574
Energy & Fuels 1995,9,574-579
Determination of Oxygenates in Gasoline by lH Nuclear Magnetic Resonance Spectroscopy W. R. Kalsi, A. S. Sarpal," S. K. Jain, S. P. Srivastava, and A. K. Bhatnagar Research and Development Center, Indian Oil Corporation Ltd., Faridabad 121 007, India Received September 23, 1994. Revised Manuscript Received May 4, 1995@
Oxygenates such as C1 to C4 alcohols and ethers like methyl tert-butyl ether (MTBE) and tertamyl methyl ether (TAME) are being used as octane boosters in motor gasoline. A quick and accurate method using lH NMR spectroscopy has been developed for their estimation based on group molecular weight (GMW) calculations. This method involves the identification and quantitation of oxygenates from their respective a carbon group (RO) resonating in the chemical shift region of 3.0-4.4 ppm, which is free from gasoline resonances. The relative GMW of aromatics (6.5-7.5and 2.1-3.0 ppm), olefins (4.5-6.5 ppm), oxygenates (3.0-4.4ppm), and saturates (0.6-2.1ppm) have been calculated from integral values of their respective regions. The percentage of each oxygenate has been determined from total GMW of sample and relative GMW of oxygenate present in straight-run (SR) and fluid catalytic cracked (FCC) gasolines. The method has also been found to be applicable to samples containing mixtures of oxygenates.
Introduction Oxygenates (C1 to C4 alcohols) and ethers (MTBE and TAME) are used as octane boosters in motor gasolines. They are added in order to replace lead alkyls for boosting octane n ~ m b e r . l - Besides ~ boosting octane number, oxygenated fuels minimize harmful exhaust emissions. Oxygenates up to the level of 20% in gasoline are being used in many c ~ u n t r i e s . ~As, ~ the use of oxygenates is becoming popular, there should be a suitable and quick method to identify and quantify their presence in the gasoline. Gas chromatographic infrared (IR),14-16m i c r o w a ~ e , ~mass ~ J ~ spectrometry (MS),18J9and nuclear magnetic resonance (NMR) spectrometryZ0methods have been reported earlier. Chromatographic methods are based on separation of peaks of oxygenates which frequently tend to overlap Abstract Dublished in Advance ACS Abstracts. June 15. 1995. (1)Europe& Conference on New Fuels and Vehicles for Clean Air, Amsterdam, June 23-24, 1992. (2)Mills, G. A,; Ecklund, E. E. CHEMTECH 1986,9,626. (3)Morandi, F.; Trotta, R.; Pecci, G.; Sposini, M. Energy Prog. 1988, 8 (1). 1. (4)Unzelman. G. H. Oil Gas J. 1988.4 . 35. (5)Sperling, D.; DeLuchi, M. A. Energi 1989,14 (81, 469-482. (6)Luke, L. A.; Ray, J. E. Analyst 1984,7,109. (7)Lockwood, A. F.; Caddock, B. D. Chromatographta 1983,17,65@
fiR
(8) Pauls, R. E.; McCoy, R. W. J . Chromatogr. Sci. 1981,19,558561. (9)Mikio Zimbo, AnaLChem. 1984,56, 224-247. (10)ASTM 4815. (11)Luhe, L. A.; Ray, J . E. Analyst 1989,109,989. (12)Goode, S.R.; Thomas, C. L. J. Anal. At. Spectrosc. 1994,9,7378. (13)Disanzo, F. P. J. Chromatogr. Sci. 1988,41 (l),30. (14)Battiste, D. R.; Fry, S. E.; White, F. T.; Scoggings, M. W.; McWilliams, T. B. Anal. Chem. 1981,53, 1096. (15) Fry, S. E; Fuller, M. P; White, F. T; Battiste, D. R. Anal. Chem. 1983,55,407-408. (16)Garcia, F . X.;Lima, L. D.; Media, J . C. Appl. Spectrosc. 1993, 7,1036-1039. (17)Ron Orlando, J. H.; Hardy, J. K. Anal. Chem. 1986,58,27882791. (18)Shoftashi, J . H.; Hardy, J . K. Anal. Chem. 1986,58, 24122414. (19)Orlando, R.;Munsor, B. Anal. Chem. 1986,58,2788-2791. (20)Renzoni, G. E.; Shankland, E. G.; Gaines, J . A,; Callis, J. B. Anal. Chem. 1985,57,2864-2867.
