March. 1934
IhDLSTRIiL
4 h D EhGIhEERING
(3) L)uniaa. .Jtin. c h m . p h , ~ s . ,[SI 55, 187 ( 1 8 5 9 ~ . (4) Gooch a n d McC'lenahan, A m . J . .Sci., [4] 17, 365 : 1 W 4 ~ . ( 5 ) Hempel. Ber., 21,897 118881 (6) H u h , French P a t e n t '13,828 ,~1917,. (7) Madorsky, IND.EXG.[ ' H E M . , 24, 233 (1932). (8) Meier, Bur. Mines, 7 e c h . P a p w 360, 40 (1925), (9) Morgan. Dissertation, (.'olurnbia University. 1919 '10) Neumann. R p r . . 18. 3 3 0 (188.5).
CHEMISTRY
359
i l l ) Ralston, Bur. Mines. Tech. Paper 321. 34 119231. (12) Reed, Fox, a n d Turrentine, IND. ENG. CHEM.,24, 910 (1932) (13) Richards, J. Am. Chem. SOC.,24,374 (1903). 1.14) Sainte-Claire-Deville. .4nn.. 120, 180 :1861)
RECEIVEDSeptember 20,1933. Preaeuted before the Division of Ferthzer Chemistry a t the 86th Meeting #of the .\rneriran Chemical 3nciety. Chicago, Ill., September 10 t o 15, 1933
Thermal Decomposition of Hydrocarbons and Engine Detonation F. 0. RICE,Johns Hopkins University, Baltimore, M d . The thermal decomposition of the f u e l probthe processes occurring Juring SURL-EY of the literatheir oxidation ( 1 3 ) . For exture (12) on the subject ably plays an appreciable role in the reactioru ample, experimental evidence is of explosions and flames occurring in the internal combustion engine. given in a paper by Pease (8) shows that the first step in such This decomposition results in one molecule of on the oxidation of propane and reactions, is usually followed by the fuel being replaced by seceral molecules; this butane. The nonexplosive oxian extremely complex series of increase in concentrat ion may greatly augment dation of these compounds at reactions; the view is widely temperatures f a r below those held that many of these subsethe rate of oxidation. Dijferent hydrocarbons required to produce appreciable quent reactions are of the chain yield differerii numbers of molecules of products cracking of the hydrocarbons in type since on this basis niany of per mole deconiposed, orid calculat ion .qhoiLv a the absence of oxygen yielded the peculiarities of explosions, strict purallelisrn betweerr this arid the knocking considerable quantities of hysuch as s e n s i t i v i t y to small tendericy. The knocking tendency increases with drogen, methane, and unsatutraces of impurities anti the rated hydrocarbons, the amounts existence of fairly sharply defined increase o j number of moles of product per molr increasing with increasing furlimits of pressure and teniperadecomposed. One effect of antiknock compounds ture, can be explained. Further nace temperature. The simiis to reduce the ririrriber of nioles of products larity between the composition evidence to this effect (4) seems formed frorti the decomposition of one nioh of to be found in the fact that these of these Droducts and those reactions are sometimes stopped hydrocarbon. formed in the ordinary thermal by surfaces, sometimes start a t d e c o m p o s i t i o n led Pease to surfaces, and sometimes are both stopped and started a t d suggest that the oxidation of hydrocarbons was accompanied surface. Experimental work on the oxidation of gaseous paraf- to a greater or less degree by a decomposition similar to fin hydrocarbons has shown (2, 9) that the oxidation results that occurring in the absence of oxygen. The theoretical in a complex series of intermediate oxidation products; the evidence which will be discussed in detail later supports the oxidation processes which occur in the internal combustion view that, even in homogeneous hydrocarbon-oxygen mixtures, engine must, therefore, be extremely complex, and it is hardly thermal decomposition accompanies the oxidation of the hypossible to offer more than the most general interpretation drocarbon to a considerable extent. of the course of the reaction. The lack of homogeneity of the gases which enter the However, it has occurred to the author that in the case of cylinder of an internal combustion engine will naturally favor hydrocarbon-oxygen