662
Energy & Fuels 1997, 11, 662-667
Estimation of Oxygenates in Gasoline by Spectroscopy
13C
NMR
A. S. Sarpal,* G. S. Kapur, S. Mukherjee, and S. K. Jain Research & Development Centre, Indian Oil Corporation Ltd., Sector-13, Faridabad 121 007, India Received December 5, 1996. Revised Manuscript Received March 5, 1997X
A direct and quick method for the estimation of alcohols (C1-C4) and ethers (C5-C6) has been developed by using 13C NMR spectroscopy. The first method involves the calibration procedure, where the percentage integral areas of oxygenates in their characteristic R-carbon signals (5075 ppm) versus concentration give a linear curve. The slope of the curve for all types of oxygenates has been found to be independent of the nature of gasoline (FCC, SR, hydrotreated, etc.) and also applicable in the case of samples containing mixture of alcohols and ethers. The second method is based on the estimation of group molecular weight (GMT) of blended gasoline containing oxygenates from their respective integral areas in the 13C NMR spectra. No standards are required in the case of the group molecular weight procedure. Both the methods have given very accurate results for methanol (MeOH), ethanol (EtOH), propanol (PrOH; normal and iso), butanol (BuOH; normal and tertiary), methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) present as a single or mixture of oxygenates in a blended gasoline samples. The pair of alcohols, such as EtOH, n-PrOH, and n-BuOH or t-BuOH, which are not possible to estimate by 1H NMR, have been quantified by the 13C NMR method. The optimum conditions for obtaining quantitative spectra have also been discussed.
Introduction In our earlier work on the estimation of oxygenates in a blended gasoline by 1H NMR spectroscopy,1 problems related to overlapping of signals and dilution effects were experienced and accordingly their solutions were found. For example, estimation of t-BuOH was carried out from the -OH signal as the R-carbon proton signals (δ 1.20) merged with the signals of gasoline. However, if water is present, this will lead to erroneous results. Also, when t-BuOH is present along with other alcohols, its estimation was not possible. Blends containing methanol or ethanol are generally hygroscopic and may eventually absorb water to cause phase separation. A suitable cosolvent such as 1-propanol, or 1- or 2-butanol (2% v/v) are added along with methanol or ethanol as stabilizing agents.2 These blends can not be quantified by 1H NMR because of overlapping of signals of ethanol with those of 1-propanol or 1-butanol. The poor resolution of mixture of oxygenates like methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME), ethanol (EtOH) and ethyl tert-butyl ether (ETBE), ethanol (EtOH) and 1-propanol (n-propyl alcohol, n-PrOH), and ethanol (EtOH) and 1-butanol (nbutyl alcohol, n-BuOH) posed difficulty in their exact quantitation by 1H NMR method. The position of -OH signal in the 1H NMR spectrum was found to be dependent upon the concentration of alcohols added in a blended gasoline (4-1.0 ppm). Sometimes, the -OH signal was found to be merged with the signals of alcohol (R-carbon) and/or gasoline Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Kalsi, W. R.; Sarpal, A. S.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Energy Fuels 1995, 9, 574. (2) Motor Gasoline Specifications 1995, 15, 2796. X
S0887-0624(96)00215-0 CCC: $14.00
(3-0.5 ppm). In order to locate its position, the sample was required to be recorded at number of dilutions. No doubt 1H NMR procedure is direct, convenient, and less time consuming compared to other analytical procedures, yet the method is inherent with scores of limitations as discussed above. In order to overcome the above-mentioned problems and to estimate all possible potential oxygenates present separately or as a mixture, the technique of 13C NMR spectroscopy has been used. Nine different types of alcohols and ethers as mentioned in the Experimental Section have been estimated in the gasoline sample. Two methods based on calibration and group molecular weight procedure have been presented. The methods are independent of the nature of gasoline, viz., FCC, SR, hydrotreated, and their blends. Experimental Details Gasoline Samples. The gasoline samples, viz., FCC, SR, HAN, and hydrotreated, were obtained from Indian refineries. Oxygenates. Methanol (MeOH), ethanol (EtOH), 1-propanol (n-PrOH), 2-propanol (isopropyl alcohol, i-PrOH), 1-butanol (n-BuOH), tert-butyl alcohol (t-BuOH), methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) were of spectroscopic grades and used as such. Blends. Blends of oxygenates and gasoline were prepared as per the details given in the Experimental Section of ref 1. Quantitative NMR Spectra. 1H-NMR. 1H NMR spectra of around 10% solution of a sample in deuterated chloroform (CDCl3) were recorded on a 300 MHz NMR spectrometer operating at 300 MHz frequency. The spectral parameter used were as follows: number of scans NS ) 64, relaxation delay 10 s, and spectral size 16K with time domain size 8K. All the chemical shift values are reported with respect to tetramethylsilane (TMS) equals 0.0 ppm as internal standard. © 1997 American Chemical Society
Estimation of Oxygenates in Gasoline by
Figure 1.
