Warmup Test 2400
f\
To determine the ozone built up in the helium pressure and whether autoignition would occur, the storage tank was allowed to warm up to -19.5' C. T h e pressure rose to 2050 p.s.i.g., 390 p.s.i.g. of which was due to ozone coming out of solution. This represents 19% ozone in the vapor phase. This tank was equipped with an explosive valve, and it was blown open after 2 hours and 5 minutes. Various pieces of protective clothing were placed under the discharge port. No fire or oxidation was evidenced on explosion. Curves of temperature and pressure us. time are presented in Figure 6.
Conclusions
\ 0
I 20
I
I
40 TIM
I
I
I
60
BO
100
120
-
MINUTES
Figure 6.
140
Warmup test
completely safe when the tank is pressurized above 1000 p.s.i.g. a t -78' C.
T h e use of Freon-12 as a solvent for ozone in concentrations up to 13 weight % ' is a convenient and safe way to store large quantities of ozone (1 to 10 pounds), provided the solution is kept a t -78' C. and overpressured with helium to at least 1000 p.s.i.g. Sensitivity tests show that the solutions are insensitive to mechanical, electrical, and high brisance shock. However, the solutions must be handled with precaution, since ozone is innately unstable.
Storage Test
Acknowledgment
A heavy-walled storage tank similar to the one used in the previous tests was used. T h e tank was instrumented with a stainless steel-sheathed thermocouple and a Statham pressure transducer. The storage tank volume was 448 inches. After the usual pressure checks, calibrations, and passivation procedures, 11.5 pounds of double-distilled Freon-12 were put into the tank, the system was cooled to -152' C., and the ozonation process was accomplished. The final weight of ozone was 1.042 pounds and the final weight of Freon-12 was 11.50 pounds, giving an 8.31y0 solution and an ullage of approximately 50%. The test tank was pressurized a t 800 p.s.i.g. a t - 145' C. and then stored i n dry ice in a closed shed. Tank conditions were checked a t least once a day for a week. The pressure and temperature held steady a t the aforementioned values for the entire time.
The authors thank A. V. Grosse for his helpful suggestions A. G. Streng for carrying out the solubility studies, and L. A. Streng, R. W. Segletes, and W. J. Liddell for assisting in carrying out much of the experimental work.
I t can be concluded from this test that no ozone decomposed during the storage period, and solutions of 10% ozone-Freon-12 can be stored for long periods of time.
literature Cited
(1) Grosse, A. V., Streng, A. G . , Research Institute of Temple University, Tech. Note 4 (Aug. 1, 1957). (2) Harper, S. A., Gordon, W. E., Aduan. Chem. Ser., No. 21, 28 (1959). ( 3 ) Potter, A. E., Jr., Stokes, C. S., National Aeronautics and Space Administration, Tech. Note, in press. RECEIVED for review February 25, 1965 ACCEPTED June 18, 1965 Work sponsored by the National Aeronautics and Space Administration, Lewis Research Center, under Contract No. NAS-3-1918.
ANTIOXIDANT INHIBITION B Y SUBSTITUTED PHENOLS Concentration and Temperature Efects in Cumene W . G.
L L O Y D ' AND
R . G. Z I M M E R M A N
Polymer Research Laboratory, The Dow Chemical Co., Midland, Mich.
major practical problem in antioxidant stabilization is that of selecting the best antioxidant (or, often, finding a reasonably good antioxidant) for a given substrate material under given conditions of exposure to an oxidizing environment. L'arious factors, such as color, compatibility, and cost, enter into actual selection processes. \Ye are here concerned, 1 Present address. The Lummus Co., Newark, N. J.
THE
180
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
however, only with the most basic factor, the actual chemical efficacy of a n antioxidant in protecting an oxidation-susceptible system from oxidative degradation. While countless specific stabilization problems have been solved by screening programs, the amount of generalized information of broad predictive value is surprisingly small. A recent review by Ingold ( 7 7 ) encompasses a good portion of
The inhibited autoxidation of'cumene i s examined in terms of the effects of temperature and of phenolic antioxidant concentration, [ A H ] , upon induction period, ti. In addition to the familiar linear dependence of t, upon [ A H ] , at higher temperatures ti increases as [AH]"', and at still higher temperatures approximately as In [ A H ] . The "activation energy of induction period" at 1 12' to 152' C. i s essentially identical for a number of phenolic antioxidants, indicating the rate of the induction period reaction to be independent of the specific chemical reactivity of AH and of AH-derived radicals. These observations are accounted for in terms of the extent of cumene hydroperoxide homolysis and the emergence at higher temperatures of the direct antioxidant-oxygen reaction. Some inferences are made relating phenol structure to inhibition stoichiometry.
