Thermal isomerization and decomposition of 1,2-butadiene in shock waves

1,2-Butadiene diluted with Ar was heated behind reflected shock waves over the temperature and the total density range of 1100–1600 K and 1.36 × 10^?5 ? 1.75 × 10^?5 mol/cm³. The major products were 1,3-butadiene, 1-butyne, 2-butyne, vinylacetylene, diacetylene, allene, propyne, C₂H₆, C₂H₄, CH₄, and benzene, which were analyzed by gas chromatography. The UV kinetic absorption spectroscopy at 230 nm showed that 1,2-butadiene rapidly isomerizes to 1,3-butadiene from the initial stage of the reaction above 1200 K. In order to interpret the formation of 1,3-butadiene, 1-butyne, and 2-butyne, it was necessary to include the parallel isomerizations of 1,2-butadiene to these isomers. The present data were successfuly modeled with a 82 reaction mechanism. From the modeling, rate constant expressions were derived for the isomerization 1,2-butadiene = 1,3-butadiene to be k₃ = 2.5 × 10¹³ exp(?63 kcal/RT) s^?1 and for the decomposition 1,2-butadiene = C₃H₃ + CH₃ to be k₆ = 2.0 × 10¹⁵ exp(?75 kcal/RT) s^?1, where the activation energies, 63 kcal/mol and 75 kcal/mol, were assumed. These rate constants are only applicable under the present experimental conditions, 1100–1600 K and 1.23–2.30 atm. © 1995 John Wiley & Sons, Inc.     

Production of 1,3-Butadiene From C4 Raffinate-3 Through Oxidative Dehydrogenation of n-Butene Over Bismuth Molybdate Catalysts

Recent progress on the bismuth molybdate catalysts for oxidative dehydrogenation of n-butene to 1,3-butadiene was reported in this review. A number of bismuth molybdate catalysts, including pure bismuth molybdates (α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉, and γ-Bi₂MoO₆) and multicomponent bismuth molybdates, were prepared by a co-precipitation method for use in the production of 1,3-butadiene from C₄ raffinate-3 through oxidative dehydrogenation of n-butene. It was observed that multicomponent bismuth molybdate catalyst was more efficient than pure bismuth molybdate catalyst in the oxidative dehydrogenation of n-butene. Various experimental measurements such as temperature-programmed reoxidation, X-ray photoelectron spectroscopy, and O₂-temperature-programmed desorption analyses were carried out to elucidate the different catalytic activity of bismuth molybdate catalysts. It was revealed that a bismuth molybdate catalyst with a higher oxygen mobility showed a better catalytic performance in terms of conversion of n-butene and yield for 1,3-butadiene. We have successfully demonstrated from experimental findings that oxygen mobility of bismuth molybdate catalyst played a key role in determining the catalytic performance in the oxidative dehydrogenation of n-butene to 1,3-butadiene.     

Thermal decomposition of 1-butyne in shock waves

1-Butyne diluted with Ar was heated behind reflected shock waves over the temperature range of 1100–1600 K and the total density range of 1.36 × 10^?5?1.75 × 10^?5 mol/cm³. Reaction products were analyzed by gas-chromatography. The progress of the reaction was followed by IR laser kinetic absorption spectroscopy. The products were CH₄, C₂H₂, C₂H₄, C₂H₆, allene, propyne, C₄H₂, vinylacetyiene, 1,2- butadiene, 1,3-butadiene, and benzene. The present data were successfully modeled with a 80 reaction mechanism. 1-Butyne was found to isomerize to 1,2-butadiene. The initial decomposition was dominated by 1-butyne → C₃H₃ + CH₃ under these conditions. Rate constant expressions were derived for the decomposition to be k₇ = 3.0 × 10¹⁵ exp(?75800 cal/RT) s^?1 and for the isomerization to be k₄ = 2.5 × 10¹³ exp(?65000 cal/RT) s^?1. The activation energy 75.8 kcal/mol was cited from literature value and the activation energy 65 kcal/mol was assumed. These rate constant expressions are applicable under the present experimental conditions, 1100–1600 K and 1.23–2.30 atm. © 1995 John Wiley & Sons, Inc.     

