Inhibition of Deterioration of Cracked Gasoline durin Storage C. J. Pedersen E. I. DU PONT D E NEMOUHS Br COMPANY, WILMINGTOS. Ub,l
Mechanism of Gum Formation
Deterioration of cracked gasoline is d u e to the autoxidation of its hydrocarbon constituents. T h e initialoxidation products, formed through a series of free radical chain reactions, are peroxidic compounds which are converted into various monomeric a n d polymeric oxygenated products. Gasoline antioxidants inhibit deterioration by reacting with chain-propagating free radicals. Because antioxidants are destroyed in the process of terminating chain reactions a n d are also oxidized by some of the peroxidic compounds, they should b e added to gasoline as soon as possible after cracking. Autoxidation of gasoline containing antioxidants is greatly accelerated by salts of certain multivalent metals, particularly copper. These catalysts should either be removed or deactivated in order to permit a minimum concentration of an antioxidant to confer adequate storage stability. Metal deactivators suppress the activity of metal catalysts by converting them into stable, chelate complexes which are devoid of pro-oxidant catalytic activity. Deactivators should be added to gasoline at the earliest moment, preferably prior to or simultaneously with the introduction of the antioxidant.
(>urn formation in cracked gasoline is priniarily duc. to I h r i i i i oxidation of olefinic hydrocarbons which induces polynirriiat ion and condensation reactions in the system (13, 15). The liqnic phase autoxidation of hvdroca~honsoccurs as follows ( 2 1 ) : f
I. 11.
Chain initiation
R-H+R’ +(He$ Chain propagation U.
R
+
0 2
+R-0-0’
+
b. R-0-0$- R’---H ----tH” R-0-0 14 I L I . Chain branching R-0-0-H +chain-initiating free radiral. I\. Phain termination a. Union of two tree radical3 6. Disproportionation of free radicals c. Destructioi~of free iadicals on ~1 alls, etc \there K--H is a hydiocaibon, 12 is a neutral hydrocarhuii-txcc radical, R-00’ is a neutial peroxide-free radical, R- 0- 0 -T!I is a hydroperoxide, a n d R’- H is a hqdrocarbon nhich i n a x hr identical to R--11.
’rh? leIlloVal 01 B t O I I l l ( h\dl(Jgell. 1 alld L I b , OCCUi-3 ILiOlP 1t’:Adily the \\raker the stieiigth- of the C - €1 bonds roncwrird, whirl die drprndent upon the i(s\oiimrp energies of the ficlc. rndic It‘ and R ” , the bond stieiigth trndi t o be vieaker t h r highr.r the [rionmce energy. The re>oriaii( c’ csnc.rpics of aornc. olt,fitiir .I I U P tiires are givrn in Tahlr IT ( A )
T
HE occuiieiice of gum in cr acked gasoline u ab hrst inentiorled by Hall in 1914 (16). Since then the production of cracked gasoline has reached enormous propoi tions and the chemiral methods of inhibiting its oxidative deterioration during stoiage have achieved great industrial importance. Many investigator s ha7.e published ieports on gum folmatlon and inhibition (16). including a recent compr eheiisive treatment by Waltrrr, Minor, and Yabroff (20). This paper presents t h r piincipal factois involved 111 ih r ouidative deterioration of cracked gasolinr and the mrthods of inhihiting it by meanb of antioxidants anti metal dractivaton. Thrw factors are summarized in Table I.
