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Composition of petroleum heavy ends. 2. Characterization of compound types in petroleum >675 °C residues
Composition of petroleum heavy ends. 2. Characterization of compound types in petroleum > 675°C residues John F. McKay, Paul M. Harnsberger, Cogswell, and Dewitt R. Latham
R. Brent
Erickson,
Thomas
E.
Laramie Energy Technology Center, US Department of Energy, P.O. Box 3395, Laramie, Wyoming 82071, USA (Received 20 December 1979)
The characterization of polar compound types in petroleum residues of b.p. >675”C are described. The fractions of acids, bases, and neutral nitrogen compounds prepared by the method described in Part 1 (Fuel 1981, 60, 14) were further separated and analysed. Major compound types identified in the acid fraction are carboxylic acids, phenols, pyrrolic nitrogen compounds, and amides. Quantitative estimates of the compound types were made by two infrared methods. Major compound types identified in the bases were pyridine benzologs, amides, and pyrrolic nitrogen compounds; types found in the neutral nitrogen fraction were amides and pyrrolic nitrogen comoounds. The average molecular weight of the >675”C residues was estimated to be 900 after being determined by four -different methods.
This paper describes the characterization of polar compound types in petroleum residues of b.p. >675”C. The fractions of acids, bases, and neutral nitrogen compounds prepared by the method described in the previous paper’ were further separated and analysed. In general, the techniques used for the analyses of compound types in the residues were the same as those used in the study of highboiling distillates2.3 so that the compositions of the two different materials could be compared. Information concerning the composition of compound types in the > 675°C residues should be of interest to refiners of high gravity petroleum and to chemists concerned with the design of catalysts used in processing petroleum because these residues contain the highest molecular weight polar compounds in a crude oil (compounds generally regarded as being the most difficult to convert to useful products). Because acids, bases, and neutral nitrogen compounds represent a large percentage of the > 675°C residues, the emphasis of this work is on the analyses of those fractions rather than the saturate hydrocarbon and aromatic hydrocarbon fractions. In this study we (1) use molecular weight data and elemental analyses to estimate the average number of heteroatoms per molecule for different classes of compounds; (2) identify some of the major compound types in residue samples from four different petroleums; and (3), where possible, estimate the amounts of each compound type in the residues. EXPERIMENTAL Appuratus
Infrared spectra were recorded using a Perkin-Elmer* model 62 1 infrared spectrophotometer. Low-resolution * Mention of specific brand names or models of equipment information Department
only and of Energy.
0016-2361/81/010017 0 198 1 IPC Business
does
10$2.00 Press
not
constitute
endorsement
is made for by the
mass spectra were recorded on a Varian CH-5 singlefocusing mass spectrometer, a field ionization mass spectrometer, and two different AEI Scientific Apparatus MS-30 double-focusing mass spectrometers, all equipped with directly heated sample introduction probes. Basic nitrogen titrations were made using a Beckman model 1063 titrimeter. A Mechrolab 301-A vapour pressure osmometer instrument was used for average molecular weight determinations. The adsorption chromatographic columns used for separation of acid, base, and neutral nitrogen fractions were gravity flow glass columns, 1.4 cm o.d. by 50 cm, packed with 50 g of adsorbent. The gel permeation chromatographic column (g.p.c.) was a waterjacketed glass tube, 1.3 cm i.d. by 150 cm, packed with 80 g of gel. A constant-volume fraction collector (Model 270. Instrumentation Specialties Co.) was used to collect 3.4 ml fractions. ReUgelltS
Poragel A-l (Waters Associates) was used with methylene chloride solvent for g.p.c. Basic alumina (Bio Rad AG-10, lOO- to 200-mesh, factory activated) was used for adsorption chromatography. Cyclohexane, benzene, methylene chloride, isopropylamine, and methanol (Burdick and Jackson Laboratories) were the solvents used for adsorption chromatography. Perchloric acid. dioxan, acetic anhydride, and benzene (Baker and Adamson) were used for potentiometric titrations. Diazald (iv-methylN-nitroso-p-toluenesulphonamide, Aldrich Chemical Company, Inc.) was used to generate diazomethane for the esterification of carboxylic acids. Routine
methods
of nitrogen,
sulphur,
utul o.u)~cqerl wu1y.si.s
Total nitrogen was determined by the micro-Dumas method, sulphur by cornbusting the sample and titrating the product with barium perchlorate to a colourimetric
FUEL,
1981,
Vol 60, January
17
Composition
of petroleum
heavy ends
(2):
J. F. McKay
endpoint, and oxygen by a modified Unterzaucher method. Base fractions were titrated potentiometrically with perchloric acid dissolved in dioxan, using acetic anhydride/benzene (2:l) solvent. Bases having a halfneutralization potential (HNP) of 350 mV or less were classified as strong bases; those with an HNP greater than 350 mV were classified as weak bases.
et al.
pound types can be estimated, the amount of each compound type in each g.p.c. subfraction may be estimated using Beer’s Law. The percentage of each compound type in the acid fraction may then be calculated by summation of the amounts in the g.p.c. subfractions.
RESULTS Procedure
Fractions of acids, bases, and neutral nitrogen compounds that had been isolated from the residues by the procedure described in Part 1’ were separated into compound types using procedures noted below. Acid fractions were separated by Separation of’ucids. two methods: (1) acids were eluted from the anion resin as described in the experimental section of Part 1’. Three acid subfractions were generated: very weakly held acids, weakly held acids, and strongly held acids; (2) a total acid fraction (150 mg) obtained from the anion resin (1) was passed through a g.p.c. column packed with Poragel A-l swollen with methylene chloride, Methylene chloride was the eluting solvent. G.p.c. subfractions of 3.4 ml volume were collected, and solvent-evaporated, and the g.p.c. weight distribution was determined. Sepuration qf buses. Residue bases (approximately 200 mg) were dissolved in benzene and passed through a column of basic alumina (50 g). The first subfraction was eluted with benzene (350 ml or until the eluate was colourless). Two additional subfractions were collected using methylene chloride: a band of material moving quickly down the column was eluted with ~450 ml, and a band of material moving slowly down the column was eluted with an additional 350 ml. A final elution was made with methylene chloride+methanol (l:l, 350 ml) to give a fourth subfraction. Separation qf neutral nitrogen compounds. Residue neutral nitrogen compounds (200 mg) were dissolved in benzene and passed through a column of basic alumina (50 g). The column was eluted successively with benzene (400 ml), methylene chloride (400 ml), dioxan (300 ml), and methylene chlorideeethanol (l:l, 250 ml) to give four neutral nitrogen subfractions. Preparation und separation of carboxylic acid esters. An acid fraction (100 mg) containing carboxylic
acids was dissolved in methylene chloride (50 ml) and esterified using diazomethane5. The methylene chloride was removed by passing a stream of nitrogen over the sample, and the sample was redissolved in cyclohexane (50 ml). The cyclohexane solution was passed over IRA904 anion resin (10 g), and esters were eluted with an additional 100 ml of cyclohexane. Nonesters retained on the resin were eluted with methylenechloride/ethanol(l:l, 50 ml). Quantitative infrared spectrometry. Quantitative estimates of the compound types in the acid fractions were made using quantitative infrared spectrometry combined with g.p.c. Details of the procedure as applied to the analysis of acid fractions from high-boiling petroleum distillates have been described’. The method involves passing a total acid fraction through a g.p.c. column and recording the infrared spectrum quantitatively for each g.p.c. sub-fraction. Because the average molecular weight of the sample and the molar absorptivities of the com-
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AND DISCUSSION
Elemental analyses and average molecular weight data are two of the most important pieces of chemical information that can be obtained for complex mixtures. In this study, these data (1) indicated that the separation scheme was separating residues into chemically meaningful fractions in a manner similar to the separation of high-boiling distillates; and (2) allowed estimates of the average heteroatom content of molecules in the acid, base, neutral nitrogen, and hydrocarbon fractions before the compound types in these fractions were known. Elemental
analyses
Table
1 shows the elemental composition of the residues from the Wilmington, Gach Saran, South Swan Hills, and Recluse oils together with the elemental composition of the fractions from the residues. The elemental compositions of the residues are quite different: Wilmington residue is high in nitrogen and oxygen; Gach Saran is high in sulphur; the Swan Hills residue is low in both nitrogen and sulphur but is relatively high in oxygen, as compared with the Gach Saran and Recluse residues; and the Recluse residue is low in nitrogen, sulphur, and oxygen, reflecting the saturate hydrocarbon nature of the residue. Elemental analyses of acid and base fractions show that nitrogen and oxygen compounds are concentrated in these fractions. The neutral nitrogen fractions are lower in nitrogen than either the acid or base fractions and in some cases must contain compounds other than nitrogen compounds. Both saturate and aromatic hydrocarbon fractions show small amounts of nitrogen and oxygen; however, sulphur, probably thiophenic compounds, is in the aromatic hydrocarbon fractions. The high percentages of hydrogen in the saturate hydrocarbon fractions are an indication that these fractions are relatively free of aromatic and heteroatom-containing compounds. Sulphur is rather evenly distributed in the acid, base, neutral nitrogen, and aromatic hydrocarbon fractions, indicating that sulphur compounds have not been concentrated in any of the fractions. In general, the nitrogen and sulphur in a total residue equalled the sum of the amounts found in separated fractions. The analyses for oxygen appear to be in error because more oxygen was found in the fractions than was found in a total residue (oxygen analyses on small samples have not been reproducible). The trends in oxygen values shown by the fractions are probably valid but the absolute percentages of oxygen reported in Table 1 may be in error. Molecular
weight data
Average molecular weights of residue fractions were determined by four different methods: (1) vapour pressure osmometry (VPO); (2) electron impact mass spectrometry; (3) field ionization mass spectrometry; and (4) quantitative infrared spectrometry. Because of the high molecular weights of the residue fractions, none of these methods by itself is reliable, however, combined the data allow an
Composition Table 1
Elemental analyses of >675’C residues and
of petroleum
heavy ends (2): J. F. McKay et al.
