Identification of polycyclic naphthenic, mono-, and diaromatic crude oil

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of 4,4’-DHAB between the isomeric species is given by Am-p (dem).Inspection of Table V reveals that such free energy differences for the meta and para isomers do exist and are appreciable. One may go further, however, in terms of understanding the mechanism of separation by examining the thermodynamic quantities which contribute to the excess free energy values obtained-uiz., the partial molar enthalpies and entropies of solution. Note that these values (Table VI) are more negative for the para isomers than for the meta isomers. More negative enthalpy values (more exothermic) are indicative of stronger solute-solvent interactions (greater e13, while more negative entropy values reveal a more ordered solution state. This behavior is consistent with the postulate that parasubstituted solutes, being more rod-like spatially, “fit” better into, and thereby interact more strongly with, the rod-like ordered solvent. In other words, in going from a completely disordered vapor state (similar for both meta and para isomers owing to the virial correction) to an ordered liquid state, the para isomer sacrifices more translational and rotational freedom, but, in return, its favorable geometry allows it to interact more strongly with the aligned liquid crystal m o l e cules. In the balance then, its entropy loss is overcome by its enthalpy gain, making it more soluble than its meta counterpart (which, although its motion is less restricted, has a greater enthalpic requirement for solution relative to an ideal solution). Finally, the longer retention associated with naphthalene relative to the previously eluted p D V B is due both to its lower vapor pressures (Table 11) and to its more

positive free energy values (Table V). Once again, as in the case of the meta isomers, solution for naphthalene is entropically favored but enthalpically disfavored because of its un-rod-like molecular structure. The liquid crystal solvent becomes far less selective in the isotropic region of temperatures (Figure 1). Although some residual short-range order is probably maintained in this region, the long range disorientation of the solvent molecules prevents any appreciable alignment (and, hence, selective r e tention) of the para isomers. The practical realization of complete separation of DVB isomers is of considerable interest to the isolation of quantities of pure isomers via preparative scale gas chromatography (6). The chromatographic data obtained in this study, taken together with considerations for liquid phase capacity, should prove valuable to approximate the requisite number of theoretical plates and appropriate conditions with which meaningful preparative scale separations of DVB isomers may be achieved on a nematic liquid crystal stationary phase. ACKNOWLEDGMENT

The authors thank Stanley P. Wasik of the Physical Chemistry Division, National Bureau of Standards, for stimulating discussions of the material in this study. RECEIVED for review September 15, 1969. Accepted November 18, 1969. Two of us (D.E.M. and L.C.C.) acknowledge grant support from the U. S. Army Research Office, Durham, N. C.

Identification of Polycyclic Naphthenic, Mono-, and Diaromatic Crude Oil Carboxylic Acids1 Wolfgang K. Seifert Chevron Oil Field Research Coo,P.O.Box 1627, Richmond, Calif. 94802 Richard M. Teeter Chevron Research Co., Richmond, Gal$ 94802 Many detailed structural features of substituted naphthenic, naphtheno-aromatic, and mono- and diaromatic carboxylic acids from a California crude oil are elucidated after isolation of fractions of derived hydrocarbons by repeated chromatography on alumina and the application of a variety of methods of high resolution molecular spectrometry. This comprehensive instrumental analytical approach results in compound class identification of many types of carboxylic acids which have not been reported previously in petroleum. The analytical methods involved are a combination of computer-averaged nuclear magnetic resonance, high resolution mass spectrometry, ultraviolet, fluorescence and infrared spectrometry, and gas chromatography combined with low resolution mass spectrometry. All acids of low hydrogen deficiency, postulated previously solely on the basis of high resolution mass measurements of acids and trihydroperfluoroalcohol esters, were confirmed by the combination of these methods of molecular spectrometry applied to hydrocarbons derived from the acids. The identified carboxylic acids represent at least 12% of all acids naturally occurring in this crude oil. A novel technique of structural analysis applicable to complex mixtures of hydrocarbons by high resolution mass spectrometry based on fragment ions combined with NMR is described and leads to detailed structural information within compound types. 180

THE importance of the identification of carboxylic acids naturally occurring in petroleum is that these acids are considered to be the precursors of petroleum hydrocarbons ( 1 , 2 ) and the knowledge of their structure is linked directly to the problem of the origin of petroleum (3-6) and of life on earth (7,8). In spite of the enormous efforts spent on this problem during the last 100 years (9), and particularly during the past several years because of the advent of modem instrumental 1 Partial preliminary communication: W. K. Seifert and R. M. Teeter, Cbem. Znd. (London), 1464 (1969).

