Composition of heavy petroleums. 2. Molecular characterization

AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 2, NUMBER 5

SEPTEMBER/OCTOBER 1988

0 Copyright 1988 by the American Chemical Society

Articles Composition of Heavy Petroleums. 2. Molecular Characterization Mieczyslaw M. Boduszynski Chevron Research Company, P.O. Box 1627, Richmond, California 94802-0627 Received November 12, 1987, Revised Manuscript Received April 2, 1988

The objective of this paper is to illustrate the variation of the chemical composition of heavy petroleums as a function of atmospheric equivalent boiling point (AEBP) up to approximately 1400 O F and further as a function of solubility down to the “bottom of the barrel”. The first paper in this series described the variation of heteroatom (S, N, 0, V, Ni, Fe) concentrations, hydrogen deficiency (H/C atomic ratio), and molecular weight distributi0n.l This paper describes the variation of the chemical composition of heavy petroleums on the molecular level. Two molecular characterization approaches are discussed. The first approach utilizes “average molecular parameters”. The data illustrate the changes in relative concentrations of the “molecular building blocks” with increasing AEBP and decreasing solubility of heavy crude components. The second approach involves the extensive use of high-performance liquid chromatography (HPLC) to separate “compound-class” fractions for further molecular characterization. The results are expressed in terms of the homologous series of compounds and their carbon number distributions by using field-ionization mass spectrometry (FIMS). The results help unravel the immense complexity of heavy petroleums. Data show the great diversity of molecular types, involving numerous homologous series of compounds, each series having a very wide carbon number distribution (extending to over Clm) and very low relative concentrations of individual carbon number homologues (from a few tenths of 1wt 5% to less than 1ppm by weight). Experimental data reveal homologous series of compounds, apparently of “biomarker” origin, greatly exceeding the carbon number reported for “biomarkers”. The results demonstrate the need for and the importance of heavy oil fractionations to produce operationally well-defined fractions having progressively higher AEBP’s, which can be then analyzed and compared on a consistent basis. Data also illustrate the importance of HPLC separations for preconcentrating and isolating compounds of interest from a complex sample matrix.

Introduction In recent years, research was undertaken in this laboratory to delineate the complexity of heavy crude oils and to provide information on the variation of their composition as a function of atmospheric equivalent boiling point (AEBP). The analytical approach involved the fractionation of heavy oils by using short-path distillation (DISTACT-volatility fractionation) followed by sequential elution fractionation (SEF-solubility fractionation) of DISTACT “nondistillable” residues. DISTACT and SEF, used together, provide a robust method for dividing 0887-0624/88/2502-0597$01.50/0

any heavy petroleum into a set of operationally well-defined fractions having progressively higher AEBP’s.~?~ The DISTACT-SEF fractions were subjected to further characterization by various analytical methods, providing data which can be compared on a consistent basis. The first paper in this series described the variation of heteroatom (S, N, 0, V, Ni, Fe) concentrations, hydrogen (1)Boduszynski, M.M.Energy Fuels 1987,1 , 2-11. (2)Schwartz, H.E.;Brownlee, R. G.; Boduszynski, M. M.; Su, F. Anal. Chem. 1987,59, 1393-1401.

0 1988 American Chemical Society

598 Energy & Fuels, Vol. 2, No. 5, 1988

deficiency (H/C atomic ratio), and molecular weight distribution, all as a function of AEBP up t o about 1400 O F and further as a function of solubility down to the “bottom of the barrel”.’ The objective of this paper is to illustrate the variation of the chemical composition of heavy petroleums on the molecular level. Two analytical approaches are discussed in this paper. The first approach utilizes elemental analysis results, 13Cnuclear magnetic resonance (NMR) data, and average molecular weight measurements and describes heavy oil composition in terms of “average molecular parameters”. The second approach involves extensive use of high-performance liquid chromatography (HPLC) to separate “compound-class”fractions for further characterization in terms of the homologous series of coFpounds and their carbon number distributions. The compositional changes are presented as a function of volatility (AEBP) and solubility (SEF) of heavy-oil components. The advantages and limitations of the two characterization approaches and the inherent ambiguity of the compositional data for heavy oils are discussed. Experimental Section Materials Studied. Atmospheric residues (ARs; -650 OF+ AEBP) derived from different crude oils have been the focus of this study. The first paper in this series describes six different feedstocks.’ In this paper, compositional data are presented for only selected samples that are identified in the text. Occasionally, for proprietary reasons, the origin of samples has not been revealed. Volatility and Solubility Fractionations. The A R s were first fractionated each into several distillate ”cuts” having progressively higher AEBP and one “nondistillable” residue (- 1300 OF+ AEBP) by using a DISTACT short-path distillation apparatus (Leybold-Heraeus, GmBH). The “nondistillable” residues were further fractionated with the SEF method, each into the following four solubility fractions: (1)n-pentane soluble (SEF-l), (2) cyclohexane solubleln-pentane insoluble (SEF-2), (3) toluene soluble/cyclohexane insoluble (SEF-3), and (4) methylene chloridemethanol(41 v/v) soluble/toluene insoluble (SEF-4). The details of the DISTACT-SEF fractionations are given in the previous paper.l Preparative Chromatographic Separations. Three preparative HPLC methods have been developed as a part of this research effort. The first separation method involves HPLC on two alumina columns. The first column is packed with basic alumina, and the second contains acidic alumina. This separation is referred to as the HPLC-BA/AA method. The objective of this separation step is to separate a sample into the following operationally defiied fractions: (a) “acidic” compounds, (b) “pyrrolic” N-compounds, (c) “basic” compounds, and (d) “neutral” compounds. The second method, HPLC-NH2,separates the “neutral” compounds into the following fractions: (a) saturates and monoaromatics, (b) diaromatics, (c) triaromatics, (d) tetraaromatics, and (e) pentaaromatics and greater. Finally, the third method, HPLC-SIL, produces fractions of saturates and monoaromatics. HPLC-BA/AA Method. The HPLC-BA/AA system consists of a programmable pump (Model 590, Waters) with an automatic solvent selection valve, a sample loop injector (Model 210, Altex), a series of two 9-mm-i.d. X 25-cm precision-bore glass columns (Altex) connected through a switching valve, and a programmable fraction collector (Foxy, ISCO). The first column is packed with basic alumina (AG-10, BioRad), and the second column is packed with acidic alumina (AG-4, BioRad). Fresh packing is used for each separation. The separation conditions are as follows: sample size, 200-400 mg dissolved in cyclohexane; flow rate, 10 mL/min. The elution sequences are as follows: (a) “neutral” compounds, a fraction eluted from the BA/AA columns with 110 mL of cyclohexane followed by 40 mL of toluene combined with a fraction eluted from the AA column with 200 mL of methylene chloride; (b) “basic”compounds, a fraction backflushed from the AA column with 100 mL of a mixture of methylene chloride and methanol (4:l v/v); (c) “pyrrolic” N compounds, a fraction eluted from the

