1 6.8 11.7 0.0 8.2
18.5 15.6 11.8 7.1 9.3 0.0 11.0 ___
14.3 25.9 20.2 7.9 5.4 0.0 0.0 ~_~~
aromatic
rings
r
3300 1800 Wavenumbers
Figure 3 1.r. spectra fractionation
L
3300 1800
3500
150
Wavenumbers (ci’)
Figure 2 fractionation
1.r.
spectra
of
fractions
derived
from
extrographic
of fractions
derived
(cni’)
from
chromatographic
to the extrographic fraction E4 developed from a chloroform/diethylether mixture (Figures 2 and 3, Tables I and 2). The shift in the solvent strength needed for eluting the phenols by extrography was caused by the basic alumina filter in the extrographic column. During chloroform elution, some of the phenolic compounds were released from the silica gel and retained on the basic alumina. Reversing the solvent flow direction’j and eluting using a chloroform/diethylether mixture meant that the phenols were not quantitatively removed from
FUEL, 1991, Vol 70, July
859
Extrographic
1
and chromatographic
1
8.0
7.0 Chemical
Figure 4
shift
Part of the ‘H n.m.r. spectrum
fractionations:
I
,
6.0
5.0
J. cernl; et al.
(ppm)
of the El/AR
fraction
basic alumina. The more polar phenols retained on alumina and those still adsorbed on silica gel were eluted in the successive extrographic fraction E5 using a tetrahydrofuran/methanol mixture. The yields of the corresponding chromatographic fractions C4 and C5 were quite low. The interaction of the basic alumina with the phenolic compounds was responsible for the higher retention of phenols in the extrographic column. Consequently, the extrographic phenolic fractions need to be eluted with stronger elution solvents. Also their yield was higher because of the overlapping of some phenols into the toluene fraction during chromatographic fractionation. Herod et al.” pointed out that the polar materials may represent 50% of the aromatic fraction. Analyses of the methanol fractions E6 and C6 were not done. Separation of saturated hydrocarbons
The mass spectrometric analyses of the n-hexane fractions El and Cl are presented in Table 3. From this table it can only be deduced that the concentration of monoaromatics is > 5 ~01% because this is the upper limit for the computing method usedi4. The analysis is also affected by the presence of olefins which were detected using ‘H n.m.r. spectroscopy. Hence, both n-hexane fractions El and Cl were separated on silica gel with 20% silver nitrate. This complexation technique has been found to be a superior method for the separation of saturates from olefins and aromatics’. The yield and elemental analysis of the separated fractions are given in Table 1. The low yields of saturates El/S and Cl/S were very surprising. Nevertheless, by collecting the 20 ml subfractions it was found that virtually all saturates were obtained at the n-hexane elution front in a narrow range of the elution volume. Thus, for the fraction El/S the yields of nine subfractions were: 0, 0 (void volume), 39.1, 0.2, 0.3, 0, 0, 0,O wt% relative to El. The separation was very sharp. There is no reason to suppose that some saturates form a complex with the silver. If this was the case, the distribution of the yields of the subfractions would have to be much broader. Therefore, the narrow elution range indicates that all saturates were eluted with n-hexane from the silica gel/silver nitrate column. The mass spectrometric analysis of the fraction El/S in Table 3 confirms the high efficiency of this separation. In addition, the separation of saturates on the silica
860
FUEL,1991,Vol
70, July
gel/silver nitrate column needs neither control by U.V. light nor optimization of elution volume7. In this work we applied 0.3 g of El or Cl per 30 g of SiOJAgNO,, i.e. the loading of the column by sample was 1 wt%. However, during subsequent fractionations of coal tar, the possibility of higher loading (up to 0.45 g of the extrographic n-hexane fraction per 7 g of SiO,/AgNO,) was successfully examinedr5. Using mass spectrometry no diaromatic hydrocarbons were detected in the El/AR fraction. It was composed of substituted monoaromatics and a high portion of olefins, which appear like cycloparaihns in the mass spectrum. The presence of olefins in the El/AR fraction is shown in the ‘H n.m.r. spectrum in Figure 4. The ratio between the aromatic and olefinic hydrogens (Ha,:H,,,_) was 0.039:0.028. From a theoretical consideration of this hydrogen ratio and knowing the average molecular weight (280 daltons) the possible composition of the fraction El/AR could be obtained. For calculation, the olefinic and monoaromatic model compounds were used, and the probable composition