(1) J. E. Cooper and E. E. Bray, Geochim. Cosmochim. Acta, 27, 1113 (1963). (2) K. A. Kvenvolden, J. Am. Oil Chem. SOC.,44, 628 (1967). (3) H. M. Smith, ibid., 44, 680 (1967). (4) E. D. McCarthy and M. Calvin, Nature, 216,642 (1967). ( 5 ) W. Henderson, G. Eglinton, P. Simmonds, and J. E. Lovelock, ibid., 219, 1012(1968). (6) T. Maclean, G. Eglinton, K. Douraghi-Zadeh, R . G. Ackman, and S . N. Hooper, ibid., 218, 1019(1968). ( 7 ) R. Robinson, ibid., 212,1291 (1966). (8) G. Eglinton and M. Calvin, Sci. Am., 216, 32 (1967). (9) H. L. Lochte and E. R. Littman, “Petroleum Acids and Bases,” Chemical Publishing Co., Inc., New York, 1955.

ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970


techniques, the information to date on the structure of carboxylic acids in petroleum is very fragmentary. The literature was summarized in our most recent paper ( I O ) , indicating that the compound classes known prior to the work in this laboratory were mainly alkanoic, 1-ring naphthenic and some simple aromatic carboxylic acids. After quantitative isolation of the acids from one California crude oil ( I ] ) , this knowledge has been greatly expanded (10,12,13) by the realization that terpenoid polynuclear saturated and polynuclear aromatic (10, 13) as well as heterocyclic (10-13) carboxylic acids occur naturally in petroleum. Because of the complexity of the mixtures and difficult separation steps previously reported, only postulation of structural types by parent peak high resolution mass spectrometry (13) of the acids and esters and by “simple” spectrometry (10) of the derived hydrocarbons was achieved. This report deals predominantly with proof of compound class structure of carboxylic acids of the naphthenic, monoand diaromatic type by high resolution molecular spectrometry. The approach taken was that of extensive separation of the acid-derived hydrocarbons by chromatography on alumina and application of several combined methods of molecular spectrometry to mixtures of about 150 or more compounds of similar polarity. The isolation and reduction scheme is repeated in Figure 1. In many cases this approach led to unequivocal compound class identification; in some cases alternates could not be ruled out. In total, the results of this study considerably enlarge the scope of the conventional concept of “naphthenk acids” and are expected to lead to most interesting speculations on the geochemical origin of these compounds and of petroleum itself. EXPERIMENTAL

Instrumentation. Infrared spectra were obtained in 0.05mm microcells using a Perkin-Elmer Model 337 infrared spectrophotometer fitted with a beam condenser. Ultraviolet (UV) spectra were obtained on a Cary recording spectrophotometer Model 14M. Fluorescence emission and excitation spectra were obtained on an Aminco-Bowman spectrophotofluorometer Model 4-8202, American Instrument Co. NMR spectra were obtained with a Varian HA-100 high resolution proton NMR spectrometer at a constant oscillator frequency of 100 MC combined with a Varian C-1024 time-averaging computer. Mass spectra were recorded on an AEI MS-9 double-focusing instrument. Isolation of Naphthenic, Mono-, and Diaromatic Hydrocarbons by Chromatography on Alumina. All solvents used were distilled and free of residue. A glass column (12.5-mm i.d. by 150 cm) was packed with 220 g of Woelm Neutral Alumina which had been deactivated with 3% water (activity Grade 11) and moistened with redistilled cyclohexane. Five hundred milliliters of cyclohexane was passed through the column and the last 100 ml was found to be free of residue on evaporation. Two hundred and eighteen milligrams of hydrocarbons from the reduction of Fraction D (representing 40% of the acids in midway Sunset 31E California crude oil, see Figure 1) was dissolved in a mixture of 250 p1 of cyclohexane and 50 p1 of Et20 and charged to the column with a wash of 0.3 ml of the same solvent mixture. Development was with cyclohexane under positive pressure of N2 (1 atm). The first 100-mlportion of eluate was free of sample. The second was found on evaporation (50 OC/l torr), (10) W. K. Seifert, R. M. Teeter, W. G. Howells, and M. J. R. Cantow, ANAL.CHEM., 41, 1638 (1969). (11) W. K. Seifert and W. G. Howells, ibid., 41, 554 (1969). (12) W. K. Seifert, ibid., 41, 562 (1969). (13) W. K. Seifert and R. M. Teeter, ibid., 41, 786 (1969).

