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Product formation in the slow oxidation of isobutane
COMBUSTION AND FLAME 40: 221-224 (1981)
221
BRIEF COMMUNICATION Product Formation in the Slow Oxidation of Isobutane G. McKAY and J. A. AGA Department of Industrial Chemistry, The Queen's University of Belfast, Belfast, BT9 5DL, Northern Ireland
Although the oxidation of isobutane has been studied extensively [1-6], no mechanism has been proposed to explain the formation of all the primary products or the relative quantities produced, The effects of varying the reaction vessel surface have been studied and a mechanistic step has been proposed to explain the formation of acrolein and methacrolein. The apparatus and experimental techniques have been previously outlined [3] and four sets
able. The scheme proposed that butylperoxy radicals are formed homogeneously and these are followed by the heterogeneous decompositions of the peroxy radicals into a complex and surface-sensirive mixture of products. Steps 2 and 3 outlined the scheme.
of experiments were performed in packed and unpacked Pyrex or quartz reaction vessels using a conventional static vacuum system. The results for the four series of oxidations are shown in Table 1. The first major attempt to explain the primary product formation during the oxidation of isobutane proposed that isobutene was formed by the homogeneous bimolecular reaction shown in step 1. C4Hs + 02 ~ C4Ha + HOz. (R1)
(R3)
The minor products were thought to be formed by the homogeneous intramolecular decompositions of isobutylperoxy and t-butylperoxy radicals. Hay, Knox, and Turner [2] observed that coating the reaction vessel wall greatly affects the kinetics of the process, hence the distribution of the minor products is highly sensitive to the nature of the reaction vessel walls. They found that during the oxidation ofisobutane between 250 and 350°C at low conversion that, although the percentage of isobutene is little affected, reactant pressure and temperature have a profound effect on the distribution of the minor products, and therefore the basic hypothesis outlined by Zeelenberg is untenCopyright © 1981 by The Combustion Institute Published by Elsevier North Holland, Inc. 52 Vanderbilt Avenue, New York, NY 10017
C4H9 + 02 ~ C4H9OO
(R2)
C4H9OO ~ Complex mixture of minor products.
Earlier work by McKay et al. [3] on the oxidation of isobutane around 600°K in the presence and absence of nitrogen and carbon dioxide, using a quartz reaction vessel, showed the initial yield of isobutene is 60-70%, and addition of inert gas lowers the ratio [isobutene]/[oxygenated products], but not systematically, indicating that simpie formation of minor products at the surface is untenable. Previous workers have explained the formation of most primary products observed during the oxidation of isobutane. However, the reactions of tbutyl hydroperoxy alkyl radicals enable the formation of acrolein and methacrolein to be explained, for example, k2 (1, 4p) C4H9 + O2 ~- t - - C 4 H 9 0 2 ~ QOOH k-2 k3 acrolein + H202 02 -~' + CH3OH -~ O2QOOH surface "~ methacrolein + H202 + OH (4)
0010-2180/81/02221+0452.50
222
G. McKAY and J. A. AGA TABLE 1 Initial Percentage of Products to 1% Isobutane Consumption
Temperature (*C) Reaction Vessel Packing Isobutane (tort) Oxygen (torr) Surface:Volume Product Acetone Propionaldehyde Isobutyraldehyde Acetaldehyde Methacrolein t-Butanol Isobutene oxide Methanol Propylene oxide Acrolein Isobutene Propylene Ethylene
Series A
Series B
Series C
Series D
300 Quartz Unpacked 150 75 0.08:1
300 Pyrex Unpacked 150 75 0.08:1
300 Quartz Packed 150 75 1.7:1
300 Pyrex Packed 150 75 1.7:1
8.00 4.80 3.10 2.30 1.87 1.37 0.64 0.60 0.04 0.46 73.00 2.40 0.07
3.00 2.50 3.50 0.52 0.25 0.15 3.20 0.04 0.18 0.25 79.00 5.00 1.00
9.50 0.40 0.32 1.20 3.00 1.20 1.50 0.60 0.25 2.10 78.00 2.00 0.03
2.40 3.30 3.40 0.15 0.50 0.14 0.50 0.20 0.28 0.60 84.00 4.00 0.80
Thus the two rearrangements of the OOQOOH radical offer possible schemes for methacrolein and acrolein formation which are independent of oxygen pressure. By considering reactions (R1) and (R4) the relative rate of acrolein to olefin formation is independent of [0 2 ]. d [acrolein]
k2k 8
d [C4Ha]
kl(k-z +ks)
Other schemes may be derived indicating that the rate of formation of acrolein and methacrolein should show a dependence on [O~ ]. In earlier work [3], using an oxidation mixture comprising 150, 75, and 240 mm Hg ofisobutane, oxygen, and nitrogen, respectively, at 600°K the maximum estimate of ks is the diffusion-controlled value of about 0.15 s- 1 . Using Ref. [7], k 6 = 107.7 exp (-41.5 kJ mol-1/RT) dm a mol-X s-1 and k 7 = 1012-1 exp (-125 kJ mol-1/RT) s- 1 the relative rates of Reactions (R5), (R6) and (R7) are 0.15, 50, and 15 s- i , where Reactions (R6) and
(R7) represent reactions giving ROOH and QOOH. k5 ROz ~ Surface products
(R5)
k6 RO2 + RH ~ ROOH + R
(R6)
k7 Re2 ~ QOOH.
