HC1: one is observed at Amax 283 mp with a value of 17,000 and the other a t 228 mp of reduced absorptivity. These results are similar to those reported previously (23). Using the value of E ~ S ~ ~ ,the , , yield of H M F after 10 hours heating at 80.0" C in 1.OM HC1 is 2 2 z . Also, H M F can be observed in chloroform at 279 mp and at 286 mp in 0.2M NaOH. (23) G. MacKinney and 0 , Temmer, J . A,,,. Chem. sot., 70, 3586 (1948).
Thin layer chromatographic studies also confirmed the identity of H M F and the chromophoric compound resulting from the acid degradation of fructose. RECEIVEDfor review March 10, 1967. Accepted May 24, 1967. This investigation was supported in part by a n institutional grant to the University of Florida by the American Cancer Society and in part by grant GM-09864-04,05 from the National Institutes of Health, u. s.Public Health Service, Bethesda, Md.
Determination of Nitrogen Compound Types and Distribution in Petroleum by Gas Chromatography with a Coulometric Detector D. Kendall Albert Research and Decelopment Department, American Oil Co., Whiting, Ind.
A recently developed selective nitrogen detector was used with gas chromatography to determine quantitatively the nitrogen compound distribution in light catalytic cycle oil. The dominant nitrogen compound types-pyridines and quinolines, indoles, and carbazoles-were determined directly on the sample without prior separations. Application of the method, which requires about 2 hours for an analysis, to three cycle oils of varying origin indicated that the relative distribution of types was similar, although the absolute concentration levels of nitrogen differed. Combination of gas chromatography with chemical methods of separation and mass spectrometry was used for detailed nitrogen compound distribution studies on fractions of light catalytic cycle oil and light virgin gas oil. The latter contained quinolines, carbazoles, and benzocarbazoles as the dominant nitrogen compound types and minor amounts of benzoquinolines, pyridines, and indoles. Other minor nitrogen compound types-e.g., nitriles-were indicated in the oils studied but were not identified. Chemical separations included a modified perchloric acid extraction method to separate indoles from carbazoles. The reaction of perchloric acid with model compounds was studied with semiquantitative results varying with the nitrogen com pou nd type.
ADVERSE EFFECTS of nitrogen compounds in petroleum on many important catalytic processes and on product stability are well recognized. To surmount these effects, sensitive and accurate analytical determinations of the different types of nitrogen compounds-e.g., pyridines, quinolines, indoles, and carbazoles-are needed. Several methods, which generally comprise a combination of various analytical techniques, for separating, identifying, and determining nitrogen compounds in petroleum, have been described ( I , 2). For example, basic nitrogen compounds were isolated from a heavy gas oil by a combination of acid extractions, alumina adsorption, and paper chromatography (3). Benzoquinolines, quinolines, and other types were identified by ultraviolet, infrared, and mass spectrometry. Nonbasic nitrogen compounds-indoles, carbazoles, and benzocarbazoles-were quantitatively determined in cracked (1) H. V. Drushel and A. L. Sommers, ANAL.CHEM., 38,19 (1966). (2) L. R. Snyder and B. E. Buell, Zbid.,36,767 (1964). (3) D. M. Jewell and G. K. Hartung, J . Chem. Eng. Data, 9,297 ( 1964).
