Anal. Chem. 1983, 55, 1685-1688
1685
Selective Isolation of Nitrogen Bases from Petroleum Jean-Marie Sc:hmitter,* Ioannis Ignatiadis, P a t r i c k Arpino, a n d Georges Guiochon Ecole Polytechnique, Laboratoire de Chimie Analytique Physique, Route de Saclay, 91120 Palaiseau, France
A method has been designed for the rapid separation of nltrogen bases firom petroleum by trapping their quaternary ammonium salts on hydrochloric acld treated silica. A chromatographic coiiumn fliied wlth this adsorbent and equipped wlth a solvent recycling system allows a selective and quantltative extraction of bask compounds from large amounts ( > I O 0 g) of crude 011. The procedure has been tested with reflerence compounds, a coker gas-oil, and a heavy biodegraded crude and appiled to 15 different oil samples.
Nitrogen bases occurring in petroleum, coal, and related materials, such as coker gas-oils or solvent-refiied coal, belong to the classes of azaarenes (nitrogenated polyaromatic hydrocarbons, NPAH) and of primary aromatic amines (PAA). The concentration of basic compounds in the investigated samples is usuailly very low: less than 500 ppm is a typical concentration for an azaarene fraction in virgin crude oils. Therefore, a selective extraction method is required to render basic fractions amenable to most characterization techniques, whether chromatographic or spectroscopic ones. Common concentration methods of basic compounds involve liquid-liquid extraction with a strong mineral acid ( I , 2 ) or ion exchange chromatography on macroreticular resins, which has been extensively used during the course of API Research Project 60 (3-5). These two methods suffer from several drawbacks: the first procedure is plagued by the formation of emulsions and requires large solvent volumes; the latter is time-consuming, has a poor selectivity, and is hardly applicable to viscous virgin crude oils. Furthermore, none of these methods is suited to the large quantities of sample which are required if the identification of individual compounds is sought. For these realsons, we have undertaken the development of a fast and reproducible isolation method, which may be easily scaled up, even with samples having API gravity values less than 20 ( X . 9 2 g/cm3). In a previous work, the selective isolation of carboxylic acids was achieved by trapping these compounds on a silica column modified with potassium hydroxide (6-8). In an analogous manner to Jewel1 et al. (9), a column equipped with a solvent-recycling system was used, resulting in an economy of solvent and a minimum operator assistance during extraction. Similarly, the property of NPAHs and PAAs to form quaternary ammonium salts was used in order to trap and selectively isolate them on an acid-modified silica column. With hydrochloric acid as a modifier, nonbmiic compounds are gathered in a single fraction by elution with dichloromethane, whereas trapped hydrochloride salts are subsequently released by percolation with methanol (ref 10, Figure 1). The selectivity of this method has been tested with reference compounds representative of the major classes of petroleum constituents, thus comprising n-alkanes, neutral PAHs, phenols, and nitrogen compounds of both basic (NPAHs and PAAs) and nonbasic (indole, carbazole) types. The recovery of nitrogen bases and the reproducibility of extraction were estimated with reference mixtures and by
spiking a coker gas-oil with acridine. EXPERIMENTAL SECTION Solvents, reagents, and adsorbents were from Merck (Darmstadt, G.F.R.) or Carlo Erba (Milano, Italy). Analytical grade solvents were distilled prior to use and silica was Soxhlet extracted with methylene chloride. Glassware was cleaned in an ultrasonic bath and rinsed twice with methylene chloride before use. Extractors equipped with a solvent recycling system (7)were of two different sizes: 8 cm i.d. X 25 cm long, used with 100-200 g of adsorbent and 500-1000-mL solvent flasks, and 1cm i.d. X 18 cm long, used with 10 g of adsorbent and 50-100-mL solvent flasks (cf. Figure 1). The acid-modified silica was prepared by thorough mixing of silica gel (grade H from Merck, 63-230 pm) with 32% concentrated acid. After 12 h of contact time, this yellowish adsorbent was loaded into the extractor and washed with three column volumes of CHzC12. Samples to be analyzed were dissolved (standards) or mixed (crude oils) with CHzClzand loaded on the top of the adsorbent. Dichloromethane was placed in the round-bottom flask and the recycling of this solvent was started. At the end of collection of the first fraction containing neutrals and acids (3-4 h), the dichloromethaneflask was replaced by another containing methanol in order to recover