Boduszynski BA column with 200 mL of methylene chloride; (d) “acidic” compounds, a fraction backflushed from the BA column with 100 mL of a mixture of methylene chloride and methanol (4:l v/v). Solvents are removed from the fractions by using standard procedures. Weight percent yields of the recovered material are determined gravimetrically. Fractions from duplicate separations are combined to produce sufficient quantities of material for further characterization work. The reproducibility between duplicate separations is about 5% or better for “neutral” fractions, and about 5-15% for “basic”, “pyrrolic”, and “acidic” fractions. HPLC-NH2Method. The HPLC system consists of a gradient pump (Model 8800, Du Pont Instruments) equipped with preparative heads, a preparative ZORBAX-NH2column (21.2 mm i.d. X 25 cm, 8-pm particle size, Du Pont), a UV/vis diode-array spectrophotometer (Model 8451A, Hewlett-Packard) as a detector, a Nelson Analytical 760 Series interface connected to a Hewlett-Packard 9920 computer system, and a plotter (Model 7470A, Hewlett-Packard). The separation conditions are as follows: sample size, 100-200 mg dissolved in heptane; flow rate, 20 mL/min. Elution sequence: heptane, 0-29.5 Min; isocratic, 29.5-54.5 min; heptane and methylene chloride (0-50%) gradient. Cut points between the ring-type subfractions are determined by using HPLC-UV profiles generated by the UV diode-array detector. Weight percent yields of the fractions are determined gravimetrically. The reproducibility between duplicate separations is about 5-15%. HPLC-SIL Method. The HPLC system involves a preparative isocratic pump (Model 8800, Du Pont Instruments), a preparative ZORBAX-SIL column (21.2 mm i.d. X 25 cm, 8-pm particle size, Du Pont), a differential refractometer detector (Model R401, Waters), a variable wavelength UV detector (Model 450, Waters), and a Nelson Analytical 760 Series interface connected to a Hewlett-Packard 9920 computer system. The separation conditions are as follows: sample size, 100-200 mg dissolved in heptane; mobile phase, heptane, isocratic, 10 mL/min flow rate. Weight percent yields of the recovered material are determined gravimetrically. The reproducibility between duplicate separations is better than 5%. Molecular Weight Measurements. Vapor pressure osmometry (VPO) was used to determine number-average molecular weights of the DISTACT-SEF fractions. The VPO measurements were conducted in a toluene solution at a temperature of 50 “C and in a pyridine solution a t a temperature of 90 “C on a vapor pressure osmometer (Model 232A, Wescan Instruments). Measurements were performed at three or four different concentrations, and the results were calculated by extrapolation to infinite dilution. It is important to note that the calibration constant a in

lime- ( A T / C ) = a / M , (where AT = temperature difference, C = concentration, and M , = number-average molecular weight) is solute independent for this i n ~ t r u m e n t . ~ In addition, field-ionization (FI) and field-desorption (FD) mass spectrometry (MS) have been used to determine molecular weight distributions of the DISTACTSEF fractions. Details were given in the earlier paper.’ I3C NMR Analysis. 13C NMR spectrometry was used to determine concentrations of aromatic and aliphatic carbons in the DISTACT-SEF fractions. The 13C NMR spectra were obtained on 40% (v/v) solutions in CDC13. The final solution contained 0.04-0.05 M Cr(acac), according to the procedure of Shoolery and B ~ d d e .Pulse ~ parameters were applied every 2.4 s. Data were acquired for 0.4 s, which required 16 384 points at the spectral width of 20 kHz. Line broadening of 3 Hz was applied prior to Fourier transformation. Each spectrum was the result of 500-2000 coadded free-induction decays. FIMS Analysis. The FIMS measurements for HPLC fractions were obtained a t SRI International, Menlo Park, CA, by using procedures previously d e ~ c r i b e d . ~The FIMS data processing (3)Burge, D.E.J. Appl. Polym. Sci. 1979,24,293-299. (4)Schoolery, J. N.;Budde, W. L. Anal. Chem. 1976,48,1458-1461. (5)Buttrill, S.E., Jr. Final Technical Report, SRI Project PYU 8903, 1981;SRI International: Menlo Park, CA.

Composition of Heavy Petroleums

Energy &Fuels, Vol. 2, No. 5, 1988 599

Gravity, “API

Fraction

50% AEBP, “F

0

*15000~~**

451

10

“500-650°F”

592

cut 1

722

E 40

Cut 2A

825

’-

Cut 2 8

887

- 20 is

30

2

u

-e

50

60

.-L

m

3 70

s

0 80 90

955 1023

Cut 4A Cut 4 8 cut 5

1091 1158 1239

SEF-1

(1365)

SEF-2 SEF-3

100

Table I. Carbon Number, Atmospheric Equivalent carbon no. n-alkane AEBP,” O F 5 97 8 258 10 345 12 42 1 15 20 25 30 35 40 45 1022 1139 60 80 1242 100 1306

Cut 3A Cut 3 8

ccc 1

Boiling Point (AEBP), and the Number of Acyclic Alkane Isomers no. of isomers* examples of Detroleum distillation cuts 3 1 gasoline 75 355)

t

82.2 x 1014 221.5 X lozo 1056.4 X lo2* 5920 X

OFrom ref 64. bFrom ref 6. and interpretation were performed in this laboratory. Results and Discussion General Considerations. Petroleum components can be viewed as two major groups of compounds, namely, hydrocarbons and nonhydrocarbons. The term hydrocarbons is used to describe molecules that comprise only carbon and hydrogen atoms. Hydrocarbons include acyclic alkanes (normal and isoparaffins), cycloalkanes (naphthenes), and aromatic hydrocarbons. Most naphthenes contain both saturated rings and side chains and are defined by the number of naphthenic rings, i.e., monocyclic, dicyclic, tricyclic, etc. Similarly, most of the aromatic hydrocarbons bear normal or branched chains and naphthenic rings. A molecule containing one aromatic ring is regarded a monoaromatic, a molecule with two aromatic rings diaromatic, with three aromatic rings as triaromatic, etc., even if several naphthenic rings and side chains are attached to the aromatic ring. Non-hydrocarbons are compounds that, in addition to carbon and hydrogen atoms, also include one or more heteroatoms such as sulfur, nitrogen, oxygen, vanadium, nickel, or iron.

.-*-

n--

---

I

vacuum residue, asphalt

+ 1+ +

1

“nondistillable” residue

The complexity of petroleum increases rapidly with increasing boiling point as a result of the increasing number of atoms in a molecule and the immense number of their possible structural arrangements. This makes the compositional analysis of high boiling petroleum fractions an extremely difficult task. Figure 1 illustrates the carbon number distribution (the total and aromatic carbon atoms) as a function of AEBP for heavy (13.6 OAPI) Kern River petroleum. (For more details see further discussion on “average molecular parameters”.) The curve representing the total number of carbon atoms covers a range from about 10 to almost 300 carbon atoms per molecule. The number of aromatic carbon atoms per molecule ranges from about 1.3 a t the “top of the barrel” to almost 150 a t the “bottom of the barrel”. The immense complexity of petroleum can be illustrated by using acyclic alkanes as an example. Table I shows the number of possible acyclic alkane isomers at a given carbon number.6 At Czo,a boiling point borderline for petroleum (6) McClellan, A. L., Chevron Research Co., unpublished data.


