(4) Dutton, H. J., Mounts, T. L., J . Catalysis 3,KO.4, 363 (1964). ( , 5 ) Faryuhar, J . W., Insull, W., Rosen, P., Stoffel, TI'., Ahrens, E. H., S u t r . A b a t ~Rev. . 17, No.8, Part 11, 1 (1959). (6) Hall, W. K., NacIver, 1). S., Weber, H . P., lad. Eng. Chem. 5 2 , 421 (1960). ( 7 ) Hoelscher, H. E., Poyner, W. G., Weger, E., Chem. Revs. 54, 575 (1953). ( 8 ) Homing, E. C., Ahrens, E. H., Jr., Lipsky, S. R., Rlattson, F. H., Mead, J . F., Turner, D. A., Goldwater, U'. H., J . Lipid Research 5, 20 (1964). ( 9 ) Humko Products, Sterick Bldg., Memphis, Tenn., Humko Tech. Bull., 1963.
(10) James, A . T., Wheatley, 5'. R., Biochem. J . 63, 269 (1956). (11) Klenk, E., Eberhagen, D.. HovveSeylers Z . Physiol. Cheh. 328, KO. 3-6, 180 (1962). 112) Kokes. R . J.. Tobin. H.. Jr.. Emmett. P. H.,J . A m . Chem. S O C . '5860 ~~, (1955). (13) Korn, E. D., J . Biol. Chem. 238, 3584 (1963). (14) Lindeman, L. P., Chem. Eng. .Vetus 40, N o . 38, 61 (1962). (15) Luddy, F. E., Barford, 11. A.,
Riemanschneider, R . W.,J . .4m. Oil Chemists' SOC.37, 447 (1960).
(16) Woodford, F. P., \.anGent, C. R l , , J . Lipid Res. 1, 188 (1960). RECEIVEDfor review September 21, 1964. Accepted Rlarch 4, 1965. Pittsburgh Conference on Analytical Chemistry and
Applied Spectroscopy, Pittsburgh, Pa., March 2 to 6, 1964. Article not copyrighted. The Northern Laboratory is part of the Korthern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture. hlention of firm names or trade products does not imply that they are endorsed or recommended by the Department over comparable products of other manufacturers.
Determination of Sulfur-Compound Distributions in Petroleum Samples by Gas Chromatography with a Coulometric Detector RONALD L. MARTIN and JOHN A. GRANT Research and Development Department, American Oil Co., Whiting, Ind.
b The distributions of sulfur compounds in a variety of petroleum' samples were determined b y a combination of gas chromatography and microcoulometric sulfur detection. The detection system responds to sulfur compounds but not to hydrocarbons. Under optimum operating conditions, it quantitatively determines individual sulfur compounds at sulfur levels down to about 5 p.p.m. Gas chromatographic separations are made on a siliconerubber column, from which sulfur compounds are eluted nearly in order of boiling point. Sulfur-compound distributions b y boiling point are shown for gasoline, coke-still naphtha, light catalytic cycle oil, virgin naphtha, kerosene, and gas-oil, Only in the lower-boiling samples can individual sulfur compounds be determined. For a crude oil, gas chromatography-in conjunction with other analytical techniques-has been used to determine the distribution of the five principal sulfur-compound types as a function of carbon number to C20. These data illustrate a typical sulfur distribution for virgin petroleums.
I
of the types and distributions of sulfur com1)ounds is needed in many phases of ~ ~ e t r o l e u processing. m As a ineans for acquiring such knowledge, gas chroniatogt,aIihj. with selective niicrocoulometric sulfur detection was investigated. 'I'he coulometric detector reslmnds to comllounds containing sulfur but not to hj.di.oc~arbonh,and thei~forecan determinp tracc amounts of sulfur com1)ounds. even if hydrocarbons are eluted with t hcni. KCRISASEI) KNOWLEDGE
644
ANALYTICAL CHEMISTRY
hficrocoulometric sulfur detectors for gas chromatography were developed independently by Coulson and Cavanagh ( 7 ) , and Klaas (IS). The detector of Coulson and Cavanagh, which followed the design of an earlier halide detector (6),has been used extensively in the analysis of pesticides ( 4 , 5 , 7 )and was suggested for petroleum analysis ( 7 ) . Klaas ( I S ) developed a similar detector and successfully applied it to petroleum samples; selective gas chromatography columns were used in clever fashion to determine sulfur-compound types- in .naphthas. Fredericks and Harlow (9) also used a coulometric detector for sulfur compounds; they modified the "halide" detector (6) to accurately and selectively determine thiols in natural gas. This type of detector was not used in our study because it does not respond to other types of sulfur compounds. In this work, microcoulometric sulfur detection was tested for quantitative performance, interferences, and applicability to all types of petroleum samples. Sulfur distributions by boiling point for seven types of petroleum samples were determined by the combination of gas chromatography and coulometric detection. The gas chromatographic separations were obtained with conventional nonselective columns. Analyses for sulfur-compound types were obtained by other techniques to supplement the gas chromatographic data. For a Middle-East crude, results from gas chromatography, mass spectrometry, and liquid-solid chromatography were combined to obtain distributions of the five principal sulfur types as a function of carbon number to Cz0.
