6457
J. Phys. Chem. 1993,97, 6457-6464
Catalytic Dehydration of Alcohols by OH. 2-Propanol: An Intetmediate Caset James R. Dunlo$ and Frank P. Tully' Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551 Received: January 28, 1993; In Final Form: March 29, 1993
We describe a laser photolysis/cw, laser-induced fluorescence kinetic study of the reaction between O H and 2-propanol, measured over the temperature range 293-745 K. The rate coefficient for hydrogen atom abstraction by OH from 2-propanol is best fit by the expression &(q= 1.044 X 10-17T1.86exp(736/7) cm3 molecule-1 s-l. Chain-catalytic dehydration of 2-propanol by OH is an important component of the reaction mechanism. By using isotopic substitution, we determine, as a function of temperature, the branching ratio for H atom abstraction by OH from the &sites of 2-propanol. Between 500 and 600 K, biexponential [OH] decays result from the unimolecular decomposition of the H2CCH(OH)CH3 intermediate. We characterize the dissociation kinetics of this HO-propene intermediate by fitting biexponential [OH] decays to a reaction model. From these results and previously established kinetic and thermodynamic data, we estimate the strength of a methyl C-H bond in 2-propanol. Measurements above T = 600 K demonstrate a role for two minor reaction channels.
Introduction Hydroxyl-radical reactions are extremely important in both atmospheric and combustion chemistries because OH is a dominant oxidant of hydrocarbons in these systems.' Alcohol reactivity impacts these chemistries because of the increasing use of clean-burning oxygenated fuels. Combustion models require accurate kinetic and mechanistic data on OH-alcohol reactions. In this paper, wereport theresults of a kineticstudy of thereaction OH 2-propanol products. Recently, we discovered a chain-reaction mechanism by which OH converts alcohols to alkenes, CnHzn+lOH CnH2, H2O (n 1 2).2J The reaction sequence begins when OH abstracts a hydrogen atom from the alcohol, OH + CnH2n++10H H2O + C,,HhOH. Every alcohol, of course, has several sites from which hydrogen atoms may be removed; a temperature-dependent distribution of isomeric radicals of empirical formula CnHh+10 results from this initiating reactive step. For a hydrogen atom abstraction that occurs from a @-siteof an alcohol, i.e., from a carbon atom immediately adjacent to that carbon (C,) that is bound to the OH functional group, the product radical is a HO-alkene intermediate that, at sufficiently high temperature, may dissociate to OH alkene without first undergoing an intramolecular rearrangement. For hydrogen atom abstractions that occur from non-8-sites of the alcohol, the resultant radicals are structured such that OH alkene products cannot be formed via simple bond-fission dissociations. In 2-propanol, 75% of the hydrogen atoms reside at the &sites of the molecule. This study extendsour investigationof the relationshipbetween the propensity for OH attack at the &site of an alcohol and the degree to which that alcohol undergoes catalytic dehydration. Prior work on the reaction kinetics of
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+
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OH + 2-propanol
k
products is limited. Lloyd et a1.4 and Kldpffer et aL5 measured k near room temperature using relative-rate techniques. Overend and Paraskevopoulos6utilized flash photolysis/resonance absorption methods to determine k(296 K). The only previous study of the temperature dependence (240-440 K) of this reaction was performed by Wallington and Kurylo7 using flash photolysis/ resonance fluorescence techniques. While these authors re"-
* Author to whom correspondenceshould be addressed.
This work is supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S.Department of Energy. t Sandia National Laboratories Postdoctoral Research Associate. t
0022-3654/93/2097-6457$04.00/0
mend the rate-coefficient expression k(7') = (5.8 i 1.9) X 10-12 exp[-(60 179) cal mol-l/RT] cm3 molecule-' s-l, it is clear that k depends only weakly on temperature, and data from all prior studies support a range for k given by 4 X 10-l25 k( T) I 7 X 10-l2 cm3molecule-1 s-l, 240 K 5 T I 440 K. The present investigation aims more toward providing mechanistic insight than to refining the above numerics, though we also accomplish the latter. We measure k(7') for the subject reaction
*
OH + (H,C),CHOH
k,
products
over the temperature range 293 K 5 T I745 K. The resulting Arrhenius plot is structured, and the structure relates directly to the catalytic dehydration mechanism. By using isotopic substitution, we determine, as a function of temperature, the branching ratio for H atom abstraction by OH from the 0-sitesof 2-propanol. Between 500 and 600 K, biexponential [OH] decays result from the unimolecular decomposition of the H2CCH(OH)CH3 intermediate. We characterize the dissociation kinetics of this HOpropene intermediate by fitting biexponential [OH] decays to a reaction model. From these results and previously established kinetic and thermodynamic data, we estimate the strength of a methyl C-H bond in 2-propanol.