was found: 60% oiefins mainly with terminal double bonds and 40% of a mixture of di- and trisubstituted monoaromatics. In spite of the inaccurate character of this calculation, analysis of the El/AR fraction verifies the superior selectivity or separation of saturates on the silica gel/silver nitrate column, CONCLUSIONS The results of the extrographic and chromatographic fractionations confirm that there are significant differences between the two techniques. The extrographic method was found to fractionate the sample of shale oil into compound classes more selectively than common chromatography on silica gel. The saturates were insufficiently separated from olefins and monoaromatics by both methods. Additional fractionation on a silica gel/silver nitrate column was needed for a superior separation of saturates. By extrography it was possible to separate the aromatic fraction from pyrrolic and phenolic compounds. The pyrrolic compounds were obtained as a discrete fraction. In the case of chromatography, the pyrrolic compounds and some phenols eluted into the aromatic fraction. Two phenolic fractions were obtained using extrographic fractionation whilst only one fraction was obtained in a substantial amount by chromatography. REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Schweighardt, F. K., Retcofsky, H. L. and Friedel. R. A. Fi& 1976, 55, 313 Farcasiu, M. Fuel 1977, 56, 9 Schwager, I. and Yen, T. F. Fuel 1979, 58, 219 Bartle, K. D., Ladner, W. R., Martin, T. G. et al. Fuel 1979, 58,413 Seshadri, K. S. and Cronauer, D. C. Fuel 1983,62, 1436 Wallace, D., Henry, D., Pongar, K. et al. Fuel 1987, 66, 44 Sebor, G., Blaiek, J. and Mitera, J. Ropauhfie 1990, 32, 373 Holmes, A. A. and Raska, K. A. Fuel 1986,65, 1539 Hayes, P. C. and Anderson, S. D. Anal. Chem. 1986, 58, 2384 Rovere, E. E., Crisp. P. T., Ellis, J. et al. Fuel 1990, 69, 1099 Herod, A. A., Ladner, W. R., Stokes, B. .J. et al. Fuel 1987,46,935 Herod, A. A., Stokes, B. J., Major, H. J. ef al. Analyst 1988, 113,797 Cerng, J., PavlikovB, H. and MachoviE, V. Fuel 1990, 69, 966 ASTM D 2786-81, Annual Book of ASTM Standards, Vol. 05.02, American Society for Testing and Materials, Philadelphia 1986 Cern$, J. Chem. f&m. submitted
Cernq,
Gustav
$ebor*
and Ji?i Mitera’
Institute of Geotechnics, Czech Academy of Sciences, V HoleSovitikktich 41, 182 09 Prague 8, Czechoslovakia *Department of Petroleum Technology and Petrochemistry, Institute of Chemical Technology, Technicki 5, 166 28 Prague 6, Czechoslovakia t Laboratory of Mass Spectrometry, Institute of Hygiene and Epidemiology, Srobdrova 48, 100 42 Prague 10, Czechoslovakia (Received 1 I February 1997)
In order to study fractionation
selectivity,
chromatography
of shale oil on silica gel was used as a reference
for comparison with the extrographic fractionation previously described. The overlapping effects between the adjacent fractions were evaluated using i.r., ‘H n.m.r. and mass spectrometry. It was found that extrographic fractionation produced better separation than chromatography. In both cases, subsequent separation of the n-hexane fractions on silica gel/silver nitrate columns into saturates and olefin/monoaromatic fractions was needed. (Keywords: fractionation;
chromatography;
separation)
The compound-class fractionations of coal-derived liquids by low pressure chromatography are often performed on silica gellp4. Using these fractionations a number of fractions can be obtained. However, the purity of the fractions is highly dependent on the activity of the silica gel. Silica gel is known to be a good sorbent for the separation of saturated hydrocarbons from aromatics, but some overlapping of aromatics with saturates occurs1’5-7. Despite the general use of silica gel for separation of saturates, in the recent work of Holmes and Raska’ separation was carried out using acidic alumina and silica gel with 20% silver nitrate. The use of silver ions has also been described by others9,10. Neutral oxygen, sulphur and pyrrolic-type heterocompounds are usually eluted from silica gel together with aromatic hydrocarbons. The efficiency of separation of aromatics from phenolic compounds depends on the elution solvent. Using toluene (or benzene), some phenolic compounds are eluted within the aromatic/ polyaromatic fraction’ “’ 2. On the other hand, the use of a weaker solvent leads to insufficient elution of polyaromatics2. In a previous paper ’ 3 the results of the compound-class fractionation of coal-derived liquids using an extrographic technique have been reported. The advantages of the extrographic technique are the possibility of high loading of the column by sample and a low consumption of both solvents and sorbents for the fractionation. Furthermore, the use of two filter sections in the fractionation column’3 allows a variety of sorbents to be used. Hence, developing the desired fractions and minimalizing the irreversible adsorption on sorbents can be optimized more easily. Thus, in the original work13, basic alumina in a one filter section was used to develop 001~2361/91;070857-04 C’ 1991 Butterworth-Heinemann
