V O L U M E 23, NO. 4, A P R I L 1 9 5 1
563
._
Table 111. Weight Per Cent Analysis of Experimental Sample Cut KO.
Feed 1 2 3 4 5 6 7 8 9
IO 11 1'7
13 14 16 16 17 18
19 Btins Loss a
A at .4 a t Total 11.42 mu 10.81 mu cis-, trans-, Decalin, (cis-) (trans-) Wt. yo W t . 70 W t . 70 0.081 0.670 40.2 58.2 98.4 0.0 0.000 1.079 93.8 93.8 0.000 0.0 1.069 92.8 92.8 0.000 0.0 1.081 93.9 93.9 0.000 0.0 1.062 92.3 92.3 1.062 0.007 3.5 92.3 95.8 93.5 0,007 3.5 1.037 90.0 5.5 1.017 93.7 0.012 88.2 0,020 9.5 0.991 86.0 95.5 96.8 0.025 12.0 0.985 84.8 100.2 19.5 0.040 0.930 80.7 96.4 0.059 0.780' 28.8 67.6 100.0 43.0 0.657 57.0 0.088 99.2 40.4 0.120 58.8 0,464 99.6 78.0 0.247 21.6 0.159 8.5 0.183 98.3 89.8 0.093 95.7 0 187 4.0 91.7 0.045 101,s 2.5 99.0 0.027 0.203 95.8 2.0 93.8 0.021 0,191 2.0 90.5 88.5 0.180 0.021 0 000 0.0 93.8 93.8 0 .lQl
Cut, Wt. %
Based on Total Wt. % cistrans-
, . .
4.9 5.0 5.0 5.0 4.9 4.9 4.9 4.8 4.9 5.0 5 0 5.0 5.0 5.0 5.1 5.1 51 5.1 2.3 3.5 4 .3 100.0
0 00 0 00 0 00 0 00 0 17 0 17 0 27 0 46 0 59 0 98 1 44 2 15 2 94 3 90 4 58 4 68 5 O i 4 78
2 04 3 28 1 81a 39.3
Assuming t h a t loss is linear over entire fractionation.
_ _ _ ~
~
~~
.~
~
zontal axis. These curves, however, can be used. Of the entire family of curves, the 11.42-micron curve, which represents a plot of absorptivity as it function of the concentration of cI'sDecalin present, and the 10.81-micron curve, which represents a plot of absorptivity as a function of the concentration of trnnsDecalin present, approach the ideal of a straight horizontal line most closely. Because none of these curvee meets the straight horizontal line criterion for any of the selected absorption bands, either a series of approximation matfices must be developed or a working curve representing :L plot of a as a function of c as determined from known synthetic blends must be utilized. From such a working curve, the concentration of either cis- or trnnsDecalin can be determined from experimental absorbance values. Such a pair of working curves were determined (11.42-micron and 10.81-micron on Figure 4) for cis- and tram-Decalin from the nine pure sgnt,hetic blends. The decision to use thePe curves for these specific ahorption bands was further justifirti hy the lack of interference due to other compounds. As a check on the accuracy of tliese curves, a sample of commercial Decalin was fractionated in a semimicrostill at a refluy ratio of 30 to 1. The percentage of cis- and trans-Decalin present i n the variouF cuts was obtained by first determining the absorbances at 11.42 and 10.81 microns and then reading the corre-
...
4.60 4.64 4.70 4 62 4.52 4.41 4.32 4.13 4.16 4.04 3.38 2.85 2.02 1.08 0.43 0.20 0.13 0 10 0 0.5 0 00
Total ~ ~ in Cut. Wt. %
... 4 60 4.64 4.70 4.62 4.69 4.58 4.59 4.58 4.74 5.01 4.82 5.00 4.96 4.98 5.01 4.88 5.18 4.89 2.08 3,28 4.43" 96.3
~
sponding percentages of cis- and transDecalin from the working curves. A weight ~ l per i cent~ analysis of this fractionation was then made, with the results shown in Table 111. From this weight per cent analysis, 96.3% us. 98.4% for the total sample, 39.3% us. 40.2% for the cis component, and 57.0% us. 58.2% for the trans component, it appears that this method fallswithin experimental accuracy. Utilizing this type of analysis, Figure 6 represents a plot of the percentage of cis- and trans-Decalin present as a function of the volume per cent overhead. CONCLUSIONS
A solution containing a mixture of cis2.62'' and trans-Decalin with other noninterfer57 0 ing components can be analyzed so as to - _ _ __ ~ ~ give the percentage of Decalin present as a sum of the percentage of cis- and trans- isomers present. The method used is that of determining the absorbance of either cis- or trans-Decalin kind then reading the representative percentage of the component present from a bvorking curve. The base-line technique is utilized so as to eliminate, as much as possible, errors that would be introduced by the interference of other compounds. The employment of illxwrptivities is not satisfactory because of deviations from the Lambert-Beer law. The results check, by a weight per cent analysis, within esperimental accuracy. This method proved satisfactory when applied to reaction products containing both Decal i n isomers. ACKNOWLEDGMENT
The author wishes to express his thanks to James Xeel of this department for his valuable assistance. LITERATURE CITED
(1) Hodgman, C. D., "Handbook of Chemistry and Physics," 30th ed.. p. 924, Cleveland, Ohio. Chemical Rubber Publishing Co., 1946.
