Energy & Fuels 1998, 12, 1223-1227
1223
Estimation of Total Aromatics and Their Distribution as Mono and Global Di-Plus Aromatics in Diesel-Range Products by NMR Spectroscopy V. Bansal, G. S. Kapur, A. S. Sarpal,* V. Kagdiyal, S. K. Jain, S. P. Srivastava, and A. K. Bhatnagar Indian Oil Corporation Limited, Research & Development Centre, Sector-13, Faridabad, Haryana, India Received March 16, 1998
In the present work, a new method based on 1H NMR spectroscopy has been developed for the estimation of total aromatics and their distribution as mono- and polynuclear (di-ring plus) aromatics in diesel-range fuel products. Multipulse NMR techniques such as, distortionless enhancement by polarization transfer and 2-dimensional heteronuclear correlation have been applied for the unambiguous assignment of the 2.0-3.5 ppm region due to R-substituents on the aromatic ring in the 1H NMR spectra. The estimation of polynuclear aromatics is based on the estimation of bridgehead aromatic (Arb) and substituted aromatic (Arq) carbons using equations developed. The proposed 1H NMR-based method correlates very well with the standard IP-391/ 90- and mass-spectrometric-based method (R2 ) 0.99).
Introduction Various performance characteristics and properties of diesel-range fuel samples (140-350 °C), such as cetane number, pour point, combustion behavior, etc., depend on their hydrocarbon composition. Diesel-fuel quality also influences emissions, and the diesel aromatic content is one such quantity which has been identified to have the greatest impact. Also, the amount of polyaromatics in such fuels is particularly important for environmental reasons because these compounds are toxic or carcinogenic. To better understand the relationships between the composition and various physical/ chemical properties of diesel fuels there is clearly a need for a robust and reliable analytical method for precise determination of total aromatics and aromatic types (mono-, di-, and polyring) content. There are various standard methods available for the determination of hydrocarbon types in liquid petroleum products.1 These are the FIA method based on elution chromatography (ASTM-1319/88), liquid chromatography (LC; IP-391/90), mass spectrometry (MS; ASTM D-2425/83, D-3239/86), and supercritical fluid chromatography (SFC; ASTM-D 5186/91). Methods based on the above techniques are well established and are now routinely used in various laboratories. However, they are quite laborious, time-consuming, and demand utmost care on the part of the analyst. Besides this, the methods require various standards for calibration. Bundt et al.2 used solid-phase extraction to separate polycyclic aromatic hydrocarbons in diesel by hydrocarbon type and analyzed the resulting fractions by capil(1) Drews, A. W. ASTM Manual on Hydrocarbon Analysis, 4th ed.; ASTM: PA, 1989. (2) Bundt, J.; Herbel, W.; Steinhart, H.; Francke, W. J. High Resolut. Chromatogr. 1991, 14 (2), 91-98.
lary GC-FID and GC-MS. Aromatics in the middle distillate have been determined by Fuhr et al.3 using SFC-FID with two packed-column systems. Chen et al.4 have proposed a new SFC method for determining the aromatic ring distribution in diesel and validated the method using MS. There is an HPLC-GC technique by Trisciani and Munari5 that allows for better separation and characterization of diesel fuels. Li et al.6 made improvements in group-type separation of diesel fuels using packed capillary column SFC. Malhotra et al.7 reported the feasibility of FIMS in combination with GC for rapid and quantitative analysis of refined hydrocarbon fuels. Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool in fuel analysis as well.8-16 However, the technique has rarely been applied for the quantita(3) Fuhr, B. J.; Klein, L. L.; Reichert, C.; Lee, S. W. LC-GC 1990, 8 (10), 800, 802-804. (4) Chen, E. N., Jr.; Cusatis, P. D.; Popid, E. J. J. Chromatogr. 1993, 637 (2), 181-186. (5) Trisciani, A.; Munari, F. J. High Resolut. Chromatogr. 