MICROCOULOMETRIC SULFUR DETECTOR
The microcoulometric detection system is manufactured by Dohrmann Instrument Co. (Model C-100) according to the design of Coulson and Cavanagh (6, 7 ) . It has two main components-combustion tube and titration cell. The combustion tube is located between the column and titration cell, and serves to osidize the column effluent; sulfur compounds form sulfur dioxide, which subsequently is titrated automatically with coulometrically generated iodine. Hydrocarbons are converted to carbon dioxide and water and generally do not affect the titration. The current used for iodine generation is recorded to give an ordinary differential chromatogram for sulfur compounds. The combustion tube, which is maint,ained a t 750" C., is 10 inches by 5/16 inch and packed with quartz chips. Xitrogen sweep and oxygen flows of 150 ml. per minute each are brought in at the head of the tube along with the column effluent. The detector is sensitive as well as selective; the minimum detectable amount is about gram of sulfur (between t'hat for thermal-conductivity and flame-ionization detectors), and samples with sulfur contents in the partsper-million range can be analyzed. The detector time constant is larger than that of conventional detectors because of the time needed for combustion and titration; this limits the effectiveness for closely-spaced 1)eaks eluted in the first several minutes, but otherwise causes no problems. The detector is easy to operate and maintain. Quantitative Performance of D e tector. F o r quantitative analyses,
sulfur dioxide production from all types of sulfur compounds must be reproducible and directly proportional t o sulfur content. Sulfur compounds are converted mainly to sulfur dioxide, but conversion never reaches 1 0 0 ~ and o depends on operation of the combustion tube. For example, the conversion to sulfur dioxide a t several combustiontube temperatures is shown in Table I for thiophene. The conversion to sulfur dioxide was calculated from the number of coulombs used for iodine generation. I t is at a maximum in the 650°-7500 C. range, and drops rather sharply as temperature is increased to 950'. The drop in conversion probably is due either to greater production of sulfur trioxide (which is kinetically possible even though opposite to the direction of equilibrium), or to less-complete decomposition of the sulfur compound because of a decrease in catalytic activity. The age of t,he quartz packing also affects sulfur dioxide production. Freshly packed tubes produced less sulfur dioxide, especially in the 500"700' C. range, than did ones used for a month or more. When alumina or 10% platinum on asbestos was used instead of quartz chips, sulfur dioxide production was only a few per cent of theory at all temperatures. The flow rate of oxygen, sweep, and column-eluting gases have little effect on sulfur-dioxide production. For example, with quartz chips a t 750' C., varying oxygen from 20 to 300 and nitrogen sweep from 0 to 300 ml. per minute had essentially no effect, nor did the use of helium instead of nitrogen as eluting and sweep gases. Even though conversion to sulfur dioxide is not complete, quantitative analyses are possible with the coulometric detection system. This is indicated by the analysis of a synthetic mixture (Table 11),for which chromatographic peak areas were assumed proportional to sulfur content and were normalized to the value for total sulfur; combustion was performed a t 750' C. The difference between added and found values averaged only 3%, which is quite acceptable. The per cent conversion to sulfur dioxide is nearly the same, apparently, for compounds of all types and boiling points. Linearity of response for the coulometric detection system extends from the minimum detectable amount (about 10-8 gram of sulfur) upward for about three orders of magnitude. Larger amounts were not tested because the 1)eaks became broad and flat-toplied as a result of insufficient speed of iodine generation. Detector Interferences. Interference has been encountered from large samples of hydrocarbons, and from
