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Negative temperature coefficient in the oxidation of butane and other hydrocarbons
COMBUSTION AND FLAME. 17, 205-214 (1971)
205
Negative Temperature Coefficient in the Oxidation of Butane and Other Hydrocarbons J. C. D6chaux, J. L. Flmnent, and M. Lucquin Laboratoirc de Chimie de la Combustion. Universit~ des Sciences et Techniques de Lille, B.P. 36, 59. Villeneuvc d'Ascq. France This paper describes a morphological study of the induction period, light emission, pressure change and cool flame explosion limits in the oxidation of n-~utane, and a broad study of the reaction products in the region of the negative temperature coefficient. The r~sults are discussed in terms of the interplay of two mechanisms involving the formation and reactions of the peroxy radica~ RO~. and RCO~. The results are explained by the competition between the chum propagation and the isomerizatioa of these radicals. A generalization is proposed.
Introduction The problem of the negative temperature coefficient may be stated briefly by recalling that in a certain temperature range the main feature of the hydrocarbon oxidation is that the rate decreases when the temperature is increased. The solution of this problem is important. In fact, any proposal of a general oxi~lation mechanism, if exact, must account for it, for the same reason as for the cool-flame, or twostage ignition lobe phenomena. Conversely, a satisfactory explanation of the phenomenon could only but reinforce the general theories in which it will be incorporated. Many authors have proposed explanations of this problem, but supporting evidence has been scanty. After the work of Pease [1]. Ubbelhode [2], and Lewis and yon Elbe [3], the first proposal which seems worthy of notice was that of Wal,~,h [4], who considered the reactivity of RO,. to change with temperature. A different explanation was given by Tcherniak, Shtern and Antonovski [5]. Bardwell and Hinshelwood [6] proposed an explanation based on competition bet';veer, the isomerization of the peroxy radical ~',O2. and H abstraction by RO='. Enikolopyan [7] based an explanation on the roles of RCO. and RCO:- radicals.
Ben-A/m and Lucquin [8 ] gave an explanation based on the variation of the branching factor with respect to the temperature. Tipper and Minkoff [9] and Semenov [10] have al,:o considered a modification of ROe', or RCO. and RCOa' with the temperature. Knox [l'.[] suggested that the phenomenon [12] mighlt result from the instability of the radical hydroperoxide
-C'-C-OeH. All authors are in general agreement ~ a t the phenomenon arises .through the increasing instability of a critical intermediate as the temperature rises: The work now describ,~d h a s been carried out to elucidate the nature of this intermediate, and consists of two parts:' first, a morphological study of the induction period, light emission, pressure change, and cooi flame and explosion limits, and second, a broad study of the reaction prod~Jcts. All experiments have been carried out in a cylindrical sili~,a reaction vessel of capacity 104 ema. M o r p h o l o g i c a l Study o f the N e g a t i v e T e m p e r a t u r e Coefficient Morphology of Cool Flames in Rdation to the
Coneentratio~,of Butane We have determined (by pressure changes and light emission) the cool flame limits for various Copyright © 1971by The Comb~stion Institute Published by American ElsevierPublishingeom0any. Inc.
2O6
J. C. DECHAUX, J. L, FLAMENT,
butane concentrations. Figure 1(a) shows thz~, resuits obtained for 5, 20, 60 and 80% butane mixtures (pressure and temperature translations have been applied to each curve so that the minima are coincident). Lhn~.t Shape In Fig. l(a). we see that passing from 5 to 80% butane, the cool flame zone. which is broad at first, shrinks and becomes narrow at 80%. This may be connected with a progressive change in the mechanism. Now, from the previous works of Antonik and Lucquin [13, 14~. towards tlae high hydrocarbon concentratiom:, mechanism L 1 would be prepcnderant. This mechanism can be represented a~; isomerization
J
product,~
and
M. LUCQUIN
PP@ssuPe
®
|empl~roPure t°C Pc
t'~c i
~oQ 360
@
i
60 450
,~.aO
RO:' hydrogen r
,°[oo
o\
hydropcroxide L I mechanism (RH ~ O 2)
Towards higher oxygen concentrations, the L2 mechanism takes over, and aldehydes and other products of isomerisation of R O f radicals are oxidized and form hydroperoxy compounds such as peracids : R C O .............. +isomerization
!
//'products
L , mechanism 10: >> RH)
Thus. we know that aldehyde cool flame zones are very broad [15]. Hydrocarbons having an important L2 mechanism, such as neepentane [13, 14]. ha,~e a broad zone. On the other hand. those which do not have an important L 2 mechanism [13. 14] h a w a nat-
++o +o
-X ot o 20
40
60
gO%C 4 H,~O
Figure (a) Cool flame limits of butane as function of hydrocarbon percentage la pressure and ~cLaperature tran.~. lation has been applied to each diagram so that the minima a;x~ coincident).(b) Variations of certain physicocbemica] values in the negative temperature coefficient zone with percentage of butane. Curt,' I. Temperature PC) at which the negative temperature coefficient starts. Curre 2. Tempcralure {"C) corresponding to the end of the cool flame: ;'otle, Curt'e 3, The temperature difference ('~C) between !he maximum ~ and the minimum ~. i.e. the size of the N T , C Curre 4. The ratio t~,,/~+., i.e., the "'importance" of the N,T.C.
row zone. as with propane in a small reaction vessel [t6]. We must also notice that additives which strengthen the lobe L t, and therefore the corresponding mechanism, also reduce the zone breadth, e.g. HBr with neopcntane [13] or N O : or nitromethane with butane [17]; this is in agreement with our previous work with pentane [18], which emphasized a broadening
207
NEGATIVE TI~IPeRATU~KE COEFFICIENT IN OXIDATIONS
of the cool flame zone on" a par with higher oxygen concentrations. Consequently. our r~ults show that an important L 2 mechanism is accompanied by a broad cool flame zone,, and that a pronounced Lt mechanism is bound to a narrow zone. One can see that the zone broadening is more important towards the high temperature limit than towards the low temperature one. Temperature Corresponding to the end of the Cool Flame Zone
The examination of Fig. l(b), curve 2 shows that the temperature corresponding to the end of ~he zone is lower when the hydrocarbon concentration is higher (324°C at 80?/0 and 35 ~°C at 5~,~). This means that the cool flame zone is shifted towards low temperatures when the L t mechanism is favored, which is in agreement with the fact that the latter occurs at lower temperatures than L z [13, 14"1. Special Study of the Negative Temperature Coefficient with 20'V. Butane From 230 ° to 450°C we have recorded the variations of the induction period (time which separates the introduction of g ~ e s from the maximum light emission), of the maximum light intensity IM, and of the presst:re change at the end of the reaction AP®,for the 52 torr isobar. These results are grouped in Fig. 2(a): the induction period increases from 350°C. whereas the light intensity and the maximum pressure change AP~ decreases from 335°C and 325°C, respectively. Therefore. there is no absolute morphological criterion to localize accurately where the negative coefficient starts. However, in the following we wilt pay particular attention to the induction period, which seems to be the most valuable criterion as its change is bound to critical analytical factors. Study of the Negative Temperature Coefficient with Variable Coneenf~ration We have measured the variations of the induction period with temperature at butane con-
150
"¢n~,b.