The pyrolysis of ethane in the presence of nitric oxide

The kinetics of the ethane pyrolysis have been studied at temperatures from 550 to 596°C and with 0 to 62% of added nitric oxide. The rates of production of various products were studied by gas chromatography; ethylene, hydrogen, methane, nitrogen, water, nitrous oxide and acetonitrile were found as primary products, with hydrogen cyanide, carbon monoxide, acetaldehyde, n-butane, 1-butene, cis- and trans-2-butene and 1,3-butadiene as secondary products. For all the primary products the orders with respect to C₂H₆and NO were determined, as were the activation energies at two different percentages of NO (15.7 and 45.5%). Nitric oxide was found to be rapidly consumed with a finite initial rate, and the rate of production of H₂O was close to that of C₂H₄ at higher nitric oxide pressures. A mechanism is proposed which gives good agreement with all of the observed results. Its main features are: (1) Initiation takes place mainly by the unimolecular dissociation of ethane; there is no evidence for or against the process NO + C₂H₆ → HNO + C₂H₅; (2) NO scavenges ethyl radicals to form acetaldoxime which decomposes, and in this way the breakdown of C₂H₅ is hastened; (3) termination takes place mainly by the unimolecular decomposition of acetaldoxime to give inactive products. Some of the relevant rate parameters are evaluated. Reactions are proposed to account for the formation of the secondary products observed.     

Rate constants for the gas-phase reactions of O3 with a series of alkenes at 296 ± 2 K

The kinetics of the gas-phase reactions of O₃ with a series of alkenes have been investigated at atmospheric pressure (ca. 740 torr) of air and 296 ± 2 K, using a relative rate method in the presence of sufficient n-octane to scavenge any OH radicals generated in these reactions. Relative to k(O₃ + propene) = 1.00, the rate constants obtained were: 1-butene, 0.975 ± 0.030; 2-methylpropene, 1.14 ± 0.04; 2-methyl-1,3-butadiene (isoprene), 1.21 ± 0.02; 1,4-cyclohexadiene, 4.75 ± 0.23; cyclohexene, 7.38 ± 0.48; cis-2-butene, 12.8 ± 0.8; trans-2-butene, 21.5 ± 1.5; 2-methyl-2-butene, 42.1 ± 2.8; cyclopentene, 64.9 ± 4.3; and 2,3-dimethyl-2-butene, 123 ± 11. These relative rate constants have been placed on an absolute basis using a rate constant for the reaction of O₃ with propene of 1.01 × 10^?17 cm³ molecule^?1 s^?1 at 296 K derived from an analysis of the available literature data. The resulting rate constants then lead to a self-consistent set of room temperature data for the reactions of O₃ with these alkenes. © John Wiley & Sons, Inc.     

Ozone–olefin reactions in the gas phase 1. Rate constants and activation energies

Reactions of ozone with simple olefins have been studied between 6 and 800 mtorr total pressure in a 220-m³ reactor. Rate constants for the removal of ozone by an excess of olefin in the presence of 150 mtorr oxygen were determined over the temperature range 280 to 360° K by continuous optical absorption measurements at 2537 Å. The technique was tested by measuring the rate constants k₁ and k₂ of the reactions (1) NO + O₃ → NO₂ + O₂ and (2) NO₂ + O₃ rarr; NO₃ + O₂ which are known from the literature. The results for NO, NO₂, C₂H₄, C₃H₆, 2-butene (mixture of the isomers), 1,3→butadiene, isobutene, and 1,1 -difluoro-ethylene are 1.7 × 10^{?1 4} (290°K), 3.24 × 10^?17 (289°K), 1.2 × 10^{?1 4} exp (–4.95 ± 0.20/RT), 1.1 × 10^{?1 4} exp (–3.91 ± 0.20/RT), 0.94 × 10^{?1 4} exp ( –2.28 ± 0.15/RT), 5.45 ± 10^{?1 4} exp ( –5.33 ± 0.20/RT), 1.8 ×10^?17 (283°K), and 8 × 10^?20 cm³/molecule ·s(290°K). Productformation from the ozone–propylene reaction was studied by a mass spectrometric technique. The stoichiometry of the reaction is near unity in the presence of molecular oxygen.     