the hydrochloride salts of the basic compounds (1-2 h elution). Free bases were regenerated in strong alkaline medium (aqueous sodium hydroxide 6 N) and extracted with CHZClp After the product was fiitered over a glass frit and dried over sodium sulfate, the solvent was removed in vacuo with a rotary evaporator. Alternatively to liquid/liquid extraction, bases regenerated in alkaline medium were recovered by chromatography over a short column packed with a reversed-phase adsorbent (LichroprepRP18 40-60 pm from Merck) with methanol as an eluent, after a first washing with distilled water. For reversed-phase liquid chromatographic purification, total basic extracts were dissolved in dichloromethane and loaded on a small amount of straight silica grade H (500 mg of extract on 1g of silica). After removal of dichloromethane (gentle heating under a stream of nitrogen), a precolumn (l/z in. i.d., 4 cm long) was packed with this material and connected to a chromatographic column (1/2 in. i.d., 25 cm long) packed with Lichroprep RP18 (40-60 pm). A first fraction (purified bases) was eluted with 100 mL of acetonitrile at a flow rate of 5 mL/min, and then the column was backflushed with dichloromethane (50 mL) at the same flow rate. Gas chromatography (GC) of basic fractions and of test mixtures were performed on a Perkin-Elmer Model Sigma 3 (Norwalk, CT) equipped with a flame ionization detector. A HewlettPackard Model 3930A integrator (Palo Alto, CA) was used for peak area measurements. RESULTS AND DISCUSSION Separation of Reference Compounds. A test mixture was analyzed to evaluate the selectivity and quantitative results of extraction of basic nitrogen compounds (cf. Table I). With a small size extractor (10 g of HC1 modified silica) without solvent recycling, fractions were collected every 50 mL and analyzed by GC. Normal undecane and tetracosane were added to the first fraction as calibration standards. As shown in Figure 2 and Table I, a 100% (&5%) recovery of nonbasic species is rapidly obtained, except for indole which polymerizes in acidic medium and is not quantitatively eluted with dichloromethane (11). All basic compounds are trapped on the column and do not interfere with constituents of the
0003-2700/83/0355-1685$01.50/00 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55,
NO. 11, SEPTEMBER 1983
Table I. Separation of a Test Mixture" % recovery for CH,Cl, elution volumes of
compd no.
nonbasic compds fluorene fluoranthene
1
2
n-c,,
3
4 5 6 7
dibenzothiophene 2,6-dimethylphenol CY -naphthol indole carbazole
8
compd no. 9
basic compounds 2-methylaniline 2,6-dimethylaniline 2,4,6-trimethylaniline 2,6-dimethylquinoline 2-aminonaphthalene benzo[h ]quinoline 2,4-dimethylbenzo[h ]quinoline N-pheny 1-N-naph t hy lamine phenazine 1 ,lo-diazaphenanthrene
10 11
12 13 14 15 16 17
% recovery
in 50 mL
50 mL
200 mL
95 90 90 90 85 90 35 90
100 100 100 100 100 100
of CH,OH 0 0 0 0 0 0 0 0
40 100
% recovery for CH,Cl, elution volumes of 50 mL 200 mL
0 0 0 0 0 0 0 0 0 0
% recovery
in 50 mL of CH,OH
0 0 0 0 0 0 0 0 0 0
pkb
95
9.55
100 100 100
10.10
7.90 9.80 95 9.75 95 95 95 95 12.77 18 95 9.73 a Amounts of individual compounds were in the range 0.5-0.8 mg. The total sample load on 1 0 g of HC1 modified silica was approximately 10mg. b Values from ref 22.
1 -
10
A ) CH2C12 4 h
B ) MeOH I
2h
I
Figure 1. Scheme of extractor equipped with a solvent-recycling
system: (1) condenser with water-coolingJacket;(2)glass frit, porosity 20-40 pm; (3) Teflon stopcock: (4) heater.
first fraction. Flushing of the column with methanol rapidly elutes the hydrochloride salts. A nearly quantitative recovery of bases is achieved with 50 mL of methanol, as controlled by GC analysis (Figure 2). This is true even for weak bases such as phenazine (pKb = 12.77) (cf. Table I). The same test mixture has been separated in the solventrecycling mode. Nonbasic species were totally recovered from the column after 1 h of elution with dichloromethane. No interference with nitrogen bases was detected when this elution time was extended up to 4 h (approximately 400 mL of CH,Cl,). Another test separation was made with the same mixture of nonbasic compounds but with only 50 pg of 2,6-dimethylquinoline and benzo[h]quinoline as basic species, each
d:
10
13:
bo
170
90
2IC
230
'cc
Flgure 2. Control of selective extraction of nitrogen bases with a test mixture. Chromatogram a, mixture before separation; b, dichloromethane fraction; c, basic function. GC separation was on an OV-73 capillary column, 70 m long, 0.28 mm i.d., and 0.15 pm film thickness. The temperature was programmed at 1.8 'C/min. For peak identification, cf. Table I; added caiibration standards n-C,, and n-C2., are labeled '3'.