gas oils and straight-run distillates by linear elution adsorption chromatography ( 2 , 4). A combination of nonlinear and linear elution adsorption chromatography was used for qualitative analysis of nitrogen compounds (5). Basic compounds, indoles and carbazoles, were isolated from a light catalytic cycle oil by a combination of silica gel adsorption and acid extraction ( I ) . The extractants included sodium aminoethoxide in ethanolamine for a variety of weakly acidic compounds and 72% perchloric acid (6) for indoles and carbazoles. The latter types also were isolated from a hydrogenated furnace oil with 72% perchloric acid (6); and, in addition, phenazines and nitriles (7) were identified. Phenazines were identified in a black precipitate which formed during the perchloric acid extractions; nitriles were identified in the acid raffinate. Many of these methods, however, are not quantitative because of interferences-e.g., hydrocarbons, oxygen compounds, and sulfur compounds-and/or incompleteness of chemical reactions. In a recently published procedure (8), the gas chromatographic separations of the nitrogen compounds of a petroleum sample can be followed, even when non-nitrogenous components are present. The separation is monitored not by a conventional detector, but by the catalytic hydrogenation of the nitrogen compounds to ammonia, which is titrated coulometrically. Consequently, only nitrogen compounds can be detected. For calibration, the nitrogen types within each gas chromatographic fraction are identified by spectral techniques and, where possible, by the retention times of individual compounds. This procedure has now been applied directly to light catalytic cycle oil (LCCO) and to fractions of LCCO and light virgin gas oil (LVGO) obtained by acid extraction. Nitrogen compound distribution in LCCO can be quantitatively determined in about 2 hours. Except for the separation of indoles from carbazoles, the acid extraction scheme is conventional ( I , 6). Pyridines and quinolines are extracted from benzene solution with hydro(4) L. R. Snyder and B. E. Buell, Anal. Chim. Acta, 33,285 (1965). ( 5 ) L. R. Snyder, ANAL.CHEM., 38, 1319 (1966). (6) G. K. Hartung and D. M. Jewell, Anal. Chim. Acta., 26, 514 (1962). (7) Zbid., 27,219 (1963). ( 8 ) R. L. Martin, ANAL.CHEM., 38, 1209 (1966). VOL. 3 9 , NO. 10, AUGUST 1967
1 1 13
Table I. Gas Chromatographic Analysis of Synthetic Blends 1.0 kl of benzene solution Nitrogen. uum A Added Found 2,4,6-Trimethylpyridine 111 112, 112, 108 Quinoline 98 104. 105, 102 CMethylquinoline 244, 40, 240 1,2,3,4-Tetrahydroquinoline 125, 27, 127 2,4-Dimethylquinoline 129 118, 19, 123 1,2,3,4-Tetrahydrocarbazole 120
SILICA GEL
--I
tfNZENE.MElHANOL
I
HCXANC C L U A T I . D I S C A I D ELUATl
W A I E R EXIRACIION
WATER PHASC, DlSCAlD
AQUEOUS HCL
I
NOOH, k E X A N e
607. HI C I O (
I
B
Indole 1,2-Dimethylindole Carbazole 2-Methylcarbazole Phenazine a Not separated.
109 114 118
108
115 118
73
72
144
145
chloric acid, indoles with 6 0 z perchloric acid, and carbazoles and phenazines with 72 perchloric acid. The recoveries of indoles and carbazoles are 75-85 % and 55-70 %, respectively; the other types are recovered quantitatively.
x
I
H Y D R O 1Y S I'S , H EX AN E
7 2 % HCtO4
I FEXANE NoOH,
I
P H E N A Z I N E IN HEXANE
EXPERIMENTAL
Figure 1. Separation scheme for nitrogen compounds in gas oils
Materials. n-Hexane was Phillips pure grade. Silica gel was Davison Code 950, 60-200 mesh, activated at 750' F for 12 hours. Other reagents were Reagent Grade. Reference nitrogen compounds, at least 98 pure by gas chromatography, were obtained from commercial sources and used without further purification. The light catalytic cycle oils had a general boiling range of 400' to 640" F and were of varying origin. The light virgin gas oil had a boiling range of 400" to 750" F. Apparatus. Details of the detection system (including preparation of the ter Meulen catalyst) and apparatus have been described (8). A separate alkaline scrubber (to remove acidic interferences) is not needed with the ter Meulen catalyst because of the alkalinity of the magnesium oxide catalyst support. The titration cell (Model T-400H) and auxiliary apparatus are available from Dohrmann Instruments Co., Mountain View, Calif. The cell employed a lead/ lead sulfate reference electrode and was operated with a bias of about 100 mV and with the sensor electrode positioned at the sample inlet. The magnetic stirrer was operated a t about two thirds of the maximum stirring rate. A 1-mV recorder with a chart speed of 0.5 inch per minute was used. The gas chromatography column [20 feet of */4-inch0.d. stainless steel tubing packed with 9 polyethylene (mol. wt. 12,000) and 3 x potassium carbonate on Chromosorb W (S)] and the catalyst section were installed in a conventional temperature-programmed gas chromatograph with the catalyst section inside the detector oven. The catalyst tube extended through the oven and cabinet walls to the titration cell. A column by-pass enabled samples to be charged directly to the catalyst for total nitrogen determinations. The catalyst bed was 2 inches long. The carrier gas was hydrogen. Mass spectra were measured a t the conventional 70 volts and a t 7.5 volts (uncorrected) with a Consolidated model 21-103c mass spectrometer, with the inlet system a t 325" C. Relative intensities of the parent peaks obtained a t low voltage were taken as a first approximation to relative concentrations. Gas Chromatographic Method. The sample-vaporizer temperature was 275" C and the carrier gas flow rate was 200 cc per minute. The column temperature and sample sizes were varied according to the nitrogen content. In general, 30 t o 50 11 of an oil sample was charged t o the
