Determination of Hydrocarbon-Type Distribution’and Hydrogen/ Carbon Ratio of Gasolines by Nuclear Magnetic Resonance Spectrometry Mark E. Myers, Jr., Janis Stollstelmer, and Andrew M. Wims Research Laboratories, General Motors Technical Center, 12 Mile and Mound Rds, Warren, Mich. 48090
information on the hydrocarbon-type composltion of gasoline is required for continued research on Internal combustion engines, including efforts to Improve efficiency and reduce emissions. The standard method for obtainlng thls Information Is based on liquid chromatography and has been used for many years. An Improved technlque of greater precision was sought that would require less analysis time, whfle serving as an independent method of anaiysls. A nuclear magnetic resonance (NMR) technique has been developed for determlnlng the composltion (aromatlc, parafflnic, and oleflnlc) and hydrogenharbon ratio of gasoline. The equations requlred for the caiculatlons are derived. Results from the NMR method on 36 commerclai gasoiines are presented along with the results from the more familiar fluorescent indicator adsorption (FIA) method and the combustion method. The absolute standard devlations between the NMR and FIA methods are l.Q%, 3.2%, and 2.4% for the aromatics, paraffins, and olefins, respectively. A standard deviation of 0.055 is obtained on hydrogen/carbon ratios,
’
Gasolines, which are complex mixtures of several hundred compounds, are usually characterized by hydrocarbon type rather than by complete analysis. Hydrocarbon types are usually determined by the fluorescent indicator adsorption (FIA) method. This method is a liquid chromatographic technique which separates the sample on silica gel into aromatic, paraffinic, and olefinic components. The FIA method is described fully in ASTM Method D-1319 ( 1 ) .The need for a rapid, independent technique of greater precision has been recognized for some time. High resolution NMR is a potentially useful tool f?r the analysis of petroleum mixtures, such as gasoline (2-4), because they are rich in hydrogen atoms which provide Ztrong proton NMR signals. The principles of NMR spectrometry utilized in such an analysis are described (5-7). As the FIA method provides a chromatographic separation of the three hydrocarbon types, the NMR method provides the spectrometric measurement of the three hydrocarbon types without separations. One purpose of this paper is to describe how to interpret the NMR spectrum of a gasoline to obtain a quantitative measure of the three hydrocarbon types (aromatic, paraffinic, and olefinic) present in the sample. The second purpose of this paper is to show how the equations derived for the hydrocarbon-type distribution can be easily rearranged to yield the hydrogen/carbon ratio.
THEORY OF HYDROCARBON COMPOSITION BY NMR This NMR method is based on calculating the relative numbers of carbon atoms, classified as aromatic, paraffinic, or olefinic in a gasoline sample, from the NMR integral spectrum. For example, benzene, toluene, xylene, and ethyl benzene have, respectively, 6, 7,8, and 8 carbon atoms classified as aromatic. Isopentane, n -heptane, and isooctane 2010
have, respectively, 5, 7 , and 8 carbon atoms classified as paraffinic. 2-Methyl-2-butene has 5 carbon atoms classified as olefinic. Some carbon atoms go “uncounted” because they are completely substituted and give no NMR signal. Other carbons are erroneously counted as belonging to a given hydrocarbon class because their protons resonate in regions of the NMR spectrum where other hydrocarbon types normally appear. Finally, the densities of light paraffins and aromatics are significantly different. Correction factors for these three effects will be derived later in this paper. However, to a first approximation, the hydrocarbon composition by volume is given by: Aromatics, vol. % Paraffins, vol. % Olefins, vol.