13C
13C
NMR
Energy & Fuels, Vol. 11, No. 3, 1997 663
NMR spectrum of a gasoline sample blended with eight different types of oxygenates.
13C
NMR. 75.4 MHz 13C NMR spectra of the samples were recorded at around 30-40% solution in CDCl3. The quantitative spectra were obtained in the inverse gated mode and using 0.1 M Cr(acac)3 as relaxation agent. Under these conditions reproducible quantitative spectra were obtained. The spectral parameters were as follows: number of scans ) 3000, relaxation delay 5 s, and spectral size 16K with time domain size 8K. Spectral Integration. The spectra were integrated thrice between 5 and 160 ppm and the mean value of each region was used for the calculations. In order to measure the integral area of an oxygenate, the region 5-75 ppm was expanded horizontally and vertically. The spectral region 5-75 ppm was integrated 10-20 times the normal integral of the full spectrum and mean of the three values were used for further calculations. Distortionless Enhancement by Polarization Transfer (DEPT). 13 C NMR 135° DEPT spectra were recorded using the pulse sequence of Doddrel et al.3,4
Results and Discussion Identification of Oxygenates. 13C NMR chemical shift data of oxygenates were generated by preparing the blends (5% w/w) of each in gasoline and subsequently recording the NMR of their 30-40% v/v solution in CDCl3. Figure 1 shows the 13C NMR spectrum of a gasoline sample containing eight different types of oxygenates. The characteristic 13C NMR chemical shift data for various alcohols and ethers used as oxygenates is also given in Table 1. It is evident from the spectrum and the chemical shift data that all the alcohols and ethers give resolved signals in the region of 46-74 ppm, which are well separated from the signals of gasoline. Also, the spread of chemical shift is very large (∼25 ppm) compared to proton chemical shift (0.9 ppm). The characteristic R-carbon signal of each of the alcohols and ethers are well separated from each other and thus can conveniently be identified even in unknown blends. The chemical shift difference between n-PrOH and i-PrOH (3) Sarpal, A. S.; Kapur, G. S.; Chopra, A.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Fuel 1996, 74 (4), 483. (4) Doddrell, D. M.; Peg, D. T.; Bendall, M. R. J. Magn. Reson. 1982, 48, 323.
Table 1. Characteristic 13C/1H NMR Chemical Shift Data Generated on Gasoline-Oxygenated Blendsa oxygenate
13C
1H
methanol (MeOH) ethanol (EtOH) 1-propanol (n-PrOH) 2-propanol (i-PrOH) 1-butanol (n-BuOH) isobutyl alcohol (i-BuOH) tert-butyl alcohol (t-BuOH) sec-butyl alcohol (s-BuOH) methyl tert-butyl ether (MTBE) tert-amyl methyl ether (TAME) ethyl tert-butyl ether (ETBE)
49.7 57.6 63.9, 9.8b 63.7 62.0 68.9 67.9 68.7, 9.9b 72.2, 48.6b 74.1, 48.2b 71.9, 56.8c
3.34 3.58 3.65 3.90 3.65 3.60 1.20a 3.70 3.10 3.08 3.48
a The chemical shift values are given for R-carbon and protons. Chemical shift values for the methyl group. c Chemical shift value for the methylene carbons.