Carboxylic Acids

Ion Exchange -Weakly Basic Resin

1 /Bl n

-

ion Exchange Strongly Basic Resin

I*!

Fraction’

Phenols Plus Carboxylic Acids

Phenols

I

Ion Exchange -Weakly Basic Resin

A

Carboxylic Acids Plus Derivatives

.

2

RCH,OSO,QCH,

9 1%

Veutral AI,O,

Fraction

CISO,

Carbovlic Acids

CH, RCH,OH ~

3+4+5+6 0.1%

Naphthenes. Monodromalics

Mono-, Diaromatics

Acidic AI,O, Fraction

I

4’ Diarornatlcs

I

This extract represents 3 . M of the total crude oil; 2. % bred on crude 011 is RCOOH, see Tables I,

I I. and I I I of rderence 11. ’ Percentsses are based on total crude oil.

Represenis 40% of all Carboxylic Acids present In crude 011.

10%of all Carboxylic Aclds. ’ Represents Represents 2* ol a11 Cartoxyllc Aclds.

Figure 1. Separation scheme to contain 52.7 mg (24.1 %) of a colorless liquid designated “Neutral Alumina Fraction 2” that was later found to consist primarily of naphthenes and monoaromatics. An additional 1500 ml of cyclohexane eluted 15.4% of a yellowish mixture of aromatic compounds, “Fractions 3-6 on Neutral Alumina.” Thirty-two milligrams of the latter sample was, chromatographed in an identical fashion on Woelm anionotropic (acidic) alumina of activity Grade I. The first three fractions (1100 ml n-hexane) consisted of 8 % saturated material (UV) which was not examined further. Subsequent elution with cyclohexane containing approximately 1 % Et20 yielded Cut No. 4 (10.5 mg, 33% yield) in 20 ml of eluate. This material possessed a UV molar absorptivity of 30,000 l/mole cm at 231 mp and was later found to be primarily diaromatic. Elution with an additional 160 ml of the same eluent resulted in the further isolation of 19% of material of almost identical characteristics. Gas Chromatography Combined with Mass Spectrometry (GCMS). Acidic alumina Cut No. 4 (diaromatic fraction) was examined by the GCMS combination. The chromatograph was a 30-m SCOT column (Perkin-Elmer Corp.) coated with SE-30 silicone gum rubber in a temperatureprogrammable F and M Model 810 chromatograph oven. Effluent from the column passed directly to the mass spectrometer through 1 m of 1.6-mm 0.d. heated SS tubing, Sample pressure was reduced from atmospheric (chromatograph exit) to -2 X torr (mass spectrometer ion source) by a heated 9-mm length of 0.015-mm i.d. glass capillary. The mass spectrometer was operated at a resolving power of about 1200 and an electron accelerating voltage of 20 V, below the ionization potential of helium. Because the total ion current of the mass spectrometer was recorded, no other detector was necessary for the chromatograph. The helium flow rate was about 5 ml/min and the column temperature was programmed at 4 “C/min starting at 160 “C on injection of 0.5 p1 of sample. No injection splitting was used. Forty spectra were recorded on the mass spectrometer


181

Table I. Aromatic and Benzylic Carbon (HRMS Group Type) Analysis of Monoaromatic Hydrocarbons Derived from Carboxylic Acids and Separated by Alto3 Chromatography.

