Determination of Total Olefins and Total Aromatics In Hydrocarbon Mixtures by Raman Spectrometry J. J. HEIGL, J. F. BLACK, Esso Laboratories, Standard
AND
B. F. DUDEKBOSTEL,
JR.
Oil Development Company, Elizabeth, IV. J .
R
AMAN spectrometric data obtained at the Esso Laboratories of the Standard Oil Development, Company show that the total aromatic and total olefin content of complex hydrocarbon samples can be determined by a fast, simple procedure. This analysis is performed with a spectrograph equipped with a photoelectric detecting device and a high speed pen recorder. The data are obtained by recording the scattering intensity in a narrow spectral region which includes a peak characteristic of the olefins and another peak characteristic of the aromatics. The exact spectral positions at which t,hese peaks occur vary somewhat among the individual compounds in each series and, therefore, it is not possible to base the rnethod on absolute peak height measurements. The recorded area under each peak, however, gives a relationship between scattering intensity and the concentration of the olefin double bond and of the aromatic carboncarbon bond.
approsimately the same displacement frequency. For esample, the olefin carbon-carbon double bond stretching vibration emits Raman lines in the 1640 to 1680 If cm.-' region ( R ) , and the stretching vibration between the carbon-carbon bond in the aromatic ring gives rise to a line in the 1590 to 1615 I;cm.-' region (5). The exact positions of the peak locations within these regions are governed by the structure of the molecules. In addition to affecting the position of the peak, the remainder of the molecule acts in a manner similar to a diluent, thereby affecting the relative intensity of the Raman line. The fact that this effect is directly proportional to molecular volume has been
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BASIS OF RIETHOD
The R~~~~ effect, Figurel. Diagramof Complete Monochromator Assembl? discovered only 20 years ago ( 4 ) , is based on the fact that when a beam of light is passed through certain s u b stances, part of the light is scattered with a wave length different from that of the exciting radiation. This scattered light constitutes the Raman spectrum of the substance. Examination of this spectrum reveals that it consists of a series of lines of extremely low intensity on either side of the exciting line. The lines are located symmetrically on either side of the exciting line and are designated as Stokes and anti-Stokes lines on the longer and shorter wave length side of the exciting line, respectively. In practice, the Stokes lines are generally employed because of their greater intensity, and their spectral positions are measured in terms of distance from the exciting line, expressed in wave numbers. This frequency difference is usually designated as wave number shift ( AF cm.-1). The magnitude of the wave number shift is governed by the interatomic vibration frequencies within the molecules being excited and is independent of the frequency of the incident radiation. All hydrocarbons have characteristic Raman spectra due to differencesin the frequencies with which their atomic groupings vibrate and rotate. The types of atoms present, the arrangement of atoms within the molecule, and the positions and types of interatomic linkages all govern the positions a t which Raman spectral lines occur. Similar atomic groups emit Raman lines at
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V O L U M E 21, N O . 5, M A Y 1 9 4 9
55s
The R a m a n lines characteristie of the olefin and a r o m a t i c carbon-earhon bond vibrations are employed i n the d e t e r m i n a t i o n of total unsaturation and t o t a l aromaticity. T h e analyses are performed b y scanning the spectral region w i t h a Raman displacement of 1590 to 1680 A8 em.-' from the exciting line. Funetions of the areas u n d e r the recorded peaks rather t h a n peak heights are employed in b o t h analyses, as the positions of the peaks s h i f t slightly w i t h different olefin or aromatic compounds. Results f m m h o t h analyses m a y he obtained w i t h an accuracy of approximately *lo% of the correct value.
established in investigations of the andytical applications of the &man effect a t the Esso Lahoboratories.