mixtures, a preliminary decomposition still more the thermal decomposition of the hydrocarbon of the hydrocarbon into lower hydrocarbons, chiefly the molecule. Even under the best conditions-namely, when all lower olefins and paraffins, resulting partly from the purely the fuel is volatilized before entering the cylinder and turthermal effect and partly from attack on the hydrocarbon bulence is induced in the gaseous mixture-the mixing of by molecular fragments, probably takes place. air and hydrochbon vapor must be far from perfect. When If this preliminary decomposition were extensive, the part of the fuel is not volatilized but enters the cylinder 111 chemical composition of the mixture after the decomposition the form of small droplets, we should expect to find many but before appreciable oxidation had set in would be approxi- qegregated volumes, each consisting mainly of hydrocarbon mately the same for all hydrocarbons; the only substantial vapor. During the compression stroke and in the earlier difference would be in the number of smaller hydrocarbons stages of the explosion these segregated volumes may be exproduced per molecule of the original fuel. Since the con- pected to undergo ordinary thermal decomposition into centration of the total hydrocarbon molecules probably plays Gmpler products, independently of the decomposition induced a considerable role in determining the pressure of the hydro- by the oxidation reactions. carbon-oxygen mixture a t which detonation sets in, it seemed If thermal decomposition does occur to any appreciable exworth while to calculate the products formed in the thermal tent, it must have considerable bearing on the detonation of decomposition of different hydrocarbons and to attempt to hydrocarbon-oxygen mixtures since through this decomcorrelate the results with measurements of the knocking position process one molecule of hydrocarbon is suddenly retendencies of different hydrocarbons. placed by several smaller molecules. The effect of this is There is considerable evidence both from the experimental twofold: (1) Since the pressure exerted by a gas depends on and from the theoretical standpoint that the simple thermal the number of molecules and not on their size, this process is decomposition of hydrocarbons plays an appreciable part in equivalent to raising the compression in the engine from 5 to
A
260
INDUSTRIAL AND EN G l N E E R l N G
CHEMISTHk
Vol. 26, No. 3
TABLEI. THERMAL DECOMPOSIT~OK OF HYDROCARBON@ (Methyl, ethyl, isopropyl. and tert-butyl radical@assumed t o be stable: relative chances of loan of primary. secondary. and tertlary hydrogen atoms assumed to be 1:2,10. resDertivelv\ M o t a s PRODUCT MOLES PRODUCT PER MOLE ANILINE PER MOLE ANILINE HYDROCARBON RATIO( 6 ) HYDROCARBOS HYDROCARBOX HYDROCARBOX RATIO (6) 2.34 1 N-pentane N-octane 3.2 -2 1 2.0 2-Methylbutane 9 3-Methylheptane 2.9i 2.0 2,2-Dimethylpropane 15 2-MethvlheDtane 2.95 4-MethGlheptane 2.77 2.64 N-hexane -6 2.2-Dimethylhexane 2.65 2-hlethylpentane 2.22 4 2,5-Dimethylhexane 2.65 5 2.22 3-Methylpentane 8 3,3-Dimethylhexane 2.63 22-Dimetnylbutane 2.0 13 3-Ethylhexane 2.49 2,3-Dimethylburane 2.0 19 2,4-Dimethylhexane 2.49 2.3-Dimethvlhexane 2.41 2.94 N-heptane 14 3-Methyl-3:ethvlpentane 2.38 2.65 0 2-Methylherane 3.4-Dimethylhexane 2.33 , . 3-Methvlhexane 2.44 3 2,2,4-Trimethylpentane 2.26 16 3-Ethyipentane4 2.3 2,2,3-Trimethylpentsne 2.26 17 2.36 2,2-Dimethylpentans 8 2-Methyl-3-ethylpentane 2.23 Fi 2,4-Dimethylpentane 2.1i 2,3,3-Trimethylpentane .. 2.21 2.17 li 2.3-Dimethvl~entane 2,3,4-Trimethylpentane 2.14 .. 3,a-Dimethylpentane 13 2.3 2,2,3,3-TetramethyIbutane 2 .o 26 2,2,3-Trimethylbutane 2.0 19 N-Nonane 3.48 -28 N-decan e 3.74 -30 2,6-Dimethylheptane 2.82 -6 2,7-Dimethyloctane 3.26 10 4-Ethylheptane 2.58 ,. 3 3 4 4-Tetramethylhexane ,. 2.62 3.3-Diet hvl~entane 2.43 .. 2.42 3:4~6irnethyloctane .. 3-MethylI4Iethylhexane 2.36 2,2,6,6-Tetramethylhexane 29 2.35 2 4-Dimethyl-3-eth lpentane 2.1s 2:3.3,4-TetramethyKpent,an~ 2.16 ..