X,
2so
20
~so
300
,~o
,~o
t°c
"~mln.
L
®
11)
os
%
0s~.
i'
"
t'c
Figure 2. (a) The variation with temperature (*C) of the
induction peripd x (minutes), of the maximn~nlight intensity lt~ (arbitrary unit), and of the pressure change at the end of the rt:actionAP®(~;rbitraryunit).To~J~pressure P ~ 52 rot'r, 20% butane. (b) The variationof the induction period ~ (minutes) with temperaturefor various % of butane. Total pressure P (~emperature)/T(tort deg"1) values were: 5~, 85/623: 20%. 54/600: 80%. 75/600: 95 % l :;0/600. ~'
centrations (total molar concentration of gas constant) of 5, 20. 80 and 95~0 as shown in Fig. 2(b). If the mixture is rich in oxygen, the negative temperature coefficient starts at a high temperature, and vice versa for mixtures which are rich in hydrocarbon (385°C for 55/0 butane and 354°C for 95 % butane); the starting " temperatures are shown in curve 1 of Fig. l(b). Figure 1(b) also shows the difference between the temperature of maximum z and minimum i.e. the "size" c,r the N.T.C. (curve 3), and the ratio z.,,~/T~,l, (curve 4), i.e. the "importance'" of the N.T.C., as functions of the percentage o f
20~;
J. C. DECHAUX, J. L. FLAMENT,
butane. From these ~ariations, we conclude that: (a) With high oxy[:en concentrations, the low temperature mechanism is very active, whereas the high temperature mecl:.mism is relatively inactive until quite high temperatures. (b) With high butane concentrations, the low temperature mechanism is not very active. whereas the high temperature mechanism is active over a wide range of temperature. These conclusions are illustrated schematically in Scheme A. Scheme A. ~) o2 q
HIO
o 3SO:
N.TC
TEMPI~RATURE
.~.. . . . . .
J
~.~:t~HANI~M
~.OW/JEMI~IERAr URE
09<:
TM
I HIGIar~M~R~TtJIClE14~:tq~,Nl$~ /
J
In order to explain these conclusions, we can reason that : (a) ix' the mixture is rich in oxygen, the low temperature mechanism (L2) occurs even at fairly high temperatures, as the most important peroxy radicals (RCO3") are good hydrogen abstraetors (which is equivalent to sayiag that they are relatively stable in terms of temperature, in our case): it is also possible th~tt products which wotdd be final ones in a mixture rich in butane become reactive ones t(rwards the higher temperatures of the low temperatures mechanism, and still permit the formation of a peroxide. If the high temperature mecb.anism is not very prortounced, it is due to the ta.'~ that the RCO 3. (or RCO. ) isomerization prodacts are not very reactive, so that branching is slight. Under still higher temperatures, the~.e products would become reactive. thus explaining the ease of'normal ignition.
and
M. LUCQUIN
(b) If the mixture is rich in butane, the low temperature mechanism (L t) is more restricted in temperature, as the most important peroxy radicals (RO2-) are not very good hydrogen abstractors (which is equivalent to ~;aying that they are rather unstable in terms of temperature) and prevent the formation of a hydroperoxide. If the high temperature mechar, ism is pronounced, it is due to the fact that the RO:isomerization products are active at high temperature, Only the lack of oxygen prevents tqe reaction development. An argument in favour of our consideratior, s comes from the work of Zaikov. Howard and Ingold [19], which has shown that the peroxyacyl radicals are more active hydrogen abstractors than the peroxyalkyl radicals. This also means that the decomposition of RCO a" is more difficult than that of RO2". isomerization and abstraction being competitive. For medium concentrations, the low and high temperature mechanisms are of similar importance, none of the two extends very far in temperature, and the mechanism overlapping is weak. which explains the large "size" of the negative temperature coefficient. An analytical study of the reaction products in terms of temperature is necessary at this stage of our argument, in order to check the findings obtained by the study of the morphology. In the analytical study of the negative temperature coefficient, analyses are carried out by gas phase chromatography, for the slow reaction zone only.
Experimental Results lnvestigatiou of Peroxides in Terms of Temperature In the slow reaction zone we have pressureqime curves which tend to lose the S-st'aped form, which indicates the suppression of branching. Shtern mentioned this fact a long time ago [20]. We have endeavored to detect peroxides by potarography: In this zone ihere are im-
209
NEGATIVE TEMPERATURE COEFFIICIENT IN OXIDATIONS
portant decreases in the quantities of peroxides which can hardly be detected at temperatures above 370°C (20 ~o butane). Cnrbon Oxides In, Fig. 3, curves showing the quantities of CO and CO2 formed at the maximum light emission, in terms of temperature, are drawn for the 52 torr isobar and with the 20~o butane mixture. We see that at low temperatures the quantities obtained decrease up to approx~.mately 350°C, and that from that temperature we get a sudden increase. The shape is similar at 80~ for. the 72 torr isobar (Fig. 3). The quantities formed at 20~ are much higher than those obtained at 80% (6 times more CO and 5 times more CO~ if we allow for differences in total pressure).
16%ol, ® 4
3 ('3% 2
/
I %
"~ 250
....
300
2,., 7
400
350
t*c
IC~H8
10"emole
Olefins and Hydrogen Peroxide Major olefins are butene-l, ethylene, and pro-. pylene. With the 20% butane mixture at the maximum light emission, we note a sudden increase in the quantity of olefins around 350°C
P tonr's
/o,
®
ff
/"
- - 2 0 °/~ .... 8 0 %
,° r
/
7~
/
Q
o
,
2so
3bo
3~o
460
, °c Figure 4. (a) The variation whh temperature o:r the quantities of olefins formed at the maximum light emission In) 20",,,', butane, P = 52 torr: (b) 80% butane. P = 72 tort.
5 I
25
% .=\'"" ", " ' . . . \ ° ' - ~ o ~ '; '
"~,.
CO
Jf
,,~-.* °~'-~_/',
2
250 300 350 400 ~' % Figure 31 The variation with Icmpcrature of tile quantities of CO and CO., formed at the m.~ximum light emission (20% butane, P = 5 2 t o r r - 80~!~i butane, P = 7 2 torr).
15 CF 17
[Fig. 4(a)]. With 80% butane, the accumulation curves of the products have the same shape [Fig. 4(b)]. We shall note that we obtained between 2 and 2.5 times more olefins at 80% than at 20% in that zone (corrected for total pressure). If the formation of olefin is accompanied by the formation of HO~. radicals, the yield
210
J. C. DECHAUX.J. L. FLAM~NT,and M, LUCQUtN
of H~O2 should parallel that of olefins. The curve obtained at 80 % (72 torr isobar) is shown in Fig. 5(b), and roughly follows that of olefin accumulation, with a marked increase around 350°C.
The quantities obtained wkh the 80% nbutane mixture are 2 to 3 ti~aes higher than those with the 20% mixture. ,Analyses carried out at the end of the reactic~n show a continuous decrease from 275° to 450°C.