b
is 0.2 ppm only. The identification in these cases has been facilitated by DEPT 135° experiments. Figure 2 shows the expanded 13C NMR spectrum of the blended sample in the region 40-76 ppm along with the corresponding DEPT 135° spectrum. The DEPT spectrum is very useful in the identification of different oxygenates as signals due to CH/CH3 carbons and those due to CH2 carbons appear antiphase, whereas signals due to quaternary carbons nullify. The R-carbon signal of 1-propanol will appear slightly up field and antiphase to that of 2-propanol. 1-Propanol can also be identified from its methyl carbon signal at δ 9.8, which is also free from the methyl carbon signals of gasoline. tert-Butyl alcohol gives a well-separated signal at δ 67.9 in contrast to 1H NMR resonance (δ 1.20) and thus can easily be identified and quantified. It is also evident from Table 1 that the mixture of oxygenates can also be identified and quantified from the 13C NMR spectrum, as the difference in their respective chemical shift is large. For instance, MTBE and TAME appear at δ 72.2 and 74.1, respectively, in the 13C NMR spectra, in contrast to the 1H NMR chemical shift δ 3.10 and 3.05. Blends of any of the isomers of butanol with methanol or ethanol in gasoline also give well-resolved signals and thus pose no difficulty in their estimations. Similarly, blends of ethanol,
664 Energy & Fuels, Vol. 11, No. 3, 1997
Sarpal et al.
Figure 2. (a) Expanded 13C NMR spectrum of the gasoline sample blended with eight different oxygenates. (b) Corresponding DEPT 135° 13C NMR spectrum. 1H
NMR spectrum and resolving power of the 13C NMR spectrum is self-evident. The effect of changing the hydrocarbon composition of gasoline on the chemical shifts of oxygenates was studied. The blends were prepared in FCC, SR, hydrotreated, HAN, and LAN. No changes in the chemical shift values were observed. Also, the mixture of alcohols show little variation (0.1-0.3 ppm) in their respective chemical shift values compared to blends containing a single oxygenate. Quantitative Analyses. Two methods based on calibration procedure and group molecular weights have been developed. 1. Calibration Procedure. In this method, plots of concentration (C) of the oxygenate versus percentage integral intensity (I0) were drawn and the following linear relationship was obtained:
C ) kI0
Figure 3. (a) Expanded 1H NMR spectrum of gasoline blended with ethanol, 1-propanol, and 1-butanol. (b) Corresponding expanded 13C NMR spectrum.
1-propanol, and 1-butanol can easily be identified as their signals are well separated. Figure 3 shows the 1H and 13C NMR spectrum of a gasoline sample containing an oxygenate mixture consisting of ethanol, 1-propanol, and 1-butanol. The poor resolution of the
(1)
where k is the slope of line with respect to a particular oxygenate. I0 is the percentage integral intensity of the characteristic 13C NMR signal (Table 1) of the oxygenate. Figure 4 shows the calibration curve drawn for different types of oxygenates blended in a gasoline sample. The slope of curve, k, for different types of oxygenates as estimated from the regression analyses are given in Table 2. Different values of k were obtained for different oxygenates. This is because of the fact that the integral intensity due to a particular oxygenate will depend upon its amount in the blend, total number of carbon atoms it contains, and its molecular weight. Therefore, a gasoline blend containing the same amount
Estimation of Oxygenates in Gasoline by
13C
NMR
Energy & Fuels, Vol. 11, No. 3, 1997 665
the blended gasoline containing oxygenates can be given as
TGMW ) KIT + 17I0
(2)
