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Separation of petroleum hydrocarbons using dealuminated mordenite molecular sieve. I. Monoaromatic hydrocarbons
Org. Geochem. Vol. 18, No. 5, pp. 587-593, 1992 Printed in Great Britain.All rights reserved
0146-6380/92 $5.00+ 0.00 Copyright © 1992Pergamon Press Ltd
Separation of petroleum hydrocarbons using dealuminated mordenite molecular sieve. I. Monoaromatic hydrocarbons LEROY ELLIS, ROBERT I. KAGI and ROBERT ALEXANDER Centre for Petroleum and Environmental Organic Geochemistry, Curtin University of Technology, G.P.O. Box U1987, Perth 6001, Western Australia Abstract--A crude oil monoaromatic fraction was applied to a Pasteur pipette packed with dealuminated mordenite molecular sieve and eluted with pentane solvent, n-Alkylbenzenes, n-alkyltoluenes and some n-alkylxylene isomers were sorbed into the mordenite molecular sieve and recovered by acid digestion of the sieve and solvent extraction of the liberated hydrocarbons. The excluded fraction which was eluted in the pentane solvent was enriched in the 3,5- and 2,6-n-alkylxylene isomers and other compounds possessing a higher degree of structural complexity. The mordenite molecular sieve provides a rapid and convenient method for the fractionation of monoaromatic hydrocarbons allowing the more subtle characteristics of these fractions to be studied in greater detail. Key words--n-alkylbenzenes, n-alkyltoluenes, n-alkylxylenes, mordenite, molecular sieve, monoaromatics
INTRODUCTION Molecular sieve sorption of hydrocarbons has become a routine geochemical procedure for separating alkane components of petroleum. For many years 5A molecular sieves have been the mainstay of n-alkane removal from alkane fractions of petroleum (Murphy, 1969; Jasra and Bhat, 1987; Kerr, 1989). The larger pore silicalite molecular sieve has also proved effective in this role and has also been reported to sorb monomethyl alkanes and some cycloalkanes (Flanigen et aL, 1978; Hoering and Freeman, 1984; West et al., 1990). The even larger pore 13X molecular sieve will sorb hopane and tricyclic terpane compounds (Dimmler and Strausz, 1983). Molecular sieves containing silicon and aluminium in their structures are polar due to the replacement of silicon by aluminium in the molecular sieve resulting in a net negative charge on the molecular sieve framework. A non-framework charge-compensating cation such as sodium is then required to balance this negative charge. Molecular sieves containing a high aluminium content exhibit high polarity and hydrophilic properties, whereas high silica molecular sieves have low polarity and hydrophobic properties (Olson et al., 1980; Hoering and Freeman, 1984). One example of a high silica molecular sieve is dealuminated mordenite which is characterized by a silicon to aluminium ratio of 30 and a one-dimensional channel system consisting of two pore apertures of 6.7 x 7 and 2.9 x 5.7 ,/k (Dessau 1980; Barrer, 1984; Dwyer and Dyer, 1984; Ruthven, 1988). Molecular sieves with silicon to aluminium ratios > 10 are referred to as high silica sieves. The low polarity of the high silica mordenite molecular sieve offers promising applications to the shape-selective separation of aromatic components of petroleum. Because of the
low polarity of the sieve, shape-selective effects dominate the separation processes even with compounds as polar as monoaromatic hydrocarbons. Shape selective techniques are a potentially valuable addition to the range of techniques currently employed to further fractionate the aromatic hydrocarbons in petroleum. Conventional separation techniques such as normal and reverse phase liquid chromatography frequently fail to yield mixtures that can be fully resolved using capillary gas chromatography procedures. By further fractionation of such complex mixtures, molecular sieve separation techniques can be used to reduce the complexity of such mixtures and thereby enable analysis of their minor components. In this paper we report a simple chromatographic procedure using dealuminated mordenite molecular sieves which selectively provides n-alkylbenzenes, n-alkyltoluenes and some n-alkylxylenes in a sorbed retained fraction. The structural features of alkylxylenes and polymethyl substituted alkylbenzenes that result in exclusion from the sieves are reported and we illustrate how previously unreported minor components of the monoaromatic fraction, which were not sorbed by the molecular sieve could then be resolved using capillary gas chromatography techniques.
EXPERIMENTAL
Samples A Jurassic crude oil from the Coeper/Eromanga Basin was separated into alkane and aromatic fractions using a Merck-Lobar Grobe A (240-10) Lichroprep Si 60,40-63/~m column attached to a Waters Millipore Model 510 double piston pump. Using a flow rate of 2 ml rain -~ of n-hexane, an alkane 587
588
LEROY ELLIS et al.
fraction was collected between elution times of 8-12 min. The aromatic components were then eluted by reversing the direction of flow of the solvent for a further 25min. The alkane components were detected using a Waters Millipore Series R-400 Differential Refractometer and the aromatic components were detected using a Waters Associates Series 440 u.v. absorbance detector at a wavelength setting of 254 nm. The aromatic components were then further fractionated into monoaromatic, diaromatic and triaromatic bands using alumina thin layer chromatography with n-hexane as eluent. The monoaromatic fraction was collected by scraping bands from the plate and extracting the alumina with dichloromethane.