Carboeyl Compounds Major carbonyl compounds are acetaldehyde, propionaldehyde, acrolein, acetone, and butanone [Fig. 5(a) and (b)]. From 27'5° to 350°C, the yields of the last four of these products decrease as the temperature rises. In the region of the negative coefficient, a distinct increase in the quantity of these products is noted, apart from acetaldehyde, whose yield decreases,
Meohols We mainly detect methanol and ethanol. There is no higher alcohol. In Fig. 6 w,- see a decrease from 275°C up to 350°C. at the :maximum light emission. At the beginning of the negative coefficient we have a distinc'~ rise, especially with the 20% butane mixture. There is approximately 3 times more alcohol :formed at 20% than at 80%.
,10"~mo~e
(~
CH3CHO 1O'6mole 7,S
Discussion o f Analytical ]Results Our results can be explained assuming that RCO 3. is the preponderant peroxyl radical for mixtures which are rich in: oxygen, and RO2" for mixtures which are rich in hydrocarbon. The previous morphological study led to much the same conclusions. Formation of Carbon Oxides These bodies come from the decomposition of RCO3", RCO~., and RCO' radicals--for the
0
250
300
350
,~OO
.~50
t "c
10-o t.
~
2 0 0/0
- - - 8 0 */* t O~6mole
I
.... :-.!,._, 200
2~o
3~o
3~o
~o "-"r~
Figure 5. The v~riation with temperature of the quantlties of ox2,,gen-c(~ntaining compounds formed at the inaxirllum light emisfion : ( a ) 2 0 % butane. P =: 52 tort: (b~ 80 % butane. P = 72 tort.
ol 250
, 300
"~::~','"'~ ~ " , ....... 350
400
I
t°c
Figure 6. The variatio, v ith temperature of the quarltities o f alcohols formed al the maximum light emission: - 2 0 % butane. P 52 Iorr: . . . . . . . . 8 0 % butane, P 7? ~orr.
NEGATIVETEMPERATURECOEFFICIENTIN OXIDATIONS formation of CO, note that RCO- reacts in two different ways : oxidation
RCO. + O, ~ RCO a"
decomposition
RCO.---, R. + CO
(1) (2)
211 Formation of Olefins and of Hydrogen Peroxide To explain the formation of olefins, we find two different proposals in the literature:
isomerization and elimination RO 2' -'-,.olefin + HO:v
(5)
hydrogen abstraction R' + O., ~ olefin + HO2"
(6)
Reaction (2) is favored when l:emperature rises; it is largeiy responsible for the rate decrease beyond 350°C, as it reduces the quantity of RCO3" radicals which are the source of branching, and for the increase in CO from 350°C. This is especially important for mixtures rich in oxygen, and coincides with the fact that there is 6 times more CO formed at 20~o than at 8 0 ~ in the negative coefficient zone. F o r m a t i o n q f C O 2. The transformation of CO into CO 2 is not important for temperatures under 450°C [21]. The RCO2" radical has a negligible part to play in the formation of COz beyond 350°C (no branching) with respect to that of RCOa..
Reaction (5) has been proposed by Semenov. Reaction (6), after the work of Benson "[23], has been accepted by a number of authors, We have measured the apparent activation energy of formation of olefins. In a series of analyses, we have characterized simultaneously the degree of advancement of the reaction by the butane consumption, and the rate of olefin formation through its concentration. For constant butane conversion in terms of temperature, we have
RCO" + 0 2 ~l RCO3 . .~ RO" + CO 2
log [olefin]r = A - E.~,,t/2.303RT.
The increase in the CO/CO2 ratio beyond 350°C :is explained by the following relationship: When reaction (1) becomes less important, CO increases and COz decreases. RCO 3. behaves in two different ways :
These measurements applied to the formation of ethylene give for 80Yo butane:
decomposition
RCO a. ~ RO- + CO,
E a = t7 keal/mole (Ref. 22) propagation
(3)
RCOa" + RH ~ RCOaH + R" E.~ = 7 kcal/mole (Ref. 22)
I4)
Reaction (4) leads to branching. If the temperature rises, it seems that reaction (3) prevails, and prevents branching. We have seen that 5 times more CO2 is formed at 2 0 ~ than a~ 8 0 ~ in the negative coefficient zone, which confirms the large quantity of RC'Os" radicals present for mixtures rich in oxygen. The minor importance of the high temperature mechanism at 2 0 ~ is bound to the important accumulation of CO and CO, which are inert (at least as long as the temperature is not too high wi~h CO).
from 270'~ to 350°C: Eao p = 11.6 kcal/mole: from 350 ° to 400°C (zone of the negative coefficient): E~pp = 31 kcal/mole, • Therefore, in each of these two parametric zones we have two different mechanisms, localized with respect to their apparent activation energy. From our previous results [24] it seems that at 290°C the olefins are formed by a heterogeneous isomerization. It is known that the activation ,energy of heterogeneous reactions may be two or three times lower than the same reactions in the gas phase. We think that from 2700 to 350°C the process is mainly.a heterogeneous isomerization (E~pp = 11.6 kcal/mole0. which becomes mainly homogeneous in the negative coefficient zone. The fact that we obtain 2 to 2.5 times more olefin at 8 0 ~ than at 20~,~ indicates clearly that for the mixtures which are rich in hydrocarbnn the RO 2, concentration is greater than
212
J. C. EIECHAUX,J. L, FLAMENT,and M. LUCQUIN
that of RCOa', which explains the i.mportance of the high temperature mechanism at 80~ in which the olefins play a large part. The increase of olefins in this zone is followed by a noticeable increase of the hydrogen peroxide, whose precursor is HOz'.
toward 335°C for instance, we are very near the cool flame limit: without doubt the line drawn is the resultant of two different components-one curve with a high maximum towards 335°C (ethylene oxide) and the other one increasing with the negative coefficient zone (CHaCHO).
Formation of Carhonyl Compounds Important quantities of carbonyl compounds are formed at low temperatures, and these quantities .decrease up to 350°C approximately. WhEn we come into the negative coefficient zone w.e obtain an increase of these products. The production mechanism is obviously different in both cases: on the analogy of what has been observed in the case of formation ofolefins, it seems that heterogeneous reactions are important at low temperatures. It is known that the decomposition of molecular peroxides gives alcohols, carbonvl products and CO. Moreover the low pressure of the experiments favours diffusion to the wall. On the other hand, in the negative temperature coefficient zone ';he homogeneous process is probably preponderant in our conditions. The isomerization eo~lsidered will be of the type proposed by Shtern and Semenov. Some possibilities of isomerization have been systematized and general!zed by Fish [25]. Since we link the formation of these products to the ROz" isomerization, it is most satisfactory to establish that concentrations in the negative coefficient zone are two to three times higher at 80% than at 20%. We must also consider the carbonyl compound formation from the olefin oxidation, but this reaction is probably of negligible importance in the low temperature range. In our opinion the different behavior of CH3CH()is explained by the fact that the chromatographic column used does not separate CH~CHO and ethylene oxide. Now from the work of Berry. Cullis, Saeed and Trimm [26] and of Fish [27]. we know that o heterocyclic compounds are formed in large quantities in the cool flame zone. During the experiments.