-
.
I
-
,. ,.
different numbers of molecules of product per molecule decomposed, it seemed of interest to calculate these numbers and compare them with the experimentally determined knocking tendency of hydrocarbons. It is clear that on the basis of our premise we would expect the knocking tendency to increase with an increasing number of molecules of product formed per molecule of hydrocarbon thermally decomposed.
METHODOF CALCULATION Precise experimental measurement 6
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to the bimolecular oxidation process. However the work of Pope, Dykstra, and Edgar (9) shows that under certain c o n d i t i o n s there is considerable aldehyde formation in the oxidation of the octanes. In the following calculations of the decomposition bons the method products described of inhydrocarprevious publications (11) has been followed exactly, with one exception: it has been assumed that in addition to the methyl and ethyl radicals, the isopropyl and tert-butyl radicals are also stable. This assumption is justified because:
saturaied hydrocarbons -UP to inFIGURE1 CoMPARlsoNOF -rHERkr\I.4L must always occur under considerable D ~ H~~~~~.~~~~~ ~ ~ pressure, ~ and thip favors ~ the reaction ~ cluding the pentanes and two of the hexanes are now available (S),but there (DOTTED CURVE) A N D KNOCKING of the r a d i c a l s w i t h s u r r o u n d i n g do not seem to be any such data for the (FULLCVRPE) molecules rather t h a n their deconiTENDEUCY higher hydrocarbons. It is, however, p o s i t i o n ; and ( 2 ) these four radipossible to calculate the products to be expected on the basis c a k may lie expected to have a higher degree of stability of a theory recently proposed (IO). This theory postulate? than any other alkyl radicals because their decomposithat the decomposition of hydrocarbons proceeds through a tion can occur only through the hreaking off of atomic mechanism involving the production of free radicals and the hydrogen. I t iq not necessary to give all these calculations in detail subsequent removal of hydrogen atoms from the surrounding hydrocarbon molecules by reaction with these radicals. If, because the method has already been fully described and for example, we consider butane, the removal of a hydrogen applies to all the paraffin hydrocarbons. Consequently, atom must leave either a N-butyl or an isobutyl radical. From the method will be illustrated in detail with reference to only the pentanes and higher hydrocarbons a greater variety of two hydrocarbons, AT-octane and 2,2,4-trimethylpentane. radicals will be obtained, depending on the number of diff erent Furtherniore only the chain cycles will be given since these kinds of hydrogen atoms in the molecule. Most of these larger determine the products of the decomposition (R represents a radicale are very unstable and rapidly decompose into olefine free alkyl radical or a hydrogen atom) :
&
March, 1934
I N D U S T R I A L AND- E N G I N E E R I N G CHEMISTRY
N-octane: CH~CH~CH~CH~CHICH~CHICH, . . _ _ _ AI: CsHla R & RH CHICHZCH~CHZCHZCH~CHZCH~Az: CsHls ----t RH CzH4 CH3CHzCHzCHzCHzCH2+RH 2CzH4 CHaCHzCHzCHz+RH 3CzH4 CH~CHZ-
+ + ++
+
++ + Ai: C&Ils CsHts + R ++ CHaCHzCH~CHzCHzCHzCHzCH(CHz)CHaCHzCHzCHzCHzCHzCHzCH(CHz)+RH + C3H6 + CH3CH&HzCHzCHz-+ RH + CsHs + CzH4 CZH4 + CH3CHzCHfAs: CaHia
