aromatic hydrocarbons in some diesel fuel fractions

Aromatic Hydrocarbons in Some Diesel Fuel Fractions Ultra v io I et Spectrometric Ide n t ific a t ion

NOR3I.AN G. ADASIS ASD DOROTHY M. RICHARDSON Bureau of Mines, Petroleum Experiment Station, Bartlesville, Oklu.

The degrading effect of aromatic hj drocarbous on the combustion characteristics of Diesel fuels has stimulated an interest in their detection and analysis. The present study bas undertalcen to investi- gate the extent to which ultraviolet absorption spectra can be used in the identification of the aromatic hydrocarbons in a 400" to 500' F. distillate from West Edmond crude oil. Specific aromatic hydrocarbons identified i n this boiling range include Tetralin, naphthalene, 1- and 2-methylnaphthalene, 2-ethylnaphthalene, diphenyl, and 2,6-, 1,6-, and 1,7-dimethylnaphthalene. In addition, the

HIS study is only a part of :I Iugcr and much more extensive T program in which the Bureau of )lines Petroleum Experi- nient Station, Bartleeville, Okla , is engaged. This program, in its n idest ramifications, involves determination and study of the composition and properties, including combustion characteristics, of Diesel fuels. The fuel used in the present study was prepared iiotn a ctude oil from the West Edmond field in Oklahoma.

PROCEDURE

The Diesel fuel was prepared from 518 gallons of West Edmond (.rude oil processed in continuous (8) and batch stills, a t atmos- pherir and 15mm. pressure, to yield a gas-oil cut representing 52.75, of the crude oil. An appropriate part of this gas-oil cut JWS chargrd to a 100-gallon "superfractionating" unit described t)v TYarti (Y), the performance of which is comparable to a labora- t6ry fractionating column having a t least 80 theoretical plates. I7orty 2-liter fractions were removed a t atmospheric pressure between 300" and 400" F., a t which point the pressure in the still w:is rcduced to 50 mm. and 65 additional fractions were removed, Thc distillation was continued from this point a t further reduced pressure, but the present study involves only fractions 41 through 81 (406' to 508" F.), the first 41 fractions of the 50-mm. pressure di,dlation. For simplicity, the volume of gas oil charged to the still was so chosen that each 2-liter fraction represented 0.5y0 of the charge, making each fraction approximately 0.25YG on the basis of the crude oil. The boiling points a t various reduced pressures Tere determined for each fraction in an ebulliometer and ext,rapolated to i60-mm. pressure by means of Diihring's rule (6). An individual plot was made for each fraction and tliphenylmethane was used as the reference liquid. These hoil- ing points are presented in Table I.

Table I. Boiling Points

41 405.7 61 4 5 5 . 3 42 4 1 4 . 0 6% 4 5 8 . 0 43 419 0 63 4 6 1 . 3

Fraction No. B.P., ' F. Fraction No. B.P., r.

~~

44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

421.0 4 2 1 . 3 4 2 2 , 3 4 2 4 . 0 4 2 6 . 3 427 .3 4 3 0 . 0 4 3 3 . 3 4 3 6 . 3 438 .7 4 4 1 . 3 4 4 4 . 7 446 .0 448 .7 4 5 1 . 0 4 5 3 . 0

. . .

64 6 5 66 67 68 69 70 71 72 73 74 75 76 77 78 70 80 81

465 .3 4 6 6 . 3 473 .8 4 7 8 . 3 4 8 0 . 7 4 8 1 . 7 484 .7 4 8 6 . 7 488 5 4 8 9 . 3 4 9 0 . 3 4 9 1 . 8 4 9 3 . 3 4 9 7 . 0 500 .0 5 0 3 . 7 505 .3 5 0 8 . 0

presence of the following is indicated: 2-methyl- 5,6,7,8-tetrahydronaphthalene, 1,3-dimethylnaphthalene, and a methyldiphenyl. The data, presented in the form of spectra of 40 consecutive fractions, should provide valuable infor:nation concerning the azeotropic displacement of the boiling points of these aromatics; they should provide information useful in the development of analytical methods for the quantitative determination of these materials; and they readily show the utility and limitations of ultraviolet spectra in the study of aromatic hydrocarbons in petroleum fractions of this boiling range.