(R7)
Consequently, it is apparent that surface reactions of t - CaHgO 2 are unimportant, even allowing for uncertainties in rate constants. Surface reactions of ROOH and QOOH in competition with homogeneous decomposition will considerably complicate the mechanism and may be responsible for the observed effects. The trends in product distribution with changing reaction pressure and altering the surface to volume ratio are difficult to explain. For the unpacked vessels the product ratios from series A and B are given in Table 2 and illustrated the effects of different surfaces. Acetone, acetaldehyde, methacrolein, t-butanol, acrolein, and propionaldehyde
OXIDATION OF ISOBUTANE
223 TABLE 2 Relative Amounts of Products Formed
Product Acetone Propionaldehyde Isobutyraldehyde Acetaldehyde Methacrolein t-Butanol Isobutene oxide Methanol Propylene oxide Acrolein Isobutene Propylene Ethylene
Series A/Series C
Series B/Series D
Series A/Series B
0.84 12.00 9.70 1.90 0.62 1.14 0.43 0.07 0.16 0.22 0.94 1.20 2.33
1.25 0.76 1.00 0.35 0.50 1.00 6.40 0.20 0.64 0.42 0.94 1.25 1.25
2.70 1.90 0.88 4.40 7.50 9.10 0.20 1.00 0.22 1.84 0.92 0.50 0.07
yields are higher in the quartz vessel; whereas ring oxides, methanol, and olefins show a significant decrease; other products do not indicate a major variation. The variaiton in product distribution for these two reaction vessels of similar volume but different surface indicates .that heterogeneous processes are important, Table 2 also shows the ratio of the initial rates of product formation for unpacked Pyrex to packed Pyrex, that is, series B/series D. The influence of packing decreases considerably the formation of isobutene oxide and slightly decreases the relative amounts of acetone, propylene, and ethylene. Significant product increases occur with acetaldehyde methacrolein, methanol, and acrolein, suggesting surface destruction of radicals is important in the formation of these products. Isobutene formation increases by 5% in a packed vessel from 79 to 84 %, and if isobutene if formed solely from the homogeneous bimolecular Reaction (R1), then other reactions have decreased from 21 to 16%. An alternative explaination is that isobutene could be formed from surface reactions of QOOH and ROOH. Additional evidence for such reactions has been obtained [3] when the reaction pressure was varied to decrease the diffusion rate of radicals to the surface; the percentage isobutene fell from 73% as shown in series A to 67% by the addition of 242 mm N2 to the reaction system. Due to the lack of accurate kinetic data, it is difficult to pre-
dict which are the important product forming reactions, particularly because a product may be formed via several possible reaction paths. A general idea of the difficulty of predicting relative product yields and mechanisms in packed and unpacked vessels may be seen by comparing the general reaction features of series B and D. The induction period (the time to detectable reaction) in series B is 20 minutes and in series D, 550 minutes; furthermore, the maximum rates of pressure rise at 300°C, (ap/ar)max, are 20 mm Hg s- 1 and 8 mm Hg s- 1 for series B and D, respectively, where P is the pressure rise in mm Hg and r is the reaction time in seconds. To compare the effect of a Pyrex-packed Pyrex vessel and a Pyrex-packed quartz vessel, series A/C and series B/D in Table 2 should be considered. Major differences are observed in the relative ratios of certain products, namely, propionaldehyde, isobutyraldehyde, acetaldehyde, isobutene oxide, and propylene oxide, indicating that different surfaces can promote reactions to different extents. By examination of previous work on the oxidation of isobutane and the surface studies reported in this work, increasing the surface to volume ratio increases the yields of acetaldehyde, methacrolein, methanol, and acrolein and decreases the yields of isobutene oxide, acetone, propylene, and ethylene. Propionaldehyde and isobutyraldehyde are formed more readily in an unpacked quartz vessel