this knoivledge. The existence of a rational relationship betxveen the structure of substituted phenols and their antioxidant efficacies \vas noted by Bolland (3, 4 ) , and \vas shown by Hammond and his co1vorkers (8) to follow roughly a sigmarho correlation. This correlation has recently been confirmed and extended (9> 73). Building upon these insights, our interest is in the three-cornered relationship among molecular structure, chemical reactivity, and antioxidant efficacy of simple phenolic inhibitors. Folloiving a kinetic examination of the inhibitory mechanism of unsubstituted phenol (781, the present study is directed to the effects of temperature and antioxidant concentration upon the induction periods of cumene stabilized by several simple phenols. Experimental
Cumene \vas a center cut of the best available commercial grade, fractionated under prepurified nitrogen and stored under argon in a light-shielded glass vessel; its molar purity by gas chromatographic analysis exceeded 9'9.97c. T h e several substituted phenols were crystallized to constant melting point except as otherwise noted. Induction periods were obtained Lvith a standard gasoline oxidation apparatus ( 7 ) in \vhich the hydrocarbon was charged to a glass liner, the liner placed in a steel bomb, and the bomb immersed in a thermostated bath and pressured with oxygen to 130 p.s.i.g. In a departure from the ASTM procedure the ivater bath \vas replaced with an insulated silicone fluid bath, the temperature of \\hi& \vas controlled by interchangeable preset mercury thermostats (77). Stock solutions lvere prepared containing exactly 0.0200 mole per liter of the several substituted phenols in pure cumene. For each oxidation run an appropriate volume of stock solution \vas combined Jvith fresh cumene to make up a 50.00-ml. charge a t the desired antioxidant concentration ; this was then charged to the 125-ml. glass liner (7): and the bomb was sealed, purged, pressured, and brought quickly to temperature, During the initial stage of a run (the induction period) the pressure remained nearly constant; then, abruptly, a very constant pressure drop developed, typically about 20 p.s.i. per hour. Raw results for a representative run are sho\vn in Figure 1. Induction periods, determined graphically by tangent intercept, are defined in this system with a precision of 10.1 hour. A running series of replicated runs (Table I , noteh) indicates reproducibility within i.5%. -4lthough post-induction-period rates obtained by this procedure are likely to be diffusion-controlled, the very slow reactions occurring during the induction period itself are unaffected by this consideration ; the induction period measurements are therefore believed to be kinetically meaningful.
Table I. Induction Periods of Cumene Containing 2,6-Diferf-butyl-4-cresol at 126.5' C. and under 130 P.S.I.G. Oxygen Antioxidant Concn., M X 103a t i , Hours 0.0400 8.05 0.100 10.25
0.300 0.500
13.5~ 15.5; 20.1
2.00 3.00
27,356
1 .oo
32.35
37.15 4.00 5.00 40.4 a Commercial 2,6-di-tert-butyl-4-cresol(Eastman Organic Chemicals), used without further puriJcation. b Average of 12 replicate runs made o m 6-monthperiod, all within range 25.8 to 28.9 hours.
0 TIME I N HOURS AT 126.5.C.
Figure 1 . Induction period of cumene charged with 0.00 1 00 mole of 4,4'-isopropylidenebis(2-tert-butylphenol) at 126.5' C.