1994, 17 (6), 452-6. (6) Li, W.; Malik, A.; Lee, M. L.; Jones, B. A.; Porter, N. L.; Richter; B. E. Anal. Chem. 1995, 67 (3), 647-654. (7) Malhotra, R.; Coggiola, M. A.; Young, S. E.; Spindt, C. A. Presented before the Division of Petroleum Chemistry, Inc., 212th National Meeting, American Chemical Society, Orlando, FL, Aug 2529, 1996. (8) Myers, M. E., Jr.; Stollsteimer, J.; Wims, A. M. Anal. Chem. 1975, 47 (12), 2010-2015. (9) Von Deutsch, K. J. Prakt. Chem. 1977, 319, 439-443. (10) Ozubko, R. S.; Clugston, D. M.; Furimsky, E. Anal. Chem. 1981, 53, 183-187. (11) Muhl, J.; Srica, V.; Mimica, B.; Tomaskovic, M. Anal. Chem. 1982, 54, 1871-1874. (12) Netzel, A.; Daniel; Thompson, F. L. Fuel 1986, 65 (4), 597598. (13) Cookson, D. J.; Smith, B. E. Energy Fuels 1987, 1, 111-120. (14) Glavincevski, B.; Gulder, O. L.; Gardner, L. Presented at the American Chemical Society Symposium, Miami, FL, Sept. 10-15, 1989. (15) Lee, S. W.; Coulombe, S.; Glavincevski, B. Energy Fuels 1990, 4, 20-23.
10.1021/ef980052y CCC: $15.00 © 1998 American Chemical Society Published on Web 10/17/1998
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tive hydrocarbon type analysis of petroleum fractions. A method for the gasoline analysis using NMR has been discussed by Myers et al.,8 whereas Von Deutsch9 has established relationships by which the aromatic and paraffinic hydrocarbon content could be determined for petroleum fractions above 220 °C. Muhl et al.11 extended the methodology used by Von Deutsch and proposed a method for determining aromatics using 1H NMR and boiling-range values of the fraction. The onedimensional and two-dimensional NMR methods have been used to elucidate the structural characteristics of a set of monoaromatic fractions separated from petroleum fuels.13 A simple 1H and 13C NMR method has been reported for estimation of the molar content of the total aromatics in transformation fuels.14 For measuring aromatics in distillate fuels, Lee et al.15 coordinated a study that compares SFC, GC-MS, and NMR techniques. The work done by G. Dosseh16 describes a strategy based on short- and long-range heteronuclear shift correlation 2D spectroscopy, which allows complete and unambiguous identification of the aromatic components in four intermediate boiling fractions of an crude oil. In another recent study by Doan et al.,17 TOCSY 2D NMR has been applied for the analysis of polyaromatics in crude gas oil mixtures. In this work, a method based on 1H NMR spectroscopy has been developed for estimation of the total aromatics and their distribution as mono- and polynuclear (di-ring plus) aromatics. The multipulse NMR techniques, such as DEPT (distortionless enhancement by polarization transfer) and 2D-HetCor (heteronuclear correlation) have been applied for the unambiguous assignment of the 1H NMR spectra, particularly the 2.0-3.5 ppm region. Experimental Section Materials. Various samples of high-speed diesel (HSD), superior kerosine oil (SKO), coker kero, etc., in the boiling range of 140-350 °C were obtained from different Indian refineries. Components used for making blends, e.g., ethylbenzene, o-diethylbenzene, n-propylbenzene, amylbenzene, tert-butylbenzene, m- and p-xylenes, mesitylene, 1,2,4-trimethylbenzene, 1,2,3,4-tetramethylbenzene, diisopropylbenzene, 1-methylnaphthalene, dimethylnaphthalene, biphenyl, dibenzyl, and anthracene, were procured from Aldrich Chemicals and used as received. NMR Measurements. NMR spectra have been obtained on a 300 MHz NMR spectrometer at room temperature under the following conditions. 1H NMR: spectral width, 5000 Hz (∼12 ppm); spectral size, 16 K. Digital resolution, 0.49 Hz/ point; 90° pulse, 18 µ; relaxation delay, 10 s; number of scans, 64; solution concentration, 5% in CDCl3; reference, TMS; no paramagnetic reagent. DEPT 13C and 2D HetCor NMR spectra were recorded by using the pulse sequence of Doddrell et al.18 and Bax et al.19 using the following parameters. DEPT: spectral width, 20 000 Hz (∼200 ppm); spectral size, 16K; 135° 1H decoupler pulse, 18.8 µ 90° 13C observation pulse, 6 µ 180° 13C observation pulse, 12 µ D (0.5/J 2 XH), 3.45 ms; relaxation delay, 2s; number of scans, 1024; solution concentration, ∼3040% in CDCl3, reference, TMS; paramagnetic reagent, Cr(16) Dosseh, G.; Rousseau, B.; Fuchs, A. H. Fuel 1991, 70 (5), 64146. (17) Doan, B. T.; Gillet, B.; Blondel, B.; Beloeil, J. C. Fuel 1995, 74 (12), 1806-1810. (18) Doddrell, D. M.; Pegg, D. T.; Bendall, M. R. J. Magn. Reson. 1982, 48, 323-327. (19) Bax, A.; Morris, G. A. J. Magn. Reson. 1981, 42, 501-505.
