Claudette M. Rosado-Reyes

Atmospheric oxidation pathways of propane and its by‐products: Acetone, acetaldehyde, and propionaldehyde

[i] Propane (C 3 H 8 ) is one of the most abundant nonmethane hydrocarbons in the atmosphere. It is a fuel widely used, derived from petroleum products during oil and natural gas processing. It can be oxidized in the atmosphere via its reactions with hydroxyl (OH) radicals and chlorine (Cl) atoms and serves as an indicator for the presence of such oxidants. During the atmospheric degradation of propane, various carbonyl compounds are formed, with acetone, acetaldehyde, and propionaldehyde among the most prominent. Carbonyl compounds are relevant because of their toxicity and ability to produce free radicals by photolysis that give rise to stable products, thus providing valuable information about atmospheric oxidation processes. The exact mechanisms of the oxidation pathways of propane have not been properly characterized, although several speculations have been made that determine the oxidation products. The present study investigates the oxidation mechanism of propane, acetone, acetaldehyde, and propionaldehyde by ab inito molecular orbital methods. Detailed pathways leading to experimentally observed products are presented. Equilibrium geometries and energetics, as well as vibrational frequencies of species, transition states, and prereactive complexes are determined at the QCISD(T)/6-311G(2df,2p)//MP2(full)/6-31G(d).

Dehydration of isobutanol and the elimination of water from fuel alcohols.

Rate coefficients for the dehydration of isobutanol have been determined experimentally from comparative rate single pulse shock tube measurements and calculated via multistructural transition state theory (MS-TST). They are represented by the Arrhenius expression, k(isobutanol → isobutene + H2O)(experimental) = 7.2 × 10(13) exp(-35300 K/T) s(-1). The theoretical work leads to the high pressure rate expression, k(isobutanol → isobutene + H2O)(theory) = 3.5 × 10(13) exp(-35400 K/T) s(-1). Results are thus within a factor of 2 of each other. The experimental results cover the temperature range 1090-1240 K and pressure range 1.5-6 atm, with no discernible pressure effects. Analysis of these results, in combination with earlier single pulse shock tube work, made it possible to derive the governing factors that control the rate coefficients for alcohol dehydration in general. Alcohol dehydration rate constants depend on the location of the hydroxyl group (primary, secondary, and tertiary) and the number of available H-atoms adjacent to the OH group for water elimination. The position of the H-atoms in the hydrocarbon backbone appears to be unimportant except for highly substituted molecules. From these correlations, we have derived k(isopropanol → propene + H2O) = 7.2 × 10(13) exp(-33000 K/T) s(-1). Comparison of experimental determination with theoretical calculations for this dehydration, and those for ethanol show deviations of the same magnitude as for isobutanol. Systematic differences between experiments and theoretical calculations are common.

Unimolecular Rate Expression for Cyclohexene Decomposition and Its Use in Chemical Thermometry under Shock Tube Conditions.

The methods used in deriving the rate expressions from comparative rate single-pulse shock tube studies, recent direct shock tube studies, and high-pressure flow experiments bearing on the data for the reverse Diels-Alder decomposition of cyclohexene to form ethylene and 1,3-butadiene are reviewed. This current interest is due to the increasing need for accurate kinetics and physical data (particularly the temperature) for realistic simulations in practical areas such as combustion. The rate constants derived from the direct shock tube studies and high-pressure flow experiments are somewhat larger than those used in comparative rate single-pulse shock tube experiments. For the latter, it is shown that they have been derived from a variety of independent experiments that include rate constants for unimolecular decomposition and isomerization processes that are considered to be well understood. The possibility of non-Arrhenius behavior in the unimolecular rate constants as a consequence of the large range covered in rate constants (as much as 12 orders of magnitude) for the comparative rate experiments has been examined and ruled out as a source of the discrepancy. Our analysis shows that there is the need to consider the possibility of radical-induced decompositions for verifying the correctness of the reaction mechanisms in studying unimolecular reactions. In the case of cyclohexene decomposition, recent experiments demonstrating the presence of residual amounts of H atoms in shock tube experiments suggest that addition to the double bond can also lead to the formation of ethylene and 1,3-butadiene and hence to rate constants larger than the true values. This possibility is even more likely to occur in high-pressure flow experiments. As a result, the internal standard method must be used with care and a radical inhibitor should always be present in sufficiently large quantities to suppress possible chain reactions. The present analysis results have important implications for the determination of temperatures in shock tubes.