Reactions of ozone with olefins: Ethylene,allene, 1,3-butadiene,and trans- 1,3-pentadiene

The reactions of O₃ with ethylene, allene, 1,3-butadiene, and trans-1,3-pentadiene have been studied in the presence of excess O₂ over the temperature range 232 to 298 K. The initial O₃ pressure was varied from 4–18 mtorr, and the olefin pressure was varied from 0.1 to 4.5 torr (ethylene), 2.8 to 39.6 torr (allene), 52.7 to 600 mtorr (1,3-butadiene) or 26.2 to 106 mtorr (trans-1,3-pentadiene). The O₃ decay was monitored by ultraviolet absorption. The reactions are first order in both O₃ and olefin, and the rate coefficients are independent of the O₂ pressure. For the O₃-ethylene system, various diluent gases (O₂, N₂, air) were used and the rate coefficients were found to be independent of the nature of the diluent gas. The various rate coefficients fit the Arrhenius expressions (k in cm³ s^?1): where the reported uncertainties are one standard deviation and R is in cal/mol K.     

Reactions of C2H with 1- and 2-butynes: an ab initio/RRKM study of the reaction mechanism and product branching ratios

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311G** calculations of the C(6)H(7) potential energy surface are combined with RRKM calculations of reaction rate constants and product branching ratios to investigate the mechanism and product distribution in the C(2)H + 1-butyne/2-butyne reactions. 2-Ethynyl-1,3-butadiene (C(6)H(6)) + H and ethynylallene (C(5)H(4)) + CH(3) are predicted to be the major products of the C(2)H + 1-butyne reaction. The reaction is initiated by barrierless ethynyl additions to the acetylenic C atoms in 1-butyne and the product branching ratios depend on collision energy and the direction of the initial C(2)H attack. The 2-ethynyl-1,3-butadiene + H products are favored by the central C(2)H addition to 1-butyne, whereas ethynylallene + CH(3) are preferred for the terminal C(2)H addition. A relatively minor product favored at higher collision energies is diacetylene + C(2)H(5). Three other acyclic C(6)H(6) isomers, including 1,3-hexadiene-5-yne, 3,4-hexadiene-1-yne, and 1,3-hexadiyne, can be formed as less important products, but the production of the cyclic C(6)H(6) species, fulvene, and dimethylenecyclobut-1-ene (DMCB), is predicted to be negligible. The qualitative disagreement with the recently measured experimental product distribution of C(6)H(6) isomers is attributed to a possible role of the secondary 2-ethynyl-1,3-butadiene + H reaction, which may generate fulvene as a significant product. Also, the photoionization energy curve assigned to DMCB in experiment may originate from vibrationally excited 2-ethynyl-1,3-butadiene molecules. For the C(2)H + 2-butyne reaction, the calculations predict the C(5)H(4) isomer methyldiacetylene + CH(3) to be the dominant product, whereas very minor products include the C(6)H(6) isomers 1,1-ethynylmethylallene and 2-ethynyl-1,3-butadiene.     

Influence of the double bond position in combustion chemistry of methyl butene isomers: A shock tube and laser absorption study

Shock tube experiments have been carried out on 2-methyl-1-butene (2M1B), 2-methyl-2-butene (2M2B), and 3-methyl-1-butene (3M1B)—the three isomers of methyl butene compound. Carbon monoxide (CO) time-histories and ignition delay times are obtained behind reflected shockwaves over the temperature range of 1350-1630 K and pressures of 8.3-10.5 atm with stoichiometric mixtures of 0.075% fuel in O₂/Ar. Comparative ignition study reveals that 3M1B ignites significantly faster than the other two isomers, while 2M1B dissociates earlier but ignites later than 2M2B. Possible mechanisms for this behavior are discussed with ignition delay time sensitivity and reaction path analysis. In addition, time-resolved CO measurements are compared with three different reaction mechanisms from the literature. Sensitivity analyses have been carried out to identify important reactions that need attention to accurately predict the chemistry of these isomers. Further investigation into the rates of unimolecular fuel decomposition reactions and C₃H₃ + O₂ = CH₂CO + HCO reaction are suggested based on the current investigation.     

