Gum Formation in Gasoline 11. Control of Gum Formation in Gasoline by Antioxidants T. H. ROGERSAND VANDEVEER VOORHEES,Standard Oil Company (Indiana), Whiting, Ind.
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S TTAS b r o u g h t out in Part I of this paper (9),
a n t i o x i d a n t s by varying the concentration, and very minute amounts of the more effective compounds are used in order to bring t h e l e n g t h of the t e s t within p r a c t i c a l limits. The response t o the effect of antioxidants varies with the original induction period of the gasoline, the e f f e c t i v e n e s s of the antioxidant increasing with increasing original s t a b i l i t y of the gasoline. Table I shows the induction period resulting from the use of 0.001 per cent of p benzylaminophenol in different gasolines. In the comparative data in the s e c t i o n on antioxidants, consideration m u s t b e g i v e n b o t h t o t h e concentration of antioxidant and t o the induction period of the gasoline used for test. Unfortunately, many of the data were obtained over an extended period of time, using different stocks, so that a direct comparison of the compounds is not always possible.
The use of antioxidants is a particularly suitable method f o r prei!ention of gum formation in cracked gasoline. The antioxidant eficiency of a number of compounds has been tested by the Voorhees method, and it is found that the increase i n induction period is directly proportional to the concentration of antioxidant. I I L line with the general conclusion that readily oxidizable compounds are the most effective inhibitors, the monosubstituted p-aminophenols are shown to be particularly e@cient. p-Benzylaminophenol has marked advantages over compounds preciously proposed as inhibitors, such as a-naphthol and the polyphenols, in that it is more than twice as soluble in gasoline as in water and is quite color-stable, generally increasing the color stability of the gasoline.
the tendency of highly cracked gasolines t o form gum introduces the problem of making them sufficiently stable to remain free from gum d u r i n g the normal period of storage and distribution. Sulfuric acid, the traditional refining a g e n t , has not proved entirely s u i t a b 1e , largely on account of more or less severe losses of both material and antiknock value. Vaporphase claying t o improve stabllity results in relatively low clay yield, and l i q u i d - p h a s e polymerization in the presence of c l a y (6) a p p e a r s to be a rather elaborate process involving rerunning. Of these and other methods which have been proposed t o attain stability, none is so attractive and economical as the use of antioxidants ( I O ) , by means of which cracked gasoline may be kept from oxidizing, even without rerunning. The application of antioxidants for some autoxidizable materials is a familiar topic because of the scientific work of Moureu ( 7 ) and co-workers. The National Benzole Association ( 8 ) has done extensive work with the use of cresol as an inhibitor for motor benzene. Some of the earlier patents (1) on the use of inhibitors in gasoline were concerned with the object of color stabilization rather than gum stabilization and the work by Egloff, Faragher, and Morrell ( 2 ) dealt to some extent with antioxidants, but also with compounds such as urea, anthracene, . phenanthrene, tetralin, and others, which have no appreciable antioxidant effect. More recently Egloff and others (3) presented the results of induction period tests with a large number of organic compounds as inhibitors in gasoline, using the metal bomb test, and showed a general relation between that test and aging. They showed that a-naphthol, the polyphenols, and the aminophenols are the most effective inhibitors. The work on which this paper is based has yielded results which are in general agreement with those reported in the latter paper, and it is not the purpose to present data on a wider range of compounds, but to develop certain gerieralizations as to the relation between the chemical nature of antioxidants and their effects, to point out a particularly effective class of compounds, and to deal with certain practical features of the use of antioxidants in gasoline.
TABLEI. EFFECTOF ~-BESZYLAMINOPHEXOL o s INDUCTION PERIOD
230 260 490 490 ( 0 . 0 0 0 5 ~ oused)
120 130 200 315
RELATION OF CHEMICAL COMPOSITION TO ANTIOXIDANT EFFICIEKCY Table I1 gives results obtained with various aromatic hydroxy and amino compounds. TABLE 11. EFFECTO F
AROM.4TIC COMPOCSDS
HYDROXY AND AMINO INDUCTION
ANTIOXIDANT
CONCN.
