Kinetics and Thermochemistry of the Cl(2PJ) + C2Cl4 Association Reaction

Reaction of Cl Atom with c-C5F8 and c-C5HF7: Relative and Absolute Measurements of Rate Coefficients and Identification of Degradation Products

Partially and fully fluorinated olefins are a class of compounds with relatively short atmospheric lifetimes and low 100-year global warming potentials, compared to their saturated predecessors, which are used or considered as refrigerants, propellants, solvents, and other end-uses. The cyclic unsaturated compounds c-C₅F₈ and c-C₅HF₇ are currently under consideration as etching agents for the semiconductor industry. In this study, we expand on our previous work on the reaction of the OH radical with c-C₅F₈ and c-C₅HF₇ and report the rate coefficients, k, for the gas-phase reaction of the Cl atom with c-C₅F₈ and c-C₅HF₇ over a range of temperature (245-367 K) and pressure (100-200 Torr of He or N₂ and 0 to 4.8 Torr O₂) using a pulsed laser photolysis-resonance fluorescence (PLP-RF) technique. In addition, a relative rate (RR) technique, employing multiple reference compounds, was used to study the Cl atom reactions at 296 K, 100 and 630 Torr (N₂ or air) total pressure. Reaction rate coefficients, k₁, of the Cl atom reaction with c-C₅F₈ were found to be independent of pressure, over the pressure range used in this work, with k₁(296 K), derived as an average of results from the PLP-RF and RR techniques being (1.07 ± 0.02) × 10^-12 cm³ molecule^-1 s^-1 and k₁(T) = (7.76 ± 0.73) × 10^-13 × (exp[(98 ± 26)/T]) cm³ molecule^-1 s^-1, where the quoted error limits represent the 2σ data precision. Rate coefficients, k₂, for the Cl atom + c-C₅HF₇ reaction were measured to be k₂(296 K) = (4.61 ± 0.10) × 10^-12 cm³ molecule^-1 s^-1 and k₂(T) = (7.42 ± 0.89) × 10^-13 × (exp[(540 ± 32)/T]) cm³ molecule^-1 s^-1. The Cl atom temporal profiles, observed with the PLP-RF technique, indicate that the Cl atom with c-C₅F₈ and c-C₅HF₇ reactions lead to adduct formation. The equilibrium constants for adduct formation were derived in this work, and a second-law analysis was used to obtain ΔH and ΔS values of -18.5 ± 0.4 kcal mol^-1, -30.9 ± 1.2 cal K^-1 mol^-1, and -13.9 ± 0.5 kcal mol^-1, -27.6 ± 1.1 cal K^-1 mol^-1 for the c-C₅F₈ and c-C₅HF₇ reactions, respectively. The Cl-initiated degradation of c-C₅F₈ and c-C₅HF₇ in the presence of O₂ was studied and stable products were identified via infrared spectroscopy using experimental or theoretically derived spectra from our previous OH reaction work. For c-C₅F₈, FC(O)CF₂CF₂CF₂C(O)F and FC(O)C(O)F were observed with molar yields of 0.80 and 0.10, respectively. For c-C₅HF₇, we observed the formation of HC(O)CF₂CF₂CF₂C(O)F and HC(O)C(O)F with a combined molar yield of 0.72. Carbonyl difluoride, F₂CO, was also a major product in the decomposition of c-C₅F₈ and c-C₅HF₇. The oxidation mechanism of the Cl-initiated degradation of c-C₅F₈ and c-C₅HF₇ is discussed. Based on the combined findings from this and our previous work, the atmospheric implications from the use of c-C₅F₈ and c-C₅HF₇ are presented.