“heavy ends” (-650 O F + AEBP), the possible number of alkane isomers is already tens of thousands. The numbers become astronomical as the boiling point and the carbon number increase. Although alkanes are not a major component of heavy petroleums, a large percentage of the carbon atoms in other types of molecules is represented by paraffiiic chains. The molecules having naphthenic and aromatic rings also have a huge number of possible structural arrangements. The problem is further complicated by the presence of heteroatoms involving a variety of functional groups at an immense number of possible locations within a molecule. Even if only a very small percentage of the thermodynamically favored isomers were actually present in crude oil, the number of compounds comprising high boiling fractions of petroleum would be immense. This is one of the key factors limiting our ability to characterize those complex mixtures. It was recognized long ago that the characterization of high-boiling petroleum fractions in terms of the individual components is impossible. Even a combination of gas chromatography and mass spectrometry (GC/MS), one of the most powerful analytical tools, is limited to relatively low-boiling fractions. This limitation is due to the immense number of isomers rather than to the limited sample volatility. For example, petroleum middle distillates (350-650 O F AEBP) are already complex enough to result in chromatograms consisting of hundreds of poorly resolved peaks. This makes the GC/MS analysis, particularly the interpretation of the data, an extremely tedious and difficult task. Analysis of vacuum gas oils (VGO’s), which are also completely volatile (-650-1000 O F AEBP) under GC/MS conditions but which contain significantly more compounds that are more difficult to resolve, is a practical impossibility. It is obvious from the above discussion that the molecular characterization of heavy oils must involve some “grouping” of compounds, which inevitably leads to ambiguity of the compositional data. “Average Molecular Parameters”. The concept of an “average molecule” has been widely used to characterize fossil fuel liquids. It typically utilizes elemental analysis results and number-average molecular weight measurements to calculate the formulae of an “average molecule”. This approach is an extreme example of “grouping”. This analytical approach can be expanded by utilizing results generated by ‘H and 13CNMR spectrometry, and it allows for describing a complex mixture in terms of the “average molecular parameter^".^ When it is combined with high-resolution mass spectrometry (HRMS), it can lead to significant insights into molecular structure.E In this work, the “average molecular parameters” have been obtained by utilizing elemental analysis (C, H, N, S, 0, V, Ni, and Fe) results, percent aliphatic and aromatic carbon from 13C NMR, and number-average molecular weight measurements. This approach provides a convenient means for comparing complex mixtures without revealing their actual components. The “average molecular parameters” are being used here only to illustrate the changes in relative concentrations of the “molecular building blocks” with increasing AEBP. No attempt should be made to derive the “average molecular structure” (7) Petrakis, L.; Allen, D. NMR for Liquid Fossil Fuels; Elsevier: Amsterdam, Oxford, England, New York, Tokyo, 1987;pp 91-162 and references therein. (8) Aczel, T.; Williams, R. B.; Brown, R. A.; and Pancirov, R. J. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed., Academic: New York, San Fracisco, CA, London, 1978; Vol. 1, pp 499-539 and references therein.

Boduszy nski

from those data. A more detailed discussion of the limitations of the “average molecular structure” determinations can be found el~ewhere.~ The molecular weight has a very significant effect on the “average molecular parameters”. Figure 2 shows molecular weight distribution curves for two heavy petroleums: (top) Kern River, San Joaquin Valley, California; (bottom) Offshore California. The curves were obtained by plotting number-average molecular weight values measured for fractions having progressivelyhigher AEBP. Interestingly, data in Figure 2 show higher molecular weights for the “less heavy” (22.5 “API) Offshore California crude oil than for the ”heavier” (13.6 OAPI) Kern River petroleum. (See ref 1 for more details.) The average molecular weight values for distillate cuts (up to about 1300 O F AEBP) were obtained by VPO in toluene and were in excellent agreement with those obtained earlier by using FIMS.’ Thus, these results demonstrate that the VPO measurements for distillate fractions are not affected by intermolecular associations. The results for the SEF fractions, which were derived from DISTACT “nondistillable”residues (about 1300 O F + AEBP), were obtained by using VPO both in toluene and in pyridine. Data in Figure 2 illustrate the dramatic effect of intermolecular associations on the molecular weight measurements by VPO in toluene. The latter produced much higher average molecular weight values than those obtained in solutions of pyridine. These results are consistent with those reported earlier by Moschopedis et al.’O The previously reported FIMS data’ for the SEF-1 fractions are in a fairly good agreement with the results obtained by using VPO in pyridine. Unfortunately, in the case of the SEF-2 and SEF-3 fractions (the SEF-4 fractions were too small to analyze), such a comparison is impossible because of the limited sample volatility (about 40-70%) under the conditions of FIMS analysis, which did not allow representative average molecular weight values to be obtained. However, FIMS data obtained for those fractions indicated relatively low molecular weights extending from about 500 to 2000 amu.l The number-average molecular weights for the SEF-2 and SEF-3 fractions obtained by using VPO in pyridine, although significantly lower than those obtained by VPO in toluene (see data in Figure 2), are beyond the molecular weight range indicated by FIMS. The average molecular weight values obtained for those fractions using VPO in pyridine may still be affected by intermolecular associations. This contention is supported by the literature data on the effects of solvent polarity and temperature on the VPO molecular weight measurements for “asphaltenes”. For example, the measurements in nitrobenzene gave significantly lower values than those obtained in pyridine. A higher temperature further reduced the molecular weight values.1° Data in Tables I1 and I11 illustrate the complexity of heavy petroleums using “average molecular parameters”. The results obtained for DISTACT-SEF fractions that were derived from Kern River (Table 11) and Offshore California (Table 111) AR’s are used as examples. The results in Table I1 show that, for cut 1 (722 O F 50% AEBP), on average every molecule is composed of 24.1 carbon atoms (6.4 aromatic and 17.7 aliphatic) and 39.3 hydrogen atoms. The data also indicate (if we assume one heteroatom per molecule) that for every 1000 molecules, there are 104 sulfur-, 75 nitrogen-, and 88 oxygen-con(9)Boduszynski, M.M.Liq. Fuels Technol. 1984,2,211-232. (10)Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976,55, 227-232. Speight, J. G.In The Chemistry and Technology of Petroleum; Dekker: New York, 1980;p 211, Table 7-6.

