Production of Xylidines by High Pressure Hydrogenation QUALITY AND PERFORMANCE CHARACTERISTICS J. F. KUNC, JR., AND W. C. HOWELL, JR. Standard Oil Decelopment Company, Elizabeth, N. J . C. E. STARR, JK. Esso Standard Oil Company (Louisiana Division), Baton Rouge, La, good antiknock T h e projected use of xylidines to extend supplies of hydrogenation of nitroxylT t z l i t y of aniline long aviation gasoline during World War 11 led to the applicaenes; this process is dchas becn known, but the use tion of the high pressure hydrogenation process as one scribed in a companion papcr of this compound in aviation phase of a government-industry cooperative project for (6). Preliminary investigagasoline has been hampered their manufacture. Evaluation of the xylidines produced tions on the nature of the by certain disadvantages, from the high pressure hydrogenation process showed xylidines produced by high such as poor solubility in them to be similar to other xylidines produced in the joiiit pressure Iiy-drogenation rehydrocarbon mixtures. Uneffort with respect to imparting markedly improved rich vealed that these materials prccedcnted demands for high mixture antiknock performance to aviation gasoline did not differ significantly blends. Their relatively poor color stability, however, from xylidines produced by octane number aviation gasoline during lT70rld n'ar 11, necessitated improvement by treating methods. Xyliother processes with respect howcvcr, stimulated interest dines gasoline bIends have satisfactory freezing point, to properties such as antiin the possibility of using water solubility, and gum stability characteristics, alknock value and engine persome derivativks of aniline, though reactive toward synthetic rubber membranes. formance o h a r a c t e r i s t ic 6 , such as xylidines, which solubility in hydrocarbons, would not possess the disand stability of their blcnds advantages of aniline and with aviation gasoline. Mawhich could be used t o extend supplies and increase the antiterials produced in the pilot plant during the early stages of t,he high pressure hydrogenation process development were, knock quality of this type of fuel. To procure iapidly a large supply of xylidines for blending with however, soniexThat poorcr in color stability than xylidines deaviation gasoline during the mi, the United States War Departrived from several other types of processes. In view of the general similarity of xylidines from various sources, this paper ment, early in 1943, invited a nunibei of chemical and petioleum companies t o investigate t h e possibillty of manufacturing them includes some data on xylidincs from sources other than the on a large scale. Several chemical and two petroleum companies high pressure hydrogenation process, in order to illustrate the various factors t o be considered in the utilization of these macooperated by producing xylidines by iron reduction or catalytic terials as aviation gasoline additives. hydrogenation of nitrosglenes which M ere produced in ordnance I n t,he manufacture of xylidines by high pressure hydrogenaTST plants. The application of the high pressure hydrogenation tion, the nitroxylenes employed as feedstocks 'Ivcre derived from process a t the Baton Rouge refinery of the Esso Standard Oil Company (Louisiana Division), as one phase of the governmentthe nitration of Cg aromatics from pet,roleum fractions (0-,m-, and p-xylene, and ethylbenzene) ; the nitration was conducted industry cooperative effort, provided a means for utilizing existing plant equipment for rapid and large scale continuous producin several of the U. S. Army ordnance works. The xylidincs product was therefore a mixture of isomers, boiling bet,ween 415 tion of xylidines. T h e chief advantage obtained by the use of aromatic amines in and 439" F. and having a specific gravity at GO" F.,lGOo F. beaviation gasoline is a marked improvement in rich mixture antitween 0.976 and 0.988; the exact nature of the mixture depended on the composition of the C, aromatics feedstock and the condiknock quality. However, the folloJving disadvantages also arc associated n ith their use: a n adverse effect on synthetic rubber tions of t>henitrat,ion process. From the m- and p-xylenes tl e used in fuel system gaskets and carburetor fuel pumps and in expected xylidines are l-amino-2,4-dimethyIbenzene, 1-amino2,6-dimethylbenzene, and 1-amino-3,G-dimethylbenzene.From self-sealing fuel tanks relatively high freezing point or separation 0-xylene the expected xylidines are l-amino-3,4-diniethglbmzent., temperatures; increased water solubility of amine fuels, which mag result in increased difficulties a t low temperatuies due to and l-amino-2,3-diniethylbenzcne.From ethylbenzene, o-aminoicing of the fuel screens, etc; relatively poor color stability of ethylbenzene and p-aminoethylbcnzene should be obtained. amine blends caused by the unstable nature of commercial grades The mixed xylidines product is rcfcrred to in this paper as xyliof amine compounds; and the poisonous nature of the amines. dine. Most of the disadvantages listed may be overcome t o a large extent if the concentrations of the amines are kept within reason4VTIKh OCK QU.kLITY able limits (up t o about 3 volume 70) and by careful selection of The degree of improvement in antiknock quality obtained by the amine compounds. Based on a consideration of all of these the addition of xylidine t o aviation gasoline depends on the anlifactors and on manufacturing and raw materials requiiements, knock quality and lead content oE t h e base stock, and the conxylidines are among the most attractive aromatic amines for use centration of xylidine employed. The measure of improvement in aviation gasolines. in antiknock quality \vas found to be dependent on the engine This paper presents the results of studies which were conducted test method employed. In general, xylidine exerts its greatest primarily t o evaluate the xylidines produced by high pressure O