Table 11. Resonance Energies of Olefinic Structures Resonance E n e i g j Calories/hlolc 6,900 18,700 80,400
30,400
Table I. Principal Factors in Inhibition of Deterioration of Cracked Gasoline A. Autoxidation of hydrocarbons I. Chain initiation 11. Chain propagation 111. Chain branching IV. Chain termination B. Inhibition of autoxidation by antioxidants V. Increasing chain termination, thus suppressing I1 and 111 C. Acceleration of autoxidation by metal catalysts VI. Increasing chain branching VII. Increasing the rate of destruction of antioxidaiits D. Suppression of metal catalysis by metal deactivatots VIII. Deactivating metal catalysts, thus eliminating V 1 and
hccordingly, h) drocarbona containing no olefimc doublt. t ) o i i d h \\ill either be inert or play only a minor pait in the b i o i age' trinpeiature autoxidation of clacked gasolines which normally m m rain Prom 10 t o 60% olefins ( I ? ) . The peroxides containing weak 0--0 bonds, in lhr C ‘ O U I W 0’ being converted into various monomeric and polynicrir ox) geria ted compounds, yield free radicals to initiate additional chain< thereby increasing the rate of oxidation. This process ia rallrd degenerative chain branching and, in the liquid phase, is rvriilually counterbalanced by inci rase in the rate of chain termination How autoxidative chains are initiated a t normal tempt~raiU P B ~ in the absence of known promoters, such as light, free i,+dical wuices, and metals, is still unknown. George and Robrrisox (1.4) have discussed thia point and Farmer (28) has suggested that the easily formed peroxides resulting from the 1,4 additioii of itiolecular oxygen to conjugated dienes may be the normal iinr 1atois of autoxidatiori. It may be assumed that cracked g a d i i i t is never completely fire of peroxides and metal catalj EorniPr being formed during its passage froin the high tenilJw-ulU T c of cracking to normal temperature and the latter bring f a i l ht-. originally present in the crude oil or picked up from thr n w t n eyuipmrnt .
VI1
The experimental data were obtained with fully refined, UIJdoped, thermally cracked gasolines from different sources. The Voorhees-Eisinger accelerated test method was used and the antioxidants were purified derivatives of p-aminophenol and p phenylenediamine. The conclusions are generally valid even if quantitatively varying results are obtained owing to differencein the composition of the gasoline, the antioxidants, and t h e methods of testing. 924
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
May 1949
Reduction in Antioxidant Effectiveness in Aged Gasoline
Table IV.
300-
5
I
f’
I
0
(Cracked gasoline, induction period of control 90 minutes) Concentration of Total Induction Period of Aging at looo C. .?I-Benzyl-p-aminoPeriod:Including Prior t o Addition of AntiDhenol, Period of Aging, oxidant, Min. Weight % Mint 0
0
15 200-
a
20
a
p.
90 30 30
2 2 +
925
BO
w
70 70
0.001
0.001 0.001 0.001 0 002
0.003
0,003
o no.?
310 130 100 90 130 230 100 I30
100-
% rr Q
I. No antioxidant added 11. 0.1% N-benzyl-g-aminophenol added at A 111. 0.1% N-benzyl-g-aminopbenol added at B
The autoxidation of olefinic hydrocarbons, particularly conjugated diolefins, is usually accompanied by the formation of polymeric peroxides (12, 18), many of which are thermally stable at normal temperatures. They probably constitute a large portion of the gum in aged gasoline and are readily decomposed a t the higher temperatures of the engine to form adherent deposits.
Gasoline Antioxidants A description of the use of antioxidants in cracked gasoline was first published by Egloff, Faragher, and Morrell in 1929 (IO). All the known efficient gasoline antioxidants are found among aromatic amino, phenolic, and aminophenolic compounds, such as N,N’-di-(sec-butyl)-p-phenylenediamine, 1,5-dihydroxynaphthalene, and N-butyl-p-aminophenols. These compounds retard autoxidation by reacting with the chain-propagating free radicals, generating in turn free radicals of low energy content, which are incapable of initiating chain reactions but are converted into relatively stable compounds. Based on the results of kinetic studies on the inhibiting effect of hydroquinone on the thermal oxidation of ethyl linoleate, Bolland and ten Have (3) concluded that hydroquinone interferes only by inis, by terminating reacteraction with peroxide radicals-that tion IIb. There is no reason, however, t o assume that antioxidants are unable to terminate reaction IIa. They are destroyed in the autoxidative system in several ways, as shown in Table 111.