Table 11. Hydrogen NMR Data of Selected Hydrocarbon Fractions Derived from Carboxylic Acids Naphthenes Plus Monoaromaticsa

Predominant Structural TypesAverage Statistical Empirical Formula+

C,,Hu ..

Mono- Plus D iarom at icsh

.. '.

__

C,,H,

ppm

Type Of Hydrogen

Atom % H

AtomslMole

0.7-1.1

Aiiphatic -CH3

44.5

19.6

NO. Of H-

Atom % H 17.0

No. Of HAtomslMoie 5.0

2.0-2.4 2.4-3.4

Aromatic ($HZ Pius -{HI

3.4-4.6

Aromatic -CHz- Aromatic

3.9

6.4-8.3

Aromatic -CH=

13.0

3.8

a Fraction 2 isolated by chromatographyon neutral alumina.

b -traction 4 isolated by chromatography on acidic alumina. FrKtlon 2 isolated by chromatographyon neulril alumina l h e averale molecular weight 01 Ihe sample was estimated le be 3 4 The member of each homologous s e r ~ e sclosest .10 141 In maleculdr wiah! was chosen lo reorerent ils series


Table 111. Combined NMR/MS Structural Data of Naphthenesa and Monoaromatics@Derived from Carboxylic Acids oscillograph recorder of which the first five and the last four were of too low intensity to be useful. High Resolution Mass Spectrometry. For the hydrocarbon group-type analyses, the mass spectrometer was operated at a resolving power of about 5000 as has been described previously (14). Exact mass measurements were performed at a resolving power of about 17,000 using the peak-matching technique with perfluorotributylamine as the mass reference. The samples were vaporized in an all-glass heated inlet system without contacting any metal surfaces (15) other than those in the ion source.

Derived From

TYP Carbon

hlS MS

Total Aromdic TOlai Aiiphatlc

NMR

Totai -CH,

lMR+MS Tolai -CH,-

NMR

Normalized Atom % C 5.6

Plus -{I

6.6

8.0

17.0

3,7

NMR

Aromatic -CH,

0.24

hlS

Aromatic Fused With Aromatic

NMR

Aromatic -CH,-

RESULTS AND DISCUSSIONS

MS

Aromatic -CH IMinImum

Isolation, which was achieved without contamination, is illustrated in Figure 1. The two predominantly naphthenk and diaromatic fractions were then subjected to the following methods of molecular spectrometry: UV and IR spectrometry, high resolution group-type mass spectrometry combined with time averaged nmr spectrometry, gas chromatography combined with low resolution mass spectrometry, fluorescence emission and excitation spectrometry, and high resolution mass spectrometry. Naphthenes and Monoaromatics. uv SPECTROMETRY AND GENERAL METHODS.The general composition as analyzed by high resolution group-type mass spectrometry is given in Table I. Approximately one-fourth of the compounds contain one benzene ring, and three-fourths are mono- and polycyclic naphthenes. Absence of sulfur and nitrogen compounds was confirmed by elemental analysis. The UV spectrum was in general agreement with the assignments of Table I and showed absorbance from 230 to 280 mp with a molar absorptivity of 80 l/mole cm characteristic (16) of rnonaromatics-e.g., benzenes, indanes, tetralins, and octahydrophenanthrenes (benzdinaphthenes>-which have molar absorptivities of 230, 1200, 600, and 800, respectively. Table I indicates the presence of about 23 % of the sum of these types, which accounts roughly for the observed absorptivity. The observed absorbance near 230 mp can be explained by the presence of a trace of naphthalenic material, as indicated in Table I (1%) and which was eluted in this fraction. The

NMR

Total Aromatic

182

Structural Implications IAverege Statistical Basis 1

ab

94.4 !6.4

Aromatic -H

(14) E. J. Gallegos, J. W. Green, L. P. Lindeman, R. L. LeTourneau, and R. M. Teeter, ANAL.CHEM., 39,1833 (1967). (15)R. M.Teeter and W. R. Doty, Rev. Sci. Znstr., 37, 792 (1966). (16) R. A. Friedel and M. Orchin, "Ultraviolet Spectra of Aromatic Compounds," John Wiley and Sons, New York, 1951.

lumber of C-Atoms Per Mole

0.9 0.06

C,hb 99%ol CH, Attached 10 Aliphatics, 6 6-CH,iMoie. P o l p p l i c Terpanes

,,,,.C6.3-CH,-~ole.