The kinetics of the gas-phase reaction between ozone and alkenes

The rate law ? d[O₃]/ dt = k₁[A][O₃] + k₃[A][O₃]²/ (k₄ + k₅[O₂]) has been found to obtain for the reaction of ozone with allene and with 1,2-butadiene. We now find that this rate law also applies to the reaction of ozone with ethylene and presumably with all lower alkenes. This generalizes the inhibiting effect of oxygen and accounts for the simplifed rate law found in the presence of excess oxygen. Oxygen itself is a product of the ozone–ethylene reaction, and we find that as [O₃]₀ increases, the (O₂ formed)/(O₃ used) ratio approaches 1.5. Values of k₁, k₃/k₅ for ethylene are compared with those for allene, 1,3-butadiene, and propene. A generalized mechanism is postulated for the reaction of ozone with alkenes involving a chain sequence that produces oxygen and which accounts for the observed rate law. A specific mechanism is postulated for the reaction of O₃ with ethylene, and the thermochemistry of the chain sequence is examined in detail.     

Fourier transform infrared study of the self reaction of C2H5O2 radicals in air at 295 K

Fourier transform infrared spectroscopy was used to identify and quantify products of the self reaction of ethylperoxy radicals, C₂H₅O₂, formed in the photolysis of Cl₂/C₂H₆ mixtures in 700 torr total pressure of synthetic air at 295 K. From these measurements, branching ratios for the reaction channels (1) of k_1a/(k_1a + k_1b) = 0.68 and k_1c/(k_1a + k_1b + k_1c) ? 0.06 were established. Additionally, using the relative rate technique, the rate constant for the reaction of Cl atoms with C₂H₅OOH was determined to be (1.07 ± 0.07) × 10^?10 × cm³ molecule^?1 s^?1. Results are discussed with respect to the previous kinetic and mechanistic studies of C₂H₅O₂ radicals.     

Shock tube and modeling study of 1,3-butadiene pyrolysis

1,3-Butadiene (1,3-C₄H₆) was heated behind reflected shock waves over the temperature range of 1200–1700 K and the total density range of 1.3 × 10⁻⁵ −2.9 × 10⁻⁵ mol/cm³. Reaction products were analyzed by gas-chromatography. The concentration change of 1,3-butadiene was followed by UV kinetic absorption spectroscopy at 230 nm and by quadrupole mass spectrometry. The major products were C₂H₂, C₂H_4, C₄H₄, and CH₄. The yield of CH₄ for a 0.5% 1,3-C₄H₆ in Ar mixture was more than 10% of the initial 1.3-C₄H₆ concentration above 1500 K. In order to interpret the formation of CH₄ successfully, it was necessary to include the isomerization of 1,3-C₄H₆ to 1,2-butadiene (1,2-C₄H₆) and to include subsequent decomposition of the 1,2-C₄H₆ to C₃H₃ and CH₃. The present data and other shock tube data reported over a wide pressure range were qualitatively modeled with a 89 reaction mechanism, which included the isomerizations of 1,3-C₄H₆ to 1,2-C₄H₆ and 2-butyne (2-C₄H₆). © 1996 John Wiley & Sons, Inc.     