Bond cleavage during isobutanol thermal decomposition and the breaking of C-C bonds in alcohols at high temperatures.

Isobutanol was thermally decomposed in a single pulse shock tube under conditions where chain processes were suppressed. The main reaction is the breaking of C-C bonds. Literature rate expressions, experimentally determined, are found in some cases to be in disagreement. The rate expressions for the decomposition processes at temperatures of 1090 to 1240 K and pressures of 1.5 and 6 atm are k(isobutanol → isopropyl + hydroxymethyl) + k(isobutanol → methyl + 1-hydroxypropyl-2) = 10(16.7±0.3) exp(-41097 ± 750) s(-1), where k(isobutanol → isopropyl + hydroxymethyl) = 10(16.45 ±0.3) exp(-40910 ± 750/T) s(-1) and k(isobutanol → methyl + 1-hydroxypropyl-2) = 10(16.38±0.3) exp(-41560 ± 750/T) s(-1). These values permit comparisons with recent estimates including those from ab initio calculations. A new procedure is presented that uses information on the kinetics of bond breaking reactions of alkanes and the effect of OH substitution to derive rate coefficients for similar reactions of alcohols. This leads to the following rate expression for the smaller alcohols, at temperatures of 1090 to 1240 K and pressures of 1.5 and 6 atm, k(ethanol → methyl + hydroxymethyl) = 10(16.42±0.3) exp(-43496 ± 750 K/T) s(-1), k(isopropanol → methyl + 1-hydroxyethyl) = 10(16.54±0.3) exp(-42495 ± 750 K/T) s(-1), k(n-propanol → ethyl + hydroxymethyl) = 10(16.43±0.3) exp(-41696 ± 750 K/T) s(-1), and k(n-propanol → methyl + 2-hydroxymethyl) = 10(16.53±0.3) exp(-42945 ± 750 K/T) s(-1). Extension of this approach to other alcohols is straightforward. The resulting correlations along with the data on dehydration of alcohols provide novel information of the kinetic stability of alcohols.

H Atom Attack on Propene

The reaction of propene (CH(3)CH═CH(2)) with hydrogen atoms has been investigated in a heated single-pulsed shock tube at temperatures between 902 and 1200 K and pressures of 1.5-3.4 bar. Stable products from H atom addition and H abstraction have been identified and quantified by gas chromatography/flame ionization/mass spectrometry. The reaction for the H addition channel involving methyl displacement from propene has been determined relative to methyl displacement from 1,3,5-trimethylbenzene (135TMB), leading to a reaction rate, k(H + propene) → H(2)C═CH(2) + CH(3)) = 4.8 × 10(13) exp(-2081/T) cm(3)/(mol s). The rate constant for the abstraction of the allylic hydrogen atom is determined to be k(H + propene → CH(2)CH═CH(2) + H(2)) = 6.4 × 10(13) exp(-4168/T) cm(3)/(mol s). The reaction of H + propene has also been directly studied relative to the reaction of H + propyne, and the relationship is found to be log[k(H + propyne → acetylene + CH(3))/k(H + propene → ethylene + CH(3))] = (-0.461 ± 0.041)(1000/T) + (0.44 ± 0.04). The results showed that the rate constant for the methyl displacement reaction with propene is a factor of 1.05 ± 0.1 larger than that for propyne near 1000 K. The present results are compared with relevant earlier data on related compounds.