Ltd
the fractions of saturated and aromatic hydrocarbons, and compounds with pyrrolic nitrogen. The elution of polar compounds, namely those with acidic sites, was then directed through the second filter section filled with deactivated silica gel. The aim of this paper is to compare the separation efficiency of both fractionation techniques, i.e. chromatographic and extrographic, in relation to the overlapping effects between individual compound classes. Column chromatography of shale oil on fully active silica gel was chosen for comparison with the extrographic fractionation previously described’ 3. EXPERIMENTAL Both the extrographic technique and the fractionation procedure have already been described in detail’ 3. From the samples fractionated, shale oil (KoS&lov region, Czechoslovakia) was chosen for comparative fractionation using column chromatography. The chromatographic fractionation was performed on a glass column (21 mm i.d., 1000 mm long) filled with silica gel 60 (0.063-0.200 mm) that was activated in uucuo for 16 h at 130°C. Sea sand (30 g, 0.14.3 mm) was pre-coated by mixing with a solution of 3 g of shale oil in dichloromethane and subsequently evaporating the solvent. The elution solvents13 were pumped into the column using a plunger pump at a flow rate of The elution volumes collected were: lOmlmin_‘. n-hexane, 1000 ml; toluene, 700 ml; chloroform, 1500 ml; chloroform/lO% diethylether, 500 ml; tetrahydrofuran/ 15% methanol, 400 ml; methanol, 400 ml. The cut-off points between fractions were determined by the boundary between the elution solvents. The fractions
FUEL,
1991,
Vol 70, July
857
Extrographic and chromatographic fractionations: J. fern+ et al.
1 S!ale
oil
RESULTS AND DISCUSSION
1
Iextroqraphyor chromatography
n-hexanc
chromatoqra SiO,/AqN&
El.CY
tolusne
hy
n-horn.
E2,CZ
85% tetrahydrofuran 15% methanol
Figure 1 Separation fractionations
1E5,C5
1
scheme of the extrographic
and chromatographic
obtained were rotary evaporated and solvent residues were removed in a vacuum oven at 80°C. The saturated hydrocarbons were separated from the n-hexane fractions by chromatography on a column (13.5 mm id., 450 mm long) filled with silica gel 60 with 20% silver nitrate. The column was wetted with n-hexane, and nine 20 ml n-hexane subfractions of saturates were collected. Aromatics were back-flushed with 100 ml of toluene. The fractions were treated as described above. The separation scheme is shown in Figure 1.
The reagent grade solvents were distilled before use. The ethanol content in chloroform after distillation was < 0.1%. Silica gel 60 was pre-coated with silver nitrate from its solution in deionized water, and the sorbent was then dried and activated in vacua for 16 h at 130°C. The FT-i.r. spectra were recorded on a spectrometer in dichloromethane using a 0.6 mm NaCl cell at a concentration of 50 mg ml-l. The spectral regions 370&3300 and 180&l 500 cm - 1 were evaluated. The ‘H n.m.r. spectra were recorded on a spectrometer operating at 400.13 MHz in deuterated chloroform with tetramethylsilane as internal standard. Mass spectrometric analyses were carried out. The ionization energy was 70 eV in the case of saturated fraction analyses. For the aromatic fraction, ionization by low energy electrons (10 eV) was used, with the ion intensity ratio of m/z 107:91 in an ethylbenzene spectrum being adjusted to be equal to 2. Other experimental conditions were as follows: the temperature of the ion source was 200°C at a pressure of 10e4 Pa, and an acceleration potential of 3 kV. The benzene solutions (1 wt%) of analysed fractions were directly evaporated into the ion source of the mass spectromter and analysed at a probe temperature increasing from 50 to 300°C at a rate of 64°C min- ‘. Calculations were peformed on averaged mass spectra of evaporated material. The relative average molecular weight of saturated fractions was determined by vapour pressure osmometry in benzene at 37°C using an osmometer with an analogue meter. The instrument was calibrated with diphenylethane.