~
1530
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1948
1531
for War (13) on the basis of a largc number of determinations. I t is of interest to apply these data 'to an estimation of the extension of supply 1400 that might be expected by the use of xylidine in the production of gasoline having a supercharge rich rating equivalent to isoIO00 octane +1.25 ml. of tetraethyllead per gallon (grade 130). Calculations based on S 4 values shown in Table I indicate that 600 1% xylidine is capable of extending the 40 70 100 130 160 supply of grade 100/130 aviation gasoline A.G.A.C. Index Number by about 18 and 11%, respectivcly, when grade 91 and 87 gasolines arc used as the extenders, and assuming that lean mixture 600 (A.S.T.M. aviation octane number) performance will not be limiting. A comparison of the relative effectiveness 400 of xylidine and other high antiknock components for raising the rich mi@urc antiknock quality of marginal 100 octane number aviation stocks indicates that under 2ooeo 70 a0 90 io0 conditions of good distribution in engine A. S.T. M.-Aviation S + 4 Value Supercharge Rich Mixture manifolds, 1% of xylidine should be equa O.N. of Base Rating of Base to about 6% cumene or about 10% of either Figure 1. Relation between Xylidine Concentration, Base Antiknock xylenes or alkylate (15). Comparatively Quality, and Antiknock Blending Value of Xylidine in Aviation Gasoline large amounts (3.5 to 5.5%) of xylidine are required to equal the performance increase obtained with the first incremental 2 ml. of tetraethyllead antiknock blending value in low octane base stocks and in low per gallon (16). concentrations. The response of xylidine to tetraethyllead generally is not as The relation between base octane level, concentration, and high as that of typical aviation base stocks, as indicated by the blending antiknock quality for xylidine in aviation base stocks fact that its blending value is, for the most part, higher in clear containing 4.0 ml. of tetraethyllead per gallon was investigated; base stocks than in leaded stocks. Xylidine is also more tenixylidine from high pressure hydrogenation and froin other perature sensitive than most aviation fuel components; its persources was employed. The various types of xylidine gave formance is best under rich mixture conditions and comparaessentially the same average antiknock characteristics within tively mild lean mixture engine operating conditions. Table the limits of precision of the test methods. Based on all of the I1 shows data obtained in a full scale air-cooled aircraft enginc d a t a obtained, the various antiknock relations are summarized cylinder. Although the fuel containing 3 r 0 xylidine gave perin Figure 1. By the A.S.T.M. aviation test method (b), over formance markedly superior to the base fuel alone under the the range of octane numbers (85 to 100) normally encountered relatively mild operating conditions (175 F. intake air temperawith high quality aviation gasolines, the blending octane number ture), under the more severe conditions (350' F. intake air temof xylidine does not change appreciably with concentration, a t perature) its lean mixture performance was actually poorer than least up to 3% xylidine. I t s blending value, however, is markedly that of the base fuel and its rich mixture performance not apdependent on the octane number of the base stock into which it is preciably better. blended; it varies from 192 in 90 octane number base to 100 in 100 octane number gasoline. By the AFD-F4 supercharge rich mixture engine test method (5),xylidine concentration and base antiknock quality are both important; the blending value TABLE I. USE OF XYLIDINES FOR EXTENDING GRADE100/130 decreases rapidly as xylidine concentration or base octane level is AVIATIONGASOLINE SUPPLIES raised. Calculated The data of Figure 1 indicate antiknock blending values for Composition of Blendsa xylidine a t 1% concentration in grade 100/130 aviation gasoline Components, Yo A B of 100 A.S.T.M. aviation octane number and 341 supercharge Grade 100 base gasoline 100 0 0 0 84.3 89.9 rich mixture S 4 value. Grade 91 gasoline 0 100 0 0 14.7 0 I800
+
+
+
The S 4 value is the volume per cent S reference fuel (technical iso-octane) +4.0 ml. of tetraethyllead per gallon in M reference fuel (straight-run low octane number naphtha) +4.0 ml. of tetraethyllead per gallon matching the detonation characteristics of the sample by the AFD-F4 supercharge rich mixture method. By this knock test method, 61.5 S 4 valueisequivalent to 90 octane number; 71.8 B 4 value equals 95 octane number; 82 S 4 value equals 100 octane number; 96 S 4 value equals 4 iso-octane +2.0 ml.' of tetraethyllead per gallon; and 100 S value equals iso-ortane +4.0 ml. of tetraethyllead per gallon.
