Separation of Sulfur Compounds DOROTHY HARESNAPE,F.A.
F I D L E R , AND R. A. LOWRY Anglo-Iranian Oil Company, Ltd., Sunbury-on-Thames, Middlesex, England
*
A
T h e relative adsorbabilities on silica gel of the different types of sulfur compounds occurring in petroleum distillates have been determined; synthetic blends were analyzed by filtration through a 175-cm. column of the gel. A description of the apparatus and the experimental procedure employed is given. All the sulfur compounds examined were found to be more strongly adsorbed than aromatic hydrocarbons, the order of increasing adsorbability being : aromatic hydrocarbons, thiophenes, alkanethiols, alkane disulfides, aromatic and cycloalkane thiols, alkane sulfides, and cyclic sulfides. The effect of boiling point and of molecular weight on adsorbability is discussed. The separation and identification of certain sulfur COMpounds present in extracts obtained either by caustic soda washing or sulfuric acid treatment of straight-run naphthas are described. Thirteen adsorptograms are given.
separation that can be expected from complex mixtures of sulfur compounds such as may be obtained from petroleum and shaleoil naphthas. It is thus an extension of the work carried out at the Bureau of Mines and deals with the different classes of sulfur compounds in more detail. The nomenclature used in this paper is t h a t suggested by the Bureau of Mines workers (1). On the recommendation of the Nomenclature Committee of the Division of Petroleum Chemistry of the AMERICAN CHEMICAL SOCIETY,it has also been adopted by the Advisory Committee of the American Petroleum Institute, Research Project 48 on Sulfur Compounds in Petroleum, for use in the reports of the project. However, because of t h e unfamiliarity of the new nomenclature, the more common alternative name is given in parenthesis when a compound is named for the first time.
MATERIALS EMPLOYED HYDROCARBONS
A
*-
DSORPTION on silica gel has been developed t o a considerable extent in the past few years by workers at the National Bureau of Standards for the separation of different classes of hydrocarbons (3). The stage has now been reached when it can be considered as a useful analytical tool in the estimation of olefins and aromatics in mixtures containing these with paraffins and naphthenes. I n addition, it provides on occasion an invaluable method for the isolation and identification of hydrocarbons from mixtures when only relatively small quantities of materials are available, such as fractions from distillation columns of high efficiency. Considerable use has been made of this procedure both in these laboratories (unpublished data) and at the Bureau of Mines ( 2 ) for the analysis and isolation of individual hydrocarbons from shale-oil naphthas. Nonhydrocarbon constituents of petroleum such as sulfur, nitrogen, and oxygen compounds are usually more strongly adsorbed than hydrocarbons. This property is most useful for the purification of hydrocarbons which have either been isolated from petroleum or synthesized. For example, before final fractionation of a paraffin or naphthene for the preparation of a “pure” hydrocarbon, it is usual to pass the material over silica gel in order to remove polar compounds or possible traces of unsaturateds or aromatics. I n connection with a program of work which has been commenced in these laboratories on the identification of sulfur compounds in Iranian naphthas, it was thought desirable t o attempt t o develop the silica gel technique for the separation of different classes of sulfur compounds in a manner analogous t o the method already mentioned for hydrocarbons. The analytical methods now in common use for the type analysis of petroleum sulfur compounds are far from satisfactory and a physical method of separating sulfur compounds from each other and from associated hydrocarbons is needed. Since the work was commenced, a publication ( 2 )has appeared with essentially the same aim, the separation of individual sulfur compounds from aromatic hydrocarbons. The present work, in addition t o separating and comparing the adsorption of aromatic hydrocarbons with sulfur compounds, also demonstrates the
The only two hydrocarbons used in this work, iso-octane
(nv 1.3960) and toluene (%so1.4957), were commercial products. THIOLS
Alkanethiol. 1-Butanethiol (n-butyl mercaptan) was separated by fractionation from the mixed thiols extracted by caustic soda treatment of a 120’ C. end point naphtha and had a purity of 99 mole yo (nv 1.4429). Aromatic Thiol. Benzenethiol (thiophenol) was synthesized in the usual manner by reduction of benzenesulfonyl chloride 1.5888). Cycloalkanethiol. Cyclohexanethiol (cyclohexyl mercaptan) was pre ared b y treatment of cyclohexyl magfiesium bromide with sulrur 1.4932).
