Systematic Approach to the Study of Aromatic Hydrocarbons in Heavy Distillates and Residues by Elution Adsorption Chromatography R. G . Ruberto, and B. E. Davis GulfResearch & Development Company, Pittsburgh, Pa. 15230
D . M. Jewell,
[1
More significant approaches to isolating aromatic hydrocarbons from heavy distillates (>400 O F ) and residuals are being devised as a result of recently established pretreatment methods which remove the major nonhydrocarbon impurities and saturates. Exponential gradient elution absorption chromatography coupled with multiple on-line detectors (e.g., dual-channel ultraviolet) can then be used for a rapid classification into aromatic su b-types (mono-, di-, etc.). The sensitivity of the eluting system to type of alkyl substitution on the aromatic ring system can be demonstrated by model systems and spectrometric techniques.
ANYCOMPREHENSIVE STUDY of petroleum crude oils and their products is faced with the problem of isolating aromatic hydrocarbons according to their number of aromatic rings. The magnitude of the problem is dependent on the nature of the product; gasolines and residuals are two extremes. A sequence of steps is being devised to rapidly provide compositional information on any of these products with a minimum of experimental variations; in this manner, comparison of products can be meaningfully made, compositionally. Earlier studies showed the stepwise preparation of total aromatic concentrates (1). The various steps described are necessary to maximize the efficiency of the chromatographic cut-point between saturates and aromatics and to provide total aromatic fractions, sufficiently free of polar nonhydrocarbons, that are suitable for existing mass spectrometric methods of group-type analysis (e.g., Robinson and Cook) ( 2 ) . This paper discusses the further separation of these total aromatic concentrates into mono-, di-, tri-, and polyaromatic sub-fractions.
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3 MORE
Figure 1. Schematic of separation apparatus 1. 2. 3. 4. 5.
6. I. 8. 9. 10. 11. 12. 13. 14.
Solvent reservoir, 1 liter Teflon tubing, l/$-in.0.d. Teflon tubing, 1/16-in. 0.d. Adapter with two '/s-in. holes and S 24/40 100-ml round bottom flask, completely full of solvent ','.-in. magnetic bar Magnetic stirrer Milton Roy mini-pump Flow-through pressure gauge Precolumn Injection valvesample loop Chromatographic column Detectors in series Collector
EXPERIMENTAL
All petroleum distillates and residues are treated stepwise by techniques previously discussed (1) in order to provide total aromatic concentrates. The gradient elution chromatographic (GEC) separation is performed as depicted in Figure 1. Solvents consist of spectral grade n-hexane, cyclohexane, chloroform, and methanol. A satisfactory pump is a variable speed, Milton Roy instrument mini-pump (No. 61948) fitted with a tungsten carbide plunger. The pump used in this study has a maximum pump rate of 460 mllhr, and all separations discussed were performed at this flow rate. The columns used were 1/2-in.x 13-in. for pre-column and 1/2-in. X 24-in. for sample with adjustable stainless steelTeflon (Du Pont) fittings (supplied by Chromatronix Inc.). The purpose of the precolumn is to ensure the consistency of the solvents from run t o run by drying them and removing any impurity which may be present. The sample column can be (1) D. M. Jewell, J. H. Weber, J. W. Bunger, H. Plancher, and D. R. Latham, Amer. Clrern. SOC.,Dic. Petrol. Cltem. Prepr., 16 (4), C13 (1971). (2) C . J. Robinson and G. L. Cook, ANAL.CHEM., 41, 1548 (1969).
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appropriately scaled up for preparative separations. An appropriate sampling valve is inserted between the columns (e.g., Chromatronix, Waters, etc.). One or more detecting systems are used in series to provide a continuous monitor of the liquid effluent stream. The most useful detector is some type of ultraviolet spectrophotometer fitted with micro flow-through cells. Although a variable wavelength instrument is ideal for this purpose, less expensive equipment possessing adequate sensitivity is satisfactory. The ISCO Model UA-2 Ultraviolet Analyzer has been found satisfactory for this purpose. This instrument permits simultaneous monitoring a t two different wavelengths. Filters a t 254, 313, and 365 nm were used in these separations. Cells had a 2-mm path length and a volume of 100 pl. Refractive index detectors cannot generally be used in a GEC system but can be used in the linear elution preceding the beginning of a solvent gradient. A Laboratory Data Control differential refractometer (Model 1103) was utilized. The adsorbent used was basic alumina (Alcoa F-20 chromatographic grade, freshly calcined at 400 "C at 10 mm Hg for 24 hr, and designated 0 % H?O-Al?Os. Separation Procedure. Both columns are rapidly drypacked with 0% H,O-AI,O, and sealed. Traces of water
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
adsorbed during packing can be removed by in situ heating a t 200 "C under a nitrogen purge. Pure n-hexane is pumped through the system under appropriate back pressure to de-gas adsorbents and equilibrate each detector. Aromatic concentrate, 500 mg, in 5 mln-hexane is injected directly into the moving stream onto the sample column. The solvent gradient is immediately begun with cyclohexane using a 100-ml mixing chamber. The ultraviolet monitors are set at 254 and 313 nm. Monoaromatics elute with increasing cyclohexane gradient and are observed only at 254 nm. When absorbance at 254 nm reaches zero (base line), a chloroform gradient is begun and the 254 filter is replaced by a 365 nm filter. The elution of diaromatics is observed at 313 nm while the 365 signal remains at base line; a t very high concentrations of diaromatics, the latter monitor will show a weak response. When absorbance at 313 nm reaches zero (base line), a methanol gradient is begun to complete the elution of polyaromatics which are observed on both 313 and 365 monitors.