Formation of Alcohols Alcohols are essentially formed by abstraction of hydlogen by the alkoxy radicals: RO- + AH ~ ROH + A. These radicals mainly originate from the decomposition of hydroperoxides or of RCOa-, or the isomerization of RO,.. From 280° to 350°C we get more methanol at 80 ~ than at 20 ~ , but these proportions are reversed in the negative coefficient zone. At low temperatures, the decomposition of hydropernxides is an important source of RO. radicals, as well as reactic,ns of the type 2RO 2. -, 2RO. + Oz (which explains the high concentrations towards 80 ~), bu? beyond 350°C peroxides disappear, and it seems that the formation of RO' is mainly due to the decomposition or isomerization of' RO zand RCOa'. The decomposition of RCOa- is as follows: P,.COa"--+RO. + CO 2, while that of ROz', instead of following only one way, seems to be very diversified; the radicals released can then be HO:', OH" or RO'. Therefore it seems normal that the quantities of methanol formed in the negative coefficient zone are lower at 80% than at 20~. Following this discussion about the analytical results, we see that i hey allow us to reach exactly the same conclusions as those obtained by means of the morphological study: in the negative temperature coefficients zone, the isomerization reactions are predominant, and the radicals concerned are mainly RCO. and RCO 3. for high oxygen concentrations, ar,d ROz" for high hydrocarbon concentrations. It is obvious that the olefinic theory is inconsistent with our point of view. Let us examiue this theory on the precise point of the negative temperature coef-
213
NEGATIVE TEMPERATURE COEFFICIENT IN OXIDATIONS
flcient [12] : this phenomenon would be due to the instability of the radical hydroperoxide:
oxides, this scheme does not consider isomerization and calls on the branching chain only. But large quantities of these products, that we satisfactorily explain by the isomerization of RO2", RCO3" and RCO. radicals, are formed in the negative coefficient zone, where branching is suppressed.
olefin + HO 2"--)-~-C-O2H whose dissociation gives olefin and HO2.. Up to zhat point this is in agreement with the increase in olefins and hydrogen peroxide in ~he negative temperature coefficient zone. But when examining this mechanism more deeply, we establish that it is at variance with experimental facts. As a matter of fact, so as to obtain carbonyl compounds, and consequently carbon
Conclusions We ,:an sum up our proposals relatiug to the low temperature oxidation and the negative coefficien~ by the following scheme B:
Scheme B.
. . A . c . , . o . --Z~o-.oL~ BRANCHING
[ CHAIN ~ \
~ ~ "~'
ALDEHYDE5
I T,
RCO3H
I
KE~NE$
",Aco~vnE~--,..co
"*m~,
OUr"FIN ~, q.~.~ here r og ' ho,'no~ CARBONY L morner. ' - ~ ! l o m e e COMPOU~O.~
". H202 L 2 mechanism
L 1 re@shah;Sin
~mmmmm N.T.C.
N.T.C. Temp@raluee ~nc~eos~g, Oe ox~n~@n¢oncenlrofion ~¢rocB;ng
Otxe sees that carbonyl compounds, peroxides, and to a lesser extent olefins, are the principal products from the primary chain. They contribute to branching, with varying importance according to the concentration and the temperature range. Accumulation of those products depends on the stability of the precursor radicals. Therefore it is interesting to consider the competition between the isomerization of the peroxy radicals, and the chain propagation through the peroxy radicals. In the low temperature range, hydroperoxidation through RO2' or RCO 3" is the main reaction. In the negative temperature coefficient zone. peroxides
can no longer be formed, because of the iastability of their precursor radicals; only zm unbrached chain essentially remains, and leads to the accumulation of products that we have observed. RCO3' is the major peroxy radical at high oxygen concentration: RO z. is preponderent in high hydrocarbon concentration, as confirmed by the relative proportions of tile products obtained. These general ideas allow us to explain the main feature of the experimental results : If the reaction mixture is rich in oxygen, the negative coefficient starts at a high temperature; and it
214
is important because the isomerization products of RCO3" and RCO. seem to be inert with respect to the high temperature mechanism. If the mixture is rich in hydrocarbon, the low temperature mechanism is stopped at quite low temperatures. R.O2- seems to be a weaker hydrogen abstractor than RCO3-. Isomerization products of g o : . are active at high temperatures, and the corresponding mechanism being so reinforced, the importance of the negative coefficient is low. We think ~:hat our ideas, which explain at the same time the morphological and analytical facts, should be generally applicable. It can b~ said that the importance of the negative temperature coefficient of any hydrocarbon will es~,~nt[ally be ~n proportion to the activity of the isomerization products of the peroxy radicals (or of their precursors) with respect to the start of the high temperature mechanism. Subsequent to this work, the influence of coated wails on the neget[ve coefficient zone of isobutane has been studied in the laboratory ['28], and we intend to enter upon a comparative study of most hydrocarbons, in order to check on the generality of our ideas.
