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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
Determination of Sulfur Compounds in High-Boiling Petroleum Distillates by Ligand-Exchange Thin-Layer Chromatography Takashi Kaimal and Atsushi Matsunaga Nippon Mining Co., Ltd., Technical Research Center, 3 Niizo-Minami, Toda, Saitama 335, Japan
Determination of sulfur compounds in high-bolilng petroleum dlsllliates was carrled out by ligand-exchange thin-layer chromatography (LETLC). Mercaptans, sulfides, and disulfides were separated from aromatic hydrocarbons by LETLC on a mercury-loaded silica gel. It was, however, dlfficult to separate dlbenzothlophenefrom aromatic hydrocarbons. Alkyl mercaptan formed a strong complex wlth mercury and was not developed at ail, even wlth polar solvent. Sulfur compounds were isolated by this procedure from petroleum aromatlc concentrate and were Identified by GLC/MS.
T h e problem of separating sulfur compounds from the hydrocarbons and other materials in petroleum has received much attention for many years. Many separation methods were proposed but no satisfactory method for the higher boiling fractions of crude oil has been reported. Recently, procedures for class separation of organic sulfides, utilizing interaction of metals with sulfur compounds from hydrocarbons, have been developed. Nonaqueous ligandexchange column chromatography has been used for separation of sulfides from other petroleum compounds (1,2). The presence of cyclic and aryl sulfides was shown by mass spectral examination. This paper describes a simple procedure for class separation of sulfur compounds and quantitative sulfur type analyses of heavier petroleum distillates. T h e separation was achieved by ligand-exchange thin-layer chromatography (LETLC) on mercuric acetate-impregnated silica gel plate. The determination of the ratio of sulfides t o dibenzothiophenes was carried out with a densitometer.
EXPERIMENTAL Apparatus. A sandwich chamber system (CF-TLC supplied by Toyo Roshi, Tokyo, Japan) was used for development of TLC. A densitometer (Dual-Wavelength TLC Scanner CS-910 obtained from Shimadzu Seisakusho, Kyoto, Japan) was used for quantitative type analysis ( 3 ) . Materials. Absorbents used were Silica gel H (Type 601, Aluminium oxide acidic (Type T), and Aluminium oxide basic (Type E) all obtained from Merck. Procedure. Adsorbent layers of approximately 0.25-mm thickness were produced by coating 20 X 20 cm glass plates following the standard procedure. The coated plates were left to dry on the bench overnight and then sprayed with 1 wt 70 methanol solution of metal salts. These metal salts-loaded silica gel plates were left to dry on the bench and then used without any further activation. The spots on the chromatograms were revealed in either of two ways: (a) by inspection under UV light (254 nm) or (b) by spraying with palladous chloride solution. Quantification of colored spots was carried out by a densitometer. Zigzag scanning and the reflectance mode were employed for measurements. Spots colored by spraying with palladous chloride were scanned at 380 nm, compensating background at 600 nm, for sulfur compounds analyses. Diphenyl sulfide and dibenzothiophene were used as standard sulfur compounds for calibration. 0003-2700/78/0350-0268$01 .OO/O
Table I. RSaValues of Standard Compounds by TLC (Solvent; n-Hexane)
Sample n-Cetyl mercaptan p-tert-Butyl thiophenol n-Decyl sulfide Benzyl phenyl sulfide Di-n-octyl disulfide Diphenyl disulfide Dibenzothiophene Liquid paraffin Alkyl naphthalenes Alkyl t e tralins a
Silica gel
Adsorbent Alumina Alumina (acidic) (basic)
1.08
1.58
1.48
1.08
1.38
1.00
0.72
1.50
1.30
0.51
0.62
0.70
1.04
1.62
1.35
0.72
1.31
1.22
0.92
1.23
1.17
1.92
-
-
1.84
-
-
1.84
-
-
Reference compound, anthracene.