residue fractions
(wt 9/o) Sample
Carbon
Hydrogen
Nitrogen
Sulphur
Oxygen
84.5 80.2 83.2 83.0 85.5 85.3
9.7 9.1 9.6 9.4 12.7 10.8
1.62 2.33 2.24 1.78 0.11 0.63
2.57 2.53 2.31 2.71 0.82 2.37
1.5 5.5 2.7 1.3 0.8 0.7
_ -
_ _ -
1.18 1.76 1.56 1.04 0.95 0.39
3.69 3.77 3.41 4.01 4.74 3.11
0.8 1.8 2.0 2.0 3.1 none
87.0 86.3 87.6 87.9 86.8 86.0 86.3
10.6 7.8 8.7 9.0 8.9 12.9 9.9
0.55 1.40 1.47 0.86 0.76 0.29 0.32
0.53 0.77 0.78 0.88 1.13 0.01 0.63
1.2 3.6 1.4 1.3 3.5 0.6 2.7
86.5
11.6
0.56 1.10 1.23 0.77 0.77 0.11 0.62
0.37 0.36 0.45 0.52 0.22 0.12 0.37
0.8 3.6 1.5 1.9 2.5 0.3 1.5
Wilmington Total residue Acids Bases Neutral nitrogen compounds Saturate hydrocarbons Aromatic hydrocarbons Gach Saran Total residue Acids Bases Neutral nitrogen 1 Neutral nitrogen 2 Total hydrocarbons S. Swan Hills Total residue Acids Bases Neutral nitrogen 1 Neutral nitrogen 2 Saturate hydrocarbons Aromatic hydrocarbons Recluse Total residue Acids Bases Neutral nitrogen 1 Neutral nitrogen 2 Saturate hydrocarbons Aromatic hydrocarbons
Tab/e 2 VP0 molecular residue fractions
weights of Wilmington
residue and
Solvent
Benzene Total residue Acids Bases Neutral nitrogen Total hydrocarbons Saturate hydrocarbons Aromatic hydrocarbons
()-
Duplicate
1609 (1664,1452) 1755t1664.1647) 1673 (1200.1568) 1990(1754) 869 (1010) 863 (881) 1020(1138)
Tetrahydrofuran
Methylene chloride
3024 1767 2025 2010 1980
_
1398 1641 1345 1660 1015 871
990
1038
determinations
estimate of the average molecular weights of the residue fractions. Table 2 shows VP0 molecular weights of the Wilmington residue and residue fractions determined in three different solvents: benzene, tetrahydrofuran, and methylene chloride. The molecular weights were measured at three different concentrations, and the data were extrapolated to infinite dilution to obtain the average molecular weights. The values in parentheses are duplicate determinations. Two trends are shown by the data in Table 2. First the molecular weights of nonhydrocarbon fractions are often different when determined in different solvents. The nonhydrocarbon fractions are usually less associated in methylene chloride than in benzene and tetrahydrofuran, and therefore these results indicate that the nonhy-
drocarbon fractions have different degrees of intermolecular association or solubility in the three solvents. Second, in benzene and methylene chloride the hydrocarbon fractions show consistently lower molecular weights than the nonhydrocarbon fractions, indicating that (1) there is less intermolecular association of hydrocarbon fractions than of nonhydrocarbon fractions: (2) the molecular weights of the nonhydrocarbon fractions are artificially high; and (3) the molecular weights shown by the hydrocarbon fractions represent the actual average molecular weights of residue molecules. From the vapour pressure osmometry data in Table 2 it is concluded that the average molecular weight of molecules in the petroleum residue is in the range of 80@-1000. Average molecular weights determined by electron impact and held ionization mass spectrometry are shown in Table 3. The problems encountered when using mass spectrometry to determine average molecular weights are different than those of vapour pressure osmometry. In mass spectrometry the major problem is vapourizing or distilling a uniform sample into the instrument. The determination is likely to produce an artificially low average molecular weight rather than an artificially high average molecular weight found with vapour pressure osmometry. We attempted to overcome this problem by recording the electron impact mass spectra of both aromatic and saturate hydrocarbon fractions in different instruments with different operating conditions. Thus, any given spectrum may or may not be an accurate determination of the average molecular weight of a sample but overall the spectra show the region of average molecular weights. The electron impact mass spectrom-
FUEL,
1981,
Vol 60, January
19
Composition
of petroleum
heavy ends (2): J. F. McKay et al.
Tab/e 3 Average molecular weights of residue hydrocarbon determined by field ionization mass spectrometry
fractions
determined
by electron
impact mass spectrometry
Average molecular weight of parent ions
Instrument
Direct inlet probe temp. (“Cl
Ionizing voltage
Aromatic hydrocarbon Aromatic hydrocarbon Saturate hydrocarbon Saturate hydrocarbon Saturate hydrocarbon Saturate hydrocarbon Saturate hydrocarbon Total hydrocarbon
CH-5 MS-30 CH-5 CH-5 CH-5 CH-5 CH-5 MS-30
225 300-400 250 250 275 300 340 375
70 25 70 70 70 70 70 70
700 725 680 760 800a 775 700 800
Total Wilmington
Field ionization
70
780
Sample
and total residues
Molecular
weight
spread of parent ions
Wilmington residue Hydrocarbon fractions
Total
residue
Electron
Recluse residue
360
impact
Field ionization
Total Recluse residue
500-I 000 600-890 500-940 500-l 000 600-1000 600-I 000 500-900 500-l 300 400-I
1030
800
400-1800
a Sample 97 per cent volatilized
Tab/e 4 Average molecular weights of residue carboxylic acids and methyl ester derivatives determined by quantitative infrared spectrometry Average molecular weight
Sample Carboxylic acids Wilmington sample Wilmington sample Gach Saran sample Gach Saran sample
No. No. No. No.
1 2 1 2
Methyl ester derivatives Wilmington sample Gach Saran sample Gach Saran sample Gach Saran sample
of No. No. No. No.
carboxylic 1 1 2 3
940 1230 860 1110 acids 740 740 1190 940
etry data in Table 3 indicate that (1) the average molecular weight of residue hydrocarbons is in the region of 700 to 800 atomic mass units; (2) the molecular weights of individual molecules range from about 500 to 1300 amu. Field ionization mass spectra were obtained on the total > 675°C residues from the Wilmington and Recluse crude oils. For Wilmington residues, 78 wt :/, of the sample was volatile and could be introduced into the mass spectrometer; 99 wt ‘YOof the Recluse residue was introduced into the mass spectrometer. The average molecular weights of 1025 and 1030, obtained by field ionization mass spectrometry, for the Wilmington and Recluse residues are about 250 mass units higher than those obtained by electron impact mass spectrometry, and in both cases parent ions were observed to exceed 1800 mass units. Although the electron impact data and field ionization mass spectrometry data show average molecular weights in the same general range, the field ionization data is considered to be more reliable than the electron impact data, and therefore conclude that (1) the average molecular weight of a total residue is about 1000 amu; and (2) the molecular weights of individual molecules range from = 400 to over 1800 amu. Average molecular weights of carboxylic acids and their methyl ester derivatives, determined by quantitative infrared spectroscopy, are shown in Table 4. The advantage of using this method is that the average molecular
20
FUEL,
1981,
Vol 60, January
weights of polar compounds, i.e. the carboxylic acids and their methyl ester derivatives, may be determined and compared with the average molecular weights of hydrocarbon fractions. One of the most serious problems in determining average molecular weights by the infrared method is the isolation of a sample (for example, of carboxylic acids or esters) free from other compounds. Sample contamination by compounds other than the type whose functional group absorption is being measured tends to make the calculated average molecular weight too high. The second Wilmington and Gach Saran acid samples, shown in Tub/e 4, which have average molecular weights of over 1000, were known (by infrared spectroscopy) to contain compound types other than carboxylic acids but the amounts of the ‘contaminants’ were unknown. The data in Tuhle 4 show that the average molecular weight of residue carboxylic acids is in the 800 to 1200 molecular weight range; but, the most reliable values are throught to be between 800 and 1000. The esters also show an average molecular weight of -900. Overall, the data obtained by the four different methods indicate that the average molecular weight of residue molecules is most likely to be in the 800 to 1000 range. There are no compelling data indicating that the average molecular weight of hydrocarbons is different from that of nonhydrocarbons. The molecular weight spread of both hydrocarbons and nonhydrocarbons is quite large and is estimated to range from -400 to over 1800, but the upper limit is not known. Heteroatom content of‘polur molecules Elemental analyses and molecular weight data can be used together to estimate the number of heteroatoms in molecules from various residue fractions. For example, using the data in Table 1 and an average molecular weight of 900, a simple calculation shows that the Wilmington acids contain an average of 1.50 nitrogen atoms, 0.71 sulphur atoms, and 3.11 oxygen atoms, or ~5.32 heteroatoms per average molecule. The Wilmington bases, having less nitrogen, sulphur, and oxygen, show an average of 1.45 nitrogen atoms, 0.65 sulphur, and 1.50 oxygen atoms per molecule for a total of 3.6 heteroatoms per average molecule. These data are useful in estimating
Composition Tab/e 5 Weight per cent of material
in acid subfractions
elemental compositions the average molecule.
bvt 56) WilSubfraction
mington
Gach Saran
Acid 1 (very weak) Acid 2 (weak) Acid 3 (strong)
39 33 28
28 34 38
Recluse
37 39 24
27 57 16
1600cm-' Aromatic
C-C
1700 - 1650 cm-’
3L60 cm-’
pyrrotic
nitrogen
N-H
Amide
et al.