definition for uninhibited thermal oxidations. Accurate determination is possible, hoIvever, if a simple power relationship is assumed:
Results
The raw results of a series of inhibited oxidations, carried out a t 126.5' C. with varying concentrations of the antioxidant 2,6-di-tert-butyl-4-cresol, are given in Table I. As initial antioxidant concentration is reduced, the induction period falls not to zero but to a finite value, t i o . Accurate direct measurement of t i o is difficult, because of the loss of sharp ti
(ti
-
t,O)
=
k[AHO]P
(1)
From Equation 1 and Table I tio is determined by the method of minimum least squares error to be 5.0 hours. The empirical validity of this treatment is indicated by the excellent linearity obtained with these data (Figure 2). The slope, p , of this loglog plot is +0.503, standard deviation i.0.0073. Thus a t VOL. 4
NO, 3
SEPTEMBER
1965
181
126.5' C. and across a hundredfold range of concentration the induction period increases regularly with the square root of dibutylcresol concentration. T h e magnitude of the temperature effect is shown by the results of a parallel series of oxidations a t 161.5' C. (Table 11). At this temperature, above the effective ceiling for phenolic antioxidants (7, 75, 76), even massive antioxidant loadings yield only short induction periods. Here the increase in induction period is no longer proportional to the square root ~
Table II. Induction Periods of Cumene Containing 2,6-Ditert-butyl-4-cresol at 161.5' C. and under 130 P.S.I.G. Oxygen Antzoxzdant Concn., M X lo3 t,, Hours 2 00 4 00 6 00 10 0 16 0 20 0 30 0 40 0
1 55 2 40 2 60
3 30 4 10 4 10 4 60 4 45
of antioxidant concentration, but is closely proportional to the logarithm of initial antioxidant concentration : ( t i - t,') = k In [AH'] (2) This dependency is borne out also for three other simple phenols a t this temperature (Figure 3, t,' 1.5 hours). T h e stoichiometry of antioxidant inhibition by 2,6-di-tertbutyl-4-cresol has been well established : Each phenol molecule stops two peroxy radicals (2, 5, 6 ) . I n the present system a t 126.5' C. and 0.00200.1.1antioxidant charges the induction periods for the dibutylcresol and for 6-tert-butyl-2,4-xylenol lvere 23.8 and 47.7 hours, respectively. At 131.9' the induction periods were 15.55 and 31.25 hours. Thus under these conditions 6-tert-butyl-2,4-xylenol lasts t u ice as long as the standard. Parallel runs with 2-tert-butyl-4-cresol yielded induction periods of 33.0 hours a t 126.3' and 21.6 hours a t 131.9', implying an intermediate stoichiometry. An Arrhenius plot of the induction periods of cumene charged \vith 0.00200 mole of each of these three phenols, over the temperature range 126.5' to 146.7' C., is sho\vn in Figure 4 . T h e slopes are essentially linear and parallel, yielding "activation energies of induction period" of 27.0 (10.8)kcal. for 2,6-di-tert-butyl-4-cresol, 28.1 ( A 1 .O) kcal. for 6-tert-butyl2,4-xylenol. and 27.1 (A0.5)kcal. for 2-tert-butyl-4-cresol. T h e virtual identity of these activation energies prompted further runs with other substituted phenols and over a broader temperature range. T h e results of these runs are summarized in Table 111. Like the above three antioxidants, the six monohydric phenols in this table all yield activation energies within the narrow range 26.4 to 28.1 kcal. T h e three dihydric phenols all show markedly lower activation energies (22 to 23 kcal.). Discussion
T h e above observations may be economically explained in terms of inhibition by simple hydrogen transfer (3, 4 ) , a TOC
126.8
INITIAL INHIBITOR CONCN,, MILLIMOLES/LITER
Figure 2. Induction periods of cumene as a function of initial concentration of 2,6-di-tert-butyl-4-cresol a t 1 26.5" C.
6
4
L a 0
I 0
f
-
-i-2-
lo3
0
-
TOK
Figure 4. Arrhenius dependencies of induction periods of cumene containing 0.00200 mole of several antioxidants in temperature range 1 26.5' to 146.7' C. 6-ferf-Butyl-2,4-xyienol
A 2-ferf-Butyl-4-cresol W 2,6-Di-terf-butyl-4-cresol
182
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
mechanism which has found recent support from measurements of kinetic isotope effects (74, 22, 23) and retardation kinetics (78, 20). The mechanism of antioxidant inhibition by 2,6di-tert-butyl-4-cresol (I) is generally conceived of in terms of B
-I
=E - butyl
-
I1 I -
IIb -
1Ia
cumene, which is known to have a slow propagation rate (70), but also with other substrates such as paraffins which are resistant to hydrogen abstraction. With readily oxidizable substrates, however, compounds such as V cannot effectively compete with substrate propagation, and the "normal" 2 to 1 stoichiometry obtains. Thus with cracked gasoline fractions containing readily oxidizable mono- and diolefins, equal concentrations of 2,6-di-tert-butyl-4-cresol and 6-tert-butyl-2,4xylenol confer essentially identical periods of protection (24). The simplest kinetic scheme for oxidation inhibition consists of a slow thermal initiation step, a fast combination of the resulting hydrocarbon radicals with oxygen, and for the case of a good antioxidant, complete consumption of the peroxy radical by the antioxidant-e.g., wi
1Va -
R*
vc -
IVb -
nR.