0887-0624/95/2509-0574$09.00/0
with those from the hydrocarbon constituents of gasoline and thus limit the resolution of separation. The resolution and sensitivity have been improved by solvent extraction,8 dual column GC,6J0 and use of selective detectors like oxygen flame ionization detector (OFID),13 FTIR,21 microwave-induced plasma atomic emission spectrometer (MIP),12 and mass spectr~meterl~ that respond to compounds containing oxygen. The limit of detection of GC-MIP is 100 pg; but the accuracy is dependent upon the purity of He gas used for the production of plasma. The GC-mass method makes use of trimethylsilyl as the reagent gas which reacts rapidly with simple alcohols (C1 t o C4) and ethers (MTBE) without their prior separation. This technique can quantify alcohol and ether in gasoline at 0.5-10.0 vol %. Though these methods are accurate and reliable, they are tedious and time consuming. The IR technique has been applied for the determination of ethanol14and methanol and MTBE.15 These methods involve measurement of absorbance at a characteristic frequency of oxygenate followed by quantitation against a standard sample. Such measurements can be erroneous for the mixture of oxygenates like methanol and ethanol due to overlapping of frequency regions.15 However, with the use of partial least squares (PLS) regression which has been applied t o methanol and MTBE, there is a good scope for further work on mixture of oxygenates.16 Using lH NMR spectroscopy, Renzoni and co-workersZohave established a linear relationship between integral height of a carbon protons and their concentration in SR gasoline. However, it has been observed experimentally in our laboratory that slopes of such calibration curves vary with change in sensitivity of the spectrometer due to change in field stability, room temperature, and frequency of spectrometer. This means that fresh calibration curves may have to be (21)Diehl, J. W.; Finkbeiner, J . W.; Disanzo, F. P. Anal. Chem. 1992, 64, 3202.
1995 American Chemical Society
Determination of Oxygenates in Gasoline
Energy & Fuels, Vol. 9, No. 4, 1995 575
- cn;
- cn PROTONS
II
i l
I 3
I4
AROMATICS
32
PROTONS
OXYGENATES
OLEFINIC
PROTONS
PROTONS
PROTONS
I
I
I I
I
e.0
l 7.0
I
I
I
1
~
,
I
, 6.0
!
,
,
~
5.0
,I
l /
d
I
PPt.4161 4 . 0
,
, ,
I
, ,
3.0
,
,
I
,
'_ I
I I
,
,
,
,
2.0
I
,
,
,
I
'3
1.0
Figure 1. Chemical shift regions of different groups marked in the 'H NMR (60 MHz) spectrum of gasoline containing MTBE.
drawn every time, which may be time consuming for routine analysis. In order t o overcome these problems, a quick and accurate method has been developed based on group molecular weight (GMW) calculations, which has been found to be independent of calibration curves, standards, and nature of gasoline and, moreover, frequency of instrument. The method is very simple, and the NMR instrument alone can resolve, identify, and quantify oxygenates in a sample compared to gas chromatographic methods, where the use of hyphenated techniques like GC-mass, GC-FTIR, and dual column GC become necessary t o obtain better results.