Assessing the viability of electro-absorption and photoelectro-absorption for the treatment of gaseous perchloroethylene

This work focuses on the development of electro-absorption and photoelectro-absorption technologies to treat gases produced by a synthetic waste containing the highly volatile perchloroethylene (PCE). To do this, a packed absorption column coupled with a UV lamp and an undivided electrooxidation cell was used. Firstly, it was confirmed that the absorption in a packed column is a viable method to achieve retention of PCE into an absorbent-electrolyte liquid. It was observed that PCE does not only absorb but it was also transformed into phosgene and other by-products. Later, it was confirmed that the electro-absorption process influenced the PCE degradation, favoring the transformation of phosgene into final products. Opposite to what is expected, carbon dioxide is not the main product obtained, but carbon tetrachloride and trichloroacetic acid. Both species are also hazardous but their higher solubility in water opens possibilities for a successful and more environmental-friendly removal. The coupling with UV-irradiation has a negative impact on the degradation of phosgene. Finally, a reaction mechanism was proposed for the degradation of PCE based on the experimental observations. Results were not as expected during the planning of the experimental work but it is important to take in mind that PCE decomposition occurs in wet conditions, regardless of the applied technology, and this work is a first approach to try to solve the treatment problems associated to PCE gaseous waste flows in a realistic way.

Kinetic fall-off behavior for the Cl + Furan-2,5-dione (C4H2O3, maleic anhydride) reaction

Rate coefficients, k, for the gas-phase Cl + Furan-2,5-dione (C4H2O3, maleic anhydride) reaction were measured over the 15-500 torr (He and N2 bath gas) pressure range at temperatures between 283 and 323 K. Kinetic measurements were performed using pulsed laser photolysis (PLP) to produce Cl atoms and atomic resonance fluorescence (RF) to monitor the Cl atom temporal profile. Complementary relative rate (RR) measurements were performed at 296 K and 620 torr pressure (syn. air) and found to be in good agreement with the absolute measurements. A Troe-type fall-off fit of the temperature and pressure dependence yielded the following rate coefficient parameters: ko(T) = (9.4 ± 0.5) × 10-29 (T/298)-6.3 cm6 molecule-2 s-1, k∞(T) = (3.4 ± 0.5) × 10-11 (T/298)-1.4 cm3 molecule-1 s-1. The formation of a Cl·C4H2O3 adduct intermediate was deduced from the Cl atom temporal profiles and an equilibrium constant, KP(T), for the Cl + C4H2O3 ↔ Cl·C4H2O3 reaction was determined. A third-law analysis yielded ΔH = -15.7 ± 0.4 kcal mol-1 with ΔS = -25.1 cal K-1 mol-1, where ΔS was derived from theoretical calculations at the B3LYP/6-311G(2d,p,d) level. In addition, the rate coefficient for the Cl·C4H2O3 + O2 reaction at 296 K was measured to be (2.83 ± 0.16) × 10-12 cm3 molecule-1 s-1, where the quoted uncertainty is the 2σ fit precision. Stable end-product molar yields of (83 ± 7), (188 ± 10), and (65 ± 10)% were measured for CO, CO2, and HC(O)Cl, respectively, in an air bath gas. An atmospheric degradation mechanism for C4H2O3 is proposed based on the observed product yields and theoretical calculations of ring-opening pathways and activation barrier energies at the CBS-QB3 level of theory.

Rheological properties of milk-based desserts with the addition of oat gum and κ-carrageenan

A growing interest in development of milk desserts with good nutritional and rheological properties can be observed. A good and stability rheological as well as nutritional properties of such desserts can be provided by applying suitably composed gum mixtures. In this work, the effect of 0.1% κ-carrageenan addition on the rheological properties of based-milk desserts with different oat gum concentrations (0.1, 0.3 and 0.5%) was investigated. All milk desserts tested in presented study showed a time dependent and shear-thinning flow behavior. The mechanical spectra were characterized by storage module (G’) greater than loss module (G”), typical for viscoelastic materials such as gels and dispersions. The incorporation of 0.1% κ-carrageenan into milk dessert with different oat gum concentrations allows to obtain stronger gel structure compared to milk dessert with separate oat gum addition. It can be also observed that desserts systems with the 0.1% κ-carrageenan had more stable viscoelastic properties. Moreover, the use the κ-carrageenan addition caused an increase in consistency coefficient (K) and decreased in n-value for Ostwald de Waele rheological model. Combined addition of oat gum and carrageenan allows to obtain milk dessert with stronger texture. The hardness of milk desserts range from 0.32 to 0.49 N for desserts without κ-carrageenan addition and from 0.513 to 0.557 N for desserts with κ-carrageenan. The high synergistic effect of composed gum mixtures on rheological properties of milk dessert occurs at 0.1% oat gum and 0.1% κ-carrageenan concentration.