+
+
+
+
+
Grade 87 gasoline Xylidine Supercharge rich performance Octane number or iso-octane ml. TEL/Ral. S 4 value Extension of supply, %
+
0
0
+
0 0
100 0
9.1
1.0
.
1.25
1.25
75.7 6 5 . 4 341
92.6 18.5
92.6 11 2
1.25 9 6 . 8 9 1 . 7 92.6
0 100 1.0
0
,
a Xylidine blends both have a lean mixture (A.S.T.M.-aviation) octane number of 99.
+
The S 4 value is equivalent, in this case, to 957 Aviation Gasoline Advisory Committee (A.G.A.C.) index number (lb). This compares with values of 100 octane number and 1100 index number, respectively, reported by the A.G.A.C. Subcommittee on Blending Octane Numbers of the Petroleum Administration
A considerable amount of data has been amassed by the National Advisory Committee for Aeronautics (N.A.C.A.) (11) on the engine performance characteristics of xylidine blended aviation fuels; these likewise indicate appreciable temperature sensitivity on the part of this material.
1532
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 8
color and gum stability. I n no case were the xylidine blends as satisfactory as the base gasoline containing no xylidine, but amine type inhibitors; in all cases the problem of color stability was indicated to be considerably more difficult than that of gum stability. Although it has not been demonstrated that color degradation alone is any criterion of potential gum formation in xylidine fuel blends, nevertheless, color itself is an important property of aviation fuels in the field, both from 100 a psychological standpoint and because color degradation Color of Xylidene might lead t o confusion of aviation stores. Considerable % Transmittance a t 550 ;LIr work was undertaken, therefore, by the C.R.C. Gasoline Figure 2. Effect of Color of Xylidine on Color of XylidineAdditives G ~ at the ~ instance ~ ~of the, various eovernAviation Gasoline Blends mental agencies concerned with the production and utilization of xylidine, to obtain a background of data relative t o EFFECT ON EIVGIYE CONDITION the color and gum stabilities of these materials. The Standard Oil Development Company and the Esso Standard Oil Company In the course of routine testing of aviation gasoline blends containing xylidine, this type of fuel tended to make the engine oper(Louisiana Division) cooperated in this work and, in addition, ation erratic and unusual engine deposits were sometimes formed. initiated a program of their own in an endeavor to expedite a n Therefore a number of tests were conducted to determine more effective solution to the problem. conclusivdl the effect on engine condition resulting from the use The problem of xylidine stability may be resolved into two of xylidine-kended gasoline. These tests were run on a standard A.S.T.M. aviation CFR knock-testing engine, on two Lauson enphases: the stability of xylidine-gasoline blends; and the stagines (Model EF-2, manufactured by the Lauson Engine Combility of xylidine, as such. pany, New Holstein, Wis.), and on an aviation CFR engine of the type used for testing lubricating oils, using base gasolines with and without 1to 2% xylidine added. STABILITY O F XYLIDINE-GASOLINE BLEVDS The results of these laboratory tests indicated that xylidine mav exert a noticeable adverse effect on engine condition. The Storage tests and general laboratory and field test observations xylidine blends left fairly heavy deposits of tarlike material on indicated that xylidine darkens on storage when exposed to air, various parts of the intake systems, and the venturis of the carand that this discoloration is accelerated by increase in temperaburetors were coated with this material in spots. I n the CFR enture and contact with metal and by exposure to light. It was gines, heavy deposits were also formed on the intake manifold heaters. The pistons from engines operated on 1 to 2y0 xylidine found that xylidine has a deleterious effect on the color of the blends were covered with a scum or scale of metallic-appearing gasoline into which it is blended only when the xylidine itself material which was hygroscopic. Qualitative analysis of this has degraded in color t o a very low value. scumlike material gave positive tests for lead, aluminum, broColor stability tests were conducted on xylidine samples from mine, and hydrocarbons and negative tests for nitrates and nitrites, as well as soluble and insoluble sulfates. various sources to determine: differences between