E
CI
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a
OR CHOLESTANE OR SQUALANE
ELUTION
FRCM
O%H20-A1203-
Figure 2. Separation of model saturates and aromatic compounds
RESULTS AND DISCUSSION
The major problem faced in separation of aromatics is to completely separate mono- and diaromatic compounds from each other. To achieve this separation, one must be able to monitor the concentration of each ring system as the separation proceeds or calculate the overlap of each system. In liquid-solid chromatography with pure compounds and pure solvents on basic aluminas, one observes that the retention time of condensed diaromatics (naphthalenes, etc.) is related to the amount of residual moisture on the adsorbent; retention times approach a maximum as per cent water approaches zero. On zero per cent water-alumina, naphthalene is practically adsorbed irreversibly from n-hexane solution. While the retention times of monoaromatics are also related to the moisture content of the adsorbent, they are significantly shorter than those of the diaromatics. As demonstrated by Snyder (3, 4 ) and more recently by Popl et af. (5) the adsorbability of monoaromatics is also affected by the structure of alkyl substituents; Popl's studies were made with 0.5 H20-A1203. This implies that maximum difference in retention times between mono- and diaromatics can be realized on a zero per cent water-alumina. The elution characteristics of a number of model saturate and aromatic compounds were studied using both linear and gradient elution. Figure 2 shows a typical separation of saturates and aromatics using dual detectors and a linear elution with n-hexane as solvent. This separation has also been achieved with typical petroleum concentrates up through the Cs0range. The separation of types is not influenced by structure of saturate molecules or their molecular weight range. The differential refractometer detects both types; this means that any overlap of saturates in aromatic concentrates is readily observed and determined. This method has now been used to ascertain that the earlier scheme ( I ) for preparing aromatic concentrates is indeed valid and that the overlap of saturates in aromatics is negligible. The gradient elution characteristics of a ternary mixture of p-xylene, 2,6-dimethylnaphthalene,and anthracene are shown in the top half of Figure 3. Xylene is detected only at 254 nm, dimethylnaphthalene at 313 nm but not at 365 nm, and anthracene at both 313 and 365 nm. The broken curve indicates that base-line separation was actually achieved and that the curves are compressed for demonstration purposes. (3) L. R. Snyder, J . Chromatogr., 6 , 2 2 (1961). (4) Zbid.,13, 415 (1964). ( 5 ) M. Popl, J. Mosteck?, V. Dolansk?, and M. Kuras, ANAL. CHEM., 43, 518 (1971).
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DI AROMATICS
ELUTION
VOLUME
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Figure 3. Calibration of GEC procedure
0 2 4 6 n-HEXANE-CYCLOHEXANE
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Figure 4. GEC separation of model aromatics Once the gradient elution system was satisfactorily developed for known available mono-, di-, and polynuclear aromatic systems, applications to various concentrates were made. The bottom half of Figure 3 shows a typical print-out of these separations. The course of each separation varies with the nature of the aromatics depending on their origin (kerosene is usually faster than residuals) and each solvent gradient can be varied and controlled to yield the desired quality of separation. The detector response determines when to begin the elution of succeeding groups. The elution curve of monoaromatics is long for two reasons: the strength of eluting solvents is purposely kept weak to prevent desorption of diaromatics, and the nature of alkyl substituents exhibits a significant effect on total adsorption of the molecule. The second point was evaluated by studying the elution characteristics of typical, structurally different monoaromatics, under standard conditions. Figure 4 illus-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
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H
0 2 n-HEXANE-
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6 8 CYCLOHEXANE
IO 12 CHCI,
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GRADIENT ___,
ET
BINARY ET
0 2 4 6 8 n H E X A N E -+ CYCLOHEXANE
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Figure 6. GEC separation of model binary mixtures
trates the elution position and shape of various structures using a UV detector (254 nm). With respect to pure benzene, multiple substitution with short chains (triisopropyl, hexaethyl) decreases the total adsorbability of the molecule. Straight chain substituents show moderate increases while condensed cycloparaffin substituents show marked increases in total adsorption. The positive or negative contribution to adsorption made by the alkyl groups, as observed earlier by Pop1 in a linear system (3, is a manifestation of the stereochemistry of the total molecule. Diphenyl and indene are more strongly absorbed because of increased aromaticity and actually elute with the diaromatics; the effect of alkyl substitution on these ring systems has not yet been studied. Figure 5 demonstrates further the positive contribution made by noncondensed cycloparaffin substituents and sulfur heteroatoms. Noncondensed diaromatics, as diphenylalkanes, are still strongly adsorbed and usually occur in the “diaromatics.” The presence of alkylsulfide