+ R +RH + CH3CHgCHzCHzCHzCHzCH(CHzCHd-+ RH + CHaCHzCH&HzCHzCHzCH :CH2+ --LHsor -+ RH + CH3CH2CH:CHz + CHzCHzCHzCHz-+ RH + CH3CHzCH:CHz + CzH4 +
+ R -+ RH + CH3CHCHzbCHz-
or
Aa: CsHis
261
I
kH3 6 H 3 CH3CHCHIC:CHz I I
+ + CH3dH3 bH3 -+ RH + (CH3)zC:CHz+ CH3CHCHZ-+RH
I
+R + -+ RH + (CH3)&CH[CH(CH&IRH + (CHs)3CCH:CHCH3 + CHIor +RH + (CH&C:CHCH(CH& + CHa-
I__
CH3CH2-
A4: CaHls
+ R +RH + CH3CHpCHzCHzCHzCH(CHzCHZCH3)+RH + CHaCH&HzCH&H:CH* + CHBCHZor +RH + CH3CH2CH2CH:CH2+ CHaCHzCHp +RH + CH3CHzCHzCH:CHZ + GH4
+ CHa-
The following is a summary of the decomposition in which the two alternative methods of decomposition in the chains As and 114 have been given equal weight; the ratio 1:2:10 has been adopted for the relative chance of reaction of a primary, secondary, and tertiary hydrogen atom, respectively. This ratio corresponds to 600' C. and would have to be diminished slightly if the decomposition in the cylinder occurred a t a higher temperature:
Thus N-octane decomposes according to four cycles; one molecule of N-octane produces either 4 or 2.5 molecules, according to whether cycles AI or AB, or cycles A3 or A? are followed, respectively; the weighted mean of all the methods of decomposition is 3.2. There is, therefore, a considerable increase in the number of molecules present when N-octane decomposes thermally : CH3 2,2,4Trimethylpentane:
Ai:
I
CH3CCH2CHCH3 I 1 CH3 CH3
+ +RH + (CH3)aCCHZCHCHzAH3 *RH + (CH3)aCCHzCH:CHz + CH3or RH + C3Hs + (CH&CCH2-+ RH + C3H6 + (CH3)zC:CHz + CH3-
CSHIS R
Summarizing as before we obtain:
+ 3(CH3)3CCHzCH:CHz + 3CH4 + 3C3Hs + 3C4Hs(iso) Ai: 9CsHis --+ 4.5CH4 + 4.5(CH,)zCHCHzC:CHsCHz+ 4.5CH4 + 4.5CaH6 + 4.5C4H1&o) Ag: 4C8Hi8 + 2CH4 + 2(CHs)aCCH:CHCHa + 2CH4 + 2(CHa)&:CHCH(CH3)2 A,: 10CaHls+ 10C4Hla(iso)+ 10C4Hs(iso) Ai: 6CaHis + 3CH4
Thus octane also decomposes according to four methods, but in this case one molecule yields only 2.5, 2.5, 2, and 2 molecules of products, respectively; the weighted mean of these is 2.26 so that the expansion of this hydrocarbon is decidedly less than N-octane on thermal decomposition; the knocking tendency has also been found to be less, which is in agreement with the theory. The final results of these calculations are given in Table I. To obtain these results i t is not necessary to work out each hydrocarbon in the detailed way given for X-octane and 2,2,4trimethylpentane; after some practice it is possible to summarize the different chain reactions by inspection and obtain the final result readily. Table I also includes the aniline numbers of Lovell, Campbell, and Boyd (6) whenever these were available. Figure 1 shows a comparison of the calculated values for the heptanes with their experimentally determined antiknock values. The parallelism between the calculated and experimental results suggests that the thermal decomposition of hydrocarbon molecules is one of the contributing factors in engine detonation. It should be possible to submit this theory to experimental test by withdrawing samples of the reaction mixture from the cylinder in the early stages of the combustion by some such method as that used by Lovell, Coleman, and Boyd (7). The products to be expected from the thermal decomposition of several paraffin hydrocarbons are given in Table 11; on the basis of the present theory these products should be formed either before or just after the mixture is sparked.