where IT is the total integral intensity in the region 2-160 ppm (excluding solvent signals). I0 is the integral intensity of the R-carbon of oxygenates in the region between 49 and 74 ppm (Table 1). The coefficient K represents the group molecular weight of the gasoline and is given as
K ) 12Cq + 13CqH + 14CqH2 + 15CqH3
Figure 4. Calibration curve for the estimation of various oxygenates blended in gasoline as per eq 1: (O) MeOH, (4) EtOH, (0) TBA, (*) MTBE, (+) IPA, (]) TAME, (9) ETBE, and (b) BuOH. Table 2. Values of the Constant k (Eq 1) for Different Types of Oxygenates Blended in Gasoline oxygenate
k
oxygenate
k
methanol ethanol 2-propanol tert-butyl alcohol
2.19 2.70 3.94 4.55
n-butyl alcohol MTBE ETBE TAME
5.58 6.05 7.03 7.15
Table 3. Hydrocarbon Composition of Gasoline Samples by 1H/13C NMR Spectroscopy (% w/w)a sample
A
O
P
C
CH
CH2
CH3
K
G-369 G-37 G-353 G-363 G-FCC GB G-17 GRMT GFCC GFCCN GBL2 SRN
26.5 17.6 14.6 13.0 7.0 11.4 24.6 64.8 15.6 16.7 16.6 10.5
16.2 28.2 18.4 14.2 55.1 nil nil nil 57.9 63.0 28.3 nil
57.3 54.2 67.0 72.8 37.9 88.6 75.4 35.2 26.5 20.3 55.1 89.5
6.0 4.9 5.5 5.0 5.0 4.0 5.7 18.8 9.3 8.6 5.4 1.4
22.2 20.6 21.5 24.4 35.0 23.7 10.2 45.6 26.7 24.5 19.6 13.0
48.7 51.7 49.6 46.3 32.0 49.6 65.7 13.0 37.2 41.5 47.3 60.4
23.1 22.8 23.4 24.3 28.0 22.7 18.4 22.5 26.8 25.4 27.7 25.2
13.9 13.9 13.9 13.8 13.8 13.9 14.0 13.3 14.0 13.9 14.0 14.1
a
A ) aromatics, O ) olefins, P ) parafins. C, CH, CH2, and CH3 are the percentage of quaternary, methine, methylene, and methyl carbons, respectively. K is the factor as described in eq 3.
(weight percent) of two different oxygenates will show different integral intensities as their molar concentration will be different. Similar curves were also obtained for blends containing mixtures of oxygenates; however, no change in the values of k was obtained. Effect of Gasoline Composition. Blends of oxygenates, MeOH, EtOH, MTBE, n-BuOH, and n-PrOH were also prepared in five different types of gasoline having different hydrocarbon compositions (Table 3). Only very marginal changes in the slope values were observed, indicating that the method is independent of the hydrocarbon composition of the gasoline. 2. Group Molecular Weight Method. This method does not require the generation of a calibration curve prepared using standards. The outline of the method is as follows. The relative average group molecular weights of the blended gasoline and oxygenates are determined from their respective relative integral areas by using eqs 2, 3, and 4. The total group molecular weight (TGMW) of
(3)
The numbers 12, 13, 14, and 15 are the molecular weights of the CHn groups, where n ) 0, 1, 2, and 3. The q means percentages of CHn groups in the sample. The number 17 appearing in eq 2 is the molecular weight of the OH group. The quantities CqHn (n ) 0-3) are generally estimated by DEPT techniques.4 In the present study, both DEPT and direct approaches3 have been used for the estimation of CqHn quantities for variety of gasoline having varying hydrocarbon composition (Table 3). The values of K estimated by eq 3 are also given in the same table. It is very interesting to note that for different classes of gasoline, the value of K varies between 13.8 and 14.1. The exception is the sample GRMT, which is a reformate gasoline, where the value of K is 13.3. Thus, the average value of K ) 14.0 can be used in eq 2, which will make the equation further simpler.
TGMW ) 14IT + 17I0
(4)
Similarly, the group molecular weight of the oxygenate (GMWO) is estimated by eq 5.
GMWO ) MI0
(5)
where M is the molecular weight of the alcohol or ether. The content of the oxygenate is then estimated by eq 6.