858
FUEL,
1991,
Vol 70, July
The yield and elemental analysis of the individual fractions derived from the extrographic (El-E6) and chromatographic (Cl-C6) fractionations of the shale oil are given in Table 1. The yields from the chromatographic fractionation were highest in the first three fractions. On the other hand, the extrographic method gave good yields in the fourth and fifth fractions. The yields and elemental compositions of the n-hexane fractions El and Cl (Table 1) were almost the same. Hence, the quality of these fractions can be considered to be similar. The n-hexane fractions are often referred to as being composed of saturated hydrocarbons only. However,the results of ‘H n.m.r. (Table 2) and mass spectrometry (Table 3) do not support this. A high amount of olefins and monoaromatics was found in both n-hexane fractions, and the subsequent efficient separation of these fractions is discussed below. The yields of the fractions E2 and C2 differ; the differences can be seen in the elemental composition, namely the heteroatom contents, as well as in the structural parameters derived from ‘H n.m.r. spectroscopy (Tab/e 2). The significant qualitative differences between these two fractions are also apparent from their i.r. spectra in the region 370&3300 cm-’ (Figures 2 and 3). In contrast to fraction E2, the spectrum of fraction C2 shows absorption bands at 3600 and 3460 cm-’ which correspond to the hydroxylic and N-H groups, respectively. These analyses demonstrate that the aromatic fraction developed by the chromatographic techique is highly contaminated by hydroxyl- and/or pyrrolic-type compounds whereas the extrographic technique provides an aromatic fraction almost free of contamination. The extrographic fraction E3 does not have a counterpart in chromatographic fractionation. In this fraction the pyrrolic-type nitrogen compounds are highly concentrated, and the hydroxyl compounds are still not eluted. This is due to the use of the basic alumina filter13. The main portion of the chloroform-eluted phenolic fraction C3 was not obtained at the solvent front; elution started approximately at a volume of 1100 ml and continued up to 1300 ml. Fraction C3 seems to be similar
Table 1
Yield and elemental
analysis
of fractions ~-
Fraction El E2 E3 E4 E5 &S El/AR Cl c2 c3 c4 c5 :7/s Cl/AR
Yield (wt%)
C (wt%)
H (wt%)
N (wt%)
41.7 14.4 1.9 17.4 10.1 I.0 16.5 24.6
86.3 88.3 82.5 79.0 74.9 _b
0.2 4.2 2.0 3.4 _h
85.7 86.5
13.7 8.9 8.8 8.8 8.2 _b 14.3 12.8
39.8 23.6 17.7 1.1 3.3 0.9 16.0 23.4
86.6 86.5 79.2 75.9 75.5 _b
13.4 9.0 8.8 8.2 8.1 -b
0.6 2.3 3.7 4.1 _b
85.9 86.8
14.1 12.8
-
a By difference *Not determined
_
(O+S)” (wt%) _ 2.6 4.5 10.2 13.5 -b _ 0.7 _ 3.9 9.7 12.2 12.3 _b _ 0.4
C/H 0.52 0.83 0.78 0.15 0.76 0.50 0.56 0.54 0.80 0.75 0.77 0.78 0.51 0.57
Extrographic Table 2 _ ___
Structural ~_
parameters
derived
from ‘H n.m.r.
