The formation of acetone in the oxidation of neopentane

Combustion and Flume

31

The Formation of Acetone in the Oxidation of Neopentane R. R. Baker, R. R. Baldwin, and R. W. Walker Chemistry Department, The University, thdl. Enghmd

When neopentane is added in slrlall amounts to slowly reueting mixtures of 1.1:and O: ill ~lged hori,z-~lcid-eoated reaction vessels at 480 'C, the main primary products ;ire 3.3-dimethyloxetane. aeclone, isobuten¢, methane, and Ibrmaldehyde. In the early ~t~lgesof re~tetion, the r~ltios [aeetone]/[3.3-dimethyloxet~me] and ([acetone] + [3,3-dimethyloxctan¢])/ [isobutene] are ~lhnosl li0e;lrly dependent o11 the pressure o f O a in tile mixture. A nlechllnlsm is proposed lbr the I'ornltltlon of the products, and the ~lllalytitrulevidence, ohtailled over a wide range of illlxttlre composition, suggests their ac:¢tolle is Ibrmed through the (CH:~)~CCI-I~(OOH)CH~OO radical. Mechanisms given by indepeftdent workel's tire ex~mlined in the [ighl of the presenl resulD.

Introduction Several workers [ I - 4 ] have reported the formation of acetone as an important product during the oxidation of ncopentane in the temperature region of 300°C, Several mechanisms have been suggested to explain the formation of acetone, but there is little supporting evidence for any of them. Product analysis has been performed at only one mixture composition, so that the kinetic consequences of the mechanisnr have not been tested. This is partly because one of the diftieulties of studying hydrocarbon oxidation is that both the nature and the relative concentrations of the radicals involved are controlled by tire hydrocarbon and its oxidation products. As a consequence, the relative importance of reactions that are zero order, first order, and second order in radical concentration will change continuously during the course of the reaction. Recently, hydrocarbons have'been added in

trace amounts to slowly reacting mixtures of H z and Oz in aged boric-acid-coated vessels at temperatures near 500°C [5]. The course of oxidation is effectively controlled by the H 2 + 02 reaction, and large changes in the concentrations of Hz. O2, and hydrocarbon can easily be made to test the kinetic consequences of the mechanism suggested. This approach allows estimates to be made of the relative importance of attack of the radicals H, O, OH, and HO 2 on the hydrocarbon. By making a detailed examination of the reaction products. thercactions of the alkyl radical in a Hz + O, environment can also be elucidated, By plotting the absolute percentage of the individual products in the mixture against the extenl of reaction, as measured by pressure change due to H.,O formation, or against the percentage of alkane lost, it is possible to distinguish primary, secondary, and tertiary products. Study of the addition of neopentane is partic("¢md~stMn t~ b?anlt', 14, 31 3fl {IL~Tt~l ('~pyrlghl t' 1971)b~ TIle Cotllbttslion Insllnllc Puhlblled hy American Elsevier l)ublishing ('onlp;uly, 11112.

R, R. Baker, R. R. Baldwin,and R, W. Walker

32 ularly profitable, since all the hydrogen atoms are equivalent, and thus reaction paths can be traced with relative ease. A detailed account is being prepared for publication, and this paper is concerned mainly with the mode of formation of acetone.

Experimental The apparatus and general procedure have been described elsewhere [6]. A standard mixture has been used, containing 0.28 and 0.14 mole fraction of Fit and O=, respectively, the remainder being N 2. Independent variation o f H z and O2 can thus be achieved by interchange wi'th N,. Pressure changes were followed by u:;ing a Southern Electronics Laboratories S.E. I:150 pressure transducer, which gave an output o!~1 mV for a pressure differential of 2 mm Hg. A~I compounds were estimated by means of gas chromatography, except formaldehyde, which was determined co]orimetrically using chromotropic acid [7]. For the work described here, an aged boric-acid-coated reaction vessel. 55 ram in diameter, was used at 480°C.

Results and Discussion The main primary products obtained on addition of neopentane are 3,3-dimethyloxctane, isobutene+ acetone, methane, and lbrmaldehyde. With a mixture containing 0,28, 0+71+ and 0.01 mole fraction of H 2. 0 2. and neopentane, respectively, the concentration profiles shown in Fig. I are obtained. Other primary products such as isobutyraldehyde and methacrolein are detected in small amounts, the sum of these being about 5 per cent of the total neopentane lost [8"~.Methane and acetone have profiles that are slightly autocatalytic in form, and presumably these compounds are also formed as secondary products. For instance, methane and acetone are formed as primary products when isobmene is used as an additive [8]. As the primary products may be quite reactive and may

o~

I]u ~l~mmIml. Figure 1. Primaryproducts in the oxidation of neopentane st 480~C+ (CH~t+C=5, O.-~355. H== 14~1 mm Hg, ~, CH+ (× ]o):r~, HCHO: ~.. CH~COCH~: O+ 3,3-dimethyloxetane;X, isobutene,

also be formed Jn secondary reactions, it is important to determine their concentrations in the early stages of reaction, and in Table 1 the concentrations have been given for 5 per cent loss of neopentane. The ratio [3,3-dimethyloxetane]/[isobutene] increases with the mole fraction of Oz to a power just less than unity, but is independent of the mole fraction of Hz. Furthermore, the ratio ['acetone]/[3,3-dimethyloxetane] increases almost linearly with the O z mole fraction, but is independent of the H a mole fraction. Thus, kinetically, the simplest scheme for the formation of the products is np--*isobutene + CH 3 rip02 acetone