column at 120' C, and the column temperature was programmed to 330" at 2 " C per minute. Elution was continued a t 330' until the chromatogram was completed, which generally required 1.5 t o 2 hours. The nitrogen distribution was calculated by measuring (planimeter) the areas of individual peaks or groups of peaks and normalizing the total area t o 100 %. Recovery of nitrogen from the column was judged by comparing the area per microliter of sample charged t o the column with that of sample charged directly to the catalyst. Agreement within 5 % was considered satisfactory. The detection system was calibrated (8) and periodically checked with samples of known nitrogen content. Analyses of synthetic blends (Table I) show the same high degree of accuracy and precision (about 1 3 %) for the gas chromatographic determinations as obtained previously (8) for total nitrogen determinations. Extraction Method. The extraction method is presented in Figure 1. A 250-ml sample of oil was percolated (nitrogen pressure) through a 4-foot by 3/4-inch i.d. column of silica gel at about 3 ml per minute. After elution of most of the oil (mostly saturates) with 500 ml of n-hexane, nitrogen compounds and aromatic hydrocarbons were desorbed with 250 ml of 1 :1 benzene-methanol. Methanol was removed by aqueous extraction The remaining benzene solution was extracted with three 100-ml portions of 1 N HC1 and washed with four 100-ml portions of water. The combined washing and acid extracts were neutralized with sodium hydroxide solution, and the basic nitrogen compounds were extracted with three 50-ml portions of n-hexane. The benzene solution now containing nonbasic nitrogen compounds and aromatic hydrocarbons, was dried with anhydrous sodium sulfate, concentrated to 50 ml with low heat under a nitrogen stream, and extracted with five 10-ml portions of 60 HC104. The boundary between the two layers was not easy to see because of the formation of a black precipitate throughout the acid layer. The combined acid extracts were filtered through glass-fiber filter paper, washed with three 10-ml portions of benzene, and mixed with water (1 :4) to decompose the indole perchlorates. The free indoles were extracted with five 50-ml portions of n-hexane. The acid raffinate was neutralized with sodium hydroxide solution,
z
1 1 14
ANALYTICAL CHEMISTRY
3 0 # l SAMPLE, AITIN.:lOn
-
-
CARBAZOLIS-
INDOLES
-
-PYRIDINESQUINOLINES
IO
20
IO
40
50
b0
70
RETENTION TIME, MINUTES
Figure 2. Nitrogen compounds in light catalytic cycle oil and additional minor amounts of alkyl indoles were extracted with three 50-ml portions of n-hexane. The combined benzene raffinate and washings from the 60 % HCIOa extraction were extracted with five 10-ml portions of 72% HClOa. The combined acid extracts were washed with three 10-ml portions of benzene, mixed with water (1:4), and extracted with five 50-ml portions of n-hexane. The hexane extract contained essentially all of the carbazoles, but only about one third of phenazine. The remainder of the phenazine and small amounts of carbazoles were obtained from the acid raffinate by neutralization and extraction as described above. The various fractions were prepared for further analyses by evaporating either to a few milliliters of solution or to dryness with low heat under a nitrogen stream. The recoveries of several model compounds after extraction from benzene solution with 72% perchloric acid are shown in Table 11. The recovery is inversely proportional to the amount of precipitate formed. Alkylphenazines were not available; but, a t least, as determined with the parent compound, the study indicates that phenazine perchlorates are soluble in 72 perchloric acid. This finding is not in agreement with other findings (6) that phenazine and alkylphenazines form insoluble perchlorates. Carbazoles produced the largest amounts of precipitate. The eventual darkening of the precipitate may be due to air oxidation and acid-catalyzed polymerization, to which many pyrrole derivatives are notoriously sensitive (9). Carbazoles may well be responsible for the dark precipitate that is formed when gas oils are extracted with perchloric acid. RESULTS AND DISCUSSION
Light Catalytic Cycle Oil. Figure 2 is a chromatogram of the nitrogen compounds in a light catalytic cycle oil (240 ppm N, 300 ppm S). They are mainly: pyridines (including some quinolines), indoles, and carbazoles, with traces of benzocarbazoles (2, 8). Except for the pyridines and quinolines, the chromatogram is qualitatively similar to one obtained with a thermal conductivity detector ( I ) for indoles and carbazoles, which had been isolated from a cycle oil by extraction. However, with the present technique, a chromatogram of all the nitrogen compounds is obtained directly on the sample without prior separations, and more individual nitrogen compounds are seen because of the absence of detector response to interferences and the absence of nitrogen compound losses, which generally occur during chemical separations. Components elute in boiling point order, and separations are sufficient to obtain a relative (9) E. H. Rodd, “Chemistry of Carbon Compounds”, Vol. IV, Part A, pp. 38, 122, Elsevier, Amsterdam, 1957.