different from that calculated
for
Characterization of residue acids
S. Swan Hills
a
heavy ends (2): J. F. McKay
of petroleum
carbony
1600 cm-’
b
Acid fractions from the residues were separated into subfractions of very weak, weak, and strong acids using the method described in the Experimental section. The amounts of material in the subfractions from different oils are shown in Table 5. With the exception of the Gach Saran residue, acid subfractions 1 and 2 contain more material than acid subfraction 3. These data indicate that the predominant compound types in residue acids are very weak and weak acids, such as pyrrolic nitrogen compounds, phenols, and amides rather than strong acids such as carboxylic acids. Infrared data confirm this conclusion. Figure 1 shows partial infrared spectra of the three acid subfractions from the Wilmington residue. The infrared spectrum of acid subfraction 1 shows a small amount of absorption at 3585 cm 1 due to phenolic O-H stretching, absorption at 3460 cm-’ due to pyrrolic nitrogen N-H stretching, and absorption between 1600 and 1700 cm- ’ due to amide carbonyl compounds. The infrared spectrum of acid subfraction 2 shows the same absorption bands but with more phenolic absorption and less pyrrolic N-H absorption. Acid subfraction 3 shows pyrrolic N-H absorption at 3460 cm - ’ and strong absorption of carboxylic acid carbonyl monomer and dimer bands at 1700 cm- ’ In Figure 2 the infrared spectra of residue acid subfractions 2 and 3 are shown together
3L60 cm-’ 1600 cm-’
a
3585 cm-’ X60
& L
I
I
cm“
Distillate
J
1700 cm-‘,
Carboxylic
acid
subfraction
dimer
C
1725 err?, acid monomer 1700 cm-’
3460 cm-‘. Intermolecular H-bonding
of carboxylic
acids
I---
3700
,
3500
3300
3100
Wavelength
1600
1800
(cm-‘)
Figure 1 1.r. spectra of (a) Wilmington (b) Wilmington residue acid subfraction acid subfraction 3
residue acid subfraction 1; 2; (c) Wilmington residue
the heteroatom content of an average acid or base molecule and indicate the type of molecules to be expected in the petroleum residues. However, because these calculations are based on an estimated average molecular weight and describe an average acid or base molecule, individual species present in the fractions could have
L
3700
I
1
3500
I
I
6
3300
I
L
3100
1600
Wavelength
I
I
1600
(cm-‘)
Figure 2
I.r. spectra of residue and distillate acid subfractions. (a) Acid subfraction 2; (b) acid subfraction 3
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1981,
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21
Composition
of petroleum
heavy
Table 6 Compound type composition by quantitative infrared spectrometry
ends
(2): J. F. McKay
in residue acids as determined
Amides
Carboxylic acids
Total weight per cent of acids accounted for
30 16
42 47
28 14
118 84
16 25
30 23
24 14
83 86
fwt 96)
Residue acid Wilmington Gach Saran S. Swan Hills Recluse
Phenols
Pyrrolic nitrogen compounds
18 7 13 24
with infrared spectra from comparable 37&535”C distillate acid subfractions to demonstrate that the same compound types are found in petroleum residue acids as were found in petroleum distillate acids. Quantitative estimates of compound types in the acid fractions have been made by two different methods, both of which involve quantitative infrared spectrometry. In the first method, quantitative infrared spectra of the three acid subfractions are recorded and the amounts of each compound type in each subfraction are calculated using a method described previously’. The total amount of each acid compound type can then be determined by summation of the amounts in the three acid subfractions. Table 6 shows the amounts of phenols, pyrrolic nitrogen compounds, amides, and carboxylic acids in the residue acids. Amides and pyrrolic nitrogen compounds are the predominant compound types in each of the residue acids and account for the high levels of nitrogen observed by elemental analyses. Three of the four compound types contain at least one oxygen atom and account for the high levels of oxygen observed by elemental analyses. The amounts of compound types in the four residues differ. The total amount of acids accounted for by the infrared method generally represents -85 wt o/, of a sample. Because all the compound types in the acid fraction contain functional groups capable of hydrogen-bonding interactions, it is probable that most of the compound types in the acid fractions were isolated by forming hydrogen bonds with the anion resin. Three major problems limit the accuracy of the quantitative infrared analyses of residue acids. First, intermolecular association (hydrogen bonding) of the acid compound types reduces the absorption band areas of 0-H and N-H; for example, to the extent that phenolic hydroxyl groups hydrogen bond so that the absorption band area of free phenolic groups will be reduced and less phenols will be detected than are actually present in the sample. Efforts were made to reduce these errors by recording spectra in dilute solution using long path length infrared cells. However, molecular association could not be entirely eliminated in dilute solution. A second problem involves the accuracy of the values for average molecular weight which are used in the infrared calculations. It was shown earlier that the average molecular weight of 900 is an estimated value. We suggest that intermolecular association imparts more error to the infrared calculations than the use of the wrong average molecular weight, and that the per cent of acids accounted for in Tuhle 6 would increase from about 85% to nearly 100% if intermolecular association could be eliminated. A third source of error in the infrared method is the
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et al.
probability that some residue molecules contain more than one functional group. For example, if a single molecule contains two pyrrolic nitrogen atoms the infrared spectrometer would see the absorption of both functional groups and the infrared calculations would indicate that two molecules were present instead of one. The infrared analyses would find more carbazole molecules in a fraction than were actually present. However, if a molecule contained two nitrogen atoms, one pyrrolic and one pyridinic only the pyrrolic nitrogen atom would be detected and, therefore, error would not be introduced into the infrared calculation. In general, the magnitude of error introduced into the infrared calculations by bifunctional or multifunctional molecules is difficult to estimate because only the amount and not the distribution of heteroatoms in residue molecules is known. The error is probably large, perhaps of the order of 25x, because elemental analyses and molecular weight data shown that the average residue molecule contains considerably more than one heteroatom per molecule. The second method of estimating the amounts of compound types in the residue acids involves the use of quantitative infrared spectrometry and g.p.c. This method was applied to the analyses of the Wilmington residue acids so that the results of the two infrared methods could be compared. A total residue acid fraction was passed through a g.p.c. column, and quantitative infrared spectra of the g.p.c. subfractions were recorded. From the spectra, the amounts of phenols and pyrrolic nitrogen compounds were calculated. The g.p.c. subfractions were then treated with diazomethane to convert carboxylic acids to ester derivatives. Derivatization was required to move the carbonyl absorption of the carboxylic acids to a region of the spectrum where the absorption could be measured independently of amide carbonyl absorption. The quantitative infrared spectrum of each g.p.c. subfraction was again recorded and the amounts of amides and esters calculated. Figure 3 shows the results of this method of analysis applied to the Wilmington residue acids. The dotted curve represent the gravimetricall) determined g.p.c. weight distribution of the total acid fraction. The solid lines represent the weight distributions of individual compound types calculated from infrared analyses. The dashed curve was obtained by adding the weight distri-
20
G.p.c. fraction
number
Compound type analysis of Wilmington residue acids. Figure 3 A, Total gravimetric weight distribution; B, total calculated weight distribution; C, carboxylic acids; D, pyrrolic nitrogen compounds; E, amides; F, phenols
Composition
bution curves of the individual compound types. The purpose of the graph is to compare the total weight distribution of compound types calculated from infrared analyses with the gravimetrically determined weight distribution of the total acid fractions. The degree to which the dashed curve and the dotted curve are superimposable is a measure of the accuracy of the infrared method. If the two curves are superimposable, the percentages of major compound types may be calculated from curve areas. Figure 3 shows that the dotted curve and the dashed curve have the same approximate distribution, the differences in the area of the two curves being * 130/,, indicating that most of the acids can be accounted for using the infrared technique. Measurement of curve areas for individual compound types (solid lines) indicates that phenols are 12% of the acids: pyrrolic nitrogen compounds, 30%; amides, 25%; and carboxylic acids, 33%. A comparison of the composition of the Wilmington residue acids by the two infrared methods is shown in Tub/e 7. Pyrrolic nitrogen compounds and carboxylic acids each represent * 30”/, of the residue acids, phenols =Z15%. The analyses for amides were not in good agreement, showing 42% amides by the first method and 25% amides by the second method. Amides are difficult to analyse by any infrared method because of interference from aromatic carbon-carbon absorption centred at 1600 cm- ‘. The weight per cents of compound types in Tab/e ‘7 have not been normalized to 100% because such a calculation would assume that all molecules are monofunctional. In general, the data in Table 7 show the variation that is expected from methods based on gross assumptions of average molecular weight, and monofunctional molecules. The good agreement in the amounts of compound types shows that the infrared methods are useful in estimating the amounts of acidic compound types in petroleum residues. Samples of total Wilmington acids from two different separation experiments were titrated with perchloric acid to determine the amount of nitrogen that is basic. In the titration experiments, it was assumed that nitrogen compounds were the only compound types that titrated
Tab/e 7 Compound type analyses of Wilmington two infrared methods
Infrared method Method Method
I II
Phenol
Pyrrolic nitrogen compounds
18 12
30 30
Amides
Carboxylic acids
Material accounted for (recoveryja
42 25
28 33
118 100
a Values in Table 7 are raw data, not normalized
Tab/e 8 Titration
residue acids by
to 100%
and total nitrogen data of Wilmington
of petroleum
heavy ends (2): J. F. McKay et al
with perchloric acid. The results of the titration experiments are shown in Table 8. About 20% of the nitrogen titrates as strong base with perchloric acid. Based on previous titration work3, the strong base nitrogen is thought to be pyridine-type. The acid-compound types were not separated and then titrated individually to determine whether the basic nitrogen is evenly distributed among compound types or whether it is concentrated in one or two compound types, consequently the exact per cent of acid molecules that titrate as strong bases cannot be calculated. However, elemental analyses and titration data, together with molecular weight data, show that the maximum number of acid molecules that could behave as strong bases is about one out of every three molecules. Weak base nitrogen compounds are thought to be amide compound types. The weak base nitrogen represents -25% of the total nitrogen. Nontitratable nitrogen, obtained by difference, is %555% of the total nitrogen and is thought to be predominantly pyrrolic. Some nontitratable nitrogen may be present in diaza compounds where only one nitrogen atom is titratable. Characterization of residue bases
Residue base fractions were separated into four subfractions using the method described in the Experimental section. The amounts of material in the subfractions from different oils are shown in Treble 9. The amounts of material are different from oil to oil indicating that the composition of the bases are different. Infrared spectra of the four base subfractions from the Wilmington residue are shown in Figure 4. The infrared spectrum of subfraction 1 shows absorption bands at 1598 cm-’ and 1567 cm-’ that are characteristic of pyridine benzologs. The band at 1697 cm-’ is thought to result from carbonyl absorption of tertiary amide compounds3. The infrared spectrum of subfraction 2 shows absorption at 1600and 1697cm-‘; the aromatic absorption is shifted to a higher wavenumber than the aromatic absorption of the previous fraction and the shoulder at 1567 cm - ’ is not resolved, indicating that the fraction contains pyridine benzologs and other aromatic compound types. Carbonyl absorption at 1697 cm 1 is assigned to tertiary amides and the absorption at 3460 cm-’ is due to the N-H absorption of pyrrolic nitrogen compounds such as carbazoles. The infrared spectra of subfractions 3 and 4 show the same N-H absorption at 3460 cm- ‘, increased hydrogen bonding in the region from 3450 to 3 100 cm ’ due to secondary amides, and carbonyl absorption between 1700 cm-’ and 1640 cm - ’ assigned to amides. Figure 5 compares the infrared spectra of subfractions 1 and 2 of residue and distillate bases. The spectra show that the compound types in the residue subfractions are similar to those found previously in the distillate subfractions. The carbonyl absorption at 1697 cm-’ in the spectrum of subfraction 1 shows the separations are not as
residue acids
Wt % nitrogen Per cent of nitrogen as:
Titrated Fraction Wilmington Wilmington
Run No.1 Run No.2
Total
Strong base
Weak base
Nontitrated
Strong base
Weak base
Nontitrated
2.33 2.37
0.533 0.416
0.601 0.549
1.196 1.405
23 18
26 23
51 59
FUEL, 1981, Vol 60, January
23
Composition
of petroleum
Table 9 Titration
heavy ends (2): J. F. McKay et al.