compounds t h a t produce strong oxidizing agents during combustione.g., compounds containing bromine, chlorine, or nitrogen. Interference from hydrocarbons varies with the size and composition of the samples. I t is not noticeable in the usual-sized sample (0.1 to 5 p1, of an individual hydrocarbon or up to about 100 p l . of a mixture of a i d e boiling range). However, larger samples often give an interfering response that generally is positive, but is sometimes negative or alternately both positive and negative. The interference from hydrocarbons apparently arises because large samples are incompletely oxidized, and some of the intermediate products are either oxidized by iodine or reduced by iodide in the titration cell. Hydrocarbon response tends to diminish, but is not eliminated, as the oxygen and particularly the sweep flow rates are increased. However, detector noise also increases with flow rate, and becomes excessive when the flows exceed about 300 ml. per minute. Limiting the sample size has been the only successful means of eliminating hydrocarbon response. This usually limits the minimum detectable amount for individual sulfur compounds to about 5 p.p.m. Compounds containing chlorine, bromine, or nitrogen interfere because their combustion produces strong oxidizing agents, which in turn oxidize iodide ion in the titration cell to give a negative response. Bromine compounds interfere seriously even in trace amounts. Interference from chlorine compounds is less severe, and it can be reduced and sometimes eliminated if the oxygen is bubbled through water before combustion (Y), in which case the combustion product is mostly hydrochloric acid rather than chlorine. Kitrogen compounds interfere less severely than the halogens, and their concentrations in petroleum samples generally are too low to be troublesome.
Table 1. Conversion to Sulfur Dioxide at Different Temperatures
Combustion-tube temperature, C. 550
% Conversion to sulfur dioxide 70
650 700 750 850 950
91 93 89 74 63
enn
80
Table II. Determination of Sulfur Compounds in Synthetic Mixture
Sulfur, To Added Found Thiophene 0 0081 0 0085 Methylpropylsulfide 0 0052 0 0050 0 0059 0 0058 3-Methylthiophene 1-Pentanethiol 0 0092 0 0090 2,5-l>imethylthiophene 0 0057 0 0060 Ethyldisulfide 0 0101 0 0095 1-Heptanethiol 0 0068 0 0066 0 0035 0 0037 n-Butylsulfide Benzolblthiophene 0 0145 0 0144 2-Methvlhenzo Iblthiopheni 0 0232 0 0236 n-Am yldisulfide 0 0058 0 0057 Dibenzothiophene 0 0107 0 0109 ,
i
errors in the gas-oil range would be somewhat higher. In this work, it was important that elution time could be related to boiling point, because individual compounds generally could not be identified, and identification by boiling point had to suffice. The silicone column occasionally was used to 400' C.; column bleed begins earlier (at about 300" C.) but does not lead to significant response from the coulometric detector. The relatively high percentage of silicone rubber (150j0) is used to avoid adsorption of thiols on the Chromosorb-W support. After several months operation, thiol adsorption increases, and the column must be replaced. APPLICATIONS
GAS CHROMATOGRAPHIC SEPARATIONS
The gas chromatographic separations were made with a stainless steel column, 20 feet by inch i.d., packed with 15% by weight of silicone rubber (General Electric SE-30) on acid and caustic washed 30-60 mesh ChromosorbW. Column temperature was programmed a t 4" per minute from 60" C. to as high as 400". The flow of eluting gas was maintained a t 100 ml. per minute. The silicone column was chosen because it elutes sulfur compounds, regardless of type, nearly in order of boiling point. In the naphtha and kerosene ranges, boiling points could be predicted from elution time within 5' C.;
Characteristic sulfur distributions are illustrated by determinations on gasoline, coke-still naphtha, catalytic cycle oil, whole crude, and the heavy naphtha, kerosene, and gas oil fractions from the crude (Figures 1 through 7). The figures are plots of sulfur concentration us. boiling point of the sulfur compounds, where boiling point has been estimated from the elution times of pure compounds with known boiling points. I3ecause all sulfur t y l m often are present in virgin samldes, identification of individual mpmbers is difficult, and distributions by boiling 1)oint often must suffice. In the refined samples, however, sulfur distributions usually are less complex, and some individual VOL. 37, NO. 6, M A Y 1965