Bansal et al. (AcAc)3, 10 g L-1. 2D Heteronuclear 13C-1H Shift Correlation Spectra [1H dimensions (13C-dimensions)]: spectral width, 764.53 Hz (20 000.00 Hz); memory capacity, 512 w (4096 w); digital resolution, 2.986 Hz/pt (9.7666 Hz/pt); 90° pulse, 12.5 µ{decouple} (6.0 µ {Observation}; relaxation delay, 2 s; initial value for t1 evaluation, 3 µ; increment for t1 evaluation, 1/2sw1; number of scans, 400; number of experiments, 128; memory capacity of 2D spectra, 512 w; solution concentration, ∼3040% in CDCl3; reference, TMS; no paramagnetic reagent. Open Column Chromatography. Column chromatographic work has been carried out as per the ASTM-2549/85 method.1 Basis of Estimation. Estimation of the total aromatics in a diesel-range fuel sample using the 1H NMR spectrum is based on the group molecular weight (GMW) method and the determination of the relative number of carbon atoms from the spectrum. This, in turn, requires the unambiguous assignment of the 1H NMR spectrum with regard to CHn (n ) 1, 2, and 3) groups of both aromatics and saturates present in the sample. Besides saturates, diesel-range samples are rich in mono ring aromatics. However, the di ring aromatics are also present in an appreciable amount, whereas tri ring-plus aromatics are present in a very small concentration. On the basis of the reported data, it is also established that the di ring aromatics and tri ring-plus aromatics are present approximately in a 4 to 1 ratio. The typical structure of mono ring and di ring aromatic components are given below: R1 R2
R1 R2
R3
R3
where R1 ) CH3 and R2 and R3 > CH3. The degree of substitution (σ) and the chain length of the alkyl substituents (R2, R3, etc.) on the aromatics depend on the boiling range of the products. The total aromatic content, therefore, will be the contribution of the ring and substituents, i.e., R1, R2, and R3 groups. Knowledge about the average chain length attached to aromatics (other than R-methyl groups) is a must for estimation of the total aromatics using the present methodology. The nature and average length of the alkyl substituents greater than CH3 cannot be estimated directly as the signals are merged with the signals of paraffins in the region of 0.0-2.0 ppm. Besides this, a knowledge about the contribution of aromatic quaternary (Arq) and bridgehead aromatic carbons (Arb) is also required. However, these carbons cannot be estimated directly from the 1H NMR spectrum because they are nonprotonated. Therefore, the following indirect approaches have been adopted to estimate the total aromatics from the 1H NMR spectrum: (1) Arq carbons have been realized from the resonance in the overlapped region of 2.0-3.5 ppm. The resolution of this region due to R-CH2 and R-CH3 substituents was achieved by 2D NMR spectroscopy. (2) The average alkyl chain length (n) attached to the aromatics (other than R-CH3) has been estimated from 1H NMR analysis of the pure aromatic fraction of a number of samples. (3) Arb carbons have been realized by the equations developed, which has facilitated the estimation of polynuclear aromatics.