3,3-dimethyloxetane

where np is the neopentyl radical, Based on the kinetic evidence, the following mechanism may be suggested for the formation of acetone, 3,3-dimethyloxetane, isobutene. methane, and formaldehyde in the early stages of oxidation :

The Formation of Acetone in the Oxidation of Neopentane (CHa)aCCH,, - - ~ (CH3)~CCH202

i (CHa)2C~-----CHz

I

+

--~

CH 2

.yCH3~ CH,~

(CH3)2C--CH 2

(CH3)2CCH2OOH

I

I

+OH

H~C--O

k.I02

HCHO

(CH3)~C--CH2OOH O

J H2C--O

I CH3COCH 3 + 2HCHO + OH Confirmation of this mechanism is achieved in three ways. First, since the mechanism requires that d[acetone]/d[3,3-dimethyloxetane] = k,[O:]/kb, a plot of [aeetone]/[3,3-dimethyloxetane] against [02] should be linear through the origin for the early stages of reaction. Figure 2 confirms this, and also shows that there is no effect of varying the H : mole fraction. The slight curvature may be due to secondary formation of acetone from isobutene,

which may slightly raise the points at low O : concentrations, where the concentrations of isobutene will be relatively high Second, since two molecules of formaldehyde are formed with each molecule of acetone, and CH 3 radicals are expected to give either formaldehyde or methane, then in the early stages of reaction [ H C H O ] = 2[acetone] + [isobutene] - [CH~]

Table I. Absolute Peremltage of Products at 5 Per Cent LOSSof Ntmpentune

Absolute percentageof neopentane = 1.00, T= 480°C Mixture Composition, %

Absolute Pe~'centageof Products at 5% Loss of Neopentane

H:

O~

3,3-dmo"

28.0 28,0 85,0

14.0 71.0 14.0

0.0209 0.0185 0.0200

Acetone

Methane

0,0038 0,0150 0.0039

0.0170 0.0024 0.0165

[sobutene 0,0270 O.OIO0 0.0220

HCHO

HCHO.

(cale.)

O.O163 0.0280 O.O113

0.0176 0.0376 0.0133

3.3-dmo = 3,3-dimethyloxetane. Table 1 shows that the agreement is quite good, since the experimental value of [ H C H O ] will be lower than expected for two reasons: I. Because of its reactivity at 480'~C, some formaldehyde will be lost even in the early stages of reaction.

2. Recent studies at 500'~C. involving the addition of methane to slowly reacting mixtures of H, + Oz, have shown that CH~ radicals may form methanol as well as methane and formaldehyde [9] under the conditions used in this type of work. Unfortunately, flame-ionization

R. R, Baker, R. R. Baldwin.and R, W, Walker

34

.....

Y. . . . . . . .

l.................. I................

/z ''/ T

0lj//

i

!

/,,2' J ............... ,~o .....

~o

~iv~n Pm~m mn HI.

,~',~ Olygen Pl~a m~HO.

Figure 2. Plot of [acetonel/[3.3-dimethyloxetane] against the pressure of Oz. (CH3)aC=5 mm Hg. X. O~=70. H: = 355 tara Hg: O. H: = 140.varyingO,.

Figure 3, Plot of [acetone+3.3-d:rvethyloxetane]/[isobutene] against the pressure of O2. ((.'H~)~C= 5 mm Hg. ~'~. O, = 70, H2 = 355 mm I'[g:G. I-I~= 140.O: varying,

detectors are relatively insensitive to methanol, and its presence ii~ small ar~lounts in the neopentane studies may not have been detected.

three ways by different workcrs. Because of the large temperature difference between the present studies and those at lower temperatures, it is not possible to state categorically that the other mechanisms suggested are incorrect, but certainly these mechanisms cannot apply at 480"C. Zeelenberg [ I ] suggests the lbllowing mechanism for the formation of acetone:

Third, the mechanism predicts that _d([acetone] + [3,3-dimethyloxetane])= k~[O,] d[isobutene] /c a Figure 3 shows that the plot of the left-hand side of the equation against [02] is closely linear: the slight falloff at high O z pressures may be due to interference by secondary processes. Variation of the H 2 mole fraction produces no effect on the product ratio. There is thus strong evidence for the mechanism suggested and, m particular, for the hypothesis that the acetone is formed from the (CH3)zC'CHz(OOH)CH2OO radical. The formation of acetone in the oxidation of neopentane at about 300'~C has been explained in