% oxygenate (O) ) [GMWO/TGMW] × 100 (6) In the case of samples containing a mixture of oxygenates, the group molecular weight of each is estimated separately by applying eq 5 and their respective percentages are estimated by eq 6. The method has been demonstrated in the Appendix for a mixture of oxygenates blended in a gasoline sample. The quantitative data of blends of single oxygenate in gasoline estimated by using eqs 3-5 is given in Table 4. The method works equally well when mixture of oxygenates are blended in a gasoline, and the data is given in Table 5. The data shows that there is excellent agreement between the calculated and added values for all the different types of oxygenates in a blended gasoline, present as single or a mixture. The blends of ethanol and 1-butanol or propanol, which cannot be estimated by 1H NMR, are estimated by 13C NMR. The effect of changing the hydrocarbon composition on the reliability of data estimated by the group molecular method has also been studied in a similar way as demonstrated for the calibration procedure. The method has been found to be independent of the nature of gasoline as demonstrated in Table 6 for the estimation of methanol blended in five different gasoline
666 Energy & Fuels, Vol. 11, No. 3, 1997
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Table 4. Estimation of Single Oxygenate in Gasoline by MeOH
EtOH
i-PrOH
t-BuOH
13C
NMR Group Molecular Weight Methoda
n-BuOH
TAME
Add
Cal
Add
Cal
Add
Cal
Add
Cal
Add
Cal
Add
2.8 5.6 9.0 13.8 18.9
2.3 5.1 8.4 13.4 19.0
2.9 5.7 9.1 13.9 19.1
2.5 5.9 9.6 14.5 19.6
1.4 2.5 5.1 7.6 10.3
1.3 2.5 5.0 7.6 10.7
1.4 2.8 4.4 6.8 9.2
1.3 2.8 4.0 6.8 9.4
2.0 4.1 6.3 8.1 11.0
2.1 4.3 6.2 7.9 10.9
2.3 4.2 6.1 8.1 10.2 12.5
a
Cal 4.1 6.4 6.1 9.7 12.5
MTBE
ETBE
Add
Cal
Add
Cal
2.0 4.0 6.5 8.5 10.1
1.7 3.3 6.2 8.0 9.8
2.1 5.4 6.9 9.9 14.9
2.0 5.3 7.2 9.2 15.0
Add ) added percentage of oxygenates. Cal ) calculated percentage of oxygenates by NMR method. Table 5. Estimation of Mixture of Oxygenates in a Commercial Gasoline by 13C NMR (% w/w)a Blend 1 TAME
MTBE
t-BuOH
i-PrOH
1.3 (1.0) 4.9 (4.5) 9.9 (9.5)
1.4 (0.9) 5.1 (4.5) 10.3 (9.8)
1.4 (1.4) 5.1 (5.1) 10.3 (11.2)
1.4 (1.3) 5.1 (5.0) 10.3 (10.7)
Blend 2 EtOH
n-BuOH
n-PrOH
9.3 (9.4)
4.1 (4.3)
3.9 (3.8)
Blend 3 MeOH
EtOH
i-PrOH
TAME
MTBE
t-BuOH
5.6 (5.3)
3.9 (4.0)
4.5 (4.2)
4.1 (3.8)
3.9 (3.7)
3.6 (3.4)
a
Values in parentheses are calculated values. Table 6. Estimation of Methanol (wt %) Blended in Different Gasoline Samples (Table 3)a
Add Cal
GBL2
GFCCN
GFCCC
SRN
GB
6.5 6.9
7.0 6.6
6.6 6.4
6.3 7.0
5.6 5.3
a Add ) added percentage of methanol. Cal ) calculated percentage of methanol by NMR method.
samples. Changes in the hydrocarbon composition will relatively change the total group molecular weight of both the blends and oxygenate and, therefore, will not affect the results in the case where the hydrocarbon composition of the gasoline is not known. As a similar procedure is used for estimating the content of different alcohols and ethers, a common correlation was drawn between the added and calculated values of oxygenated content. Figure 5 shows the correlation plot between added percentages of oxygenates versus those calculated by the 13C NMR method with a correlation coefficient of 0.99 and standard error of estimation equals 0.4. Important Instructions. In order to estimate the amount of a particular oxygenate by the calibration or group molecular weight methods, it is necessary to follow the procedure in total and thoroughly. The most important is the generation of quantitative 13C NMR spectra. This can be achieved by using the calculated amount of relaxation agent, Cr(acac)3. Since the relaxation times of quaternary carbons of aromatics hydrocarbons are much higher than those of paraffinic hydrocarbons, it is advised to use higher amount of relaxation agent and optimum delay time (D1). The 0.1-0.15 M Cr(acac)3 has been found to be sufficient, irrespective of the nature, source, and composition of
Figure 5. Correlation plot between the added and calculated percentage of oxygenates (eq 5). Correlation coefficient ) 0.99, standard error of estimation ) 0.4.