Mordenite molecular sieves The high-silica mordenite molecular sieve samples were kindly provided by Dr E. M. Flanigen of UOP Tarrytown Technical Center, New York and also from Dr Jeppe Magnusson of CONTEKA, Surte, Sweden. The powder was activated by heating overnight at 350°C and was stored in an airtight container. The activity of the mordenite molecular sieve diminished slowly and was found to sorb nalkylbenzene and n-alkyltoluene components of petroleum monoaromatic fractions for a period of 5 months after initial activation.
Molecular sieve separations Separations were performed using mordenite powder (1 g) packed into a Pasteur pipette as a suspension in pentane. In a typical experiment approx. 5-10 mg of monoaromatic sample was applied to the top of the column. After standing for 10 min the excluded components of the sample were eluted with three bed volumes of pentane (approx. 5 ml). The included components were recovered by treating the mordenite with hydrofluoric acid (1-3 ml/50% w/v) and solvent extracting (3 x 5 ml pentane) the liberated components. It was found that the action of hydrofluoric acid on the mordenite molecular sieve did not affect any of the sample components or the 15 reference compounds used in the blank molecular sieve experiments.
Synthesis of reference compounds All hydrocarbon reference compounds were prepared by the reaction of the appropriate aryl Grignard reagents with aldehydes and the subsequent reduction of the alcohols. Good yields (60-90%) of the benzylic alcohols were obtained. The alcohols were subjected to hydrogenolysis using palladium/ carbon (10%) as catalyst in glacial acetic acid with a hydrogen pressure of 1.5atm for 36h. The hydrocarbons produced were further purified by chromatography using a column of silica gel with n-hexane as solvent. The isolated hydrocarbons were characterized using G C - M S techniques.
Analysis of compounds in the monoaromatic fraction using GC-MS techniques Compounds were identified by co-chromatography with authentic standards. In all cases the unknown compounds co-chromatographed with the standards using capillary columns coated with both BP-I and BP-5 stationary phases. The mass spectra of the unknowns were also measured and in all cases matched those of the authentic compounds.
Internal and external standards p-Terphenyl and 1-(4-methylpentyl)-2,3,4,5-tetramethylbenzene were used as internal included and excluded standards respectively. The internal standards were added to the monoaromatic fraction and an accurate subsample of this mixture was removed before application to the molecular sieve. After the included and excluded fractions were recovered a calculated volume of the external standard phenanthrene was added to the subsample and to both the included and excluded fractions in order to estimate recoveries.
Gas chromatography-mass spectrometry A Hewlett Packard 5897 G C - M S fitted with a 60 m x 0.25 mm i.d. (J&W) fused-silica open tubular column containing a 0.25/~m DB-1 phase was used for all analyses. The GC oven was temperature programmed for 50-90°C at 8°C/min and then from 90 to 300°C at 3°C/min. Samples were injected on-column using an OCI-3 (SGE) injector.
RESULTS AND DISCUSSION
Figure 1 shows the total ion chromatograms obtained from the monoaromatic fraction of the Jurassic crude oil together with the corresponding chromatograms for the components that were either sorbed into the molecular sieve (labelled included) or not sorbed (labelled excluded) when the monoaromatic fraction was treated with dealuminated mordenite. Two internal standards were added before the monoaromatic fraction was treated with the molecular sieve. One, p-terphenyl, was sorbed by the molecular sieve and the peak representing this compound is labelled as I.S.I., the other, l-(4methylpentyl)-2,3,4,5-tetramethylbenzene was not sorbed and is represented by the peak labelled I.S.E. The use of such standards enables the relative proportions of components in the two fractions to be determined. In addition an external standard, phenanthrene, labelled E.S. was added to the solutions before G C - M S in order to accurately determine the recovery of components subjected to the molecular sieving procedure. In the case of the included fraction, most components were recovered in 70-80% yield and those in the excluded fraction in 80-90% yield. The greater loss of components in the included fraction was attributed to
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Fig. I. Total ion chromatograms of the Jurassic crude oil monoaromatic fraction and the components of the monoaromatic fraction that were sorbed (included) and not sorbed (excluded) by the molecular sieve.
R E T E N T I O N T I M E (min),
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TOTAL MONOAROMATICS
O,x~
0
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B
0
0
0
590
LEROYELLISet al.