and total nitrogen data for residue bases Per cent of nitrogen
Wt % titrated Base sample
Wt % of base fraction
N,(wt%)
Strong
Weak
Total Wilmington bases Subfraction 1 Subfraction 2 Subfraction 3 Subfraction 4
2.24 2.09 2.58 2.69 2.78
0.93
13 13 40 34
0.59 -
Total Gach Saran bases Subfraction 1 Subfraction 2 Subfraction 3 Subfraction 4
1.56 1.36 1.64 1.61 1.56
0.69 0.93
0.50
19 21 45 15
0.68 0.53 0.46
0.23 0.66 0.77
Total S. Swan Hills bases Subfraction Subfraction Subfraction Subfraction
1 2 3 4
31 20 33 16
1.47 1.53 1.37 1.72 1.05
0.76 1.33 0.75 0.51 0.47
0.24 -
Total Recluse bases Subfraction Subfraction Subfraction Subfraction
1 2 3 4
45 17 27 11
1.23 1.07 1.07 1.12 0.67
0.84 1.02 0.68 0.40 0.24
1.25 0.86 0.49 0.54
efficient with residue samples as with high-boiling distillate samples. Quantitative estimates of the compound types in the bases were not made because functional group absorption of strong bases and nontitratable bases was not always observable by infrared spectrometry. For example, the amount of pyridine benzologs (strong bases) in base subfraction 1 could be estimated by quantitative infrared spectrometry because the fraction contained mostly pyridine benzologs and the unique infrared absorption of this compound type could be observed; however, the amounts of strong bases, presumably pyridine benzologs, in subfractions 2,3, and 4 could not be estimated because the aromatic absorption of pyridine benzologs was masked by the aromatic absorption of other compound types. Quantitative estimates of the base compound types from titration and elemental analyses data are not possible because of the presence of multi-nitrogen compounds in which only one nitrogen atom titrates. However, the distribution of strong, weak, and nontitratable nitrogen can be determined by potentiometric titration. Table 9 shows titration and total nitrogen data for residue base fractions and subfractions. Wide variability is seen in the nitrogen content of the fractions and subfractions from the four residues. For example, Wilmington bases have almost twice the nitrogen content of the Recluse bases. If we assume the molecular weights of the bases to be 900, the average Wilmington base molecule would contain * 1.5 nitrogen atoms per molecule, meaning that an average of one of every two molecules contains two nitrogen atoms. The Recluse bases, with a nitrogen content of only 1.23 per cent, have an average of one nitrogen atom in two of every three molecules. The titration data in Table 9 show qualitatively the same separation trends that were observed by infrared
24
F?IEL,
1981,
Vol 60, January
0.33 0.80
0.15 0.48 0.49
0.27 0.25 0.54
Nontitrated
as:
Strong
Weak
Nontitrated
0.72 0.84 1.72 1.87 1.44
42 60 33 18 19
26 -
32 40 67 70 52
0.37 0.43 0.73 0.42 0.33
44
0.47 0.20 0.47 0.73 0.09
52 87 55 30 44
0.12 0.05 0.39 0.47 +0.11
68 95 64 36 36
68 41 33 30
12 29
32 _ 14 41 49
16 11 28 47
22 22 81
24 32 45 26 21
32 13 34 42 09
10 05 36 42 16
spectrometry. Strong bases (pyridine benzologs) are concentrated in subfraction 1 with smaller amounts in the later fractions. Weak bases are not found in subfraction 1 but are concentrated in subfractions 3 and 4, corresponding to the amides observed by infrared. Nontitrated nitrogen is concentrated in subfractions 2 and 3 and is interpreted to be carbazole-type nitrogen and nitrogen in diaza compounds or other multiple nitrogen atom compounds that do not titrate. The observation that strong bases containing tertiary nitrogen are not held on the basic alumina as well as weak bases containing N-H bonds leads us to conclude that the separation of base compound types on basic clumina is dependent on the degree to which compound types hydrogen bond with the alumina rather than on basic strength.
Charucterization
of neutral
nitrogen
compounds
The neutral nitrogen fraction of petroleum residues has been the most difficult fraction to separate and characterize. Both the insolubility of this material in many solvents and VP0 molecular weights ranging from 1500 to 3500 show that the molecules are highly associated in solution and/or that the material contains the highest molecular weight compounds of a crude oil. The Wilmington neutral nitrogen fraction was separated into four subfractions using basic alumina. Subfractions l-4 represented lo%, 8”/,, 37% and 28% of the fraction; the total recovery of material from the column was 83:/,. Infrared spectra of the four subfractions are shown in Figure 6. Pyrrolic N-H absorption of carbazoles and carbonyl absorption of amides are the most predominant functional group bands in the spectra. Subfraction 1 shows a band at 3460 cm-’ due to carbazoles and absorption around 1695 cm-’ due to amides. The spectrum of subfraction 2 is similar to
of petroleum
Composition 159Ocm-I, Pyridine
a
CC
heavy ends (2): J. F. McKay et al.
a
1590cm-‘, Pyridine benzolog C-C
1567 cm-’
3460 cm-’
1600 cm-’
b
Aromatic
C-C
1600 cm-’ 1 Aromatlc C-C
1697 cm’ 3460 cm-‘, Pyrrolic nitrogen N-H 3L60 cm-‘, Pyrrollc N-H
Residue
subfraction
1600 cm-’ n
C
I
I
1700 -1640 cm-’ I\
Y\
34GO cm-’ 3450-3100
cm-’
_J==Qg
I’
3600
I
I
I
1600
1800
3LOO Wavelength
(cm-‘1
l.r. spectra of residue and distillate base subfractions. Figure 5 (a) Base subfraction 1: (b) base subfraction 2
u
1600 cm-‘, Aromatic C-C
d
L
3600
c
t
3400
8
,
I
3200 Wavelength
1900
1700
1500
(cm-‘)
Figure 4 Ix. spectra of (a) Wilmington residue base subfraction 1; (b) Wilmington residue base subfraction 2; (cl Wilmington residue base subfraction 3; (d) Wilmington residue base subfraction 4
subfraction 1 but the concentration of carbazoles increased. Subfractions 3 and 4 show increased amounts of amide absorption between 1600 cm-’ and 1700 cm-‘. These subfractions also show small bands at 3585 cm-’ that may be due to phenol OH obsorption and larger bands at about 1025 cm-* that may be caused by sulphoxides. Quantitative estimates ofcompound types in the neutral nitrogen fraction were not attempted by
I
L
3600
3.400
3200 Wavelength
I
1800
I
I
1600
(cm-‘)
I.r. spectra of Wilmington residue neutral nitrogen subFigure 6 fractions. A, subfraction 1; 6, subfraction 2; C, subfraction 3; D, subfraction 4
FUEL,
1981,
Vol 60, January
25
Composition
of petroleum
Tab/e 10 Titration
and nitrogen
heavy ends (2): J. F. McKay et al. data for Wilmington
residue
neutral
nitrogen
fractions
and subfractions
Wt % nitrogen
Sample Total neutral nitrogen fraction Subfraction 1 Subfraction 2 Subfraction 3 Subfraction 4
Total
Very weak base
Weak base
Nontitrated base
Very weak base
Weak base
Nontitrated base
10 8
1.78 1.26 -
0.29 0.23 0.26
0.57 0.45 0.57
0.91 0.58 _
16 18 -
32 36 _
52 46 -
37 28
0.98 1.74
0.22 0.54
0.72 0.67
0.04 0.53
22 31
74 39
4 30
AND CONCLUSIONS
Major compound types identified in the acid fraction are carboxylic acids, phenols, pyrrolic nitrogen compounds, and amides. Quantitative estimates of the compound types were made by two infrared methods. Major compound types identified in the bases were pyridine be-
26
FUEL,
1981,
as:
Wt 56 of fractron
infrared analyses because of the possibility that all of the compound types are not observable by infrared, and, in addition, even in dilute solution the fractions showed extensive intermolecular association. Titration and nitrogen data for the Wilmington neutral nitrogen fraction and subfractions are shown in Table 10. The nitrogen data indicate that the neutral nitrogen molecules contain an average of one nitrogen atom per molecule, assuming an average molecular weight of 900 for the residue. Although titration graphs of the neutral nitrogen fraction and subfractions show two break points as in the case of the residue bases, the first break point appears in an HNP range of e400 to 460 millivolts; by our previous definition, compounds titrating in this HNP range are classified as weak bases. The second break point was observed at about 600 millivolts, indicating the presence of very weak bases. SUMMARY
Per cent of nitrogen
Vol 60, January
nzologs, amides, and pyrrolic nitrogen compounds; and types found in the neutral nitrogen fraction were amides and pyrrolic nitrogen compounds. Quantitative estimates of the compound types in the latter two fractions could not be made. The average molecular weight of the > 675°C residues was estimated to be approximately 900 after being determined by four different methods. The molecular weight data and elemental analyses data show that molecules in the acid, base, and neutral nitrogen fractions contain an average of about three to five heteroatoms per molecule; however, the distribution of heteroatoms within the molecules is not known.
REFERENCES McKay, J. F., Amend, P. J., Harnsberger, P. M., Cogswell, T. E. and Latham, D. R. Fuel 1981,60, 14 McKay, J. F., Cogswell, T. E., Weber, J. H. and Latham, D. R. Fuel 1975. 54. 50 McKay, J. F., Weber, J. H. and Latham, D. R. Anal. Cheat 1976,48, x91 Cogswell, T. E., McKay, J. F. and Latham, D. R. Anal. Chem. 1971, 43, 645 Fieser, L. F. ‘Experiments in Organic Chemistry,’ D. C. Heath and Company, Boston, Mass., 1957
R. Brent
Erickson,
Thomas
E.