% Aniline Phenol o-Cresol p-Cresol @-Naphthol 1 5-Dihydroxynaphthalene &Naphthylamine Hydroquinone Hexylresorcinol p-Phenylenediamine m-Phenylenediamine Pyrogallol
TESTSOF ANTIOXIDANTS IN GASOLINE As has previously been pointed out ( 5 ) ,the effect of antioxidants is to prolong the induction period of gasoline, with practically no change in the oxidation behavior thereafter. Figure 1 shows that the induction period by the Voorhees stability test is directly proportional to the concentration of antioxidant employed. It is therefore possible to compare
INDUCTION PERIOD USINQ 0.001% p-BENZYLAMINOPXENOL Minutes
INDUCTION PERIOD O F GASOLINE Minutes
0.05 0.05 0.05 0.05 0.05 0.005 0.05 0,002 0.005
0.01 0.02 0.002
INDCCTION PERIOD PERIOD CONTROL Minutes Minutes 55 55 55 55 55 135 55 55 230 55 55 90
It has previously been shown (3) that aliphatic alcohols and amines do not increase the induction period. The simple phenols and aromatic amines have a relatively slight inhibiting action, and, when a second hydroxyl or amino group is introduced, the effectiveness is markedly increased.
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of oxidation, but again i t may indicate a specific reaction betreen the quinone and the autoxidizing catalysts in the gasoline. I n this connection, 4,4’-dihydroxydiphenyl showed considerable antioxidant activity which may be accounted for by the possibility of oxidizing to diquinone. Table I V gives the results of a miscellaneous group of compounds; some of them are of interest because of previous references in this connection and others are cited to show the effect of different structures from those shown above. But few of these compounds have a significant action. The results obtained with anthracene, phenanthrene, and SUBSTITUTED PHEXOLS AND AROMATIC urea-practically a check on the control-are not unexpected. AMINES The alkaloids have been reported as antioxidants, but no INDUCTION effect was shown in these tests. Of the hydrazines and hyIHDUCTIOS PERIOD CONCN. PERIOD CONTROL drazobenzenes shown, the only inhibitor is diaminohydrazoMinutes Minutes % benzene. It is possible that the ineffectiveness of hydrazo65 0.05 55 benzene is due to the fact that it is so readily oxidized by oxy0.01 780 90 0.002 230 90 gen itself that in a brief time none remains to act as an antioxi0.01 430 335 0.005 230 235 dant.
The ortho and para positions give better results than the meta, and, when three hydroxyl groups are introduced, for example, in pyrogallol, the antioxidant effectiveness is most pronounced. These facts indicate that inhibitory action is related to the oxidizability of the compounds, and these and various illustrations below show that antioxidant efficiency, as measured by the induction period test, is a function of the reducing power of the inhibitor. Table I11 shows the effect of various substituting groups in connection with phenols and aromatic amines. TABLE111. EFFECTOF ANTIOXIDANT p-Nitrophenol p-Nitrosophenol p-Nitrosophenol p-Phenylphenol p-Cyclohex ylphenol 4 4’-Dihydroxybiphenyl dutyloresol Reaorcinol monomethylether B u t lguaiacol H y Aoxymethyl anethol Pyrogallol dimethylether Diphenylnitrosoamine Di henylamine p-lminomonometh ylaniline o-Aminodimethvlaniline ~ c eyl-p-phenyienediamine t Dibenzal-p-phenylenediamine Dibetanaphthylamine Ben~idine Dibutalbensidine Acetalbenzidine
.