(3)
+ 02+ROO*
(4)
+ AH ROOH + A * ROO. + A inert
Y -
1
+
ROO.
(5)
+
(6)
+
If the observed induction period, ti, is taken to be the sum of the antioxidant consumption period, t' (during which the antioxidant sacrificially maintains extremely low free radical concentrations), and the uninhibited induction period, tio (during which free radical concentrations accumulate until steady-state free oxidation is attained), then the simple firstorder dependency derived from Reactions 3 to 6 may be represented as: tt
hydrogen transfer to form the resonance-stabilized "phenoxy" radical (IIa), which reacts as the cyclohexadienone-4-y1 radical (IIb) to yield the stablr dienone adduct (111) (2, 5, 6 ) . By analogy, 6-tert-butyl-2,4-xylenol will react via I V b or IVc to yield the dienone(V) or its equivalent; but V with its unhindered and allylic activated 2-methyl group can, in competition \vith a sluggish substrate, stop a third radical by hydrogen transfer, forming the radical V I which will ultimately stop a fourth radical. Thus a stoichiometry of a t least four radicals per molecule may be expected for the tert-butylxylenol in this system. If this correctly accounts for the long induction periods with the tert-butylxylenol, then 2-tert-butyl-4-cresol should have a n intermediate stoichiometry, since of the two reactive forms of its "phenoxy" radical, V I I b leads to the stable dienone (VIII) FLhile the roughly equiprobable V I I c leads to the dienone (IX) which like V is likely to stop a t least two more radicals. This intermediate stoichiometry is indeed found. These high stoichiometries may be expected not only with
-
= 2[AH0]/nwi
(7) wherein [ A H " ] is the zero-time antioxidant concentration and and n represent the rate of thermal initiation (Reaction 3) and the initiation efficiency factor, respectively. Equation 7 is probably adequate a t 75' C. I t fails at 126.5' because here cumene hydroperoxide is not an inert species ; a t this temperature the hydroperoxide decomposes homolytically with a n estimated half life of 150 to 190 hours (79, 27). T h e same may be true of the adduct peroxide formed in Reaction 6. For kinetic reasons discussed elsewhere (72, 76) it is necessary to assume that most or all of the radical products of hydroperoxide homolysis are stopped by the inhibitor. t{O
+ m(R0. + HO.) + AH -+ ROH + A .
ROOH RO. (HOG)
(8)
(9)
(HOW
Making the steady-state assumption for the radical species but not for the hydroperoxide, this scheme predicts the hydroperoxide concentration a t any time t to be:
Induction Periods of Cumene Containing 0.00200 Mole of Phenolic Antioxidants under 130 P.S.I.G. Oxygen and at Various Temperaturesa Temperature, ' C. Estimated 7 17.8' 121.6' 737.9' 747.8" 157.5" 767.5' E,, Kca1.b Phenolic Antioxidant
Table 111.
p-Ethoxyphenol p-Benzyloxyphenol 4-tert-Butyl-2-cresol 4-Benzyloxy-2-tert-butylphenol 2-Benzyl-6-tert-butyl-4-cresol
2,4,6-Trimethylphenol tert-Butylcatechol Pyrogallol Hydroquinone a Induction periods in hours.
b
110.3 104.1 70.1 111 . 5
48.2 43.0 28.6 47.6 57.5 59.65 30.3 26.4
20.2 17.4 12.1 19.55 ... 21.95 ... 23.1 60.8 13.55 55.0 11.9 26.2 ... 7.4 Standard deviations of Ea's in parentheses.
8.2 7.25 5.4 8.0 8.6 9.5 6.87 6.03 3.10
4.25 3.40 2.65 3.85 4.10 4.80 3.55 3.25 1.85
VOL. 4
1.95
...