A m o N OF AXTIKNOCK COMPOUNDS I n considering any theory of antiknock action, we must distinguish carefully between two effects which can arise
----t
TABLE11. MOLESOF PRODUCTS FORMED PER MOLE OF HYDROCARBON THERMALLY DECOMPOSED (Methyl, ethyl isopropyl and tert-butyl radicals assumed to be stable: relative chances of loss of primary, secondary, and tertiary hydrogen atoms assumed to be 1:2:Id, respecti;ely; these products should be formed in the ordinary thermal decomposition of hydrocarbons a t 600' C. and under pressure.) HYDROCARBON N-hexane 3-Methylpentane 2,3-Dimethylbutane N-heptane 2,4-Dimethylpen tane 2,2,3-Trimethylbutane N-octane 2,5-Dimethylhexane 2.2,4-Trimethylpentane N-nonane 28-Dimethylheptane N-decane 2,2,6,6-Trimethylherane
CHI 0.646 0.74 0.81 0.54 0.445 0.7 0.534 0.75 0.656 0.53 0.455 0.528 0.5
CaHd 0.91 0.223
C1H6
CsHs
0.456
1.17
0 :463
1.4 0.16
0 467
1.65 0.591 1.9 0.347
0:471
0.364 0.223 0.188 0.308 0.334 0.12 0.267 0.15 0.259 0.236 0.364 0.211
... ... ... ...
0.26
:. . .
:
0 4f4 .,.
...
C;Hs
...
0 : is8
NC4Hs 0.182 0.111
:
0 : i54
...
o:ik ...
0 Ok56 0.18 0.76
...
o:iis
... ...
... ...
0 : io5
0.646
...
IsoC4Hs
... ... ... ... ...
...
0:556 0.18
...
...
... ... ...
...
0.18
0.605
0:346
...
0.5
... ...
0:455
... ... ...
IsoC4Hio
0..347
...
... ...
...
0.5
HIQHER OLEFINS 0.182 0.52 0.81 0.308 0.278 0.7
0.4 0.35 0.397 0.471 0.41 0.626 0.654
262
INDUSTRIAL AND ENGINEERING CHEMISTRY
through the addition of small quantities of catalytic substances to a reaction mixture; a catalyst may either influence the decomposition of the hydrocarbon, or i t may act to lengthen or shorten the chain reactions occurring in the oxidation processes. I n regard to the first effect, on the basis of the theory here proposed, a substance, such as a metal alkyl, when mixed with a gasoline will initiate the decomposition of the latter a t a much lower temperature. I n general the effect of this will be that secondary and, more especially, tertiary hydrogen atoms will react relatively much more easily than the more strongly bound primary hydrogen atoms, and the lower the temperature of decomposition the more marked will be this effect. I n general the result will be fewer molecules of products per molecule of hydrocarbon decomposed, and consequently less knocking. I n view of our lack of knowledge of the binding strengths of different carbon-hydrogen bonds it is not desirable to attempt to make detailed calculations. However, even without precise calculations we obtain the surprising result that hydrocarbons, such as 2,5-dimethylhexane or 2,6dimethylheptane which have tertiary hydrogen atoms near the end of the chain, should yield more molecules when decomposed in the presence of a catalyst such as tetraethyllead than in its absence; for such hydrocarbons, therefore, tetraethyllead should act as a knock inducer in so far as i t affects the thermal decomposition of the hydrocarbon. The chief requirement of an efficient antiknock coinpound lies in its ability to produce free radicals a t a temperature such that these can initiate chains and thus promote the decomposition of the hydrocarbon, If the catalyst decomposes a t too low a temperature, the free radicals will recombine with each other; if decomposition takes place a t too high a temperature, the compound will lose in efficiency through the fact that the chains involving primary hydrogen atoms will increase in relation to those involving secondary and tertiary hydrogen atoms, since the latter predominate more and more, the lower the temperature. The actual measure of this effect is determined by the activation energy of the decomposition of the antiknock compound. On the basis of this theory a great many of the compounds suggested as catalysts to lessen detonation cannot have any value because the activation energy of the decomposition reaction is far too high. Judging from empirical experimental evidence, i t would seem that an activation energy between 30,000 and 40,000 calories is required for efficiency; of the compounds mentioned in. this capacity in the literature (5) only certain of the metal alkyls meet the requirements. A further experimental test is possible through a comparison of the results of two experiments, in one of which a given concentration of an antiknock substance such as tetraethyllead is dissolved in the gasoline, and in the other of which the tetraethyllead is vaporized a t the air intake in such amount that its concentration in the cylinder is approximately the same as in the first experiment. All the other factors in both experiments must of course be kept as nearly identical as possible. If the tetraethyllead really acts as a catalyst by