Table 11. Extraction of Nitrogen Compounds with 7 2 z Perchloric Acid ReAmount of Added Found covery, precipitate Compound ppm ppm 7Z formed 115 97 84 Smalla Indole 111 83 75 Small 2-Methylindole 106 85 80 Small 1,2-Dimethylindole 132 90 68 Largeb Carbazole 108 60 56 Very largeb 2-Methylcarbazole 144 140 97 None Phenazine Just enough to produce a white haziness or slight turbidity. b Originally white, but changed to blue to bluish gray after isolation. Q
Table 111. Nitrogen Analyses of Light Catalytic Cycle Oilsa Nitrogen, ppm Oil No. 2 Oil No. 3 Oil No. 1 1.8 1.2 C, pyridines 0.4 1.6 2.2 1.5 Ca pyridines 3.4 3.8 C4 pyridines 3.1 13.6 18.4 C6+pyridines, 9.8 quinolines 2.0 1.2 1.3 Indole 19.6 14.2 11.0 C1 indoles 35.0 36.2 C, indoles 22.8 33.4 49.3 20.8 C, indoles 19.5 34.6 Ca+indoles 6.9 21.0 21 .o 10.6 Carbazole 58.0 58.6 37.0 C, carbazoles 44.0 71.5 C2carbazoles 36.7 36.8 58.4 22.2 C3+carbazoles 29 1 370 184 Total N by GC 372 285 185 By Kjeldahl 25 21 15 Basic N by GC 24 30 13 By Titration 132 94 64 Total indoles by spectrophotometry 136 110 63 Total indoles by GC 209 160 Total carbazoles by 106 GC 345 270 169 Nonbasic N by GC 261 342 172 Nonbasic N by difference a Sulfur contents are 0.4, 1.8, and 0.4 %, respectively.
distribution by carbon number. Overlap between types (indicated by the extrapolated broken lines) is small enough that an estimation of types can be made. The largest overlap occurs with the basic compounds and indoles with CS+ pyridines and quinolines overlapping indoles as indicated in Figure 2. To test the quantitative applicability of the gas chromatographic method without prior chemical separations, three light catalytic cycle oils of varying origin were analyzed. The proportion of each nitrogen compound type and the relative distribution of each type by carbon number were estimated. The amount of area proportional to the Cb+ pyridines and quinolines was estimated from the background area between the major indole peaks. Results are shown in Table 111. Values for total, basic, and nonbasic nitrogen are in good agreement with those obtained by Kjeldahl analysis and potentiometric titration. Agreements for basic and nonbasic nitrogen are particularly good considering the possible errors in the estimation of overlap and the arbitrary nature of basic nitrogen determination. Results for indoles also compare favorably with analyses obtained spectrophotoVOL. 39, NO. 10, AUGUST 1967
11 15
{ A ) 60% HC104
4
-PYRIDINES
-
I
I
c4
1
Rt FOR QUINOLINE A N D I N D O L E
(CARBAZOLES)
I
I
I
[SI 72% HCIO4
I
-CARBAZOLES-
2
P
1 1 16
ANALYTICAL CHEMISTRY
2 -
1 1
Figure 3. Basic compounds extracted from catalytic cycle oil
(10) M. A. Muhs and F. T. Weiss, ANAL.CHEM., 30,259 (1958).