Experimental Technique We perform all experiments using the laser photolysis/cw, laser-induced fluorescence techniquedescribedin detail in previous paper^.^^^ Briefly, 160Hforms from the fast reaction of 160(lD2) with H2160following pulsed, 193-nm laser photolysis of N P O . In addition, a small fraction of the [l60H]results from the direct photolysis of H2I60 at 193 nm. In the 180H + 2-propanol experiments,N2O is absent from the reaction mixture, and l80H forms via direct photolysis of H2180 at 193 nm. Following initiation of the reaction, we measure time-resolved OH concentration profiles as functions of alcohol number density using laser-induced fluorescence. We selectively excite either the R1(3.5) line or the Ql(2.5) line of the (0,O) band of the A2E+ X% OH transition with a 6kpH(OH+ propane, T = 293 K),15 and this observation suggests that a primary C-H bond in 2-propanol may be slightly weaker than the corresponding bond in propane. In any case, we do consider our bond strength determination to be sufficiently accurate to provide support for the higher value recommended by Tsang, 101 kcal mol-'. T 1 624 K. For 160H 2-propanol experiments above T = 600 K, the diffusion-corrected [l60H] temporal profiles fit well to single-exponential functions, but the determined k, values are significantly smaller than those measured below T = 504 K. This results from the fact that at these temperatures, k-3 becomes comparable to and then exceeds ka[(H3C)2CHOH], and OH re-forms almost as rapidly as it abstracts a 8-site hydrogen atom. We could, of course, force the condition ka[(H3C)2CHOH] = 5k-3, enabling observation of biexponential [OH] profiles, by using large [(H3C)2CHOH] values; however, even at T = 624 K, k-3 = 1400 s-l, requiring [(H3C)2CHOH] = 1.25 X lOI5 molecules ~ m - a~value , fully 15 times larger than the maximum concentration of 2-propanol used in our actual experiment at this temperature. Interference from 193-nmphotolysisof 2-propanol (see Figure 8) would render such experiments uninterpretable. The phenomenological structure in ka that is evident in Figure 2 does not imply that OH reacts with 2-propanol more slowly above T = 600 K, &site hydrogen atom abstraction reactions by OH efficiently dehydrate 2-propanol, 2-propanol propene HzO,reproducing OH. Indeed, if the kinetic and spectroscopic experimentalconfigurationswere arranged such that [2-propanol] were the monitored quantity, its disappearance rate would be governed by the expression for (kl + kz)(T) given above. In similar, fashion, ISOH + (H3C)2CHI60H experiments at high temperatures would yield (kl + kz)(T). The structure in k, is a direct result of the catalytic dehydration mechanism. It is informative to consider the high-temperatureextrapolation of the dotted line in Figure 2 that represents the quantity (kaH + k o ~ ) ( T ) It. undershoots our experimental measurements of k,(T 1 624 K) by a meaningful amount. Either of two factors may contribute to this difference. First, as discussed above in
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0
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5.0 7.5 TIME (milliseconds)
2.5
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10
Figure 8. Growth and decay of [OH(v=O)] following photolysis of 2-propanol, T = 293 K, P = 750 Torr of helium, and, in molecule cm-), [(HsC)2CHOH] = 6.74 X lOI4, [NzO] = 0, [HzO] = 0.
the quantum yield for production of OH nor the resulting nascent OH-state distribution has been characterized. In our low- and high-temperaturestudies, in which we measure only single-exponential [OH] profiles, we chose concentration and photolysis conditions so as to maximize OH formation from the N20 and H2O precursors and minimize OH formation from photolysis of 2-propanol. Photolytic interference was insignificant in these experiments,since k'values, and, therefore [2-propanol], could be kept small. However,for experiments in the temperature range 504 K I T I 564 K, our measured [OH] profiles are biexponential decays. These profiles may, in fact, be the sum of a weak OH(u=O) signal such as that of Figure 8 and a strong OH(u=O) signal such as that of Figure lb. As [(H3C)2CHOH] increases, so does the fractional contribution of the OH signal resulting from (H3C)2CHOH photolysis. Thus, we might expect that the net "biexponential" [OH] profiles could produce k-3 and k5 values that vary subtly with [(H3C)2CHOH]. One could minimize this potential interferenceby working with low [ (H3C)zCHOH] ;however, in order to meaningfully sample a biexponential [OH] decay, the condition
k, [OH][(H,C),CHOH] >>
[H3CCH(OH)CH2]
must hold during the early portion of the [OH] us time profile. Since k, depends weakly on T (Figure 2) while k-3 increases very rapidly with T (Figure 7 ) ,the requiredvalues of [(H3C)2CHOH] rise dramatically with temperature. Thus, the practical temperature limit of our biexponential [OH] measurements is that point at which photolysis of (H$)2CHOH becomes too perturbing ( T > 564 K). We may derive from our data an estimate of the C-H bond dissociation energy (BDE) for a (&site) primary hydrogen in 2-propanol. In order to accomplish this, we first must compute AH?9aK(H2CCH(OH)CH3).Recognizingthat this HO-propene adduct would form if OH were to add to the central carbon in propene, we may write
BDE(H,CCH(CH,)OH) = E,(-3) - E,(3)
+ RT
which equals 29.0 f 1.5 kcal mol-' at T = 530 K. In computing this quantity, we set E,(-3) = 26.9 f 1.5 kcal mol-' from our best fit listed above. We equate E,(3) to the activation energy found
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6464 The Journal of Physical Chemistry, Vo1.97, No. 24, 1993 reference to Figure 7, HzCCH(OH)CH3 dissociates to non-OH products to a minor degree at elevated temperatures; conversion, via &site H atom abstraction, of 2-propanol to propene + H20 is highly efficient but not quantitative. Second, the expression for (kaH k o ~ ) ( T results ) from measurements at low temperatures where &OH is probably insignificant. Therefore, its extrapolation to high temperatures would not reflect a growing contribution from this channel (&OH) above T = 600 K. Both of these minor reaction channels may contribute to the “additional” reactivity observed at T 2 624 K. This study strengthens our observed correlation between the branching fraction for @-siteH atom abstraction from an alcohol by OH and the degree of catalytic dehydration that the alcohol undergoes. 2-Propanol proves to be an intermediate case, where the sharp structure evinced by &. is of a substantial magnitude consistent with the determined values of F p Computer models of alcohol combustion must consider these chain-catalytic dehydration reaction channels if they are to provide meaningful insight into the combustion process.
