Analysis of C, Olefins by a Combination of Gas Chromatography and Nuclear Magnetic Resonance Spectrometry E.
D. ARCHER, J. H.
SHIVELY, and S. A. FRANCIS
Texaco Research Cenier, Beacon, N.
Y.
b A novel combinaiion of gas chromatography and riuclear magnetic resonance spectrometry was used for the analysis of a sample of C, olefins. Sample components were identified by correlating gas chromatographic data for two columns, one polar and one nonpolar, with the structural information obtained from NMR. The NMR analyses were obtained on gas chromatographic microfractions containing from one to five components. Of 15 peaks found in the sample, six were identified as Ce olefins, eight as C7 olefins, and one a!i a Cs olefin.
S
gas chromatographic methods have been reported for determining individud olefins in the Cs to Ce range (6, 8 11, 18, 19, 22). The analysis of C7 olef ns is known to be more difficult because of the large number of isomers possible. Lichtenfels, Fleck, and Burow (IO) obtained only seven partially resolved peaks from a complex heptene mixture conttining about 19 components. ReEYERAL
Figure 1.
cently, Polgar (16) reported very successful separation of olefins through C, using a capillary column. Other investigators have determined olefins in naphthas by a variety of combinations of physical, chemical, spectrometric, and chromatographic techniques (4,13-16,17).
I n the present study, a combination of two-column gas chromatography (9) and nuclear magnetic resonance spectrometry (NMR) was used to analyze a sample of C7 olefins. Although the combination of gas chromatography and N M R has been used previously to identify an isoprenoid in petroleum (3) and CS aldehydes (12), as far as could be determined this is a unique approach to the analysis of a light hydrocarbon sample. EXPERIMENTAL
Apparatus. A oommercially available gas chromatograph equipped with a hot-wire thermal conductivity detector and a 1-mv. recorder having full scale response time of 2 seconds was used. T h e glass chromatographic columns were 6 mm. i.d. and 2.5
meters long filled, with Apiezon L grease on 30- to 60-mesh Chromosorb P (3 grams of substrate per 100 grams of support) and benzyl cyanide-silver nitrate (2 to 1 ratio) on 60- t o SO-mesh Chromosorb P (10 grams of substrate per 100 grams of support). Chromatographic peak areas were measured with a compensating polar planimeter. The gas chromatographic fractions for NMR were collected in a 6 X 50 mm. glass culture tube containing 0.25 ml. of carbon tetrachloride chilled to icewater temperature. A short piece of l/l&xh Teflon tubing was connected to the detector and immersed in the carbon tetrachloride. The NMR spectra were measured with a Varian Associates V-1311 high resolution NNIR spectrometer operating at 60 &IC.per second. Peak positions were measured by the side band technique (,?) from an internal reference material, tetramethylsilane. An integrating system similar to that described by Johnson (21) was used to measure areas under selected portions of the spectra. Procedure. GAS CHROMATOGRAPHY. The sample for which a n analysis was desired was a narrow boiling, 198" to 204" F. (92' to 95" C.),
Chromatogram of C? olefin sample on Apiezon L column
VOL 35, NO. 10, SEPTEMBER 1963
1369
distillation fraction of C, olefins. Therefore, a simple mixture of C7 olefins was expected. Very few C I olefins were available for calibration, so the sample was first run on a nonpolar Apiemn L column to obtain a boiling point analysis. The column was operated at room temperature with a helium flow rate of 100 cc. per minute. Three large partially resolved peaks were obtained (Figure I), which represented approximately 60, 30, and 7% of the sample. I n addition, eight small peaks representing a Qotal of about 3% of the sample were found. I n an attempt to characterize the peaks, log relative retention time was plotted B S . boiling point (Figure 2 ) using the available CSand C7olefins as calibrants. The chromatographic peaks could not be identified from this graph because too many compounds have boiling points in this range. However, three of the small peaks were apparently Cg olefins since they eluted in the Cg boiling range. The sample was then run on the highly polar benzyl cyanidesilver nitrate column which was known to separate olefins according to their structure ( I , 90). When first used, shortly after preparation, this column gave poorly separated, asymmetrical peaks. Peak symmetry was improved by further conditioning of the column, by allowing it to sit in the chromatograph for several days under the conditions to be used for the analysis. On the well-conditioned column, operated a t room temperature with a helium flow rate of 100 cc. per minute, the sample separated into 15 peaks (Figure 3), four more than on the Apiezon L column. Since the benzyl cyanidesilver nitrate column has been shown to be especially selective for cis-trans isomers ( I ) , it was suspected that the additional peaks were due, at least in part, to cis-trans isomers. As on the
Apiezon L column, two major peaks were found in an approximate 2 to 1 ratio and representing a total of about 90% of the sample. A plot of log
relative retention time us. carbon number was drawn (Figure 4) using the available C6--C7 olefins as calibrants. Because of the selectivity of the column, 10 0
8.0 6.0
4.0
- 3.0
-
w 2.0
z z
P
- z - 0.8 : W
1.06
2
X-CALleRANTS A-CHROMATCGRAPHIC PEAKS
-G6+
-0.5 - 0 4 a
0.3 0.2
I
I I10
I20
I
100
1
I
Figure 2.
I
lo
t 60
70
80
90
BOILING POINT.
1
'C
Log relative retention time
v5.
boiling point
Apiezon 1 column
Table 1.
Cotrected Relative Retention Times and Boiling Points of Calibrant Olefins
Boiling point. "C.