Laramie Energy Technology Center, US Department of Energy, P.O. Box 3395, Laramie, Wyoming 82071, USA (Received 20 December 1979)
The characterization of polar compound types in petroleum residues of b.p. >675”C are described. The fractions of acids, bases, and neutral nitrogen compounds prepared by the method described in Part 1 (Fuel 1981, 60, 14) were further separated and analysed. Major compound types identified in the acid fraction are carboxylic acids, phenols, pyrrolic nitrogen compounds, and amides. Quantitative estimates of the compound types were made by two infrared methods. Major compound types identified in the bases were pyridine benzologs, amides, and pyrrolic nitrogen compounds; types found in the neutral nitrogen fraction were amides and pyrrolic nitrogen comoounds. The average molecular weight of the >675”C residues was estimated to be 900 after being determined by four -different methods.
This paper describes the characterization of polar compound types in petroleum residues of b.p. >675”C. The fractions of acids, bases, and neutral nitrogen compounds prepared by the method described in the previous paper’ were further separated and analysed. In general, the techniques used for the analyses of compound types in the residues were the same as those used in the study of highboiling distillates2.3 so that the compositions of the two different materials could be compared. Information concerning the composition of compound types in the > 675°C residues should be of interest to refiners of high gravity petroleum and to chemists concerned with the design of catalysts used in processing petroleum because these residues contain the highest molecular weight polar compounds in a crude oil (compounds generally regarded as being the most difficult to convert to useful products). Because acids, bases, and neutral nitrogen compounds represent a large percentage of the > 675°C residues, the emphasis of this work is on the analyses of those fractions rather than the saturate hydrocarbon and aromatic hydrocarbon fractions. In this study we (1) use molecular weight data and elemental analyses to estimate the average number of heteroatoms per molecule for different classes of compounds; (2) identify some of the major compound types in residue samples from four different petroleums; and (3), where possible, estimate the amounts of each compound type in the residues. EXPERIMENTAL Appuratus
Infrared spectra were recorded using a Perkin-Elmer* model 62 1 infrared spectrophotometer. Low-resolution * Mention of specific brand names or models of equipment information Department
only and of Energy.
0016-2361/81/010017 0 198 1 IPC Business
does
10$2.00 Press
not
constitute
endorsement
is made for by the
mass spectra were recorded on a Varian CH-5 singlefocusing mass spectrometer, a field ionization mass spectrometer, and two different AEI Scientific Apparatus MS-30 double-focusing mass spectrometers, all equipped with directly heated sample introduction probes. Basic nitrogen titrations were made using a Beckman model 1063 titrimeter. A Mechrolab 301-A vapour pressure osmometer instrument was used for average molecular weight determinations. The adsorption chromatographic columns used for separation of acid, base, and neutral nitrogen fractions were gravity flow glass columns, 1.4 cm o.d. by 50 cm, packed with 50 g of adsorbent. The gel permeation chromatographic column (g.p.c.) was a waterjacketed glass tube, 1.3 cm i.d. by 150 cm, packed with 80 g of gel. A constant-volume fraction collector (Model 270. Instrumentation Specialties Co.) was used to collect 3.4 ml fractions. ReUgelltS
Poragel A-l (Waters Associates) was used with methylene chloride solvent for g.p.c. Basic alumina (Bio Rad AG-10, lOO- to 200-mesh, factory activated) was used for adsorption chromatography. Cyclohexane, benzene, methylene chloride, isopropylamine, and methanol (Burdick and Jackson Laboratories) were the solvents used for adsorption chromatography. Perchloric acid. dioxan, acetic anhydride, and benzene (Baker and Adamson) were used for potentiometric titrations. Diazald (iv-methylN-nitroso-p-toluenesulphonamide, Aldrich Chemical Company, Inc.) was used to generate diazomethane for the esterification of carboxylic acids. Routine
methods
of nitrogen,
sulphur,
utul o.u)~cqerl wu1y.si.s
Total nitrogen was determined by the micro-Dumas method, sulphur by cornbusting the sample and titrating the product with barium perchlorate to a colourimetric
FUEL,
1981,
Vol 60, January
17
Composition
of petroleum
heavy ends
(2):
J. F. McKay
endpoint, and oxygen by a modified Unterzaucher method. Base fractions were titrated potentiometrically with perchloric acid dissolved in dioxan, using acetic anhydride/benzene (2:l) solvent. Bases having a halfneutralization potential (HNP) of 350 mV or less were classified as strong bases; those with an HNP greater than 350 mV were classified as weak bases.
et al.
pound types can be estimated, the amount of each compound type in each g.p.c. subfraction may be estimated using Beer’s Law. The percentage of each compound type in the acid fraction may then be calculated by summation of the amounts in the g.p.c. subfractions.
RESULTS Procedure
Fractions of acids, bases, and neutral nitrogen compounds that had been isolated from the residues by the procedure described in Part 1’ were separated into compound types using procedures noted below. Acid fractions were separated by Separation of’ucids. two methods: (1) acids were eluted from the anion resin as described in the experimental section of Part 1’. Three acid subfractions were generated: very weakly held acids, weakly held acids, and strongly held acids; (2) a total acid fraction (150 mg) obtained from the anion resin (1) was passed through a g.p.c. column packed with Poragel A-l swollen with methylene chloride, Methylene chloride was the eluting solvent. G.p.c. subfractions of 3.4 ml volume were collected, and solvent-evaporated, and the g.p.c. weight distribution was determined. Sepuration qf buses. Residue bases (approximately 200 mg) were dissolved in benzene and passed through a column of basic alumina (50 g). The first subfraction was eluted with benzene (350 ml or until the eluate was colourless). Two additional subfractions were collected using methylene chloride: a band of material moving quickly down the column was eluted with ~450 ml, and a band of material moving slowly down the column was eluted with an additional 350 ml. A final elution was made with methylene chloride+methanol (l:l, 350 ml) to give a fourth subfraction. Separation qf neutral nitrogen compounds. Residue neutral nitrogen compounds (200 mg) were dissolved in benzene and passed through a column of basic alumina (50 g). The column was eluted successively with benzene (400 ml), methylene chloride (400 ml), dioxan (300 ml), and methylene chlorideeethanol (l:l, 250 ml) to give four neutral nitrogen subfractions. Preparation und separation of carboxylic acid esters. An acid fraction (100 mg) containing carboxylic
acids was dissolved in methylene chloride (50 ml) and esterified using diazomethane5. The methylene chloride was removed by passing a stream of nitrogen over the sample, and the sample was redissolved in cyclohexane (50 ml). The cyclohexane solution was passed over IRA904 anion resin (10 g), and esters were eluted with an additional 100 ml of cyclohexane. Nonesters retained on the resin were eluted with methylenechloride/ethanol(l:l, 50 ml). Quantitative infrared spectrometry. Quantitative estimates of the compound types in the acid fractions were made using quantitative infrared spectrometry combined with g.p.c. Details of the procedure as applied to the analysis of acid fractions from high-boiling petroleum distillates have been described’. The method involves passing a total acid fraction through a g.p.c. column and recording the infrared spectrum quantitatively for each g.p.c. sub-fraction. Because the average molecular weight of the sample and the molar absorptivities of the com-
18
FUEL,
1981,
Vol 60, January
AND DISCUSSION
Elemental analyses and average molecular weight data are two of the most important pieces of chemical information that can be obtained for complex mixtures. In this study, these data (1) indicated that the separation scheme was separating residues into chemically meaningful fractions in a manner similar to the separation of high-boiling distillates; and (2) allowed estimates of the average heteroatom content of molecules in the acid, base, neutral nitrogen, and hydrocarbon fractions before the compound types in these fractions were known. Elemental
analyses
Table
1 shows the elemental composition of the residues from the Wilmington, Gach Saran, South Swan Hills, and Recluse oils together with the elemental composition of the fractions from the residues. The elemental compositions of the residues are quite different: Wilmington residue is high in nitrogen and oxygen; Gach Saran is high in sulphur; the Swan Hills residue is low in both nitrogen and sulphur but is relatively high in oxygen, as compared with the Gach Saran and Recluse residues; and the Recluse residue is low in nitrogen, sulphur, and oxygen, reflecting the saturate hydrocarbon nature of the residue. Elemental analyses of acid and base fractions show that nitrogen and oxygen compounds are concentrated in these fractions. The neutral nitrogen fractions are lower in nitrogen than either the acid or base fractions and in some cases must contain compounds other than nitrogen compounds. Both saturate and aromatic hydrocarbon fractions show small amounts of nitrogen and oxygen; however, sulphur, probably thiophenic compounds, is in the aromatic hydrocarbon fractions. The high percentages of hydrogen in the saturate hydrocarbon fractions are an indication that these fractions are relatively free of aromatic and heteroatom-containing compounds. Sulphur is rather evenly distributed in the acid, base, neutral nitrogen, and aromatic hydrocarbon fractions, indicating that sulphur compounds have not been concentrated in any of the fractions. In general, the nitrogen and sulphur in a total residue equalled the sum of the amounts found in separated fractions. The analyses for oxygen appear to be in error because more oxygen was found in the fractions than was found in a total residue (oxygen analyses on small samples have not been reproducible). The trends in oxygen values shown by the fractions are probably valid but the absolute percentages of oxygen reported in Table 1 may be in error. Molecular
weight data
Average molecular weights of residue fractions were determined by four different methods: (1) vapour pressure osmometry (VPO); (2) electron impact mass spectrometry; (3) field ionization mass spectrometry; and (4) quantitative infrared spectrometry. Because of the high molecular weights of the residue fractions, none of these methods by itself is reliable, however, combined the data allow an
Composition Table 1
Elemental analyses of >675’C residues and
of petroleum
heavy ends (2): J. F. McKay et al.