xylidines By contrast with the results obtained on the xylidine-blended manufactured by various processes; the effect of inhibitors in fuels, the base gasolines without xylidine gave much cleaner inimproving color stability; and the effect of treatment. Tests take systems and pistons were practically free of carbon and in nere conducted by measuring the color degradation after exrelatively good condition. The scale particles were much smaller, covered a larger portion of the piston area, and showed little posing the xylidine to air in a constant temperature oven at evidence of being hygroscopic. elevated temperatures or a t room temperature. Color changes It is concluded from the laboratory engine tests that xylidine were determined by measuring the per cent transmittance of blends may cause serious engine deposits under the conditions of light through the sample, compared to distilled water. The operation employed. However, it has not been demonstrated that the performance of xylidine blends in these tests is a critetime in hours for the xylidine to degrade to 10% transmittance rion of their performance in aircraft engines. For example, it is of light was used as a measure of color stability in these tests conceivable that the deposits in the carburetors and intake systems because lower transmittance values could not be determined were due largely to the fact that the mixture temperatures in the accurately and, below 10 to 2070 transmittance, the xylidine laboratory engines 1Tei-e too low to ensure evaporation of all the xylidine; hence it remained in the intake system and eventually begins to have a deleterious effect on the color of gasoline into formed gum. If this is true, the problem of intake system deposits which it is blended. The latter point is illustrated by the data may be considerably less serious for full scale engines using xyliplotted in Figure 2 for 1 and 3% concentration blends of high dine blends, as mixture temperatures are usually higher than those pressure hydrogenation xylidine in grade 100/130 aviation gasoof the test engines. Thls latter presumption is given support by the fact that in three full scale engine tests run by the U. S. Army line. Air Forces Rlat6riel Command a t Wright Field (IO), careful inAlthough, as demonstrated by the data in Figure 2, the color spection of all carburetors and intake systems revealed no harmof xylidine a t the time of blending into aviation gasoline may ful deposits resulting from the use of aviation gasoline containing 1% xylidine. The effect of xylidine on the power section (coinbustion chambeis, pistons, valves, rings, and bearings) of aircraft engines, however, may be harmful under certain conditions, although data on this subject ale not sufficiently extensive to draw TABLE 11. EFFECTO F XYLIDINE ON P E R F O R M A N C E O F GRADE definite conclusions in this regard. l00/130 AVIATION GASOLINE IN FULL SCALEAIRCRAFT ENGINE CYI.INDER L
STABILITY
Desert storage tests carried out by the Gasoline Additives Group of the Coordinating Research Council (C.R.C.) during 1943 and 1944 ( 7 , 8) disclosed that aviation fuels containing xylidine may not possess satisfactory color and gum stability under severe storage conditions. Xylidine blends containing phenylenediamine and/or aminophenol type oxidattion inhibitors were shown by these tests to have poor color and gum stability whereas those blends containing xylidine and alkyl phenol type inhibitors, or no synthetic inhibitor a t all, had considerably better
P r a t t & Whitney R-2800-8, air cooled; compresRion ratio: 6.65/1; speed: 2250 r.p.m.) Knock-Limited Power, IMEP" a t Furl/& Ratio 0 . 0 7 (Lean) 0.10 (Rich) 175 350 178 350' Intake air temp., O F. 450 450 4.50 450 Cylinder head temp., F. 35 35 35 36 Spark advance, degrees 250 175 170 85 Grade 100/130 fuel 198 82 303 1803 Grade l00/130 fuel 3% xylidine +16 -4 1-21 +3 Change in power due t o xylidine addition, 70 (Cylinder:
-
~
+
Q
Indicated mean effective pressure. pounds per square inch.
-
August 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 111. RELATION OF COLOR OF XYLIDINE TO GUMCONTENT OF
XYLIDINE-GASOLINE BLEND
(Clear, uninhibited 100/130 aviation gasoline base stock) Color of Glass Dish Gum, Xylidinen Mg./100 M1. Gasoline base 0.2 1% Xylidine 100 (wit'&white) 0.2 10 (Black) 0.2 1 (Black) 0.2 < 1 (Black) 0.6
+
Per cent transmittanoe of light with North Sky filter.