645
DISULFIDES SULFIDES THIOLS THIOPHENES BENZOTHIOPHENES
Coke-Still Naphtha. I n coke-still naphtha, which is formed from residuum by thermal cracking, the distribution of sulfur compounds is more complex, as shown in Figure 2 for a typical sample containing 0.7201, sulfur. Thiophenes still predominate, but significant amounts of hydrogen sulfide, thiols, and sulfides also are present. Most of the thiols could be identified. They are concentrated in the lowerboiling part of the sample and decrease in concentration from ethyl to amyl. Most of them are straight chained; isopropyl is the main exception. Two peaks (thiacyclopentane and methylthiacyclopentane) were tentatively identified as sulfides by emergence times. Most of the other sulfides form a background under the thiophenes; they are probably cyclic, because the unidentified peaks, like those in gasoline, do not match the retention times of chain sulfides. Chain sulfides probably are not sufficiently stable to survive the cracking reactions. Light Catalytic Cycle Oil. I n light catalytic cycle oil, which come5 from catalytic cracking of gas oil, the distribution of sulfur compounds also is relatively uncomplicated, as shown in Figure 3 for a typical sample containing 1,42y0 sulfur. Although this chromatogram has many unresolved peaks, the sulfur distribution is less complicated than those of virgin stocks of the same boiling range. Most of the sulfur is thiophenic; the analyses for sulfur types (14), which are included on the chromatogram, show that two- and three-ring thiophenes account for 79% and 14%, respectively, of the total sulfur. Only a few individual compounds could be identified with certaintybenzothiophene, 2- and 3-methylbenzothiophene (In the peak labeled CS BaThs), and dibenzothiophene. However, many of the peak groupings were tentatively identified by carbon number. Such identifications were achieved by comparing mass spectrometric and gas
2 II I
61
25 100%
250
1
350 BOILING POlN T, O F
Figure 1 .
I
I00
HYDROGEN SULFIDE THIOLS DISULFIDES SULFIDES THIOPHENES BENZOTHIOPHENES
I
200
300
400
BOILING POINT,
Figure 2.
5 15 3 20 52
5
500
I
OF
Sulfur compounds by boiling point in coke-still naphtha
components or groups of components can be determined or at least estimated. Gasoline. The distribution of sulfur compounds in gasoline is relatively simple, as shown in Figure 1 for a regular-grade sample containing 0.090% sulfur. Most of the compounds are thiophenes, a few of which-thiophene, 2- and 3-methylthiophenej benzo[blthiophene, and 2- and S-methylbenzothiophene-were identified by emergence times and are labeled on the chromatogram. The others are identified only by carbon number because individual reference compounds were not available. The breakdown of sulfur types in the gasoline, as determined by specific tests (S, 10, 11, 1 4 , 15, I T ) , is shown a t the upper left of the chromatogram. The sulfides, thiols, and disulfides are distributed throughout the boiling range. About one third of the sulfur is in com1)ounds boiling above the normal gasoline end 1)oint of about 415' F., most of which are benzothiophenes. This I)rol)ortion is higher than might be suspected, but is characteristic of most gasolines. 646
5
Sulfur compounds by boiling point in gasoline
2MTh
,ISH
4 50
ANALYTICAL CHEMISTRY
About 20 gasolines and catalytically cracked naphthas (which provide the bulk of sulfur in most gasolines) have been surveyed by gas chromatography. Interestingly, similar sulfur profiles were obtained in each case, even though the samples were produced by different companies from different crudes and with different catalytic cracking processes.
cg
BrThs
IO ' CBzThs
'
I - R I N G THIOPHENES 2-RING THIOPHENES CII
3-RING THIOPHENES
I 79 14
0
Table 111. Thiophenic Compounds by Carbon Number in Light Catalytic Cycle Oil
Sulfur, c/L
BENZOTHIOPHENES C* C9 ClO
0 014 0 15
c 1 2 e 1 3
Ci4 and higher
DIBENZOTHIOPHENES~ ClZ
0 08
Clband higher
0.07 0 05
One-ring thiophenes and nonthiophenic sulfur*
BOILING POINT,OF Sulfur compounds by boiling point in crude oil
0 017
Cl?
e;; Figure 4.