224 than a Pyrex-packed quartz vessel. The higher yields of isobutene formed suggest it is predominanfly formed by the bimolecular Reaction (R1), but evidence presented suggested that QOOH or
G. M c K A Y and J. A. A G A
REFERENCES
2. Hay, J., Knox, J. H., and Turner, J. M. C., Tenth Symposium (lnternationaO on Combustion, The Combustion Institute, Pittsburgh, 1965, p. 331. 3. McKay, G., Norrie, K. M., Poots, V. J. P., and Turner, J. M. C., Combust. Flame 25:219 (1975). 4. Irvine, G. W., and Knox, J. H., Symposium on Mechanism of Hydrocarbon Oxidations, Siofok, Hungary, 1973, p. 513. 5. Luckett, G. A., and Pollard, R. T., Combust. Flame 21:265 (1973). 6. Baldwin, R. R., Plaistowe, J. C., and Walker, R. W., Combust. Flame 30:13 (1977). 7. Baldwin, R. R., Bennett, J. P., and Walker, R. W., Symposium (International) on Combustion Processes, 16:1041 (1977).
1. Zeelenberg, A. P., and Bickel, A. F., J. Chem. Soc. 4014 (1961).
Received 18 January 1979; revised 19 March 1980
ROOH radicals may result in some heterogeneous formation of isobutene. Surface reactions of QOOH and ROOH in competition with homogeneous decomposition considerably complicate the mechanism and make the interpretation of the observed effects difficult.
221
BRIEF COMMUNICATION Product Formation in the Slow Oxidation of Isobutane G. McKAY and J. A. AGA Department of Industrial Chemistry, The Queen's University of Belfast, Belfast, BT9 5DL, Northern Ireland
Although the oxidation of isobutane has been studied extensively [1-6], no mechanism has been proposed to explain the formation of all the primary products or the relative quantities produced, The effects of varying the reaction vessel surface have been studied and a mechanistic step has been proposed to explain the formation of acrolein and methacrolein. The apparatus and experimental techniques have been previously outlined [3] and four sets
able. The scheme proposed that butylperoxy radicals are formed homogeneously and these are followed by the heterogeneous decompositions of the peroxy radicals into a complex and surface-sensirive mixture of products. Steps 2 and 3 outlined the scheme.
of experiments were performed in packed and unpacked Pyrex or quartz reaction vessels using a conventional static vacuum system. The results for the four series of oxidations are shown in Table 1. The first major attempt to explain the primary product formation during the oxidation of isobutane proposed that isobutene was formed by the homogeneous bimolecular reaction shown in step 1. C4Hs + 02 ~ C4Ha + HOz. (R1)
(R3)
The minor products were thought to be formed by the homogeneous intramolecular decompositions of isobutylperoxy and t-butylperoxy radicals. Hay, Knox, and Turner [2] observed that coating the reaction vessel wall greatly affects the kinetics of the process, hence the distribution of the minor products is highly sensitive to the nature of the reaction vessel walls. They found that during the oxidation ofisobutane between 250 and 350°C at low conversion that, although the percentage of isobutene is little affected, reactant pressure and temperature have a profound effect on the distribution of the minor products, and therefore the basic hypothesis outlined by Zeelenberg is untenCopyright © 1981 by The Combustion Institute Published by Elsevier North Holland, Inc. 52 Vanderbilt Avenue, New York, NY 10017
C4H9 + 02 ~ C4H9OO
(R2)
C4H9OO ~ Complex mixture of minor products.