c
= u *
RETENTION T I M E , M I N U T E S
metrically (10). The latter method employs p-dimethylaminobenzaldehyde, which reacts with indoles-but not carbazoles-to form a dye. The relative distribution of types is similar, although the concentration levels of nitrogen differ considerably. For example, the average ratio of carbazoles to indoles is about 1.6 =k 0.1. Other light catalytic cycle oils which we have analyzed show carbazole-indole ratios in the same general range. Cycle oils which differ appreciably in boiling range would, of course, differ appreciably in nitrogen distribution. For example, one cycle oil with a boiling range of 396" to 606" F showed a carbazole/indole ratio of only 0.2, which agrees with the finding (2) that relatively little carbazoles appear in fractions boiling below about 600" F. Also, in light cycle oils that we have examined, the amount of basic nitrogen has been relatively constant with basic nitrogen in the general range of 6 to 12x of the total nitrogen. Light cycle oils with larger amounts of basic compounds have not been available; therefore, the extent of the error in judging the basic compounds-indoles overlap in such stocks is not known. The potential source of error would, of course be greater with such stocks because of the greater difficulty in resolving the amount of overlap. The results in Table I11 also include unidentified minor nitrogen compound types. However, types of nitrogen compounds other than pyridines, quinolines, indoles, carbazoles, and benzocarbazoles were not identified. Gas chromatograms of chemically separated fractions show in more detail the extent of overlap between types. Figure 3 is a chromatogram of the basic compounds extracted by HCI. They are predominantly pyridines with relatively smaller amounts of quinolines, although all of the quinolines may not have been completely extracted. The latter is suggested by the relatively larger amounts of Csf pyridines and quinolines that was obtained by direct analysis (Table 111). Minor amounts of C6+substituted pyridines and quinolines overlap in the region corresponding to retention times of the indoles, but not beyond Cs indoles. No indoles were detected by mass spectrometry among the pyridines and quinolines. The overlap of indoles into carbazoles is relatively small. Figure 4A shows that only small amounts of CI carbazoles are present in the indole fraction. Figure 4B shows that no indoles are present in the carbazole fraction. The chromatograms of the indole and carbazole fractions
1
I
I
I
L ,\
in Figure 4 are qualitatively similar to the respective portions of the chromatogram of the original sample in Figure 2. However, there is some alteration of components during the chemical separations. For example, comparison of the relative areas of the two peaks for Cz indoles in Figure 4A with the relative areas of the same peaks in Figure 2 indicates a larger loss of one component during the chemical separation. Similarly, the relative area of the first peak in the C1carbazole group (Figure 4B] is less than in the original sample, A more detailed analysis of the distribution of nitrogen compounds in each fraction was made by mass spectrometry. These results were combined with gas chromatographic analyses to show the distribution by carbon number for each type and the relative amount of each type. Results, which are summarized in Figure 5, are semiquantitative (about 75 total nitrogen recovery) because of losses of nitrogen during the chemical separation. Some components, particularly quinolines, were present in amounts which were too small for accurate determinations. Both of the major classes, carbazoles and indoles, reach a maximum distribution with derivatives that are substituted with two carbon atoms, which is as others have found (1, 2, 11). Mass spectrometric analyses of the raffinate from 72% perchloric acid extractions indicated the present of nitrilese.g., dicyanoindanes (n-but these results were not confirmed. Phenazines were not detected. Light Virgin Gas Oil. The nitrogen compound distribution in a light virgin gas oil (Figure 6 4 is much more complex than in cycle oil. The major types-quinolines, carbazoles, and benzocarbazoles-are not separated because of the predominance of highly substituted derivatives of each class. Because of lack of separations and excessive peak tailing of higher boiling components, quantitative determination of nitrogen distribution was not possible. The distribution of the major types of compounds was estimated qualitatively, however, with the aid of chemical separations. A chromatogram (Figure 6B) of a basic fraction shows distribution of basic compounds over essentially the full boiling range of the oil, but most of the compounds are (11) R. W. Sauer, F. W. Melpolder, and R. A. Brown, Ind. Eng. Chem., 44,2606 (1952).