RECEIVED May 1, 1960. Presented before the Pittsburgh Conference on Analytical Chemistry and .4pplied Spectroscopy, Pittsburgh Pa., February 1950.
Properties of High Boiling Petroleum Products Physical and Chemical Characterization Studies with Relation to Polynuclear Aromatic Components G. G. WANLESS, L. T. EBY, AND JOHN REIIiVER, JR. Esso Laboratories, Standard Oil Development Co., Linden, W. J.
A
3' PART of a broad investigation on certain properties of high
boiling petroleum products, a preceding paper (8) has dealt with the distillation behavior with emphasis on the polynuclear aromatic components The present paper describes a variety of physical and chemical studies that have been carried out in search of improved methods for characterizing the polynuclear aromatic components of high boiling petroleum products. Esamples of the kinds of petroleum products and streams of
primary interest, and a definition of what is meant by high boiling, have already been given (8). Preliminary examinations of various products revealed that the usual measures of petroleum aromaticity-based on simple determinations of refractive index, aniline point, density, boiling point, and some combinations of these properties-do not meet current needs. Rather, what seemed to be required was a suitable characterization more specifically related to the amounts and types of certain, as yet
564 largely unknown, polynuclear aromatic compounds present in some of the products. However, it is not possible a t this time to determine simply and unambiguously all of the classes of compounds known or expected to occur in high boiling refinery products. This uncompleted task possesses analytical difficulties that doubtless transcend even those classically illustrated by the case of coal tar. Furthermore, even if such an analysis were possible, there would still remain to be established, through isolation techniques, which of the classes are responsible for particular properties of the oils, with the added likelihood that many specific molecules within a given class are probably unimportant by virtue of isomeric or structural factors. Thus, while it may be theoretically possible to resolve the aromatic fraction of a high boiling oil into its individual molecular species, the effort would be Herculean, and probably of long duration. A .somewhat less academic approach was undertaken in deference to practical limitations. Explorations were made of some new and more complex methods for characterizing the aromaticity of high boiling petroleum products, and some previously known techniques were extended. The results have shown considerable promise as newer methods for process and product evaluations. The remainder of this paper is concerned with these methods. REFRACTOMETRY OF CHROMATOGRAPHIC FRACTIONS
The refractive index of a high boiling petroleum oil provides only limited information as to the types and concentrations of the polynuclear compounds present. With primary interest in the compounds having four or more aromatic rings per molecule, some quantitative data on the content of these could be obtained by extending refractometric analysis to oil fractions that had been prepared by chromatographic separation. The method is based on the idea that inasmuch as the refractive indexes of polynuclear aromatic hydrocarbons are higher (Table I ) than that of a solvent such as toluene, if a toluene solution of the oil is chromatographically fractionated and the refractive indexes of small cuts are plotted as the ordinate against the volume of effluent as the abscissa, the width and height of the curve subtended by the refractive index line for the solvent should be measures of the concentration and polynuclearity, respectively, of the higher aromatics present. Integration of the subtended area would therefore be expected to combine these two factors into a single useful characterization factor. Development of the foregoing idea into a practicable laboratory method led to the use of solvent mixtures in the chromatographic procedure, for the folloffing reasons: Waxes and other insoluble materials often found in high boiling products should be rendered completely soluble; the fluidity of the products must usually be increased in order to facilitate the exchange processes on the adsorption column; and elution of the more tightly bound components should be complete. A method found to be widely applicable to a variety of high boiling petroleum products has been developed which involves a combination of chromatographic elution and displacement techniques. The nonaromatic portions of the sample are removed with n-heptane while the aromatic hydrocarbons are removed in toluene, pyridine being used as the displacement solvent. The following standard procedure meets the abovementioned requirements. The chromatographic column consists of a graded borosilicate glass tube 6.5 feet long (top section 2.75 feet long and 30 mm. in inside diameter, middle section 2.75 feet long and 20 mm. in inside diameter, bottom section 1 foot long and 10 mm. in inside diameter) equipped with a 2-mm. pressure stopcock a t the bottom. The tube is filled to within 75 mm. of the top with silica gel (Davison commercial desiccant grade, 28- to aOO-mesh, ovendried overnight a t 150" (3.). A dropping funnel with a squarecut stem, inserted a t the top of the column through a rubber stopper, is mounted axially so that its contents will fall onto the center of the gel surface, thus preventing channeling. The gel is prewetted with 200 ml. of n-heptane. Without allowing air to enter between successive additions of the liquids, the heptane is