affect t h e color of the resultant fuel blend, apparently it has little effect on preformed gum content. This is4Jhown by Table 111, which indicates t h a t color forming bodies are so small a part of the xylidine as t o escape detection as preformed (glass dish) gum, even when the color is extremely dark. (The glass-dish procedure is the same as (14) except t h a t a glass evaporating dish is employed.) The preformed gum content of the gasoline in this case was not affected by the addition of 1% xylidine. No consistent difference was found to exist between the high pressure hydrogenation xylidine and that produced by other processes, with respect t o their effect on copper dish (14), glass dish, and A.S.T.M. gum (I), but the addition of either type of xylidine in all cases increased the copper dish gum value t o a greater extent than the preformed (glass dish or A.S.T.M.) gum. The presence of tetraethyllead apparently has no effect on the gum values obtained. The base fuel into which the xylidine is blended appears to have a definite influence on the gum blending value; paraffinic base stocks of low copper dish gum content (1 mg. per 100 ml.) are much better in this respect than cracked gasolines having a higher copper dish gum residue (5.7 mg. per 100 ml.). Additional data on the stability characteristics of xylidine aviation fuel blends are shown in Table IV. These data were obtained by testing various xylidine-inhibitor combinations in a base stock of marginal stability. This base stock was one t h a t could be made satisfactorily stable in the absence of xylidine by the addition of normal amounts (0.8 pound per 5000 gallons) of aminophenol type gasoline inhibitor. Some data obtained with a less stable base gasoline are shown also. Table IV shows that the addition of 1% xylidine t o the base fuel has a definite beneficial effect on potential gum stability as measured by the army accelerated test method (9). This beneficial effect is lessened somewhat by addition of higher concentrations (3%) of xylidine. Xylidine alone apparently does not stabilize against lead precipitation, as measured by the nitrogen bomb test (16),although all the gasolines containing xylidine plus inhibitor were satisfactory i n this regard, I n the nitrogen bomb test the sample, contained in a glass bottle in a sealed metal bomb, is subjected t o nitrogen under a pressure of 100 pounds per square inch and a temperature of 212' F. for 16 hours. This test has been found useful for predicting the high temperature storage stability of t,etraethyllead in aviation gasoline; the formation of precipitate is noticeable with unstable fuels. Table IV shows that from the standpoint of gum stability and lead stabilization, the aminophenol and alkyl phenol inhibitors are about equally effective.
1533
The effect of xylidine on the color stability of aviation gasoline blends is illustrated by the data in Table V. These data, which were obtained in a test program in which xylidine-blended aviation gasolines were stored at 125' F. in rusted iron pots, show that addition of xylidine t o green grade 100/130 aviation gasoline causes appreciable color degradation. The data of Table V also lead t o the conclusion that the ability of a n inhibitor to suppress color degradation of xylidine, as such, is not related necessarily t o its ability t o protect xylidine gasoline blends against color degradation. For example, some sulfur-type compounds, which are potent inhibitors for protecting unblended xylidine, are inferior to alkyl phenol type inhibitors in protecting the gasoline blends. Table V shows that 2,6-di-tert-butylC-methylphenolwas successful in protecting some of the original green coloration of these fuels, although some color degradation occurred even with this inhibitor. To determine the reasons for the relatively poor color stability of xylidine aviation fuel blends, laboratory experiments were conducted from which the following observations were made: Base stock type has a marked influence on the color stability of xylidine-gasoline blends. Paraffinic bases are considerably more stable in this respect than olefinic or aromatic bases, and aromatic hydrocarbons are more stable than olefins. Clear xylidine blends made with wholly paraffinic base show no tendency t o develop color. There is no evident relation between the color stability of xylidine when stored by itself and the color stability of the xylidinegasoline blend. Gasoline blends containing either a highly colorstable or a color-unstable xylidine have substantially the same color stability characteristics. The presence of small amounts of peroxides in xylidine blended fuels is sufficient t o cause significant formation of color bodies. This color probably is associated with oxidation products of xylidine or xylidine impurities; the oxidation process is accelerated by peroxides. Reduction of peroxide content t o a low figure greatly restricts, but does not entire1 eliminate, the formation of color in aromatic or olefinic xylidine-glended fuels. This suggests some sort of reaction between unsaturated gasoline constituents (or oxidation products thereof) and xylidine. Alkyl phenol inhibitors, as typified by 2,6-di-tert-butyl 4methylphenol (2,6 B4M), restrict the formation of color bodies in base stocks initially free of peroxides. When peroxides are present however, alkyl phenol inhibitors in the concentrat ions normaily employed in aviation gasoline (up t o 1.0 pourid per 5000 gallons) do not prevent color degradation of xylidine-gasoline blends, although t,hey do reduce the rate at which the color degradation occurs; the effectiveness is increased a t higher concentration levels. n-Butyl-p-aminophenol inhibitor, which is used extensively as a gasoline antioxidant, actually accelerates the color degradation of xylidine blends in the laboratory test. The yellow dye used to produce the green coloration of grade 100/130 gasoline containing blue tetraethvllead fluid, is more reactive t o xylidine and/or xylidine oxidatcon products than is the blue dye contained in the tetraethyllead fluid.