0.01 0.005 0.005 0.005 0.005 0.005 0.01 0.06 0.01 0.003 0.002b 0.002 0.005 0.04
0.003 0.003
835 250 135 165 107 155 30 155 400 155 105 160 135 80 130 115
335 230 135 230 114 135 90 55 90 90 90 180 60 55 90 90
I n general, the introduction of a hydrocarbon radical in phenol has no improving effect. Similarly the nitro group usually reduces the activity of phenols as illustrated by the fact that nitrophenol is less effective than phenol. However, the nitroso group has a decided activating effect on phenol which is in line with the well-known sensitivity of the resulting compound to atmospheric oxidation. It mill also be noted that the ethers of the polyphenols have little or no antioxidant action, which coincides with the fact that those compounds are not readily oxidized like the free phenols. Usually replacing the hydrogen of the hydroxyl or both of the hydrogens of the amino group greatly reduces the antioxidant activity. The stability manifested by acetylated amines and phenols, which is frequently utilized in organic syntheses, operates to decrease the antioxidant efficiency. The inactivity of p-aminodimethylaniline, compared t o p-phenylenediamine, has been observed by Egloff et al. (3).
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The aminophenols have very low oxidation-reduction potentials (4) and are commonly used as photographic developers. These facts, together with the consideration mentioned above, led to a thorough study of this class of compounds as antioxidants. Their effectiveness is more marked than any of the compounds mentioned previously, as will be seen by a consideration of Table V. TABLEV. EFFECTOF AMINOPHENOLS ANTIOXIDANT
COHCN.
%
INDUCTION PERIOD
Minutea
INDUCTION PERIOD CONTROL
Minutes
p-Aminophenol 0.005 270 55 o-Aminophenol 0.005 55 280 TABLEIV. EFFECTOF MISCELLANEOUS COMPOUNDS 4-Amino-8-naphthol 205 0.002 135 0.0005 130 90 INDUCTIOS2,Q-Diaminophenol 0.002 95 495 INDUCTIOXPERIOD p-Methylaminophenol 0.001 305 90 AHT~OXIDAST CONCN. PERIOD CONTROL p-Methylaminophenol w-Methvlaminoohenol 0.01 1800 55 Minutes Minutes % p-Ber.z) larnino&enol 0.005 665 120 Anthracene 0.016 116 125 p-Renz).lainiiiopherol 340 0,002 120 Phenanthrene r.-Phenylaminophencl 0.016 125 116 550 0,005 120 Urea 0 02 65 Ilenzal-p-aminophetlLI 60 120 0.002 80 Furfuramide 0.005 110 90 Cinnanirr!-p-aininop~.ello! 0.002 110 80 Aldol-a-naphthylamine 0.05 255 55 Acetone-ani1 0.005 150 135 Phenylhydrazine 0.01 55 55 The data show that the simple aminophenols themselves Bensalphenylhydrasine 0.005 80 90 Cinnamalphenylhydrasine 0.005 145 90 are quite effective antioxidants and this fact was previously Hydrazobenzene 0.05 65 55 p,p‘-Diaminohydrazobensene 0,005 300 90 pointed out by Egloff et al. (3). The replacement of one p-Aminoazobenzene 90 0,005 90 of the amino hydrogens, however, has a remarkable effect Hydroxyasobensene 0.01 90 70 &,fur Accelerator on the activity of these compounds, making them far more E t h y l mercaptan 35 0.05 .. 55 Thiourea effective gasoline antioxidants than the parent compounds. 0.02 70 60 p-Thiocresol 0.005 90 90 In addition, it is possible by selecting suitable substituents o-Thiodiphenylamine 0.005 210 135 Benzaldehyde 0.02 55 60 to increase their gasoline solubility. Whereas methylaminoNicotine 0.01 90 90 Brucine phenol is soluble only to the extent of 0.008 per cent in cracked 0.02 60 60
As a rule, it appears that those compounds which may revert to a quinonoid structure on oxidation are the most effective antioxidants. Possibly this merely signifies ease
gasoline a t ordinary temperature, the benzyl derivative has a solubility of 0.2 per cent. When both hydrogens of the amino group are replaced, as in benzal-p-aminophenol, the activity falls off consider-
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ably. This phenomenon resembles the result observed with the diamines in which complete coverage of an amino group gave lower antioxidant activity. I n order to permit direct comparison of their effectiveness, Table VI shows the results of the use of 0.002 per cent of several inhibitors, when used in the same gasoline (a raw vapor-phase gasoline with an induction period of 130 minutes).