and chromatographic
fractionations:
J. Cerng et al.
spectroscopy C”” PT
Cil0Jh ar
0.52
0.25 _
0.14
0.42
0.14
0.44
0.58
0.21
0.17
0.07
0.65
0.24
0.10
0.32
0.38
0.13
0.48
0.21
0.20
0.07
0.49
0.88
0.13
0.28
0.45
0.14
0.42
0.17
0.19
0.06
0.53
0.86
c4
0.10
0.30
0.45
0.15
0.42
0.13
0.19
0.10
0.59
0.76
C5
0.12
0.33
0.43
0.12
0.44 ~~
0.15 _~
0.21
0.08
0.58
0.82
Fraction
H,,
H*
H,
H,
El
0.04
0.08
0.63
0.25
0.08
E2
0.21
0.33
0.34 _
0.12 _
E4
0.13
0.27
0.46
E5
0.15
0.31
0.40
El/AR
0.07
0.14
Cl
0.04
c2
0.17
C3
E3
.L
’ CFr = H,, x H/C, fraction of protonated aromatic carbon ’ Cp” = HJ2 x H/C, fraction of alkyl substituted aromatic carbon or condensed aromatic carbon r c:, =j: - C;:b, fraction of aryl substituted ” H,,,/C,, = (Cyr+Ci:b)/f,, atomic H/C ration of the hypothetical unsubstituted 0 = C~“/(C~‘, + Czb). degree of substitution of the aromatic rings
-.
CC< dr
d
H,,,/C,,
0.20
0.07
0.44
0.87
0.17
0.18
0.07
0.51
0.83
0.19
0.21
0.04
0.53
0.91
~~
c::
Table 3 (vol%) Compound
Mass spectrometric
type
n- and lsoalkanes Cycloalkanes monobitritetrapentahexaMonoaromatics
type analysis
of the n-hexane
fractions“’
Cl
El
El/S
27.9
26.2
26.2
20.4 16.9 8.1 6.8 11.7 0.0 8.2
18.5 15.6 11.8 7.1 9.3 0.0 11.0 ___
14.3 25.9 20.2 7.9 5.4 0.0 0.0 ~_~~
aromatic
rings
r
3300 1800 Wavenumbers
Figure 3 1.r. spectra fractionation
L
3300 1800
3500
150
Wavenumbers (ci’)
Figure 2 fractionation
1.r.
spectra
of
fractions
derived
from
extrographic
of fractions
derived
(cni’)
from
chromatographic
to the extrographic fraction E4 developed from a chloroform/diethylether mixture (Figures 2 and 3, Tables I and 2). The shift in the solvent strength needed for eluting the phenols by extrography was caused by the basic alumina filter in the extrographic column. During chloroform elution, some of the phenolic compounds were released from the silica gel and retained on the basic alumina. Reversing the solvent flow direction’j and eluting using a chloroform/diethylether mixture meant that the phenols were not quantitatively removed from
FUEL, 1991, Vol 70, July
859
Extrographic
1
and chromatographic
1
8.0
7.0 Chemical
Figure 4
shift
Part of the ‘H n.m.r. spectrum
fractionations:
I
,
6.0
5.0
J. cernl; et al.
(ppm)
of the El/AR
fraction
basic alumina. The more polar phenols retained on alumina and those still adsorbed on silica gel were eluted in the successive extrographic fraction E5 using a tetrahydrofuran/methanol mixture. The yields of the corresponding chromatographic fractions C4 and C5 were quite low. The interaction of the basic alumina with the phenolic compounds was responsible for the higher retention of phenols in the extrographic column. Consequently, the extrographic phenolic fractions need to be eluted with stronger elution solvents. Also their yield was higher because of the overlapping of some phenols into the toluene fraction during chromatographic fractionation. Herod et al.” pointed out that the polar materials may represent 50% of the aromatic fraction. Analyses of the methanol fractions E6 and C6 were not done. Separation of saturated hydrocarbons
The mass spectrometric analyses of the n-hexane fractions El and Cl are presented in Table 3. From this table it can only be deduced that the concentration of monoaromatics is > 5 ~01% because this is the upper limit for the computing method usedi4. The analysis is also affected by the presence of olefins which were detected using ‘H n.m.r. spectroscopy. Hence, both n-hexane fractions El and Cl were separated on silica gel with 20% silver nitrate. This complexation technique has been found to be a superior method for the separation of saturates from olefins and aromatics’. The yield and elemental analysis of the separated fractions are given in Table 1. The low yields of saturates El/S and Cl/S were very surprising. Nevertheless, by collecting the 20 ml subfractions it was found that virtually all saturates were obtained at the n-hexane elution front in a narrow range of the elution volume. Thus, for