A N A L Y T I C A L CHEMISTRY followed with a solution of 50 ml. of sample in 100 ml. of n-heptane and 100 ml. of toluene, this being followed by 500 ml. of pyridine; these solvents are all of reagent grade. The effluent is collected in approximately 5-ml. fractions of equal volume. The latter can be collected manually or, more economically, by employing a Technicon automatic fraction collector. The refractive index a t 20' C. is determined for each fraction. The measurements are made with an Abbe refractometer, using reflected light supplied through the upper prism from a Bausch and Lomb microscope illuminator. This has a logwatt ribbon filament light source, and is equipped with a borosilicate glass water cell for cooling the light beam, and filters for controlling intensity. This technique reduces the undesirably large errors that are often encountered in measuring the refractive index of a colored solution by means of transmitted light. A chromatogram is constructed by plotting the refractive index of each fraction as the ordinate against volume of effluent as the abscissa. Chromatograms need not be plotted if one is merely interested in determining the area under the curve subtended by a given refractive index level. It is obvious that this value can be computed for, say, the pyridine level by merely taking the sum
Ap =
(nao -1.5100) (ml. of fraction)
(1)
for all fractions having n2,0 > 1.5100, the refractive index of pyridine. A similar area above the toluene level, At, can be computed by carrying out the summation for fractions having %so >1.4970, and using this value in Equation 1. However, such analytical evaluations fail to reveal certain informative features that are disclosed by a plot.
A simple theoretical treatment of an idealized case serves to explain the principles of the method. Consider 100 ml. of a toluene solution containing as solutes, Mi granls of mononuclear, M 2 grams of binuclear, - - -, and M , grams of n-nuclear aromatic hydrocarbons. Let R and Re be the refractive indexes of the solution and of the pure solvent, respectively. For purposes of calculation, it is assumed that each species of solute has an additive effect on the refractive index. Then, if ARI, A&, ---denote the increments in refractive index obtained on dissolving 1 gram of each respective species in 100 ml. of toluene, one has n
R
- R.
=
CN,ARj j - 1
Suppose now that the original solution has been fractionated by a physical procedure such as chromatography, into z equal fractions having a total volume V (expressed as multiples of 100 ml.) without a change in solvent composition-that is, in the caBe considered, it is assumed for simplicity that toluene was employed to elute all the solute species from the adsorption column. Then each fraction will, in general, contain a certain number of grams, mid, of each of the aromatic species, and the compositions of the fractions can be represented by the following array, where the double subscripts identify the degree of poljmuclearity and the fraction number, respectively.
1
Binuclear, Grams mu
2
mlt
Fraction
Mononuclear, Grams
3
2
Total volume = V
mlr
=
34,
- - - n-Nuclear, Grams
---
-----
mni
mu
mu
--
mw
=
MI
-
mnz mm
- - - E = Mn
Then for any fraction i, whose refractive index is represented by
Ri,
V O L U M E 23, NO. 4, A P R I L 1951
565
different processes; (3) a procedure for the selective extraction of some polynuclear aromatic components from oils by means of aqueous caffeine solution, and the spectrometric analysis of the extract; (4) a procedure for characterizing components having an anthracene nucleus, employing a Diels-Alder reaction of the oil with maleic anhydride; and ( 5 ) some preliminary attempts to investigate and further resolve the higher aromatic components that are not reactive toward maleic anhydride, through reaction rate studies involving oxidizing agents such as lead tetraacetate and osmium tetroxide. Some of these characterization methods are useful in studying refinery processes such as thermal and catalytic cracking, and physical and chemical methods for processing refinery streams containing high boiling products. They are also useful for examining the product behavior of tars, lubricating oils, asphalts, waxes, and residua.