Energy & Fuels, Vol. 2, No. 5, 1988 601


Cut Point,

KERN RIVER CRUDE OIL (13.6 "API)

0 By VPO in Toluene

By VPO in Pyrldlne

Numbedverage Molecular Weight

NumberAverage Molecular Weight

Figure 2. Number-average molecular weight distributions for Kern River and Offshore California heavy petroleums. Table 11. "AverageMolecular Parameters" for Kern River AR 50% AEBP, fraction

O F

cut 1 cut 2A cut 2B cut 3A cut 3B cut 4A cut 4B cut 5 SEF-1

722 825 887 955 1023 1091 1158 1239 (1365)

cumwt% from AR 21.6 32.8 42.8 51.4 58.3 64.3 68.9 74.3 90.3

SEF-2

96.0

SEF-3

99.1

SEF-4

99.6

mol wt 3375 420" 471" 5275 594" 682' 755" 876" 1464" 1112b 3064" 2518b 5625" 3619*

"2"' -8.9 -12.0 -14.7 -16.9 -19.7 -23.6 -27.0 -31.5 -57.9 -44.1 -170.5 -140.3 -334.6 -215.4

c,

car

24.1 30.0 33.7 37.7 42.3 48.6 53.8 62.4 103.8 78.9 218.1 179.3 395.8 254.7

6.4 8.0 9.0 10.0 12.4 14.0 16.1 17.7 38.6 29.3 97.3 80.0 214.1 137.8

a Number-average molecular weight determined by VPO in toluene. '"Z" in the general formula C,H,,,+zX.

taining molecules. This means that 26.7% of all molecules in cut 1 could be represented by non-hydrocarbons.

€3 39.3 48.0 52.7 58.5 64.9 73.6 80.6 93.3 149.7 113.7 265.7 218.3 457.0 294.0

av no. of atoms/molecule S N 0 0.075 0.088 0.104 0.118 0.134 0.156 0.163 0.215 0.177 0.196 0.293 0.198 0.249 0.378 0.264 0.349 0.296 0.492 0.235 0.316 0.615 0.515 0.356 0.745 1.464 0.873 0.608 0.737 0.462 1.112 2.298 1.388 4.815 1.888 1.141 3.957 4.641 2.514 9.643 2.986 1.617 6.204

V

Ni

Fe

0.001 0.001 0.008 0.001 0.001

0.001 0.002 0.001 0.003 0.002 0.023 0.019 0.038 0.025

0.001 0.001 0.015 0.013 0.042 0.027

0.010

0.009 0.030 0.019

* Number-average molecular weight determined by VPO in pyridine. These are, of course, only average estimates. The actual distribution of heteroatoms in a molecule is not known.


Boduszynski

Table 111. “Average Molecular Parameters” for Offshore California AR

fraction cut 1 cut 2A cut 2B cut 3A cut 3B cut 4A cut 4B cut 5 SEF-1

50 % AEBP, OF 697 804 887 955 1026

1100 1153 1243 (1372)

cumwt% from AR 18.9 30.4 38.4 45.4 49.4 54.2 57.0 62.7 78.4

SEF-2

87.9

SEF-3

98.6

SEF-4

99.7

av no. of atoms/molecule mol w t 329” 399” 469” 549” 609” 704” 785” 955” 2254“ 1911b 5697” 4146b 16348’ 6069*

“Z”‘

ct

c,

-7.6 -10.1 -11.7 -13.8 -16.8 -21.3 -25.6 -30.8 -79.3 -67.3 -278.2 -202.6 -908.9 -337.5

23.0 28.1 32.9 38.3 42.7 48.8 55.1 66.2 154.6 131.1 387.5 282.1 1112.1 412.9

3.9 5.5 6.4 7.8 8.5 10.7 14.3 16.4 41.4 35.1 151.9 110.6 487.1 180.8

H 38.3 46.1 54.1 62.7 68.6 76.3 84.6 101.6 229.9 194.9 496.8 361.5 1315.3 488.3

S

N

0

V

Ni

Fe

0.282 0.353 0.462 0.624 0.807 0.990 1.185 1.611 4.120 3.487 11.358 8.250 34.739 12.871

0.056 0.108 0.141 0.188 0.239 0.332 0.387 0.532 1.481 1.255 7.121 5.180 23.938 8.882

0.062 0.090 0.117 0.137 0.164 0.189 0.235 0.310 0.873 0.740 2.991 2.177 12.261 4.552

0.001 0.004 0.007 0.008 0.007 0.006 0.104 0.076 0.452 0.168

0.001 0.001 0.002 0.002 0.002 0.024 0.017 0.103 0.038

0.001 0.001 0.016 0.012 0.088 0.033

*

a Number-average molecular weight determined by VPO in toluene. Number-average molecular weight determined by VPO in pyridine. “2” in the general formula C,H2,+zX.

For example, should oxygen-containing species be represented only by carboxylic acids (two oxygen atoms per molecule) and nitrogen-containing species be represented only by porphyrins (four nitrogen atoms per molecule), then the number of oxygen- and nitrogen-containing molecules in cut 1 would be reduced by a factor of 2 and a factor of 4, respectively. Data for the higher boiling cut 3B (1023 OF 50% AEBP) derived from the same crude oil (Table 11) show that the “average molecule” is composed of 42.3 carbon atoms (12.4 aromatic and 29.9 aliphatic) and 64.9 hydrogen atoms, and for every lo00 molecules, there are already 249 sulfur-, 378 nitrogen-, and 264 oxygen-containing molecules. This brings the concentration of non-hydrocarbons in cut 3B to a total of 891 molecules/lOOO or 89.1%. For comparison, “an average molecule” in cut 3B (1026 OF 50% AEBP) derived from Offshore California AR (Table 111) is composed of 42.7 carbon atoms (8.5 aromatic and 34.2 aliphatic) and 68.6 hydrogen atoms, and for every 1000 molecules there are 807 sulfur-, 239 nitrogen-, and 164 oxygen-containingmolecules and one vanadium-containing molecule. This brings the total concentration of heteroatoms to 1.21 per molecule, implying that some molecules must contain more than one heteroatom. The number of heteroatoms per molecule increases rapidly with increasing AEBP. Results in Table I11 show the following trend for Offshore California AR:1.516 heteroatoms/molecule for cut 4A (1100 OF 50% AEBP), 1.815 heteroatoms/molecule for cut 4B (1153 OF 50% AEBP), 2.462 heteroatoms/molecule for cut 5 (1243 OF 50% AEBP), and 5.490 heteroatoms/molecule for the SEF-1 fraction (1372 OF 50% AEBP). Two sets of data are reported in Tables 11 and I11 for the SEF fractions derived from “nondistillable” residues. The first set of “average molecular parameters” was obtained by using erroneously high average molecular weight values determined by VPO in toluene. The second set of data is based on average molecular weight values determined by VPO in pyridine, which, as discussed earlier, are considerably lower. (See also Figure 2.) The results illustrate the dramatic effect of intermolecular associations on VPO molecular weight measurements and, consequently, on the calculated “average molecular parameters”. For example, data in Table I11 show that an “average molecule” in the SEF-3 fraction of Offshore California crude oil is composed of the following number of atoms: 412.9 carbon (of which 180.8 are in aromatic rings), 488.3 hydrogen, 12.9 sulfur, 8.9 nitrogen, 4.5 oxygen,