RESULTS A N D D I S C U S S I O N Separation of S t a n d a r d S u l f u r Compounds. Separation of sulfur compounds from aromatic hydrocarbons was investigated with TLC on silica gel and alumina (acidic and basic) layers. Sulfur compounds and hydrocarbons used in these examinations are listed in Table I. n-Hexane was used as developing solvent. R, values (reference compound: anthracene) of standard compounds are indicated in Table I. As shown, it was difficult to separate any sulfur compound from aromatic hydrocarbons and to make class separation of sulfur compounds with conventional adsorbents. Table I1 shows R, values of sulfur compounds in TLC on various metal salts-loaded silica gel layers with n-hexane as developer. Mercuric acetate and silver nitrate were effective for class separation of sulfur compounds. Any of the metal salts in Table I1 was effective for separation of n-cetyl mercaptan. n-Cetyl mercaptan was not developed because of its formation of strong complexes with metal salts, but not because of the formation of mercaptides. This was supported by the fact that rz-cetyl mercaptan was developed by TLC on mercuric acetate-loaded basic alumina or florisil with n-hexane as developer. Zinc chloride, cadmium acetate, and copper acetate were effective for separation of p-tert-butyl thiophenol, but not so effective for sulfides and disulfides. This means that thiophenols form complexes with metal salts easily. Table I11 shows R, values of standard compounds in TLC on mercuric acetate-loaded silica gel with various developing solvents. Figure 1 shows the chromatogram developed with n-hexane. Alkyl mercaptan was not developed a t all. Thiophenol and sulfides were developed a little. Disulfides 0 1978
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Table 11. R , Values of Sulfur Compounds by TLC on Various Metal Salts-Loaded Silica Gel (Solvent, n-Hexane)
Sulfur compound
Mercuric acetate
Zinc chloride
0 0.13 0.05
0 0.32 0.82 0.52 1.64 1.12 1.28
n-Cetyl mercaptan p-tert-Butyl thiophenol n-Decyl sulfide Benzyl phenyl sulfide Di-n-octyl disulfide Diphenyl disulfide Dibenzothioohene
0.05 0.86 0.41 1.05
Table 111. R , Values o f s t a n d a r d Compounds by TLC on Mercuric Acetate-Loaded Silica Gel Solvent Carbon tetran-Hexane chloride
Sample n-Cetyl mercaptan p-tert-Butyl thiophenol n-Decyl sulfide Benzyl phenyl sulfide Di-n-octyl disulfide Diphenyl disulfide Dibenzothiophene Liquid paraffin Dialkyl benzenes Alkyl naphthalenes Alkyl te tralins
Metal salt Cadmium acetate 0 0.19 1.00 0.42
1.30 1.00 1.00
Copper acetate
Silver nitrate
0 0.10 0.85 0.35 1.30 0.90
0 0.18 0 0.18
1.00
0.53 0.88 1.18
Table IV. Sulfur Compounds Isolated from High-Boiling Distillates of Kuwait Crude Oil (Identified by GLC/MS) Molecular ion ( m i e ) 250, 264, 278, 292, 306, 320
Benzene
0
0
0
212, 226, 240, 254, 268, 282, 296, 310, 324
0.13
0.24
0.40
214, 228, 242, 256, 270, 284
0.05
0.18
0.19
0.05
0.29
0.76
210, 224, 238, 252, 266, 280, 294, 308, 322
0.86
0.78
1.00
0.41
0.33
1.00
1.05
1.00
1.00
2.45
1.24
1.00
2.09
1.13
1.00
2.09
1.13
1.00
2.09
1.13
1.00
Table V. Color Reaction with PdCl, Color
Sulfur compound
White Yellow Orange
n-Decyl sulfide Benzyl phenyl sulfide, n-cetyl mercaptan Diphenyl disulfide, dibenzothiophene, p-tert-butyl thiophenol Di-n-octyl disulfide Benzothiophene
Brown Dark brown I
i
I
fl 0
I
3
I
0
3
o
c
1
2
3
4
5
6
7
8
3
lC""2
Flgure 1. Chromatogram of standard compounds by TLC on mercuric acetate loaded silica gel. (1) n-Cetyl mercaptan, (2) p-tert-butyl thiophenol, (3) n-decyl sulfide, (4) benzyl phenyl sulfide, (5)di-n-octyi disulfide, (6)diphenyl disulfide, (7) dibenzothiophene,(8)liquid paraffin, (9) anthracene, (10)dialkyl benzenes, (1 1) alkyl naphthalenes, (12)alkyl
tetralins were developed with tailing. Dibenzothiophene was developed to the similar position of anthracene without tailing. Hydrocarbons were developed without tailing and gave larger R, values. I t is clear that the separation of sulfur compounds except dibenzothiophene from hydrocarbons is possible by the procedure mentioned above. I t was reported ( 4 ) that low molecular weight sulfides form complexes with mercuric ions and these complexes can be extracted from the organic to the aqueous layer. Such interaction may be responsible for the TLC separation procedure. Sulfur compounds may be developed while forming complexes. This analytical procedure could be expressed as ligand-exchange TLC.