Table 111. Destruction of Antioxidants active free radicals +relatively inactive
+ produots B. Antioxidant + peroxide -+- nonantioxidant products C. Antioxidant + 02 ----f nonantioxidant products A. Antioxidant
In A the antioxidant performs its function by eliniinating chain carriers, but in C it is uselessly destroyed. B can occur t o a significant extent only in the presence of preformed peroxides. Hence, antioxidants should be added t o cracked gasoline as soon as possible after cracking. Their effectiveness rapidly decreases with the extent t o which the gasoline has been autoxidixed prior to addition, as demonstrated in Table IV. The autoxidation, hoyever, can be inhibited at any time by the
introduction of a sufficiently large quantity of antioxidant, as shown in Figure 1. There was a n immediate reduction in the rate of oxygen absorption on adding the antioxidant. The data indirate that gum formation was inhibited by the use of a n impractically large amount of antioxidant. Attempts have been made t o correlate the efficiencies of antioxidants with their apparent oxidation potentials (8, 11). T h e results obtained in a series of determinations with a sample of acked gasoline are given in Table V. The beneficial effect of the secondary butyl groups in p-phenylenediamine is also possessed by secondary alkyl groups containing 3 t o 7 carbon atoms. The series was not carried higher. Although the apparent oxidation potentials of the more effective antioxidants fall within rather narrow limits, the method has not yet proved t o be a reliable guide in the search for new antioxidants, possibly because of lack of precision. It is probable that a relationship exists between the apparent oxidation potential and the chain-terminating efficiency and also the tendency t o react with peroxides and molecular oxygen. The monohydric phenols with potentials between 740 and 1090 are poor antioxidants because they are inefficient chain terminators although resistant t o direct oxidation. On the other hand, ringisopropylated N-methyl-paminophenol with a potential of 530 fails t o be a satisfactory antioxidant because, in spite of a high chain-terminating efficiency, i t is directly oxidized too readily. Much is known about gasoline antioxidants and their practical application, h i t t h e exact mechanism of their action ha.: yet t o be ducidated.
Copper Catalysis Copper and many of its compounds are powerful pro-oxidant catalysts in cracked gasoline ( 4 , 8, 20). The catalytic effects of some of them are shown in Table VI. These data merely demonstrate that copper and many of itn
930
INDUCTION PERIOD IN ABSENCE OF CU I NO ANTIOXIDANT : I10 MINUTES IT 0002% N-BENfYL-P-AMINOPHENL42OMN COPPER ADDED AS CUPRIC OLEATE
a
i
CONCENTRATIONOF DISSOLVED COPPER PARTS PER MILLION
Figure 2. Catalytic Effects of Dissolved Copper at Different Concentrations on Induction Period
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
826 Table V.
Apparent Oxidation Potentials and Efficiencies of Antioxidants
Apparent Oxidation Molecular Potential, Antioxidants Weight Millivolts 1v-(n-8utyl)-paminol,henol 165 664 T- Isobnty1)-p-aminop~ienol 165 652 ;i'-jsec-Buty*)-p-aminophenol 165 N N ' - D i ( n - b u t y l ) - p - p h e n y l e n e d i a n ~ ~ ~220 ~~ 697 'V' V'-Di(isobuty1)-p-phenylenediamine 220 684 N::Y '-Di-(see-butyl)-p-phenylenediamine220 630
Table VI.
Efficiency Wt. Molar
'%
%
100 J ':
100
34 36
45 48 120
90
:!
Catalytic Effects of Copper and Its Compounds on Induction Period Amount Added per 100 Ml. Gasoline. Gram
Catalyst
Percentage Reduction i n Induction Period 87
0,0072 0,0072 0,0072 0 5 sq. inch of niirfacr
Cupric chloride Cupric oleate Cuprous oxide Metallic copper Cupric sulfide Cupric oxide Babbitt (3% Cu)
73 57
0 2.5
55 38
0,0072 0 . 7 sq. inch of surface
Effect of Dissolved Copper on Extracted Gasoline
sample Copper absent Copper preaent
Oxygen Absorbed, Mg./100 311. 297
Air J e t Giim, hlg. '100 311. 186
13
51
Another sample of cracked gasoline was extracted in the same manner and both the unextracted and the extracted portions, containing and not containing 10 p.p.m. of copper, were stored in glass under an ample supply of air a t atmospheric temperature. The development of gum was followed during 173 days. The rewlts obtained are given in Table VIII. Cn spite of the great reduction in the storage stahility of the
Table VIII.