...
1.63 1.90
2.15 1.75 1 .57
...
NO. 3
26.8(fO.6) 28.0(10.4) 26.7(f0.2) 28.l(f0.4) 27.3 ( f 1 . 3 ) 26.4(11.2) 23.3(10.3) 23.2(+0.2) 22.1(11.2)
SEPTEMBER
1965
183
T h e rate of inhibitor consumption is:
-d[AH]/dt =
3f 2
2 mks[ROOH]
(11)
Replacement of [ROOH] by Equation 10 and integration yields :
[AH’] = (m
+
1/2)
nqt‘
-
mnwi - (1 ks
- e-ksl’)
(12)
For this system a t 126.5’ ks is known to be about 0.004 hour-’ (79, 27). For induction periods of less than 50 hours the product kat’ is much less than unity, and therefore the exponential may be replaced by the first three terms of its series expansion. This yields:
Equation 13 describes the induction period dependency for inhibited, degenerately branching systems. Where the branching reaction is sufficiently important that the second term dominates, Equation 13 collapses to :
This accounts for the observed dependency of induction period upon the square root of antioxidant concentration in cumene a t 126.5’. White oil inhibited with I-naphthol yields a similar induction period dependency, which has been similarly interpreted (72). At 161.5’ the observed induction periods are not only much shorter but exhibit a completely different kinetic dependency (Table 11, Figure 3, Equation 2). This marked change in kinetics can be explained in terms of two known changes in the system chemistry. First, in this temperature range cumene hydroperoxide has a short half life (79) and this system thereby acquires a nondegenerate branching character, for which hydroperoxide concentration may be subjected to the steadystate approximation. Secondly, phenolic antioxidants are known to lose their efficacy above about 150’ (7, 75, 76), because of the emergence of ciirect oxidation of the phenol by molecular oxygen. AH
+
0 2 -+
(products)
(15)
The rate of antioxidant consumption for the scheme embracing Reactions 3, 4, 5, 6, 8, 9, and 15 is:
-d[AH]/dt
=
+ klj[Os] X
[AH]
where s is a stoichiometric efficiency factor, n(m f 0.5). gration yields:
(16) Inte-
Among good phenolic inhibitors of varying efficiencies and stoichiometries the “activation energies of induction period” remain essentially constant, implying that the important ratedetermining steps in inhibitor consumption do not involve the phenolic inhibitor itself. This is in agreement with the kinetic model of Equation 14, which predicts this over-all activation energy to be simply related to the true activation energies of the thermal initiation and hydroperoxide branching reactions (72). Thus for these simple phenols Reaction 15 is unimportant except a t the top of the temperature range, The polyhydric phenols on the other hand show lower activation energies, suggesting either a basic change in inhibitory mechanism or a greater susceptibility to the direct antioxidantoxygen reaction. 4-tert-Butylcatecho1, which was examined in more detail a t 161.5’ (Figure 3), was found to be markedly more susceptible to this side reaction. The varying kinetic dependencies of induction period upon antioxidant concentration (first-order, half-order, and logarithmic) may thus be accounted for in terms of homolysis of the substrate-derived hydroperoxide and the participation, a t higher temperatures, of a direct antioxidant-oxygen reaction. The variation in induction period with equimolar concentrations of several good monohydric phenol antioxidants can be accounted for in terms of formation (and in appropriate cases, further reactions) of cyclohexadienones. Our future attention will be directed to the relationship between the molecular structure of simple phenolic antioxidants and their inhibitory efficacies. Nomenclature
AH
[AH”] A.
