Energy & Fuels 1989, 3, 455-460
455
NMR and GC/MS Investigation of the Saturate Fractions from the Cerro Negro Heavy Petroleum Crude Daniel A. Netzel* and Frank D. Guffey Western Research Institute, P.O. Box 3395, University Station, Laramie, Wyoming 82071 Received July 11, 1988. Revised Manuscript Received April 6, 1989 The Cerro Negro crude oil is a biodegraded crude with a very low concentration of acyclic alkanes. Because of its compositional complexity, new analytical methods and approaches are needed for complete chemical characterization. Saturate hydrocarbon fractions isolated from the 200-425, 425-550, and 550-700 "C distillate cuts and the >700 "C residue of the Cerro Negro heavy crude were characterized by using the nuclear magnetic resonance techniques DEPT (distortionless enhancement by polarization transfer) and QUAT (quaternary-onlycarbon spectrum). Average molecular structural parameter calculations from NMR data give the following average number of rings per molecule for the four saturated hydrocarbon fractions: 2.5 in 200-425 "C; 2.4 in 425-550 "C; 4.7 in 550-700 "C; 3.8 in >700 "C. Gas chromatography/mass spectrometry (GC/MS) was used to determine the amounts of acyclic alkanes in the 200-425 and 425-550 "C saturate fractions. The amounts of acyclic alkanes by GC/MS in two of the saturate fractions are 0.22% and 7OO0C
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Figure 2. 13C NMR spectra of the four saturate fractions from the Cerro Negro heavy petroleum crude. The 'H resonances (Figure 1)between 0.8 and 0.9 ppm and at 1.25 ppm are due to methyl and methylene groups, respectively, for normal and branched alkanes. The broad resonances between 1.4 and 2.0 ppm are mostly associated with methine and cyclic methylene hydrogens. It is interesting to note that as the boiling point of the crude fraction from which the saturate fraction was separated increases (increase in average molecular weight), the ratio of methyl to methylene hydrogens decreases. This decrease suggests an increase in the number of saturate rings or an increase in. the normal alkane carbon chain length with a corresponding decrease in branched alkanes. These normal and branched alkanes may exist either independently or as a substituent on a cycloalkane moiety. Many of the resonances in the 13CNMR spectra (Figure 2) can be identified as carbons belonging to normal alkanes (14,23,32,30,and 29.5 ppm) and branched alkanes of the isoprenoid class of compound^.'^ The 13C spectra show (14) Netzel, D. A.; McKay, D. R.; Heppner, R. A.; Guffey, F. D.; Cooke, S. D.; Varie, D. A.; Linn, D. E. Fuel 1981, 60, 307-320.
Energy & Fuels, Vol. 3, No. 4, 1989 457
Cerro Negro Heavy Petroleum Crude
Table 11. Carbon and Hydrogen Type Distributions for the Saturate Fractions from the Cerro Negro Heavy Petroleum Crude saturate fraction 200-425 "C 425-550 OC 550-700 "C >700 o c carbon tvDe C H C H C H C 'H C aromatic aliphatic 0.052 0.035 0.010 0.0 CH aromatic 0.025" 0.012" 0.006" 0.002" aliphatic 0.285 0.154 0.254 0.133 0.291 0.158 0.271 0.144 CHZ 0.425 0.460 0.477 0.499 0.544 0.590 0.573 0.607 CH3 0.238 0.387 0.235 0.368 0.155 0.253 0.156 0.249 "Benzene concentration not included in the calculation of the fractional amounts of carbon and hydrogen types.