Olefin trans-2-Pentene 36.3 2-Methyl-2-butene 38.6 2,3-Dimethyl-l-butene 55.7 36.9 n's-2-Pentene 67.3 %Methyl-2-pentene 67.9 trans-2-Hexene 60.7 2-hfethyl-1-pentene 64.7 2-E t hyl- I-bu k n e 63.5 1-Hexene cis-2-Hexene 68.8 98.0 trans-2-Heptene 98.5 cis-2-Heptene Not determined on this column.
RRT on benzyl cyanide-silver nitrate 0.15 0.15 0.31 0.34 0.34 0.35 0.47 0.58
0.62
0.72 0.80 1.68
RRT on Apiezon L a
0.71 0
1.00 1.00 0.87 0.97 0.94 1.11 2.82 2.84
5
N
15
Figure 3.
1370
10 Chromatogram of
ANALYTICAL CHEMISTRY
C7 olefin
5 sumple on benzyl cyanide-silver nitrate column
TIM E (MINI
10 0
I-t.-&-Z-METHYL-2 OLEFINS 2- 2-METHYL-1 O L E F I N S 3 - 2-ETHYL-I O L E F I N S 4-C-6-n -a-OLEFINS
NMR Assignments for Peak 1 1 and Peak 12 Fractions Speptral region, No. Group Peak c.p.s. of H assignment 11 40-70 3.2 1 CH3,P"or > 70-95 4 . 1 2 CHq. B or > 95-105 2 . 8 1 CHa; h 105-140 2 . 0 1 CHz, cy 270-280 1.9 C=CHz 12 40-80 6.1 2 CH3, P or > 80-105 2 0 1 CH2, p or > 105-140 3 . 9 2 CHz, CY 270-280 2 . 0 C=CHz a: and p refer t o positions relative to an olefinic carbon atom. Table II.
80
60
x -CALIBRANT A-I'EAK
I D E N T I F I E D BY NMR
40 3 0
-
W
2.0
I.
z
e
-
0.8
-
0.6
-
-
z
1.OL 2
2
'1
0.5J
0.4 0 3
1
rnatography, microsize fractions w t w collected from the benzyl cyanidco'2 silver nitrate analytical column. The 1 I I I 1 0. I two major peaks (peaks 11 and 12 in 8 7 6 5 4 CARBON NUMBER Figure 3) were collected individually. The minor peaks, because of their Figure 4. Log relative retention time vs. carbon number small size, were collected in groups. Benzyl cyanide-silver nitrate column Group I included the four peaks eluted just before the major peaks and group most of the points obtained fell on one SUCLEAR MAGNETICRESONANCE I1 included the three peaks eluted of four lines representing different olefin SPECTROMETRY. Since the identity O f after the major peaks. KMR spectra of structures. The corrxted relative rethe major components could not be these four fractions are shown in Figures tention times of the calibrant olefins determined by the gas chromatographic 6 and 7. for both columns are aihown in Table I. The spectrum of the peak 11 fraction technique alone, nuclear magnetic resdivides naturally into five regions which From this informal ion, seven peaks onance (NMR) spectra were obtained. were identified. Correlation of these can be associated with hydrogens on The spectrum of the original sample is shown in Figure 5, The basis for asidrntifications with thl: boiling point inolefinic carbon atoms (275 c.p.s.), formation from the Apiezon L column CH2 groups (120 c.P.s.) and CH3 signing resonances in particular spectral showed them t o be ccnsistent with the groups (100 c.P.s.) alpha to olefinic regions t o olefinic protons has been deApiezon I, chromatogram. Thus the carbon atoms, and CH2 (80 c.p.s.) scribed (5, ?', 16). The olefinic hydropeaks numbered 1. 2, 3, 5, 6, 8, and 14 gen resonance near 276 c.p.s. indicates and CH3groups (55 c.P.s.) separated by in Figure 3 werr identified as 2,3that the predominant constituents have one or more carbon atoms from olefinic dimethyl-1-butene, tim~s-2-hexene or carbons. The number of hydrogens the general structure R1R2*CH2. contributing to each of these regions, 2-methyl-2-pentenr, 2-methyl-1-penThe areas in the various regions of this trnr, 2-cthyl-1-huter e, cis-2-hexene. based on the empirical formula C7H14 spectrum could be accounted for by for the sample, is given in the third truns-2-heptene, and (is-2-heptene. repostulating the presence of two princicolumn of Table 11. These numbers spectively. Even though about half pal components, 2-methyl-1-hexene and the componentc: preseit in the sample 2-ethyl-l-pentene, in an approximate lead t o the structural group assignments shown in the last column. The could he identified in this manner, both 2 to 1 ratio. of the major components and most of To confirm thesc itlentificationi and only structure which is consistent with to attempt t o identify some of the the presence of these structural groups what appeared t o be the C7 olefins remained unidentified. minor components shown by gas chrois that of 2-methyl-I-hexene.
Table 111.
I
11 10 8
7
9
NMR Identifications of Group I and Group II Fractions
Cornpound 2-Methyl-1-hexene 5-Methyl-1-hexene trans-2-Heptene 2-Methyl-2-hexene 3-lllethyl-3-hexene 'Total
IT
14 13 15
1r;egrttls (from spectrum) cis-2-Hep tene PMeth yl-cis-2-hexene 2-1:thyl-1-hexene
Total Integrals (from spectrum)
M't.
mo
15 .5
35
22.5
22.5 100
50
30
20 100
CHs
CHz
0.45 0.30 1.05 0.67 1.35
0.60
3.82
2.65
1.0 1.50 1.80 1.20 4.50
2.8 2.00 0.60 0.80
3.40
2.40
4.4
3.8
2.6
(0or > ) (Por > ) CHI ( a ) CH*(o/) 0.20
1.40
0.45
...
0.45
...
1.05
1.35 0.67 3.52 3.4 1.50 0.90
...
0.30 0.10
11IRzC-
I