(nv
DISULFIDES
3,4-Dithiahexane (diethyl disulfide, n Z D O 1.5074) was obtained by oxidation of ethanethiol (ethyl mercaptan). SULFIDES
Alkane Sulfides. Two samples of 4-thiaheptane (di-n-propyl sulfide) were used, one a commercial product and the other a synthetic material 1.4480). 3-Thiahexane (ethyl-n-propyl 1.4455) was synthetic and 3-thiapentane (diethyl sulfide, 1.4428) was a commercial material. sulfide, Cyclic Sulfides. Specimen3 of synthetically prepared thiacyclohexane (pentamethylene sulfide, nV 1.5066) and tlriacyclopentane (tetramethylene sulfide, 1.5042) of purity 99.8 mole % were available.
nv nv
(nv
nso
THIOPHENE
The 3-methylthiophene was a product of the Socony-Vacuum Oil Company (ny 1.5204). ALCOHOLS
Commercial absolute ethyl alcohol and amyl alcohol of analytical reagent purity were used as the eluents. SILICA GEL
I n hydrocarbon work, it had bsen shown t h a t the sharpness of separation obtained between various types depended upon the amount of very fine particles in the gel. The commercial gel used, which all passed through a 100-mesh sieve, contained 20% of fines passing through a 330-mesh sieve; this was satisfactory
2691
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2692
Vol. 41, No. 12
I'
50
- 1-40
-1.40
5 3
-1.44
g 5 2 Y
-1.42
0
IO 20 W E I G H T OF FILTRATE I N GRAMS
I
30
Figure 1. Relative Adsorption of 1-Butanethiol and Toluene Temperature, 0' C.
I
I
I
I
I
I
0
IO WEIGHT
I
I
I
20 OF FILTRATE I N GRAMS
I 30
Figure 3. Relative Adsorption of 3Methylthiophene and Thiacyclohexane
-11.52
The second apparatus differed slightly in that the actual adsorption column was of uniform diameter (12 mm.) throughout, its length and had a ground-glass flange joint a t the top instead of a spherical joint, The tube was surrounded in each case by a jacket of Pyrex pipe line which served the dual purpose of cooling jacket and safety shield. The filtration was normally carried out a t room temperature, but sometimes improved separations were obtained by lowering the temperature. The method originally used for supporting the gel was by means of a sinteredglass disk a t the bottom of the column, but o\ving tmot'he tendency for cracks to develop around the glass plate this was later modified. The disk was removed and 2 inches of capillary tubing (approximately 1.5 mm. in inside diameter) were sealed to the foot of the column. A few grams of coarse silica gel were placed in the column before the main bulk of gel in order to support the finer material and to prevent its passing out wit'h the filtrate.
WEIGHT
OF F I L T R A T E IN G R A M S
Figure 2. Relative Adsorption of 1 -Butanethiol and Thiacyclohexane for bringing about the relatively easy separation of a paraffin from an aromatic, but was not sufficiently selective for more difficult se arations of hydrocarbon mixtures containing olefins. The use o r v e r fine gel alone was not practicable, mainly because of the dif&ulty of packing the column satisfactorily, as the gel tended to adhere to the side of the tube and cause channeling with consequent poor separation. The adsorbent finally adopted for all hydrocarbon work was a blend made by mixing equal weights of the original gel and fine gel passing through a 200mesh sieve, prepared by grinding some of the coarser material in a Raymond mill. This blend had the following approximate sieve analysis: 100 to 200 mesh 200 t o 330 mesh Finer than 330 mesh
40 t o 20% 20 to 10% 40 t o 70%
This gel was of satisfactory efficiencyfor the separation of paraffins, olefins, and aromatics and was also used throughout the sulfur-type separations described here. Activation of the gel was carried out by heating a t 300 C. foi 3 to 5 hours, not more than 2 days before use. The activated gel was stored in a bottle fitted with a tight rubber bung.