the additional procedures involved in the release and recovery of the sorbed components. The losses during processing are probably due mainly to evaporation during solvent removal. The partial total ion chromatograms of the included and excluded fractions shown in Fig. 1 are markedly different and indicate that selective sorption occurred when the monoaromatic fraction was treated with the molecular sieve. Approximately 40% (w/w) of the monoaromatic fraction was contained in the included fraction. The identities of the major components in the included fraction was established by co-chromatography of authentic standards with the unknowns and by comparison of their mass spectra. The Cl6 components were examined in detail and shown to consist of four major components (peaks 1-4 in Fig. 1). The compounds represented by peaks 1, 2 and 4 were shown to be meta, para and ortho nonyltoluenes respectively. The compound represented by peak 3 was n-decylbenzene. Although the nonyltoluenes and other alkyltoluenes have previously been reported in petroleum (Philp, 1985; Albaiges et al., 1986; Williams et al., 1988) this is the first report of the elution order of the isomers having been established using authentic reference compounds*. The groups of compounds in the included fraction with retention times less than and greater than the C16 compounds were identified by their mass spectra. The first, second and fourth eluting compounds in each set have mass spectra indicating dominant m / z 105 and 106 ions characteristic of alkyltoluenes (Gallegos, 1973; Philp, 1985; Albaiges et al., 1986; Williams et al., 1988). The third eluting compound at each carbon number had a mass spectrum with a dominant m / z 92 ion indicative of alkylbenzenes (Gallegos, 1973; Philp, 1985). The abundance of the homologous series of n-alkyltoluenes and n-alkylbenzenes in the included fraction and their absence from the
excluded fraction (Fig. 1) clearly shows the efficiency of sorption of these compound types on dealuminated mordenite and the selectivity of this molecular sieve towards these components in the monoaromatic fraction. The total ion chromatogram of the excluded fraction (Fig. 1) shows a lesser number of prominent peaks than that of the included fraction, however this fraction accounted for 60% (w/w) of the monoaromatic fraction and contains a significant amount of complex unresolved hydrocarbons of unknown molecular structure. The major components represented by peaks 5 and 6 were examined in more detail. The compound represented by peak 5 was shown to be 1-(4-methylpentyl)-2,3,6-trimethylbenzene by co-chromatography with an authentic standard using BP-I and BP-5 capillary columns and by comparison of mass spectral data. This compound has not previously been reported as a constituent of petroleum. The identity of the compound represented by peak 6 is unknown at this time, however mass spectral data suggests that it is related to the earlier eluting compound and possibly contains an additional substituent on the aromatic ring. A detailed discussion of these compounds and their geochemical significance is beyond the scope of this paper and will be published elsewhere. Table 1 shows the results obtained when the newly identified constituent of petroleum, 1-(4-methylpentyl)-2,3,6-trimethylbenzene and some other alkylbenzenes were treated with dealuminated mordenite molecular sieves. The various compounds have been classified as excluded from the molecular sieve if they were eluted from the sieve column with three bed volumes of solvent. The included fractions were obtained by acid digestion of the sieves and solvent extraction of the liberated hydrocarbons. Components that were present in both fractions have been classified as partially included. It is apparent from the
Table 1. Substituentpatterns of isomersof each compoundtypethat wereeither included, partially includedor excludedfromthe molecular sieve channels in blank experiments
i
ll3
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Fig. 2. Selected ion c h r o m a t o g r a m s
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by the molecular sieve.
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(m/z 119 120) o f the crude oil monoaromatic fraction and the components o f the m o n o a r o m a t i c fraction that were sorbed (included) and not sorbed (excluded)
I J
TOTAL MONOAROMATICS m/z 119-120
2,5 2,4 l / 3 , 4 \ ~2,
N g.
0 0
&
0 D
0
592
LEROYELLISet al.
data that compounds with n-alkyl groups can be sorbed into the sieve channels provided only one methyl group is attached to the aromatic ring or, if two methyls are attached, partial exclusion occurs when these two groups are at positions 3 and 5 or 2 and 6. With three methyl groups on the aromatic ring a change from n-alkyl to iso-alkyl for the alkyl chain does not have a significant effect on sorption. However addition of a fourth methyl group on the aromatic ring, irrespective of the substitution, resulted in exclusion from the sieve. These observations suggest that sorption in the sieve channels occurs only when the molecules have molecular cross sections similar to those of the sieve channels and the critical values are near those for the 3,5- and 2,6-heptylxylencs. Compound types that occur in the monoaromatic fraction at lower concentrations than alkyltoluenes and alkylbenzenes were analysed using selected ion monitoring GC-MS techniques. The partial m / z 119-120 mass chromatograms for the monoaromatic
fraction together with the corresponding sections of the mass chromatograms from the included and excluded fractions are shown in Fig. 2. Regular sets of peaks are apparent in these chromatograms that represent compounds with carbon numbers from 13 to 23. Minor amounts of compounds with greater carbon numbers were also present in the sample. The identity of the group of Cls compounds was established by co-chromatography with authentic standards and comparison of mass spectral data. The inset shown in Fig. 2 shows the substitution patterns of the six isomers of n-heptylxylene that were identified. As observed in the blank experiments only the 3,5- and 2,6- substituted isomers are appreciably excluded from the sieve with all other isomers occurring in the included fraction. These features established for the C]s compounds can also be observed for the groups of compounds with other carbon numbers and confirms the suggestion that substitution patterns of the methyl groups on the aromatic ring rather than the length of the alkyl chain is the
TOTAL MONOAROMATICS PeakA 119
......°'+Jr,+ j,.i +° INCLUDED 159
]
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0 188
,,,
. . . . . . . . fro. . . . . . i+6 . . . . . i + + " EXCLUDED
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26
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RETENTION TIME (rain).,
29
....
,-.. ' " " a'o
190
.i f , .
. . . . . I~o . . . . . ~0 . . . . .
26o
~,
Fig. 3. Partial total ion chromatograms of the crude oil monoaromatic and the components of the monoaromatic fraction that were sorbed (included) and not sorbed (excluded) by the molecular sieve together with the mass-spectra of the compounds represented by the shaded peaks.