residue fractions
(wt 9/o) Sample
Carbon
Hydrogen
Nitrogen
Sulphur
Oxygen
84.5 80.2 83.2 83.0 85.5 85.3
9.7 9.1 9.6 9.4 12.7 10.8
1.62 2.33 2.24 1.78 0.11 0.63
2.57 2.53 2.31 2.71 0.82 2.37
1.5 5.5 2.7 1.3 0.8 0.7
_ -
_ _ -
1.18 1.76 1.56 1.04 0.95 0.39
3.69 3.77 3.41 4.01 4.74 3.11
0.8 1.8 2.0 2.0 3.1 none
87.0 86.3 87.6 87.9 86.8 86.0 86.3
10.6 7.8 8.7 9.0 8.9 12.9 9.9
0.55 1.40 1.47 0.86 0.76 0.29 0.32
0.53 0.77 0.78 0.88 1.13 0.01 0.63
1.2 3.6 1.4 1.3 3.5 0.6 2.7
86.5
11.6
0.56 1.10 1.23 0.77 0.77 0.11 0.62
0.37 0.36 0.45 0.52 0.22 0.12 0.37
0.8 3.6 1.5 1.9 2.5 0.3 1.5
Wilmington Total residue Acids Bases Neutral nitrogen compounds Saturate hydrocarbons Aromatic hydrocarbons Gach Saran Total residue Acids Bases Neutral nitrogen 1 Neutral nitrogen 2 Total hydrocarbons S. Swan Hills Total residue Acids Bases Neutral nitrogen 1 Neutral nitrogen 2 Saturate hydrocarbons Aromatic hydrocarbons Recluse Total residue Acids Bases Neutral nitrogen 1 Neutral nitrogen 2 Saturate hydrocarbons Aromatic hydrocarbons
Tab/e 2 VP0 molecular residue fractions
weights of Wilmington
residue and
Solvent
Benzene Total residue Acids Bases Neutral nitrogen Total hydrocarbons Saturate hydrocarbons Aromatic hydrocarbons
()-
Duplicate
1609 (1664,1452) 1755t1664.1647) 1673 (1200.1568) 1990(1754) 869 (1010) 863 (881) 1020(1138)
Tetrahydrofuran
Methylene chloride
3024 1767 2025 2010 1980
_
1398 1641 1345 1660 1015 871
990
1038
determinations
estimate of the average molecular weights of the residue fractions. Table 2 shows VP0 molecular weights of the Wilmington residue and residue fractions determined in three different solvents: benzene, tetrahydrofuran, and methylene chloride. The molecular weights were measured at three different concentrations, and the data were extrapolated to infinite dilution to obtain the average molecular weights. The values in parentheses are duplicate determinations. Two trends are shown by the data in Table 2. First the molecular weights of nonhydrocarbon fractions are often different when determined in different solvents. The nonhydrocarbon fractions are usually less associated in methylene chloride than in benzene and tetrahydrofuran, and therefore these results indicate that the nonhy-
drocarbon fractions have different degrees of intermolecular association or solubility in the three solvents. Second, in benzene and methylene chloride the hydrocarbon fractions show consistently lower molecular weights than the nonhydrocarbon fractions, indicating that (1) there is less intermolecular association of hydrocarbon fractions than of nonhydrocarbon fractions: (2) the molecular weights of the nonhydrocarbon fractions are artificially high; and (3) the molecular weights shown by the hydrocarbon fractions represent the actual average molecular weights of residue molecules. From the vapour pressure osmometry data in Table 2 it is concluded that the average molecular weight of molecules in the petroleum residue is in the range of 80@-1000. Average molecular weights determined by electron impact and held ionization mass spectrometry are shown in Table 3. The problems encountered when using mass spectrometry to determine average molecular weights are different than those of vapour pressure osmometry. In mass spectrometry the major problem is vapourizing or distilling a uniform sample into the instrument. The determination is likely to produce an artificially low average molecular weight rather than an artificially high average molecular weight found with vapour pressure osmometry. We attempted to overcome this problem by recording the electron impact mass spectra of both aromatic and saturate hydrocarbon fractions in different instruments with different operating conditions. Thus, any given spectrum may or may not be an accurate determination of the average molecular weight of a sample but overall the spectra show the region of average molecular weights. The electron impact mass spectrom-
FUEL,
1981,
Vol 60, January
19
Composition
of petroleum
heavy ends (2): J. F. McKay et al.
Tab/e 3 Average molecular weights of residue hydrocarbon determined by field ionization mass spectrometry
fractions
determined
by electron
impact mass spectrometry
Average molecular weight of parent ions
Instrument
Direct inlet probe temp. (“Cl
Ionizing voltage
Aromatic hydrocarbon Aromatic hydrocarbon Saturate hydrocarbon Saturate hydrocarbon Saturate hydrocarbon Saturate hydrocarbon Saturate hydrocarbon Total hydrocarbon
CH-5 MS-30 CH-5 CH-5 CH-5 CH-5 CH-5 MS-30
225 300-400 250 250 275 300 340 375
70 25 70 70 70 70 70 70
700 725 680 760 800a 775 700 800
Total Wilmington
Field ionization
70
780
Sample
and total residues
Molecular
weight
spread of parent ions
Wilmington residue Hydrocarbon fractions
Total
residue
Electron
Recluse residue
360
impact
Field ionization
Total Recluse residue
500-I 000 600-890 500-940 500-l 000 600-1000 600-I 000 500-900 500-l 300 400-I
1030
800
400-1800
a Sample 97 per cent volatilized
Tab/e 4 Average molecular weights of residue carboxylic acids and methyl ester derivatives determined by quantitative infrared spectrometry Average molecular weight
Sample Carboxylic acids Wilmington sample Wilmington sample Gach Saran sample Gach Saran sample
No. No. No. No.
1 2 1 2
Methyl ester derivatives Wilmington sample Gach Saran sample Gach Saran sample Gach Saran sample
of No. No. No. No.
carboxylic 1 1 2 3
940 1230 860 1110 acids 740 740 1190 940
etry data in Table 3 indicate that (1) the average molecular weight of residue hydrocarbons is in the region of 700 to 800 atomic mass units; (2) the molecular weights of individual molecules range from about 500 to 1300 amu. Field ionization mass spectra were obtained on the total > 675°C residues from the Wilmington and Recluse crude oils. For Wilmington residues, 78 wt :/, of the sample was volatile and could be introduced into the mass spectrometer; 99 wt ‘YOof the Recluse residue was introduced into the mass spectrometer. The average molecular weights of 1025 and 1030, obtained by field ionization mass spectrometry, for the Wilmington and Recluse residues are about 250 mass units higher than those obtained by electron impact mass spectrometry, and in both cases parent ions were observed to exceed 1800 mass units. Although the electron impact data and field ionization mass spectrometry data show average molecular weights in the same general range, the field ionization data is considered to be more reliable than the electron impact data, and therefore conclude that (1) the average molecular weight of a total residue is about 1000 amu; and (2) the molecular weights of individual molecules range from = 400 to over 1800 amu. Average molecular weights of carboxylic acids and their methyl ester derivatives, determined by quantitative infrared spectroscopy, are shown in Table 4. The advantage of using this method is that the average molecular
20
FUEL,
1981,
Vol 60, January
weights of polar compounds, i.e. the carboxylic acids and their methyl ester derivatives, may be determined and compared with the average molecular weights of hydrocarbon fractions. One of the most serious problems in determining average molecular weights by the infrared method is the isolation of a sample (for example, of carboxylic acids or esters) free from other compounds. Sample contamination by compounds other than the type whose functional group absorption is being measured tends to make the calculated average molecular weight too high. The second Wilmington and Gach Saran acid samples, shown in Tub/e 4, which have average molecular weights of over 1000, were known (by infrared spectroscopy) to contain compound types other than carboxylic acids but the amounts of the ‘contaminants’ were unknown. The data in Tuhle 4 show that the average molecular weight of residue carboxylic acids is in the 800 to 1200 molecular weight range; but, the most reliable values are throught to be between 800 and 1000. The esters also show an average molecular weight of -900. Overall, the data obtained by the four different methods indicate that the average molecular weight of residue molecules is most likely to be in the 800 to 1000 range. There are no compelling data indicating that the average molecular weight of hydrocarbons is different from that of nonhydrocarbons. The molecular weight spread of both hydrocarbons and nonhydrocarbons is quite large and is estimated to range from -400 to over 1800, but the upper limit is not known. Heteroatom content of‘polur molecules Elemental analyses and molecular weight data can be used together to estimate the number of heteroatoms in molecules from various residue fractions. For example, using the data in Table 1 and an average molecular weight of 900, a simple calculation shows that the Wilmington acids contain an average of 1.50 nitrogen atoms, 0.71 sulphur atoms, and 3.11 oxygen atoms, or ~5.32 heteroatoms per average molecule. The Wilmington bases, having less nitrogen, sulphur, and oxygen, show an average of 1.45 nitrogen atoms, 0.65 sulphur, and 1.50 oxygen atoms per molecule for a total of 3.6 heteroatoms per average molecule. These data are useful in estimating
Composition Tab/e 5 Weight per cent of material
in acid subfractions
elemental compositions the average molecule.
bvt 56) WilSubfraction
mington
Gach Saran
Acid 1 (very weak) Acid 2 (weak) Acid 3 (strong)
39 33 28
28 34 38
Recluse
37 39 24
27 57 16
1600cm-' Aromatic
C-C
1700 - 1650 cm-’
3L60 cm-’
pyrrotic
nitrogen
N-H
Amide
et al.