N
c, x 102 c, + c, + c, c, c x 102 +
c,
c/c
+
x 102 +
c,
( 1) ( 2) ( 3)
where C, is the total number of aromatic carbon atoms, C, is the total number of paraffinic carbon atoms, and C, is the total number of olefinic carbon atoms. Hydrocarbon Types and NMR Spectral Regions. Table I indicates the various organic functional groups commonly found in gasolines, the spectral regions in which their protons resonate, and letters designating the integrals of these regions. The NMR spectrum of a typical premium grade gasoline is shown in Figure 1. From the considerations in Table I, we have the following, uncorrected, expressions for C,, C,, and C,: C, C,
N
- K(A + C/3),
K(D
+
E/2
C,
+
F / 3 ) , and
- KB
( 4) ( 5) ( 6)
where K is an instrument constant. With these explicit expressions for C,, C,, and C,, Equations l through 3 can now be rewritten as: Aromatics, vol. % = (A + C/3) X lo2 ( 7) ( A + C / 3 ) + ( D + E / 2 -!- F / 3 ) + B Paraffins, vol. $ = (D + E / 2 + F / 3 ) X 10’ ( 8) ( A + C / 3 ) + (D + E / 2 + F’3) + B Olefins, vol. % = B x lo2 (A C / 3 ) + (D + E / 2 + F / 3 ) + B ( 9) Because the instrument constant K cancels out, the composition can be determined from the NMR spectrum integrals without the need for quantitative preparation of sample reference solutions, or for quantitative addition of internal reference materials. Corrections for Density and Carbon Count. Correc-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
+
‘f
Table I. Organic Functional Groups and NMR Spectral Regions Spectral region,
Integral
Total No, of carbons
ppm from TMSa designation (of type indicated)
Proton type
Aromatics
A
t t Q c y 5 *- Chr,m8eal Shift From TYS lppm)
Figure 1. High resolution NMR spectrum of a premium grade gasoline Proton tyFbe
Paraffins
CH,
I
Chemical shift region
cn,ccn,cH.cn.cH
Ring aromatic: Olefin a -Methyl Methine (paraffins) Methylene (paraffins) Methyl (paraffins)
6.6 to 4 . 5 to 2.0 to 1.5 to 1.0 to 0.6 to
C
2.0-3.0
and
I
8.0 ppm ( A )
6.0 ppm 3.0 ppm 2.0 ppm 1 . 5 ppm 1.0 ppm
(B) (C)
H
(D)
CH,
1.5-2.0
1)
1.0-1.5
E
0.6-1 0
F
t I 4 4 4
(E) (F)
CH,CCH,CH,CHJX,
I
n
tions for Aromatics. A ) The densities of aromatic hydrocarbons are significantly greater than those of light paraffins. Thus, Equation 7 gives a high estimate of the volume percent aromatic content, and the aromatic term ( A C/3) in Equations 7 through 9 needs to be multiplied by a constant to correct for this density difference. Table I1 contains gas chromatographic (GC) data for gasolines indicating the average volume percents of 50 individual components for 25 typical gasolines. These data were obtained a t the General Motors Research Laboratories. The table also contains the weighted average density for each hydrocarbon class, defined as:
+
CH
+
+ I
4
CHJXH CH cn CH
I
H Olefins
CH
I
cn C=CCH
4.5-6.0
I n
B
t TMS, Tetramethylsilane. Carbons of the type $J
- CH2-R are counted as methyl groups. Because methyl groups are much more numerous than methylene groups, only a slight error is introduced. (Y
[vol. %,(i)] [density (i)]
Weighted av density =
(Y
i
The weighted average density for the aromatics is 0.8732 g/ml and for the paraffins is 0.6612 g/ml. The aromatic term (A C/3) must therefore be multiplied by 0.7572 (the ratio of 0.6612/0.8732) to correct for the density difference. B) Not all of the aromatic carbon atoms are counted by NMR spectrometry. The ring carbons to which the methyl groups are attached in toluene and xylene, for example, have no remaining hydrogen atoms and give no NMR signal. The ratio of the carbon atoms counted to those actually present for each component is also given in Table 11. The weighted average rat io, defined as
+
Weighted av r a t i o (carbons counted/’carbons present) = no. carbons counted (9 [vol. %; (i)] no. carbons p r e s e n t (i) i vel. %(91
Z----C[
(11)
i
is also indicated in Table I1 for each hydrocarbon class. This weighted ratio for the aromatics is 0.7835. The aromatic term (A C/3) must be multiplied by 1/0.7835 = 1.2763 to correct for the uncounted carbon atoms. The overall constant to be applied to the aromatic term ( A C/3) is, therefore, 0.7572 X 1.2763 = 0.97. Corrections for Paraffins. No density corrections are necessary for the paraffins because the aromatic and olefinic components are both normalized to the paraffin density.