an increase in the amount of the normal alkane moiety relative to branched alkanes with an increase in the distillation temperature. The direct identification of cycloalkanes in the 13C spectra of the saturate fractions is difficult to ascertain. Some information about the average number of rings and branch sites of a cycloalkane can be obtained from 13C subspectral analysis of the saturate fractions. The normal 13Cand carbon-type (CH,, n = 0-3) spectra for the 200-425 "C saturate fraction are shown in Figure 3. This set of spectra is typical of the DEPT/QUAT spectra obtained for each of the four saturate fractions. Integration of each carbon-type spectrum yields the fractional amount of carbon for a given carbon type in the sample. The C and H atom fractions for the carbon types in each of the saturate fractions are listed in Table 11. The atom fraction of hydrogen listed for each saturate fraction was calculated from the 13C data and normalized to 1.0. The average number of carbon atoms per molecule (Nc) was calculated from the fraction of carbon atom of a given carbon type, the average molecular weight, and the molecular weight of the carbon type by using the relationship
Nc =
h h H n
MWA/WCH,MWCH,
(1)
where fcHn = the fraction of carbon atoms of carbon type CH, from DEPT/QUAT spectral analysis (n = 0-3), MWcH, = the molecular weight of carbon type CH,, nhH, = the number of carbon atoms of carbon type CH,, MWA = the average molecular weight of sample, and Nc = the number of carbon atoms in an average molecule. Equation 1 is valid for any type of hydrocarbon molecule. The average number of branching sites (BS), branches (NB),and saturate rings (NR)per molecule were obtained by using eq15 2-4, where f c , f C H , and fcHB are the frac-
BS = Nc(fcc + ~ C H NB = NC(2fC -k f C H )
)
NR = 0.5Nc(2fc + PCH - ~ c H , )+ 1
CDCI)
"C-Normal
(2) (3)
(4)
tions of carbon atoms present as C, CH, and CH3 groups, respectively. The average molecular parameters for the four saturate fractions are listed in Table 111. The H/C ratios were obtained from the lSC data and from conventional elemental analyses. The ratios obtained by the two techniques are in good agreement and, thus, validate the carbon-type analysis by NMR. The carbon number range and the average carbon number per molecule for each saturate fraction are also listed in Table 111. The carbon number range corresponding to the distillate fraction boiling point range was determined by comparing the initial and final tempera(15)Cookson, D. J.; Smith, B. E. Anal. Chem. 1985, 57, 864-871.
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Figure 3. Normal 13C NMR spectrum and carbon-type spectra for the 200-425 O C saturate fraction from the Cerro Negro heavy petroleum crude.
tures of each fraction to the boiling point for n-alkylsubstituted cycloalkanes shown graphically in Figure 4. The appropriate curve used in Figure 4 for each fraction corresponded to the average number of rings per molecule determined from the NMR data. Estimates of boiling points for alkyl-substituted cycloalkanes with two or four rings (Figure 4,dash lines) were based on the relationship between the boiling point and the carbon number for nalkyl-substituted cyclohexanes and n-alkanes obtained from the literature16J7(Figure 4,solid lines). The boiling (16)TRC-Thermodynamic Research Center, Thermodynamic Tables, Hydrocarbons: Texas A&M University: College Station, TX, 1974;Vol.
V.
458 Energy & Fuels, Vol. 3, No. 4, 1989
Netzel and Guffey
Table 111. Average Molecular Structural Parameters for the Saturate Fractions from the Cerro Negro Heavy Petroleum Crude saturate fraction 425-550 "C structural parameter 200-425 "C 550-700 "C >700 "C 1.91 (1.84) 1.85 (1.88) H/C ratio" 1.89 (1.88) 1.84 (1.87) 25-41 carbon no. rangeb 10-25 41-63 >74 av no. of carbon atoms per molecule ( N J 17 32 using midpoint of the temp rangeb 45 74 from NMR data 19.6 31.1 47.4 49.1 271 (268) 432 (450) av mol wt (MWq)'vd 656 (618) 682 (688) av no. of branching sites per molecule (BS) 6.6 9.0 14.3 13.3 av no. of branches per molecule (NB) 10.1 14.7 7.6 13.3 2.5 2.4 av no. of rings per molecule (NB) 4.7 3.8 av no. of quaternary carbons per molecule 1.0 1.1 0.5 0.0 av no. of methine carbons per molecule 5.6 7.9 13.8 13.3 av no. of methylene carbons per molecule 8.3 14.8 25.8 28.1 av no. of methyl carbons per molecule 4.7 7.3 7.3 7.7 OH/C ratios in parentheses are from ref 5. *Eachfraction based on average number of rings per molecule from NMR data and Figure 4. dNumbers in parentheses are from GC-simulated distillation; boiling point corresponds to 50% offpoint!
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Figure 4. Boiling point versus carbon number for n-alkanes and n-alkyl-substituted cycloalkanes: (A)n-alkanes,l6Tables 23-2-
(1.101)-a,23-2-(1.102)-a,23-2-(1.103)-k,23-2(1.105)-k;( 0 )n-alkyl-substituted cy~lohexane,'~ Tables 23-2-(3.1110)-a,23-2(3.1111)-a,23-24 3.11ll)-k; (m) decalin,I7Table 23-2-(3.5212)-a; (0) perhydr~anthracene,'~ Table 23-2(43.5409)-a;(A)perhydrochrysene?l (0)perhydropentacene.'* point for a cycloalkane with five rings were not available and, thus, was estimated by group contribution methods.18 The estimated boiling point can easily be in error by 20 "C or more. For the higher boiling fractions, the carbon number for a n-alkyl-substituted cycloalkane may be considerably lower than that of a n-alkane at a given temperature. Because the saturate fractions have been determined (vide infra) to be composed mostly of cycloalkanes, it is more precise to use the relationship of boiling point versus carbon number for alkyl-substituted cycloalkanes than the boiling point data of n-alkanes when the carbon number for oil fractions that are predominantly cycloalkanes is calculated. The average carbon number for each fraction was determined from (1)the molecular weight and NMR data (eq 1)and (2) Figure 4 by using the temperature a t midpoint of the distillate range and the average number of saturate rings per molecule calculated from NMR data. (17) TRC-Thermodynamic Research Center, Thermodynamic Tables, Hydrocarbons: Texas A&M University: College Station, TX, 1974; Vol. I. (18) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, NY, 1987; Chapter 2, pp 14, 21.