groups on benzenes (thioanisole types) increases adsorption and these compounds are shifted down to the di- and polyaromatic regions. Figure 6 shows that with very simple molecules, the GEC of different types of monoaromatics on zero per cent wateralumina can actually yield base-line separation ; there is n o synergistic effect upon mixing since position and shape of elution curve are unchanged from that shown for the single solute cases. A blend of thirty-one pure aromatic compounds, representing many different ring systems and substitution patterns, has also been separated by this scheme. This separation also conformed to predictions based on all model systems demonstrated. Evaluation of GEC Method. The examples demonstrated in Figures 2, 4-6 provide a basis for predicting gross features of aromatic species that are separated by this technique. 2320
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PPM 18)
Figure 5. GEC separation of model aromatics and sulfur compounds
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Figure 7. Partial ‘H NMR spectrum of monoaromatics
Any overlap of saturates in the aromatic concentrates can be determined. The alkyl substitution pattern of mononuclear aromatics will vary with increasing elution time in the sequence: short multi-branched chains < straight chain < condensed cycloparaffin. Two or more benzene rings insulated by several saturated carbons are isolated as diaromatics. Benzenes containing more than three condensed cycloparaffin rings may appear in “diaromatic” fraction. “Monoaromatics,” as isolated by this technique, are defined as molecules containing only one benzene ring and three or less cycloparaffin rings. Few alkyl substituted di-, tri- and polyaromatic model compounds have been evaluated on this GEC system to determine their influence on elution time. Since the strength of eluting solvent is quite high in this region, the positive or negative contributions of substituents to total adsorption of these ring systems should be minimized. However, alkyl indenes are shifted into the monoaromatic region and alkylphenanthrenes are shifted into the diaromatic region. For this reason, the second group is referred to as ditriaromatics. Alkylanthracenes are found in the polyaromatic region. The question of overlap between mono- and diaromatics is approached by critically examining the cut-point region by various spectrometric methods. If the sensitive on-line detectors and model compound calibrants are indeed accurate, few if any diaromatics (condensed) should be in the mono fraction and only traces of benzo-polycycloparaffins should be in the diaromatic fraction. A low sulfur (0.5 %), they may erroneously be reported as “monoaromatics”. When high sulfur aromatic concentrates are separated, the presence of the heteroatom does not affect the monoaromaticdiaromatic separation, as described above. Only traces of sulfur, if any, are ever found in the monoaromatics; all sulfur compounds appear in the di- and polyaromatic fractions, consistent with the model compounds shown in Figure 5 . The sulfur compounds, however, increase the difficulty in obtaining base-line separation of the polyaromatics. The sulfur compounds also create interpretation problems in the mass spectra of di- and polyaromatics by conventional low resolution methods since they may erroneously be called “monoaromatics”. For example, benzothiophenes cannot be differentiated from alkylbenzenes by low voltage mass spectrometry.
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(6) J. M. Duswalt and T. J. Mayer, Amer. Chem. Soc., Diu. Petrol. Chem. Prepr., 16 (l), A17 (1971). (7) B. J. Mair and T. J. Mayer, ANAL.CHEM., 36, 351 (1964).
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Table 111. Distribution of Aromatic Sub-Types in Petroleum Products (Weight Per Cent of Total Oil) Di PolyMonoaromatic triaromatic aromatic Venezuela middle gas oil 14.6 18.5 1.8 Nigerian gas oil 10.7 13.6 2.3 So. La atrn residual 12.7 20.4 2.3 Kuwait vacuum residual 2.7 37.7 3.7 Kuwait gas oil 12.2 22.0 0.34 Cracked gas oil 1.8 30.4 2.5 Kerosene 20.2 12.2 2.8 Hydrogenated coal distillate 55.1 26.4 1.5
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Application. The GEC described has been found satisfactory for aromatic concentrates from kerosene, cracked or virgin gas oils, FCC feedstocks, and residuals. Only subtle experimental diflerences exist between extremes (keroseneresiduals) since their pretreatment steps are identical. Table I11 shows aromatic-type distributions for a variety of petroleum systems; one example from hydrogenated coal is also cited. The only limitation is volatility since chloroform and methanol must be removed from the polyaromatics. The method is rapid, flexible, and can be made analytical with appropriate micro columns and detectors. The total sequence of steps described earlier and in this paper provides an analytical and quantitative approach to the following eight major classes of compounds: acids, bases, neutral nitrogen compounds, total saturates, total aromatics, monoaromatics, di triaromatics, and polyaromatics.
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ACKNOWLEDGMENT NMR spectra were kindly supplied by R . K. Jensen. The experimental assistance of R . C. Query is deeply appreciated. RECEIVED for review March 30, 1972. Accepted July 11, 1972. Presented at the 163rd National Meeting, American Chemical Society, Boston, Mass., April 9-14, 1972.
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