Effects of Dissolved Copper on Unextracted and Extracted Gasolines
Gasoline
Copper
Air J e t G u m Present, (R.Id100 &11.), Days 7 27 56 100 173
Unextracted Extracted Extracted
Present Absent Present
7
9
1
62 63 24
Days t o 20 h ~ ~ . / i oMI. o (Interpolated)
164
173
117
237
gasoline caused by the extraction, the extracted sample 15 lesa sensitive to copper than the uncxtracted samplc. Thcrr is an indication that copper is actually beneficial in the formc~r. This 13 not completeiv unexpected, as mppei and its compounds arc antioxidants for certain tt3rpcnex (9). The mechanism of roppcr ratslysip is descrihcd in Table IX. The reactions of metallic copper and monovalent copper with peroxide> formed in cracked g;a3ollnf?, and divalent copppr \\it11 an efficient antioxidant are extremely rapid. The reaction of antioxidant with peioxides at rnom temperature is relatively slow but greatly acceleiated by the addition of even tlaces of coppcxr, Metallic copper can proniotv degeneiative chain hranchmg but the catalytic states lie between cuprous and cuprw, for c.upric i q reduced to cuprous but not to the metallic state.
95 81
coninion derivatives are pro-oxidant catalysts, but they do not pretend to establish a relative order of catalytic potency. The effects of different concentrations of dissolved copper in the i'orm of cupric oleate on the induction period of cracked gasoline are shown in Figure 2. These results, particularly a t the lower concentrations, indicate that the antioxidant ie more sensitive to copper than the gasoline itself, which contained only the naturally occurring phenolic antioxidants of low efficiencies. I t appears possible that the hydrocarbon constituents of cracked gasoline are not of themselves susceptible to copper catalysis at ordinary temperatures. -A sample of thermally cracked gasoline u-as treated with a mixture of aqueous potassium hydroxide and potassium isobutyrate in order t o remove the extractible phenols and other acidic compounds. The changes caused by this treatment are shown in Figure 3. The extracted sample containing no added antioxidant was stored in glass under oxygen for 233 hours a t room temperature and the amounts of oxygen absorbed and gum formed during this period were quantitatively determined. To one portion 10 p.p.m. of dissolved copper as cupric oleate were added before storage. The results obtained are given in Table S'II.
Table VII.
Vol. 41, No. 5
~,
.
26
'OOt
/
V 1
0
1
I
0.003
0.0015
CONCN. OF N-(N-BUTYL)-P-AMINOPHENOL
--J
: WT. X
Figure 3. Comparison of Properties of Unextracted and Extracted Gasoline Samples I t is likcly that during the earlier stages of autoxidation of pure hydrocarbons a t low temperatures, no compound capable of reducing divalent copper exists in the system and the catalytic power of copper is thus suspended. The addition of an aiitioxidant able to effect the reduction permits the copper to function as a catalyst. Once autoxidation i s in full progress after the destruction of the antioxidant, oxidation products, such as aldehydcs, are formed which are capable of reducing cupric copper. It has been suggested that peroxides ma> also cause rrduction according to the following equation (1.9): cu++
+ R-0-O--
-+CU-
-I- r{--o--o'
but this reaction is likely t o be negligible in t h e presence of an efficient antioxidant. The harmful effect of copper ran be counteracted by the addition of enough antioxidant, as shown in Figure 4.
Table IX. Mechanism of peroxide --+ Cui 4 or C u + peroxide+ Cu++ B, C u + + + reducing agent +Cu t C u + + + antioxidant+ CUT A.
Cu
+
+-
+
Copper Catalysis chain-initiating free radicals chain-initiatingfxee radicals
+ + (antioxidant - 1 election)
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1949
Nearly four times the concentration of antioxidant was required to achieve the same degree of stabilization in the presence of 1 p.p.m. of copper as in its absence. This ratio will vary for different gasolines and antioxidants, but this method of overcoming the effect of copper is not an economical one. I n order to permit a minimum concentration of an antioxidant to confer adequate storage stability, the effect of copper must be eliminated as soon as possible.