C,,,,H, -$H ' -CH,:

m. 66% Unsubstitution 5.6 Disubstitutedd Ar0;alIcs 1%of

Mb

f

0. l b

10.7-~HMoie,

Fused Naphthenes

-CH, Attched to Aromdics

o( Aromatics

Subst. by 1 -CH,

0.03

0.

.

NMR :oh MS

Tetraiins

MS

Benzdinaphthenes

1.4b

-

4.3

8.7

'Fraction 2 0 1 separdion on neutral alumina. :MS: Calculated from grouptype analysis; compare Table I, Assuming zeroquaternary aiiphdic carton. lndanes and tetralins are defined as disubstituted aromatics. Assuming no substitution on benzylic carton.

structures containing one aromatic ring fused with saturated rings have previously (13) been postulated as major components of an acid fraction of low polarity separated from Fraction D by a combination of column silica gel and thin layer chromatography (Table VI1 of Reference 13). The assignments, which were based on high resolution mass spectra of the acids themselves and their trihydroperfluoroalcohol ester derivatives, are now further confirmed by the information on the derived hydrocarbons given here. The infrared spectrum of this naphthenic/monoaromatic hydrocarbon fraction shows no evidence of nonhydrocarbon functional groups, in agreement with the UV and MS assignments. COMBINATION OF NMR WITH MS. In order to obtain more detailed structural information on this complex mixture of naphthenic and monoaromatic compounds, the information derived from high resolution group type mass spectrometry (14) (Table I) was combined with that derived from time averaged NMR spectra (Table 11). The structural implications of this combination approach are given in Table 111. The objective was to obtain information on the degree of substitution on the aromatic rings, the degree of substitution on



benzylic carbon, the distribution of methyl groups on aromatic rings us. attachment on aliphatic structures, the location of aromatic rings in fused aromatic naphthenic structures, and the amount of naphthalene bridgehead carbon. NMR alone established that there are no detectable olefins present. The upper part of Figure 2 shows the NMR spectrum of the naphthenic plus monoaromatic fraction from neutral alumina after 468 accumulations on a computer. Atom percent hydrogen is listed in Table 11. I n order to translate this information into the number of hydrogen atoms and further into the number of carbon atoms per mole for structural information, it is necessary to know the relative amounts of methylene (CHJ and methine (CH) hydrogens which can be determined only as the sum of both by NMR (1.1-2.0 ppm, Table 11). Without this breakdown, the accurate amount of total aliphatic carbon cannot be determined, which means that the accurate amount of total aromatic carbon is not obtainable which, in turn, prohibits the calculation of the degree of substitution on the aromatic rings. Assuming equal sensitivities for all compounds, the average statistical empirical formula and molecular weight are determined by MS (Table I). Combination of this information with the atom per cent hydrogen derived from hydrogen NMR yielded the number of hydrogen atoms per mole for each NMR species (Table 11, each H-atom equals 2.27%). In order to further translate the hydrogen data into types of carbon species, the group type mass spectrum was used to calculate the total amount of aromatic carbon (5.6%, Table I) and aliphatic carbon (94.4% by difference). Now the breakdown of the sum of methine plus methylene carbon into its individual components is possible as follows. The 0.18, Table 11) total number of methyl hydrogens (19.6 gives the number of methyl carbon atoms (6.6, Table 111) and per cent methyl carbon (26.4). Therefore, 68.0% (94.426.4z) of the carbon atoms are methylene plus methine (Table 111). This information is now combined with the NMR-derived information (23.1 0.22, Table 11) leading to the empirical formula C17.0Ha3.3.3 which fits only one ratio of methylene to methine [Table 111). The conclusion is that the number of methine groups (10.7 per mole) is larger than the number of methyl groups (6.6 per mole) which is indirect evidence for the presence of fused naphthenes. The previous evidence for naphthenes was based exclusively on mass spectrometry; however, the latter cannot distinguish between rings and double bonds. The NMR spectrum of the naphthenic fraction shown in the upper part of Figure 2 shows the absence of any signal in the olefin region and is particularly meaningful because 468 spectra were recorded. This information combined with the above is further proof for the MS assignment of naphthenes (IR and other methods also did not show olefinic double bonds; however, these methods are less sensitive than NMR). The high concentration of methyl groups attached to aliphatic carbon (6.6 per mole) further supports the previously postulated presence of polycyclic terpanes (13)-e.g., gammacerane possesses eight methyl groups. The degree of substitution on aromatic carbon is now easily calculated. NMR (Table 11) gives a value of 2.1 for the atom per cent of hydrogen attached to aromatic carbon. This corresponds to 3.7 % of unsubstituted aromatic carbon (Table 111). Inasmuch as 5.6% of the total carbon was aromatic (MS, Table I), the aromatic rings are, therefore, disubstituted on an average. A further implication of this conclusion is that the benzdinaphthenes are predominantly of