Experimental Section The C1 to C4 alcohols (methanol, ethanol, 2-propanol, and tert-butyl alcohol (TBA)) and ethers, methyl tertbutyl ether (MTBE) and tert-amyl methyl ether (TAME), were of analytical grade. Carbon tetrachloride and tetramethylsilane (TMS) were of spectroscopic grade and were used without purification. Gasolines of different aromatic, paraffin, olefin, and naphthenes [PONAI contents were obtained from refineries. The spectra were recorded on neat samples and also in carbon tetrachloridelchloroformd solution containing TMS as an internal standard on 60 MHz T60A Varian and 300 MHz ACP Bruker instruments. A sweep width of 500 Hz and scanning time of 250 s were used for recording 'H NMR spectra on a continuous wave 60 MHz spectrometer. For spectral recording on 300 MHz FTNMR, the spectral width (SW) 5000 Hz, spectral size (SI) 16K, time domain (TD) 8K, delay time (RD) 10 s, and number of scans (NS) 32 were used and kept the same for each recording. All NMR spectra were integrated thrice and the mean of integrated areas was taken for plotting the calibration curves and estimating total group molecular weight (GMW). Blends of oxygenates in gasolines upto 15% were prepared both by weight and volume.
Results and Discussion The hydrocarbon composition for total aromatics, olefins, and saturates for SR and FCC gasoline samples
Table 1. Chemical Composition of Gasolines (wt %) sample gasoline A gasoline B
saturates
aromatics
41.2 84.5
12.6 15.5
olefins 46.2 nil
determined by the reported method22is given in Table 1. It is quite clear from the NMR spectra of gasoline containing oxygenates that alcohol and ether resonances fall in the region of 3.0-4.4 ppm and do not interfere with gasoline resonances (Figures 1 and 2). Chemical shift data in Figure 2 show that alcohols and ether give separate signals without overlapping. Thus, it is evident that these oxygenates can be identified and quantified accurately when present in a gasoline. It has been observed that as the concentration of alcohol varies from 1t o 15%, the -OH proton overlap with a carbon protons or methyymethylene groups. Since the position of -OH is concentration dependent and for low concentration of alcohols, it may overlap with the methyymethylene signals and interferes in the estimation of group molecular weights. Removal of -OH by D20 exchange has not helped in this case. On addition of an equivalent amount of D20 in the sample, a substantial fall in the intensity of alpha carbon protons was observed due to formation'of separate layer at the top. The layer formation takes place as D2O is insoluble in gasoline and extracts alcohol from the sample and thereby decreases its actual concentration. The effect of dilution on shift in the chemical shift of -OH signal of alcohol has been studied in detail. The data for various alcohols at two different concentrations in the neat sample and their subsequent dilutions in C c 4 are given in Table 2. The chemical shift values of alcohol for neat samples fall in the region of 3.18-4.40 ppm which is free from the signals of gasoline. As the samples are'diluted in the range of 50-75% (v/v), the position of -OH signal was found to shift to the higher field due to decrease in the intermolecular hydrogen bonding. The shift (Ad) is low for methanol (1.84, 0.62 ppm) and 2-propanol (1.5,0.8 ppm) and high for ethanol (2.82, 2.26 ppm) and TBA (r3.0, 1.82 ppm) for 6.0 and (22) Sarpal, A. S.; Apparao, N. V. R. Res. Ind. 1986,31,64-69.
Kalsi et al.
576 Energy & Fuels, Vol. 9, No. 4, 1995
Figure 2. Partial 'H NMR spectrum of FCC gasoline containing (a) 2% MTBE, (b) 4% 2-propanol, (c) 6% ethanol, (d) 4% methanol, and (e) 10% tert-butyl alcohol. Table 2. Chemical Shift Data of -OH Signal of Alcohols at Different Dilutions at 300 MHz
concentration alcohol
neata
50%
66%
75%
Adb(ppm)
4.25 4.45
3.64 4.18
3.02 4.02
2.41 3.83
1.84 0.62
4.32 4.66
2.57 3.49
1.50 3.10
-' 2.40
2.82 2.26
4.00 4.20
3.60 4.00
3.02 3.70
2.50 3.40
1.50 0.80
3.18 3.84 4.40
1.40 2.83 4.21
-'
-c
methanol 6% 12%
ethanol 6% 12%
2-propanol 6% 12%
tert-butyl alcohol 6% 12%
+
methanol(7%) ethanol (7%)
2.41
4.12
2.01 3.81
>3.0 1.82 0.59
Ad = d(neat - 75%). Signal merged a Neat = no dilution. with the -CH2/CH3 signals.