Dual-Level Direct Dynamics Studies on the Hydrogen Abstraction Reactions of CH2CH3 - n Xn+HBr (X=Cl, Br and n=1, 2)

The reactions of the HBr molecule with CH2CH2Cl (reaction R1), CH2CHCl2 (R2), CH2CH2Br (R3) and CH2CHBr2 (R4) are investigated by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points were calculated at the MPW1K/6–311+G(d,p) and BMK/6–311+G(d,p) levels. To refine the reaction enthalpy and energy barrier height of each reaction, single-point energy calculations were carried out by the G2M(RCC5) method based on the geometries optimized at the above-mentioned two levels. Using the canonical variational transition state theory or the canonical variational transition state theory with the small-curvature tunneling correction, the rate constants of HBr with CH2CH2Cl (R1), CH2CHCl2 (R2), CH2CH2Br (R3), and CH2CHBr2 (R4) were calculated over a wide temperature range of 200–2000 K at the G2M(RCC5)//MPW1K/6–311+G(d,p) level. The effect of chlorine or bromine substitution on the ethyl radical reactivity is discussed. Finally, the total rate constants are fitted by two models, i.e. three-parameter and four-parameter expressions.

Chlorine atom initiated reactions of selected tropospheric halocarbons — Kinetic and product studies

Halogens are suggested as important atmospheric oxidants in the marine boundary layer. The room-temperature kinetics of the chlorine-initiated reactions of three biogenic brominated hydrocarbons and four anthropogenic chlorinated ethenes was investigated by gas chromatography with flame ionization detection (GC–FID) at a pressure of 1 atm (1 atm = 101.325 kPa) in air, using the relative rate technique. The rate constants (× 1013 cm3 molecule–1 s–1) for CH2Br2, CHBr2Cl, and CHBr3 reactions at 298 ± 2 K were found to be 4.25 ± 0.65, 2.03 ± 0.31, and 2.81 ± 0.41, respectively, using methane as a reference compound. Room temperature rate constants (±1011 cm3 molecule–1 s–1) obtained for 1,1-dichloroethene, cis-dichloroethene, trans-dichloroethene, and trichloroethene using ethene as a reference are 13.4 ± 3.3, 9.1 ± 2.3, 7.4 ± 1.8, and 7.7 ± 1.9, respectively. The rate constants of chlorine-atom reactions with various hydrocarbons obtained in this work and taken from literature were correlated with corresponding rate constants of the OH radical available in the literature. The temperature dependences for the reactions of chlorine atoms with chlorinated ethenes were studied within the 298–358 K range. The corresponding Arrhenius expressions for the rate constants are (cm3 molecule–1 s–1): ln k = (–25.26 ± 0.17) – (758 ± 55)/T for 1,1-dichloroethene, ln k = (–25.79 ± 0.10) – (799 ± 34)/T for cis-dichloroethene, ln k = (–26.74 ± 0.09) – (1018 ± 28)/T for trans-dichloroethene, and ln k = (–26.10 ± 0.26) – (846 ± 83)/T for trichloroethene. In addition, product studies for the chlorine-initiated gas phase oxidation reactions of CHBr3 and CHBr2Cl were performed using gas chromatography with mass spectrometric detection (GC–MS). The only identified product for the reaction of CHBr3 with Cl reaction was COBr2, while for the CHBr2Cl + Cl reaction, COBrCl and COCl2 were observed, indicating the possibility of halogen atom release. The atmospheric implications of the results obtained are discussed.Key words: tropospheric reactions, kinetics, chlorine atoms, chlorinated hydrocarbons, brominated hydrocarbons.