APPARATUS Two adsorption columns were used for this work, the first being similar to that used by Mair ( 3 ) . The actual dimensions were: Reservoir Wide part of adsorption tube Narrow part of adsorption tube
10 cm. long, 30 mm. diameter 75 cm. long, 15 mm. diameter 100 om. long, 10 mm. diameter
PROCEDURE The column was packed with t,he activated gel by filling t o the top and tapping the spherical or flange joint with a. rubbertipped rod until the upper level of the gel was just below the reservoir and did not fall appreciably with further tapping. The filling usually occupied 10 to 15 minutes. The charge of gel held by these columns was approximately 200 grams. If the experiment was to be carried out below room temperature, cooled met,hanol was circulated through the outer jacket. The material t o be examined was introduced into the reservoir and a few pounds of nitrogen pressure were applied through the glass joint. When the sample had completely entered the el the reservoir was filled with eluent, the joint was sealed wit8 d low melting point wax, and a nitrogen pressure of the order of 30 to 40 pounds per square inch was applied. After 3 to 4 hours, filtrate issued from the bottom of the tube and was collected by weight in approximately 1-gram portions. When t,he pure eluent commenced to filter through, the experiment was stopped by releasing t'he nitrogen pressure. On the completion of each run, the column was cleaned out by laying it on the ground and displacing the gel with water applied t,hrough a copper tube inserted a t the ground-glass joint. ,4 fresh batch of gel was used for each separat,ion and no attempt was made to reactivate uscd gpl. To determine the behavior of various types of sulfur compounds and their ease of separation with silica gel, a number of synthetic blends were made. Some of these contained an aromatic, toluene, which represented the most strongly adsorbed class of hydrocarbon found in straight-run petroleum naphthas. The blends were composed of approximately 20 grams of adsorbable material (aromatics plus sulfur compounds) mixed with 10 to 15 grams of iso-octane. The latter was employed because it served to dissipate the heat of wetting and previous experiments with hydrocarbons had shown that t,he separat'ion between an aromatic and an olefin was much improved if the sample contained a certain amount of a parafin or a naphthene.
December 1949
'
CHEMISTRY
I N D U S T R I A L AND, E N G I N E E R I N G
2693
0 In preparing blends for analysis, the various compounds were
mixed in approximately equal proportions, because in the case of substances only slightly different in adsorbability, i t is possible t o modify to some extent the order of filtration through the gel by having one or other of the components in large excess. As is usual in this type of work, progress of the separations was followed by refractive index determinations using an Abbe type refractometer. I n certain cases, however, the refractive indexes of two components were almost identical, as with 3,4-dithiahexane and thiacyclohexane, and it was necessary to use dispersion measurements. Because the latter are dependent on the drum reading on a n Abbe refractometer, it was found perfectly satisfactory to follow the separation using these drum readings, which in these cases showed a greater spread than refractive index (Figure 5). The results were calculated in the usual manner by estimating the cut points from the adsorptogram visually or, if the separation was not very sharp, they could usually be determined with reasonable accuracy by the method of equal areas. It was found most convenient to carry out the experiments on a weight basis and, although it was realized t h a t refractive index was more nearly proportional to volume per cent than weight per cent, the errors from this cause were not significantly large, and the results were usually far more useful in this form.
ETHYL ALCOHOL ISOOCTANE 40
36 3'4 -DlTHIWEXANE
36
TOLUENE
-
3.4-DIMIAHEXfflE
-TOLUENE
-4
I
I
RESULTS OBTAINED WITH SYNTHETIC
".
I 10
Figure 4.
I
BLENDS
Detailed results of the analyses obtained with synthetic blends are given in Table I and Figures 1 t o 9. In each case, t h e weight per cent compositions obtained from the silica gel experiments are compared with the actual blends as prepared. T h e separation obtained between toluene and 1-butanethiol a t 20" C. was very poor, the maximum refractive index being 1.474. Considerable improvement was brought about, however, in a repeat
PARAFFIN 10
I36
20 30 WEIGHT OF F I L T R A T E I N GRAMS
Relative Adsorption of Toluene a n d 3,4-Dithiahexane Temperature, ' 0 C.
