GulfResearch & Deuelopment
eo.,Pittsburgh, Pa.
WIDE-RANGEpetroleum fractions can be separated into aromatics, olefins, and paraffin-cycloparaffins by liquid chromatography on silica gel ( I ) . Further fractionation of the paraffins and cycloparaffins into n-paraffins and branched paraffins can be made by treatment with urea ( I ) or molecular sieves (2). However, complete removal of branched paraffins from cycloparaffins and separation OS the various types of cycloparaffins are much rnore difficult. Methods which have been applied to these systems are thermal diffusion, azeotropic distillation, and liquid chromatography with added components (3). Although fractionation of fairly complex mixtures of branched parafins and cycloparaffins has been obtained by these methods, the techniques have generally been limited to narrow distillation cuts. The separation of the cycloparaffins according to ring content is equally as difficult to achieve as the branched paraffins from cycloparaffins (3). Also, there does not appear to be an easy method for separating condensed cycloparaffins from noncondensed cycloparafins in complex mixtures. Mair et al. (4) investigated the separation of petroleum hydrocarbons with Sephadex LEI-20. Sephadex EH-20, a methylated cross-linked dextran, has been used primarily for gel filtration, a technique which segregates compounds according to size. These workers studied a number of model cornpounds and demonstrated that Sephadex LH-20 separates n-paraffins from cycloparaffins and alkylbenzenes from cyclanobenzenes when the components are of similar molecular weight. Also separated were branched paraffincycloparaffin petroleum fractions near CZ1. By combining and reprocessing, a final fraction containing 96 ,% branched paraffins was obtained. This investigation was undertaken to extend the Sephadex LH-20 separation of branched paraffins from cycloparaffins to wide-range petroleum fractions. During the study three other separations were also found to occur: condensed from noncondensed cycloparaffins, condensed cycloparaffins according to the number of rings, and separation of branched paraffins according to carbon number. E
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Two batches of Sephadex EH-20 were prepared. The first was allowed to stand for 16 hours in contact with an excess of acetone and then transferred to a glass column 6 feet in length and 1 inch in diameter. A plug of glass wool inch thick was added to the top of the packing to avoid disturbing the surface when adding the sample or solvent. The second batch was allowed to stand for 16 hours in contact with an excess of tetrahydrofuran and then transferred to a glass column 1 foot in length and 1 inch in diameter. The ends
(I) B. J. Mair, W. J. Mareulaitis, and F. D. Rossini, ANAL.CHEM., 29, 92 (1957). (2) M. Norris and S. G. O’Connor, ibid., 31,275 (1959). (3) R. W. Sauer: T. A. Washall, and F. W. Melpolder, ibid., 29, 1327 (1957). (4) B. J. Mair, P. T. R. Hwang, and R. G. Ruberto, ibid., 39, 838 (1967). 2 192
ANALYTICAL CHEMISTRY
Figure 1. Separation of Cll to Cs cycloparaffin
e 1 6
branched paraffins from a
0 2-Methyldeeane(eII) 2,2,4,6,6-Pentamethylheptane( C d Heptamethylnanane (CX)
A 1,2-E)imethylcyclohexane(C,) of this column were movable and covered with finely woven Teflon fabric to serve as diffusing plates. A solid pack was obtained by compressing the gel between the diffusing plates. A flow rate of 0.8 ml per minute was maintained in both columns throughout these studies, and the effluent was collected in 6-ml fractions. The fractions were monitored by means of gas-liquid chromatography, differential refractrometry, ultraviolet spectrophotometry, and mass spectrometry. The 6-foot column containing the acetone expanded gel was used to study model compounds and the naphtha distillate. A charge of 1 ml was used for the model studies; 0.5 ml, for the naphtha studies. The I-foot column containing the tetrahydrofuran expanded gel was used to study the crude distillate material. A charge of 0.5 ml was used. To obtain representative branched paraffin and cycloparaffin mixtures, a naphtha distillate (C, to Cls) and a crude distillate (C14 to CeB+) were prepared as follows: The distillates were passed through Grade 923 silica gel and the effluent percolated through a column containing silica gel coated with silver nitrate (5). This was used to remove as completely as possible all olefinic, aromatic, and polar materials. The normal paraffins were then removed using 5 A molecular sieves, as described by O’Connor and Norris (2). The nonnormal portions of the distillates were collected, and the sol-
(5) L. R. Chapman and D. F. Kuemmel, ANAL.CHEM.,37, 1598 (1965).