different from that calculated
for
Characterization of residue acids
S. Swan Hills
a
heavy ends (2): J. F. McKay
of petroleum
carbony
1600 cm-’
b
Acid fractions from the residues were separated into subfractions of very weak, weak, and strong acids using the method described in the Experimental section. The amounts of material in the subfractions from different oils are shown in Table 5. With the exception of the Gach Saran residue, acid subfractions 1 and 2 contain more material than acid subfraction 3. These data indicate that the predominant compound types in residue acids are very weak and weak acids, such as pyrrolic nitrogen compounds, phenols, and amides rather than strong acids such as carboxylic acids. Infrared data confirm this conclusion. Figure 1 shows partial infrared spectra of the three acid subfractions from the Wilmington residue. The infrared spectrum of acid subfraction 1 shows a small amount of absorption at 3585 cm 1 due to phenolic O-H stretching, absorption at 3460 cm-’ due to pyrrolic nitrogen N-H stretching, and absorption between 1600 and 1700 cm- ’ due to amide carbonyl compounds. The infrared spectrum of acid subfraction 2 shows the same absorption bands but with more phenolic absorption and less pyrrolic N-H absorption. Acid subfraction 3 shows pyrrolic N-H absorption at 3460 cm - ’ and strong absorption of carboxylic acid carbonyl monomer and dimer bands at 1700 cm- ’ In Figure 2 the infrared spectra of residue acid subfractions 2 and 3 are shown together
3L60 cm-’ 1600 cm-’
a
3585 cm-’ X60
& L
I
I
cm“
Distillate
J
1700 cm-‘,
Carboxylic
acid
subfraction
dimer
C
1725 err?, acid monomer 1700 cm-’
3460 cm-‘. Intermolecular H-bonding
of carboxylic
acids
I---
3700
,
3500
3300
3100
Wavelength
1600
1800
(cm-‘)
Figure 1 1.r. spectra of (a) Wilmington (b) Wilmington residue acid subfraction acid subfraction 3
residue acid subfraction 1; 2; (c) Wilmington residue
the heteroatom content of an average acid or base molecule and indicate the type of molecules to be expected in the petroleum residues. However, because these calculations are based on an estimated average molecular weight and describe an average acid or base molecule, individual species present in the fractions could have
L
3700
I
1
3500
I
I
6
3300
I
L
3100
1600
Wavelength
I
I
1600
(cm-‘)
Figure 2
I.r. spectra of residue and distillate acid subfractions. (a) Acid subfraction 2; (b) acid subfraction 3
FUEL,
1981,
Vol 60, January
21
Composition
of petroleum
heavy
Table 6 Compound type composition by quantitative infrared spectrometry
ends
(2): J. F. McKay
in residue acids as determined
Amides
Carboxylic acids
Total weight per cent of acids accounted for
30 16
42 47
28 14
118 84
16 25
30 23
24 14
83 86
fwt 96)
Residue acid Wilmington Gach Saran S. Swan Hills Recluse
Phenols
Pyrrolic nitrogen compounds
18 7 13 24
with infrared spectra from comparable 37&535”C distillate acid subfractions to demonstrate that the same compound types are found in petroleum residue acids as were found in petroleum distillate acids. Quantitative estimates of compound types in the acid fractions have been made by two different methods, both of which involve quantitative infrared spectrometry. In the first method, quantitative infrared spectra of the three acid subfractions are recorded and the amounts of each compound type in each subfraction are calculated using a method described previously’. The total amount of each acid compound type can then be determined by summation of the amounts in the three acid subfractions. Table 6 shows the amounts of phenols, pyrrolic nitrogen compounds, amides, and carboxylic acids in the residue acids. Amides and pyrrolic nitrogen compounds are the predominant compound types in each of the residue acids and account for the high levels of nitrogen observed by elemental analyses. Three of the four compound types contain at least one oxygen atom and account for the high levels of oxygen observed by elemental analyses. The amounts of compound types in the four residues differ. The total amount of acids accounted for by the infrared method generally represents -85 wt o/, of a sample. Because all the compound types in the acid fraction contain functional groups capable of hydrogen-bonding interactions, it is probable that most of the compound types in the acid fractions were isolated by forming hydrogen bonds with the anion resin. Three major problems limit the accuracy of the quantitative infrared analyses of residue acids. First, intermolecular association (hydrogen bonding) of the acid compound types reduces the absorption band areas of 0-H and N-H; for example, to the extent that phenolic hydroxyl groups hydrogen bond so that the absorption band area of free phenolic groups will be reduced and less phenols will be detected than are actually present in the sample. Efforts were made to reduce these errors by recording spectra in dilute solution using long path length infrared cells. However, molecular association could not be entirely eliminated in dilute solution. A second problem involves the accuracy of the values for average molecular weight which are used in the infrared calculations. It was shown earlier that the average molecular weight of 900 is an estimated value. We suggest that intermolecular association imparts more error to the infrared calculations than the use of the wrong average molecular weight, and that the per cent of acids accounted for in Tuhle 6 would increase from about 85% to nearly 100% if intermolecular association could be eliminated. A third source of error in the infrared method is the
22
FUEL,
1981,
Vol 60, January
et al.
probability that some residue molecules contain more than one functional group. For example, if a single molecule contains two pyrrolic nitrogen atoms the infrared spectrometer would see the absorption of both functional groups and the infrared calculations would indicate that two molecules were present instead of one. The infrared analyses would find more carbazole molecules in a fraction than were actually present. However, if a molecule contained two nitrogen atoms, one pyrrolic and one pyridinic only the pyrrolic nitrogen atom would be detected and, therefore, error would not be introduced into the infrared calculation. In general, the magnitude of error introduced into the infrared calculations by bifunctional or multifunctional molecules is difficult to estimate because only the amount and not the distribution of heteroatoms in residue molecules is known. The error is probably large, perhaps of the order of 25x, because elemental analyses and molecular weight data shown that the average residue molecule contains considerably more than one heteroatom per molecule. The second method of estimating the amounts of compound types in the residue acids involves the use of quantitative infrared spectrometry and g.p.c. This method was applied to the analyses of the Wilmington residue acids so that the results of the two infrared methods could be compared. A total residue acid fraction was passed through a g.p.c. column, and quantitative infrared spectra of the g.p.c. subfractions were recorded. From the spectra, the amounts of phenols and pyrrolic nitrogen compounds were calculated. The g.p.c. subfractions were then treated with diazomethane to convert carboxylic acids to ester derivatives. Derivatization was required to move the carbonyl absorption of the carboxylic acids to a region of the spectrum where the absorption could be measured independently of amide carbonyl absorption. The quantitative infrared spectrum of each g.p.c. subfraction was again recorded and the amounts of amides and esters calculated. Figure 3 shows the results of this method of analysis applied to the Wilmington residue acids. The dotted curve represent the gravimetricall) determined g.p.c. weight distribution of the total acid fraction. The solid lines represent the weight distributions of individual compound types calculated from infrared analyses. The dashed curve was obtained by adding the weight distri-
20
G.p.c. fraction
number
Compound type analysis of Wilmington residue acids. Figure 3 A, Total gravimetric weight distribution; B, total calculated weight distribution; C, carboxylic acids; D, pyrrolic nitrogen compounds; E, amides; F, phenols
Composition
bution curves of the individual compound types. The purpose of the graph is to compare the total weight distribution of compound types calculated from infrared analyses with the gravimetrically determined weight distribution of the total acid fractions. The degree to which the dashed curve and the dotted curve are superimposable is a measure of the accuracy of the infrared method. If the two curves are superimposable, the percentages of major compound types may be calculated from curve areas. Figure 3 shows that the dotted curve and the dashed curve have the same approximate distribution, the differences in the area of the two curves being * 130/,, indicating that most of the acids can be accounted for using the infrared technique. Measurement of curve areas for individual compound types (solid lines) indicates that phenols are 12% of the acids: pyrrolic nitrogen compounds, 30%; amides, 25%; and carboxylic acids, 33%. A comparison of the composition of the Wilmington residue acids by the two infrared methods is shown in Tub/e 7. Pyrrolic nitrogen compounds and carboxylic acids each represent * 30”/, of the residue acids, phenols =Z15%. The analyses for amides were not in good agreement, showing 42% amides by the first method and 25% amides by the second method. Amides are difficult to analyse by any infrared method because of interference from aromatic carbon-carbon absorption centred at 1600 cm- ‘. The weight per cents of compound types in Tab/e ‘7 have not been normalized to 100% because such a calculation would assume that all molecules are monofunctional. In general, the data in Table 7 show the variation that is expected from methods based on gross assumptions of average molecular weight, and monofunctional molecules. The good agreement in the amounts of compound types shows that the infrared methods are useful in estimating the amounts of acidic compound types in petroleum residues. Samples of total Wilmington acids from two different separation experiments were titrated with perchloric acid to determine the amount of nitrogen that is basic. In the titration experiments, it was assumed that nitrogen compounds were the only compound types that titrated
Tab/e 7 Compound type analyses of Wilmington two infrared methods
Infrared method Method Method
I II
Phenol
Pyrrolic nitrogen compounds
18 12
30 30
Amides
Carboxylic acids
Material accounted for (recoveryja
42 25
28 33
118 100
a Values in Table 7 are raw data, not normalized
Tab/e 8 Titration
residue acids by
to 100%
and total nitrogen data of Wilmington
of petroleum
heavy ends (2): J. F. McKay et al
with perchloric acid. The results of the titration experiments are shown in Table 8. About 20% of the nitrogen titrates as strong base with perchloric acid. Based on previous titration work3, the strong base nitrogen is thought to be pyridine-type. The acid-compound types were not separated and then titrated individually to determine whether the basic nitrogen is evenly distributed among compound types or whether it is concentrated in one or two compound types, consequently the exact per cent of acid molecules that titrate as strong bases cannot be calculated. However, elemental analyses and titration data, together with molecular weight data, show that the maximum number of acid molecules that could behave as strong bases is about one out of every three molecules. Weak base nitrogen compounds are thought to be amide compound types. The weak base nitrogen represents -25% of the total nitrogen. Nontitratable nitrogen, obtained by difference, is %555% of the total nitrogen and is thought to be predominantly pyrrolic. Some nontitratable nitrogen may be present in diaza compounds where only one nitrogen atom is titratable. Characterization of residue bases
Residue base fractions were separated into four subfractions using the method described in the Experimental section. The amounts of material in the subfractions from different oils are shown in Treble 9. The amounts of material are different from oil to oil indicating that the composition of the bases are different. Infrared spectra of the four base subfractions from the Wilmington residue are shown in Figure 4. The infrared spectrum of subfraction 1 shows absorption bands at 1598 cm-’ and 1567 cm-’ that are characteristic of pyridine benzologs. The band at 1697 cm-’ is thought to result from carbonyl absorption of tertiary amide compounds3. The infrared spectrum of subfraction 2 shows absorption at 1600and 1697cm-‘; the aromatic absorption is shifted to a higher wavenumber than the aromatic absorption of the previous fraction and the shoulder at 1567 cm - ’ is not resolved, indicating that the fraction contains pyridine benzologs and other aromatic compound types. Carbonyl absorption at 1697 cm 1 is assigned to tertiary amides and the absorption at 3460 cm-’ is due to the N-H absorption of pyrrolic nitrogen compounds such as carbazoles. The infrared spectra of subfractions 3 and 4 show the same N-H absorption at 3460 cm- ‘, increased hydrogen bonding in the region from 3450 to 3 100 cm ’ due to secondary amides, and carbonyl absorption between 1700 cm-’ and 1640 cm - ’ assigned to amides. Figure 5 compares the infrared spectra of subfractions 1 and 2 of residue and distillate bases. The spectra show that the compound types in the residue subfractions are similar to those found previously in the distillate subfractions. The carbonyl absorption at 1697 cm-’ in the spectrum of subfraction 1 shows the separations are not as
residue acids
Wt % nitrogen Per cent of nitrogen as:
Titrated Fraction Wilmington Wilmington
Run No.1 Run No.2
Total
Strong base
Weak base
Nontitrated
Strong base
Weak base
Nontitrated
2.33 2.37
0.533 0.416
0.601 0.549
1.196 1.405
23 18
26 23
51 59
FUEL, 1981, Vol 60, January
23
Composition
of petroleum
Table 9 Titration
heavy ends (2): J. F. McKay et al.