27 25 18 12 14
0 0 0 0 0
cu
0.10 __ 1.42
Includes naphthothiophenes. Determined by catalytic-decomposition method (14). a
chromatographic data on two- and three-ring aromatic concentrates obtained by liquid-solid chromatography. With gas chromatography, compounds were found, as a first approximation, to group together by carbon number. An analysis by carbon number for benzothiophenes and dibenzothiophenes, as calculated from the Figure 3 chromatogram, is given in Table 111. These values were obtained by integrating the areas of peaks attributed to each carbon number, and normalizing the areas to the value for total sulfur less that determined as nonthiophenic sulfur ( 1 4 ) . Although such analyses are only qualitatively valid, they have been very useful in following the progress and patterns of desulfurization reactions. Crude Oil. Figure 4 shows the distribution of sulfur compounds boiling to 1000" F. in the Middle-East whole crude (containing 2.72y0 sulfur), a n d illustrates t h e complexity of sulfur distributions in -virgin samples. Only a few components can be identified with much certainty, and essentially none can be determined. Qualitative information, however, can be deduced. Sulfur in the naphtha range is low. The big rise begins in t'he kerosene range with the two-ring thiophenes, most of which are benzothiophenes and are visible over the background of other peaks. A second and steeper rise start's in the light gasoil range wit,h the three-ring thiophenes, which are mostly dibenzothiophenes and naphthothiophenes. The sulfur compounds boiling through 1000° F., as determined by the chart area and also by analyzing distillation fractions, account for 40% of the total sulfur. Virgin Naphtha. .i better look a t the naphtha-range sulfur compounds is provided by Figure 5, which shows the heavy naphtha (containing 0 . 0 9 0 ~ o sulfur) from the Middle-East crude before and after removal of thiols and disulfides (after reduction) with silver nitrate.
The lower-boiling portion of the total naphtha contains mainly thiols, most of which are identified in Figure 5. Unlike the thiols in coke-still naphtha, these are mainly branched, which is consistent with the findings of others on virgin naphthas ( 3 ) . Some of these thiols were not originally present in the crude oil, but rather were formed during distillation. The sulfur compounds in the thiolfree naphtha are mainly sulfides, since the thiophene analysis (14) represented only 1% of the total sulfur. A few of the major peaks are definitely identified as straight-chain sulfides. Most of the I
,
I
peaks between the straight-chain members probably are branched-chain sulfides. Cyclic sulfides apparently are present in smaller amounts than chain sulfides, which is reverse to the usual situation ( 1 , 2, 18). Cycloparaffins in this naphtha also are present only in small amounts, which suggests that proportions of cyclic and chain sulfides in crude oils may follow the proportions of cyclic and chain paraffins.
I
I
I
1
I
nII
THIOL-FREE NAPHTHA
T O T A L NAPHTHA DISULFIDES SULFIDES THIOPHENES
100
200
300
6 57 I 100%
5 00
4 00
BOILING POINT , O F
Figure 5.
Sulfur compounds by boiling point in heavy naphtha VOL. 37, NO. 6, M A Y 1965
0
647
THIOLS D I S U L F l DES SULFIDES I-RING THIOPHENES 2-RING THIOPHENES 3-RING THIOPHENES
I
1
I
6 3
I
I
I
1
I
450 550 B O I L I N G P O I N T , OF
I
I
650
i
Sulfur compounds b y boiling point in kerosene
A nearly complete resolution of naphtha-range sulfur compounds might be obtained with selective liquid phases, such as those suggested by Karchmer ( l a ) and Klaas ( I S ) . Long, wide-bore capillary columns might be particularly effective for such work. Kerosene. The distribution of sulfur compounds in the kerosene (containing 0.50% sulfur) from the Middle-East crude is shown in Figure 6. The major peaks extending above the background represent chiefly the 9-, lo-, 1I-, and 12-carbon benzothiophenes; benzothiophene itself is too small to be positively identified. The higher-boiling portion contains some dibenzothiophenes, and dibenzothiophene itself is identified. The background consists mostly of sulfides, which account for about 42Q/,of the total sulfur. Again, one-ring thiophenes are present only in small amounts. Thiols account for only 6% of the total sulfur, which is a lower percentage than in the naphtha range, but a similar absolute amount. Quantitative use of this chromatogram is limited to determining sulfur distribution by boiling point. Individual compounds or compounds by carbon number cannot be determined with certainty. An estimate of the proportion of thiophenic sulfur can, however, be made. Gas Oil. The distribution of sulfur compounds in the light vacuum virgin gas oil (containing 2.60% sulfur) from the Middle-East crude is shown in Figure 7 . Again, thiophenic compounds extend above the background of other peaks. The first series of peaks (between 450" and 600" F.) consists mostly of benzothiophenes. The second series (from about 630' to 750" F.) consists mostly of dibenzothiophenes, nhich project above a n intense ound that includes benzothiolthenes as well as sulfides. In contrast to cycle oil, two- and three-ring thiophenes cannot be determined, because the background of other sulfur compounds is too great. The distribution of thio1)henic types must be obtained by another method (14). 648
CII
EzThs
2
350
Figure 6.