Re~'erences 1. PI~E. R. N., J. Anwr. Chcul. Soe.. 51, 1839 (1929), 2. U3BELHODE.A. R.. Proc. Roy. Soc.. A. 152, 354 (1935). 3. Lt~wls, B., ~md Vo.~ EL,E. G.. d. Anlt,r. Chet~t. Sot',. 59. 970 (1937), 4. WAI.Stl, A. D,. Trans. Faraday Soc.. 43, 297 (19.~7). 5 TCHERNIAK. N.. and SIla'EP,,'q. V. Y^., Dokl..4kad. Nau/c. SSSR. 78. I (1951): ANTONOVSKLV. L,. and Snl I~RN.V. YA,, F~okl,Akad. ?/auk. SSSR. 78, 2 (1951 }. 6. 8^~IOWI~LL.J.. and HINSlII~LW
J. C. DECHAUX, J. L. FLAMEN'T, and M. LUCQUIN 7. ENIKOLOPYAN.N. S., DokL Akad. Nauk. SSSR. 119, 520(1958). 8. BEN-A~M. R., and LucouI~, M.. J. Chim. Phys.. 56, 649 (1959), 9. MINKOrF, G. J.. an~ TIPPIw,, C. F. H.. Chemistry of Combustion Reactions. p. 130. Butterworths: London , (1962). 10. SEMV,NOV, N, N.. in Photochemistry and Reaction Kim.ties. (ed. P. G, AS.MORIL F. S. DAL'CroN, and T. M. StJq3ueN), p. 229. The University Press: Cambridge (1967), II. KNox. J. H., Combustion & Flame. 9, 297 (1965). 12. KNOX. J. H., in Ref. 10. 13. ANTONIK. S.. and LUCQUIN. M.. Bull. So~. Chim. Fr., 7. 2796 (1968). 14. ANTONIK. S.. and LUCQUIN, M.. Bull Soc. Chi~n. Fr.. 10. 4043 (1968). 15. CHAMr~OUX.J., and LUCQUIN. M.. J. Chim. Pl,gs.. 59. 797 (1962). 16. BONNER. '6. Fl., and TIPPER. C. F. H., Combustion & Flame, 9. 387 (1965). 17. DI-:CHAUX,J. C., FLA~IE,',rr. J. L., and Lucoultq. M.. Bull Soc. Chim. France 7. 2491 (1971). 18. LUCQtrlN. M.. Rev. Inst. Franc^is P~trole. 12, 1079 (1957). 19. ZAmOV, G. E.. HOW^gO, J. A., and INGOLD. K. U.. Canad. d. Chem.. 47, 3(H7 (1969). 20. SI-I'rL:RN,V. YA.. Tsepn Rcakt OkisL Uglev.. Akad. Nauk. SSSR. 37 (1955). 21. NEUM~,N. M, B.. El:n~,'qov, V. Y., S~t,'cog. N. T.. and LUKO~NIKOV, A. F.. Izvest. dkad. Nauk. SSSR, OtdeL Khim. Nauk.. 408 (1956). 22. M^cMmt,^N. G. R., and C^LVEItr. J. G.. Oxidation and Combustion Reviews (ed. C. F. H. TII,pER). p. 121. Elsevier : Amsterdam (1965). 23. BL~SON, S. W.. J. Amcr. Chem. Soc.. 87. 972 (1965). 24. DI~CHAUX. J. C,. LANGRAND, F., HERMANT, G., and LuCOUIN, M,, Bull. Soc. Chim. Ft.. 10, 4031 (1968), 25. FISH. A., Quarterly Rer.. 18. 243 (1964). 26. BEgRV. T.. CI.ILLIS,C. F.. Sz,~Eu. M.. and TrUMM.D. L., ddvan. Chem. Ser.. 76, p. 86. American Chemical Society: Washin~,'~n (1968). 27. FISH. A.. Proc. Roy. Soc.. A. 298. 204 (1967): At~gew. Chem.. 7, 45 (1968). 28. v^r~ 13E S'rr~Fr~E. M,, Thfi~ de Troisi6me Cycle. Lille 1970). (Received J u n e 1970: revised t,ersion ~,eceived S e p te m b e r 1970)
205
Negative Temperature Coefficient in the Oxidation of Butane and Other Hydrocarbons J. C. D6chaux, J. L. Flmnent, and M. Lucquin Laboratoirc de Chimie de la Combustion. Universit~ des Sciences et Techniques de Lille, B.P. 36, 59. Villeneuvc d'Ascq. France This paper describes a morphological study of the induction period, light emission, pressure change and cool flame explosion limits in the oxidation of n-~utane, and a broad study of the reaction products in the region of the negative temperature coefficient. The r~sults are discussed in terms of the interplay of two mechanisms involving the formation and reactions of the peroxy radica~ RO~. and RCO~. The results are explained by the competition between the chum propagation and the isomerizatioa of these radicals. A generalization is proposed.
Introduction The problem of the negative temperature coefficient may be stated briefly by recalling that in a certain temperature range the main feature of the hydrocarbon oxidation is that the rate decreases when the temperature is increased. The solution of this problem is important. In fact, any proposal of a general oxi~lation mechanism, if exact, must account for it, for the same reason as for the cool-flame, or twostage ignition lobe phenomena. Conversely, a satisfactory explanation of the phenomenon could only but reinforce the general theories in which it will be incorporated. Many authors have proposed explanations of this problem, but supporting evidence has been scanty. After the work of Pease [1]. Ubbelhode [2], and Lewis and yon Elbe [3], the first proposal which seems worthy of notice was that of Wal,~,h [4], who considered the reactivity of RO,. to change with temperature. A different explanation was given by Tcherniak, Shtern and Antonovski [5]. Bardwell and Hinshelwood [6] proposed an explanation based on competition bet';veer, the isomerization of the peroxy radical ~',O2. and H abstraction by RO='. Enikolopyan [7] based an explanation on the roles of RCO. and RCO:- radicals.
Ben-A/m and Lucquin [8 ] gave an explanation based on the variation of the branching factor with respect to the temperature. Tipper and Minkoff [9] and Semenov [10] have al,:o considered a modification of ROe', or RCO. and RCOa' with the temperature. Knox [l'.[] suggested that the phenomenon [12] mighlt result from the instability of the radical hydroperoxide
-C'-C-OeH. All authors are in general agreement ~ a t the phenomenon arises .through the increasing instability of a critical intermediate as the temperature rises: The work now describ,~d h a s been carried out to elucidate the nature of this intermediate, and consists of two parts:' first, a morphological study of the induction period, light emission, pressure change, and cooi flame and explosion limits, and second, a broad study of the reaction prod~Jcts. All experiments have been carried out in a cylindrical sili~,a reaction vessel of capacity 104 ema. M o r p h o l o g i c a l Study o f the N e g a t i v e T e m p e r a t u r e Coefficient Morphology of Cool Flames in Rdation to the
Coneentratio~,of Butane We have determined (by pressure changes and light emission) the cool flame limits for various Copyright © 1971by The Comb~stion Institute Published by American ElsevierPublishingeom0any. Inc.
2O6
J. C. DECHAUX, J. L, FLAMENT,
butane concentrations. Figure 1(a) shows thz~, resuits obtained for 5, 20, 60 and 80% butane mixtures (pressure and temperature translations have been applied to each curve so that the minima are coincident). Lhn~.t Shape In Fig. l(a). we see that passing from 5 to 80% butane, the cool flame zone. which is broad at first, shrinks and becomes narrow at 80%. This may be connected with a progressive change in the mechanism. Now, from the previous works of Antonik and Lucquin [13, 14~. towards tlae high hydrocarbon concentratiom:, mechanism L 1 would be prepcnderant. This mechanism can be represented a~; isomerization
J
product,~
and
M. LUCQUIN
PP@ssuPe
®
|empl~roPure t°C Pc
t'~c i
~oQ 360
@
i
60 450
,~.aO
RO:' hydrogen r
,°[oo
o\
hydropcroxide L I mechanism (RH ~ O 2)
Towards higher oxygen concentrations, the L2 mechanism takes over, and aldehydes and other products of isomerisation of R O f radicals are oxidized and form hydroperoxy compounds such as peracids : R C O .............. +isomerization
!