Structural studies on platinum alkene complexes and precursors

A number of platinum complexes, precursors to alkene complexes (Pt₂Cl₄(PPh₃)₂ and cis-PtCl₂(CH₃CN)(PPh₃)), alkene complexes (cis-PtCl₂(C₂H₄)(PPh₃), cis-PtCl₂(C₃H₆)(PPh₃) and cis-PtCl₂(1-C₆H₁₂)(PPh₃)), the diamination product of a 1,3-butadiene platinum complex and the 1,2,3,4-tetramethylcyclobutadiene complex resulting from dimerization of 2-butyne have been synthesized, characterized and the structures determined by X-ray diffraction. The ethylene complex, cis-PtCl₂(C₂H₄)(PPh₃), has been a useful reagent for preparing other alkene complexes. Reaction of a bound butadiene complex with diethylamine yielded a diamination product with anti-Markovnikov stereochemistry. An attempt at binding cis-butyne to the metal center resulted in metal-assisted formation of 1,2,3,4-tetramethylcyclobutadiene with previously unreported geometry.     

The thermal reaction of 2-pentene around 500°C at low extent of reaction

The thermal reaction of 2-pentene (cis or trans) has been performed in a static system over the temperature range of 470°–535°C at low extent of reaction and for initial pressures of 20–100 torr. The main products of decomposition are methane and 1,3-butadiene. Other minor primary products have been monitored: trans-2-pentene, trans- and cis-2-butenes, ethane, 1,3-pentadienes, 3-methyl-1-butene, propylene, 1-butene, hydrogen, ethylene, and 1-pentene. The initial orders of formation, 0.8–1.1 for most of the products and 1.5–1.8 for 1-pentene, increase with temperature. The formation of the products and the influence of temperature on their orders can be essentially explained by a free radical chain mechanism. But cis–trans or trans–cis isomerization and hydrogen elimination from cis-2-pentene certainly involve both molecular and free radical processes. The formation of 1-pentene mainly occurs from the abstraction of the hydrogen atom of 2-pentene by resonance stabilized free radicals (C₅H₉.).     

FTIR study of the reaction of ethyl radicals with O2 and Cl2 in air at 295 K

Using Fourier transform infrared spectroscopy, the ethene yield from the reaction of C₂H₅ radicals with O₂ has been determined to be 1.50 ± 0.09%, 0.85 ± 0.11%, and <0.1% at total pressures of 25, 50, and 700 torr, respectively. Additionally, the rate constant of the reaction of C₂H₅ radicals with molecular chlorine was measured relative to that with molecular oxygen. (1) A ratio k₆/k₇ = 1.99 ± 0.14 was measured at 700 torr total pressure which, together with the literature value of k₇ = 4.4 × 10^?12 cm³ molecule^?1s^?1, yields k₆ = (8.8 ± 0.6) × 10^?12 cm³ molecule^?1s^?1. Quoted errors represent 2σ. These results are discussed with respect to previous kinetic and mechanistic studies of C₂H₅ radicals.     

Formation yields of epoxides and O(3P) atoms from the gas-phase reactions of O3 with a series of alkenes

Reactions of ozone with propene, 1-butene, cis-2-butene, trans-2-butene, 2,3-dimethyl-2-butene, and 1,3-butadiene were carried out in N₂ and air diluent at atmospheric pressure and room temperature and, by monitoring the formation of the epoxides and/or a carbonyl compound formed from the reactions of O(³P) atoms with these alkenes, the formation yields of O(³P) atoms from the O₃ reactions were investigated. No evidence for O(³P) atom formation was obtained, and upper limits to O(³P) atom formation yields of <4% for propene, <5% for 1.3-butadiene, and <2% for the other four alkenes were derived. The reaction of O₃ with 1,3-butadiene led to the direct formation of 3,4-epoxy-1-butene in (2.3 ± 0.4)% yield. These data are in agreement with the majority of the literature data and show that O(³P) atom formation is not a significant pathway in O₃—alkene reactions, and that epoxide formation only occurs to any significant extent from conjugated dienes. © 1994 John Wiley & Sons, Inc.     