It is concluded from these studies that the principal reasona for color degradation of green aviation fuels containing xylidine are: reaction between xylidine and the oxidation produc-ts of the gasoline hydrocarbons (such as peroxides) ; and reaction between
TABLE IV. EFFECT OF XYLIDINE ON STABILITY OF AVIATIONGASOLINE Gum. Mg./100 M1. N2 Glass Copper 5-hr. 16-hr. Bomb Inhibitor, 0 . 8 Lb./5000 Gal. dish dish Army Army Test A None 1 3 19 247 DNP A 0 n-Butyl-p-sminophenol 1 1 3 12 Pass A H,P.'H. b 1 None 3 4 7 13 DNP A 3 None H.P.H.6 5 7 17 21 DNP H.P.H. b 1 Mixed tert-butyl cresols 2 2 6 7 Pass Other process 1 None 2 5 7 17 DNP Other process 3 None 4 6 12 22 DNP Other process 1 n-Butyl-p-arninophenol 3 4 5 5 Pass Other process 1 2,6-Di-tert-butyl-4-methylphenol 4 1 4 7 Pass Other process 1 2,4-Dimethyl-6-tert-butylphenol 3 3 4 6 Pass Other process 1 Mixed tert-butyl cresols 1 1 5 13 Pass B 0 None 2 6 131 B H.$.'&. 1 None 2 17 15 .. B H.P.H. 1 2,6-Di-tert-butyl-4-methylphenol 4 15 9 B H.P.H. 1 2.4-Dimethyl-6-tert-butylphenol 4 13 9 .. .. B H.P.H. 1 p-Thiocresol 4 10 20 Rase stocks were bote grade 100/130 aviation gasolines containing 4 . 8 ml. tetraethyllead per gallon and finished to different oxidation stability levels. b H.P.H. deaignates high pressure hydrogenation.
Base Gasolinea
Xylidine Source Vol. % 0
...
.. .. ..
..
.. .. ..
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
1534
T.4BLE
v.
AGIlVG
(Base gasoline:
TESTS.4T 125" F.
I N I i U S T E U I R O K POTS
25% straight-run, 75% cracked base
+ 4 . 0 ml. TEL/gal.) 7
Xylidine Souroe Voi. yo
..
?
n-Butyl-p-aminophenol 2,6-di-tei t-but~l-4-i1ietliylpheiiol
H.P.H.Q Otherprocess Other process Other process
1 1 1
Other process Other process
1 1
Other process
1
a
1
1
A S . T.M. G u m , 1Ie./l00 hll. After 3 1110. Original Original storage Visual .1.S.T.II.
Inhibitor, 1 . 0 Lb./3000 Gal. Concn. None n-Butyl-p-aminophenol
Sone Xone None None H.P.H." H.P.H.a H.P.H.a
1.8 2.2 3.8 3.6
Vol. 40, No. 8
Color of Gasoline After 3 nio. storage Visual A.S.T.ll.
-
1.8
10.6
Green Green Green Green Green Green Green
Mixed m o n o - and di-ieri-butyl cresols Sone n-Butyl-p-aininoplienol 2,6-di-te~t-butyl-4-nietliylplienol
2.4 1.8 2.4 2.2
12.0 13.0 10.4 4.8
Green Green Green Green
0
Mixed mono- and di-teit-butyl cresols 2,6-di-tert-butyl-4-1netli~l~lienol, reacted with 5 % P& 2,6-di-tert-butyl-4-ii~ethylphenol containing 57* S a sec-butyl xanthate
2.2 0.4
10.4 14.0
Green Green
0 0
Green Green Green Green Brown Brown Greenbrown Brown Brown BroTvn Greenbrown Brown Brown
2.4
l5,O
Green
0
Brown
2,6-di-te~t-buty1-4-1iieth?.lplienol
Mixed mono- and di-teit-butyl cresols
-.
N o;-." no
8:8 4.0 8.0
13.2
1.8 1.8
6.0
0 0 0
Before storage, after 16-hour Army gum test A.S.T.bl.
0
3
23/a
3
3:/4
n
?lip 2 11:
3 11/z 2"l'i
lJ/4?
2112 2 lB/4
2
H.P.H. designates high pressure hydrogenation
instrument ivas equipped x i t h a grating and was adjusted to pass light in the range 535 to 565 mp when set for 550 mp. The Korth Sky filter used m4th the Helige-Diller instrument had a maximum transmittance of light a t 510 mp arid covered the visible spectrum from about 400 t o 700 mp. Color measurements have been reported either as per cent transmittance or as optical denZ sity-that is, loglo 2,where Io is 100O-', and represents the light I transmitted by xylidine containing no color bodies (water white) an+ I is the per cent light transmitted through the sample. An optical density of 1 is equivalent to lOV0 transmittance of light and represents a n almost opaque sample under these conditions. To enhance the accuracy of this test method, all tests were made on xylidine which had been freshly redistilled in an atmosphere of inert gas (nitrogen or carbon dioxide) immediately prior to addition of inhibitors and testing.