Results and Discussion Figure 1 shows the 1H NMR spectrum of a high-speed diesel (HSD) sample along with the expanded 2-9 ppm region. Various resonance signals assigned to different groups have been marked in the spectrum. The spectrum has been divided into various regions: aromatic ring protons (A), 6.5-9.0 ppm; R-alkyl (CH2, CH3) to aromatic ring groups (B and C), 2.0-4.0 ppm; β-CH and
Estimation of Total Aromatics and Their Distribution
Figure 1. 1H NMR (300 MHz) spectrum of a representative diesel (HSD) sample.
X
Y
Z
Figure 2. Two-dimensional heteronuclear 13C-1H (Hetcor) NMR spectrum of the aromatic fraction of a diesel sample.
CH2 groups to aromatic rings and -CH and CH2 groups of cycloalkanes and normal and isoparaffins (D), 1.02.0 ppm; and (-CH3 to aromatics and other paraffinic CH3 (E), 0.5-1.0 ppm. On average, the D region is assumed to be due to CH2 groups. The integral areas of these regions have been denoted by letters A, B, C, D, E. Estimation of Substituted Aromatic Carbons (Arq). For estimation of substituted aromatic carbons (Arq), assignment of the signals in the highly overlapped regions of 2.0-3.5 ppm, which contains signals due to R-substitution (-CH3, -CH2, -CH) is absolutely essential. Theoretically, the number of R-substituent carbons is equal to the number of substituted aromatic carbon (Arq) atoms. The broad envelope between 2 and 2.4 ppm is mainly due to signals from R-CH3 of mono aromatics, whereas the signals in the 2.4-3.5 ppm region may be due to R-CH3, R-CH2, and R-CH of both mono- and polynuclear aromatics and the naphthenic ring attached to the aromatics. The assignment of this region has been facilitated with the help of the 2D Hetcor experiment as described below. Two-Dimensional Hetcor NMR Studies. Twodimensional NMR techniques have found great utility for peak assignment in much the same way as multiplicity NMR techniques such as INEPT/DEPT. Figure 2 shows the 1H-13C shift-correlated 2D NMR spectrum of an aromatic fraction separated from a HSD sample.
Energy & Fuels, Vol. 12, No. 6, 1998 1225
The 1H NMR spectrum is shown along one axis (2.03.5 ppm), whereas the other axis includes the 13C DEPT 135° spectrum of the fraction. In the DEPT spectrum, signals of the -CH and -CH3 carbons appear as negative and those due to -CH2 carbons appear as positive peaks. It is clear from the spectrum that signals between 18 and 20 ppm in the 13C NMR spectrum are due to the methyl substituents of the aromatic carbons, as confirmed by the DEPT spectrum. These signals and their corresponding correlations with the 1H signals in the spectrum have been used to assign the 1H NMR region from 2.0 to 3.5 ppm. Careful examination of the Hetcor spectrum (contour “X”) shows that the 1H NMR region 2.0-2.4 ppm (C, Figure 1) is due to R-CH3 groups only and is free of any overlap. However, the presence of contour “Y” shows that few R-CH3 signals also appear between 2.4 and 2.6 ppm. Contour “Z” shows the correlation between the -CH2 carbons (25-40 ppm) with the protons in the region 2.4-3.5 ppm. This shows that the 2.4-3.5 ppm region contains signals due to both R-CH2 and R-CH3 substituents on the aromatics. There may be R-CH protons in this region also; these could not be visualized due to their very small concentration. Since it is not possible to individually quantify the -CH, -CH2, and -CH3 groups due to overlap, the 2.4-3.5 ppm region (B, Figure 1) is assumed to be due to an average of two protons. Thus, the relative number of substituted aromatic carbons (Arq) is given by the following equation:
Arq ) (B/2 + C/3)
(1)
Average Alkyl Chain Length of Aromatic Substituents. The average alkyl chain length (n) of the aromatic substituents (other than R-methyl) has been estimated from the 1H NMR spectral analysis of pure aromatic fractions of a variety of samples separated by silica-gel column chromatography. The 1H NMR spectra of the aromatic fractions of a representative sample is given in Figure 3(a and b). The spectrum in Figure 3a clearly indicates signals mainly due to monoaromatics. However, the spectrum in Figure 3b mainly depicts signals due to diaromatics. Both methyl and longer alkyl chain lengths are visible in the monoaromatic fraction, which is evident from the intense signals due to CH2 (1.4 ppm) and CH3 (0.9 ppm). However, the fraction rich in di-plus-ring aromatics contains methyl substituents in abundance. The average chain length (n) of the aromatic substituents excluding R-methyl groups has been calculated from the 1H NMR spectrum of the aromatic fraction using the following equation
n) [(I3.5-2.4/2) + (I2.0-1.0/2) + (I1.0-0.5/3)]/(I3.5-2.4/2) (2) where I3.5-2.4, etc., is the integral intensity of the 3.52.4 ppm region. The R-CH3 groups have been excluded because their contribution can be directly realized from the 2.0-2.4 ppm region (C, Figure 1). However, the value of n is required to be known in order to realize the contribution of longer alkyl chains (β, γ, δ) attached to the aromatics whose signals overlap with those of paraffin chains.