Pressure dependence of the reaction Cl + C2H2 over the temperature range 230 to 370 K

The pressure dependence of reaction (1), Cl + C₂H₂ + M → C₂H₂Cl + M, has been measured by a relative rate technique using the pressure independent abstraction reaction (2), Cl + C₂H₆ → C₂H₅ + HCl, as the reference. Values of k₁/k₂ were measured at pressures between 25 and 1300 torr at four temperatures ranging from 252 to 370 K, using air, N₂, or SF₆ diluent gases. Low pressure measurements (10–50 torr) were performed at 230 K. Assuming a temperature-independent center broadening factor of 0.6 in the Troe formalism and using the established value of k₂, these data can be used to determine the temperature dependent high and low pressure limiting rate constants over the range of conditions studied in air for reaction (1): k_∞(1) = 2.13 × 10^?10 (T/300)^?1.045 cm³/molecule-s; and k₀(1) = 5.4 × 10^?30 (T/300)^?2.09 cm⁶/molecule²-s. Use of these expressions yields rate constants with an estimated 20% accuracy including uncertainty in the reference reaction. The data indicate that the rate constant for a typical stratospheric condition at 30 km altitude is approximately 50% of that previously estimated.     

Alkenoxy radicals in gas-phase reactions of alkenes with oxygen atoms or ozone

Observations in the O₃ + trans-2-butene reaction system and in the O + trans-2-butene + O₂ reaction system suggest the intermediacy of alkenoxy radicals. A mechanism is proposed for the production of C_n and C_m (m <n) alkenoxy radicals by the reaction of C_nH_2n alkenes with oxygen atoms or with ozone.     

The reaction of O(1D) with CH4

The room-temperature photolysis of N₂O (10–100 torr) at 2139 Å to produce O(¹D) has been studied in the presence of CH₄ (10–891 torr). The reactions of O(¹D) with CH₄ were found to be The method of chemical difference was used to measure the rate constant ratio k₄/(k₂ + k₃), where reactions (2) and (3) are The CH₃ radicals produced in reaction (4) react with the O₂ and NO produced in reactions (2) and (3). Thus, near the endpoint of the internal titration, ?{C₂H₆} gives an accurate measure of k₄/(k₂ + k₃). For the translationally energetic O(¹D) atoms produced in the photolysis, k₄/(k₂ + k₃) = 2.28 ± 0.20. However, if He is added to remove the excess translational energy, then k₄/(k₂ + k₃) drops to 1.35 ± 0.3.     

Arrhenius constants for the reactions of ozone with ethylene and acetylene

The kinetics of ozonation of C₂H₄ and C₂H₂ have been studied in the gas phase from ?40 to ?95°C (C₂H₄) and +10 to ?30°C (C₂H₂). The O₃ concentrations were near 10^?4 M, and the hydrocarbons were present in 2- to 25-fold excess. A few experiments with propylene were also carried out. The reactions were followed by observing the rate of decay of O₃ absorption at 2537 Å. Reaction stoichiometries and effects of added O₂ were investigated. The second-order rate constant for C₂H₄ was log k(M^?1 sec^?1) = (6.3 ± 0.2) – (4.7 ± 0.2)/θ (θ = 2.3RT). The rate was independent of the presence of excess O₂. Rate measurements for C₃H₆ were less accurate because of aerosol interference. Combined with room temperature measurements of other workers, the C₃H₆ rate constant was log k(M^?1 sec^?1) = (6.0 ± 0.4) – (3.2 ± 0.6)/θ. The C₂H₂ rate constant was log k(M^?1 sec^?1) = (9.5 ± 0.4) – (10.8 ± 0.4)/θ. In the case of C₃H₆ the major product was propylene ozonide. Ethylene did not yield the ozonide, and the products of the O₃–C₂H₄ and O₃–C₂H₂ reactions were not identified. Pre-exponential factors for the olefin reactions are consistent with a five-membered ring transition state formed by 1,3 dipolar cycloaddition of O₃. For C₂H₂, however, the much higher observed A factor suggests a different mechanism. Possible transition states for the O₃–C₂H₂ reaction are discussed.