xylidine and the yellow dye. Obviously, the latter difficulty could be overcome by merely leaving out the yellow dye, or if a green fuel were desired, by using a more stable yellow dye or by using a stable green dye in place of the blue-yellow combination. The former difficulty can be avoided t o a large extent by using a gasoline initially free from oxidation products, a xylidine product having good color at the time of blending, and an alkyl phenol inhibitor which is highly potent in restricting oxidation of unstable gasoline components and, at the same time, is fully compaiible with xylidine. STABILITY OF XYLlDINE
The laboratory investigation of the color stability of xylidine centered around a relatively simple method of test which consisted of storing the xylidine in open 16-mm. inside diameter Pyrex test tubes in an oven held at elevated temperature and periodically determining photoelectrically the degree of transmittance of light through the sample. Color measurements were made using either a Coleman model 1 1 Universal spectrophotometer provided mith a PC-4 purple filter or a Helige-Diller model 400 soectroDhotometcr with a Korth 91rv filtcr. The Coleman
Using this method of test, it was found that color degradation may differ markedly mith different xylidine materials, and that although xylidine of original fair color stability (40 to 90 hours 10% transmittance time a t 158" F.) may be improved considerably by the use of an appropriate inhibitor, the highly stable and highly unstable materials were either insensitive to or actually harmed by certain types of inhibitors. Accordingly, the program dealing with the stability of xylidine was divided into t x o parts: O F INHIBITORS O X ACCELERATED C O L O R SThBILITY O F TABLE VI. EFFECT the first phase was an attempt to increase XYLIDINE the color stability of xylidine of original fair Time in Hours Required for Degradation of Sample t o 10% Transmittance of Light a t Indicated Temcolor stability by the use of inhibitors; the perature and Inhibitor Concentration (HelligeDiller Colorimeter, S o r t h Sky Filter) second phase was t o improve xylidine of poor 1 5 8 O F. 167' F. . 194O F. original color stability t o a satisfactory stability Concn., 10% Concn., 10% Concn., 10% Inhibitor Used wt.% Time wt.% Time wt.% Time ' level by treating methods. 70
T n n m
.,_..l
Alkyl phenols Mixed mono-tert-butyl cresols 2,6-di-tert-buty1-4-methy!phenol 2 4-dimethyl-6-tert-butylphenol Slixed mono- and di-teit-butyl cresols Sulfur compounds Carbon disulfide p-Thiocresol Isopropyl xanthic acid Xanthomethyl diisobut,ylphenol Sodium isopropyl xanthate Sodium sec-butyl xanthate Sulfur Thiourea Mixed inhibitors Mixed mono-tert-butyl cresols containing 23% CSZ 2.6-di-tert-butyl-4-methPlphenol Containing 23% CS? 2,6-di-tert-buty1-4-methyl phenol treated with 5% PzSa Petroleum &enols treated with 5 % P2S6 n-Butyl-p-aminophenol in 5070 alcoholic solvent (methyl f isopropyl alcohol)
...
..
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0.01
270 131
0.2
...
0.14
... ...
90
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..
0.01
403
0.1 0.2
if5 450
... ...
...
... ... 0.4
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34
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0.2 0.2 0.2 0.2
88 43 96
0.2
60 186
, . .
0.06 0.2 0.05
200
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,.
,..
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, . .
0.2 0.2
87
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0.2 I
.
21 40 46 23
48
.
0.2 0.01 0.01
5s 63 62
..
...
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0.13
229
... ...
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0.13
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0.2
110
0.2
59
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EFFECT OF INHIBITORS ON COLOR STABILITY O F XYLIDINE
The results of tests t o determine the effectiveness of various inhibitors for improving the color stability of xylidines indicated that: Color stability decreases rapidly with increase in storage temperature. The alkyl phenols exert varying effects. They are effective color stabilizers in xylidine of original fair (intermediate) color stability; they are without effect in xylidine of original poor color stability and actually may degrade the color stability of xylidine of original high stabilitv. Sulfur compounds appear to be effective colo- stabilizers in both the poor and intermediate stability xylidines; insufficient data arc
August 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
available t o assess accurately their value in protecting the highest stability xylidine, although inhibitor CX-4 which was made by recrystallizing the room temperature reaction product of xylidine and carbon disulfide, almost doubled the time required for degradation t o 10% transmittance of light at, 194' F. There is some evidence t h a t mixtures Of alkyl phenol and certain sulfur compounds may be more effective color stabilizers than either compound by itself under these accelerated test conditions. Aliphatic amines and hydroxylamines are effective color stabilizers regardless of the original color stability level of the xylidine. Aminophenol and phenylenediamine type inhibitors, which are used extensively as gasoline .antioxidants, have an adverse effect on color stability. D a t a on the effect of a number of inhibitors in xylidine of originally fair color stability are presented in Table VI.