0.17 vanadium, 0.04 nickel, and 0.03 iron when the number-average molecular weight value of 6069 determined by VPO in pyridine is used. However, when the numberaverage molecular weight value of 16 348 determined by VPO in toluene is used, the average number of atoms per molecule for the same SEF-3 fraction is as follows: 1112.1 carbon (of which 487.1 are in aromatic rings), 1315.3 hydrogen, 34.7 sulfur, 23.9 nitrogen, 12.3 oxygen, 0.45 vanadium, 0.10 nickel, and 0.09 iron. In the past, erroneously high average molecular weights have led to many misconceptions regarding the molecular nature of heavy crudes and, particularly, of so-called “asphaltenes” (a fraction insoluble in alkane solvents) for which molecular weight values extending up to 100000 or more have been reported.lG15 “Compound-Class Composition”. Many attempts have been made in the past to separate “heavy ends” of petroleum into groups or types of compounds. Numerous separation methods and entire separation schemes have been developed over the years.16 Characterization approaches based on chromatographic separations describe heavy oils in terms of weight percent concentrations of operationally defined fractions, providing so-called “compound-class”or “compound-type” composition. The SARA method, S = “saturates”, A = “aromatics”, R = “resins”, and A = “asphaltenes”, is a classic example.” This kind of compositional data, although useful for a rough comparison of different feedstocks, does not reveal the complex chemistry of heavy oils. It is obvious that both the concentration of those fractions and the molecular nature of their components are likely to vary with increasing AEBP and the origin of the crude oil. (11) Bunger, J. W. Prepr.-Am. Chem. SOC.,Diu.Pet. Chem. 1977,

22(2), 716.

(12) McKay, J. F.; Amend, P. J.: Cogswell, T. E.; Harnsberger, P. M.; Erickson, R. B.; Latham, D. R. In Analytical Chemistry of Liquid Fuel Sources, Tar Sands, Oil Shale, Coal, and Petroleum; Uden, P. C., Siggia, S.,and Jensen, H. B., Eds.;Advances in Chemistry Series 170; American Chemical Society: Washington, DC, 1978; pp 128-142. (13) Bunger, J. W.; Thomas, K. P.; Dorrence, S.M. Fuel 1979,58,183. (14) Boduszynski, M. M.; McKay, J. F.; Latham, D. R. Asphalt Pauing Technol. 1980.49, 123-143. (15) Chemistry of Asphaltenes; Bunger, J. W., and Li, N. C., Eds.; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1981. (16)Altgelt, K. H.; Jewell, D. M.; Latham, D. R.; Selucky, M. L. In Chromatography in Petroleum Analysis; Altgelt, K. H., Gouw, T. H., Eds.; Chromatographic Science Series: Dekker: New York and Basel, 1979; pp 185-214 &d references therein. (17) Jewell, D. M.; Albaugh, E. W.; Davis, B. E.; Ruberto, R. G. Ind. Eng. Chem. Fundam. 1974,13, 278.



I

Sample

I

+ HPLC-BA/AA

“Neutral“

“Basic”

“Pyrrolic”

”Acidlc”

HPLC-NHs

Monoaromatics HPLk-SiL

Figure 3. HPLC separation scheme.

One of the most recent systematic studies on the composition of “heavy ends” of petroleum was the American Petroleum Institute Research Project 60.18-39This research effort greatly advanced the knowledge of the composition of high-boiling petroleum fractions and demonstrated the need for extensive separations followed by analyses of the fractions in order to derive meaningful compositional information. In this work, the goal of chromatographic separations was to prepare chemically meaningful and operationally Hinds, G. P. Proc.-Am. Pet. Inst., Sec. 3, 1970,50, 279. Hainea, W. E.; Ward, C. C.; Sugihara, J. M. Proc.-Am. Pet. Znst., Diu.Refin. 1971, 51, 375. (20) Jewell, D. M.; Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D. R. Anal. Chem. 1972,44, 1391. (21) Hirsch, D. E.; Hopkins, R. L.; Coleman, H. J.; Cotton, F. 0.; Thompson, C. J. Anal. Chem. 1972,44,915. (22) McKay, J. F.; Jewell, D. M.; Latham, D. R. Sep. Sci. 1972, 7,361. (23) McKay, J. F.; Latham, D. R. Anal. Chem. 1972,44, 2132. (24) Cogswell, T. E.; McKay, J. F.; Latham, D. R. Anal. Chem. 1971, (18) (19)

43, 645. (25) Coleman, H. J.; Dooley, J. E.; Hirsch, D. E.; Thompson, C. J. Anal. Chem. 1973,45, 1724. (26) Dooley, J. E.; Hopkins, R. L.; Hirsch, D. E.; Coleman, H. J.;

Thompson, C. J. “Compound Type Separation and Characterization Studies for a 370 to 535 OC Boiling Distillate of Gach Saran, Iran, Crude Oil”; BuMines RI 7770; US. Bureau of Mines: Pittsburgh, PA, 1973. (27) Hirsch, D. E.; Dooley, J. E.; Coleman, H. J.; Thompson, C. J. “Compound Type Separation and Characterization Studies for a 370 to 535 OC Distillate of Wilmington, Califomia, Crude Oil”,BuMiies RI 7893, US. Bureau of Mines: Pittsburgh, PA, 1974. (28) Hirsch, D. E.; Dooley, J. E.; Vogh, J. W.; Thompson, C. J. “Compound Type Separation and Characterization Studies for a 370 to 535 OC Distillate of Recluse, Wyoming, Crude Oil”, BuMines RI 7945, U.S. Bureau of Mines. Pittsbureh. PA. 1974. (29) Dooley, J. E.; Hirsch, DrE.; Thompson, C. J.; Ward, C. C. Hydrocarbon Process. 1974,53(11). 187 (30) Haines, W. E.; Hirsch,-D. E.; Thompson, C. J. “Separating and Characterizing High-Boiling Petroleum Distillates: The USBM-API Procedure”: RI LERC/RI-75 / 2, BERC/RI-75/ 2, July 1975. (31) Thompson, C.’J.; Ward, C. C.( Ball, J:S. “Characteristics of World‘s Crude Oils and Results of API RP 60”, RI BERC/RI-76/8, July 1976. (32) McKay, J. F.; Cogswell, T. E.; Weber, J. H.; Latham, D. R. Fuel 1975,54, 50. (33) McKay, J. F.; Weber, J. H.; Latham, D. R. Anal. Chem. 1976,48, 891. (34) McKay, J. F.; Amend, P. J.; Harnsberger, P. M.; Cogswell,T. E.; Latham, D. R. Fuel 1981,60, 14. (35) McKay, 3. F.; Harnsberger, P. M.; Erickson, R. B.; Cogswell, T. E.; Latham, D. R. Fuel 1981,60, 17. (36) McKay, J. F.; Latham, D. R.; Haines, W. E. Fuel 1981, 60, 27. (37) Boduszynski, M. M.; McKay, J. F.; Latham, D. R. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1981,865-881. (38) Grizzle, P. L.; Green, J. B.; Sanches, V.; Murgia, E.; Lubkowitz, J. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1981,839-850. (39) Sturm, G. P.; Jr.; Grindstaff, Q.G.; Hirsch, D. E.; Scheppele, S. E.; Hazos, M. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1981,851-856.