of these azaarenes accounting for approximately 1%by weight of the starting mixture. The recovery of basic compounds was estimated to be 90%, still without detectable interferences between basic and nonbasic fractions. This high recovery level was also found to remain constant for reference di- and triaromatic azaarenes with sample loads ranging from 0.5 to 10 mg (10). Two conclusions, which are of main concern for the analysis of energy source related materials, can be drawn from the result of these test separations. First, a quantitative extraction of basic compounds can be achieved without artificial alteration of their original composition, i.e., no specific enrichment of monoazaarenes vs. diaza compounds or vs. PAAs is to be
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
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~-~ Table 11. Extraction of Nitrogen Bases from Hydrotreated Coker Gas-Oil, Estimation of Acridine Recovery after Spiking
extract no. 1
wt of basic fraction, mg 44
P, =
A(acridine)/ A(n-C,4)b 0.865
A(2,:igM+)/ .A(n-C,, 1 0.550
2
45
0.885
0.570
3 4
47 46a
0.865 0.870
0.545 0.565
5 av, i.RSI)
47a 45.8, * 3%
0.875 0.8-72, k 1%
0.590
comments acridine and n-CZ4added after extraction, recovery of bases on RPLC adsorbent same conditions as in 1,but bases recovered by liquid-liquid extraction same conditions as in 2 acridine added before extraction, n-CZ4 after; bases recovered as in 2 same conditions as in 4
01.564, *3.2%
Total weights minus 1 mg accounting for the addition of standards. limit, + 5%; A, peak areas; 2,6-DMA is 2,6-dimethylaniline. a
expected. In particular, no decomposition or irreversible adsorption of standards NPAHs and PAAS was evident with our method, wlhereas such problems are often encountered with procedures involving the use of alumina (12, 13). Secondly, the selectivity of the method allows the isolation of low amounts of basic compounds dispersed in complex mixtures, which is a basic requirement for the analysis of petroleum nitrogen bases. Test Separations of a Coker Gas-Oil. The reproducibility of extraction was evaluated with a hydrotreated coker gas-oil as starting material. Both NPAHs and PAAs were found in this sample, but the analysis of the same gas-oil after catalytic hydrotreatment revealed that all unsubstituted azaarenes such as acridine had been removed (14). Therefore, this sample, which still contains 1800 ppm of nitrogen bases (PAAS > NPAHs), w a chosen for recovery tests with acridine spiking. The results, summarized in Table 11, indicate that variations in the separaticm of the total basic fraction of this sample do not exceed *5%, which is within the limits of experimental precision. In spiked samples, acridine was recovered with a 95% yield (GC analysis, tetracosane was added as calibration standard). As expected, no discrimination of acridine vs. PAkS is to be noticed, as estimated from the ratio of peak areas of acridine and 2,&dimethylaniline, this latter compound being the major constituent of the fraction (14). These test separations were made with 25 g of gas-oil in small size solvent-recycling columns, using 4 h of elution with dichloromethane and 2 h for methanol. Hydrochloride salts, of red-brown color, were trapped within the first two centimetres of the adsorbent, but the silica was left colorless after percolation of methanol. The concentration of basic compounds was found to be in agreement with previous results of extractions effected with 150 g of hydrotreated gas-oil loaded on 100 1; of HC1 modified silica (14). As mentionedl in the Experimental Section, after conversion of hydrochloride salts to free bases, a recovery by percolation with methanol through a short reversed-phase (RP) column (15) was effected alternatively to liquid/liquid extraction (cf. Figure 3). With both methods, nitrogen bases were extracted from the hydrotreated gas-oil sample in nearly equal amounts (Table 11). The formation of emulsions was negligible during liquid/liquid extraction and, in the absence of asphaltenes, no precipitation occurred in methanol with the second method. Test Separations of a Virgin C r u d e Oil. The applicability of the extraction method to crude petroleum samples was evaluated with an oil from the Emeraude basin (Congo). This sample, of Upper Cretaceous age, is an example of heavy biodegraded crude having a low sulfur content; it contains approximately 32% by weight of resins and asphaltenes (16). These high mollecular weight polar constituents are troublesome a t various stages of the separation. In particular, they are partly trapped on HC1 modified silica during percolation
Mean values of three measurements; confidence
ii SiOp/HCI
salts it MeOH removal
it aqueous NaOH
@ k RPLC H1O,CH,CN
so
Iiq; liq. extraction
fraction li RPLC Mew
I uurified bases I Figure 3. Schematic diagram of the extraction and puriflcation of nitrogen bases from complex mixtures. Method B is used for crude oils, whereas method A Is restricted to samples which do not contain