Fractionation of monoaromatic petroleum hydrocarbons key factor in determining sorption of n-alkylxylenes into dealuminated mordenite. Shape selective separation of hydrocarbons can sometimes enable compounds to be separated which are difficult to separate using high efficiency gas chromatography techniques. Figure 3 shows partial total ion chromatograms of the monoaromatic fraction together with the corresponding sections of the chromatograms for the included and excluded fractions after treatment with molecular sieves. The mass spectra of the components represented by the shaded peaks are also shown. The compound shown in the excluded fraction with major m / z 120 and 190 ions is suggested as 3,5-hexylxylene, by comparison of the mass spectral data of the authentic 3,5-heptylxylene and the chromatographic retention order of the 3,5alklyxylene series as shown in Fig. 2. Although the identity of the included compound in question is unknown, it is apparent from the mass spectra that Peak A in the monoaromatic fraction represents at least two compounds, one of which has been included and the other excluded when the monoaromatic fraction was treated with the molecular sieve. This example serves to illustrate the unique properties of molecular sieves to separate compounds which co-chromatograph even on high resolution columns.
CONCLUSIONS n-Aikylbenzenes, n-alkyltoluenes and some nalkylxylenes have been shown to be selectively sorbed from a pentane solution of the monoaromatic fraction of a crude oil using dealuminated mordenite molecular sieve, n-Alkylxylenes with the 3,5- or 2,6substitution patterns were partially sorbed as were trimethylalkylbenzenes with the 2,3,6- substitution pattern. All tetramethylalkylbenzenes were excluded from the sieve. *Note added in proof. Since this paper was accepted for publication a report has been made of the use of authentic reference compounds to establish the elution order of the alkyltoluene isomers cf. Sinniglie Darmst6 et al. (1991) Geochim. Cosmochim. Acta 55, 3677-3683.
593
REFERENCES
Albaiges J., Algaba J., Clavell E. and Grimalt J. (1986) Petroleum geochemistry of the Taragona Basin (Spanish Mediterranean off-shore). In Advances in Organic Geochemistry 1985 (Edited by Leythaeuser D. and Rullk6tter J.), pp. 441-450. Pergamon Press, Oxford. Barrer R. M. 0984) Zeolite Structures. In Zeolites: Science and Technology (Edited by Ramoa Ribeiro F. et al.) NATO ASI Series E, pp. 35-81. Martinus Nijhoff, The Hague. Dessau R. M. (1980) Selective sorption properties of zeolites. In Adsorption and Ion Exchange with Synthetic Zeolites (Edited by Flank W. H.) ACS Symposium Series 175, 123-135, Washington, D.C. Dimmler A. and Strausz O. P. (1983) Enrichment of polycyclic terpenoid, saturated hydrocarbons from petroleum by adsorption on zeolite NaX. J. Chromatogr. 270, 219-225. Dwyer J. and Dyer A. (1984) Zeolites for industry. Chem. Ind. 2, 237-244. Flanigen E. M., Bennett J. M., Grose R. W., Cohen J. P., Patton R. L. and Kirchner R. M. (1978) Silicalite,a new hydrophobic crystalline silicamolecular sieve. Nature 271, 512-516. Gallegos E. J. (1973) Identification of phenylcycloparaffin alkanes and other monoaromatics in Green River Shale by gas-chromatography-mass spectrometry. Anal. Chem. 45, 1399-1403. Hoering T. C. and Freeman D. H. (1984) Shape selective sorption of monomethyl alkanes by silicalite, a zeolitic form of silica. J. Chromatogr. 316, 333-341. Jasra R. V. and Bhat S. G. (1987) Sorption kinetics of higher n-paraffins on zeolite molecular sieve 5A. Ind. Engng Chem. Res. 26, 2544-2546. Kerr G. T. (1989) Synthetic zeolites. Scient. Am. 261, 82-87. Murphy M. T. J. (1969) Analytical Methods. In Organic Geochemistry (Edited by Eglinton G. and Murphy M. T. G.), pp. 74--88. Springer, Berlin. Philp R. P. (Editor) (1985) Fossil Fuel Biomarkers. Applications and Spectra, Methods in Geochemistry and Geophysics No. 23, pp. 106-i 11. Elsevier, Amersterdam. Olson D. H., Haag W. O. and Lago R. M. (1980) Chemical and physical properties of the ZSM-5 substitutional series. J. Catal. 61, 390-396. Ruthven D. M. (1988) Zeolites as selective adsorbants. Chem. Engng Prog. 84, 42-50. West N., Alexander R. and Kagi R. I. (1990) The use of silicalite for rapid isolation of branched and cyclicalkane fractions of petroleum. Org. Geochem. 15, 499-501. WilliamsJ. A., Dolcater D. L., Torkelson B. E. and Winters J. C. (1988) Anomalous concentrations of specific alkylaromatic and alkylcycloparaffin components in West Texas and Michigan crude oils. In Advances in Organic Geochemistry 1987 (Edited by Novelli L. and Mattavelli L.), pp. 47-60. Pergamon Press, Oxford.