well-defined “compound-class” fractions for further molecular characterization. One must bear in mind, however, that even the most efficient chromatographic separation can only produce operationally well-defined fractions. The mere separation of a complex mixture into fractions does not reveal their composition. I t accomplishes, however, an important goal of “grouping”and isolating components of interest from a complex sample matrix. Thus, it makes the fractions more amenable to further molecular characterization. Of particular interest was the effect of increasing AEBP, and thus increasing complexity, upon (a) the performance of chromatographic separations and (b) the ability of mass spectrometry to describe the chromatographic fractions in terms of homologous series of compounds. Figure 3 shows the HPLC separation scheme used in this study. A schematic diagram in Figure 4 illustrates the DISTACT-SEF-HPLC-FIMS separation and characterization steps, using Kern River petroleum as the example. The UV/vis profiles for the “solubility” fractions (SEF) derived from DISTACT “nondistillable”residue are shown in Figure 5. The first HPLC step (Figure 3) involves separation on a dual basic alumina/acidic alumina column (HPLCBA/AA). Conceptually, this separation step is similar to the API RP 60 scheme because it first separates the sample on the basis of acidity and basicity. The major difference, however, is the use of basic and acidic alumina instead of anion- and cation-exchange resins. Although the latter were believed to be more selective than alumina (due to simpler in nature interactions with solute molecules), they require tedious preparations, were difficult to reproduce from batch to batch, and were reported to introduce artifacts due to the deterioration of r e s i n ~ . ~ ~ ~ * ~ The HPLC-BA/AA method uses fresh, “as received”, basic and acidic alumina for each separation. The separation requires less than 2 h to complete. The fractions are operationally very well-defined due to the use of automated HPLC equipment. The method has been developed primarily for separations of DISTACT distillable cuts covering a boiling point range from about 650 to 1300 OF AEBP. However, in this work the method has been also used for the SEF-1 fractions, which were derived from DISTACT “nondistillable” residues (see Figure 4). These solubility fractions had a 50% AEBP of approximately 1370 OF as determined by simulated distillation methods.l2 The results obtained for fractions boiling up to about 1300 O F AEBP showed material recoveries of 98.5 wt % or better. Losses for the SEF-1 fractions did not exceed about 8 wt %. The separation of nitrogen-containing species into “acidic”, “basic”, and “pyrrolic” compounds using the HPLC-BA/AA method was excellent, with nitrogen balance of 95% or better. Further separation of “neutral” compounds involved two additional steps as shown in Figure 3. The HPLC-NH2 method provides separation of aromatics according to the number of double bonds in the aromatic ring system with the simultaneous survey and identification of aromatic ring cores by the photodiode-array detector. This is illustrated in Figure 6, which shows an example of the HPLC-UV map for “aromatics” in a straight-run VGO (-650-1000 OF AEBP). The data show well-resolved ring-type fractions for which the predominant aromatic ring cores, namely, benzene ( l ) , naphthalene (2), phenanthrene (3), and pyrene followed by chrysene (4), can be recognized. (40) Green, J. B.; Hoff, R. J.: 1984,63, 1290-1301.

Woodward, P. W.; Stevens, L. L. Fuel

(41)Strachan, M. G.; Johns, R. B. Anal. Chem. 1987, 59, 636-639.


Boduszynski

KERN RIVER PETROLEUM Wt %*

Fraction O C

I.D.

-

13.6”API

HPLC

O F

Homologous Series CnHzn+z

204

4001

260

500

3

---

5 6-500F-

Dicyclic 0.182 Wt ‘10 Monocyclic 0.112 Wt %

G3

Tricyclic 0.252 Wt

O/o

Tetracyclic 0.385 Wt VO

h

0

m W

3

1 2 4 -- 500-65OF-

--

Acyclic 0.105 Wt Oh*

Pentacyclic 0.399 Wt O/o

e

-

2

-427

]177-CUT

800 92--CUT

e 0

2

482

I

2A

8.2 - CUT 2 8

900

7.0 -CUT

3A

5.7 -CUT

38

4.9 -CUT

4A

--@ -

Hexacyclic 0.217 Wt O h Saturates “Compound-Class” Fractions FlMS /

\

-@B--

i

n

g

3

593 1100

E

f w

5

3.8-CUT

48

649 1200

0

=t

v)

4.4 -CUT

704 1300

7 6 0 1400

CnHzn-6 Tetracycllc Alkanes

5

---e3

-

1.6 I c30

L .n

I-”

I

0

t

L

2 13.1-SEF-1 3

4.7-SSEF-2

-

2.5 - SEF-3

-

0.4

0)

K n 0 Carbon Number

- SEF-4

*Wt % from Crude Oil

Figure 4. Schematic diagram of the DISTACT-SEF-HPLC-FIMS separation and characterization steps for Kern River petroleum. Key to HPLC fractions: (1) “saturates”;(2) ”monoaromatics”;(3) “diaromatics”;(4) ‘triaromatics”, (5) “tetraaromatics”;(6) “pentaaromatics and greater”; (7) “basic”compounds; (8) ”pyrrolic”N compounds; (9) “acidic”compounds.

The effect of alkyl substituents on ring-type separation is an obvious concern. Results obtained with model compounds indicated the following trend. A paraffinic side chain slightly decreases the retention time, while a naphthenic ring attached to the aromatic ring slightly increases the retention time as compared with the unsubstituted aromatic hydrocarbon. When both types of alkyl substituents are present, there is a tendency for those two factors to cancel out. However, the effect of alkyl substituents upon the efficiency of this separation step becomes more significant with increasing AEBP. This is illustrated in Figure 7A,B, which shows HPLC-UV maps for fractions having progressively higher AEBP. The increasing sample complexity with increasing AEBP and consequently increasing degree of alkyl substitution on aromatic rings cause some overlapping between adjacent ring-type fractions. Analyses of the fractions derived from distillates covering about 650-1000 O F AEBP range indicate less than 20% overlapping between ring-type fractions. The overlapping between the fractions boiling above 1000 O F AEBP is greater, and it is difficult to quantitate because of the lack of adequate analytical methods. The “neutral“ sulfur-, nitrogen-, and oxygen-containing compounds generally show chromatographic behavior similar to that of aromatic hydrocarbons having the same aromatic ring number. The results show, however, that only the sulfur-containingcompounds are major interfering species during the separation step. The separation of saturates from monoaromatics cannot be accomplished by