Many high boiling petroleum products, especially those produced by cracking, contain various types and concentrations of polynuclear aromatic hydrocarbons that influence product properties. New methods for characterizing such polynuclear components are required because the usual measures of petroleum aromaticity heretofore employed fail to give an adequate evaluation of the influence of such components. The authors have developed a number of physical and chemical methods; some are described herein in considerable detail. They include the following: (1) a combination procedure based on the refractometry of chromatographic fractions, a description of the chromatograms of various kinds of petroleum products, and an indication of the possible utility of the procedure in developing a method of analysis for such products; (2) the patterns observed in the ultraviolet absorption spectra of fractions of high boiling products derived from
and Equation 5 can be added to give the final result
(3) M , = (l/AR,)[V
If, now, one hrts reason to be interested in those particular polynuclear compounds for which, for example, n 2 4, the latter can be combined into a special group, designated by the subscript e, and Equation 3 can be written in the form
where AR, is the mean refractive index increment for all polynuclear components having n 2 4, and mci is the summation of the mji for these components. Equation 4 can be solved to give 3
mei = V(Ri
- Ro)/zARe
- xmjiARj/ARc
(5)
j - 1
I f z is allowed to denote the mean excess refractive index of the fractions, relative to the value for the pure solvent, then by definition E
x(Ri
- Ro)
zhR
d = l
I
1.520 1.5001.4801.460-
nE0
-
1.440-
1.4201.4001.380-
,
1
I
a
I
,
a - (MlAR1 + M2ARz + MjARt)]
(7)
Equation 7 relates the concentration, M,, of the group having n 2 4 to the total volume of effluent, V , the height of the refractive index curve above the solvent level, and to products of the concentrations and refractive index increments for the mono-, bi-, and trinuclear species. Since V i s the width of the refractive index curve a t the solvent level, the first term in the right member of Equation 7 is proportional to the area under the curve subtended by the refractive index line for the solvent. This area should be approximately proportional to Me, provided the last three terms in Equation 7 are comparatively small, and provided the divisor, ARo, is sufficiently constant for different oils. For the first condition, Mi will be considerably smaller than V because it represents, by definition, mononuclear solute species present in the mixture, and does not include the toluene used as solvent and eluent. For the second condition, the ARCof different oils should not differ much since the increase in refractive index with ring number for polynuclear aromatic hydrocarbons tends to level off above three fused rings, as indicated in Table I. The areas obtained from the refractive index curves can be expected to be proportional to M,, within the conditions mentioned.
a,
EXAMINATION OF CHROMATOGRAMS 1
A4chromatogram of a high boiling oil or tar is analogous to an optical spectral curve, in the sense that the positions and heights of its peaks, valleys, and contours appear to give patterns more or less characteristic of the product examined. The information gained from observation of the plot is only semiquantitative because of solvent volatility losses suffered during collection of the fractions. In the procedure described above, about 10% of the toluene is thus lost; probably a comparable figure applies to n-heptane losses. However, the chromatograms can be closely duplicated; the volatility losses under standard conditions must remain constant and thus do not jeopardize the utility of the plots for making comparisons. Also, the area values discussed are subject to considerably less error because the volume and refractive index of
A N A L Y T I C A L CHEMISTRY
566 each fraction are read more or less simultaneously: furthermore, errors resulting from evaporation that might occur at this stage tend to be compensated because the resulting abscissa1 compression of the plot is accompanied by a corresponding dilatation of the ordinate values arising from concentration of the solute species. The area values could be reproduced within about 5%. Figure 1 shows the chromatogram obtained in a control run employing n-heptane as the oil, and following the procedure out-
The chromatograms shown in Figures 5, 6, and 7 are for high boiling oils derived from catalytic cracking. This group of products gives a strikingly similar pattern. They contain a considerable proportion of nonaromatic components which are eluted with the heptane. These are followed by a sharp line of demarcation between the heptane and toluene zones, the toluene fractions containing material of high refractive index in the forecuts. The aromatics content of the products shown in Figures 5 , 6, and 7 are 30, 40, and 70%, respectively. Their boiling range is approximately 640" to 1000" F., with a small portion of undistillable residue.
,
I
--
t
1.500
1.480 1.460 1.440
Another characteristic of these calalytically produced oils is their content of material showing a high refractive index in the fractions between the toluene
II.--.Rnt I
I
__ such uui;ioiirs. This'rJeak can be destroved bv" sul" furic acid treatmeniofthc oil, ,I-ithiut influencingthe remainder ofthe chromatogram. Vel.?. likely, the effect of other types of
lined. The initial refractive index level corresponding to 12heptane elution is folloned by an abrupt rise which introduces the toluene zone. The early fractions in the latter zone have rethe toluene, and partly to a slight carry-over of nheptane. The next rise in the curve corresponds to the appearance of the pyridine zone. The exact point, in this and related figures, at which pyridine first occurs in t,he effluent fractions is indicated on the chromatograms. This point is readily established in the labo-. ratory by noting the first fraction in which the odor of pyridine is detectable. Figure 2 shows the chromatogram of a West Texas residuum (boiling range about 800f" F.) from the distillation of a virgin crude. The product c,ontains a large proportion of aromatic hydrocarbons, but very little of the latter appears to be of the higher polynuclear types-that is, above the range of, say, alkylated naphthalenes. Certain alkylated benzenes appear to separate with the heptane fractions, as might be expected for mononuclear aromatics with a high degree of long-chain aliphatic substitution.
.