+
+
The weighted average ratio (carbons counted/carbons present) is calculated to be 0.9831 (Table 11). Therefore, the paraffinic term (D E/2 F/3) in Equations 7 through 9 must be multiplied by 1/0.9831 = 1.02. Corrections for Olefins. The weighted average density for the olefins is 0.6808 g/ml. The weighted ratio (carbons counted/carbons present) is calculated to be 0.2917 for the olefins (Table 11).The overall constant to be applied to the olefinic term B is (0.6612/0.6808) X (1/0.2917) = 3.33. The olefin integral must be determined with considerable precision because it is being multiplied by a large constant and because it comprises only about 2 to 3% of the total raw integral for the gasoline sample. Cross Correction Terms. The alkyl parts of the olefin molecules produce signals in the D, E , and F regions of the NMR spectrum; these signals are sources of error in the determined paraffin content. The most serious problem is caused by olefinic contribution t,o the paraffin methine region D because methyl and methylene groups a to C=C groups resonate here. T o correct for this, the quantity 2B is subtracted from region D so that the paraffinic term becomes (D - 2B E/2 F/3) 1.02. (The correction 2B can be shown to be approximately correct on the basis of the components listed in Table 11.) Although numerous other cross corrections could be considered, we believe the one mentioned is the most important; the others are excluded for reasons of simplicity.
+
+
+
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After taking into account all of these correction factors, Equations 7 through 9 become:
(A
+
(A C/3)0.97
+
+
C/3)0.97 x l o 2 + E/2 + F/3)1.02
(D - 2B
+
Aromatics, vol. % =
3.33B (12)
Table 11. Individual Hydrocarbons in a Typical Gasoline 0
Ratio, carbons counted1
Vol. ?4
Component
Densifi, ‘g/ml
carbons present
Aromatics 1.5 0.8845 Benzene 5.9 0.8719 Toluene 1.3 0.8717 Ethyl benzene 5.9 0.8730 Xylene 1-Methyl -3 -ethyl benzene i 1.5 0.8673 1-methyl -4 -ethyl benzene t e v f -Butyl b e n z e n e + 1.7 0.8756 1,2,4 - t r i methyl benzene Weighted average density = 0.8732 g/ml. Weighted ratio, carbons counted/carbons present = 0.7835. Paraffins 7.0 0.5844 n -Butane 9.3 0.6248 Isopentane 4.6 0.6312 n -Pentane 1.4 0.6664 2,3 -Dimethylbutane 3.8 0.6579 2 -Methyl pentane 2.4 0.6690 3 -Methyl pentane 3.3 0.6640 n -Hexane Methyl cyclopentane + 1.8 0.7159 2.2 d i m e t h y l pentane 2,4-Dimethyl pentane + 1.4 0.6858 2,2,3 - t r i m e t h y l butane 2,3 -Dimethyl pentane + 3.9 0.6912 2 -me thvl hexane 3-Methyl hexane + unknown 2.4 0.6915 3 -Ethyl pentane + 4.9 0.6994 2 , 2 . 4 - t r i m e t h y l pentane n -Heptane 2.0 0.6882 Methyl cyclohexane + 1.0 0.7368 2,2 -dimethyl hexane 2,2,3 - T r i m e t h y l pentane + 2,5 d i m e t h y l hexane + 1.9 0.7076 2,4 d i m e t h y l hexane 3,3 -Dimethyl hexane + 2,3,4-trimethyl pentane + 3.6 0.7211 2,3,3 - t r i m e t h y l pentane + 2.3 d i m e t h y l hexane 1.1 0.7101 3 -Methyl heptane Weighted average density = 0.6612 g/ml. Weighted ratio, carbons counted/carbons present = 0.9831. Olefins 2 -Methyl -1 -butene 0.8 0.6557 trans -2-Pentene 0.8 0.6533 2 -Methvl-2 butene 1.7 0.6676 2 -Methyl-1 -pentene + 0.6814 0.5 1-hexene 2 -Methyl -2 -pentene 1.0 0.6913 4,4 -Dimethyl -1 -pentene 0.6 0.6871 2 -Methyl -1 -hexene 1-heptene + 0.9 0.7092 2 -ethyl -1 -pentene 3 -Methyl -trans -2 -hexene + 2 -methyl-2 -hexane + 1.0 0.7053 trans -3 -heptene + cis -3 -heptene Weighted average density = 0.6808 g/ml. Weighted ratio, carbons counted/carbons present = 0.2917. a I n instances where two or more components appear unresolved in the GC chromatogram, they are assumed to be amounts, and the average densities and average ratio values are used. ~~~