using the amino-bonded phase column (HPLC-NH2). The third separation step, involving the HPLC-SIL method, was required to provide good separation of saturates from monoaromatics. Figure 8 illustrates the “compound-class” composition of Kern River petroleum. This is a composite picture obtained by plotting the results of chromatographic separations for fractions having progressively higher AEBP up to approximately 1400 O F . The data in Figure 8 show a dramatic decrease in “saturates” (no. 1in Figure 8)with increasing AEBP. The “saturates” account for 28.3 wt % of Kern River crude oil. Approximately 41% of the total “saturates” are represented by distillate cuts boiling below 650 OF AEBP. Almost 92% of the “saturates” do not exceed lo00 O F AEBP, and 99% do not exceed about 1300 OF AEBP. The “aromatics”having one through four aromatic rings (mono, no. 2;di, no. 3; tri, No. 4; and tetra, No. 5 in Figure 8) account for 30.8 wt % of Kern River petroleum and show a unique distribution pattern. Monoaromatics (no. 2) and diaromatics (no. 3) dominate. The distillation cuts boiling below 650 OF AEBP contain only about 19% of the total “aromatics”, while the 650-1000 O F AEBP portion of this crude oil accounts for almost 62% of the total “aromatics”. Approximately 98% of the “aromatics” do not exceed 1300 O F AEBP. The fraction designated “pentaaromatics and greater” (no. 6 in Figure 8) was found to contain considerable amounts of “neutral” nitrogen compounds. This fraction


Composition of Heavy Petroleums SEF-1

SEF-2 SEF-3

81.o

Time, Min.

648.0

SEF-1

SEF-2

SEF-3

0.0‘

Time, Min.

81.0 648.0

Figure 5. UV/vis profiles for the “solubility” (SEF) fractions derived from Kern River “nondistillable” residue.

20*ol

225.0

F45.0

Wavelength, nm

398.0 0.0

Time, Min.

Figure 6. HPLC-UV map of “aromatics” in a straight-run vacuum gas oil: (1) monoaromatics; (2) diaromatics; (3) triaromatics; (4) tetraaromatics.

accounts for 6.5 wt % on a crude oil basis. Approximately 25% of this fraction is represented by the VGO (650-1000 O F AEBP), and almost 50% is present in the 1000-1300 OF AEBP range. The “basic” compounds (no. 7 in Figure 8) account for 7.5 wt % of Kern River petroleum. Data show that about equal portions of this “compound-class” fraction, each accounting for 28%, are represented by the 650-1000 and 1000-1300 O F AEBP ranges. The “pyrrolic” N compounds (no. 8 in Figure 8) account for 10.1 wt % of this crude oil. The VGO (-650-1OOO O F AEBP) accounts for about 30% of the total “pyrrolic” N compounds, and the 1000-1300 O F AEBP range accounts for 37%. The “acidic” compounds (no. 9 in Figure 8) account for 7.7 w t % on the crude oil basis and show the following distribution pattern: 25% in distillates below 1000 O F AEBP, 35% in the 1-1300 O F AEBP range, and 40% in the fraction above 1300 O F AEBP. The SEF-2, SEF-3, and SEF-4 fractions, derived from “nondistillable” residue, together account for 7.6 wt % of Kern River petroleum (see Figures 4 and 5) and by definition (all three being pentane insoluble) belong to the

so-called pentane asphaltenes, which are conventionally produced by precipitation. However, there is not a 1:l correspondence between the above-noted SEF fractions and the conventional pentane aspha1tenes.l These fractions could not be separated by using the separation scheme shown in Figure 3 without significant losses due to irreversible adsorption. Homologous Series of Compounds. The term homologous series is used to describe compounds belonging to the same class and having the same general formula CnH2rl+zX where C and H are carbon and hydrogen, respectively, n is the number of carbon atoms, 2 is the homologous series value or hydrogen deficiency number, and X refers to heteroatom(s) (S, N, 0, V, Ni, Fe). Members of the homologous series differ by the homologous unit “CH2”(14 m u ) and are referred to as homologues. The carbon number homologues are defined by their molar mass/charge value m/z. The isomers are not distinguished. For example, the acyclic alkanes C5H12(m/z 72), C6H14 ( m / z 86), C7H16( m / z loo), etc., all belong to the same homologous series, which is described by the general formula CnH2n+2 and includes n- and isoalkanes (paraffins). The concept of “grouping” of compounds on the basis of their general formulas has been utilized for years in so-called “type The early method of Brown42 involved mass spectrometry capable of unit resolution up to only a few hundred in molar mass. Soon, higher resolution mass spectrometers allowed the analyzing of higher molar mass mixtures.43 However, it became evident that only a slight increase in the molar mass of the component molecules in a complex mixture creates a formidable problem. Namely, compounds with different structures and belonging to different homologous series can have the same general formula. This greatly complicates the analysis. For example, alkylbenzenes and tetracyclic alkanes (naphthenes) both share the identical empirical formula CnH2n-6. In the case of relatively low-boiling hydrocarbon mixtures, the above problem was partly solved by the use of chromatographic separations prior to the mass spectral analysis. Separate “type analyses” were devised for aroand saturate& fractions. However, those methods were still incapable of resolving such homologous series of compounds as, for example, alkylnaphthalenes and tetracyclic monoaromatics (e.g., alkyldecahydropyrenes),both sharing the same formula CnH2n-12.Efforts have also been made to develop the high-resolution electron-impact mass spectrometry (HR EIMS) “type analysis”, which does not require prior chromatographic separation.& However, all “type analyses”, by their very nature, cannot provide accurate compositional data. Those methods are also limited to hydrocarbon mixtures boiling below about 950 O F AEBP. In this work, FIMS has been used as a routine method to describe the HPLC-derived “compound-class” fractions in terms of “apparent” 2 series and their carbon number distribution profiles. This characterization approach, (42)Brown, R. A. Anal. Chem. 1951,23,430. (43)O’Neal, M. J., Jr.; Wier, T. P. Anal. Chem. 1951,23,830. (44)Hastings, S. H.; Johnson, B. H.; Lumpkin, H. E. Anal. Chem. 1956,28,1243. (45)Hood, A.; O’Neal, M. J., Jr. Proceedings of Joint Conference Organized by Hydrocarbon Research Group, Institute of Petroleum, and ASTM Committee E-14,University of London, September 24-26,1958. (46) Gallegos, E. G.; Green, J. W.;Lindeman, L. P.; LeTourneau, R. L.; Teeter, R. M. Anal. Chem. 1967,39,1833. (47)Robinson, C. J.; Cook, G. L. Anal. Chem. 1969,37,792.

Boduszynski


A

1

100.0

B

592°F 50% AEBP

80.0

OF

Avg. Carbon Number = 17

60.0 40.0 20.0 225.0

40.0

Wavelength, nm

)\40.0 Wavelength, nm

398.0 0.0

Time, Min.