JUNCTlON Box
COAX CONNECT,
APPARATUS
A complete description of the Raman spectrometer employed in the development of this analysis will he the subject of an additional publication. A brief description of this apparatus is presented below. The monochromator fallows conventional lines and consists of a lens and prism assembly mounted in accordance with a modified Young-Thallon design (6). The design of the light source and sample cell follows that previouslx described by Fenske et a2. (1 ). The mercury lines around 4358 A. are employed as the exciting frequencies. A diagram of the complete assembly is shown in Figure 1. A photograph of the instrument is given in Figure 2. The most important feature of the equipment is the detecting, amplifying and recording system, whivh was designed at the Research hivision of the Esso Lahorstories. A photoniultiplier, mounted in accordance with the desirn shown in Figure 3,is employed as the detecting element. The photomultiplier is cooled with dry ice. The output from the photomultiplier is fed to 8. direct ourrent amplifier whose output is recorded o n a two-recorder system. The recorders operate with a sensitivity ratio of 1to 3, 80 that peaks with a wide range in intensity can he recorded with maximum accuracy. A view of the amplifying and recording system is given in Figure 4.
Figure 3. Photomultiplier Housing
ploying a standard reference procedure (I),which involves scanning the 459A8 em.-' line of carbon tetrachloride hefore and after the spectral trace of each sample. Scattering intensities MEASUREMENT O F RAMAN LINE INTENSITIES are then converted to scattering coefficients by dividing the Pure Compounds. For the quantitative applications of Raman recorded height of the sample peak by the average of the heights spectrometry, a reproducible method for expressing scattering of the two carbon tetrachloride peaks. Both the carbon tetraintensities is required. Because of slow changes in instrument chloride standard and the hydrocarbon sample must he measured sensitivity, absolute scattering intensities are not comparable in cells of the same dimension. ovw an extended period. This limit,ation is overcome by emIn the analysis of hydrocarbon mixtures for individual isomers, the Raman peaks selected are due to single constituents ( 1 , 5 ) . For these samples, scattering coefficients based on peak height alone are employed. However, in the analysis of complex hydrocarbon mixtures for total aromatio and total olefin content, the Raman peaks c h a r m teristic of the aromatic and olefinic carhoncarbon hands are the result of a Imge number of individual compounds. Because these Raman displacements are found to vary among the various compounds, a broad band is recorded a t the position characteristic of each of these bond types. Scattering coefficients based on recorded peak height cannot he employed under these conditions. It has heen established a t this laboratory that the area under the recorded peak can he employed as a measurement of scattering intensity. The praerdure that hcts been developedfor cdculating the scattering intensity from the recorded peak area is illustrated in Figure 5. The peak height (PB,Figure 5) is divided by the height of the 459 A8 em.-' carbon tetrachloride peak to give the scattering coefficient previously described. Figure 2. Photoelectric Recording Raman Spectrograph
ANALYTICAL CHEMISTRY
555 Table I. R a m a n D a t a Used in D e t e r m i n a t i o n of Total MolQo-olefinsby M e a s u r e m e n t of R a m a n Line Characteristic of - C 4 Stretclh i n g Vibration Olefin
Olefin Hydrookrbon
Type
8-Methyl-1-pentene 4-Methyl-1-pentene cir4-Methyi-Z-~entene 2.3-Dimethyl-1-butene tmna-4-Methyl-2-pentene 2-Methyl-1-pentene l-HeXe"c OslnaJ-Herene cia-2-Hexene 2.3-Dimethy1-2-butene
I I I1 111 I1 111 I
I1 I1 V
Ram%" shift.
Cm.?
1640 1643 1663 1647 1647 1650 1650 1670 1660 1680
Aversce for hexenes
2.4-Dimethyl-2-pentene 4.4-Dimethyl-1-pentenr 1-Heptene 3-Ethyl-Z-pentene eans-2-Heptene cia-2-Heptene
IV I
I
IV
I1 I1
I1
3-Heoteno Average for heptenea 2,4.4-Trimethyl-l-pentene 2,4,4-Trimethyl-2-pentene
111
IY I
6-Methyl-1-he~tene 2-Methyl-1-heptene 2-Ethyl-I-hexene 1-0otene I-Methyl-3-he~tene eo"s-2-Octe"e eia-2-oetene
111
I11 I IV
I1 I1
1675
1543 1645 1665 1660 1650 1660 1647 1675
1643
1550
1647
1640
1665 1670 1670
Scattering Coefficient
0.294 0.332 0.265 0.260 0.261 0.338 0.314
(H.B.1-
0.2~8
*
0.028
0.298 0.216 0.306 0.317 0.226 0.253 0.034
0.192
0.225 o.190
I-DeCene I 0.178 0.198 Average for deoenes Over-hll average Distinguished as low boiling and high boilinp isomers.