Table I. Adsorption Analysis o n S y n t h e t i c Blends a
b
o
d
e
(Weight % composition) f g h
i
k
j
M
.r
A
+
9 I -2
-
k
I I1
I11 IV
V VI VI1 VI11
IX X
XI XI1 XI11
XIV XV XVI
XVII XVIII
Blend Synthesis Analysis Synthesis Analysis Synthesis Ana 1y si4 Synthesis Analysis Synthegis Anal sis Synt%e?is Analysis Synthesis Analysis Synthesis Analysis Synthesis Analysi! Synthesis Analysis Synthesis Analysis Synthesis Analysis Synthesis Analysis Synthesis Analysis Synthegis Anal 81s SyntEesis Analysis Synthesis Analysis Synthesis Analysis
42.2 42 41.9 42 40.2 41.8 35.4 35 35.3 36 36.8 37.2 33.5 30 38.9 38.9 37.0 37 36.2 36.1 29.7 29.8 37.7 38.0 35.8 35.8 38.0 38 38.8 40.0 40.0 40.4 43.0 41.3 40.8 41.0
28.6
30
..
__
29
..
.. ,. 3812 39.4
..
23:7 23.6 24.0 22.1
22.8 22.8 21.3 21.4 1
.
20:5 21.4
29.2
28 26.8 26.9
.. .. 37:4
.. .. .. .. ... ... ..
., .. ..
..
..
.. 3i:3 31.1
.. ..
..
.,
.. ..
..
.. ..
.. ..
..
..
.. ..
.. ..
..
..
35 32.9 34.0
.. ..
..
.. ..
..
.. ..
I .
, .
.
I
..
..
.. .. ..
..
.. .. .. .. . ,. 29:1 .. .. .. .. . .. No analysis of individual sulfur compounds possible .. .. .. .. .. 22.9 .. .. .... 21.7 .. .. .. .. .. 26:o 37.0 .. .. .. .. .. 25 38 .. .. .. .. 24:o .. . . .. .. 16:l .. 25.6 .. .. .. 14.7 .. .. 23.7 .. . . 22:s . . .. .. No'analysis of individual sulfur compounds possible .. .. .. .. 16.3 . . 23.2 . .. .. .. _. ._ 16.4 22.8 .. .. .. .. . . 2i:3 21.6 ..
30:3 28.8
.. ..
.. .. ..
..
I
.
,
..
..
.. ..
20: 8 19.8
.. ..
..
..
zs: 5 27
21.4
.. .. ..
21 . 4
.
33:5 35 19.9 . 18.8 .. .. 25.5 34:6 No'analysis of indikidual &fur compounds possible 28.8 . No'snalysis of indikidual suifur compounds possible' .. .. 33.4 .. .. .. 32.8
.. .. ..
..
..
..
..
..
.. ..
..
..
V
......
..
.. .. .. .. .. .. .. ..
Slur
......
Sharp cut Sharp'cut a-e Slur e-c Sharp cut
...... Slur ......
.. ..
. *
Sharp cut
.. ..
Bad slur
.. .. .. .. .. .. .. .. ..
Slur
..... ......
..
Sharp cut
.....
..
..
..
4
..
5
..
I .
.. ..
..
6
Shrtrp'c'uts a-g Sharp c u t g-c Slur
'7 8
a, g , i
Sharp cuts
a,' h; i
Sharp cuts
.. .. ..
Slur
..
...
...... ......
..
......
..
......
Sharp cuts
......
No separation
28.2
..
...
..
25.8 26.2
...
..
.. .. ..
h; c , .. a, g, c ...
a, j, c
..
2
'3
a,
... j, g ...
..
'i ..
k, h 1, e
......
Slur Fairly sharp cut
..
9 .. ..
..
.. ..
..