Earlier work by McKay et al. [3] on the oxidation of isobutane around 600°K in the presence and absence of nitrogen and carbon dioxide, using a quartz reaction vessel, showed the initial yield of isobutene is 60-70%, and addition of inert gas lowers the ratio [isobutene]/[oxygenated products], but not systematically, indicating that simpie formation of minor products at the surface is untenable. Previous workers have explained the formation of most primary products observed during the oxidation of isobutane. However, the reactions of tbutyl hydroperoxy alkyl radicals enable the formation of acrolein and methacrolein to be explained, for example, k2 (1, 4p) C4H9 + O2 ~- t - - C 4 H 9 0 2 ~ QOOH k-2 k3 acrolein + H202 02 -~' + CH3OH -~ O2QOOH surface "~ methacrolein + H202 + OH (4)
0010-2180/81/02221+0452.50
222
G. McKAY and J. A. AGA TABLE 1 Initial Percentage of Products to 1% Isobutane Consumption
Temperature (*C) Reaction Vessel Packing Isobutane (tort) Oxygen (torr) Surface:Volume Product Acetone Propionaldehyde Isobutyraldehyde Acetaldehyde Methacrolein t-Butanol Isobutene oxide Methanol Propylene oxide Acrolein Isobutene Propylene Ethylene
Series A
Series B
Series C
Series D
300 Quartz Unpacked 150 75 0.08:1
300 Pyrex Unpacked 150 75 0.08:1
300 Quartz Packed 150 75 1.7:1
300 Pyrex Packed 150 75 1.7:1
8.00 4.80 3.10 2.30 1.87 1.37 0.64 0.60 0.04 0.46 73.00 2.40 0.07
3.00 2.50 3.50 0.52 0.25 0.15 3.20 0.04 0.18 0.25 79.00 5.00 1.00
9.50 0.40 0.32 1.20 3.00 1.20 1.50 0.60 0.25 2.10 78.00 2.00 0.03
2.40 3.30 3.40 0.15 0.50 0.14 0.50 0.20 0.28 0.60 84.00 4.00 0.80
Thus the two rearrangements of the OOQOOH radical offer possible schemes for methacrolein and acrolein formation which are independent of oxygen pressure. By considering reactions (R1) and (R4) the relative rate of acrolein to olefin formation is independent of [0 2 ]. d [acrolein]
k2k 8
d [C4Ha]
kl(k-z +ks)
Other schemes may be derived indicating that the rate of formation of acrolein and methacrolein should show a dependence on [O~ ]. In earlier work [3], using an oxidation mixture comprising 150, 75, and 240 mm Hg ofisobutane, oxygen, and nitrogen, respectively, at 600°K the maximum estimate of ks is the diffusion-controlled value of about 0.15 s- 1 . Using Ref. [7], k 6 = 107.7 exp (-41.5 kJ mol-1/RT) dm a mol-X s-1 and k 7 = 1012-1 exp (-125 kJ mol-1/RT) s- 1 the relative rates of Reactions (R5), (R6) and (R7) are 0.15, 50, and 15 s- i , where Reactions (R6) and
(R7) represent reactions giving ROOH and QOOH. k5 ROz ~ Surface products
(R5)
k6 RO2 + RH ~ ROOH + R
(R6)
k7 Re2 ~ QOOH.