low solubility in gasoline. This may even cause difficulty in getting the compound in solution, but, in general, the amounts required are so minute that this could probably be accomplished satisfactorily. The particular difficulty is that the solubility of such compounds in water is generally many times that in gasoline, so that, if the gasoline is in contact with water, the inhibitor is apt to be largely removed from the gasoline phase. I n the normal handling, storage, TABLEVI. EFFECTOF ISHIBITORS and distribution of gasoline, i t is practically impossible to I N D ~ C T PERIOD ION insure the absence of water in the bottoms of tanks, etc., FOR 0.002% CONCN ANTIOXIDAXT and for this reason the use of highly water-soluble antioxiMinutes Hydroquinone 140 dants seems inadvisable. Hydroquinone, catechol, pyroCatechol 150 gallol, and the polyphenols, in general, are soluble in gasoline a-Naphthol 190 Pyrogallol 310 to the extent of only a few thousandths of a per cent, but p-Benaylaminophenol 345 in water the least soluble will dissolve to the extent of about These results indicate p-benzylaminophenol as the most 10 per cent. Experiments in shaking gasoline containing effective inhibitor of this group. As shown by the compari- pyrogallol, for example, with water, show that it is fairly son in Figure 1, p-methylaminophenol is somewhat more readily removed. Samples of gasoline to which 0.01 per effective on the weight basis; on the molar basis they are cent pyrogallol had been added were stored in gallon cans, about equivalent. in which about a half-inch of water was added. After long storage the gasoline from the can containing water was OTHERCONSIDERATIONS IN CHOICEOF found to have a gum content of 24.0 mg. The sample withANTIOXID.4NTS out water had a gum content of 6.4 mg. The color of the Aside from gum-inhibiting efficiency, there are other very gasoline and water was quite brown. It is not necessary that the inhibitor have a high soluimportant practical considerations involved in the use of antioxidants. I n addition to cost, these include color sta- bility in gasoline, but the important point is that the parbility and solubility in gasoline and in water. Many of the tition ratio with water be low. This has been accomplished effective antioxidants cause color formation in the gasoline, in the case of benzylaminophenol, for, while its gasoline both in the light and in the dark. This is undoubtedly due solubility is relatively low (0.2 per cent a t 83" F.),the soluin part to oxidation of the inhibitor but is a rather compli- bility in water is appreciably less (about 0.08 per cent). cated phenomenon, as it may vary with the nature of the Shaking experiments were made with gasoline containing this compound and i t was found that the induction period gasoline and the conditions of storage. Storage tests of various cracked gasolines containing a- of a gasoline containing pbenzylaminophenol is not decreased naphthol have shown marked color development (worse even after 10 minutes of vigorous shaking with 10 per cent than the control gasoline) particularly in the light, but also of water. The following tests in steel barrels a t atmospheric temperain the dark. Other antioxidants which are quite effective, such as nitrosophenol, the phenylenediamines, the simple ture show that stabilization in practical storage is effected aminophenols, and naphthylamine condensation products by benzylaminophenol, either with or without water present. develop color to such an extent after relatively brief storage The fact that the aging of the stabilized gasoline is slower that their use would appear to be impractical. I n the case with water present than