I N D U S T R I A L A N D E N G I N E E.R I N G C H E M I S T R Y
2694
42 ETHYL ALCOHOL 40
L
- 3'4-DITHIAHUIME -
i-
THIACIUOHUANE
PARAFFIN I2 0 8
DISULFIDE 9 3
I
8'
CYCLIC SULFIDE II 0
-138
8' I
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20 30 WEIGHT O F F I L l R A T E I N
2 B
THIACYCWUIANC
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Vol. 41, No. 12
tive blends with toluene (blends I and V) it appears that the disulfide is slightly more strongly adsorbed than the thiol. This is in agreement with the results of Dinneen el al. ( 2 ) , Rho used similar mixtures. The separation between toluene and 3-thiapentane (blend VIII), 3-thiahexane (blend XI), and 4-thiaheptane (blend X) is good, giving very clear-cut points on the adsorptograms. I n placing the alkane sulfides in their position relative to the compounds already discussed, the blend of 3-thiapentane and 3,4-dithiahexane (blend IX) shows the former t o be the more strongly adsorbed of these t n o types. This is confirmed by the fact that a much bcttcr separation is achieved between toluene and Sthiapentane (blend VIII), than between toluene and 3,4-dithiahesane (blend V). Thiacyclohexane is sharply separated from toluene, 1-butanethiol, 3-methylthiophene, 3,4-dithiahexane, and 4-thiaheptane; this enables good agreement t o be ohtained between the observed and theoretical results in these cases. It appears therefore that cyclic sulfides are the most strongly adsorbed class of sulfur compound likely to be present in a straight-run petroleum naphtha When specimens of benzenethiol and ryclohesanethiol became available, blends of these with other sulfur compounds were passed over gel in order t o place them relative t o the classes already described. From blends with toluene, 3,4-dithiahexane, 3-thiahexane, 4-thiaheptane, and thiacyclohexane, it is found t h a t both these thiols came between alkane disulfides and alkane sulfides in the strength of their adsorption on silica gel.
40 CRAMS
Figure 5. Relative Adsorption of 3,4-Dithiahexane and Thiacyclo hexane
EFFECT O P ELUENTS
In the early runs carried out with sulfur compounds, amyl alcohol was used as the eluting liquid rather than ethvl alcohol. which is normallv used in hydrocarbon tvne separations. It was thought that because the viscosities of sulfur compounds are higher than the hydrocarbons, a more viscous eluent would assist in getting sharper definitions and eliminate any tendency for the desorbant to creep into the sample. Actually, the back end cut was very poor in some of the run? using amyl alcohol, particularly when the sample contained the strongly adsorbed cyclic sulfide, thiacyclohexane (see Figure 3). "
run carried out a t 0" C., although the cut between the hydrocarbon and thiol was still not perfectly sharp (blend I). By comparing the separation achieved between toluene and 1butanethiol (blend I ) on one hand and Bmethylthiophene and 1-butanethiol (blend VII) on the other, it is possible to place these three compounds in the order, toluene, 3-methylthiophene, 1-butanethiol, of increasing adsorption on the gel. The adsorp%ivepower of silica gel for butanethiol and 3,4-dithiahexane is very close indeed, but from the separation achieved in their respec-
52
F
I
3 ' 4 - DITHIAHEXANE
-
- I 50
THIACYCLOHEYANE 50
48
-1.48
- 1'48 O P
,46
R.
F'
.
@
?5
3-THIA PENTANE
4 4
-I44
w
: 2
42
E
0
ALKANE
PARAFFIN l O . O a .
DlSlLFlDE
10,5t8
SULFIDE
6.7F
10 W E I G H T OF
20
40
,
a0
,3b
FILTRATE I N GRAMS
Figure 6. Relative Adsorption of 3-Thiapentene and 3,4-Dithiahexane
a Y m
(r
1
P
w
1 I \ 1;: -I42
E
38
I IO
I
I 20
I
I
30
ar
W E I G M OF F I L T R A T E IN GRAMS
Figure 7. Relative Adsorption of Toluene, 4-Thiaheptane, and Thiacyclohexane
&
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1949
2693
~IACTCLOWANE
J
-8ENZEHETHIOL
,i j,..*
1 L-l
B I40
VARAFFIN
s31T
ARQMAnC 6
TDrAL SULFUR COMPOUNDS
'98
I WEIGHT
158. I
I OF FILTWATE
IL
L
-
I I
I 36
I N GRAMS
1.44
Figure 8. Relative Adsorption of 'Toluene, 3-Thiahexane, and Thiacyclohexane
142
Replacement of the amyl alcohol by ethyl alcohol brought about
I40
a much sharper cut point (see Figure 5). Consequently, for all but a few of the early blends, ethyl alcohol was employed.