37.2 33.1
0.031
37.3
.
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~~
~
~~
~
~~
%"
".,
27.0 25.5 27.0
* 4.7
..
25.0 24.6 24.0
24.0 24.0 25.0 24.0 25.0 28.0
*
5 0
~
. .
I
term(37.3)istheslopeofthelinepresented in Fieure 6 and is constant over the entire
mojecuiar weignr range as xnucawu III Table 11, + 1.0 The use of the molal scattering coeffieient provides a simple and direct method for converting the Imeamred scattering coefficient into moles of unsaturates nermilliliter. The conversion procedure and its validit,y are indicated by the following considerations.
*
0.018
The line in Figure 6 must, by definition, pass through zero of per milliliter of sample. Inspection f, ~i~~~~6 indicates that, within experimental error, this behavior is observed. According to this curve, a scattering coefficient is therefore equal to the moles of unsaturates per milliliter times the slope of the line-i.e., moles of unsaturstes per milliliter times 37.3. It is conversely true that a n observed scattering coefficient divided by 37.3 (the average molal scattering coefficient) is equal to the males of unsaturates per
scatte,.ingcoefficient at zerO
~
OLEFIN ANALYSIS
Calibration. The total olefin analysis PIP sented in this paper was developed from data abtsjned in the examination of the Raman speotra of 29 pure mono-olefins, which ranged from hexenes to decenes and included all olefin types as defined by the classifierttionsystemof Schmidt and Boord (7). The Raman scattering characteristic of the carbon-carbon double bond vibration in the mono-olefins examined a t the Esso Laboratories is presented in Table I. The data in Table I reveal that the magnitude of the scattering coefficient characteristic of the
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The ttase width of the peak is then measured on a line, DBE, . ~ u epea^ neignt, . , ~ L I . r n e 1nmits . drawn perpenanuar LO of the base width (points D and E)are defined by the intersection of line DBE with the linear extrapolation of the sides of the peak. The base width observed for the sample is divided by the average base width calculated from the spectra of pure compounds. The observed scattering coefficient is tlnen multiplied by this quotient' to correct for the width of the peak The use of relative base widtkL to correct the soatt,ering eoefficient8 of the Raman lines employed for the total olefin aud total aromatic analyses is justified by the constant base width observed for pure compound data. The average base width of the olefinic double band peak, calculated from Raman data on 29 pure olefins, is 23.8 + 1.0 mm. and is independent of molecular weight or double bond position. The average base width of the Raman line characteristic of the conjugated carbon-carbon bond in the aromatic nucleus is 15.3 * I d mm. This observation is based upon the Fiaman spectra of 22 pure aromatic hydrocarbons.
..
24.5 27.0 23.0 25.0
23.0 42.6 35.9 0" F 33.7 " _ 37.4 * 6.2 37.3 * 4 . 2 23.8
-
..
23.5
* 3.4
30.9 29.7
0.198
1655 1643
23.5 24.0 23.5 23.5
35.2 36.9 43.0 35.5 45.9 45.8
0.230 0.293 0.297
1655
23.5
43.0 31.5 35.0 35.7
0.267 * 0.212 0.230 0.235 0.277
IV
23.5
41.9 30.2 43.0
0.255
IV
23.0
42.2
38.9 35.3 38.3 37.3
~
peak nase width. Mm.
24.5 25.0
32.1
0.240 *
Averare . for ootenes 3.4.4.5-TetramethyI-2-herene (L.B.)* 3.4.4.5-Tetrsmethy1-2-hexene
i
36.9 41.8 42.3 32.8 32.5
0.338
0.323 0.301
scattering Coefficient x Mol. Yo1.
olefinic double bond decreases with inoreingmolecularweight