1535
TABLE VII. EFFECT O F TEMPERATURE O N COLOR STABILITY O F XYLIDINE PLUS INHIBITORS 70' F. 1.0%
Xylidine Sample and Inhibitor Used poor stabiljty xylidine p-Thiocresol Carbon disulfide 2,6-di-tert-butyl-4-methylphenol treated with 5% PiSs 2,4-dimethyl-6-tert-butylphenol Mixed mono-tert-butyl cresols 2,6-di-tert-butyl-4-methylphenol
+ + + ++ +
Fair stability xylidine 2,4-dimethyl-6-tert-butylphenol p-Thiocresol Mixed mono-tert-butyl cresols 4- Carbon disulfide 2,6-di-tert-butyl-4-methylphenol 2,6-di-tert-butyl-4-methylphenol treated with 5% Pass
++ + ++
time, Concn. hr.
o:i
0.06
0.2
0.2 0.2
0.2
158' F. 10%
time, Concn. hr.
505 650 575 480
012
430 410 310
Optical density after 2650 hr. 0.20 0:2 0.14 0.1 0.26 0.2 0.27 0.06 '0.49 0.2 0.91 0 . 2 >1.0
,
l67O F.
-
IO%,
time Concn. hr.
o:i
..
35 49 67
..
0.06 0.2
24
0.2
35
0.2 0.2
27
0.2
16 16 14
0.1
012
..
..
70
o:i4
iii
0:Ol
270
0.2
..
,.
90
13 32 33
34 0:i 0.2 0.2 0.06 0.2
0.2
96 200 88
185 43 110
A comparison of the effects of various inhibitors on the color stability of xylidine at elevated temperatures and a t room temperatures indicat,ed that the relative effectiveness of the inhibitors is not the same at all temperatures. This is shown by Table VII, which compares data obtained at 70" t o 167" F. on xylidine plus various inhibitors. Thus, while in the poor stability xylidine, the various inhibitors maintained approximately their same relative order of effectiveness at 70°, 158", and 167" F., some of those indicated t o be either neutral or slightly beneficial at the higher temperatures were actually harmful at 70" F. In the fair stability xylidine, only the 2,4-dimethyl-6-tert-butylphenol was effective in improving color stability at 70" F., although based on the higher temperature data, all the inhibitors shown in Table VI1 were effective. This is especially noteworthy in the cases of carbon disulfide and p-thiocresol, and the phosphorus pentasulfide-treated alkyl phenol, which showed outstanding performance in the higher temperature tests. It is concluded that for protecting xylidine against color degradat,ion at relatively high temperatures, certain types of sulfur compounds are very effective whereas for prolonged protection at lower temperatures none of the inhibitors tested is outstanding. It appears that, under low temperature storage conditions, the color stability of the xylidine material itself far OvershaiJows any improvement that can be achieved through the use of inhibitors. IMPROVEMENT
IN COLOR STABILITY BY OTHER METHODS
Inasmuch as the xylidine produced in t,he pilot plant operations during the development of the process for high Pressure hydrogenation of nitroxylenes was found t o be inferior in color stability t o xylidines from other sources, a n extensive program of investigation was undertaken t o determine the reason for its relatively poor color stability and t o develop methods for improving it. Although, the color stability of xylidine itself is apparently of littJe significance from the standpoint of affecting the color stability of gasoline blends containing this additive, nevertheless it is important t o have a color-stable product t o Prevent its going off color during transit and storage prior t o blending in aviation gasoline. Tests were run on a number of batches of xylidine produced by the Esso Standard Oil Company (Louisiana Division) in pilot plant and full scale equipment, Using a supported sulfur-resistant catalyst, a t approximately 3000 pounds per square inch pressure ( 6 ) . Effect Possible Impurities and/or Contaminants. A study of the reduction mechanism for aromatic nitro shows that the following classes of intermediates may be formed and may be present in small concentrations in the xylidine produced by high pressure hydrogenation: pheny~hydroxy~aminetype; nitroso compounds; aminophenols; azoxy compounds; 820 compounds; and hydrazo compounds.
In. addition, the presence of unconverted nitroxylenes, hydrogen sulfide, diamines, and cyclohexylamines' is also possible. The effects of ail of these possible contaminants, which might be present as impurities in xylidine produced by high pressure hydrogenation, were not investigated with respect to effect on color stability but i t was determined t h a t aminophenols, diamines, dicyclohexylamines, azoxy and hydrazo compounds have a very harmful effect on color stability when present. I n low concentrations (