I
RESULTS OBTAINED WITH PETROLEUM PRODUCTS
li
%
An investigation was in progress in these laboratories into the composition of a tar oil obtained from a spent acid which had been used for refining a n aromatic concentrate. A naphtha boiling over the range 100" t o 160°C. was solvent-extracted with liquid sulfur dioxide and the extract, containing 75% aromatics, was refined by treatment with 2.5 to 4% of 98% sulfuric acid. Dilution of the spent acid with water threw out the usual t a r oil which was composed almost entirely of sulfur compounds with a small quantity of aromatic hydrocarbons. Fractionation of this oil in a 100-plate column gave the true boiling point curve shown in Figure 10. An examination 01 the fractions on the plateaus showed them to consist primarily of the cyclic sulfides, thiacyclopentane, 2- and 3methylthiacyclopentanes, and thiacyclohexane with small amounts of the alkane sulfides. Because the sulfides are the most strongly adsorbed of the sulfur compounds examined, a sharp separation was obtained between these and aromatic hydrocarbons, and silica gel analysis was found very useful in estimating the amount of hydrocarbon contaminant in any fraction. The adsorptogram given in Figure 11 shows the separation between aromatics and sulfur compounds which was obtained with a wide boiling cut -namely, 86" t o 143" C. The number of sulfides which boil in this range is so large that it was not expected to obtain a really sharp separation between the aliphatic and cyclic compounds. However, with relatively narrow boiling cuts, the separation between these two types of sulfides was marked and it was possible to give an estimate of the proportion of the two types of sulfides in any particular cut. Figures 12 and 13 show typical results obtained on fractions of boiling point 118" and 137"C., respectively. The former contained no aromatic hydrocarbons and consisted mainly of alkane sulfides which from the boiling point were presumably 2- or Sthiahexanes.
IARCUATIC
PARAFFIN
""1.
038.
I
INZENE
THIOL 5 58.
I30
:vCLIC SULFIM
"8
11
L
Figure 9. Relative Adsorption of Toluene, Benzenethiol, and Thiacyclohexane The cyclic sulfide in this fraction was thiacyclopentane, which was shown to be the main constituenf of the 120' C. flat. T h e sulfide in the 137" C. boiling material was almost entirely cyclic in nature. The plot of refractive index against weight of filtrate showed distinctly CS aromatics followed by a small percentage of alkane sulfide and finally the cyclic sulfide. The latter was isolated and shown to be 3-methylthiacyclopentane.
by 1
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2696
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ETHYL ALCOHOL ISOOCTANE
THIACYCLOHEXANE
i3.
TOLUEHE THIACYCLOHEYANE
4t
Vol. 41, No. 12
strongly adsorbed and would therefore tend to filter through easier. The first fraction of thiol obtained on treatment of this 63" to 65' C. cut with silica gel had a refractive index of 1.424 and a freezing point of -6" C. and gave a green color with the Rheinboldt color test ( 4 ) )showing the presence of a tert,iary t,hiol. The refract,ive index of t,hc filtratc gradually increased as the run progressed and finally reached 1.431. The fraction with this refractive index gave a red coloration with the Rheinboldt test, 1Thic.h is characteristic of a primary thiol. This cut t,herefore contained both 2-methyl-2-propanethiol and 1-propanethiol. =Inalysis by silica gel was of considerable value in following the purification of 2-butanethiol. Although the majority of the t,hiols separated by fractionation of t,he mixture described were of high purity, the 2-butanet'hiol cut appeared to be impure and vould not crystallize on cooling but set to a glass. Filtration of the material through silica gel showed it to contain a small percentage of impurities wit8hboth lower and higher refractive indexes t,han the main bulk. Extractive distillation of the thiol in the presence of aniline served t o remove the inipurit'ies and gave a. thiol of satisfactory purity.
DISCUSSION
I
I
0
10
I
Figure 1 1 .