12.0% of alcohols in gasoline, respectively. It is also evident from the chemical shift data a t different dilutions that for neat samples the -OH protons are not likely to overlap with those of either a carbon protons or gasoline. However, the -OH signal of TBA may overlap with gasoline signals for samples containing ethanol > methanol > 2-propanol. It is also evident from Table 1that it is very difficult t o ascertain the exact position of -OH signal in an unknown concentration. For concentration range less than 5%,the -OH signals are more likely to overlap with gasoline signals. Some important findings emerged from this dilution studies which would be very useful for locating the exact position of -OH signal and estimation of group molecular weight of -CH, -CH2, and -CH3 groups accurately. The following procedure should be adopted: 1. Record IH NMR spectrum of a neat sample and identify the nature of alcohol and locate the position of -OH signal. If the signal is found to be merged with the main a carbon proton signals, dilute the sample with CDCldCC14 so that -OH signal comes out and falls in the region free from gasoline signals. 2. If in any case, the -OH signal is found to overlap with the hydrocarbon signal (-CH, -CHz, -CH3) for a neat or diluted sample, its contribution from these signals can be taken out in proportion to the integral area of the main a carbon proton signal by mathematical treatment as explained in Appendix. This procedure will accurately estimate both, group molecular weights and percentage of alcohol. The calculations in Appendix clearly reveals that if -OH merges with gasoline signals, the standard deviation is only 1.25%. From the above exercise, it appears that overlapping of -OH signal with those of gasoline introduces a very small error in the determination of oxygenate content, which may be considered insignificant in light of the high concentration range of alcohols present in gasoline. Before an outline of the present method is described, the oxygenates in the gasolines have been determined by the published method reported by Renzoni and coworkers.20 The difficulties encountered during the determinations have also been mentioned. The reported method by Renzoni and co-workers20 involves plotting of graphs of integral values of a carbon proton versus concentration of oxygenates in gasoline, which gives a straight line over a range of 1-25 ~ 0 1 % . It has been found that the slope of the calibration curves varies with the change in sensitivity of the machine (CW NMR spectrometer where the field is temperature dependent) due to change in room temperature and frequency of NMR spectrometer. Consequently, it is observed that integral values of a carbon proton change t o a large extent for the same solution when recorded after a span of time, say a week or a month (slope as 0.56and 0.76)thereby affecting the repeatability of the results. To circumvent the exercise of plotting standard calibration curves, a new and direct method based on GMW calculation has been developed. This method has been tested on a variety of gasoline/oxygenate blends and found to be applicable t o all types of gasoline samples. The interesting aspect of this method is that the standard calibration curves are not required for the determination of oxygenates. This method has also been found to be independent of the sensitivity of the machine. The outline of the method is given below.
Determination of Oxygenates in Gasoline
~
~
4.0
.
3.5
~
Energy & Fuels, Vol. 9, No. 4, 1995 577
~
3.0
I
2.5
~
~
PPM
2.0
.
1.5
~
1.0
I
~
~
.s
~
0.0
~
I
Figure 3. Partial 'H NMR (300 MHz) spectrum of a blend of FCC gasoline and oxygenates: (a) 2-propanol, (b) ethanol, ( c ) methanol, (d) MTBE, and (e) TAME.