Vol. 26, No. 3
inducing the thermal decomposition of the hydrocarbon at a lower temperature than would otherwise be the case, it should be more efficient in the first experiment, where it is mixed directly with the hydrocarbon, than in the second, where it is first mixed with air. There is the further possibility that some substances may exist which stabilize paraffin hydrocarbons by terminating the chains of their thermal decomposition. The effect of such a substance would be to raise the temperature a t which the hydrocarbon decomposes and in this way to favor, relatively, the reaction chains involving primary hydrogen atoms, thus causing an increase in the number of molecules of products per molecule of hydrocarbon, and therefore increasing the tendency towards knocking. S o substances which diminish the rate of thermal decomposition of hydrocarbons have so far been reported, but, in view of the property of some organic nitrites of inducing knocking, it would seem desirable to test this class of compounds for this effect. So far only the possible effect of catalysts on the ordinary thermal decomposition of the fuel has been discussed. There is, however, a second effect that the catalyst may have by affecting the chain reactions that constitute the oxidation processes. The most characteristic property of these oxidation chains is their sharply defined critical limit of pressure and of temperature, a t which the rate of the oxidation suddenly changes from a very slow reaction to a violent detonation. It seems entirely possible that catalysts introduced into the cylinder may suppress or accelerate knocking by their effect on these oxidation chains. Since such oxidation chains are usually very sensitive to the effect of surface, it is quite conceivable that lead or tellurium dust formed by the thermal decomposition of the alkyls may favor destruction of the oxidation chains and thus diminish the violence of the explosion. This hypothesis could be tested by comparing the effect of tetraethyllead with that of another compound, the thermal decomposition of which has the same activation energy but does not produce a nonvolatile dust.
LITERATURE CITED (1) Bates and Spence, J . Am. Chem. SOC.,53, 1689 (1931). (2) E~loffand Schaad, Chem. Rev., 6, 91 (1929). (3) Frey and Hepp, IND. EXQ.CHEM., 2 5 , 4 4 1 (1933). (4) Hinselwood, Trans. Faraday SOC.,28, 184 (1932).
(6) Kalichevsky and Stagner, “Chemical Refining of Petroleum,” p. 281, A. C. S. Monograph KO.63, Chemical Catalog CO,
1933. (6) Lovell, Campbell, and Boyd, (1931).
IND.ENQ.CHEM.,23, 26, 555
(7) Lovell, Coleman, and Boyd, I b i d . , 19, 373 (1927). Pease, J. Am. Chem. Soc., 51, 1839 (1929) Pope, Dykstra, and Edgar, Ibid., 51, 2203 (1929). Rice, Ibid., 53, 1959 (1931). Rice, Ibid., 55, 3035 (1933). Rice and Urey, “Treatise on Physical Chemistry,” p. 1012, Van Kostrand, 1931. (13) Steele, Nature, 131, 725 (1933). (14) Thompson and Hinselwood. Proc. Rog. Soc. (London), A125, (8) (9) (10) (11) (12)
277 (1929).
RECEIVED September 22, 1933
for pavement provided a good running surface when the road was USE OF WOOLFOUND NOTFEASIBLE IK ROAD CONSTRUCTION. Experiments with wool as a material for road construction con- dry and beaten down by traffic, but that in wet weather it colducted in Australia have not proved successful, according t o a lected on the wheels of vehicles and made progress extremely consular report from Melbourne, made public by the Commerce difficult. The experiment has now had nine months of trial and the conDe artment. $he experiments were undertaken as a result of reports that clusions are that the wool does not impregnate the soil to any exgraziers had been making use of low-grade wool for the improve- tent but rather tends t o work through and disassociate itself from ment of private roads on their properties. The Department of the soil in dry periods. Reenforcement of the soil has not been Roads of New South Wales constructed a short trial section of apparent. It is probable, as a result of the experiments, that the road in a district where the soil was typical of that of the country Department of Roads will abandon any further attempts to emroads in the state. It was found that the wool used as a binder ploy wool in road construction, the report declares.