(R7)
Consequently, it is apparent that surface reactions of t - CaHgO 2 are unimportant, even allowing for uncertainties in rate constants. Surface reactions of ROOH and QOOH in competition with homogeneous decomposition will considerably complicate the mechanism and may be responsible for the observed effects. The trends in product distribution with changing reaction pressure and altering the surface to volume ratio are difficult to explain. For the unpacked vessels the product ratios from series A and B are given in Table 2 and illustrated the effects of different surfaces. Acetone, acetaldehyde, methacrolein, t-butanol, acrolein, and propionaldehyde
OXIDATION OF ISOBUTANE
223 TABLE 2 Relative Amounts of Products Formed
Product Acetone Propionaldehyde Isobutyraldehyde Acetaldehyde Methacrolein t-Butanol Isobutene oxide Methanol Propylene oxide Acrolein Isobutene Propylene Ethylene
Series A/Series C
Series B/Series D
Series A/Series B
0.84 12.00 9.70 1.90 0.62 1.14 0.43 0.07 0.16 0.22 0.94 1.20 2.33
1.25 0.76 1.00 0.35 0.50 1.00 6.40 0.20 0.64 0.42 0.94 1.25 1.25
2.70 1.90 0.88 4.40 7.50 9.10 0.20 1.00 0.22 1.84 0.92 0.50 0.07
yields are higher in the quartz vessel; whereas ring oxides, methanol, and olefins show a significant decrease; other products do not indicate a major variation. The variaiton in product distribution for these two reaction vessels of similar volume but different surface indicates .that heterogeneous processes are important, Table 2 also shows the ratio of the initial rates of product formation for unpacked Pyrex to packed Pyrex, that is, series B/series D. The influence of packing decreases considerably the formation of isobutene oxide and slightly decreases the relative amounts of acetone, propylene, and ethylene. Significant product increases occur with acetaldehyde methacrolein, methanol, and acrolein, suggesting surface destruction of radicals is important in the formation of these products. Isobutene formation increases by 5% in a packed vessel from 79 to 84 %, and if isobutene if formed solely from the homogeneous bimolecular Reaction (R1), then other reactions have decreased from 21 to 16%. An alternative explaination is that isobutene could be formed from surface reactions of QOOH and ROOH. Additional evidence for such reactions has been obtained [3] when the reaction pressure was varied to decrease the diffusion rate of radicals to the surface; the percentage isobutene fell from 73% as shown in series A to 67% by the addition of 242 mm N2 to the reaction system. Due to the lack of accurate kinetic data, it is difficult to pre-
dict which are the important product forming reactions, particularly because a product may be formed via several possible reaction paths. A general idea of the difficulty of predicting relative product yields and mechanisms in packed and unpacked vessels may be seen by comparing the general reaction features of series B and D. The induction period (the time to detectable reaction) in series B is 20 minutes and in series D, 550 minutes; furthermore, the maximum rates of pressure rise at 300°C, (ap/ar)max, are 20 mm Hg s- 1 and 8 mm Hg s- 1 for series B and D, respectively, where P is the pressure rise in mm Hg and r is the reaction time in seconds. To compare the effect of a Pyrex-packed Pyrex vessel and a Pyrex-packed quartz vessel, series A/C and series B/D in Table 2 should be considered. Major differences are observed in the relative ratios of certain products, namely, propionaldehyde, isobutyraldehyde, acetaldehyde, isobutene oxide, and propylene oxide, indicating that different surfaces can promote reactions to different extents. By examination of previous work on the oxidation of isobutane and the surface studies reported in this work, increasing the surface to volume ratio increases the yields of acetaldehyde, methacrolein, methanol, and acrolein and decreases the yields of isobutene oxide, acetone, propylene, and ethylene. Propionaldehyde and isobutyraldehyde are formed more readily in an unpacked quartz vessel
224 than a Pyrex-packed quartz vessel. The higher yields of isobutene formed suggest it is predominanfly formed by the bimolecular Reaction (R1), but evidence presented suggested that QOOH or
G. M c K A Y and J. A. A G A
REFERENCES
2. Hay, J., Knox, J. H., and Turner, J. M. C., Tenth Symposium (lnternationaO on Combustion, The Combustion Institute, Pittsburgh, 1965, p. 331. 3. McKay, G., Norrie, K. M., Poots, V. J. P., and Turner, J. M. C., Combust. Flame 25:219 (1975). 4. Irvine, G. W., and Knox, J. H., Symposium on Mechanism of Hydrocarbon Oxidations, Siofok, Hungary, 1973, p. 513. 5. Luckett, G. A., and Pollard, R. T., Combust. Flame 21:265 (1973). 6. Baldwin, R. R., Plaistowe, J. C., and Walker, R. W., Combust. Flame 30:13 (1977). 7. Baldwin, R. R., Bennett, J. P., and Walker, R. W., Symposium (International) on Combustion Processes, 16:1041 (1977).
1. Zeelenberg, A. P., and Bickel, A. F., J. Chem. Soc. 4014 (1961).
Received 18 January 1979; revised 19 March 1980
ROOH radicals may result in some heterogeneous formation of isobutene. Surface reactions of QOOH and ROOH in competition with homogeneous decomposition considerably complicate the mechanism and make the interpretation of the observed effects difficult.