(CH3)3C--CHz--->-(CHz)3C--O

'

+ HCHO

1

O--O CH3COCH 3 + CH 3 However, as 3,3-dimethylo×etane is also formed from the (CH3)3CCH2OO radical, the ratio [aeetone]/[3,3-dimethyloxetane] should be independent of mixture composition. Antonik and Luequin [2] give the following scheme:

(CH3)3CCH20.~ R-~E~"(CH3)3CCHyC)2H----~(CH~)3CCH,O (CH~)3CO2H ~-RI-L-~ (CH3)3COz

~

OH + (CH3)3CO---,CH3COCH 3 ~- CH 3

(CH3)3C

+

+

OH

HCItO

35

The Formationof Acetonein the Oxidationof Neopentane With this scheme, the only way an oxygen dependenc e can be introduced into the [acetone]/ [3,3-dimethyloxetane'l ratio is to assume that the (CH3)3C radical reacts only occasionally with 02 to give the (CH3)zCO 2 radical, so that only an oxygen-dependent fraction of the (CH3)3C radical leads to acetone. It is difficult, however, to find a plausible alternative reaction for the (CHa)~C radical. Drysdale and Norrish [3] consider that acetone is formed as a secondary product, although examination of their data suggests that it is also lbrmed in a primary process. With their reaction scheme, it is not possible to obtain the correct O z dependence for the product ratio at 480'~C. The formation of acetone through the (CH 3)2CH.~

I

CH3--C-~-CH:OOH ~

I

H,CO0

CCH2(OOH)CH2OO radical is of considerable interest since, although there is ample evidence [10-12] for the existence of this type of radical at temperatures near 300°C, it is generally considered that it is unlikely to be formed above 400°C. Further work is in progress to confirm the existence of the (CH3),CCH.,(oOH)CH2OO radical and similar species at high temperatures, and two other important facts may be reported here. First, with neopentane, the [methacrolein]/[acetone] ratio is independent of mixture composition. The simplest exphmalion of this fact is that the (CH3)2CCH 2(OOH)CH,OO radic~I can decompose in two ways, giving either acetone, as above, or methacrolein : CH 3

{

CH3--C--CHOOH ~

CH~--C--CHO

l

{I

H2COOH

Second, when isobutane is used as additive, acetaldehyde and 3-methyloxetane are formed in small amounts as primary products, and the: ratio [acctaldehyde]/[3-methyloxetane] is proportional to the pressure of O, used. These

H2C

observations may be explained by considering that the (CH3):CHCH, radical undergoes reactions similar to those involved in the formation of acetone and 3,3-dimethyloxetane from neopentyl radicals. CH2OOH

{

(CH 0:CHCH ~--~(CH,0~CHCH 2 O O ~ C H

3C--- H -----~CFI.~CH --CH 2

I

HzC CH2OOH

I

CH ,CHO + 2HCHO + O H ~ C H 3 C - . H

{

FI.~COO

P

H2C ~-0

36

This work was supported in port b y the United States Office o f Scientific Research under Grant A F E O A R 68-0013. through the European OJ$ce o f Aerospace Research, United States Air Force.

References I. ZEELENnERG,A. P,, Rec. Troy. Chim,. 81. 720 (I962). 2, AN~ONIK,S.. and LUCQUtN,M.. Bull. Soc. Chim., 2796 (1968). 3. DRYSOALE,D. D.. and NORRISr[. R. G. W.. Pl'oc. Re)'. See. (London), Ser. A. 308. 305 (1969). 4. KNOX,J. H . Advan. Chem. Ser.. 2. No. 76, I (1968).

R. R. Baker. R. R. Baldwin, and R. W. Walker 5, BALDWIN,R. R., EVERETT,C. J., HOPKINS, D. E., and WALKER, R. W., Advon. Chem. Ser,, 2. No. 76, 124

(1968), 6. BALDWIN.R. R., and SIMMONS.R. F.. TrciiM, Faraday See.. 51. 680 (1955). 7. BP,ICKER, C. E., and JOHNSON, H. R., Ind. Eng. Chem. (Anal.). 19, 400 (1945). 8. BAKER, R. R.~ BALDWIN.l~.. R.. and WALKER,R. W., unpublished work. 9. BALDWIN.R. R.~ LONGTHORN,D.~ and WALKER.R. W., unpublished work. 10. FISH. A.. Prec. Roy. So¢. (London), Ser. A. 298, 204 (1967). [I, CARTLIDGE.J. alld TIPPER.C. F~ H., Prec. Roy. Soc. ( Lomton ), Set. ~1, 26L 388 (1961L 12. KNOX,J. H.. Cou~bustlon &Flamc. 9. 297 ( ]965),