1226 Energy & Fuels, Vol. 12, No. 6, 1998
Bansal et al.
Experimentally, an average value of K can be estimated from the 1H NMR spectrum by using following equation:
K ) (A + B/2 + C/3)/A
(4)
The value of K was estimated for a number of samples, which when substituted in eq 3 yielded the relative value of PAC ) 3.13Id. Theoretically, the percentage of bridgehead carbons (Arb) in unsubstituted diring and triring aromatics (i.e., aromatic carbon content ) 100%) is 20.0% and 28.6%, respectively. If diring and triring aromatics are assumed to be present in a 4:1 ratio, the theoretical percentage of Arb in the sample becomes 21.7%. Hence, the relative contribution due to bridgehead carbons in the 1H NMR spectrum can be given as
Arb ) (21.7PAC)/100 ) (21.7 × 3.13Id)/100 ) 0.68Id (5)
Figure 3. 1H NMR (300 MHz) spectra of diesel fractions: (a) fraction rich in monoaromatics and (b) fraction rich in di-ringplus aromatics.
Contribution of Bridgehead Carbons (Arb). It was observed from the 1H NMR of the pure diaromatic fraction (Figure 3b) that the chemical shift value 7.5 ppm acts as a dividing point and that the integral intensity on either side in the 6.5-10.0 ppm region is exactly the same. Therefore, in the 1H NMR spectrum of a HSD sample, twice the integral intensity in the region 7.5-10.0 ppm (Id) should give a contribution due to protons of di-plus-ring aromatics. The rest of the integral intensity, i.e., I6.5-10.0 - 2Id, should correspond to protons of monoring aromatics. On the basis of this observation and other structural considerations, the total relative number of di-plusaromatic (i.e., polynuclear aromatic) carbons (PAC) can be obtained from the 1H NMR spectrum by using following equation:
Estimation of Total Aromatics. After the complete assignment of the 1H NMR spectrum has been achieved and the relative contribution of Arb and Arq carbons has been realized, the total aromatic content of a sample can be estimated. The first step involves the estimation of the total relative number of carbons (TC) and total group molecular weight (Tw) of the sample. The TC for the sample is calculated by dividing the individual regions in the 1H NMR spectrum by the number of protons causing the signal. Tw is then obtained by multiplying these numbers with the respective molecular weight of the groups (12 for C, 13 for CH, 14 for CH2, and 15 for CH3 group). Therefore, TC and Tw are given as
TC ) A/1 + B/2 + C/3 + D/2 + E/3 + Arb + Arq (6) Tw ) (A/1)13 + (B/2)14 + (C/3)15 + (D/2)14 + (E/3)15 + (Arb+ Arq)12 (7) Substituting the contributions of Arq and Arb from eqs 1 and 5, respectively, the above equation simplifies to
Tw ) 13(A+B) + 9C + 7D + 5E + 8.2 Id
(8)
PAC ) (2Id × K × 1.25 × 0.8) + (2Id × K × 1.4 × 0.2) (3)
For estimating the total aromatic content, the group molecular weight of the aromatics (Aw) is also required, which is given as
The basis of eq 3 can be understood as follows: In a diring aromatic molecule (excluding side chains), e.g., naphthalene, the total number of carbon atoms (i.e., PAC) is 10. Similarly, in a triring aromatic molecule (excluding side chains), e.g., anthracene, PAC ) 14. The first part of eq 3 gives the contribution from diring aromatics and other parts from triring aromatics. The factor K is directly related to the degree of substitution and is given as (ArH + Arq)/ArH. The factors 1.25 and 1.4 come from the ratio (ArH + Arq + Arb)/(ArH + Arq) in diring and triring aromatics, respectively. For example, the 1,3-dimethylnaphthalene molecule has 6 of ArH, and 2 of each Arq and Arb carbons. Similarly, the parameters 0.8 and 0.2 are the average mole fractions of diring and triring aromatics in the sample, respectively, based on the assumption that diring and triring aromatics are present in a 4:1 ratio.