//'products
L , mechanism 10: >> RH)
Thus. we know that aldehyde cool flame zones are very broad [15]. Hydrocarbons having an important L2 mechanism, such as neepentane [13, 14]. ha,~e a broad zone. On the other hand. those which do not have an important L 2 mechanism [13. 14] h a w a nat-
++o +o
-X ot o 20
40
60
gO%C 4 H,~O
Figure (a) Cool flame limits of butane as function of hydrocarbon percentage la pressure and ~cLaperature tran.~. lation has been applied to each diagram so that the minima a;x~ coincident).(b) Variations of certain physicocbemica] values in the negative temperature coefficient zone with percentage of butane. Curt,' I. Temperature PC) at which the negative temperature coefficient starts. Curre 2. Tempcralure {"C) corresponding to the end of the cool flame: ;'otle, Curt'e 3, The temperature difference ('~C) between !he maximum ~ and the minimum ~. i.e. the size of the N T , C Curre 4. The ratio t~,,/~+., i.e., the "'importance" of the N,T.C.
row zone. as with propane in a small reaction vessel [t6]. We must also notice that additives which strengthen the lobe L t, and therefore the corresponding mechanism, also reduce the zone breadth, e.g. HBr with neopcntane [13] or N O : or nitromethane with butane [17]; this is in agreement with our previous work with pentane [18], which emphasized a broadening
207
NEGATIVE TI~IPeRATU~KE COEFFICIENT IN OXIDATIONS
of the cool flame zone on" a par with higher oxygen concentrations. Consequently. our r~ults show that an important L 2 mechanism is accompanied by a broad cool flame zone,, and that a pronounced Lt mechanism is bound to a narrow zone. One can see that the zone broadening is more important towards the high temperature limit than towards the low temperature one. Temperature Corresponding to the end of the Cool Flame Zone
The examination of Fig. l(b), curve 2 shows that the temperature corresponding to the end of ~he zone is lower when the hydrocarbon concentration is higher (324°C at 80?/0 and 35 ~°C at 5~,~). This means that the cool flame zone is shifted towards low temperatures when the L t mechanism is favored, which is in agreement with the fact that the latter occurs at lower temperatures than L z [13, 14"1. Special Study of the Negative Temperature Coefficient with 20'V. Butane From 230 ° to 450°C we have recorded the variations of the induction period (time which separates the introduction of g ~ e s from the maximum light emission), of the maximum light intensity IM, and of the presst:re change at the end of the reaction AP®,for the 52 torr isobar. These results are grouped in Fig. 2(a): the induction period increases from 350°C. whereas the light intensity and the maximum pressure change AP~ decreases from 335°C and 325°C, respectively. Therefore. there is no absolute morphological criterion to localize accurately where the negative coefficient starts. However, in the following we wilt pay particular attention to the induction period, which seems to be the most valuable criterion as its change is bound to critical analytical factors. Study of the Negative Temperature Coefficient with Variable Coneenf~ration We have measured the variations of the induction period with temperature at butane con-
150
"¢n~,b.
X,
2so
20
~so
300
,~o
,~o
t°c
"~mln.
L
®
11)
os
%
0s~.
i'
"
t'c
Figure 2. (a) The variation with temperature (*C) of the
induction peripd x (minutes), of the maximn~nlight intensity lt~ (arbitrary unit), and of the pressure change at the end of the rt:actionAP®(~;rbitraryunit).To~J~pressure P ~ 52 rot'r, 20% butane. (b) The variationof the induction period ~ (minutes) with temperaturefor various % of butane. Total pressure P (~emperature)/T(tort deg"1) values were: 5~, 85/623: 20%. 54/600: 80%. 75/600: 95 % l :;0/600. ~'
centrations (total molar concentration of gas constant) of 5, 20. 80 and 95~0 as shown in Fig. 2(b). If the mixture is rich in oxygen, the negative temperature coefficient starts at a high temperature, and vice versa for mixtures which are rich in hydrocarbon (385°C for 55/0 butane and 354°C for 95 % butane); the starting " temperatures are shown in curve 1 of Fig. l(b). Figure 1(b) also shows the difference between the temperature of maximum z and minimum i.e. the "size" c,r the N.T.C. (curve 3), and the ratio z.,,~/T~,l, (curve 4), i.e. the "importance'" of the N.T.C., as functions of the percentage o f
20~;
J. C. DECHAUX, J. L. FLAMENT,
butane. From these ~ariations, we conclude that: (a) With high oxy[:en concentrations, the low temperature mechanism is very active, whereas the high temperature mecl:.mism is relatively inactive until quite high temperatures. (b) With high butane concentrations, the low temperature mechanism is not very active. whereas the high temperature mechanism is active over a wide range of temperature. These conclusions are illustrated schematically in Scheme A. Scheme A. ~) o2 q
HIO
o 3SO:
N.TC
TEMPI~RATURE
.~.. . . . . .
J
~.~:t~HANI~M
~.OW/JEMI~IERAr URE
09<:
TM
I HIGIar~M~R~TtJIClE14~:tq~,Nl$~ /
J
In order to explain these conclusions, we can reason that : (a) ix' the mixture is rich in oxygen, the low temperature mechanism (L2) occurs even at fairly high temperatures, as the most important peroxy radicals (RCO3") are good hydrogen abstraetors (which is equivalent to sayiag that they are relatively stable in terms of temperature, in our case): it is also possible th~tt products which wotdd be final ones in a mixture rich in butane become reactive ones t(rwards the higher temperatures of the low temperatures mechanism, and still permit the formation of a peroxide. If the high temperature mecb.anism is not very prortounced, it is due to the ta.'~ that the RCO 3. (or RCO. ) isomerization prodacts are not very reactive, so that branching is slight. Under still higher temperatures, the~.e products would become reactive. thus explaining the ease of'normal ignition.
and
M. LUCQUIN
(b) If the mixture is rich in butane, the low temperature mechanism (L t) is more restricted in temperature, as the most important peroxy radicals (RO2-) are not very good hydrogen abstractors (which is equivalent to ~;aying that they are rather unstable in terms of temperature) and prevent the formation of a hydroperoxide. If the high temperature mechar, ism is pronounced, it is due to the fact that the RO:isomerization products are active at high temperature, Only the lack of oxygen prevents tqe reaction development. An argument in favour of our consideratior, s comes from the work of Zaikov. Howard and Ingold [19], which has shown that the peroxyacyl radicals are more active hydrogen abstractors than the peroxyalkyl radicals. This also means that the decomposition of RCO a" is more difficult than that of RO2". isomerization and abstraction being competitive. For medium concentrations, the low and high temperature mechanisms are of similar importance, none of the two extends very far in temperature, and the mechanism overlapping is weak. which explains the large "size" of the negative temperature coefficient. An analytical study of the reaction products in terms of temperature is necessary at this stage of our argument, in order to check the findings obtained by the study of the morphology. In the analytical study of the negative temperature coefficient, analyses are carried out by gas phase chromatography, for the slow reaction zone only.
Experimental Results lnvestigatiou of Peroxides in Terms of Temperature In the slow reaction zone we have pressureqime curves which tend to lose the S-st'aped form, which indicates the suppression of branching. Shtern mentioned this fact a long time ago [20]. We have endeavored to detect peroxides by potarography: In this zone ihere are im-
209
NEGATIVE TEMPERATURE COEFFIICIENT IN OXIDATIONS
portant decreases in the quantities of peroxides which can hardly be detected at temperatures above 370°C (20 ~o butane). Cnrbon Oxides In, Fig. 3, curves showing the quantities of CO and CO2 formed at the maximum light emission, in terms of temperature, are drawn for the 52 torr isobar and with the 20~o butane mixture. We see that at low temperatures the quantities obtained decrease up to approx~.mately 350°C, and that from that temperature we get a sudden increase. The shape is similar at 80~ for. the 72 torr isobar (Fig. 3). The quantities formed at 20~ are much higher than those obtained at 80% (6 times more CO and 5 times more CO~ if we allow for differences in total pressure).