0146-6380/92 $5.00+ 0.00 Copyright © 1992Pergamon Press Ltd
Separation of petroleum hydrocarbons using dealuminated mordenite molecular sieve. I. Monoaromatic hydrocarbons LEROY ELLIS, ROBERT I. KAGI and ROBERT ALEXANDER Centre for Petroleum and Environmental Organic Geochemistry, Curtin University of Technology, G.P.O. Box U1987, Perth 6001, Western Australia Abstract--A crude oil monoaromatic fraction was applied to a Pasteur pipette packed with dealuminated mordenite molecular sieve and eluted with pentane solvent, n-Alkylbenzenes, n-alkyltoluenes and some n-alkylxylene isomers were sorbed into the mordenite molecular sieve and recovered by acid digestion of the sieve and solvent extraction of the liberated hydrocarbons. The excluded fraction which was eluted in the pentane solvent was enriched in the 3,5- and 2,6-n-alkylxylene isomers and other compounds possessing a higher degree of structural complexity. The mordenite molecular sieve provides a rapid and convenient method for the fractionation of monoaromatic hydrocarbons allowing the more subtle characteristics of these fractions to be studied in greater detail. Key words--n-alkylbenzenes, n-alkyltoluenes, n-alkylxylenes, mordenite, molecular sieve, monoaromatics
INTRODUCTION Molecular sieve sorption of hydrocarbons has become a routine geochemical procedure for separating alkane components of petroleum. For many years 5A molecular sieves have been the mainstay of n-alkane removal from alkane fractions of petroleum (Murphy, 1969; Jasra and Bhat, 1987; Kerr, 1989). The larger pore silicalite molecular sieve has also proved effective in this role and has also been reported to sorb monomethyl alkanes and some cycloalkanes (Flanigen et aL, 1978; Hoering and Freeman, 1984; West et al., 1990). The even larger pore 13X molecular sieve will sorb hopane and tricyclic terpane compounds (Dimmler and Strausz, 1983). Molecular sieves containing silicon and aluminium in their structures are polar due to the replacement of silicon by aluminium in the molecular sieve resulting in a net negative charge on the molecular sieve framework. A non-framework charge-compensating cation such as sodium is then required to balance this negative charge. Molecular sieves containing a high aluminium content exhibit high polarity and hydrophilic properties, whereas high silica molecular sieves have low polarity and hydrophobic properties (Olson et al., 1980; Hoering and Freeman, 1984). One example of a high silica molecular sieve is dealuminated mordenite which is characterized by a silicon to aluminium ratio of 30 and a one-dimensional channel system consisting of two pore apertures of 6.7 x 7 and 2.9 x 5.7 ,/k (Dessau 1980; Barrer, 1984; Dwyer and Dyer, 1984; Ruthven, 1988). Molecular sieves with silicon to aluminium ratios > 10 are referred to as high silica sieves. The low polarity of the high silica mordenite molecular sieve offers promising applications to the shape-selective separation of aromatic components of petroleum. Because of the
low polarity of the sieve, shape-selective effects dominate the separation processes even with compounds as polar as monoaromatic hydrocarbons. Shape selective techniques are a potentially valuable addition to the range of techniques currently employed to further fractionate the aromatic hydrocarbons in petroleum. Conventional separation techniques such as normal and reverse phase liquid chromatography frequently fail to yield mixtures that can be fully resolved using capillary gas chromatography procedures. By further fractionation of such complex mixtures, molecular sieve separation techniques can be used to reduce the complexity of such mixtures and thereby enable analysis of their minor components. In this paper we report a simple chromatographic procedure using dealuminated mordenite molecular sieves which selectively provides n-alkylbenzenes, n-alkyltoluenes and some n-alkylxylenes in a sorbed retained fraction. The structural features of alkylxylenes and polymethyl substituted alkylbenzenes that result in exclusion from the sieves are reported and we illustrate how previously unreported minor components of the monoaromatic fraction, which were not sorbed by the molecular sieve could then be resolved using capillary gas chromatography techniques.
EXPERIMENTAL
Samples A Jurassic crude oil from the Coeper/Eromanga Basin was separated into alkane and aromatic fractions using a Merck-Lobar Grobe A (240-10) Lichroprep Si 60,40-63/~m column attached to a Waters Millipore Model 510 double piston pump. Using a flow rate of 2 ml rain -~ of n-hexane, an alkane 587
588
LEROY ELLIS et al.
fraction was collected between elution times of 8-12 min. The aromatic components were then eluted by reversing the direction of flow of the solvent for a further 25min. The alkane components were detected using a Waters Millipore Series R-400 Differential Refractometer and the aromatic components were detected using a Waters Associates Series 440 u.v. absorbance detector at a wavelength setting of 254 nm. The aromatic components were then further fractionated into monoaromatic, diaromatic and triaromatic bands using alumina thin layer chromatography with n-hexane as eluent. The monoaromatic fraction was collected by scraping bands from the plate and extracting the alumina with dichloromethane.
Mordenite molecular sieves The high-silica mordenite molecular sieve samples were kindly provided by Dr E. M. Flanigen of UOP Tarrytown Technical Center, New York and also from Dr Jeppe Magnusson of CONTEKA, Surte, Sweden. The powder was activated by heating overnight at 350°C and was stored in an airtight container. The activity of the mordenite molecular sieve diminished slowly and was found to sorb nalkylbenzene and n-alkyltoluene components of petroleum monoaromatic fractions for a period of 5 months after initial activation.
Molecular sieve separations Separations were performed using mordenite powder (1 g) packed into a Pasteur pipette as a suspension in pentane. In a typical experiment approx. 5-10 mg of monoaromatic sample was applied to the top of the column. After standing for 10 min the excluded components of the sample were eluted with three bed volumes of pentane (approx. 5 ml). The included components were recovered by treating the mordenite with hydrofluoric acid (1-3 ml/50% w/v) and solvent extracting (3 x 5 ml pentane) the liberated components. It was found that the action of hydrofluoric acid on the mordenite molecular sieve did not affect any of the sample components or the 15 reference compounds used in the blank molecular sieve experiments.