The ultraviolet absorption sprctrum throughout the 240- to :330-mp region wts determined for each of the 4 I oonsitwtive fractions 41 through 81 on a Beckman Model DU photoelectric quartz spectrophotometer (Y), using "iso-octane" (2,2,4-t'riniethyl- pentane) as the diluent. Measurement8 were made at 1-nip in- tervals, with the maxima and minima located to the nearest 0.5 mp. Whenever feasible, quantitative measurements mere t'aken to determine the approximate concentrations of the specific aromatic hydrocarbons present,

DISCUSSION OF RESULTS

By plotting the spectra of this series of distillation cutA in terms of absorptivity z?s. wave length, it is powil)lc t o .;how the stepwise increase and decrease in the concentrations of the specific aromatics identified (Figures 1 to 11). From this series of conmcutive-fraction spectra, one can immediatoly ascertain which fraction contains the maximum concentration of each aro- inatic hydrocarbon and also determine the optimum distillation cut point for the separation of two such compounds.

8 , I I I 1

04

02

W A V E LENGTH .my

Figure 1. Ultraviolet Spectra of Fractions 41 through 45

In the following discussion (:itch of the spectra of the, 41 fractions is numbered to correspond to the fruction numbers.

Fraction 41. The spectrum of fraction 41 is influenced pre- dominantly by the presence of Tetralin (tetrahydronaphthslcne), and a smaller amount of naphthalene. The small peak at 311 nip and the shoulder at 286 nip are caused solely by naphthalene, while both naphthalene and Tetralin contribute to the two peaks at 266 and 273.5 mp. .4 more detailed analysis gave 1.270 Tetralin and 0.5% naphthalerw. -4s a check on this analyeis the spectrum was run on a blend of Tctralin, naphthalene, and iso- octane with these respective concentrations. This spectrum (4113) closely duplic:ttes that of fractiori 41, except for a higher 1,ackgwund : i ~ ~ s ~ r p t i o n in the latter, especially in thr, 240- to 260-

129

130

I , '

A N A L Y T I C A L C H E M I S T R Y

I I I mp region. This background is probably caused by a significant quantity of a benzenoid-type aromatic containing 10 or 11 carbon atoms. Mair et al. (6) isolated 1,2,3,4-tetramethylbenzene from a corresponding boiling range of another mid-continent crude oil. Fraction 41 is the last fraction containing naphthalene, as seen by the absence of the 311-mp peak in subsequent fractions. The maximum concentration of naphthalene occurred in fraction 39 (not included in this survey). The boiling point of this fraction was 25" F. lower than the boiling point of naphthalene.

0 0

0.4 c z

0.2 0

m 0.0

1 I I I I 0 00

0.08

o.oe

0 .04

0.02

0.00

WAVE LENGTH , m#


Fractions 42 through 50. Spectra 42 through 50 show a gradual decline in the Tetralin influence, with a progressive shift in the spectra to the form exhibited in spectrum 50. Although identity of all of the aromatic hydrocarbons giving rise to this spectrum has not been determined, the presence of 2-methyl-5,6,7,8-tetrahydronaphthalene is inferred by the distinct band at 278.5 mp.

WAVE LENGTH I mp

Figure 3. Reference Ultraviolet Spectra of 1- and 2- Methylnaphthalene

The presence of this material was suspected because of its identification in a similar crude oil by Mair et al. (6). It is altogether possible that this band is caused by an aromatic other than 2- methyl-5,6,7,8-tetrahydronaphthalene. However, a survey of the available spectra of aromatics in this boiling range failed to show another spectrum with this band occurring as high as 278.5 mp. Correlations have shown that this peak (at 269 mp in toluene), characteristic of benzenoid aromatics, shifts to longer wave lengths as the number of substituent groups on the benzene nucleus increases-for example, it appears a t 268.5 mp in n-butyl- benzene (2) in contrast with 277 mp for 1,2,3,4-tetramethylbenzene (2). Therefore, inasmuch as the spectra of several of the possible components were not available to the authors, the pre- diction is made that if the 278.5-mp band in spectrum 50 is not caused by 2-methyl-5,6,7,8-tetrahydronaphthalene, it is caused by an alkylbenzene with four or more substituent groups and with a total of 11 carbon atoms. The absence of alkylnaph-

! I I

I I

I I 0 2 4 - 2 4

I - 0 2 2 2 2 /q

thalenes is indicated by the lack of specific absorption in the 300- to 330-mp region.