Y 398.0 0.0

100.0-

100.0Avg. Carbon Number = 30

Time, Min.

i

825°F 50% AEBP

80.0.

50% AEBP

Carbon Number = 49

1239°F 50% AEBP

80.0.

60.0-

60.0.

40.0.

40.0-

20.0-

20.0-


,‘40.0

Wavelength, nm 398.0

0.0

l i m e , Min.

398.0

100.0~

100.01I 955°F

80.0-

Time, Min.

0-0

1

A

I

1‘G

50% AEBP


60.0-

1365°F 50% AEBP Avg. Carbon Number = 85

40.0-

20.0. 225.0%

pi

Wavelength, nm

e40.0 v

398.0 0.0

Time, Min.

Time, Min. 398.0’

Figure 7. HPLC-UV maps: (A) uaromatics”derived from DISTACT distillate cuts having 50% AEBP of 592 (top), 825 (center), and 955 O F (bottom); (B) “aromatics”derived from DISTACT distillate cuts having 50% AEBP of 1091 (top) and 1239 OF (center), and from the SEF-1 fraction having 50% AEBP of 1365 O F (bottom).

utilizing more efficient HPLC separation methods than those available in the past, allows the unravelling of changes in composition of heavy oils with increasing AEBP up to about 1400 OF. Despite some ambiguity in the FIMS data, the results provide improved understanding of the immense complexity of these mixtures. The great advantage of FIMS is that it produces essentially fragmentation-free molecular ion spectra for most organic compound^.^^^^ This technique has been well ~~

(48) Beckey, H. D. Field Ionization Mass Spectrometry; Pergamon: Oxford, England, 1971. (49) Scheppele, S. E.; Grizzle, P. L.; Greenwood, G. J.; Mariott, T. D.; Pereira, N. B. Anal. Chem. 1976,48,2105.

(50) Scheppele, S. E.; Greenwood, G. J.; Benson, P. A. Anal. Chem.

1977,49,1847. (51) Buttrill, S . E., Jr. Org. Coat. Plast. Chem. 1980, 43, 330. (52) Yoshida, T.; Maekawa, Y.; Higuchi, T.; Kubota, E.; Itagaki, Y.; Yokoyama, S. Bull. Chem. SOC.Jpn. 1981,54,1171. (53) Yoshida, T.; Maekawa,Y.; Shimada, T. Anal. Chem. 1982,54,967. (54) Boduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Anal. Chem. 1983,55, 232.

established a t SRI International, and it is available on a routine bash5 However, the nominal-mass resolution of FIMS ( M / U = -1400) is an obvious limitation. “Saturates”.Figure 9 shows examples of FIMS molar mass profiles for “saturates” separated from VGO’s (650-1000 OF AEBP) that were derived from three different crude oils. All spectra have been corrected for the natural abundance of the carbon-13 isotope. Each spectrum consists of even-numbered molecular ion peaks that correspond to the following “apparent”2 series: CnHk+,, acyclic alkanes; CnHan,monocyclic alkanes; CnH2n-2,dicyclic alkanes; CnH2n-4,tricyclic alkanes; CnHan4,tetracyclic alkanes; CnH2n-8,pentacyclic alkanes; CnH2n-10, hexacyclic alkanes. The molar mass peaks belonging to each 2 series are 14 amu (CH,) apart. Intensities of the peaks have been (55) Boduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Fuel 1985,64, 242. (56) Boduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Fuel 1986, 65, 223.



Fraction

Gravity, “API

g

13.6

50% AEBP, “F

~~~oooF-”

451

“500-650°F”

592

20 30

-

cut 1

722

40

-

Cut 2A

825

S 50

-

Cut 28

887

1 0

10

20 30 40 50 60 70 80 Cumulative Wt % from Fraction

90

Cut 3A

955

Cut 3 8 Cut 4A cut 4 8 cut 5

1023 1091 1158 1239

SEF-1

(1365)

SEF-2 SEF-3 SEF-4

100

Figure 8. “compound-class”distributions in Kern River petroleum: (1) “saturates”;(2) “monoaromatics”;(3) “diaromatics”;(4) “triaromatics”;(5)”tetraaromatics”;(6)“pentaaromaticsand greater”;(7) “basic” compounds; (8) “pyrrolic” N compounds; (9) “acidic”

compounds; (10) losses.

converted to weight percent concentrations by using relative ionization sensitivities for each 2 series. Data in Figure 9 demonstrate that a mere comparison of weight percent yields of “saturates” (90.5 wt % in VGO “A”, 36.3 wt % in VGO “B”, and 58.6 wt % in VGO “D”) has limited analytical value because it does not reveal their molecular nature. The FIMS results show dramatic differences in compositions of the three “saturate” fractions. The “saturates” from VGO “A”consist predominantly (72.6 wt %) of acyclic alkanes (paraffins, 2 = +2) covering the carbon number range from C1, to about C4@Note that isomers are not distinguished. Each molar mass peak may be represented by a large number of isomers. (See Table I.) The cycloalkanes (naphthenes) in this sample include mainly monocyclics (2 = 0, 18.7 wt %) and dicyclics (2 = -2, 5.8 wt %). The higher ring-number naphthenes account for a very small portion of this sample. (See Figure 9, top.) Conversely, the “saturate” fraction from VGO “B” contains mainly naphthenes and only 3.5 wt 90 of paraffins (2 = +2). Tetracyclic naphthenes (2 = -6) are most abundant in this sample (37.5 w t %) and show an unusual carbon number distribution with peaks a t m l z 372,386, 400, and 414 that are characteristic of C27-C30steranes. (See Figure 9, center.) Those were confirmed by the fragmentation pattern in the EIMS spectra. The third sample of “saturates”,derived from VGO “D” (Figure 9, bottom), contains relatively high concentrations of both paraffins and naphthenes. Figure 10 illustrates the distribution of alkane homologous series in Kern River petroleum as a function of AEBP. This immature and biodegraded crude oil has a very low concentration of acyclic alkanes (paraffins), and it is rich in cycloalkanes (naphthenes). The relatively high concentration of tetra- and pentacyclics (no. 5 and no. 6 in Figure 10, respectively) is of particular interest because those include such “biomarkers”as tetracyclic steranes and pentacyclic triterpanes (e.g., hopane~).~’ (57) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: West Berlin, 1984.

The use of HPLC and FIMS for characterization of “saturates” is particularly attractive for high-boiling petroleum fractions. This is illustrated in Figure 11, which shows an unusual carbon number distribution pattern for tetracyclic naphthenes in high-boiling cuts derived from Kern River petroleum. (See also Figure 10 for the distribution of the alkane homologous series.) Interestingly, the FIMS data in Figure 11 reveal the presence of tetracyclics covering the carbon number range from CI9 ( m l z 260) to CS (mlz 1198). These data are significant as they greatly exceed the carbon number reported for “biomarkers” and the carbon number that is amenable to GC/MS (-

Composition of heavy petroleums. 2. Molecular characterization

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