Synthesis of reference compounds All hydrocarbon reference compounds were prepared by the reaction of the appropriate aryl Grignard reagents with aldehydes and the subsequent reduction of the alcohols. Good yields (60-90%) of the benzylic alcohols were obtained. The alcohols were subjected to hydrogenolysis using palladium/ carbon (10%) as catalyst in glacial acetic acid with a hydrogen pressure of 1.5atm for 36h. The hydrocarbons produced were further purified by chromatography using a column of silica gel with n-hexane as solvent. The isolated hydrocarbons were characterized using G C - M S techniques.
Analysis of compounds in the monoaromatic fraction using GC-MS techniques Compounds were identified by co-chromatography with authentic standards. In all cases the unknown compounds co-chromatographed with the standards using capillary columns coated with both BP-I and BP-5 stationary phases. The mass spectra of the unknowns were also measured and in all cases matched those of the authentic compounds.
Internal and external standards p-Terphenyl and 1-(4-methylpentyl)-2,3,4,5-tetramethylbenzene were used as internal included and excluded standards respectively. The internal standards were added to the monoaromatic fraction and an accurate subsample of this mixture was removed before application to the molecular sieve. After the included and excluded fractions were recovered a calculated volume of the external standard phenanthrene was added to the subsample and to both the included and excluded fractions in order to estimate recoveries.
Gas chromatography-mass spectrometry A Hewlett Packard 5897 G C - M S fitted with a 60 m x 0.25 mm i.d. (J&W) fused-silica open tubular column containing a 0.25/~m DB-1 phase was used for all analyses. The GC oven was temperature programmed for 50-90°C at 8°C/min and then from 90 to 300°C at 3°C/min. Samples were injected on-column using an OCI-3 (SGE) injector.
RESULTS AND DISCUSSION
Figure 1 shows the total ion chromatograms obtained from the monoaromatic fraction of the Jurassic crude oil together with the corresponding chromatograms for the components that were either sorbed into the molecular sieve (labelled included) or not sorbed (labelled excluded) when the monoaromatic fraction was treated with dealuminated mordenite. Two internal standards were added before the monoaromatic fraction was treated with the molecular sieve. One, p-terphenyl, was sorbed by the molecular sieve and the peak representing this compound is labelled as I.S.I., the other, l-(4methylpentyl)-2,3,4,5-tetramethylbenzene was not sorbed and is represented by the peak labelled I.S.E. The use of such standards enables the relative proportions of components in the two fractions to be determined. In addition an external standard, phenanthrene, labelled E.S. was added to the solutions before G C - M S in order to accurately determine the recovery of components subjected to the molecular sieving procedure. In the case of the included fraction, most components were recovered in 70-80% yield and those in the excluded fraction in 80-90% yield. The greater loss of components in the included fraction was attributed to
l
® "7"
,s~l
,,,
/
i.s.i.
LS.I.
® ®
I
60
Fig. I. Total ion chromatograms of the Jurassic crude oil monoaromatic fraction and the components of the monoaromatic fraction that were sorbed (included) and not sorbed (excluded) by the molecular sieve.
R E T E N T I O N T I M E (min),
I
I
E.S.
'lJi' IJ,I
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40
(~,
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,
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,A,
lit
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20
EXCLUDED
,,
,~,.=~°
TOTAL MONOAROMATICS
O,x~
0
g
g-
B
0
0
0
590
LEROYELLISet al.
the additional procedures involved in the release and recovery of the sorbed components. The losses during processing are probably due mainly to evaporation during solvent removal. The partial total ion chromatograms of the included and excluded fractions shown in Fig. 1 are markedly different and indicate that selective sorption occurred when the monoaromatic fraction was treated with the molecular sieve. Approximately 40% (w/w) of the monoaromatic fraction was contained in the included fraction. The identities of the major components in the included fraction was established by co-chromatography of authentic standards with the unknowns and by comparison of their mass spectra. The Cl6 components were examined in detail and shown to consist of four major components (peaks 1-4 in Fig. 1). The compounds represented by peaks 1, 2 and 4 were shown to be meta, para and ortho nonyltoluenes respectively. The compound represented by peak 3 was n-decylbenzene. Although the nonyltoluenes and other alkyltoluenes have previously been reported in petroleum (Philp, 1985; Albaiges et al., 1986; Williams et al., 1988) this is the first report of the elution order of the isomers having been established using authentic reference compounds*. The groups of compounds in the included fraction with retention times less than and greater than the C16 compounds were identified by their mass spectra. The first, second and fourth eluting compounds in each set have mass spectra indicating dominant m / z 105 and 106 ions characteristic of alkyltoluenes (Gallegos, 1973; Philp, 1985; Albaiges et al., 1986; Williams et al., 1988). The third eluting compound at each carbon number had a mass spectrum with a dominant m / z 92 ion indicative of alkylbenzenes (Gallegos, 1973; Philp, 1985). The abundance of the homologous series of n-alkyltoluenes and n-alkylbenzenes in the included fraction and their absence from the