In this method, the first task is to identify the nature of oxygenate from their characteristic chemical shift values and splitting patterns. The chemical shift values of a carbon protons of methanol, ethanol, 2-propanol, MTBE, and TAME are 3.34, 3.58 , 3.9, 3.11, and 3.08 ppm, respectively, and the chemical shift value of hydroxy group (OH) in case of TBA changes from 1.0 to 4.4 ppm, which is found to be concentration dependent. The splitting pattern of each oxygenate is also different and specific (Figure 2). The methanol and MTBE give sharp singlet with different chemical shift values. Similarly, ethanol and 2-propanol could easily be distinguished and identified as the former gives a quartet at 3.58 ppm and the latter gives a septet a t 3.9 ppm. There is very little difference between the chemical shift value of MTBE (3.11) and TAME (3.08). In order t o achieve better resolution, the 300 MHz NMR spectrometer was used for obtaining spectra of blends containing these alcohols (Figure 3). The identification problem is faced in the case of TBA which does not possess any a carbon protons (RO)and, moreover, the chemical shift of the -OH group is concentration dependent. However, this can easily be identified by the -OH group and a sharp singlet at 1.2 ppm. After the identification of oxygenates, integral value is measured for types of hydrocarbons from their chemical shift regions shown in Figure 1. The corresponding integral values of alkyl protons of oxygenates which resonate in the region of saturates are subtracted when calculating the integral values of saturates (CH, CH2, and CH3 groups). The total GMW of aromatics, olefins, saturates, and oxygenates is calculated by the equations given below. The percentage of an oxygenate is then calculated from total GMW and relative GMW of an oxygenate in the sample. 1. Group molecular weight of aromatics (GMWa):
(a) group molecular weight of -CH (6.57.5 ppm) = 131, (1)
(b) group molecular weight of substituents (2.1-3.0 ppm) (CH, from 2.4-3.0 ppm and CH, from 2.1 to 2.4 ppm) = (lU3)/2 (151,)/3 = 71, 51, (2)
+
+
(c) group molecular weight of substituted carbons of aromatic ring = (12IJ2 (1Z4)/3=
+
613 + 414 (3)
Therefore, the relative group molecular weight of aromatics (GMWa) is obtained by adding the eqs 1, 2, and 3; i.e. GMW, = 131,
+ 131, + 91,
(4)
2. Group molecular weight of olefinic groups (4.56.5 ppm):
(5)
GMW,, = 1 3 1 2
In the present study, blends have been prepared in FCC gasoline. However, visbreaker gasoline is also commercially available. This type of gasoline predominantly contains a-olefin of the type -CH=CH2 or -C(CH+CH2 (4.7-5.0 ppm). Accordingly, eq 5a should be used: GMW,, = [I312 = 131,
+ (141',)/21
+ 7r,
(54
where T2 is the integral intensity of the region 4.7-5.0 PPm. 3. Group molecular weight of saturates (GMW,) 0.62.1ppm(CH, 1.5-2.1ppm,-CH2, 1.1-1.5ppm,-CH3, 0.6-1.1 ppm):
+
GMW, = (1315)/1 (1416)/2
+ (1517)/3
= 131, -t 716 -k 51, 4. Group molecular weight of oxygenates:
(6)
~
Kalsi et al.