the fraction El/S the yields of nine subfractions were: 0, 0 (void volume), 39.1, 0.2, 0.3, 0, 0, 0,O wt% relative to El. The separation was very sharp. There is no reason to suppose that some saturates form a complex with the silver. If this was the case, the distribution of the yields of the subfractions would have to be much broader. Therefore, the narrow elution range indicates that all saturates were eluted with n-hexane from the silica gel/silver nitrate column. The mass spectrometric analysis of the fraction El/S in Table 3 confirms the high efficiency of this separation. In addition, the separation of saturates on the silica
860
FUEL,1991,Vol
70, July
gel/silver nitrate column needs neither control by U.V. light nor optimization of elution volume7. In this work we applied 0.3 g of El or Cl per 30 g of SiOJAgNO,, i.e. the loading of the column by sample was 1 wt%. However, during subsequent fractionations of coal tar, the possibility of higher loading (up to 0.45 g of the extrographic n-hexane fraction per 7 g of SiO,/AgNO,) was successfully examinedr5. Using mass spectrometry no diaromatic hydrocarbons were detected in the El/AR fraction. It was composed of substituted monoaromatics and a high portion of olefins, which appear like cycloparaihns in the mass spectrum. The presence of olefins in the El/AR fraction is shown in the ‘H n.m.r. spectrum in Figure 4. The ratio between the aromatic and olefinic hydrogens (Ha,:H,,,_) was 0.039:0.028. From a theoretical consideration of this hydrogen ratio and knowing the average molecular weight (280 daltons) the possible composition of the fraction El/AR could be obtained. For calculation, the olefinic and monoaromatic model compounds were used, and the probable composition was found: 60% oiefins mainly with terminal double bonds and 40% of a mixture of di- and trisubstituted monoaromatics. In spite of the inaccurate character of this calculation, analysis of the El/AR fraction verifies the superior selectivity or separation of saturates on the silica gel/silver nitrate column, CONCLUSIONS The results of the extrographic and chromatographic fractionations confirm that there are significant differences between the two techniques. The extrographic method was found to fractionate the sample of shale oil into compound classes more selectively than common chromatography on silica gel. The saturates were insufficiently separated from olefins and monoaromatics by both methods. Additional fractionation on a silica gel/silver nitrate column was needed for a superior separation of saturates. By extrography it was possible to separate the aromatic fraction from pyrrolic and phenolic compounds. The pyrrolic compounds were obtained as a discrete fraction. In the case of chromatography, the pyrrolic compounds and some phenols eluted into the aromatic fraction. Two phenolic fractions were obtained using extrographic fractionation whilst only one fraction was obtained in a substantial amount by chromatography. REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Schweighardt, F. K., Retcofsky, H. L. and Friedel. R. A. Fi& 1976, 55, 313 Farcasiu, M. Fuel 1977, 56, 9 Schwager, I. and Yen, T. F. Fuel 1979, 58, 219 Bartle, K. D., Ladner, W. R., Martin, T. G. et al. Fuel 1979, 58,413 Seshadri, K. S. and Cronauer, D. C. Fuel 1983,62, 1436 Wallace, D., Henry, D., Pongar, K. et al. Fuel 1987, 66, 44 Sebor, G., Blaiek, J. and Mitera, J. Ropauhfie 1990, 32, 373 Holmes, A. A. and Raska, K. A. Fuel 1986,65, 1539 Hayes, P. C. and Anderson, S. D. Anal. Chem. 1986, 58, 2384 Rovere, E. E., Crisp. P. T., Ellis, J. et al. Fuel 1990, 69, 1099 Herod, A. A., Ladner, W. R., Stokes, B. .J. et al. Fuel 1987,46,935 Herod, A. A., Stokes, B. J., Major, H. J. ef al. Analyst 1988, 113,797 Cerng, J., PavlikovB, H. and MachoviE, V. Fuel 1990, 69, 966 ASTM D 2786-81, Annual Book of ASTM Standards, Vol. 05.02, American Society for Testing and Materials, Philadelphia 1986 Cern$, J. Chem. f&m. submitted