excluded fraction (Fig. 1) clearly shows the efficiency of sorption of these compound types on dealuminated mordenite and the selectivity of this molecular sieve towards these components in the monoaromatic fraction. The total ion chromatogram of the excluded fraction (Fig. 1) shows a lesser number of prominent peaks than that of the included fraction, however this fraction accounted for 60% (w/w) of the monoaromatic fraction and contains a significant amount of complex unresolved hydrocarbons of unknown molecular structure. The major components represented by peaks 5 and 6 were examined in more detail. The compound represented by peak 5 was shown to be 1-(4-methylpentyl)-2,3,6-trimethylbenzene by co-chromatography with an authentic standard using BP-I and BP-5 capillary columns and by comparison of mass spectral data. This compound has not previously been reported as a constituent of petroleum. The identity of the compound represented by peak 6 is unknown at this time, however mass spectral data suggests that it is related to the earlier eluting compound and possibly contains an additional substituent on the aromatic ring. A detailed discussion of these compounds and their geochemical significance is beyond the scope of this paper and will be published elsewhere. Table 1 shows the results obtained when the newly identified constituent of petroleum, 1-(4-methylpentyl)-2,3,6-trimethylbenzene and some other alkylbenzenes were treated with dealuminated mordenite molecular sieves. The various compounds have been classified as excluded from the molecular sieve if they were eluted from the sieve column with three bed volumes of solvent. The included fractions were obtained by acid digestion of the sieves and solvent extraction of the liberated hydrocarbons. Components that were present in both fractions have been classified as partially included. It is apparent from the
Table 1. Substituentpatterns of isomersof each compoundtypethat wereeither included, partially includedor excludedfromthe molecular sieve channels in blank experiments
i
ll3
(CHz) s
CH3
CH(CH3)2
I
I
(CH2) 6
(CHz) 3
ill3
CH(CH3)2
I (CH2) s
(CH2) 3
Compound classes
CH 3
Included Partially included Excluded
2- ,3- ,4-
(CH3) 2
(CH3) 3
(CH3) 3
(CH3) 4
2,3- 2,4- 2,53,43,5- 2,6-
2,3,6-
2,3,6r 2,3,4,5-2,3 ,4,6-2,3,5,6-
I
1..,
]
~
Fig. 2. Selected ion c h r o m a t o g r a m s
2O
EXCLUDED
INCLUDED m/z 119-120
Aj~. ~ " ~
C13 ~ ~
! I
I
I C14
i
~ ,~
3,4 2,4 /
J
I
C15/........ ---'''~
~. . . . . . . .
3,5
i
I
I I
40
1.,
C17 "
RETENTION TIME (min),
CI6
1
~1
>
C18
~
,A.,
C19
C20
C21 C22
by the molecular sieve.
I
6O
C23
(m/z 119 120) o f the crude oil monoaromatic fraction and the components o f the m o n o a r o m a t i c fraction that were sorbed (included) and not sorbed (excluded)
I J
TOTAL MONOAROMATICS m/z 119-120
2,5 2,4 l / 3 , 4 \ ~2,
N g.
0 0
&
0 D
0
592
LEROYELLISet al.
data that compounds with n-alkyl groups can be sorbed into the sieve channels provided only one methyl group is attached to the aromatic ring or, if two methyls are attached, partial exclusion occurs when these two groups are at positions 3 and 5 or 2 and 6. With three methyl groups on the aromatic ring a change from n-alkyl to iso-alkyl for the alkyl chain does not have a significant effect on sorption. However addition of a fourth methyl group on the aromatic ring, irrespective of the substitution, resulted in exclusion from the sieve. These observations suggest that sorption in the sieve channels occurs only when the molecules have molecular cross sections similar to those of the sieve channels and the critical values are near those for the 3,5- and 2,6-heptylxylencs. Compound types that occur in the monoaromatic fraction at lower concentrations than alkyltoluenes and alkylbenzenes were analysed using selected ion monitoring GC-MS techniques. The partial m / z 119-120 mass chromatograms for the monoaromatic
fraction together with the corresponding sections of the mass chromatograms from the included and excluded fractions are shown in Fig. 2. Regular sets of peaks are apparent in these chromatograms that represent compounds with carbon numbers from 13 to 23. Minor amounts of compounds with greater carbon numbers were also present in the sample. The identity of the group of Cls compounds was established by co-chromatography with authentic standards and comparison of mass spectral data. The inset shown in Fig. 2 shows the substitution patterns of the six isomers of n-heptylxylene that were identified. As observed in the blank experiments only the 3,5- and 2,6- substituted isomers are appreciably excluded from the sieve with all other isomers occurring in the included fraction. These features established for the C]s compounds can also be observed for the groups of compounds with other carbon numbers and confirms the suggestion that substitution patterns of the methyl groups on the aromatic ring rather than the length of the alkyl chain is the
TOTAL MONOAROMATICS PeakA 119
......°'+Jr,+ j,.i +° INCLUDED 159
]
<
Ii iI119 ~II I I[
7~
132
0 188
,,,
. . . . . . . . fro. . . . . . i+6 . . . . . i + + " EXCLUDED
120
91
,,.,
25
26
27
28
RETENTION TIME (rain).,
29
....
,-.. ' " " a'o
190
.i f , .
. . . . . I~o . . . . . ~0 . . . . .