z 0
I-
a
K I-
z
0 W
z V 0
W
z I-
< .J W
K
Figure 2. Simultaneous separation of branched paraffins according to size and of branched paraffins from cycloparaffins
0 (1) S-~-Butylhexadecane(Go) (2) Heptamethyhonane (C1d A (3) n-Decylcyclohexane(CX) vent used in the molecular sieve separation was carefully distilled off. The non-normal mixtures were then considered ready for fractionation on the Sephadex columns. RESULTS AND DISCUSSION Several model systems were studied first to determine the feasibility of separating branched paraffins from cycloparaffins in wide-range petroleum distillates. A nearly complete separation of a Cs branched paraffin from a CS cycloparaffin and of a more complex mixture consisting of c6 and CS branched paraffins and two C8cycloparaffins is easily accomplished with one pass. Figure 1 shows a separation of a mixture of branched paraffins ranging from C11 to c16 from a CS cycloparaffin. In Figure 2, a gel filtration separation of a CZO branched paraffin from a cl6 branched paraffin is shown as well as the separation of both as a class from a c 1 6 cycloparaffin. The gel filtration separation is not apparent in Figure 3 because the branched paraffins are presented as a composite system. Wilk et al. (6) showed that Sephadex LH-20 resolves aromatic compounds according to the number of rings, the ring content increasing with elution volume. This same type of separation is obtained with cyclic paraffins. A one-ring cyclic paraffin was separated from a two-ring cyclic paraffin and both were separated from branched paraffins of similar molecular weight. Figure 3 shows an unexpected separation. In this system 1,Zdiphenylethane was almost completely resolved from 2,6dimethylnaphthalene in one pass. This suggests the possibilities of separating condensed from noncondensed cycloparaffins in petroleum fractions. The studies on the naphtha and crude distillates show this to be true. The separation of a five-membered ring cycloparaffin (cyclopentane) from a six-membered ring cycloparaffin(cyclohexane) was not complete. However, some separation did occur even though the differences in size and configuration were small. Based on experience with the other model compounds, recycling should improve the separation.
FRACTION
NUMBER
-3
Figure 3. Separation of a condensed cyclic compound from a noncondensed cyclic compound 0 1,2-Diphenylethane A 2,6-Dimethylnaphthalene
NAPHTHA DISTILLATE The branched paraffin and cycloparaffin fraction of the naphtha was analyzed by mass spectrometry and gas chromatography before separation on Sephadex LH-20. The fraction contained 63.1 % branched paraffins and 36.9% cycloparaffins with a carbon number range of CSto C16. The cycloparaffin portion contained 27.7 noncondensed cycloparaffins and 8.7 % condensed cycloparaffins. The mass spectrometric results of the separated fractions are shown in Table I. The first 40 fractions contained only acetone and the next nine fractions naphtha and acetone. Three fractions-41, 42, and 43-contained only branched paraffins. Fraction 44, which represented less than 10% of the sample, contained 70.8% branched paraffins and 29.2% cycloparaffins. Fractions 45 and 46 were blended together before analysis. The composite showed only 2.9% branched paraffins and 97.1% cycloparaffins. Fractions 47, 48, and 49 were also blended before analysis and were found to contain only cycloparaffin. The results in Table I also indicate a separation of noncondensed from condensed cycloparaffins. In fraction 44, the
Table I. Mass Spectrometric Analysis of Separated Fractions of the Naphtha Distillate Reported as Mole Per Cent Fraction number 41-43 44 45 & 46 47-49 Paraffins 100 70.8 2.9 0.0 Noncondensed cycloparaffins 0.0 27.8 92.9 71.2 Condensed 0.0 1.3 4.1 27.4 cycloparaffins 0.0 1.3 4.1 25.9 2-Ring condensed 0 0 0.0 0.0 1.5 3-Ring condensed 0.0 0.1 0.0 1.3 Benzenes 0.0 0.0 0.1 0.1 Naphthalenes
(6) M. Wilk and J. Rochlitz, 2.Nuturforsch, 216,975 (1966). VOL. 40,
NO. 14, DECEMBER 1968 * 2193
CRUDE DISTILLATE Table 11. Gas Chromatographic Analysis of Separated Fractions of the Naphtha Distillate Reported as Weight Per Cent Fraction number 41 42 43 44 45 & 46 0.0 0.0 0.0 0.4 0.0 0.7 4.3 7.4 0.0 0.0 2.6 20.2 34.6 24.3 0.0 18.2 35.8 25.4 26.0 1.5 45.8 28.7 19.0 24.3 23.8 46.4 27.3 11.1 12.5 16.9 61 . O 3.5 4.2 3.5 28.3
c9 ClO c 1 1
Cn Cia Ci4 Cia+
A cursory look was taken at extending the separation into a higher molecular weight range. For this, the branched 47-49 0.3
1.3 13.9 35.2 26.4 14.1 7.3
cycloparaffins are primarily noncondensed; whereas, the composite of fractions 47, 48, and 49 contained 27.4z condensed cycloparaffins. A separation of the condensed cycloparaffins according to the number of rings is also shown in Table I. In fraction 44, the condensed cycloparaffins contain only two-ring compounds; whereas, in the composite of fractions 47, 48, and 49 both two-ring and three-ring condensed cycloparaffins appear. This type of separation was obtained with n-butylcyclohexane and decahydronaphthalene in the model compound studies. The carbon number distribution data, as obtained by gas chromatography, are presented in Table 11. A gel filtration type of separation is clearly shown in fractions 41, 42, and 43-i.e., as the elution volume increases, the average carbon number of the isoparaffins decreases. With the elution of the cycloparaffins,the trend is reversed.