= phenolic antioxidant = zero-time antioxidant concentration = antioxidant-derived “phenoxy” radical
E,
= activation energy k = a n empirical constant of proportionality ks, k l S , etc. = rate constants for the indicated reactions = kilocalories per gram mole kcal. M = gram moles per liter m = branching efficiency factor, radicals per molecule n = initiation efficiency factor, radicals per molecule P = exponent in an empirical power dependency R. = hydrocarbon radical (here, cumyl radical) RO. = alkoxy radical (here, cumyloxy radical) ROH = an alcohol (here, cumyl alcohol) ROO. = peroxy radical (here, cumylperoxy radical) ROOH = hydroperoxide (here, cumyl hydroperoxide) t = elapsed time t’ = antioxidant consumption period t, = observed induction period with antioxidant present t,’ = uninhibited induction period = rate of thermal initiation w,
Acknowledgment
Whenever the direct antioxidant-oxygen reaction is substantially more important than thermal initiation, Equation 17 shows that the induction period will vary with the logarithm of zero-time antioxidant concentration. This accounts for the observed data a t 161.5’. I t also permits a crude interpretation of the slopes of these semilogarithmic plots: The relative susceptibilities of the several phenolic antioxidants to direct oxygen-oxidation (Reaction 15) can be estimated from the reciprocals of the slopes of d(ti)/d(ln [AH’]). From the data of Figure 3, for example, if the oxidation susceptibility of 2,6di-tert-butyl-4-cresol is taken as unity, the susceptibilities of 6tert-butyl-2,4-xylenol, 2,4-xylenol, and 4-tert-butylcatechol under these conditions are 0.67 (=t0.03), 1.3 (+0.15), and 6.5 ( k 2 ) , respectively. 184
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
We are indebted to A. J. Dietzler for providing purified samples of the several phenolic antioxidants used in this study. Literature Cited (1) Am. SOC. Testing Materials, “Standards,” ASTM Test D
525-55, Part 7, pp. 263ff, 1958. (2) Bickel, A. F., Kooyman, E. C., J . Chem. SOC. 1953, 3211. (3) Bolland, J. L., ten Have, P., Discussions Faraday Soc. 2, 252 (1947). (4) Bolland, J. L., ten Have, P., Trans. Faraday Soc. 43, 201 (1947). (5) Boozer, C. E., Hammond, G. S., Hamilton, C. E., Sen, J. N., J . Am. Chem. SOC.77, 3233 (1955). (6) Campbell, T. W., Coppinger, G. M., Zbid., 74, 1469 (1952). (7) Cohen, G., Murphy, C. M., O’Rear, J. G., Ravner, H., Zisman, W. A., Ind. Eng. Chem. 45, 1766 (1953). (8) Hammond, G. S., Boozer, C. E., Hamilton, C. E., Sen, J. N., J . Am. Chem. SOC.77, 3238 (1955). (9) Hedenburg, 3 . F., IND.ENG. CHEWFUNDAMENTALS 2, 265 (1963).
Howard, J. .4.,Robb, J. C., Trans. Faraday Soc. 59, 1590
n/=\ YUJ).
Ingold, K. U., Chem. Revs. 61, 563 (1961). Ingold, K. U., J . Inst. Petroleum 45, 244 (1959). (13j Ingold, K. U., J . P h y . Chem. 64, 1636 (1960). (14) Ingold, K. U., Howard, J. A., ,\‘ature 195, 280 (1962). (15) Kennerly, G. \\.’., Patterson, \V. L., Jr., Znd. Eng. Chem. 48, 1917 (1956). (16) Lloyd, W. G., J . Chem. Eng. Data 6, 541 (1961). (17) Lloyd, \.V. G., J . Polymer Sei. A-1, 2551 (1963). (18) Lloyd, \V. G., Lange, C. E., J . Am. Chem. Soc. 86, 1491 (1964).
(19) Mageli, (I. L., Stengel, S. D., Doehnert, D. F., M o d . Plastics 36. No. 7, 135 (1959). (20) ‘Mahoney, L. R.,’ Ferris, F. C., J . Am. Chem. Soc. 85, 2345 (1963). (21) Mapstone, G. E., Chem. Processzng, 26 (25), 47 (1963). (22) Shelton, J. R., Ofic. Dig.,Federation Soc. Paint Technol. 34, 590 (1962). (23) Shelton, J. R.. Vincent, D. N., J . Am. Chem. Soc. 85, 2433 (1963). (24) Il’right, K. B., Petroleum (London) 2 5 , 412 (1962). RECEIVED for review December 21, 1964 ACCEPTED June 22, 1965
SWEETENING OF PETROLEUM DISTILLATES WITH HYDROGEN PEROXIDE H U G H
R. E I S E N H A U E R
Research Centre, Du Pont of Canada, Ltd., Kingston, Ontario
High boiling sour petroleum distillates can b e sweetened with hydrogen peroxide in the presence of a basic catalyst. Aqueous hydrogen peroxide may b e used when a common solvent, such as acetone or dioxane, is added to make the oil and aqueous phases completely miscible. Alternatively, the peroxide-common solvent mixture may b e replaced with a solid adsorbate of hydrogen peroxide on an anion exchange resin. A highly basic organic amine, such as piperidine, serves as a catalyst. Under suitable conditions, the mercaptan sulfur content of a light atmospheric gas oil can b e readily reduced to 0.001%. The effect of reaction temperature and time, and hydrogen peroxide and catalyst concentration, on reaction efficiency is discussed. The use of organic peroxides in place of aqueous hydrogen peroride is also described.