+

+

‘FRACTION 2 I S O L A T E D BY CHROMATOGRAPHY O N NEUTRAL ALUMINA

NAPHTHENES PLUS MONOAROMATICS

468 A C C U M U L A T I O N S ON COMPUTER

S E N S I T I V I T Y 16

SENSITIVITY I

,

OLEFINS

__

I

..\

I

I

A

&-iH,’

I

I

?FRACTION 4 ISOLATED BY CHROMATOGRAPHY ON ACIDIC A L U M I N A

J

I

I

I

I

I

,

,

,

,

MONO- PLUS DIAROMATICS 65 ACCUMULATIONS O N COMPUTER

*’ +=AROMATICS

1,

+ - ~ H Z ~ J J ’

SENSITIVITY I

I

80

70

60

50

40

30

20

,

,

I O

CHEMICAL SHIFT, pprn

Figure 2. NMR spectra of carboxylic acids

,

,

0 (TMS)

derived from

the type in which the aromatic ring is arranged in the terminal position rather than the middle position because the latter would be a tetrasubstituted monoaromatic. A phenanthene shape is preferred to an anthracene shape because the completely aromatic phenanthrenes have been shown (10) to be present in other fractions of this same acid extract, whereas anthracenes have not been detected. An alternative possibility is a bridged structure such as benzbicycloheptene (Table 111) or -octene. Another implication of the requirement of disubstituted aromatic rings on an average basis is that the indane and tetralin compounds which, by this definition, are disubstituted aromatics already, must have additional substituents attached to the naphthenic portion of the molecule rather than to the aromatic ring. Information on substitution on benzylic carbon was obtained from the difference of apparent NMR-derived (assuming no substitution) benzylic carbon (0.4%) and of true MS-derived benzylic carbon (1.4%). The conclusion is that for each hydrogen on benzylic carbon there must be 3.5 substituents (Table 111), which means that structures such as benzbicycloheptene or octahydrophenanthrene with substituents on benzylic carbon are preferred over those having no substituents on benzylic carbon (Table 111). A final check on the validity of these calculations is the observation that the NMR-derived carbon totals 98.7%. The missing 1.3 % is mainly (1 %) caused by invisibility of that portion of the aromatic carbon which is attached to substituted benzylic carbon. A small amount of aromatic carbon (0.1%), not accountable for by NMR alone, is due to naphthalene bridgehead carbon (Table I). The above technique is new, and we believe it to be applicable to any mixture of saturated and aromatic hydrocarbons meeting the requirements of the 19 X 19 MS grouptype analysis (15)-namely, absence of olefin, nitrogen compounds, and sulfur compounds other than the three types of thiophenes listed. The sample must be >C17 and vaporize in the MS inlet system (

Identification of polycyclic naphthenic, mono-, and diaromatic crude oil

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