A \ TOTAL NITROGEN I30 pI L V G O . A T T E N . : l O n l
CARBAZOLES
QUINOLINES
f'\
----.INDOLES----CARBAZOLES-
30-
E
-
BEN ZOC ARBAZO LES-
B j BASIC NITROGEN (HCL EXIRACT1
P
,Z.METHYLQUlNOLlNE
,z,
I.DIM~THYLQUINOLINE
!z
z
,c) C A R B A Z O L E S a
10-
INDOLES
(PERCHLORIC A C I D LXTRACTSI
-----INDOLES-----
Q
*CARBAZOLES-
-, c, 9%
P
I
CARBAZOLE,
NUMBER
Figure 5. cycle oil
i
-
B E N Z O CA R B AZOL ES-
'I
OF CARBON ATOMS P E R MOLECULE
Nitrogen
distribution
in catalytic
concentrated in the same range as the nonbasic types. A relatively small portion of the basic compounds elute before carbazoles, and trace compounds elute before quinoline. The bulk of the compounds are C3+substituted derivatives. Mass speotrometric analyses indicated quinolines were the most abundant class of basic compounds with benzoquinolines and pyridines present in relatively smaller amounts. Molecular weights of quinolines ranged from that of quinoline to CI3 substituted derivatives, with C6-Cs most abundant. Infrared analyses indicated that the number of adjacent hydrogens on the aromatic rings (either carbocyclic or heterocyclic) ( I ) was predominantly four, which suggests that derivatives with unsubstituted fused benzene rings are probably most abundant. Chromatograms (Figure 6C) of the nonbasic fraction indicate carbazoles as the major nitrogen compound type; there is a marked similarity to the carbazole fraction of LCCO (Figure 2 ) . Mass spectrometry indicated that they ranged from carbazole to Cs substituted derivatives, with C3 derivatives most abundant. Indoles were present in relatively small amounts [4.7 as determined spectrophotometrically (IO)]; in contrast to LCCO, Figure 6 C shows that they are mainly C3+-substituted. Benzocarbazoles were not definitely indicated in the extract; probably they are not extracted with perchloric acid. CONCLUSIONS Nitrogen compound distribution in light catalytic cycle oils can be quantitatively determined by gas chromatography without prior sample treatment. The method is fast, accurate, and suitable for routine use. For more complex stocks, such as virgin gas oils, the technique is useful qualitatively. In the present stage of development, the gas chromatographic technique is most useful with stocks having end points below about 650" F. Applications to higher boiling stocks should be improved with higher temperature catalyst systems-e.g., with unsupported nickel (8). With the ter Meulen catalyst, about 440" to 4.50' C is the highest tem-
0
b0
0
PO
RETENTION TIME, MINUTES
Figure 6. Nitrogen compounds in a light virgin gas oil perature that can be used; however, with granular nickel, for example, temperatures up to about 950" are possible. (A catalyst system that employs granular nickel is available from the Dohrmann Instruments Co., Mountain View, Calif.). Also, smaller gas chromatography columns with lightlyloaded (e.g., 0.5 liquid phase) packings should facilitate separation of higher boiling compounds. Such columns would necessitate small sample charges, however, and might require a concentration step prior to analysis. The technique can be used advantageously to follow the effectiveness of chemical (or other) separation procedures. The modified perchloric acid extraction method separates indoles from carbazoles and partially separates phenazines from carbazoles. However, separations with perchloric acid have disadvantages of side-reactions and incompleteness of reactions. Identification of minor nitrogen compound types in perchloric acid extracts is particularly troublesome. Major types of nitrogen compounds-primarily pyridines, quinolines, indoles, and carbazoles-reported in 400-750" F petroleum fractions were confirmed; but some minor types, particularly phenazines, were not confirmed. The black precipitate that forms as a by-product from perchloric acid extractions appears to be primarily carbazole derivatives and not phenazines, as previously reported. Combination of the gas chromatographic technique with other separation methods, such as liquid-solid chromatography, particularly linear elution adsorption chromatography, and thin layer chromatography, is now under investigation. ACKNOWLEDGMENT
I am grateful to R. L. Martin for helpful counsel and assistance with the gas chromatographic system, and to Seymour Meyerson who interpreted the mass spectra. RECEIVED for review February 23, 1967. Accepted May 12, 1967. VOL. 39, NO. 10, AUGUST 1967
1 1 17