Aw ) (A/1)13 + n(B/2)14 + (C/3)15 + (Arb+ Arq)12 (9) Substituting the value of n ) 3.0 from eq 2 and Arb and Arq from eqs 1 and 5
Aw ) 13A + 27B+ 9C+ 8.2 Id
(10)
The total aromatic content of the sample (weight %) can then be estimated as
total aromatics (TA) ) (Aw/Tw)100
(11)
The total aromatic content for various samples obtained from the 1H NMR-based equation is given in Table 1. As shown in Appendix 1, it is clear that by taking different ratios of diring and triring aromatics
Estimation of Total Aromatics and Their Distribution Table 1. Comparison of Quantitative Data for Monoaromatics, Di-Plus-aromatics, and Total Aromatics Estimated by 1H NMR and Other Methodsa
Energy & Fuels, Vol. 12, No. 6, 1998 1227
plus-ring aromatics (PA) have been made for a number of samples, and the results are tabulated in Table 1. Validation
percentage, w/w 1H
NMR method
other methods
sample mono di-plus total mono di-plus H-1 H-2 H-3 H-4 H-5 H-6 H-7 C-1 C-2 C-3 C-4 S-1 S-2 S-3 K-1 K-2 K-3 K-4 K-5 K-6 BL-1 BL-2 BL-3 BL-4
18.6 20.3 17.1 27.2 22.7 19.0 17.0 19.4 19.2 22.0 21.7 19.5 19.4 26.0 16.0 17.9 15.9 17.5 18.5 18.6 38.0 32.5 57.2 74.7
8.5 8.9 6.9 8.1 7.6 8.9 9.4 4.1 2.8 3.9 3.0 10.5 9.8 8.4 1.2 0.7 1.3 0.5 1.2 0.5 26.0 25.6 42.8 25.3
27.1 30.4 24.0 45.0 30.3 27.9 26.4 23.5 22.0 25.9 24.7 30.0 29.2 34.4 17.2 18.6 17.2 18.0 19.7 19.1 63.8 58.7 100 100
17.1 19.5 13.5 25.4 19.9 18.0 17.9 22.0 18.5 22.0 22.1 20.7 20.1 27.5 16.9 18.2 16.8 18.4 17.3 19.8 38.5 34.3 58.6 73.1
8.9 9.5 7.7 9.1 8.9 9.7 8.9 4.6 3.6 4.2 3.6 11.6 11.0 7.8 1.7 0.8 1.6 0.9 2.3 1.0 25.4 22.0 41.4 26.9
total 25.5 (HPLC) 29.0 (HPLC) 21.2 (HPLC) 44.5 (HPLC), 43.8 (OC) 28.8 (HPLC), 30.0 (OC) 27.7 (HPLC), 29.4 (OC) 26.8 (HPLC) 25.6 (HPLC) 22.1 (HPLC) 26.2 (HPLC) 25.7 (HPLC) 32.3 (MS) 31.1 (MS) 35.3 (HPLC), 34.3 (OC) 18.6 (MS) 19.0 (MS) 18.4 (MS) 19.3 (MS) 19.6 (MS) 20.8 (MS) 63.5 (actual) 58.5 (actual) 100 (actual) 100 (actual)
a HPLC ) IP 391/90 method, MS ) high-resolution mass spectroscopic method, OC ) open column chromatographic method, Actual ) blends prepared, H ) HSD, C ) coker kero, S) straight run gas oil, K ) kero cuts, BL) blend.