high molecular weight compounds. of methylene chloride and, partly also, eluted with the hydrochloride salts of the bases when methanol is percolated through the column. Thus, total basic fractions from this oil sample had a bituminous aspect and free bases could not be recovered, after addition of aqueous sodium hydroxide, by means of percolation through a R P adsorbent as described above for a gas-oil (method A in Figure 3). Indeed, because of its insolubility in methanol or acetonitrile, a part of the high molecular weight material of the basic fraction is hardly removable from the walls of the flask which served for collection and also tends to clog the R P adsorbent. Therefore, a liquid-liquid extraction step could not be avoided when virgin crude oils were analyzed. The major drawback of this latter procedure is the formation of emulsions, but this problem occurred only to a limited extent with the small amount of solvent used (e100 mL) and could be readily solved by filtration of the dichloromethane layer over a glass frit and subsequent drying with sodium sulfate. The occurrence of high molecular weight species in the basic fractions results in a poor reproducibility of measurements of weights of these fractions when different ratios of sample load over adsorbent weight were used (cf. Table 111). Other problems are related to subsequently applied characterization techniques. In particular, total basic fractions could not be analyzed as such by gas chromatography without heavy contamination of injectors and columns. For these reasons, an additional cleanup step involving reversed-phase liquid chromatography (RPLC) was necessary. In RPLC, the elution order of azaarenes closely follows the number of carbon atoms of the solute molecules when methanol is used as an eluent (17);acetonitrile introduces some
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
Table 111. Extraction of Nitrogen Bases from Emeraude Crude Oil (Congo) with Various Ratios of Crude Oil/Modified Silica wt of
crude oil, g
wt of silica, g
% total
20 50 50 50 100
10 100 100 50 100
1.4 0.6 0.8 1.2 1.1
bases
RPLC purified bases, ppm
av RSD
400 410 440 430 450 426 4.9
Table IV. Nitrogen Bases Contents in Investigated Crude Oil Samples
name
origin
geological age
Likouala Baliste Batiraman Hassi-Messaobud
Congo Guinea Turkey Algeria Nigeria Iran Iran Aquitaine Aquitaine California Venezuela Indonesia Indonesia
Up. Cretaceous
Agha-Jari Lacq Midway Boscan Pematang Handil a
Cretaceous Cretaceous Miocene Cretaceous Cretaceous Cretaceous Up. Cretaceous Pliocene Eocene Miocene Miocene
nitrogen bases, ppm 630 340 330 1060 1620 280 240 1070 2050 950 1550 >400 700a
Mean values for three samples at different depths.
perturbations to this sequence but can be used as well if only gross separations or sample cleanup are required. By use of 1/2 in. i.d., 25 cm long column packed with Lichroprep RP18 (40-60 Km), standard reference azaarenes with up to seven fused rings were eluted within 100 mL of methanol or acetonitrile. This corresponds to the upper limit of size of solutes which can be routinely analyzed by gas chromatography (18). Thus, the volume to be collected, defining the fraction of purified bases, was fixed to 100 mL. Because of their poor solubility in methanol or acetonitrile, samples of basic fractions were dissolved in methylene chloride and loaded on a small amount of straight silica. After removal of this solvent, which is undesirable in such type of purification, the obtained material was packed in a precolumn (cf. Experimental Section). In this way, purified fractions were amenable to GC and further spectroscopic techniques of characterization. As indicated in Table 111, values measured for the concentration of purified nitrogen bases from Emeraude crude oil were reproducible, with a standard deviation not greater than 5%. A blank test was run in the conditions currently applied to the extraction of 100 g of crude oil through all steps of the selective isolation method, including RPLC purification. No contaminants were detected by GC when operating at the sensitivity used for the analysis of a crude oil extract.
This method has been in use in our laboratory over a period of 4 years, has been applied to the analysis of 15 different crude oil samples (Table IV) and was a requisite for the detailed analysis of various azaarene fractions (14, 19-21). It is quantitative for solutes having boiling points above 250 "C; losses of low boiling compounds are mostly caused by the use of a rotary evaporator for sample concentration. If the investigation of indole derivatives is required, in parallel to that of nitrogen bases, it is recommended to extract these compounds first, as discussed above. Except for this latter point, no limitation seems to hinder the application of the same extraction procedure to the separation of nitrogen bases from other complex mixtures such as tobacco smoke or environmental samples.
ACKNOWLEDGMENT We thank D. Joniaux for technical assistance and A. Y. Huc (Institut Frangais du PBtrole, Rueil Malmaison, France), J. L. Oudin (Total CFR, Talence, France), and J. Orrit (SNEA(P), Boussens, France) for kindly providing crude oil samples. Registry No. HC1, 7647-01-0; SOz, 7631-86-9.
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RECEIVED for review March 8,1983. Accepted May 9, 1983. This work was supported by grants of DBl6gation GBnBrale 5 la Recherche Scientifique et Technique (Paris, France) No. 77-7-1116 and 79-7-1306.