The spectrum of fraction 51, showing a sharp departure from that of spectrum 50, is influenced primarily by a small quantity (0.7%) of 2-methylnaphthalene, the lower boiling of the two monomethyl derivatives. For purposes of comparison, the spectra of both the 1- and 2-methylnaphthalene through the 300- to 330-mp region are presented in Figure 3. These spectra are different enough to allow identification and quantitative measurement of each in binary mixtures of the two, using the 314- and 319-mp absorption maxima for the alpha and beta isomers, respectively.

Fractions 51 and 52.

I I I I I I I I I I

PAVE L E N G T H , mp

Figure 5 . Ultraviolet Spectra of Fractions 55 through 60

V O L U M E 23, NO. 1, J A N U A R Y 1 9 5 1 131

Fractions 53 through 55. As 2-methylnaphthalene increases to its maximum concentration (4.iY0) in fraction 53, the concentration of the alpha isomer, as identified by the increasing shoulder a t 314 mp, becomes significant (0.7%). The boiling point of

0 I4

0 12

0 IO

008

006

004

0 02

ow WAVE LENGTH, mp


0.38

0.36

0.34

0 .32

0.30

0.28

0 .26

0 . 2 4

0 .22

0.20

3.18

3.18

0. I 4

3.12

3.10

3.08

0.08

0.04

0.02

0.00

WAVE L E N G T H , my


this fraction is 27" F. below the normal boiling point of 2-methylnaphthalene.

As the concentration of 2-methylnaphthalene decreases in fractions 54 and 55, the concentration of the alpha isomer reaches a maximum-that is, 3.5% in fraction 55. This fraction had a boiling point 28" F. below the normal boiling point of l-methylnaphthalene. This maximum concentration coincides with a maximum in the 314-mp peak in spectrum 55.

Fractions 54 through 64. Beginning with spectrum 54 and persisting through 64, the 305-mp peak for 2-methylnaphthalene has been replaced by a similar yet smaller peak at 304 mp, This shift is due to the initial presrnce of 2-ethylnaphthalene, whose spectrum exhibits a peak at the 304-mp position. The 310-mp peak coincides in the spectra of both the methyl and ethyl dr- rivatives. Determination of the fraction containing the niaui- mum concentration of %-ethylnaphthalene from these spectra h:aa been unsuccessful owing to the probable presence of remnants of the 2-methylnaphthalene. However, fraction 55 probably contains the maximum 2-ethylnaphthulene Concentration, although the boiling point is 41 ' F. below that of the pure compound.

Although the presence of 1-ethylnaphthalene is assumed, ita identification from the spectra was believed impossible. It mav be present, however, in small amounts in fractions 58 through 64

132 A N A L Y T I C A L C H E M I S T R Y

Spcctrum 64 appears as an anomaly, in that it resembles 61 more closely than it resembles 63 or 65. Owing to operational difficulties, the distillation was stopped after removal of fraction 63. Upon resuming operations, a &hour reflux period was allowed before removal of fraction 64.

Fractions 61 through 66. In spectrum 61 is seen the appearance of a broad absorption band in the region of 250 mp, which exerts its maximum influence in spectrum 65, with an absorption maximum a t 247 mp. This spectrum corresponds to that of diphenyl, the spectrum of a 0.3% solution being shown in the same figure for comparison. Its maximum concentration appears in fraction 65, with a boiling point 25' F. below that for the pure compound. Although prominent in appearance, this spectrum represents less than 1% diphenyl. Through these five fractions the concentration of 2-ethylnaphthalene drops to negligible proportions, as indicated by the 319-m~ band, but rises abruptly in fraction 66. The reason for this anomaly has not been deter- niiiicd.

0 20

0 I8

0 I b

0 14

0 I 2

0 I O

o o n

0 06

I 0 4

0 2

2 4 0 250 2!0 270 280 290 300 3 WAVE LENGTH, mp


Fractions 67 through 69. Spectra 67, 68, and 69 show a sharp departure from that of 66, caused by the presence of consider- able quantities of 2,6-dimethylnaphthalene, some of which was separated from fraction 69 ( 4 ) and identified by its melting point, the melting point of its picrate, and a mixed melting point of it and a known sample of 2,6-diniethylnaphthalene. Fraction 68 shows the largest Concentration of this naphthalene derivative (a%), and has a boiling point 19 O F . lower than the normal boiling point of this compound.