Molecular weights for each fraction (Table 111) were determined directly by vapor-phase osmometry (VPO) using dichloroethane as a solvent and from the average carbon number determined from gas chromatography simulated distillation data.6 As shown in Table 111, the molecular weights determined by the two methods for the >700 "C saturate fraction are very close to the molecular weights determined for the 550-700 "C fraction. The reason that the molecular weights of the two saturate fractions are nearly the same is that their GC simulated distributions profiles are similar and cover essentially the same temperature range. Also, the GC-simulated distribution profiles for the 550-700 "C saturate fractions occur over a lower temperature range than the corresponding GC-simulated distribution profiles of the whole distillate (550-700 "C) and whole residue (>700 "C) from which the saturate fractions were isolatede6 Apparently, the presence of the saturates in the 550-700 "C whole distillute and >700 "C whole residue is due to the existence of some type of intermolecular association of the saturates with other highly polar components of the whole distillate and whole residue. This association causes the saturates to distill at higher effective atmospheric equivalent boiling points (AEBP) than the saturates would if separated.6 A similar phenomenon was reported by Boduszynski' in which the pentane soluble fraction of a "nondistillable" residue exhibited appreciable volatility under the vacuum thermal gravimetric analysis conditions. 200-425 "C Saturate Hydrocarbon Fraction. This saturate fraction represents 8.7 wt % of the crude oil. The molecules in this fraction range from 10 to 25 carbons with an average carbon number of about 19 and an average molecular weight of 270. The GC/MS results for this sample indicate that neither normal nor branched alkanes were present in this saturated hydrocarbon fraction above 1.0% by weight. We estimate that the concentration of normal alkanes in this fraction is 0.22% by weight. Brown et a1.6 used the ASTM mass spectrometric method D-2786 to determine that this sample contains 11.28% by volume or about 9.0% by weight of acyclic alkanes (assuming a specific gravity of 0.8). This value is significantly larger than the value determined by the GC/MS method. Possible reasons for the difference in the values are discussed in a later section. The normal 13C NMR spectrum of the 200-425 "C saturate fraction shown in Figure 2 does not have the characteristic five-line pattern of n-alkanes with a carbon chainlength greater than 10 carbons. Alkane structures with a carbon number less than 10 may be present, but
Energy & Fuels, Vol. 3, No. 4, 1989 459
Cerro Negro Heavy Petroleum Crude because of boiling point constraints, they must be bonded to another molecular moiety. If, according to the GC/MS and NMR results, normal or branched alkanes are not present in appreciable quantities, then the alkane present must be due mainly to alkyl-substituted cycloalkanes. Reduction of the 13C DEPT/QUAT spectral data suggests that the average molecular structure is a molecule containing 19 carbons, with two saturate rings, seven branch sites, and eight branches (Table 111). A correlation of the methyl 13C chemical shift data of this sample with model compounds suggests that two branch sites and three branches are due to a molecular configuration such as that of 1,1,2-or 1,1,3-trimethylcyclohexane.1g Brown et alS5reported that cycloalkanes with one to four rings are present in the 200-425 "C saturate fraction in which 32% by volume of the fraction is composed of dicycloalkanes. Monocycloalkanes and tricycloalkanes are almost equally distributed and make up 43% by volume of the fraction, while 14% by volume of the sample is made up of tetracycloalkanes. The weight average number of rings per saturate molecule for this fraction calculated from the mass spectral data given by Brown et alS5is 2.7. This value compares favorably with the NMR value of 2.5 listed in Table 111. The weight average number of rings per molecule was calculated fro-m mass spectral data by using equations 514 and 6,where C = the weight average number of ring carbon
c = CCi2Xi/CCiXi NR= 1 + (e- 6)/4
(5) (6)
atoms per polycondensed cycloalkane, Ci = the number of ring carbon atoms per cycloalkane i, X i = the weight or volume percent of cycloalkane i from mass spectral data, and NR= the weight average number of rings per molecule. 