The flow reactor oxidation of C1−C4 alcohols and MTBE

Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 179-185

THE

FLOW

REACTOR

OXIDATION

OF

CI-C 4 ALCOHOLS

AND

MTBE

THOMAS S. NORTON AND FREDERICK L. DRYER Department of Mechanical and Aerospace Engineering Princeton University, Princeton, New Jersey, 08544 Experimental results are presented for the flow reactor oxidation of methanol, ethanol, normal- and iso-propanol, tert-butyl alcohol (TBA), and methyl tert-butyl ether (MTBE) at initial temperatures of 1020-1120 K and at atmospheric pressure. In comparison to alkanes, alcohols have a more complex oxidation mechanism, which involves the production of both oxygenated and nonoxygenated intermediates directly from the fuel. The ratio of dehydration to dehydrogenation observed depends on the molecular structure of the fuel. Primary alcohols are more susceptible to dehydrogenation than to dehydration, because of the weakness of the a C--H bonds. The direct production of aldehydes from primary alcohols causes these fuels to have much shorter reaction times than do the corresponding alkanes. By contrast, tertiary alcohols are highly susceptible to unimolecular dehydration. Since the dominant intermediates are alkenes, the chemistry closely resembles that of non-oxygenated hydrocarbons. Secondary alcohols react both by dehydration to alkenes and by dehydrogenation to ketones. MTBE decomposes directly to methanol + isobutene. These findings are summarized in a general mechanism for alcohol fuel oxidation. Methanol is observed to have the unique property that nearly half of its heat release occurs before the CO peak.

Introduction Studies of homologous series of fuels elucidate the oxidation pathways that are common to related fuels, and thereby make possible the prediction of the oxidation mechanisms of larger fuels in the same series.1 The current study uses this approach to explore the chemical pathways of alcohol oxidation. Data are presented from flow reactor oxidation experiments for five simple alcohols (methanol, ethanol, n-propanaol, i-propanol, and t-butyl alcohol (TBA)). Data for one ether (MTBE) are also presented, since it is closely related to TBA. Based on these experimental results, a general mechanism is developed to describe the oxidation characteristics of alcohol fuels. Only one similar study of the combustion of a series of alcohols has been reported previously. 2 The order of bond energies3 in alcohol molecules is typically O---H > C - - H > C---O > C---C. The strengths of C---O (92-93 kcal) and O---H (103-105 kcal) bonds are similar for all alcohols, including primary (methanol, ethanol, n-propanol), secondary (i-propanol), and tertiary (TBA) structures. The strengths of C - - H bonds in alcohols are more variable, becoming weaker in the proximity of the electrophilic O atom. The two most obvious differences between alkanes and alcohols are the reduced symmetry and reduced C - - H bond energies of the alcohols. The reduced symmetry of alcohol molecules rel179

ative to hydrocarbons increases both the variety of their decay pathways and the variety of the intermediate species which they produce. Whereas alkanes may undergo only dehydrogenation (loss of H by abstraction) or decomposition (bond cleavage) reactions, alcohols may also undergo dehydration (loss of OH or H20). Both the dehydrogenation of. alkanes and the dehydration of alcohols produce alkene intermediates (or methyl radicals), which are subsequently oxidized to aldehydes in reactions with O atoms, Oz or HOz. Dehydrogenation of an alcohol, however, leaves the O atom of the original molecule intact and therefore leads directly to the production of an oxygenated intermediate (aldehyde or ketone). These processes are summarized in Figure 1. Since the oxidation of alkenes or CH3 to form aldehydes is typically slow, the ability of alcohols to bypass this step is expected to decrease their overall reaction time. The magnitude of the difference between alcohol and a|kane chemistry depends on the branching ratio of dehydrogenation and dehydration pathways for a given fuel (Fig. 1). If an alcohol reacts primarily by dehydration, then its chemistry should closely resemble that of an alkane. If, on the other hand, dehydrogenation occurs to a significant extent, then the alcohol's chemistry should differ markedly from alkane chemistry, because aldehydes or ketones will appear as principal reaction intermediates. Both aldehydes and ketones are more reactive than alkenes. Aldehydes contain a weak C - - H

180

REACTION KINETICS ethanol + X--->CHzCH2OH---~ C2H4 + OH

Primary Alcohol

n-propanol + X---~CHaCHCH2OH---~ C3H6 + OH De-H De-OH

Alkane

i-propanol + X--->CHaCH(OH)CH2---~ C3H6 + OH.