Kinetics, Mechanism, and Thermochemistry of the Gas Phase Reaction of Atomic Chlorine with Dimethyl Sulfoxide

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction of chlorine atoms with dimethyl sulfoxide (CH3S(O)CH3; DMSO) as a function of temperature (270-571 K) and pressure (5-500 Torr) in nitrogen bath gas. At T = 296 K and P > or = 5 Torr, measured rate coefficients increase with increasing pressure. Combining our data with literature values for low-pressure rate coefficients (0.5-3 Torr He) leads to a rate coefficient for the pressure independent H-transfer channel of k1a = 1.45 x 10(-11) cm3 molecule(-1) s(-1) and the following falloff parameters for the pressure-dependent addition channel in N2 bath gas: k(1b,0) = 2.53 x 10(-28) cm6 molecule(-2) s(-1); k(1b,infinity) = 1.17 x 10(-10) cm3 molecule(-1) s(-1), F(c) = 0.503. At the 95% confidence level, both k1a and k1b(P) have estimated accuracies of +/-30%. At T > 430 K, where adduct decomposition is fast enough that only the H-transfer pathway is important, measured rate coefficients are independent of pressure (30-100 Torr N2) and increase with increasing temperature. The following Arrhenius expression adequately describes the temperature dependence of the rate coefficients measured at over the range 438-571 K: k1a = (4.6 +/- 0.4) x 10(-11) exp[-(472 +/- 40)/T) cm3 molecule(-1) s(-1) (uncertainties are 2sigma, precision only). When our data at T > 430 K are combined with values for k1a at temperatures of 273-335 K that are obtained by correcting reported low-pressure rate coefficients from discharge flow studies to remove the contribution from the pressure-dependent channel, the following modified Arrhenius expression best describes the derived temperature dependence: k1a = 1.34 x 10(-15)T(1.40) exp(+383/T) cm3 molecule(-1) s(-1) (273 K < or = T < or = 571 K). At temperatures around 330 K, reversible addition is observed, thus allowing equilibrium constants for Cl-DMSO formation and dissociation to be determined. A third-law analysis of the equilibrium data using structural information obtained from electronic structure calculations leads to the following thermochemical parameters for the association reaction: delta(r)H(o)298 = -72.8 +/- 2.9 kJ mol(-1), deltaH(o)0 = -71.5 +/- 3.3 kJ mol(-1), and delta(r)S(o)298 = -110.6 +/- 4.0 J K(-1) mol(-1). In conjunction with standard enthalpies of formation of Cl and DMSO taken from the literature, the above values for delta(r)H(o) lead to the following values for the standard enthalpy of formation of Cl-DMSO: delta(f)H(o)298 = -102.7 +/- 4.9 kJ mol(-1) and delta(r)H(o)0 = -84.4 +/- 5.8 kJ mol(-1). Uncertainties in the above thermochemical parameters represent estimated accuracy at the 95% confidence level. In agreement with one published theoretical study, electronic structure calculations using density functional theory and G3B3 theory reproduce the experimental adduct bond strength quite well.

High temperature oxidation of C2Cl4/CH4 mixtures

Experiments on high temperature oxidation of multi-chlorinated hydrocarbons, tetrachloroethylene (C2Cl4), with hydrocarbon fuels, CH4, were performed in a 15 mm i.d. tubular flow reactor. Temperatures ranged from 700 to 850 degrees C, with the average residence time in the range from 0.3 to 1.5s. Three equivalence ratios, phi=0.87 (fuel-lean (FL)), phi=1 (stoichiometry (S)), and phi=1.3 (fuel-rich (FR)), were studied. The global Arrhenius equations for the decomposition of C(2)Cl(4) for each reactant set ratio are: k(lean)=5.77 x 10(15) exp(-30447/RT), k(stoi)=5.15 x 10(15) exp(-30421/RT), and k(rich)=6.32 x 10(14) exp(-28879/RT). The important reactions for destruction of parent C2Cl4 include: C2Cl4 --> C2Cl3 + Cl, C2Cl4 + H--> C2Cl3 + HCl and C2Cl4 + H --> C2HCl3 + Cl. The resulting reactant loss, and intermediate and final product profiles were determined. C2HCl3, C2Cl2, CO, CO2 and HCl are the major products for the reaction of C2Cl4/CH4/O2 mixtures for these three reaction systems. Minor intermediates include C2H3Cl, C2HCl, COCl2, CH3CHCl2, C2H4, C2H6, CCl2CHCH3 , trans-CHClCHCl, cis-CHClCHCl, trans-ClHC=CClCH(3), C6H6, and Cl2. The experimental data showed that as the oxygen concentration increased, the temperature needed to detect the resulting products decreased.