26o
~,
Fig. 3. Partial total ion chromatograms of the crude oil monoaromatic and the components of the monoaromatic fraction that were sorbed (included) and not sorbed (excluded) by the molecular sieve together with the mass-spectra of the compounds represented by the shaded peaks.
Fractionation of monoaromatic petroleum hydrocarbons key factor in determining sorption of n-alkylxylenes into dealuminated mordenite. Shape selective separation of hydrocarbons can sometimes enable compounds to be separated which are difficult to separate using high efficiency gas chromatography techniques. Figure 3 shows partial total ion chromatograms of the monoaromatic fraction together with the corresponding sections of the chromatograms for the included and excluded fractions after treatment with molecular sieves. The mass spectra of the components represented by the shaded peaks are also shown. The compound shown in the excluded fraction with major m / z 120 and 190 ions is suggested as 3,5-hexylxylene, by comparison of the mass spectral data of the authentic 3,5-heptylxylene and the chromatographic retention order of the 3,5alklyxylene series as shown in Fig. 2. Although the identity of the included compound in question is unknown, it is apparent from the mass spectra that Peak A in the monoaromatic fraction represents at least two compounds, one of which has been included and the other excluded when the monoaromatic fraction was treated with the molecular sieve. This example serves to illustrate the unique properties of molecular sieves to separate compounds which co-chromatograph even on high resolution columns.
CONCLUSIONS n-Aikylbenzenes, n-alkyltoluenes and some nalkylxylenes have been shown to be selectively sorbed from a pentane solution of the monoaromatic fraction of a crude oil using dealuminated mordenite molecular sieve, n-Alkylxylenes with the 3,5- or 2,6substitution patterns were partially sorbed as were trimethylalkylbenzenes with the 2,3,6- substitution pattern. All tetramethylalkylbenzenes were excluded from the sieve. *Note added in proof. Since this paper was accepted for publication a report has been made of the use of authentic reference compounds to establish the elution order of the alkyltoluene isomers cf. Sinniglie Darmst6 et al. (1991) Geochim. Cosmochim. Acta 55, 3677-3683.
593
REFERENCES
Albaiges J., Algaba J., Clavell E. and Grimalt J. (1986) Petroleum geochemistry of the Taragona Basin (Spanish Mediterranean off-shore). In Advances in Organic Geochemistry 1985 (Edited by Leythaeuser D. and Rullk6tter J.), pp. 441-450. Pergamon Press, Oxford. Barrer R. M. 0984) Zeolite Structures. In Zeolites: Science and Technology (Edited by Ramoa Ribeiro F. et al.) NATO ASI Series E, pp. 35-81. Martinus Nijhoff, The Hague. Dessau R. M. (1980) Selective sorption properties of zeolites. In Adsorption and Ion Exchange with Synthetic Zeolites (Edited by Flank W. H.) ACS Symposium Series 175, 123-135, Washington, D.C. Dimmler A. and Strausz O. P. (1983) Enrichment of polycyclic terpenoid, saturated hydrocarbons from petroleum by adsorption on zeolite NaX. J. Chromatogr. 270, 219-225. Dwyer J. and Dyer A. (1984) Zeolites for industry. Chem. Ind. 2, 237-244. Flanigen E. M., Bennett J. M., Grose R. W., Cohen J. P., Patton R. L. and Kirchner R. M. (1978) Silicalite,a new hydrophobic crystalline silicamolecular sieve. Nature 271, 512-516. Gallegos E. J. (1973) Identification of phenylcycloparaffin alkanes and other monoaromatics in Green River Shale by gas-chromatography-mass spectrometry. Anal. Chem. 45, 1399-1403. Hoering T. C. and Freeman D. H. (1984) Shape selective sorption of monomethyl alkanes by silicalite, a zeolitic form of silica. J. Chromatogr. 316, 333-341. Jasra R. V. and Bhat S. G. (1987) Sorption kinetics of higher n-paraffins on zeolite molecular sieve 5A. Ind. Engng Chem. Res. 26, 2544-2546. Kerr G. T. (1989) Synthetic zeolites. Scient. Am. 261, 82-87. Murphy M. T. J. (1969) Analytical Methods. In Organic Geochemistry (Edited by Eglinton G. and Murphy M. T. G.), pp. 74--88. Springer, Berlin. Philp R. P. (Editor) (1985) Fossil Fuel Biomarkers. Applications and Spectra, Methods in Geochemistry and Geophysics No. 23, pp. 106-i 11. Elsevier, Amersterdam. Olson D. H., Haag W. O. and Lago R. M. (1980) Chemical and physical properties of the ZSM-5 substitutional series. J. Catal. 61, 390-396. Ruthven D. M. (1988) Zeolites as selective adsorbants. Chem. Engng Prog. 84, 42-50. West N., Alexander R. and Kagi R. I. (1990) The use of silicalite for rapid isolation of branched and cyclicalkane fractions of petroleum. Org. Geochem. 15, 499-501. WilliamsJ. A., Dolcater D. L., Torkelson B. E. and Winters J. C. (1988) Anomalous concentrations of specific alkylaromatic and alkylcycloparaffin components in West Texas and Michigan crude oils. In Advances in Organic Geochemistry 1987 (Edited by Novelli L. and Mattavelli L.), pp. 47-60. Pergamon Press, Oxford.