> Alkene

> Aldehyde

> CO - - >

CO:

Secondary Alcohol

>

Ketone

De-H

The role of H-atom abstraction from the OH group of alcohols remains unclear, but may be larger than previously thought. 9 Such a process is expected to produce both a hydrocarbon radical and an oxygenated species: methanol + X ~ CH30 + XH;

FIG. 1. A comparison of the possible reaction pathways (dehydrogenation (De-H), dehydration (DeOH), and oxidation (Ox)) for alkanes and primary and secondary alcohols.

CHaO + M ~ formaldehyde + H ethanol + X ~ CHaCH20 + XH; CHaCHzO ~ formaldehyde + CH3 n-propanol + X ~ C2HsCH20 + XH; C2HsCH20 ~ formaldehyde + C2H~

bond in the HCO group, which makes them very susceptible to radical attack. 4 H atom abstraction from an aldehyde leads quickly to CO formation. Ketones are similar to alkanes in molecular structure and bond strengths, but their reactions lead to CO formation via the intermediates CHACO and CH2CO, rather than through an aldehyde intermediate. 5,6,7 The relative weakness of the a C - - H bonds in an alcohol molecule (those attached to the same carbon atom as the OH group) encourages the formation of oxygenated intermediates: methanol + X--->CH2OH + XH; CH2 OH + 02 ~ formaldehyde + HO2 ethanol + X ~ CHaCHOH + XH; CHaCHOH + 02 ~ acetaldehyde + HO2 n-propanol + X ~ C2HsCHOH + XH; C2HsCHOH + 02 ~ propionaldehyde + HO2 i-propanol + X ~ (CHa)2COH + XH; (CHa)2COH + O2---> acetone + HO2. Note that while the O - - H bonds are the strongest bonds in the original alcohol molecules, they are weakened by the attraction of the unpaired electrons in the dehydrogenated radicals. These radicals react very readily with 02 to produce an aldehyde or ketone and HOz. Beta C - - H bonds are stronger than a bonds, but are more numerous in molecules such as ethanol and i-propanol. H-atom abstraction at a fl carbon site can lead to dehydration of the alcohol radical by /3-scission of the OH group, producing an alkene. s

i-propanol + X ~ (CHa)2CHO + XH; (CHa)2CHO ~ acetaldehyde + CHa TBA + X ~ (CHa)aCO + XH; (CH3)aCO --->acetone + CH3. The experimental data presented in the following sections show that the ratio of production of oxygenated to non-oxygenated intermediate species from alcohol fuels varies dramatically with fuel structure.

Experimental Results Experiments were performed using the Princeton atmospheric pressure flow reactor, which has been documented elsewhere, l~ Gas samples extracted at fifteen positions along the reactor duct. centerline were quenched in the hot-water-cooled probe and stored at 70 C for later gas-chromatographic analysis. The heated sample storage system, which includes a multi-port valve for automatic operation and stainless steel storage loops, 12 represents a significant improvement over the method used in earlier methanol studies. 13 The purifies of the five alcohol fuels .were greater than 99%; the MTBE purity was 97%. All fuels were used without further purification. The experimental results are presented in Figures 2-7. The conversion of reactor duct distances to elapsed gas residence times was made on the basis of hot-wire anemometry data. The "C4"', "C5" and "Cfi" profiles shown in Figures 4-7 represent small, unidentified GC peaks with retention times characteristic of non-oxygenated 4- to 6-carbon species. The C4 species are the same for n- and i s o propanol, while the C6 species are different. Fur-

FLOW REACTOR STUDY OF ALCOHOLS AND MTBE 1200

1.0

181

1.0

~I1250

O.B

12oo ~

0.8

1150

~

~

0.6

0.6

~o.4 11oo

~

t100

O.B 20.4

_~,-'~1 to5o 10

1050

BO

30

40

50

60

30

40

50

60

30 40 TIME {msec)

50

60

350

0.2

H6

300

250 o_ 200 0

20

3~i

40

60

0

CH4

~ ~

~OO 50

o~ to

0

15o x

~B~I 0

iO00 iO0

80

'

.

20

~ ~ ' - ~ . 40 TIME

60 (msec)

A,

0

. . . .