578 Energy & Fuels, Vol. 9, No. 4, 1995 Table 3. Amount of Oxygenates in FCC Gasoline Determined by the GMW Method (wt %) 2-propanol
ethanol
methanol added
calcd
added
calcd
1.00 2.10 4.20 6.30 8.40 10.5
1.10 2.12 4.25 6.15 8.19 10.26
1.00 2.10 4.20 6.30 8.40 10.5
0.98 2.25 4.56 6.41 8.29 10.51
added -
2.10 4.20 6.30 8.40 10.50
calcd 2.22 4.12 6.51 8.31 10.21
MTBE
TBA added 5.30 7.90 10.50 13.10 15.80
calcd 5.28 8.19 10.25 13.20 15.46
TAME
added
calcd
added
calcd
1.00 2.00 4.00 6.00 8.10 10.10
0.94 2.01 4.20 5.89 7.93 9.96
1.50 1.80 4.20 7.10 9.40 11.20
1.40 2.00 4.00 7.30 9.00 11.10
Table 4. Results for the Mixtures of Oxygenates FCC Gasoline (wt %) mixture methanol methanol methanol methanol methanol methanol
added 5.20 5.20 5.20 5.20 5.20 5.10 5.20 4.90 5.4+5.11+5.35 2.8 5.6 5.7 5.6
+ + + + + +
+ ethanol + 2-propanol + TBA + MTBE + TAME + MTBE + TAME + MTBE + ethanol + 2-propanol GMW,, = MI,Jn
(7)
where M is the molecular weight of oxygenates, n is the number of protons in the alpha carbon group, I,, is the integral intensity of a carbon group of the oxygenate, the numbers 12, 13, 14, and 15 are the molecular weights of -C, -CH, -CHz, and -CH3 groups, respectively, and 11,IZ ..., I,, are the integral intensities (11) of the region as marked in Figure 1. If more than one type of oxygenates are present in a blend, the total relative molecular weight of all is considered. 5. Total group molecular weight of the sample: GMW = GMW,
+ GMW,, + GMW, + GMW,,
(8)
6. Weight percentage of oxygenate = iGMW,, x loo)/ GMW. The methodology has been explained for the determination of methanol in gasoline in the Appendix. The results of oxygenates determined by this method are given in Table 2. The regression analysis showed a correlation coefficient between 0.98 and 1.0 for all types of oxygenates. The method described above can be applied to both FCC and SR gasolines without knowing the source and composition. The change in the composition will not require any change in the methodology because change in composition will change the relative GMW, which will be taken care of by eqs 1-6. Also, it avoids the use of standard calibration curves as the percentage of oxygenate will depend upon its relative GMW,, and its contribution to total GMW of the sample. The blends containing more than 15%wlw of oxygenate were also analyzed. The results showed that there was no limit on concentration ranges. However, alcohols were found to form a separate layer when the concentration exceeded 30% wfw.
Statistical Correlation The above analysis data have been compared with those from the Renzoni method. Fresh calibration curves of concentration vs integral area of each oxygenate in gasoline were plotted. The concentrations of oxygenates in the blends as given in Table 3 were measured using these calibration curves. The data from both the methods have been computed using a computer program developed in Fortran language based on an
+
+ 2.1
calcd 5.02 5.40 4.91 5.26 4.78 5.26 4.78 4.98 5.0+5.6+5.7 2.7 6.3 5.6 5.6
+ + + + + +
+
+ 1.9
Table 5. Detection Limits for Oxygenates (mol %) oxygenate
detection limit
methanol ethanol 2-propanol MTBE TBA TAME
0.10 0.22 0.30 0.33 0.71 0.20
algorithm by Constantinide~.~~ The relationship between the two variables x and y can be expressed in a C, which is called the mathematical form: y = ax regression line of y on x . In the present study, y and x are data from calibration and GMW methods. The extent of relationship between the two variable will be given by correlation coefficient R:
+
R = covarianceiqy)/= where Sx and Sy are the standard deviation of x and y, respectively. The regression analysis between the values by GMW and calibration methods showed correlation coefficient ( R )in the range of 0.98-1.0. This indicates a good agreement between the two methods. Mixture of Oxygenates. We have also quantified mixtures of oxygenates in gasoline by this method. The 300 MHz H NMR spectrum of a blend of five oxygenates in a gasoline shows that each alcohol appears separately and thus can be quantified accurately (Figure 3). The position of the -OH signal varies from 4.0 to 2.6 ppm as the neat sample containing these alcohols is diluted with carbon tetrachloride from 0 t o 80% vfv, as shown in Figure 3. On further dilution, the -OH signal was found to overlap with -CH, CHZ, -CH3 proton resonances. The mixtures of MTBE and TAME, which were poorly resolved by the 60 MHz instrument, have been completely resolved by the 300 MHz instrument. The results of blends containing two or more oxygenates in a gasoline are given in Table 4. Again the agreement is quite convincing. The detection limit for different oxygenates in gasoline have been calculated and are given in Table 5. The detection limit for tertbutyl alcohol is the highest while for methanol it is the lowest. The sharp singlet representing three protons of the methyl group in contrast t o multiplet of other oxygenates contributes to the high signal-to-noise (Sf N) ratio and, therefore, low detection limits for methanol. ( 2 3 )Constantinides, A. Applied Numerical Methods with Personal Computer; McGraw-Hill: New York, 1987.