c= CLASS I
927
N
H
R
R
TWO MOLECULES PER ATOM CU
Copper Deactivators Aside from using a large amount of antioxidant, the troubles arising from copper may be avoided by three methods: prevention, removal, and deactivation. The last method is frequently the most convenient, and can be made to provide a certain degree of protection against future contamination. The first account of copper deactivators for gasoline was given by Downing, Clarkson, and Pedersen in 1939 (6). The active ingredient of one of the better known products is N,N'-disalicylidene-1,2-diaminopropane, which functions by forming a stable chelate compound of cupric copper which is totally devoid of prooxidant catalytic activity. The method of applying the deactivator has been fully described ( 6 , 7 ) . Most of the copper deactivators obtained by condensing salicylaldehyde with different primary-amino compounds may be divided into three classes forming the chelate derivatives shown in Figures 5 and 6. The most effective copper deactivators are found in class 2, less efficient ones in class 1, and all those tested in class 3 were found t o be practically ineffective.
*
0
0
H
CLASS 2
-0- R"
-0
I ONE MOLECULE PER ATOM CU I
Figure 5.
5
Chelate Compounds of Copper
TWO MOLECULES PER TWO ATOMS CU
Figure 6.
I /
I
& 300
/
I PART PER MILLION AS CUPRIC OLEATE
0 a w n
z 0
g 200 0 T3
Chelate Compounds of Copper
noethane effected 35% deactivation, 33 moles per atom increased the deactivation to only 38OJo0. Although copper deactivators will accelerate the solution of metallic copper under the conditions of active oxidation, they will preserve gasoline in contact with metallic copper from catalytic deterioration under certain conditions of storage. This has been demonstrated by both accelerated and storage tests. By determining the induction periods at 78.5" and 100" C.: the over-all, apparent energies of activation for the autoxidation of a number of cracked gasoline samples were determined (Table
X).
z Table X.
Apparent Energies of Activation for Autoxidation of Cracked Gasoline
0.0020.004 0006 0,0080.01
CONCN OF N-(N-BUTYL)-P-AMINOPHENOL:
WT. X
Figure 4. Counteracting Effect of Copper with Antioxidant
I t is not possible to list deactivators simply on the basis of weight or molar efficiencies as in the case of antioxidants, because some of them will completely deactivate copper but at different minimum required concentrations, whereas others will only partially deactivate copper even when used a t high concentrations. For example (see Figure 7 ) ,in a particular sample of cracked gasoline, 1.8 moles of N,N'-disalicylidene-1 ,a-diaminoethane and 7.7 moles of 2-hydroxy-5-methylacetophenoxime were required, respectively, to deactivate completely 1 atom of dissolved copper, but although 3.3 moles of N-salicylidene-N'-dimethyl-l,2-diami-
Gasoline Sample 1 No added antioxidant or copper Added antioxidant Added antioxidant and copper Added antioxidant, copper, and deactivator
Sample 2 No added antioxidant or copper Added antioxidant Added antioxidant and copper
Activation Energy, Calories per Mole 10,900 12,000 9,800 11,900 10,300 11,200 9,400
Although the values are considerably lower than those previously reported (do), the differences in each series permit the conclusion that the beneficial effects of the addition of antioxidants and metal deactivators will be relatively greater under storage conditions than under accelerated conditions.
INDUSTRIAL AND ENGINEERING CHEMISTRY
928
-
110a W
w
p: 1T
a
2-HYDROXY-%METHYLACETOPHENOXIME
n
c
Vol. 41, No. 5
If
METALS ADDED AS T H E OLEATES
P
I,2-DIAMINOETHANE
W P 0
200
0 Io 3 0
z-
PHENOLt I PART PER MILLION CU AS THE OLEATE I
I
I
0001 0002 0003 CONCENTRATION OF CU DEACTIVATOR. WEIGHT PERCENT
0
Figure 7. Effects of Copper Deactivators
I
Effects of Other Metals
Figure 8.