------+
3enzene
Flgure 2. Chromatogram of standard sulfur compounds (two-dimensional development). (A) n-Cetyl mercaptan (white); (B) p-terl-butyl thiophenoi, n-decyl sulfide (orange); (C) benzyl phenyl sulfide (brown); ( D ,D') di-n-octyl disulfide, diphenyl disulfide (brown); (E) dibenzothiophene
(orange) Dibenzothiophene was different from other types of sulfur compounds and developed to the similar position of anthracene. For this reason, it is considered that dibenzothiophene cannot form a coordination compound with mercuric ion because of its steric hindrance. This is supported by the fact that alkyl thiophenes which have less steric hindrance than dibenzothiophene were separated from aromatic hydrocarbons in high-boiling petroleum distillates by preparative LETLC. (See Table IV.) Sulfur compounds were classified as shown in Table V by color reaction with palladous chloride. Hydrocarbons do not react with palladous chloride. Acidic alumina was found to be as effective a support for separation of sulfur compounds from hydrocarbons as silica
270
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
C
Cora,,,
J
Yellow
White ___)
0epzeoe
Figure 3. Chromatogram of sulfur compounds in transformer oil (two-dimensional development) gel, but basic alumina, neutral alumina, molecular sieves, and Florisil were not effective. This means that the formation of complex can be accomplished under acidic state. Two-dimensional development was carried out to attain a clear class separation of sulfur compounds on the mercuric acetate-loaded silica gel. Figure 2 shows a chromatogram of standard sulfur compounds by two-dimensional development. Dibenzothiophene was separated clearly from other sulfur compounds. S e p a r a t i o n of S u l f u r Compounds in Lube-Stocks. Figure 3 shows a chromatogram of sulfur compounds in an aromatic concentrate of transformer oil by two-dimensional development. Two-dimensional development was effective for class separation of sulfur compounds in refined oil. An orange spot indicates the presence of dibenzothiophenes. A yellow part indicates the presence of sulfides and little presence of disulfides. This means that the main types of sulfur compounds in the transformer oil are dibenzothiophenes and sulfides. Qualitative analysis was carried out on sulfur compounds in aromatic concentrate from high-boiling distillates (bp 280-440 "C) of Kuwait crude oil. Sulfur compounds except dibenzothiophenes were separated from aromatic hydrocarbons by LETLC on a preparative scale with n-hexane as developer. After separation, sulfur compounds except dibenzothiophenes on silica gel were scratched up (aromatic hydrocarbons and dibenzothiophenes are seen to be blue by inspection under UV light) and then dissolved in methanol. Table IV shows sulfur compounds identified by GLCIMS. Existence of thiophenols was confirmed by strong UV absorption at 316 nm. Q u a n t i t a t i v e Analyses. Figure 4 shows the result of the densitometric scanning of the spots of diphenyl sulfide and
Figure 4. Densitometric scanning of the spots of diphenyl sulfide and dibenzothiophene (at 380 nm)
dibenzothiophene after separation by LETLC and visualization. The extinction coefficient of colored complex varied with the difference of sulfur compound. For example, that of diphenyl sulfide was seven times as large as dibenzothiophene a t 380 nm. The repeatability of this measurement was not so good, mainly because of the instability of the extinction coefficient of the complex. Therefore, it was necessary to standardize t h e interval from spraying to scanning. But even so, the variations of the results were in the range 2 to 4 % . The ratios of sulfides to dibenzothiophenes in several lubricant basestocks (150 neutral) were determined. Two type sulfur compounds were separated with two-dimensional development of LETLC. After visualization, densitometric scanning of the spots was carried out. The contents of sulfides in lubricant basestocks were 7 to 20 wt % of total sulfur compounds and dibenzothiophenes were 80 to 93 wt (IC. The ratios of sulfides to dibenzothiophenes depended on the crude source and refining process. ACKNOWLEDGMENT The authors thank Takeda Rika, Tokyo, for use of the densitometer. The authors are also grateful to K. Fujimori, and T. Akada for helpful discussions. LITERATURE CITED (1) L. R. Snyder, Anal. Chern.. 41, 314 (1969). (2) J. W. Vogh and J. E. Dooley, Anal. Chem., 47, 816 (1975). (3) H.Yamamoto, T. Kurita, J. Suzuki, R. Hira, N. Nakano, H. Makabe, and K. Shibata, J . Chromatogr., 116, 29 (1976). (4) T. Yotsuyanagi, T. Kamidate, and K. Aomura, Jpn. Anal., 18, 1487 (1969).
RECEIVED for review August 10, 1977. Accepted September 30, 1977.