Kinetics of the Thermal Reaction of H Atoms with Propyne

The reaction of hydrogen atoms with propyne (CH[triple bond]CCH(3)) was investigated in a heated single pulse-shock tube at temperatures of 874-1196 K and pressures of 1.6-7.6 bar. Stable products from various reaction channels (terminal and nonterminal H addition, and by inference H abstraction) were identified and quantified by gas chromatography and mass spectrometry. The rate constant for the channel involving the displacement of methyl radical from propyne (nonterminal H addition) was determined relative to the methyl displacement from 1,3,5-trimethylbenzene (135-TMB), with k (H + 135-TMB --> m-xylene + CH(3)) = 6.70 x 10(13) exp(-3255/T[K]) cm(3)/mol x s, k(H+propyne-->CH[triple bond]CH+CH3))=6.26 x 10(13) exp(-2267/T[K]) cm3/mole x s. Our results show that the acetylene to allene yield is approximately 2 at 900 K, and decreases with increasing temperature. The rate expression is: k(H+propyne-->CH2=C=CH2+H))=2.07 x 10(14) exp(-3759/T[K]) cm3/mole x s. This is a lower limit for terminal addition. Kinetic information for abstraction of the propargylic hydrogen by H was determined via mass balance. The rate expression is approximately k(H+CH3C[triple bond]CH-->CH[triple bond]C-CH2+H2))=1.20 x 10(14) exp(-4940/T[K])cm3 /mole x s and is only 10% of the rate constant for acetylene formation. All channels from H atom attack on propyne at combustion temperatures have now been determined. Comparisons are made with results of recent ab initio calculations and conclusions are drawn on the quantitative accuracy of such estimates.

Hydroxyl-Radical-Initiated Oxidation Mechanism of Bromopropane

Bromopropane has been considered as a replacement for chlorofluorocarbons used as the active component of industrial cleaning solvents, more specifically for HCFC-141b. The proposed mechanism for the atmospheric oxidation of bromopropane is studied via ab initio methodology. Ab initio molecular orbital methods at the CCSD(T)/6-311++G(2df,2p)//MP2/6-31G(d) level of theory have been used to determine the structure and energetics of the 58 species and transition states involved in the atmospheric oxidation of bromopropane. The calculations show that the major oxidation species is bromoacetone. Other brominated species that result from the oxidation are BrCH 2CH 2C(O)H, BrC(O)CH 2CH 3, and BrC(O)H, potential new bromine reservoir species that result from bromopropane in the atmosphere.

Isomerization of cis-1,2-dimethylcyclohexane in single-pulse shock tube experiments.

Cyclic hydrocarbons are major constituents of jet fuels and reference compounds in jet fuel surrogates. The kinetic and thermal stability and reaction mechanisms of fuel molecules are essential input parameters in the models and simulations used in the design of novel fuels, renewable energy technologies, and devices. A detailed study and analysis of the pyrolytic chemistry of cis-1,2-dimethylcyclohexane has been performed in single-pulse shock tube experiments. The investigations are carried out over the temperature range of 1100 to 1200 K at about 2.5 atm pressure. The isomeric products are trans-1,2-dimethylcyclohexane, 1-octene, and (cis + trans)-2-octene. The three octene isomers can be attributed to internal disproportionation processes. Assuming a diradical mechanism and that cis-1,2-dimethylcyclohexane is formed in equal amount with respect to its trans isomer, the total rate expression for isomerization is kC-C = 10(15.5±0.8) exp(-38,644 ± 2061 K/T) s(-1). The rate constants are over an order of magnitude smaller than the equivalent noncyclic hydrocarbon system. The presence of the isomeric octenes suggests that internal disproportionation is an important component of the isomerization process.

Computational Study of the Reaction of n-Bromopropane with OH Radicals and Cl Atoms

Abstract Ab initio molecular orbital theory is utilized to study the hydrogen abstraction reaction of n -bromopropane with hydroxyl radical and chlorine atom. The stability of the trans and gauche isomers of n -bromopropane is explored. The potential energy surface of both reactions is characterized by pre- and post-reactive complexes, as well as transition state structures in both trans and gauche isomeric forms. The importance of these two reactions relies on the ultimate product distribution from both reactions. Differences in the reactivity of 1-bromopropane toward OH and Cl are observed. The reaction of n -bromopropane with OH radical favors the abstraction of β hydrogen atoms while the reaction with Cl atoms favors the abstraction of hydrogen atoms at the α and β carbon sites.