, 1.5201.500-
1.480201.460-
",1.440-
,
7 +PrRtmm
1.420-
1.400c
1.380-
I
Figure 8 is a chromatogram of a tar boiling above 500" F. from the steam cracking of a paraffinic gas oil. While this tar is almost completely aromatic and contains polynuclear aromatic hydrocarbons, its chromatogram is different in pattern from those of the catalytically cracked oils. In this case, much material of high refractive index is present which is difficult t o elute with toluene, and which persists in the later cuts of the toluene zone.
1
'""i 1.480
1.460 1.440 1.4201 1.400 1.3 I
0
50
100
I50
200
,
250 300 350 ML. OF EFFLUENT
400
450
500
550
Figure 3. Chromatogram of a Slack Wax from a Paraffinic Distillate
The foregoing discussion illustrates the usefulness of the method in gaining some insight into the composition of high boiling petroleum products, and it clearly indicates the added possibility of developing an analytical scheme that could be based on the multiple determination of the characteristic peak positions, heights, widths, and perhaps other contour features, for a systematic group of t-ypical materials.
V O L U M E 2 3 , NO. 4, A P R I L 1 9 5 1
"'1
567 terials were distilled in a 3-inch packed colunui a t reduced pressure. Cuts having atmospheric boiling points above 800' F. were taken as a side stream from the still pot. The positions and relative heights of absorption peaks in the various distillation cuts indicate t,he types of rompourids which may predominate in certain cuts, and the boiling range in n-hirh they may he found.
1.500
I
1.4801 1.460
1
Although an absorption maximum is maintained at 325 mp over the entire boiling range investigated, other maxima a t longer wave lengths will vary with the boil1.420 ing range. Principal peaks a t 360 and 380 mp occur in 1.400 the 600" to 700" F. boiling range, with these peaks shifting toward the red through the next 50" F. rise. 1.380 A strong ahsorption appears a t 340 nip in the 700" 0 50 KM I50 200 250 300 350 400 450 500 550 to 800" F. boiling range, and this also shifts toward ML. OF EFFLUENT the red in the next 50" F. cut. A characteristic abeorpFigure 5 . Chromatogram of a Clarified Oil tion peak a t about 440 mp occurs in the boiling range from 800" to 1OOO" F. Another example of peak absorption at about 440 mp is found in the highest boiling fraction derived from catal!-t,ic cracking (clarified oil) discussed in connection with Figure 6. This was subjected to distillation at 0.1 mni. of mercury in a straight path still (81, and narrow cuts were selected for spectral examination. Some properties of these cuts are given in Table 11. Many of 1.480 the cuts were unstable; they darkened on standing 1.460 in stoppered bottles, even though they xere light in color when first distilled. Figure 11 shows the ultra1.440 violet absorption curves for the freshly distilled samples of Table 11, as measured in benzenci. Each of the fractions examined showed a definite and large peak a t 1.400 440 mp, the highest peak occurring with the highest boilcut. ing 1.380 I I Oil fractions of specific boiling ranges can be charac0 50 100 I50 200 250 300 350 400 450 500 550 ML. OF EFFLUENT tcrized hp absorption data a t selected wave lengths. .klthough such data alone do not reveal much as to Figure 6. Chromatogram of a Clarified Oil caomDositional features. it has nevertheless been found that'they can be usefull;. corwlated with certain oil propULTRAVIOLET ABSORPTION CH 4RhCTERISTICS erties that are known to depend 011 both the tTpe. and amounts of pol\-nuclenl aromatic Lompourids p~r m i t High boiling aromatic oils absorb ultravlolet radiation over a broad spectral region, as would be expected from their molecular complexitj, it appears difficult to obtain information of special significance from the spectrum of a whole oil. Although the Table 11. Distillation Cuts of Clarified Oil spectrum does not serve to identify specific compounds in these Extrap. Atm. complex miutures, it is valuable in following separations of general cut C u t Point, Boilig Point, 2" NO. % of Charge %. n ii types by various treatments such as distillation or contacting with 9 1 7 . 5 t o 20.1 779 t o 781 1.5370 solid adsorbents. 1.440
I
~
Aisomewhat parallel behavior is observed in the changes of the
26 39
5 6 . 2 to58.8 88.9t091.7
~~
836 t o 838 959 to 961
1 5448
(Solid)
absorption spectra during the course of distillation of different pptroleuni products, as shown in Figures 9 and 10. The two ni:L
Table I. Refractive Indexes of Some Polynuclear Aromatic Hydrocarbons at 20" C. Compound
nn I 5014 1 630 1 622 1 693 1.743 1,724 1 731 1 674 1 93 t o % . O i
Reference
Naphthalene Anthracene Phenanthrene Pyrene 1,2-Bensanthracene HI lO-Dimethy1-1,2-benzanthracene 20-Met hvlcholanthrene Graphite a Extrapolated to 20' C. b Extrapolated to 20' C. Averane of d u e s of 1.767 and 1.719 eytrapolatel i r o l r l nieasure I n i . i i t , of toluene and m-dioxane solutions. d Average of values of 1.713 and 1.734 extrapolated from nleasllrements of toluene and m-dioxane solutions. * Extrapolated from measurements of m-dioxane solutions / Averagp of values of 1.716 and 1.631 extrapolated from IneasurelnentJ of toluene and m-dioxane solutions. Tenipernture uncertain: measurements are referred t o molten sulfur and phosphorus as immersion liquids.