TEM~/~CB2 fp

1250

30.S

OB xB

20

~ 5 O0

1.0

~0

~ ~0

1300 TOTAL CARBON .

20

I00

BO

FIG. 2. Flow reactor results for methanol oxidation at T~ = 1031 K and q~ = 1.18. The H20 profile is calculated by H atom balance. 1.2

tO

-

FIG. 4. Flow reactor results for n-propanol oxidation at T~ = 1084 K and 4, = 0.64.

1200

CLO.5 1150

0 XO.4

II00 0.2 1050

12OO(

10

20

30

40

50

I0

20

30

40

50

10

20 30 TIME (msec)

40

50

t000

800

BOO d x

400 200

0

~ 1Bi

0" 0

ther details of the experimental methods, including gas chromatograph calibration methods, are available elsewhere, la Conditions for most of the experiments were chosen such as to show complete fuel disappearance within the duct residence time. This choice was made for purposes of surveying the oxidation phenomenology of these fuels, the majority of which had never before been studied in the flow reactor. As a result, significant amounts of fuel decay occurred in the diffuser for some fuels. Also, the data for methanol, ethanol, and n-propanol exhibit large temperature increases and concentration gradients in the vicinity of the CO peak. Diffusional transport should be considered 14 if these experimental data are to be compared with modeling results in future investigations.

Methanol

FIG. 3. Flow reactor results for ethanol oxidation at Ti = 1092 K and 4~ = 0.61. The H~O profile is calculated by H atom balance; 02 is calculated by O atom balance.

Methanol has been the most extensively studied of all the alcohols. 14 The current results (Fig. 2) are presented here primarily for comparative purposes. Under a wide range of experimental conditions, methanol oxidation proceeds by successive dehy-

182

REACTION KINETICS

1400

-

i-C3H70H

_

1140

~

~_-4~'TEMP

1600 /IdCO x.5

1030

1400

1120

1200

9 =

= =TEMP

:

= :

:

~

:

1200 I000

t100~ rn

BOO

>=

x

600

~1000

t020

Q BOO

1080 c 600

400

CO

4O0

I010

x2

1060

200 0 200 q

20O

20

40

60

80

1040

20

100

40

60

CH3CHO

60

~

3

B0

H

100

4

150 C2H2

/

~_40

1oo

~

20

.

5 csCb2H4

20

40 TIME

60 (msec)

BO

0

t00

FIG. 5. Flow reactor results for i-propanol oxidation at T~ = 1121 K and q5 = 0.78. Concentrations are scaled for total carbon = 1.0% throughout, because of fuel supply fluctuations during this experiment.

20

40 TIME

60

80

t00

(msec)

FIG. 6. Flow reactor results for TBA oxidation at 1027 K and ~b = 0.67.

T~ =

2500

t040 ~TEMP

2000

-

-

I020

x 1500

drogenation of CHaOH ~ CH20 ~ CO, followed by oxidation of CO to CO2. Because methanol has no /3 carbon sites, dehydration can occur only by C - - O H bond cleavage. Only very small amounts of CH4 or other hydrocarbon species are observed in methanol oxidation, except at the highest temperature conditions. It has been shown recently I~'16 that the sources of CH3 in methanol oxidation and pyrolysis include CH30 + CO = CH3 -4- CO2

Q

g8o N

~

5OO

-

.

~

I

H

4 3H(

20

40

60

B0

IOO

940

C3H4 100

C5

BO

CH30 + H = CH3

+ OH 2H,4

CH2OH + H = c a 3

+ OH

4O

6 C02

and

CH3OH -I- M = CH 3 -4- OH + M,

but not CH3OH + H = CH3 + H~O, as had been previously suggested. Methanol chemistry is dominated by reactions of HO2,15 produced in the fast reaction of CH2OH with 02.

0O

~0

40 riO TIME (msec)

BO

~.00

FIG. 7. Flow reactor results for M T B E oxidation at T i =

1 0 2 4 K a n d ~b =

0.96.