T h e etieots of six metals, present as the soluble oleateh at
0011-
oeritratioii atomically equivalent to I p.p.m. of copper, on 1 he induction period of cracked gasoline containing antioxidant are given in Figure 8. T h e nwtala are catalytically active in the following diminiikiiiiy ardei : copper, cobalt, manganese, iron, nickel, and chromium. Chromium was found to be practically inert in the form of chromic okate. The effects of the addition of some copper deactivator. to tht. catalytically active metals are shown in Table XI. T h r iiiinui values denote pelcentage deactivation and the plus v d l r w s pelcentage activa tion. The+ trini. are drfined as follows:
4 1- the indurtion pel iod of gasoline cwritainiiig ;rtrtioxidant, and metal. l3 is the induction pmiod of gasoline containiiig antioxidant, metal, and metal deactivator. C is the induction period of gaholine containing ailtioxidant, metal, and metal activator. D is the induction pcariod of gaiolirw cmkaininy CIA antioxidant. Percentage deactivation = 100 (I3 A)/(D - -\) Peicentagr activation = 100 i 4 - I’) /Cy
-
Table XI. Effects of Copper Deactivators on Different Metals Parts
-.
AI n
Copper deactivator (concn. O.OOL%) N,N‘-Disalicylidene 1,2-diaminopropane
Balioylaldoxime
N-Saiicyiidene o-amhopheno1 %,2’4‘ Trihydroxyazobenzene Npl?r N”’-Tetrasalicyiidenr tet;a-(~minomethyl)-methane
0.86
0.87
1 .0
N“: +833 + 96 --I00 SB + 55 4-124
- 100 - 100
- 100 - 100 - I00
- 100
/ .
+
100
c: U
Fe
-i:84 - 100 c 73 - 100 ~
--
.___.
+ 103
t
,
I
J
MN NI FE GO CU 0.81 0.92 0.87 0.86 0.92 1.0 CONCN. PARTS PER MILLION
NO METAL CR
~
- 100
-100
Because these cornpounds form chelate compounds with Ilir metals, the mere formation of a chelate compound is no rriterion of metal deactivation. The over-all effectiveness of the last compound as a metal deactivator is noteworthy. Much interest in chelate compounds has been aroused in COILuection with work on biological s y s t e m ( 1 ) and agents capable of reversibly absorbing molecular oxygen (6). The determination of bhe catalytic activities of the chelate compounds should prove ari aid in such studies.
Effect of Dissolved Metals an Induction Period
Acknowledgment The author wishes to express his appreciation T O F. B. Downing and B, M. Sturgis, both of Jackson Laboratory, for their interest and advice in the preparation of this paper.
Literature Cited (1) .Albert, X., arid (:ledhill, Rr.S., Biochem. J ., 41, 529 (1947). (2) Bolland, J. L., and Gee, G., Tmns. F a r a d a y Soc., 42, 244 (1946). (3) Bolland, J. L., and ten Have, P., I b i d . , 43, 201 (1947). (4) Bridgeman, C . O., S.A.E. J o u r n a l , 30, 207 (1932). (6) Calvin, hl., and Ba,iles, R. H., J . Am. Chem. Soc., 69, 1886 (1947) ; also previous publications by Calvin and collaborntors in J . Am. Chem. SOC. (6) Downing, F,E., Clarkson, R. G., a,nd Pedersen, C . ,I,, Oi/ & Gas J., 38,No. 11,97 (1939). ( 7 ) Downing, F. B., and Prderaen, C . 5., L;. S. .Par.rntm3,IX1,12i (Nov. 28, 1939). (8) Ilryer, C. G., Morrell, J. C., Zgloff, G., and Lovry, C . D., 1 h . D . R P G . CHEX., 27, 15 (1935). (9) Dupoiit, G., and Crouaet, J., BUZZ.inst. p i n , No. 58, 101 (1929). (10) Egloff, G . , Faragher, W.I?., a.nd Morroll. *J, C., Natl. P~iroleum, Kews, 21, No. 49, 4 0 (1929), (11) Elley, H. W ~Electrochem , Soc., Preprint 69-21 (1936). (12) Farmer, E. If., T r a n s . F a r a d a y SOC.,42, 228 (1946). (13) Flood, I). T., Hladkey, J. W . , and Edgar, G., TKD. E ~ oCHEM.~ . 25, 1234 (1933). (14) George, P., and Robertson, A., P m c . Hou. Soe. (T,ondo.n), 185A,309 (1946). (15)