and total nitrogen data for residue bases Per cent of nitrogen
Wt % titrated Base sample
Wt % of base fraction
N,(wt%)
Strong
Weak
Total Wilmington bases Subfraction 1 Subfraction 2 Subfraction 3 Subfraction 4
2.24 2.09 2.58 2.69 2.78
0.93
13 13 40 34
0.59 -
Total Gach Saran bases Subfraction 1 Subfraction 2 Subfraction 3 Subfraction 4
1.56 1.36 1.64 1.61 1.56
0.69 0.93
0.50
19 21 45 15
0.68 0.53 0.46
0.23 0.66 0.77
Total S. Swan Hills bases Subfraction Subfraction Subfraction Subfraction
1 2 3 4
31 20 33 16
1.47 1.53 1.37 1.72 1.05
0.76 1.33 0.75 0.51 0.47
0.24 -
Total Recluse bases Subfraction Subfraction Subfraction Subfraction
1 2 3 4
45 17 27 11
1.23 1.07 1.07 1.12 0.67
0.84 1.02 0.68 0.40 0.24
1.25 0.86 0.49 0.54
efficient with residue samples as with high-boiling distillate samples. Quantitative estimates of the compound types in the bases were not made because functional group absorption of strong bases and nontitratable bases was not always observable by infrared spectrometry. For example, the amount of pyridine benzologs (strong bases) in base subfraction 1 could be estimated by quantitative infrared spectrometry because the fraction contained mostly pyridine benzologs and the unique infrared absorption of this compound type could be observed; however, the amounts of strong bases, presumably pyridine benzologs, in subfractions 2,3, and 4 could not be estimated because the aromatic absorption of pyridine benzologs was masked by the aromatic absorption of other compound types. Quantitative estimates of the base compound types from titration and elemental analyses data are not possible because of the presence of multi-nitrogen compounds in which only one nitrogen atom titrates. However, the distribution of strong, weak, and nontitratable nitrogen can be determined by potentiometric titration. Table 9 shows titration and total nitrogen data for residue base fractions and subfractions. Wide variability is seen in the nitrogen content of the fractions and subfractions from the four residues. For example, Wilmington bases have almost twice the nitrogen content of the Recluse bases. If we assume the molecular weights of the bases to be 900, the average Wilmington base molecule would contain * 1.5 nitrogen atoms per molecule, meaning that an average of one of every two molecules contains two nitrogen atoms. The Recluse bases, with a nitrogen content of only 1.23 per cent, have an average of one nitrogen atom in two of every three molecules. The titration data in Table 9 show qualitatively the same separation trends that were observed by infrared
24
F?IEL,
1981,
Vol 60, January
0.33 0.80
0.15 0.48 0.49
0.27 0.25 0.54
Nontitrated
as:
Strong
Weak
Nontitrated
0.72 0.84 1.72 1.87 1.44
42 60 33 18 19
26 -
32 40 67 70 52
0.37 0.43 0.73 0.42 0.33
44
0.47 0.20 0.47 0.73 0.09
52 87 55 30 44
0.12 0.05 0.39 0.47 +0.11
68 95 64 36 36
68 41 33 30
12 29
32 _ 14 41 49
16 11 28 47
22 22 81
24 32 45 26 21
32 13 34 42 09
10 05 36 42 16
spectrometry. Strong bases (pyridine benzologs) are concentrated in subfraction 1 with smaller amounts in the later fractions. Weak bases are not found in subfraction 1 but are concentrated in subfractions 3 and 4, corresponding to the amides observed by infrared. Nontitrated nitrogen is concentrated in subfractions 2 and 3 and is interpreted to be carbazole-type nitrogen and nitrogen in diaza compounds or other multiple nitrogen atom compounds that do not titrate. The observation that strong bases containing tertiary nitrogen are not held on the basic alumina as well as weak bases containing N-H bonds leads us to conclude that the separation of base compound types on basic clumina is dependent on the degree to which compound types hydrogen bond with the alumina rather than on basic strength.
Charucterization
of neutral
nitrogen
compounds
The neutral nitrogen fraction of petroleum residues has been the most difficult fraction to separate and characterize. Both the insolubility of this material in many solvents and VP0 molecular weights ranging from 1500 to 3500 show that the molecules are highly associated in solution and/or that the material contains the highest molecular weight compounds of a crude oil. The Wilmington neutral nitrogen fraction was separated into four subfractions using basic alumina. Subfractions l-4 represented lo%, 8”/,, 37% and 28% of the fraction; the total recovery of material from the column was 83:/,. Infrared spectra of the four subfractions are shown in Figure 6. Pyrrolic N-H absorption of carbazoles and carbonyl absorption of amides are the most predominant functional group bands in the spectra. Subfraction 1 shows a band at 3460 cm-’ due to carbazoles and absorption around 1695 cm-’ due to amides. The spectrum of subfraction 2 is similar to
of petroleum
Composition 159Ocm-I, Pyridine
a
CC
heavy ends (2): J. F. McKay et al.
a
1590cm-‘, Pyridine benzolog C-C
1567 cm-’
3460 cm-’
1600 cm-’
b
Aromatic
C-C
1600 cm-’ 1 Aromatlc C-C
1697 cm’ 3460 cm-‘, Pyrrolic nitrogen N-H 3L60 cm-‘, Pyrrollc N-H
Residue
subfraction
1600 cm-’ n
C
I
I
1700 -1640 cm-’ I\
Y\
34GO cm-’ 3450-3100
cm-’
_J==Qg
I’
3600
I
I
I
1600
1800
3LOO Wavelength
(cm-‘1
l.r. spectra of residue and distillate base subfractions. Figure 5 (a) Base subfraction 1: (b) base subfraction 2
u
1600 cm-‘, Aromatic C-C
d
L
3600
c
t
3400
8
,
I
3200 Wavelength
1900
1700
1500
(cm-‘)
Figure 4 Ix. spectra of (a) Wilmington residue base subfraction 1; (b) Wilmington residue base subfraction 2; (cl Wilmington residue base subfraction 3; (d) Wilmington residue base subfraction 4
subfraction 1 but the concentration of carbazoles increased. Subfractions 3 and 4 show increased amounts of amide absorption between 1600 cm-’ and 1700 cm-‘. These subfractions also show small bands at 3585 cm-’ that may be due to phenol OH obsorption and larger bands at about 1025 cm-* that may be caused by sulphoxides. Quantitative estimates ofcompound types in the neutral nitrogen fraction were not attempted by
I
L
3600
3.400
3200 Wavelength
I
1800
I
I
1600
(cm-‘)
I.r. spectra of Wilmington residue neutral nitrogen subFigure 6 fractions. A, subfraction 1; 6, subfraction 2; C, subfraction 3; D, subfraction 4
FUEL,
1981,
Vol 60, January
25
Composition
of petroleum
Tab/e 10 Titration
and nitrogen
heavy ends (2): J. F. McKay et al. data for Wilmington
residue
neutral
nitrogen
fractions
and subfractions
Wt % nitrogen
Sample Total neutral nitrogen fraction Subfraction 1 Subfraction 2 Subfraction 3 Subfraction 4
Total
Very weak base
Weak base
Nontitrated base
Very weak base
Weak base
Nontitrated base
10 8
1.78 1.26 -
0.29 0.23 0.26
0.57 0.45 0.57
0.91 0.58 _
16 18 -
32 36 _
52 46 -
37 28
0.98 1.74
0.22 0.54
0.72 0.67
0.04 0.53
22 31
74 39
4 30
AND CONCLUSIONS
Major compound types identified in the acid fraction are carboxylic acids, phenols, pyrrolic nitrogen compounds, and amides. Quantitative estimates of the compound types were made by two infrared methods. Major compound types identified in the bases were pyridine be-
26
FUEL,
1981,
as:
Wt 56 of fractron
infrared analyses because of the possibility that all of the compound types are not observable by infrared, and, in addition, even in dilute solution the fractions showed extensive intermolecular association. Titration and nitrogen data for the Wilmington neutral nitrogen fraction and subfractions are shown in Table 10. The nitrogen data indicate that the neutral nitrogen molecules contain an average of one nitrogen atom per molecule, assuming an average molecular weight of 900 for the residue. Although titration graphs of the neutral nitrogen fraction and subfractions show two break points as in the case of the residue bases, the first break point appears in an HNP range of e400 to 460 millivolts; by our previous definition, compounds titrating in this HNP range are classified as weak bases. The second break point was observed at about 600 millivolts, indicating the presence of very weak bases. SUMMARY
Per cent of nitrogen
Vol 60, January
nzologs, amides, and pyrrolic nitrogen compounds; and types found in the neutral nitrogen fraction were amides and pyrrolic nitrogen compounds. Quantitative estimates of the compound types in the latter two fractions could not be made. The average molecular weight of the > 675°C residues was estimated to be approximately 900 after being determined by four different methods. The molecular weight data and elemental analyses data show that molecules in the acid, base, and neutral nitrogen fractions contain an average of about three to five heteroatoms per molecule; however, the distribution of heteroatoms within the molecules is not known.
REFERENCES McKay, J. F., Amend, P. J., Harnsberger, P. M., Cogswell, T. E. and Latham, D. R. Fuel 1981,60, 14 McKay, J. F., Cogswell, T. E., Weber, J. H. and Latham, D. R. Fuel 1975. 54. 50 McKay, J. F., Weber, J. H. and Latham, D. R. Anal. Cheat 1976,48, x91 Cogswell, T. E., McKay, J. F. and Latham, D. R. Anal. Chem. 1971, 43, 645 Fieser, L. F. ‘Experiments in Organic Chemistry,’ D. C. Heath and Company, Boston, Mass., 1957