16%ol, ® 4
3 ('3% 2
/
I %
"~ 250
....
300
2,., 7
400
350
t*c
IC~H8
10"emole
Olefins and Hydrogen Peroxide Major olefins are butene-l, ethylene, and pro-. pylene. With the 20% butane mixture at the maximum light emission, we note a sudden increase in the quantity of olefins around 350°C
P tonr's
/o,
®
ff
/"
- - 2 0 °/~ .... 8 0 %
,° r
/
7~
/
Q
o
,
2so
3bo
3~o
460
, °c Figure 4. (a) The variation whh temperature o:r the quantities of olefins formed at the maximum light emission In) 20",,,', butane, P = 52 torr: (b) 80% butane. P = 72 tort.
5 I
25
% .=\'"" ", " ' . . . \ ° ' - ~ o ~ '; '
"~,.
CO
Jf
,,~-.* °~'-~_/',
2
250 300 350 400 ~' % Figure 31 The variation with Icmpcrature of tile quantities of CO and CO., formed at the m.~ximum light emission (20% butane, P = 5 2 t o r r - 80~!~i butane, P = 7 2 torr).
15 CF 17
[Fig. 4(a)]. With 80% butane, the accumulation curves of the products have the same shape [Fig. 4(b)]. We shall note that we obtained between 2 and 2.5 times more olefins at 80% than at 20% in that zone (corrected for total pressure). If the formation of olefin is accompanied by the formation of HO~. radicals, the yield
210
J. C. DECHAUX.J. L. FLAM~NT,and M, LUCQUtN
of H~O2 should parallel that of olefins. The curve obtained at 80 % (72 torr isobar) is shown in Fig. 5(b), and roughly follows that of olefin accumulation, with a marked increase around 350°C.
The quantities obtained wkh the 80% nbutane mixture are 2 to 3 ti~aes higher than those with the 20% mixture. ,Analyses carried out at the end of the reactic~n show a continuous decrease from 275° to 450°C.
Carboeyl Compounds Major carbonyl compounds are acetaldehyde, propionaldehyde, acrolein, acetone, and butanone [Fig. 5(a) and (b)]. From 27'5° to 350°C, the yields of the last four of these products decrease as the temperature rises. In the region of the negative coefficient, a distinct increase in the quantity of these products is noted, apart from acetaldehyde, whose yield decreases,
Meohols We mainly detect methanol and ethanol. There is no higher alcohol. In Fig. 6 w,- see a decrease from 275°C up to 350°C. at the :maximum light emission. At the beginning of the negative coefficient we have a distinc'~ rise, especially with the 20% butane mixture. There is approximately 3 times more alcohol :formed at 20% than at 80%.
,10"~mo~e
(~
CH3CHO 1O'6mole 7,S
Discussion o f Analytical ]Results Our results can be explained assuming that RCO 3. is the preponderant peroxyl radical for mixtures which are rich in: oxygen, and RO2" for mixtures which are rich in hydrocarbon. The previous morphological study led to much the same conclusions. Formation of Carbon Oxides These bodies come from the decomposition of RCO3", RCO~., and RCO' radicals--for the
0
250
300
350
,~OO
.~50
t "c
10-o t.
~
2 0 0/0
- - - 8 0 */* t O~6mole
I
.... :-.!,._, 200
2~o
3~o
3~o
~o "-"r~
Figure 5. The v~riation with temperature of the quantlties of ox2,,gen-c(~ntaining compounds formed at the inaxirllum light emisfion : ( a ) 2 0 % butane. P =: 52 tort: (b~ 80 % butane. P = 72 tort.
ol 250
, 300
"~::~','"'~ ~ " , ....... 350
400
I
t°c
Figure 6. The variatio, v ith temperature of the quarltities o f alcohols formed al the maximum light emission: - 2 0 % butane. P 52 Iorr: . . . . . . . . 8 0 % butane, P 7? ~orr.
NEGATIVETEMPERATURECOEFFICIENTIN OXIDATIONS formation of CO, note that RCO- reacts in two different ways : oxidation
RCO. + O, ~ RCO a"
decomposition
RCO.---, R. + CO
(1) (2)
211 Formation of Olefins and of Hydrogen Peroxide To explain the formation of olefins, we find two different proposals in the literature:
isomerization and elimination RO 2' -'-,.olefin + HO:v
(5)
hydrogen abstraction R' + O., ~ olefin + HO2"
(6)
Reaction (2) is favored when l:emperature rises; it is largeiy responsible for the rate decrease beyond 350°C, as it reduces the quantity of RCO3" radicals which are the source of branching, and for the increase in CO from 350°C. This is especially important for mixtures rich in oxygen, and coincides with the fact that there is 6 times more CO formed at 20~o than at 8 0 ~ in the negative coefficient zone. F o r m a t i o n q f C O 2. The transformation of CO into CO 2 is not important for temperatures under 450°C [21]. The RCO2" radical has a negligible part to play in the formation of COz beyond 350°C (no branching) with respect to that of RCOa..
Reaction (5) has been proposed by Semenov. Reaction (6), after the work of Benson "[23], has been accepted by a number of authors, We have measured the apparent activation energy of formation of olefins. In a series of analyses, we have characterized simultaneously the degree of advancement of the reaction by the butane consumption, and the rate of olefin formation through its concentration. For constant butane conversion in terms of temperature, we have
RCO" + 0 2 ~l RCO3 . .~ RO" + CO 2
log [olefin]r = A - E.~,,t/2.303RT.
The increase in the CO/CO2 ratio beyond 350°C :is explained by the following relationship: When reaction (1) becomes less important, CO increases and COz decreases. RCO 3. behaves in two different ways :
These measurements applied to the formation of ethylene give for 80Yo butane:
decomposition
RCO a. ~ RO- + CO,
E a = t7 keal/mole (Ref. 22) propagation
(3)
RCOa" + RH ~ RCOaH + R" E.~ = 7 kcal/mole (Ref. 22)
I4)
Reaction (4) leads to branching. If the temperature rises, it seems that reaction (3) prevails, and prevents branching. We have seen that 5 times more CO2 is formed at 2 0 ~ than a~ 8 0 ~ in the negative coefficient zone, which confirms the large quantity of RC'Os" radicals present for mixtures rich in oxygen. The minor importance of the high temperature mechanism at 2 0 ~ is bound to the important accumulation of CO and CO, which are inert (at least as long as the temperature is not too high wi~h CO).
from 270'~ to 350°C: Eao p = 11.6 kcal/mole: from 350 ° to 400°C (zone of the negative coefficient): E~pp = 31 kcal/mole, • Therefore, in each of these two parametric zones we have two different mechanisms, localized with respect to their apparent activation energy. From our previous results [24] it seems that at 290°C the olefins are formed by a heterogeneous isomerization. It is known that the activation ,energy of heterogeneous reactions may be two or three times lower than the same reactions in the gas phase. We think that from 2700 to 350°C the process is mainly.a heterogeneous isomerization (E~pp = 11.6 kcal/mole0. which becomes mainly homogeneous in the negative coefficient zone. The fact that we obtain 2 to 2.5 times more olefin at 8 0 ~ than at 20~,~ indicates clearly that for the mixtures which are rich in hydrocarbnn the RO 2, concentration is greater than
212
J. C. EIECHAUX,J. L, FLAMENT,and M. LUCQUIN
that of RCOa', which explains the i.mportance of the high temperature mechanism at 80~ in which the olefins play a large part. The increase of olefins in this zone is followed by a noticeable increase of the hydrogen peroxide, whose precursor is HOz'.
toward 335°C for instance, we are very near the cool flame limit: without doubt the line drawn is the resultant of two different components-one curve with a high maximum towards 335°C (ethylene oxide) and the other one increasing with the negative coefficient zone (CHaCHO).