CIO
42 2 45
number of the five principal sulfur types in the Middle-East crude oil. Data for thiophenic types came from analysis of liquid-solid chromatographic fractions by gas chromatography and/or lowvoltage mass spectrometry, and analysis of distillation fractions by the catalytic decomposition method ( 1 4 ) . Sulfide and thiol values came from specific tests on distillation fractions (8, 10, I Y ) , and were verified by values for nonthiophenic sulfur obtained by catalytic decomposition ( 1 4 , 1 5 ) ; carbon numbers were estimated from the boiling ranges of the fractions. One-ring thiophenes are not included in Figure S because their levels are insignificantly low. Disulfides are not included because they mere not found in freshly distilled fractions. Because some assumptions had to be made to prepare the plots, the data should be considered only as semiquantitative. The data for fourring thiophenes probably are the least accurate. Two- and three-ring thiophenes are the principal compound types in the crude oil. Mass spectrometric analysis of chromatographic fractions indicates
ANALYTICAL CHEMISTRY
SULFUR TYPES BY CARBON NUMBER IN CRUDE OIL
Gas Chromatography can be more effective when used in combination with other analytical techniques than by itself. The effectiveness of combining gas chromatography, mass spectrometry, liquid-solid chromatography, and specific tests is illustrated by Figure 8, which shows distributions by carbon
'
cI3
1 22
THIOLS SULFIDES I-RING T H I O P H E N E S 2 - R I N G THIOPHENES 3 - R I N G THIOPHENES 4-RING THIOPHENES
D i 8TI hs
DiBzThs
I
35 37
DiBzTh
4
E
4
I0 0 %
BOILING POlNTpF
Figure 7.
Sulfur compounds b y boiling point in gas oil .06
I
I
I
I
I
I
I
I
I
.os
01 0:
Figure 8. Sulfur types b y Carbon Number in MiddleEast crude
,03
g
I\ 4I \ ...-..--
2-RING
\/
\
.02
'
,01
I / -
-\
THIOPHENE^ d
B
11
16
C A R B O N NUMBER
20
14
that 80 to 90% of these thiophenes are tully aromatic. Two-ring thiophenes are a t a mauimum a t CI1 (three carbons above the first member of the series), and then decrease gradually with increasing carbon number. Three-ring thiophenes are a t a maximum a t CI4 (two carbon numbers above the first member of the series) and then decrease rather sharply with increasing carbon number. Benzothiophene itself is present in very small proportion (0.0002~o) ; dibenzothiophene, however, is about 0.009%. Sulfides increase in amount to about CQ0and then level off; they are the principal type in the heavy naphtha (C,-C,) range. Thiols are a t a maximum a t about Cs; they then decrease and are negligible in the gas-oil range. The sulfur compounds through Cz0 plotted in Figure 8 account for 0.67% sulfur, which is only 25y0 of the total sulfur in the crude. Hydrocarbons through C?, account for 607, of the total crude. Figure 8 illubtrates a typical distribution of sulfur compounds in petroleum. Crude oils contain different absolute amounts of sulfur and slightly different proportions of the various sulfur types (f4), but their distributions by carbon number, in our euperience, usually are similar. Two- and three-ring thiophenes, for example, generally possess the same maxima and similar rates of
decline as those shown in Figure 8. This crude has a slightly smaller proportion of sulfides than most crudes (f4),but the sulfides show the characteristic steady rise in amounts into the gas-oil range. The crude also shows a fairly typical distribution of thiols; of all the compound types, thiols probably are the most variable among crude oils.