FLOW REACTOR STUDY OF ALCOHOLS AND MTBE

Ethanol Fewer studies of ethanol than of methanol chemistry are available, 14 and even fewer offer species concentration evolution data. The previous results demonstrate that the ratio of ethanol dehydration to dehydrogenation depends strongly on temperature. Whereas low temperature static reactor studies 17As found acetaldehyde to be the dominant product of ethanol oxidation, the diffusion flame study of Smith and Gordonz observed significantly more ethene than acetaldehyde. In the current data (Fig. 3), acetaldehyde, ethene and methane are all major intermediate species. Since acetaldehyde is more reactive than ethene, the approximate equality of the concentrations observed here indicates that dehydrogenation of ethanol is significantly faster than dehydration under flow reactor conditions. A recent modeling study 14A9 has shown that ethanol oxidation, like methanol, is dominated by HOz reactions and that all three of the isomeric radicals CH2CH2OH, CHaCHOH and C2H50 play important roles in the reaction mechanism. This modeling study also suggests that significant amounts of formaldehyde are produced in ethanol oxidation. However, formaldehyde was not observable in the current ethanol or propanol experiments, due to a problem with CH20 polymerization during gas sample storage.

183

ethanol oxidation (Fig. 3). In each case, the fuel is completely consumed within 40 msec and the reaction is complete to COg within 60 msec. By contrast, isopropanol oxidation is much slower, even at an initial temperature 40 K higher (Fig. 5). The difference in overall reaction times for the two propanols is due primarily to the difference in reactivity of the major intermediate species for the two fuels. The acetone produced by isopropanol is less reactive than the propionaldehyde produced by n-propanol. Isopropanol has more/3 C - - H sites than n-propanol and therefore produces more C3H6. Isopropanol also produces less acetaldehyde than does n-propanol.

TBA and MTBE Previous studies of the pyrolysis of TBA and MTBE have shown that both of these fuels decay by a unimolecular mechanism over the temperature range 400-1300 K. It is generally agreed that these unimolecular reactions proceed through a four-center activated complex. 24-~7 The products are isobutene and either water or methanol: H3C H3C--C...OH --->i-C4Hs + HzO H2C.. 9H

N-Propanol and I-Propanol The literature on propanol chemistry is limited to a very few studies of pyrolysis in low-temperature static reactors2~ and in a diffusion flame. 2 No previous oxidation studies have been reported. The previous studies of n-propanol z'zl observed the major hydrocarbon intermediate species to be ethene, methane, propene, acetylene, ethane, acetaldehyde, and formaldehyde. The same species, with the exception of formaldehyde (as explained above) and with the important addition of propionaldehyde (C2H~CHO), are observed in the current data (Fig. 4). Since propionaldehyde is very reactive, the observation of even small quantities of this species indicates that it is an important intermediate during the early stages of fuel decay (the concentration peak occurs prior to the first sampling point). Propionaldehyde is produced following H abstraction at the a carbon site of n-propanol, as discussed above. The observation of propene indicates that some H abstraction also occurs at the /3 site. The decomposition of propionaldehyde2a is likely to be a major source of the C2H4 observed in the current samples. Possible additional sources of ethene include H abstraction at the CH3 or OH sites (see Fig. 8 and Refs.2"21). The general characteristics of the lean n-propanol oxidation of Figure 4 are similar to those found for

and H3C

I

H 3 C - - C . . . O - - C H 3 --->i-C4Hs + CH3OH HzC.. 9H The current experimental data (Figs. 6-7) show that isobutene is a dominant intermediate species in the oxidation of both fuels and that CH3OH is also prominent in MTBE oxidation. (H20 was not measured.) Fuel decay rates are essentially first order after the first 10 msec of residence time. Although TBA decay is considerably slower than MTBE decay, both are fast compared to the evolution of final products. These results suggest that TBA and MTBE decay primarily by the same unimolecular elimination mechanism in both the presence and absence of oxygen. However, the observation of small amounts of acetone in these experiments indicates that secondary fuel decay routes play a minor role as well. z4"zs

Discussion The experimental data presented above show that all alcohols undergo both dehydrogenation and de-