c
oils contain trace amounts of a variety of sulfur compounds kvhich must be removed during the refining process. The term “siveetening” refers to the removal of sulfur compounds from sour petroleum distillates. Sulfur compounds are objectionable because of their obnoxious odor, their potential corrosiveness, and their reduction of the antiknock characteristics of tetraethyllead ( 9 ) . Among the sulfur compounds normally present, hydrogen sulfide and mercaptans are particularly offensive. \Vhile hydrogen sulfide may be readily removed, the removal of mercaptans varies in ease depending on their molecular structure. Each petroleum distillate fraction contains mercaptans boiling in the same range as the hydrocarbon material. Different fractions require different mercaptan removal or slveetening treatments. Simple caustic extraction is in common use for nveetening the more volatile fractions and a variety of methods are available for fractions having intermediate volatility. For the higher boiling fractions, \vhich are more difficult to sweeten, the preferred technique is hydrogenation. This is very effective but can be rather expensive and a cheaper method lvould be desirable. This paper describes the development of a sweetening method involving the treatment of sour petroleum distillates with hydrogen peroxide. Hydrogen peroxide will react with mercaptans to form disulfides : RUDE
2 RSH
+ 131202
-+
RSSR
+ 2 H20
This reaction occurs in both acid (4, 7) and alkaline solutions ( 7 , 3 ) , although in the latter, alkaline cleavage of the resulting disulfide can occur. Both hydrogen peroxide and sodium peroxide have been proposed as sweetening agents in combination with aqueous alkaline solutions of phenol ( 8 ) or acetic acid (6).
Experimental
Preliminary results were obtained using prepared solutions of pure mercaptans in heavy naphtha. The solvent was purified before use by shaking it with 10 volume 76 of a 575 aqueous solution of silver nitrate. The organic layer was separated and filtered to remove the solid silver mercaptides \vhich had formed. T h e filtrate was subsequently distilled at 50’ to 60” C. and 4 mm. Tests showed that the heavy naphtha was free of mercaptans. The individual mercaptans, obtained from the Fisher Scientific Co., lvere used \vithout further purification. T h e solutions were prepared a t a concentration of approximately 0.0270 mercaptan sulfur. The petroleum distillate used was a sour light atmospheric gas oil (LAGO) obtained from Imperial Oil, Ltd., Sarnia, Ontario. O u r analysis of the distillate gave: boiling range, 425’ to 540’ F.; d?0, 0.831 ; mercapran sulfur, 0.01687,. For evaluation, a 300-ml. round-bottomed creased flask immersed in a constant temperature bath a t 50’ C. was fitted with a high speed disperser stirrer, nitrogen inlet tube, and reflux condenser. The flask \vas flushed out ivith nitrogen and charged xvith 150 ml. of the sour distillate. To this were added the catalyst and 1 mole of hydrogen peroxide, as a lO7c aqueous solution, per mole of mercaptan. T h e contents were stirred throughout the run by the high speed disperser rotating a t about 1600 r.p.m. Samples \vere withdrawn periodically and analyzed by a potentiometric method based on ASTM D 1323-61. For the experiments \vith pure mercaptan solutions, control runs were conducted in rhe absence of the sweetening agent to determine the loss of mercaptan by evaporation. The experimental results were corrected by a factor taking this loss into account. \Vhen single-phase sveetening \vas employed, sufficient organic common solvent was added to the petroleum distillate to dissolve the aqueous hydrogen peroxide and form a single homogeneous solution. When 10% hydrogen peroxide was employed with LAGO, the quantity of common solvent required was: acetone, 20 volume yo; tetrahydrofuran, 40 volume 7 0 ; dioxane, 35 volume 70. VOL. 4
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SEPTEMBER 1965
185