without is presumably due to the of the aminophenols this tendency is lessened with substi- irregularities of barrel storage results and is not believed tution in the amino group; with gasolines containing p- to be significant. benzylaminophenol, no color difficulties due to the inhibitor GUM OCT.4NE STOCK CONTENT5 NO.' have been experienced, and, in general, the color stability Mu. of the gasoline is improved. For example, a group of five Sweetened vapor-phase gasoline (control) 17.4 68 finished gasolines was prepared by mild acid treatment of Control + 0.5% water 22.0 68 Control + 0.00157 benzylaminophenol 10.0 69 vapor-phase and cross-cracked gasolines, followed by re- Control + 0.00154 benzylaminophenol + 0.5% water 4 . 9 70 running and sweetening. These were stored a t room temAfter 25 weeks of barrel storage. perature in 2-gallon cans with and without 0.005 per cent of this antioxidant. The colors after one year, of samples Several other barrel tests showing the effectiveness of this containing the inhibitor, were 14 to 17 Saybolt; the control antioxidant are cited in Table I, Part I (9). gasolines had much poorer color, ranging from -9 to 12 Although the substituted aminophenols are fairly expenSaybolt. Samples of gasoline containing the same antioxi- sive compounds, their effectiveness as antioxidants is so dant have been stored in barrels, both with and without great that the cost for a given degree of inhibition is less water. These samples consistently show better color than than that for almost all other inhibitors tested. From the control gasoline. Sunlight tests with p-benzylamino- data available it appears that the color stability and the soluphenol are also favorable as illustrated by the folIowing com- bility characteristics of p-benzylaminophenol are such that parative tests on a finished mixed cracked gasoline. The it is the most suitable antioxidant of this class. tests were made by direct exposure in quart bottles. Each ACRNOWLEDGMENT of the antioxidants was used a t 0.085 per cent concentration: Acknowledgment is made to F. W. Sullivan, Jr., for advice and suggestions; and t o M. A, Youtz, J. L. Bussies, SATBOLTCOLORS ANTIOXIDANT Initial 1 hour 3 hours and P. T. Ward for assistance in carrying out some of the work. LITERATURE CITED Since most of the effective antioxidants are quite polar compounds, one of the practical difficulties in their use is
(1) Bjerregard, U. S. Patent 1,761,810 (June 3, 1930); Standard Oil Co. of N. Y . , British Patents 348,011 (April 29, 1931) and 349,427 (May 21, 1931).
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(2) Egloff, Faragher, and Morrell, Am. Petrolewm Inst., Proc. 10th Ann. Meeting, 11, KO. 1, Sect. 111, 124-32 (1930). (3) Egloff, Morrell, Lowry, and Pryer, IND.EXG.CREX.,24, 1375 (1932). (4) Fieser, J. Am. Chem. Soc., 52, 4915 (1930). ( 5 ) Flood, Hladky, and Edgar. Paper presented before Division of Petroleum Chemistry at 80th Meeting of American Chemical Society, Cincinnati, Ohio, September 8 to 12, 1930. (6) Holland, Oil G a s J . , 30 (33), 22 (1931). (7) Moureu and Dufraisse, Chem. Rev., 3, 113 (1926).
(8) (9) (10)
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Natl. Benzole Assoc. (England), Joint Benrole Research, R e p t . 4 (1927) et seq.; Hofferts (to Natl. Benzole Xssoc.) U. 6 . Patent 1,816,811 (July 28, 1931). Rogers, Bussies, and Ward, ISD.ENG.CHEM.,25, 397 (1933). Thomas, J . Inst. Petroleum Tech., 18, 350 (1932).
RECEIVED September 20, 1932. Presented before the Division of Petroleum Chemistry at the 84th Meebing of the American Chemical Society, Denver, Colo., August 2 2 t o 2 6 , 193%.
Solubility of Ethyl Alcohol in Gasoline OSCA4R
c.