(as 3:1 and 2:1), there is not much of a difference in the total aromatic content. To simplify the calculations and minimize the arithmetic error, the whole scheme has been programmed in “Basics”, which just needs integrals A, B, C, etc. from the 1H NMR spectrum as the input. Estimation of Monoring (MA) and Global DiPlus-Ring Aromatics (PA). Since the contribution of the bridgehead carbons (Arb) can be obtained from the 1H NMR spectrum, the percentage of Ar has been used b for the estimation of di-plus-ring aromatics (PA) in a sample. The percent of Arb is given as
% bridgehead carbons (%Arb) ) (0.68Id × 100)/TC (12) As discussed above, an average value of %Arb equal to 21.7 correspond to 100% di-plus-ring aromatics in a diesel sample. Therefore, knowing the experimental value of %Arb, the di-plus-ring aromatic content (PA) can be estimated using
PA ) (%Arb × 100)/21.7
(13)
And the monoring aromatic content (MA) is given as
MA ) (TA - PA)
(14)
The quantitative estimation of monoring (MA) and di-
The validity of the proposed method has been done by mass spectrometry, open column chromatography, and HPLC (IP-391/90) techniques. The data obtained using the above techniques along with that from the proposed 1H NMR method is given in Table 1. The following statistical data (where R2 is the correlation coefficient) are derived from regression analysis of data sets in Table 1. aromatics
R2
standard deviation
total aromatics monoaromatics di-plus-aromatics
0.998 0.994 0.995
1.2 1.4 1.1
The statistical data indicate a good correlation among the values obtained by NMR and those by other methods. The data of the blends has not been included for drawing correlations as the amount of aromatics are rather high (60-100%) compared with the actual diesel samples (20-40%). However, the data has been included in Table 1 to observe the validity of the equations for estimating even higher amounts of aromatics using the equations developed. Conclusions 1H
A NMR-based method has been proposed for estimation of the total aromatic content in diesel-range fuel samples. The method provides not only the total aromatic content, but also their distribution as monoring and global di-plus-ring aromatics, which can be determined conveniently for different varieties of dieselrange fuel products. The method is fast compared to the established methods and is independent of any standards. Compositional analysis in terms of total aromatics and saturates can be obtained with in a short span of time, i.e., ca. 10 min. Appendix 1. Calculation of Total Aromatic Content by Taking Different Ratios of Diaromatics and Triaromatics. For example, in the 1H NMR spectrum, the following values of the integrals are obtained (sample H-5): A ) 5.2, B ) 6.5, C ) 4.0, D ) 81, E ) 38.5, Id ) 1.4. Case 1. Diaromatics:Triaromatics ) 4:1. Theoretical % Arb ) 21.7, PAC ) 3.13Id (eq 3), Arb ) 0.68Id (eq 5), Tw ) 958.6 (eq 8), Aw ) 290.6 (eq 10), TA ) 30.3% (eq 11). Case 2. Diaromatics:Triaromatics ) 3:1. Theoretical % Arb ) 22.15, PAC ) 4.82Id (eq 3), Arb ) 1.068Id (eq 5), Tw ) 965.0 (eq 8), Aw ) 297.0 (eq 10), TA ) 30.8 % (eq 11). Case 3. Diaromatics:Triaromatics ) 2:1. Theoretical % Arb ) 22.8, PAC ) 4.89Id (eq 3), Arb ) 1.117Id (eq 5), Tw ) 965.9 (eq 8), Aw ) 297.8 (eq 9), TA ) 30.9% (eq 11). EF980052Y