REACTION KINETICS

184

hydration reactions under conditions of intermediate temperature oxidation. The ratios of these processes differ dramatically among fuels of different molecular structure, but all alcohols yield some of both oxygenated and non-oxygenated species as intermediates. Even methanol, which reacts almost entirely by dehydrogenation to formaldehyde, produces some methane. Likewise, even TBA, which reacts almost entirely by unimolecular dehydration to isobutene, produces some acetone. The fundamental processes of H-atom abstraction and unimolecular decomposition that lead to the formation of aldehydes, ketones, and alkenes in alcohol oxidation are summarized in Fig. 8. Primary alcohols (methanol, ethanol, and n-propanol) produce more aldehydes than alkenes, because of the weakness of the C ~ H bonds at the a carbon site. Secondary alcohols (isopropanol) produce more alkenes than ketones, because they contain many more /3 C - - H bonds than t~ C - - H bonds. Tertiary alcohols (TBA) produce almost exclusively alkenes, because their molecular geometry favors the unimolecular dehydration reaction. While methanol conforms to the reaction scheme presented in Fig. 8 (formaldehyde is produced by the abstraction of H from either the CH 3 or the OH group), it also possesses a number of unique characteristics that are worthy of mention. First, the sole carbon atom in methanol exists in an already partially oxidized state. The C---O bond is rarely broken in methanol reactions and only trace quantities of any hydrocarbon intermediates are formed. Second, the major intermediate species, formaldehyde, is more reactive than the fuel itself. Therefore, the concentration of CHzO always remains lower than that of CHaOH. By contrast, the al-

kenes anad ketones formed in the oxidation of other fuels quickly surpass those fuels in concentration. Third, a much higher proportion of 02 is consumed during the early portion of fuel decay in methanol oxidation than in the oxidation of other fuels, due to the very high reactivity of the CH2OH radical toward Oz in the reaction CH2OH + Oz = CH20 + HO2. The net result of the special characteristics of methanol listed above is that nearly half of the heat release in methanol oxidation occurs before significant oxidation of CO begins. This result is in distinct contrast to those for all other hydrocarbon fuels. Figure 9 shows that the early heat release in methanol oxidation is associated with the fact that the exothermie production of H20 is nearly a linear function of fuel consumption. Even ethanol shows much slower evolution of HzO and heat during fuel decay. Despite the occurrence of the very fast reaction CHaCHOH + O~ = CHaCHO + HOz in ethanol oxidation, less H20 is formed because a significant percentage of the H atoms resides in the ~.O HEAT RELEASE 0.[3 1

== ,,>,

u._ 0.2

~ 0 C.O C

R1

~

~

~

o

P

o.,

H20 g

~

,

0.6

~

o 0.8

~ 3.0

, @ ~ ]

0.8

H 0.6

~

h

Aldehyde Alkene

Alkene (orHorCH3)

._/o

_,.

i

...........,.o, /

g

Ab~ractlon

FIG. 8. General mechanism for alcohol oxidation. R denotes any hydrocarbon radical (or H).

:

LL O. i

)

i

i

0.2 0.4 0.6 O.B EXTENT OF FUEL CONSUMPTION

~

3.0

FIG. 9. The fractional evolution of heat and of H20, as functions of fractional fuel consumption, in the methanol, ethanol and n-propanol oxidation experiments of Figures 2-4. Initial temperatures are calculated by extrapolation of each temperature profile to zero extent of fuel decay.

FLOW REACTOR STUDY OF ALCOHOLS AND MTBE unreactive hydrocarbon intermediates CgH4 and CH4. Furthermore, some of the HgO forms via the endothermic dehydration process: 8 CgH5OH + OH = CHgCHgOH + HgO CHgCHgOH = C2H4 + OH, rather than by the exothermic consumption of Oz. Therefore, it is the combination of the fast production of HOe (via CHgOH + 02) and the low concentrations of CHgO and all hydrocarbon intermediates that gives methanol the unique characteristic of continuous heat release.

Conclusion In summary, experimental data obtained for a homologous series of oxygenated molecules has led to the development of a general mechanism for alcohol fuel oxidation. This mechanism shows the results of H-atom abstraction at each C - - H bond in an alcohol molecule and the processes of formation of aldehydes, ketones, and alkenes as intermediate species. The oxidation characteristics of larger alcohol molecules may be predicted on the basis of these results. In general, the longer the chain of an alcohol molecule or the more highly branched its structure, the greater is its hydrocarbon character. The smaller the alcohol, the greater its deviation from alkane chemistry.

Acknowledgments The authors are grateful to Joe Sivo and Don Peoples for their technical assistance with the experimental work. This work was supported by a National Science Foundation Graduate Fellowship (TSN) and by the U.S. Dept. of Energy, Chemical Sciences Division, Office of Basic Energy Sciences, under contract #DEFG02-86ER-13503.

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185

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