The uItraviol(3t absorption sprrtrum of a high biding aromatic oil can be selectively modified in various spectral regions by contacting with appropriate solid a.dsorbents. Table I11 shows the results obtained on contacting a clarified oil with various adsorbents. The oil was stirred with 10% by weight of adsorbent while heating to 392" F. in about 24 minutes. This mixture was then rooled to 212" F. in about 13 minutes and filtered through a pad of Hyflo filter aid on a Buchner funnt:l. A control run showed that the filter aid had no effect on the results. Table I11 shows that the selectivity and the over-all adsorption efficiency both depend on the adsorbent used. For example, Magnesol and Norite '4 are both effective in reducing the general ultraviolet adsorp tion, but the Sorite does not remove material absorbiiig at 420 to 430 mp as well as does the Magnesol; in contrast, the adsorptive capacities of the two agents are about, equal for 440 IIW material. Effectiveness of the various agents in removing color bodies can be judged from the values a t 500 mp, which are a good indication of the color of the treated oil. Further contacting with fresh adsorbent reduces the absorptivity values to even lower values, as would be espect,ed. Composition studies of petroleum fractions are cffectively carried out by ultraviolet examination of highly resolved fractions
I
I
I
I
I
,
I
has but little effect on the nature of the aqueous extract. 3. A single extraction removes little of the extractables from the oil phase. 4. Repeated extractions are required in order t o demonstrate an appreciable change in the nature of the extract, as judged by the ultraviolet spectrogram of the extract. 5. The type of solvent diluent used for the oil affects the amount, but not the type, of material extracted. 6. n-Heptane is superior to benzene as the diluent in serving to release a larger amount of the higher aromatics t o the aqueous phase, and thus providing better measurements of the extractables. 7. Aqueous solutions of caffeine are chemically unstable. 8. The oil should be diluted and extracted within a limited period of time, with control of the ex-
1.5401.5201.500-
1.480a1.460"D 1.440-
1.4201.400-
1.380I
150
t
200
250
I
I
300350
I
,
400 4 5 0 5 0 0
BEHAVIOR IN HIGH FREQUENCY FIELDS
Certain polynuclear aromatic hydrocarbons are characterized by the presence of molecular sites or regions having excessively high values of mobile electron density (17, 19). It was therefore felt that, if such molecules were present in an aromatic oil, they might be revealed by dielectric or polarization effects resulting from interaction with a high frequency electromagnetic field. In order to explore this possibility, dilute benzene solutions of various pure polynuclear aromatic hydrocarbons related to anthracene, phenanthrene, and pyrene, and containing from two to five aromatic rings per molecule, were examined for beat frequency change in a high frequency oscillator ( I S ) . Within the limits of experimental error, no changes in beat frequency could be detected for solutions of about 1%concentration; the method was therefore abandoned as unpromising.
550
1
"
"
1
,
,
,
,
1
"
,
,
,
,
,
,
'
,
4.800-850% 0. sm-m #I C. 700-750 D. 75+000' E. 8 0 0 - 9 0 0 ~ ~ F. 900-9(10. e. R)+IOIO "
EXTRACTION WITH AQUEOUS CAFFEINE SOLUTIONS
The solubility of the higher polynuclear aromatic hydrocarbons in pure water is exceedingly low. However, the solubility is greatly enhanced by the presence of certain dissolved purines such as caffeine and 1,3,7,9-tetramethyluric acid (5,21). The effect
WAVE LENGTH, m y Figure 9. Ultraviolet Absorption Spectra of Distillation Fractions from a Clarified Oil
569 Table IV. Effect of Caffeine on Aqueous Solubility of Some Hydrocarbons
A. e600.F. 0.6@3-WO* c. 650-700. 0.700-750-
E.750-800.
Solubility, Micromoles/LiterD 0.6% aqueous Water caffeine
Hydrocarbon Phenanthrene Pvrene Chiysene 1,2-Benzanthracene 3,4-Benapyrene 9,10-Dimethylanthracene Anthracene 9,10-Dimethyl-1,2-benzanthracene Coronene 1,2.5,6-Dibenzanthracene 20-Methylcholanthrene Naphthacene Rubrene
226 171 20.1 13.1 10.6 9.6 7.0
3
2.13 1.64 1.37 0.58 Practically insoluble Practically insoluble
4 7 5 4 4 4
16 1.0
3.4 0.04316 0.24 .... 0.5
.