paraffin and cycloparaffin fraction from a Cl&ze+ crude distillate was used. The mass spectrometric analysis of this paraffin fraction showed 93.1 cycloparaffins and 6.9 branched paraffins with the cycloparaffins consisting of 54.8 noncondensed and 34.3 condensed. The mass spectrometric results of the separated fractions showed that the initial fractions contained only tetrahydrofuran. The distillate appeared in six fractions. The first two were combined before analysis. The separations obtained were similar to, but not as complete as, those found for the naphtha using the longer column. The first three fractions contained both branched and cycloparaffins, but the last three contained only cycloparaffins. A separation of noncondensed from condensed cycloparaffins is again indicated. A discrepancy was noted in the combined fractions 1 and 2 but as the elution continued through fractions 3, 4, 5, and 6, the percentage of noncondensed cycloparaffins decreased and that of condensed cycloparaffins increased. A separation of the condensed cycloparaffins according to the number of rings is also shown. Except for the composite fraction 1-2, the ring content increased with elution volume.
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ACKNOWLEDGMENT
The authors express their gratitude to H. T. Best, P. W. Mazak and R. E. Swab for their efforts in obtaining the mass spectrometric and gas chromatography data. RECEIVED for review July 27. 1968. Accepted September 3, 1968.
Quantitative Determination of Qrganic Halides in Dimethylsulfoxide Joe A. Vinsonl and James S. Fritz Institute f o r Atomic Research and Department of Chemistry, Iowa State Unioersity, Ames, Iowa 50010 NUMEROUS methods have been proposed for the determination of organic halides. The Carius, Pregl, and Schoniger methods make use of various oxidants to convert the organic halide to carbon dioxide, water, and halide ion, which is then determined titrimetrically or gravimetrically. These methods have been reviewed by Schoniger (1). Reduction methods include the use of potassium fusion ( 2 ) and sodium biphenyl (3). These are general methods for organic halides. Hydrolysis methods offer the possibility for the selective determination of certain halides in the presence of others. Rates of hydrolysis of organic halides in basic solution vary considerably, depending on the structure of the compound. In many instances it should be possible to find conditions such that one halide would react quantitatively while another would not react perceptively. With water and alcoholic solvents, 1 Present address, Department of Chemistry, Washington & Jefferson College, Washington, Pa. 15301
(1) W. SchiSniger in “Advances in Analytical Chemistry and Instrumentation,” Vol. 2, C. N. Reilley, Ed., Interscience, New York, N. Y., 1960. (2) G. Kainz, Mikroclzeinie Der. Mikrochim. Acta, 39, 1 (1952). (3) L. M. Liggett, ANAL.CHEM., 26,748 (1954). 2194
e
ANALYTICAL CHEMISTRY
however, the time and temperature required for quantitative reaction of many types of organic halides is excessive. It has been shown that rates for the reaction of base with organic halides are greatly accelerated in dimethylsulfoxide (DMSO) ( 4 ) . For example, the rate of reaction of methyl iodide with hydroxide ions is 5 x 106 faster in DMSO than in water. This may be explained by the decreased solvation of hydroxide ions in DMSO compared with protic solvents. Other aprotic solvents such as dimethyl formamide or dimethyl acetamide also possess large rate enhancing characteristics (5). However, these are not as stable as dimethylsulfoxide toward base and tend to hydrolyze. Therefore, dimethylsulfoxide appears to be the solvent of choice. The procedures described below make use of the hydrolysis reaction with dimethylsulfoxide as the solvent and potassium hydroxide as the base. The reaction may be either or both of the following : R-X
+ OH-+
R-OH
f X-
(1)
(4) E. Tommila and L. Hamalainen, Acta Chern. Scand.. 17, 1985 (1963). (5) A. J. Parker, Quart. Rev. (London), 16, 163 (1962).