(2.BRIDGEMAK AKD DALEQUERFELD, Bureau of Standards, Washington, D. NE of the major difficultieswhich
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has been encountered in the use of alcohol-gasoline b l e n d s a s motor fuels arises from the limited solubility of ethyl alcohol in gasoline. This is particularly serious i n c l i m a t e s in which freezing temperatures occur during the winter, necessitating the addition of some material to increme the solubility. Preparatory to a n investigation of such materials, it, has seemed desirable to obtain a comprehensive set of data on the solubility of ethyl alcohol, containing various percentages of water. in a number of gasoiines. There appears to be little published information on the subject. -4number of investigators (4) have used the solubility in kerosene as a criterion of the water content of ethyl alcohol and have given some d a t a on critical s o l u t i o n temperatures for alcohol c o n t a i n i n g various percentages of water. A limited set of data on the solubility of aqueous ethyl alcohol solutions in six gasolines has been given by Ormand) and Craven ( 3 ) . Hubendick (1) gives a few values for the solubility of alcohol in gasoline Sicolardot ( 2 ) has done considerable FIGERE1 . A w 4 - work on the solubility problem when RATES F O R hIE.4sUREVENT OF CRITI- a r o m a t i c s o r o t h e r niaterials are c . 4 ~SOLUTIOK TEJI- added, but little on s t r :I i g h t blends PER4TURE of ethyl alcohol and gasoline. Data on one aviation gasoline have been reported by Sparrow ( 5 ) .
SOLUBILITY TESTS The procedure used in the present m-ork consist,ed in preparing quantitatively solutions of ethyl alcohol, of I;no\Tn water content, in gasoline (warming if necessary) and in measuring the critical solution temperature by noting the appearance of a second phase on cooling. A diagram of the .apparatus is shown in Figure 1. I t consists simply of' a glass test tube sealed into 'an air jacket, which is provided with an outlet to the vacuum line for controlling the air pressure and hence the rate of cooling. The stirrer operates through a packing gland in the cork stopper used to prevent absorption of moisture during the determination. A mercury-in-glass thermometer graduated in 0.2" C . is used for temperature measurements don-n to -36" C.; for temperatures below this a toluene thermometer similarly graduated is employed. Both thermometers are calibrated and stem corrections are applied to the temperature readings when necessary.
In making an experiment, approximately 15 ml. of solution mere introduced into the solubility tube which was then closed with the stopper holding the thermometer and stirrer. The tube was placed in a special holder and introduced into a Dexar flask containing liquid air or water, depending upon the critical solution temperature. The rate of cooling was maintained at about 2" C. per minute, either by adjusting the depth of immersion in the cooling liquid or by controlling the pressure in the air jacket surrounding the solution. As the temperature dropped, the solution was st'irred vigorously until at some definite temperature for each solution a turbidity suddenly appeared. This temperature was recorded as the critical solution temperature. A check determination was made on each solution without removing it in order to insure that no absorption of moisture was occurring during the determination. Duplicate values agreed \%-ell,the average difference being less than 0.2" C. Data were obtained on twenty-three gasolines described in Table I. I n every case critical solution temperatures were measured on blends with aqueous ethyl alcohol solutions containing approximately 99 to 93 per cent of alcohol by
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20 30 40 30 €4 70 80 P e r c e n t G o s o l i n e in Blend
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FIGLRE 2.
CRITIC.4L SOLUTION TEJIPER.4TCRES O F BLESDSO F ETHYLA L COHOL AND
GASOLIKE1
volume. No attempt was made t o measure critical solution temperatures lying outside of the range from +65" to -65" C. The compositions of the alcohol solutions were determined from density measurements to 0.01 volume per cent. Check determinations were made, and fresh solutions were prepared from time to time, but the values did not differ by more than *0.03 volume per cent. Mean values are given in Table 11. In all calculations the actual value, and not the mean, was used. As applied to the alcohol solutions, volume per cent refers to the volume of alcohol (or of water, respectively) a t 60" F. (15.6" C.) in 100 ml. of solution a t 60" F.