Fractions 70, 71, 72, and 73 exhibit spectra distinguishably different from that of 2,6-dimethylnaphthalene in that the peak at 317 mp has virtually disappeared and the peaks at 309.5 and 324 mp have shifted to 308 and 322 mp, respectively, in these four spectra. This particular form of spectrum persists through spectrum '73, but the absence of the 308- mp band in spectrum 74 indicates a change of the naphthalene- type components a t this point.

Fractions 70 through 73.

W A V E L E N G T H , rnp

Figure 10. Ultraiiolet Spectra of Subfractions 1 through 8 from Fraction 79

I I

D

2 0 D ; e o

: I 5 1 5

2 1 0 1 0 > I-

c K

: : 5 0 5 4

0 0 0 2 4 0 250 260 2 7 0 280 290 300 310 320

W A V E L E N G T H , mp

Figure 11. Ultraviolet Spectra of Subfractions 9 through 13 from Fraction 79

In an attempt to circumvent the difficulties presented by the lack of differentiating structure in the ultraviolet spectra of the dimethylnaphthaleneE, recourse was made to infrared spectra. This necessitated the concentration of these dimethylnaph- thalenes to minimize interference from other hydrocarbon types. An aromatic concentrate was separated from fraction 71 by adsorption column technique using silica gel. This was treated with picric acid, forming solid picrates with the naphthalenes present. The solid material was separated and decomposed to yield the naphthalenes originally present in this fraction. The infrared absorption spectrum was then determined on this material and was compared with spectra of the ten dimethylnaph- thalenes, and 1- and 2ethyl- and n-propylnaphthalene (1) This comparison indicated the presence of both 1,6- and l,7-dimethylnaphthalene in approximately equal proportions, with only traces of other naphthalenes. Indications point to the 1,3- isomer as constituting the major portion of the other naphthalenes present.

In the naphthalenic region of the spectrum (300 to 330 mp) very little difference is noted in spectra 74, 75, 76, and 77, except that the absorptivity progressively decreases, indicating a decrease in concentration of these naphthalenic-type aromatics.

Fractions 78, 79, 80, and 81 exhibit a broad band centering around 251 mM, probably caused by a methyldiphenyl. Little more than this speculation can be obtained from this particular spectrum, and the position of the methyl group is not known.

Throughout this study, in fractions containing both benzenoid (including Tetralins) and naphthalenic types of aromatics, little or no information was gained concerning the former type because of the obscuring effect of the much stronger absorbing naphthalenes. That such benzenoid aromatics exist in these fractions is clearly shown in Figures 10 and 11. In this instance, fraction 79, which contained both diphenyl and naphthalenic types, as indicated by its ultraviolet spectrum, was further treated with silica gel, and the ultraviolet spectra were obtained for all of the re-



V O L U M E 2 3 , N O . 1, J A N U A R Y 1 9 5 1 133

Table 11. Azeotroping Effect

Compound Displacement, F. Boiling Point

Saplithalene 2 2

Diphenyl 25 2-Ethylnaphthalene 41 '.6-Diniethylna),hthalene 19

1 -Methylnaphthalene 28 2-hfethylnaphthalene 27

1,6-Diinethylnaphthalene 17 1,7-DirnethylnaphtIialene 17

-~ ~~.. ~-..

d t i i i g subfrartions. That a separation of aronxttic typcs was effected and benzenoid aromat,ics are present is s1ion.n bl- the three distinct types of spectra. Subfractions 1 and 2 represent t,he p:iraffin-naphthene portion from the silica gel separation; 3 through 8 show spectra characteristic of benzenoid aromatics (including Tetralins); 9 through 11 show naphthalenic-type spectra; and 12 and 13 show spectra representing a mixture of naphthalenic and diphenyl derivatives. Although subfractions 3 through 8, containing benzenoid aromatics, const,itute roughly one third of the aromatic portion, their contribution to the spectrum of the whole fract,ion is negligible. To study these alkylated benzenes and Tetralins, separations of the naphthalenes niust first be effected.

I t was found that the maximum concentration of each arom:itic hydrocarbon identified occurred in a fraction boiling considcrably

lower than the boiling point of the pure hydrocarbon. tent of this azeotroping effect is shown in Table 11.