I
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13b
3e
20 WEIGHT
OF FRTRATE IN
GRAMS
Passage of Acid Tar Oil over Silica Gel
Showing presence of aromatics, with mixture of alkane and cyclic
sulfides
The separation of hydrocarbons from thiols by means of silica gel adsorption also provided a useful tool in the isolation of pure thiols from a mixed thiol feed stock. The mixed thiols recovered from the spent caustic soda used for the extraction of a 120" C. end point straight-run naphtha was found t o be a good source of pure thiols in relatively large quantities. Fractionation of this material in 100-plate columns gave most of the C?, CsI and C1 thiols in a reasonable state of purity. However, constant boiling flats were occasionally obtained having lower rrfractive indexes than was expected. On filtration through silica gel it was shown that the thiols were contaminated vi ith small amounts of paraffins or naphthenes which had presumably distilled over with the sulfur compounds as azeotropes. A t j pica1 example of this was shown with 2-methyl-1-propanethiol (isobutyl mercaptan), the refractive index of which by fractionation mas 1.4340. Silica gel analysis showed it to contain 5yoof a paraffin and the refractive index of the thiol portion from the gel was in actual fact 1.4387.
Table 11. Physical Properties of I-Propane- and 2-Methyl2-propane Thiols Thiol 1-Propane %Met hyl-2-propane
Boiling Point, 0
c.
67.82 64 35 I
Freezing Point,
c.
- 113.80
+
0.82
n
so
1,4380 1.4230
The presence of 2-methyl-2-propanethiol (ter t-butyl mercaptan) in this crude mixture was also detected by the use of silica gel One fraction from the 100-plate distillation of the mixed thiols boiled b e t m e n 63' and 65O C., indicating the possibility of either 2-methyl-2-propanethiol or 1-propanethiol. The physical properties of these are given in Table 11. Inasmuch as there is a small difference in the adsorbability of compounds of the same type but of different molecular weight, if a blend of two such materials was analyzed by this method, the higher molecular weight compounds would be somewhat less
The behavior of the various types of sulfur conipounds on passage over silica gel in synthetic blends has been fairly completeljcovered in the runs described, although all the possible combinations have not, been tried. It is possible, by comparing the ease with which members of hhe various classes are separated from one another, to compile a list, showing the order of their increasing adsorbability on gel in the same way as has been donc with hydrocarbons. As far as possible, compounds of similar boiling point have been used, for it is mainly as closely cut, fractions from highly efficient, dist,illat,ioncolumns they would be met in practice. The effect of boiling point was demonstrated in the separation betil-een t,he alkane and cyclic sulfides. If blends were made with sulfides of the same boiling point, the cyclic sulfide x-as of lower molecular weight (blends X and X I I ) and sharp separations were achieved, the alkane sulfide emerging through the gel before the cyclic sulfide. If, however, t,he tvio sulfides had similar molecular vieights, which meant that the cyclic sulfide had a higher boiling point (blend XI), there was less difference in adsorbability between the two compounds and the separation was adversely effected. I n blend XIII, where the cyclic sulfide had a lower boiling point and a considerably lower molecular weight, the break between the two types of sulfide was very sharp, leading t,o an analysis that agreed exactly with the blend as prepared. Other workers have shown that 'for hydrocarbons an important factor in separation by silica gel is the molecular weight; with t y o compounds of the same type, the one of lower molecular weight is more strongly adsorbed. The difference in adsorbability between alkane and cyclic sulfides of the same molecular weight was insufficient t'o separate them completely, but when the more strongly adsorbed cyclic sulfide was also of lower molecular weight than the alkane sulfide, as was the case when the two sulfides mere of the same boiling point, then the difference in adsorbabilitjbetween the two compounds was increased sufficiently to produce sharp separation. After examining the results of t,hese blends of alkane and cyclic sulfides, one run was carried out using a mixture of 3,4-dithiahexane and isopropylbenzene (blend XVIII), an aromatic hgdrocarbon of approximately t,he same boiling point as the disulfide. The separation was not absolutely sharp, but was a considerable improvement on the blend of toluene and 3,4-dithiahexaize. This also agrees 114th the above generalization regarding molecular weight and adsorbabilit,y. All the sulfur compounds described in this paper were more strongly adsorbed than iso-octane or toluene and from the separations achieved with synthetic blends, the order of adsorbability on