Formation of Carhonyl Compounds Important quantities of carbonyl compounds are formed at low temperatures, and these quantities .decrease up to 350°C approximately. WhEn we come into the negative coefficient zone w.e obtain an increase of these products. The production mechanism is obviously different in both cases: on the analogy of what has been observed in the case of formation ofolefins, it seems that heterogeneous reactions are important at low temperatures. It is known that the decomposition of molecular peroxides gives alcohols, carbonvl products and CO. Moreover the low pressure of the experiments favours diffusion to the wall. On the other hand, in the negative temperature coefficient zone ';he homogeneous process is probably preponderant in our conditions. The isomerization eo~lsidered will be of the type proposed by Shtern and Semenov. Some possibilities of isomerization have been systematized and general!zed by Fish [25]. Since we link the formation of these products to the ROz" isomerization, it is most satisfactory to establish that concentrations in the negative coefficient zone are two to three times higher at 80% than at 20%. We must also consider the carbonyl compound formation from the olefin oxidation, but this reaction is probably of negligible importance in the low temperature range. In our opinion the different behavior of CH3CH()is explained by the fact that the chromatographic column used does not separate CH~CHO and ethylene oxide. Now from the work of Berry. Cullis, Saeed and Trimm [26] and of Fish [27]. we know that o heterocyclic compounds are formed in large quantities in the cool flame zone. During the experiments.
Formation of Alcohols Alcohols are essentially formed by abstraction of hydlogen by the alkoxy radicals: RO- + AH ~ ROH + A. These radicals mainly originate from the decomposition of hydroperoxides or of RCOa-, or the isomerization of RO,.. From 280° to 350°C we get more methanol at 80 ~ than at 20 ~ , but these proportions are reversed in the negative coefficient zone. At low temperatures, the decomposition of hydropernxides is an important source of RO. radicals, as well as reactic,ns of the type 2RO 2. -, 2RO. + Oz (which explains the high concentrations towards 80 ~), bu? beyond 350°C peroxides disappear, and it seems that the formation of RO' is mainly due to the decomposition or isomerization of' RO zand RCOa'. The decomposition of RCOa- is as follows: P,.COa"--+RO. + CO 2, while that of ROz', instead of following only one way, seems to be very diversified; the radicals released can then be HO:', OH" or RO'. Therefore it seems normal that the quantities of methanol formed in the negative coefficient zone are lower at 80% than at 20~. Following this discussion about the analytical results, we see that i hey allow us to reach exactly the same conclusions as those obtained by means of the morphological study: in the negative temperature coefficients zone, the isomerization reactions are predominant, and the radicals concerned are mainly RCO. and RCO 3. for high oxygen concentrations, ar,d ROz" for high hydrocarbon concentrations. It is obvious that the olefinic theory is inconsistent with our point of view. Let us examiue this theory on the precise point of the negative temperature coef-
213
NEGATIVE TEMPERATURE COEFFICIENT IN OXIDATIONS
flcient [12] : this phenomenon would be due to the instability of the radical hydroperoxide:
oxides, this scheme does not consider isomerization and calls on the branching chain only. But large quantities of these products, that we satisfactorily explain by the isomerization of RO2", RCO3" and RCO. radicals, are formed in the negative coefficient zone, where branching is suppressed.
olefin + HO 2"--)-~-C-O2H whose dissociation gives olefin and HO2.. Up to zhat point this is in agreement with the increase in olefins and hydrogen peroxide in ~he negative temperature coefficient zone. But when examining this mechanism more deeply, we establish that it is at variance with experimental facts. As a matter of fact, so as to obtain carbonyl compounds, and consequently carbon
Conclusions We ,:an sum up our proposals relatiug to the low temperature oxidation and the negative coefficien~ by the following scheme B:
Scheme B.
. . A . c . , . o . --Z~o-.oL~ BRANCHING
[ CHAIN ~ \
~ ~ "~'
ALDEHYDE5
I T,
RCO3H
I
KE~NE$
",Aco~vnE~--,..co
"*m~,
OUr"FIN ~, q.~.~ here r og ' ho,'no~ CARBONY L morner. ' - ~ ! l o m e e COMPOU~O.~
". H202 L 2 mechanism
L 1 re@shah;Sin
~mmmmm N.T.C.
N.T.C. Temp@raluee ~nc~eos~g, Oe ox~n~@n¢oncenlrofion ~¢rocB;ng
Otxe sees that carbonyl compounds, peroxides, and to a lesser extent olefins, are the principal products from the primary chain. They contribute to branching, with varying importance according to the concentration and the temperature range. Accumulation of those products depends on the stability of the precursor radicals. Therefore it is interesting to consider the competition between the isomerization of the peroxy radicals, and the chain propagation through the peroxy radicals. In the low temperature range, hydroperoxidation through RO2' or RCO 3" is the main reaction. In the negative temperature coefficient zone. peroxides
can no longer be formed, because of the iastability of their precursor radicals; only zm unbrached chain essentially remains, and leads to the accumulation of products that we have observed. RCO3' is the major peroxy radical at high oxygen concentration: RO z. is preponderent in high hydrocarbon concentration, as confirmed by the relative proportions of tile products obtained. These general ideas allow us to explain the main feature of the experimental results : If the reaction mixture is rich in oxygen, the negative coefficient starts at a high temperature; and it
214
is important because the isomerization products of RCO3" and RCO. seem to be inert with respect to the high temperature mechanism. If the mixture is rich in hydrocarbon, the low temperature mechanism is stopped at quite low temperatures. R.O2- seems to be a weaker hydrogen abstractor than RCO3-. Isomerization products of g o : . are active at high temperatures, and the corresponding mechanism being so reinforced, the importance of the negative coefficient is low. We think ~:hat our ideas, which explain at the same time the morphological and analytical facts, should be generally applicable. It can b~ said that the importance of the negative temperature coefficient of any hydrocarbon will es~,~nt[ally be ~n proportion to the activity of the isomerization products of the peroxy radicals (or of their precursors) with respect to the start of the high temperature mechanism. Subsequent to this work, the influence of coated wails on the neget[ve coefficient zone of isobutane has been studied in the laboratory ['28], and we intend to enter upon a comparative study of most hydrocarbons, in order to check on the generality of our ideas.
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