gasoline. The concentration of oxygenate will not matter as the relaxation times of R-carbons are much shorter compared to hydrocarbons. The delay time of 4-5 s along with 0.1-0.15 M relaxation agent is sufficient to overcome the nuclear Overhauser effect (NOE). The second most important parameter is the integral areas of oxygenates. Since the amount of oxygenates and cosolvents is 2-15% w/w in gasoline, it would be required to measure 0.2-9% of the total integral areas of the sample for ethers and alcohols. Thus, the minimum requirement will be to read or measure 0.4 cm. height of the integral, which may lead to erroneous results. Therefore, it is advised to amplify the normal integral by 20-30 times and repeat at least thrice to get the average integral. The base-line and phase correction and higher signal to noise ratio of the spectrum will also enhance the accuracy of the results. The number of scans should be increased in the case of samples containing low content of oxygenates (1-5%). Choice of the Method. From our earlier1 and present work it is quite evident that these methods offer a direct, accurate, and fast method for the identification and quantitation of all potential oxygenates in a blended gasoline. The 1H NMR method is less time consuming compared to the 13C NMR method as the recording time is much higher for obtaining the quantitative 13C NMR spectra. Therefore, it must be preferred over 13C NMR
Estimation of Oxygenates in Gasoline by
13C
NMR
Energy & Fuels, Vol. 11, No. 3, 1997 667
methods for gasoline containing a single oxygenate and for samples containing few of the mixtures of oxygenates as discussed in the Introduction. The 13C NMR method is a very convenient choice for both single and mixture of oxygenated gasoline, particularly blends containing ethanol and all isomers of butanol and 1-propanol. The 13C NMR method based on relative group molecular weight should be given preference compared to the one based on calibration procedure, as the slope of a curve may change in cases of uncontrolled gasoline containing a portion of higher boiling hydrocarbons than the specified range. This will introduce serious errors in the results. As discussed, the group molecular weight method is independent of the nature of the gasoline as these changes would be well taken care of in the estimation of group molecular weight.
and quantified by the 13C NMR method. The method has been found to be independent of the nature and composition of gasoline.
Conclusions
I0 (n-BuOH, 62.0 ppm) ) 1.3
Appendix The following is the procedure for the estimation of oxygenates in a gasoline containing 1-propanol (nPrOH), 1-butanol (n-BuOH), and ethanol (EtOH) (Table 5) using the group molecular weight 13C NMR method.
IT (5-63 + 105-160 ppm) ) 148.3 I0 (EtOH, 57.6 ppm) ) 4.5 I0 (n-PrOH, 63.9 ppm) ) 1.4
13C
NMR methods can be applied with great ease and convenience for the estimation of alcohols and ether types of oxygenates (Table 1) in a blended gasoline. Both the calibration and group molecular methods can cover a large range (1-20%). These methods can be used on any NMR spectrometer irrespective of the frequency, as the slope and group molecular weights are dependent only upon the relative integral areas of the oxygenates and gasoline samples. The mixture of oxygenates, particular ethanol and propanol or butanol, can easily be analyzed. tert-Butyl alcohol, whose 1H NMR resonance signals are overlapped with those of gasoline, can easily be identified
total I0 for oxygenates ) 7.2 TGMW ) 14 × 148.3 + 17 × 7.2 ) 2198.6 (eq 4). GMWO of ethanol, propanol, and butanol ) 4.5 × 46, 1.4 × 60, and 1.3 × 74 respectively (eq 5).
% ethanol ) [4.5 × 46/2198.6] × 100 ) 9.4% (eq 6) % 1-propanol ) [1.4 × 60/2198.6] × 100 ) 3.8% % 1-butanol ) [1.3 × 74/2198.6] × 100 ) 4.3% EF960215N