..
.....
0,002156
..... . ....
0.00666
N,,. ti^ Rings per hlolecule
a Measurements by Weil-Malherbe (9f)at 18" t o 20' C., except those indicated. b Measurements by Klevens (1 6) at 25' C.
WAVE LENGTH, m y
Figure 10. Ultraviolet Absorption Spectra of Distillation Fractions from a Steam-Cracked Tar "
' ' '
"
'
I
'
I
'
' ' ' '
IO
' ' 1
The following standard procedure was based on these findings.
A 2.00-gram sample of the oil is weighed into a 250-ml. glassstoppered Erlenmeyer flask, and 10.0 ml. of reagent grade toluene are added. The flask is closed and manually swirled in order to
0.11 ' $0
3&
&
3 b 4bo' 420 4 0 4& WAVE LENGTH, m p
I &
5&3
'o'ol
Figure 12. Ultraviolet Absorption Spectra of a Clarified Oil and Its Aqueous Caffeine Extract
A. 779-701 OF.
6.836-838 " C- 959-961
0.01'
&
' 360
11
I
I
I
380 400 420 WAVE LENGTH, m p
440
460
Figure 11. Ultraviolet Absorption Spectra of Sarrow Distillation Fractions from B Clarified Oil
Table 111. Contacting Clarified Oil with Solid Adsorbents Adsorbent None Hy00 Pumice Attapulgus fines Activated alumina Super-Filtrol Silica gel (28 to 200 mesh) Santocel-58 Santocel-45 Magnesol Magnesol A Activated charcoal (Merck) Norite A ~
Ultraviolet Absorptivity, ms 430 440 500
420
Absorptivity Ratio, 440/430 ms 0.86 0.88 0.86 0.87 0.86 0.93 0.91 0.86 0.88 0.97 0.96 0.86 0.84
dissolve the sample. When this has occurred (or when only a wax remains), 10.0 ml. of reagent grade n-heptane are added, and manual swirling is continued until complete solution has occurred. Fifty milliliters of a 0.1 M solution of reagent grade caffeine in distilled water are now added. The steps up to this point should all be taken on the same day, and the caffeine solution should be not more than 3 days old. The stoppered flask is now shaken mechanically for 30 minutes on a Burrell wrist action shaker, with the setting a t 100 (the most vigorous action); this step and the subsequent steps are conducted in a constant temperature room at 25' C. The mixture is poured into a 125-ml. separatory funnel, allowed to settle for 15 minutes, and about 10 ml. of the aqueous layer are collected in a 15 X 125 mm. test tube. The test tube i s then closed with a foil-covered cork stopper and centrifuged for 15 minutes. The clear aqueous solution is pipetted into another clean test tube. If this solution is not perfectly clear, it must be recentrifuged until the transferred solution is clear. The resulting solution is placed in a clean 1-ml. optical absorption cell, and a matched reference cell containing 0.1 M caffeine solution is compared with the former in a Beckman Model DU uartz spectrophotometer. The optical absorbance (or optical tensity) of the extract is determined a t 340 and 460 mp, with a 0.3 mm. slit width ( 1 ) . If dilution of the extract is necessary, this should be done with the standard caffeine solution, and the resulting readings should be appropriately corrected. A "caffeine number," (CN)::;, may be defined as the difference between the optical absorbances a t the two wave lengths; thus
( C J V ) ; ~= ;
A340
-
(8)
The selection of the wave lengths used in this definition arises from the fact that an aqueous caffeine solution is transparent in the near ultraviolet, whereas some higher polycyclic aromatic hydrocarbons of particular interest display characteristic absorption in the neighborhood of 340 mp. The second term in Equation 8 is a correction for background color. This was selected,
570
ANALYTICAL CHEMISTRY
somewhat arbitrarily, to fall beyond the absorption threshold for polycyclic aromatic hydrocarbons having from four to six aromatic rings per molecule. The caffeine numbers are reproducible t o within 5%.