December 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY I. so
THIACYCLOPENTANE
2-THIAHEXANE OR
-
3-THIAHEXANE
WEIGHT
OF
FILTRATE I N GRAMS
Figure 12. Passage of 118' C. Cut from Acid Tar Oil (after Removal of Aromatics) over Silica Gel silica gel of the various classes of sulfur compounds and aromatic hydrocarbons was therefore shown t o be: Aromatic hydrocarbons Thiophenes Alkanethiols Alkane disulfides Aromatic and cycloalkanethiols Alkane sulfides Cyclic sulfides Aromatic hydrocarbons can be sharply separated from any sulfur compound more strongly adsorbed than a disulfide. Thiophenes, alkanethiols, alkane disulfides, aromatic thiols, and cycloalkanethiols have only slight differences in the degree to which they are adsorbed and cannot be separated from each other, b u t a sharp separation is obtained between any of them and a cyclic sulfide. Alkane sulfides and cyclic sulfides are sharply separated, provided the cyclic compound is lower in molecular weight than the alkane sulfide.
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with their aromatic nucleus, are much less strongly adsorbed on silica gel than the sulfides having completely saturated molecules. On the other hand, if the adsorption on gel were due to the attachment of the sulfur atoms to the adsorbent, it might be supposed that disulfides would be more strongly adsorbed than monosulfides, but the reverse is the case. By comparing the results of the analyses with the composition of the blends synthesized, where a sharp separation was possible, the calculated results were well within the error of ~ 2 of % the actual composition and in many cases the agreement was better than this. However, where the separation was only a slur and no sharp definition was obtained between the components, the results could sometimes be calculated t o an accuracy of about 15%. I n a few cases, analysis of individual sulfur compounds was impossible as, for example, between 3-methylthiophene and 1-butanethiol. Difficult separations could almost certainly be improved by using a much longer column of gel, but no work has been carried out along these lines u p to the present because of the practical difficulties involved, such as reactivation of the gel in situ and the very much longer time of filtration which would be required. One of the principal aims of this work was t o determine whether a simple and rapid laboratory technique could be developed for analyzing mixtures of sulfur compounds by means of silica gel adsorption. Both the apparatus and procedure were simple and only about 20 grams of the sulfur compounds were used. The experiments generally occupied 6 to 8 hours but, after the initial charging, attention was necessary for only the 2 t o 3 hours during which the fractions were collected from the foot of the column. Inasmuch as this method of analysis would be employed mainly with narrow boiling cuts from fractionations, the sulfur compounds which could possibly be present in the fraction would probably be known. Even if they had not been synthesized, the physical properties, such as refractive index, could probably be estimated from the properties of the other members of the same class, so that the identification of the actual compounds as well as the type analysis may sometimes be possible. B y employing the method of adsorption on silica gel not only may an analysis be calculated, but a small sample of the sulfur compound may be obtained in a reasonable state of purity for further examination.
LITERATURE CITED (1) Ball and Haines, Chem. Eng. News,24, 2765 (1946). (2) Dinneen, Bailey, Smith, and Ball, IND. ENG.CHEM.,ANAL.ED., 19, 992 (1947).
(3) Mair, J . Research Natl. Bur. Standards, 34, 435 (1945). (4) Rheinboldt, Ber., 60B,184 (1927). RECEIVED March 22, 1949. Y
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Figure 13. Passage of 137" C. Cut from Acid Tar Oil over Silica Gel Comparing this order of adsorption with that for hydrocarbons, the position of thiophenes and aromatic thiols with respect to the sulfides is a little surprising. I n the hydrocarbon series, aromatics and olefins are more strongly adsorbed than either paraffins or naphthenes, but in the case of the sulfur compounds, thiophenes, with two double bonds in the molecule, and aromatic thiols,
Stainless Steel Columns Filled with Silica Gel Separate Types of Sulfur Compounds at Bartlesville, Okla., Experiment Station