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summuy We describe a laser photolysis/cw, laser-induced fluorescence kinetic study of the reaction between OH and 2-propanol, measured over the temperature range 293-745 K. The experiments c o n f i i and extend our mechanism for the (chain) catalytic dehydration of alcohols by OH. By using isotopic substitution, we determine, as a function of temperature, the branching ratio for H atom abstraction by OH from the &sites of 2-propanol. Between 500 and 600 K,biexponential [OH] decays result from the unimolecular decomposition of the H2CCH(OH)CHp intermediate. We characterize the dissociation kinetics of this HOpropene intermediate by fitting biexponential [OH] decays to a reaction model. From these results and previously established kinetic and thermodynamic data, we estimate the strength of a methyl C-H bond in 2-propanol. Measurements above T = 600 K demonstrate a role for two minor reaction channels.
Dunlop and Tully
Acknowdlpmnt. We thank Dr. Andrew McIlroy of Sandia National Laboratories for many helpful discussions.
Retcreaas md Notes (1) Atkinson,R. Chem. Rev. 1986,86,69. (2) Hess, W. P.; Tully, F. P. Chem. Phys. Lett. 1988, 152, 183. (3) Tully,F. P. Twenty-ThirdSympium(Inimmtionaf)onCombustlon, The Combustion Institute 1990, 147. (4) Lloyd, A. C.; Darnall, K. R.; Winer, A. M.;Pit&, J. N., Jr. Chem. Phys. Lett. 1976, 42, 205. ( 5 ) Kldpffer, W.; Frank, R.; Kohl, E.-G.; Haag,F. Chem.-Z. 1986,l IO, 57. (6) Overmd, R.; Paraskevopoulos, G. J. Phys. Chem. 1978,82, 1329. (7) Wallington, T. J.; Kurylo, M.J. Int. J. Chem. Kinet. 1987,19,1015. (8) Tully, F. P.; Goldsmith, J. E. M.Chem. Phys. h i t . 1985,116,345. (9) Tully, F. P.; Droege, A. T.; Koszykoweki, M.L.; Meliu, C.F.1. Phys. Chem. 1986,90,691. (10) Tully, F. P.; Droege, A. T. Int. J. Chem. Kinet. 1987, 19, 251. (11) Droege, A. T.; Tully, F. P. 1.Phys. Chcm. 1986,90, 1949. (12) Tully, F. P.; Goldsmith, J. E. M.;Droege,A. T. J. Phys. Chem. 1986, 90,5932. (13) Tully, F. P.; Koszykowki, M.L.; Binklsy,J. S . TwentiethSymparium (Intematio~I) on Combustion, The Combustion Insritute 1984, 715. (14) Heas, W. P.; Tully, F. P. J. Phys. Chem. 1989,93, 1944. (15) Tully, F. P. Chem. Phys. Lett. 1983,96, 148. (16) Tully, F. P. Chem. Phys. Lett. 1988, 143, 510. (17) Tully, F. P. Work in progress. (18) Harrison, A. J.; Cederholm, B.J.; Terwilliger, M.A. J . Chem.Phys. 1959, 30, 355. (19) Atkinson, R. J. Phys. Chem. Re$ Data 1989, Monograph 1. (20) Baaon, S. W. Thermochemical Kinetics, 2nd 4.; Wiley-Interscience: New York, 1976. (21) Chase, M.W., Jr.; Davies, C. A.; Downey, J. R., Jr.; F ~ r i pD. , J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed.; American Chemical Society and American Institute of Physics: New York, 1986. (22) Stein, S. E. NISTStructures & Properties Databaseand Estimation Program; NIST Standard Reference Database 25; National Institute of Standards and Technology: Washington, DC,1991; Vol. 1.1. (23) Tsang, W. J. Am. Chem. Soc. 1985,107,2872. (24) McMillen, D. F.; Golden, D. M.Annu. Rev.Phys. Chem. 1982,33, 493.