Table V. Caffeine Numbers of Some High Boiling Petroleum Products Description of Product
CCNY,::;
West Texas residuum Slack wax from paraffinic distillate Thermal reformer t a r Heavy catalytic cycle gae oil Steam-cracked aromatic tar Clarified oil
0.05 0.05 0.13
0.29 0.50 0.82
Preliminary correlations basc:d on certain oil studies show considerable promise for the caffeinenumber as an assay tool. A few values of the caffeine iiuml-wr are given in Table 1. for a variety of products. A comparison of the tabulatrtl caffeine numbers with the corresponding chromatograms shows a strong parallelism, in that t,he high caffeine numbers can be associated with aromat,ic components of high refractivc iiides m d high degree of polynuclearity. From the standpoint of routine anal! traction method enjoys an advantage over the chromatographic method, which is primarily a research tool. Sinc.e the tabulated caffeine number;: can lie expected to depcnd on the nature of the feed stocks and the conditions of manufacture, they should not be regarded as quantitatively characteristic of d l products bearing the same general descriptions. REACTION WITH .MALEIC ANHYDRIDE
Certain typrs of polycyclic aromatic hydrocarbons, particularly those containing an anthracene nucleus, are rapable of undergoing the Diels-Alder reaction with maleic anhydride to form adducts that can be thermally derompowd to yirkl the parent hydrocarbons (16). In contra.st to anthracene and its derivativcs, Jones and coworkers have shown ( 1 4 )that phenanthrene, fluorene, fluorapthene, pyrene, chrysene, and benzpyrene are practically nonreactive under the same esperimental conditions. This sharp delineation in the diene reactivity of the two groups of compounds enables one to make a partial characterization of the polynuclear components of high boiling oils, based on anthracenic struct,ures. Inasmuch as the reaction follows the law of mass action (b),the formation of t,he adducts is favored by the use of a high molar ratio of the anhydride to the reactive components of the oil. -.ilso, the attainnient of the equilibrium is favored by an increa,-e in temperature, but the latter is limited by the thermal dissociation reaction. The optimum reaction condit,ions employed in this work followed closely those established by previous investigators ( 2 , 14, 16), although some modifications were dictated by the occasional interfcrmce from asphaltic and tarry reaction products. In a typical run, 40 grams of the oil and 20 grams of maleic anhydride are heated for 8 hours at 115" C. in a heavy-walled glass pressure vessel fitted with a removable cap. The flasks are shaken mechanically The reaction mixture is then allowed to cool, is diluted with 100 ml. of benzene, and 30 grams of potassium hydroxide are added as a 40% aqueous solution. The mixture ie centrifuged to separate asphaltic residues that might be present, and the decanted liquid is separated into a benzene and a water layer. The benzene layer is washed once with dilute potassium hydroxide and the washings are added t o the water layer. The combined water layers are extracted once with fresh benzene, and are then neutralized by adding a 33% solution of sulfuric acid dropwise, with constant stirring. The neutral adduct is separated and washed on a suction filter. It is then redissolved in aqueous alkali, and the neutralization and subsequent stpps are repeated. The purified product is dried in a
vacuum a t 50 O original oil.
c. and its weight is expressed as per cent of
the
Because the molar ratio of anhydride employed will obviously depend on the amount of reactive components in the oil, a run was made to learn whether the formation of the adducts is complete, under the above conditions. .4 heavy catalytic cycle oil was reacted with maleic anhydride according to the above procedure, and yielded 2.80% of purified adduct. The residual oil was then heated with nearly three times its weight of maleic anhydride. This run was carried out for 16 hours at' 115" to 120" C. in a strongly agitated stainless steel bomb. The reaction mixture yielded only 0.015% of purificd adduct, which is negligible in comparison with the original yield. It was possible to demonstrate that the mixed adducts obtained from the heavy catalytic cycle oil were, a t least in part, of anthracenic origin. By means of an elaborate series of chromatographic separations and fractional sublimations, a minute fraction TvaR eventually obtained that could be unmnistakably identified as an impure specimen of an alkylated l,%benzanthracene. This work will be more completely reported in a later publication. The examination of a wide variety of high boiling petroleum products showed that no proportionalit>- esists betn-een the maleic anhydride adduct yield and the gross aromaticity of the product. Some illustrative esamples are given in Table VI. The lack of proportionality betmen the two values is not surprising hecause the products are of great variety and of considerably different boiling ranges, and also because the aromatic components consist in part of substances, such as pyrene derivatives, that do not undergo the Diels-Alder reaction with maleic anhydride.
Table VI. Maleic .4nhydride Adduct Yields and Gross 4romaticities of Some High Boiling Petroleum Products
Description of Product
Maleic Anhydride Gross Adduct Yield, Aromaticity", % VOl.
""
West Texas residuum 0.04 -50 Slack wax from paraffinic distillate 0.10 9.7 Heavy catalytic cycle gas oil, middle c u t 0.58 30 Thermal reformer tar 1.4 -90 Heavy catalytic cycle gas oil 2.80 31 Clarified oil 12.2 -70 Steam-cracked aromatic tar 15.6 -95 a The values in this column indicated as approximate were estimated from chromatograms as described elsewhere in this paper. The other values were determined by conventional separation on a silica gel column. The probable error of the estimated values is within i 5 units.
l\Iultiplication of the adduct yield value by the fraction of the