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
1.000 0.857 0.750 0.750 0.666 0.583
1.000 1.000 1.000 1.000 1.000 1.000 1.000 0.928 0.928 1.000
1* 000 0.937 1.000 0.937 0.958
0.937 1.000
0.400 0.400 0.200 0.416 0.166 0.428 0.333
0.214
present in equal
Paraffins, vol. W = ( D - 2B + E / 2 + F/3)1.02 x l o 2 ( A + C/3)0.97 + ( D - 2 B + E / 2 + F / 3 ) 1 . 0 2 + 3.33B
Table 111.1974 Winter Gasolines Analyzed Sample No.
Olefins, vol. $& = (A
+
C/3)0.97
+
3.33B x 10’ (D - 2B + E/2
+ F/3)1.02
+
3.33B (14) These are the final theoretical equations used in this experimental work for determining hydrocarbon-type distribution. An implicit assumption in this development has been that the relative amounts of the various individual components remain approximately the same for gasolines with both high and low aromatic and olefin contents. The results of NMR surveys of commercial gasolines over the past three years indicate this to be the case for the aromatics and olefins. The paraffins are a quite different matter. The relative amounts of normal paraffins and isoparaffins depend upon whether the gasoline is a regular or premium grade, a low lead, or an unleaded grade. Because the paraffin constant is so close to unity (1.02), however, the equations presented adequately provide the total paraffin content regardless of how it is distributed among normal paraffins, isoparaffins, or cycloparaffins. (The ratio of normal paraffins to isoparaffins, which can also be obtained from the NMR spectrum, can be correlated with the octane number of the gasoline. This work will appear in a future publication.) It should also be emphasized that the method developed applies only to full boiling range gasolines and, therefore, Equations 12 through 14 will probably not give the hydrocarbon-type distribution accurately for other petroleum products such as kerosenes, for example. Another set of equations would have to be derived for such materials. Determination of Hydrogen/Carbon Ratios. The total number of carbon atoms per unit sample volume can be determined from the denominator in Equations 12 through 14. T o do this, the density correction factor must be removed from each term, and the resulting terms multiplied by the NMR spectrometer constant K . Performing these operations, we have for the total number of carbon atoms:
+ C/3)1.28 + F/3)1.02 + 3.42B] ( 1 5 )
C (all carbon atoms) = K [ ( A
(D - 2 B
+
E/2
+
The total hydrogen content p e r unit sample volume is given by: C (all hydrogen atoms) =
K(A t B + C + D + E + F )
(16)
hydrogen/carbon == A + B + C + D + E + F (A + C/3)1.28 + ( D - 2B + E / 2 + F/3)1.02
+
(The instrument constant K again cancels out.) EXPERIMENTAL Automotive Gasolines. Thirty-six winter grade gasolines representing most of the various brands and grades available in the Detroit, Mich., area in January 1974 were used in this study. An identification number and the grade for each sample are given in Table 111.