Determination of Oxygenates in Gasoline
Energy & Fuels, Vol. 9, No. 4, 1995 679
Table 6. Detailed Method for the Estimation of Group Molecular Weights and Percentage of Methanol in Gasoline= chemical shift regions (61, ppm gasoline region group IR GR GMW
6.5-7.5 arom (-CHI 3 3 39
methanol
2.4-3.0 a-CH2
2.1-2.4 a-CH3
-C-
4.5-6.5 CH=CH
1.5-2.0 -CH
1.0-1.5 -CH2
0.6-1.0 -CH3
3.34 -0CH3
1 0.5 7
7 2.3 34.5
2.8 33.6
7 7 91
29 29 377
13 6.5 91
29 9.7 145.5
6.5 2.16 69.3
a IR = integral ratio; GR = group ratio; GMW = group molecular weight = GR x molecular weight of groups, e.g., GMW of methanol = (6.5/3) x MW of methanol (32) = 69.33.
Sources of Errors. The accuracy of determination depends upon the accuracy in the measurement of integral area of each region in a spectrum. This can be achieved by proper base line correction and better signal to noise ratio (S/N). The errors can be minimized by integrating the spectrum thrice and taking the mean value for group molecular weight calculations. The errors in determination may be introduced if the oxygenate concentration is less than 1% and spectrum of a dilute solution is recorded. This is particularly true for the CW NMR spectrometer where a noisy signal will be obtained. In such cases integral areas will not be reproducible. The best solution will be t o record the spectra on an FTNMR instrument with increased number of scans. Generally, the problem of rolling base line is experienced when neat or very concentrated solutions are recorded on FTNMR spectrometer which results in the nonreproducibility of integral areas. In such cases, analysis should be carried out on a CW NMR instrument. When MTBE and TAME are required to be analyzed together, the spectrum should be expanded horizontally to observe both the signals clearly as shown in Figure 3. This will reduce the error in determination. If the TBA is required to be estimated in the presence of other alcohols, then the signal at 1.2 should be considered for the determination of GMW. Conclusions A direct, quick, and accurate method based on group molecular weight (GMW) has been presented for the determination of oxygenates in a gasoline. The method is independent of the nature and source of gasolines and
standard calibration curves of each oxygenate are not required. The method is also applicable to samples containing mixtures of oxygenates in a gasoline. A blend containing TAME and MTBE gives complete separation of resonances corresponding to each with the higher frequency NMR spectrometer.
Appendix Calculations (Table 6).
GMW, (eq 4) = 39
+ 7 + 34.5 + 33.6 = 114.1
GMW,, (eq 5) = 91 GMW, (eq 6) = 377
+ 9 1 + 145.5 = 613.5
GMW,, (eq 7) = 69.3 total GMW (eq 8) = 114.1
+ 9 1 + 613.5 + 69.3 = 887.9
Case 1. If -OH is not overlapped, % oxygenate (methanol) = (69.3B87.9) x 100 = 7.8. Case 2. If -OH is overlapped with methyl signals (0.6-1.0 ppm), integral of -CH3 = 29 - 2.16 = 26.84 (because integral of -OH = 2.16). Therefore, GMW of the -CH3 group = (26.84/3) x 15 = 134.2. Total GMW = 876.6; and % methanol = (69.3B76.6) x 100 = 7.9. Case 3. If -OH signal is overlapped with -CH2 or -CH protons, similarly, % methanol = 7.94 or 8.05. Average % of methanol = 7.92; standard deviation = 0.103 (1.28%). EF9401810