425-550 "C Saturate Hydrocarbon Fraction. The amount of saturate hydrocarbons in the Cerro Negro crude within the temperature range of 425-550 OC is 3.3% by weight. The carbon number range in this fraction is 25-41. The carbon number range reported by Boduszynski8 for the saturates from the vacuum gas oil fraction of the Kern River petroleum crude is 21-40. The Kern River petroleum is also a biodegraded crude oil, and the vacuum gas oil fraction had a distillation temperature range of 343-538 OC. The difference in the boiling point of the crude oils can easily account for the small difference in the carbon number range between their two saturate fractions. The GC/MS data show that no normal or branched alkanes are present at or above the detection limit of the method ( 10 are present in the 13C NMR spectrum of the 550-700 "C saturate fraction (Figure 2). From the integration of these resonances, assuming that the resonances are due to n-alkanes, the average carbon chain length is equal to 14 carbons. An n-alkane of this carbon number has a boiling point of 253 "C, which is far too low to be present in this sample. Thus, we conclude that the observed resonances associated with long, straight carbon chains are due to long, straight carbon chains bonded to a polycondensed cycloalkane moiety. The 13C DEPT/QUAT spectral data suggest that the average molecular structure is a molecule containing 47 carbons with five saturate rings, 14 branch sites, and 15 branches. There is evidence from the 13C chemical shift data for a small amount of 1,1,2- and 1,1,3-trimethylcyclohexane configuration as observed for the other two fractions. The average number of carbon atoms per molecule calculated from NMR data is in good agresment with the number of carbon atoms (45) determine from the midpoint of the temperature range and the iling point data for cycloalkanes (see Figure 4). >700 "C Saturate Hydrocarbon Fraction. The saturate concentration is only a small fraction of the total residue (1.2% by weight). The normal 13C NMR spectrum of the >700 "C saturate fraction in Figure 2 shows the characteristic five resonances associated with carbon chain length greater than 10 carbons. From the integration of these resonances, the carbon chain length is 15 carbons. An n-alkane of 15 carbons has a boiling point of 271 "C, which is well below the distillation temperature for this fraction, and would not be present. As with the other fractions, the presence of a long, straight-chain alkane as noted from the 13Cspectrum must arise from an alkane substituent on a polycondensed cycloalkane moiety. The average molecular structure derived from the 13C DEPT/QUAT spectral data is a molecule containing 49 carbons with four saturate rings, 13 branch sites, and 13 branches (Table 111). The average number of carbon atoms per saturate molecule is estimated a t 74 based upon the boiling point versus carbon number for alkyl substituted tetracycloalkanes (approximately four rings per molecule from NMR data see Figure 4). However, if the carbon number range from the 550-700 "C saturate fraction is 41-63, then the initial carbon number
Netzel and Cuffey for the saturate fraction should be 63 or lower. The average number of carbon atoms per molecule of 49 was calculated from NMR data. The NMR result is directly related to the average molecular weight, which for this saturate fraction is 682. The boiling point of a saturate hydrocarbon with a molecular weight of 682 is considerably less than 700 "C, the atmospheric equivalent boiling point temperature at which the residue was obtained. The average molecular weight of 991 was determined for the total residue! The reasons why the extracted saturate molecules that have a lower molecular weight than the total residue are found in the residue fraction may be due to some type of chemical association of the saturate molecules with the polar aromatic molecules that make up the bulk of the residue fraction or due to overlapping between the two distillate cuts. Thus, it would be erroneous to conclude that the average carbon number of the saturate fraction is >74 based on the AEBP temperature (>700 "C) at which the residue fraction was obtained. The simulated distillation results for this sample2 indicate that 5 % by volume distills a t a temperature of 506 "C. The distillate temperature attainable by the GC/MS method (482 "C) is significantly below the temperature of the initial boiling material in this sample (506 "C) and, thus, not applicable to analysis by GC/MS.13
Conclusions It has been demonstrated in this study that quantitative 13C NMR subspectral analysis can provide detailed molecular structural information on saturate fractions that have atmospheric equivalent boiling points above the temperature for effective use of the GC/MS and ASTM D-2786 mass spectral techniques. For the 200-425 and 425-550 "C distillate fractions, the NMR data correlate reasonably well with mass spectral data. However, a significant difference in the acyclic alkane content of the saturate fractions occurs between GC/MS and ASTM D-2786 methodology. We suggest that this difference may be due to the assumptions used in the initial development of the ASTM mass spectrometric method. That is, the assumptions may not be valid for a compositionally complex petroleum crude such as the Cerro Negro, in which the saturates are essentially cycloalkanes. Acknowledgment. We wish to express thanks and appreciation to the United States Department of Energy for funding of this work under Cooperative Agreement No. DE-FC21-83FE60177 and to Jim Reynolds of the Department of Fuel Chemistry a t the National Institute for Petroleum and Energy Research for preparing the highquality saturate fractions of the Cerro Negro heavy petroleum crude.