( 2 ) Brown, R., Meyerson, S., Ibzd., 44, 2620 (1952). ( 3 ) Coleman, H. J., Thompson, C. J., Hopkins, R. L., Rall, H. T., J . Chem. Eng. Data 10, 80 (1965). ( 4 ) Challacombe, J. A,, AIcNulty, J. A., Residue Rev. 5, 57 (1964). ( 5 ) Coulson, D. M.,in “Gas Chromatography,” L. Fowler, ed., p. 213, Academic Press, New York, 1963. ( 6 ) Coulson, I>. Al., Cavanagh, L. A,, AIVAL. CHEM.32, 1245 (1960). ( 7 ) Coulson, D. AI., Cavanagh, L. A.,
CONCLUSION
“Mcrocorilometric Detection in Gas Chromatography,” Pittsbur: h Conference on Analytical Chemistry and Applied Spectroscopy, JIarch 1961. ( 8 ) Ilrushel, H. \’., \[iller. J. F.. ANAL. CHEM.27. 495 i1955). ( 9 ) Fredericks, E. AI., Harlow, G. A.,
Gas chromatography with coulometric detection has proved to be an effective means for characterizing sulfur distributions, even though individual compounds generally cannot be determined. I n future work, gas chroniatography should be used more in combination with other analytical techniques. Two excellent techniques would be linear elution adsorption chromatography ( I C ) , and mass spectrometry; fractions separated by linear chromatography would be analyzed by both gas chromatography and mass spectrometry. Such a combination should yield much more information than the techniques could produce individually, and should be amenable to routine application. LITERATURE CITED
( 1 ) Birch, 8. F., Cullum, T. V., Dean,
R. A , , Ilenyer, R. L., I n d . Eng. Chem.
47, 240 (1955).
Ibid., 35, 263 (1964). (10) Hastingr, S. H., Ibid., 25,420 (1953). ( 1 1 ) Hubbard, R. L., Haines, W. E., Ball, J. S.,Ibid., 30, 91 (1958). (12) Karchmer, J. H., Ibid., 31, 1377 (1959). (13) Klaaq, P. J., Ibid., 33, 1851 (1961). (14) Martin, R. L., Grant, J. A., Ibid., 37, 649 (1965). (15) lIcCoy, R. N., Weiss, F. T., Ibid., 26, 1928 (1954). (16) Snyder, L. R., Ibid., 33, 1527, 1538 f 1961).
RuFeau ’of M i n e s Rept. Invest. 6252 (1963).
RECEIVED for review December 14, 1964. Accepted January 28, 1965. Division of Petroleum Chemistry, 149th LIeeting, ACS, Detroit, Nich., April 1965.
Determination of Thiophenic Compounds by Types in petroleum Samples RONALD L. MARTIN and JOHN A. GRANT Research and Development Department, American Oil
A method for determining thiopheniccompound types in petroleum samples of all types and boiling ranges has been developed. Nonthiophenic sulfur compounds are decomposed over alumina a t 500” C. to form hydrogen sulfide and aromatic thiols, which are collected and titrated to determine total nonthiophenic sulfur. Thiophenic compounds, extensively dealkylated in the decomposition reaction, are separated b y gas chromatography according to number of rings and detected by microcoulometric titration; thiophenic types with one, two, three, four, and five or more rings are determined. Accuracy of the method is good as judged by analyses on test samples and by comparison with other analytical techniques. Distillation fractions and residua from seven crude oils were analyzed to provide sulfur-
Co., Whiting,
Ind.
type characterizations heretofore unattainable. Differences among crudes in the distribution of sulfur-compound types usually are not large. Thiophenic compounds typically account for 50 to 7070 of the sulfur; except in residua, most of these compounds have either two or three rings.
I
ri THE REFINING of
high-sulfur crude oils, the determination of sulfurcompound types often is important. The need for such analyses is particularly great in the gas oil and residuum ranges, where available methods have not been completely satisfactory. One particular use is to follow the Iirogress of desulfurization processes, which proceed a t different rates for each of the sulfur types. I n the first relatively complete delineation of the sulfur types in gas
oils, Lumpkin and Johnson (13) showed that compounds containing condensed thiophene and aromatic rings accounted for iiiost of the sulfur. Their work provided the basis for the first method for thiophenic types-the mass-sl)ectrometric method of Hastings, Johnson, and Lumpkin (10)-which gives semiquantitative determinations of benzo-, dibenzo-, and naphthobenzothiophenes. Although this method has been very valuable in following composition trends and reaction behavior, its accuracy and alq)licability leave something to be desired. In our experience, values for thiophenic sulfur frequently are high, paiticularly for samples with limited boiling ranges. Mass sl)ectronietry also has been used to determine onering thiophenes ( 9 ) ,but only in naphtharange samples. Liquid-solid chromatography on VOL. 37, NO. 6 , M A Y 1965
649