The ex-

ACKNOWLEDGMENT

The authors are indebted to R. L. Hopkins for making thr separat'ions of aromatic types by silica gel and picrate formation, and to H. 11. Smith and H. T. Rall for their encouragement and helpful suggwtions in the course of the research.

LITERATURE CITED

(1) American Petroleum Institute, Research Project 44, Sational Bureau of Standards, "Catalog of Infrared Spectral Data," Serial Nos. 764 through 777, contributed by Research Lahora- tories, Trinidad Leaseholds, Ltd., England.

(2) American Petroleum Institute, Research Project 44, Kational Bureau of Standards, "Catalog of Ultraviolet Spectral Data."

(3) Cary, H. H., and Brcliman, A. O., J . Optictrl Suc. Am., 31, 682 (1941).

(4) Hopkins, R. L., and Adanis, ?;. G., Proc. Okluiioma Acad. of S c i . 27 (1947).

( 5 ) Hougen, 0. h., and TYatson, K. M., "Industrial Chemical Cal- culations," pp. 95-101, Xew l o r k , John Wiley & Sons, 1931.

. and Streiff. 9. J.. J . Research S u t l . Bur. Stnndrirds. 27,'343 (1941 1.

(7) W a d , C . C., Gooding. 11. AI., and Eccleston, B. H., Inrl. h'ug.

(8) TYard, C. C., and 8chwal.ta. F. G., Petro leum P r o c c s s h g , 5, 164 them., 39, in5 (1947).

(1950). R E C E I V E D May 27, 1950. Presented before the Division of Petroleum Chemistry at the 117th l lee t ing of t h e A v E R I c A N CHEMICAL S O C I E T Y ,

Houston, Tex.

Infrared-Transmitting Solvents Triethylamine Addition as a Means of Increasing Applicability

J. S. 4 R D AYD THOMAS D. FONTAINE Rureau of Agricultural and Zndustrial Chemistry , Agricultural Research Center, Reltsville, M d .

The solvents considered best for infrared investiga- tions-carbon disulfide and carbon tetrachloride- are not suitable solvents for most organic acids. .4 nonaqueous solvent suitable for solubilizing acids was needed in order to characterize plant-growth regulating acids. When carbon disulfide and carbon tetrachloride are modified by the addition of 0.5 to 2.5% triethylamine, the resulting mixtures transmit sufficiently for easy use in all the regions from 2 to 15 microns, except those already obliterated by the unmodified solvents. Some plant-growth regulat-

HEN infrared spectra are desired of solids in the solution state, it is frequently impossible to find a suitable solvent.

Carbon disulfide and carbon tetrachloride are outstanding ex- amples of the type of solvents that have sufficient transparency for u5e over extensive portions of the salt region, but a large pro- poi tion of the organic substances do not dissolve to the necessary extent in these liquids. The inability to find a suitable solvent need not, however, prevent one from obtaining the spectra of a solid. Mull and film techniques give spectra that are satisfactory for many purposes, and those who have developed skill with these techniques state that they allow great flexibility in contrast to the limited applicability of solvent techniques. The mull and film techniques are not satisfactory for all purposes, however, and there remains a critical need for increasing the applicability of solvent techniques.

ing acids, for which no other suitably transmitting solvent could be found, readily dissolved in these mixtures. Analogous solvation methods offer a possible way for extending the solubility of other types of solutes in infrared-transmitting solvents. The method facilitates studies of the neutralized state of acids by offering a practical nonscattering medium for obtaining their spectra, and the spectra indicate the effects of neutralization on the vibration of bonds at specific locations along the molecular chains.

Barnes, Liddel, and Willianis ( 1 ) and Torkington and Thonip- son ( 8 ) have discussed some of the requirements of infrared-transmitting solvents. Because the desired infrared transparency of solvents is believed to be associated with a simple moleculiir structure in which the bonds are as few as possible in both type and number, such as CXa, (CX,),, or partially appropriate struc- tures, like X.( CX$),.X, all the solvents that seem appropriate fall into narrow portions of formula indexes and are readily searched out. From such a search and follow-up tests, it appears that important improvements can be expected only for applira- tions over limited spect,ral regions, or to eliminate ccrtain skips, and that there is littlc chance t o discover new solwnts that would a l l o ~ one to get the fairly complete spectra of additional typcs of s o 1 U t C P .

An alternative possibility i3 t o w e a mixed solvent, in which a

aromatic hydrocarbons in some diesel fuel fractions

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