Unleaded Low Lead
Aromatics, vol. % ;ample
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
DA:
3.42B (17)
Regular Premium Sub Regular Super Premium
Table IV. NMR a n d FIA Hydrocarbon Composition Results on Commercial Gasolines
SD:
Thus, the expression for the hydrogenlcarbon ratio is
Grade
1-14 15-28 29 30 31-33; 3 6 34-35
Paraffins, vol. %
Olefins, vol. %
FIA
N MR
FIA
K MR
FIA
NMR
21 20 26 24 25 27 26 27 19 25 23 18 23 23 29 18 22 15 9 14 7 18 22 25 28 33 15 11 20 29 26 23 25 11 20 16
20 21 27 25 24 25 26 26 21 24 21 19 24 24 30 19 25 16 11 14 9 17 27 28 28 38 17 13 20 32 27 21 26 12 22 17
65 64 67 68 66 54 65 53 72 63 67 67 69 70 64 68 72 76 81 70 82 67 73 68 64 64 76 83 68 59 65 56 67 81 67 74
66 64 63 66 66 60 63 58 71 63 69 68 66 68 60 74 67 77 81 69 84 67 68 65 61 56 72 80 69 60 63 59 69 79 65 73
14 16 7 8 9 19 9 20 9 12 10 15 8 7 7 14 8 9 10 15
14 15 10 9 10 15 11 1’6 8 13 10 13 10 8 10 7 8 7 8 17 7 16 5 7 11 6 11 7 11 8 10 20 5 9 13 10
1.9 -0.92
3.2 +0.72
11
15 5 7 8 3 9 6 12 12 9 21 8 8 13 10 2.4 +0.22
Sample Preparation. The gasolines were diluted in deuterated chloroform to a concentration of about 30 vol. %. Tetramethylsilane (TMS) was added as a chemical shift reference and for field locking; the concentration of TMS, about 3%, is not critical. Spectrometer Conditions. The determinations were performed on a Varian Model HA-100D NMR spectrometer. The absorption and integral spectra were obtained in separate scans from 0 to 10 parts per million (ppm) from TMS a t room temperature. The spectrometer was operated in the field sweep mode to obtain the most accurate integrals. Sweep times of 500 seconds were used for the absorption spectra and 50 seconds for the integrals. The R F attenuator setting was 40 dB “in” and the manual oscillator field milligauss setting was 0.1 X 2.5 for both the absorption and inte-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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Table V. Comparison of Hydrogen/Carbon Ratios Determined by Combustion and NMR HydrogenICarbon Ratios Sample
Combustion
1.83 1.84 1.79 1.73 1.82 1.76 1.80 1.74 1.82 1.78 1.81 1.84 1.76 1.74 1.77 1.93 1.84 1.97 SD = 0.055. DA = + 0.024. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Hydrocarbon t y p e
HydrogenICarbon Ratios
SMR
Sample
Combustion
NMR
1.72 1.74 1.76 1.79 1.79 1.71 1.75 1.71 1.85 1.75 1.79 1.78 1.80 1.80 1.77 1.92 1.83 1.99
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
2.07 1.93 2.08 1.90 1.84 1.80 1.75 1.67 1.96 2.03 1.83 1.72 1.82 1.88 1.81 2.00 1.89 1.97
2.01 1.88 2.07 1.84 1.86 1.82 1.77 1.66 1.91 2.01 1.77 1.76 1.78 1.70 1.82 2.00 1.82 1.92
gral spectra. No apparent saturation effects were observed on the absorption spectra and the much faster integral sweep ensures that such effects are absent from the integral spectra. Determination of HydrogedCarbon Ratios. The ratios were obtained by an independent (combustion)method using a Perkin-
Elmer, Model 240, Elemental Analyzer.
RESULTS AND DISCUSSION Hydrocarbon-Type Determination. The 6 spectral regions ( A , B, C, D, E, F ) and the types of protons they represent are indicated in Figure 1. The integral values for the A, B, C, D, E, and F spectral regions were obtained from the spectra; a computer program was used to determine the hydrocarbon compositions (Equations 1 2 through 14) and the hydrogenlcarbon ratios (Equation 17). The results obtained on the 36 samples of commercial gasolines are given in Table IV, with results obtained by FIA for comparison. Indicated for each hydrocarbon type are the standard deviations ( S D ) and the difference of the averages ( D A ) ,defined as
SD = I "
The standard deviations are 1.9, 3.2, and 2.4 for the aromatic, paraffin, and olefin volume percents, respectively. The differences of the averages are -0.92, +0.72, and +0.22, for the aromatic, paraffin, and olefin volume percents, respectively. Out of the total of 36 samples, differences between the NMR and FIA results for all three hydrocarbon types were
