Formation of Products of Incomplete Destruction (PID) from the Thermal Oxidative Decomposition of Perfluorooctanoic Acid (PFOA): Measurement, Modeling, and Reaction Pathways.

The thermal decomposition of perfluorooctanoic acid (PFOA) under oxidative conditions was investigated using air (O2) and N2O as oxidants over temperatures ranging from 400 to 1000 °C in an α-alumina reactor. In the presence of air, PFOA was found to decompose into perfluorohept-1-ene (C7F14) and perfluoroheptanoyl fluoride (C7F14O) in addition to HF, CO, and CO2. At temperatures above 800 °C, both C7F14 and C7F14O were no longer detected. A comprehensive analysis of the reaction mechanisms through quantum chemical analysis and kinetic modeling in combination with experimental observations was utilized to identify key reaction pathways. Quantum chemical analysis led to the conclusion that oxygen atoms are crucial in decomposing perfluoroalk-1-enes, especially the stable perfluorohept-1-ene (C7F14). Under oxidative conditions, it was found that significant quantities of C2F6 and CF4 were formed. Further quantum chemical analysis suggests that the O atoms facilitate the formation of volatile fluorinated compounds (VFCs) such as tetrafluoromethane (CF4) and hexafluoroethane (C2F6), particularly at higher temperatures. By elucidating these key reactions, an improved understanding of the potential formation products of incomplete combustion (PICs) or products of incomplete destruction (PIDs) is made.

Pyrolysis of Glyphosate and Its Toxic Products.

With health concerns developing about the use of glyphosate (phosphonomethylglycine, PMG), the world's most used herbicide, the possibility of destruction of stockpiles via incineration arises. Little is known, however, about the possible toxic products of decomposition. We have performed a quantum chemical computation of the mechanism of thermal decomposition of PMG. Two initiation channels, one producing sarcosine and the other producing N-methylaminomethylphosphonic acid, have been located. Both the initial products decompose to dimethylamine (DMA), and the mechanism of further decomposition and toxic products is explored. Global potential energy surfaces for the initial decomposition of DMA are presented together with chemical kinetic modeling wherein the rate constants employed have been calculated from the quantum chemical data. Time and temperature evolution of the expected toxic products are presented and discussed.

Mechanism of the Thermal Decomposition of Chlorpyrifos and Formation of the Dioxin Analog, 2,3,7,8-Tetrachloro-1,4-dioxino-dipyridine (TCDDpy).

Thermal decomposition of the pesticide, chlorpyrifos (CPf) and its major degradation product, 3,5,6-trichloro-2-pyridinol (TCpyol), has been studied by quantum chemical calculation using density functional methods at the M06-2X/GTLarge//M06-2 X/6-31+G(d,p) level of theory. Chlopyrifos was found to undergo a series of unimolecular stepwise elimination reactions releasing two molecules of ethylene and finally HOPOS to form TCpyol. TCpyol underwent oxidative decomposition initiated by abstraction of its phenolic H atom by O2. Two phenoxy radicals so produced underwent combination leading to the formation of 2,3,7,8-tetrachloro-[1,4]dioxinodipyridine (TCDDpy). Via Smiles rearrangement both cis and trans TCDDpy are formed. Kinetic models have been constructed to model the decomposition of CPf into TCpyol and of the latter into cis and trans TCDDpy. Modeled results are compared with the experiments of Sakiyama et al. ( Organohalogen Compounds, 2012, 74, 1441-1444).

Mechanism and Rate of Thermal Decomposition of Hexachlorocyclopentadiene and Its Importance in PCDD/F Formation from the Combustion of Cyclodiene Pesticides.

Thermal decomposition of hexachlorocyclopentadiene (HCCP) has been studied in inert gas and under oxidative conditions in a silica flow reactor at a residence time of 5.0 s between 690 and 923 K and 1 atm pressure. Pyrolysis was initiated by Cl bond fission to form pentachlorocyclopentadienyl radical; two such radicals then combined to undergo a series of intramolecular rearrangements and Cl fissions, producing principally octachloronaphthalene (8ClNP) and Cl2. This process has been studied by quantum chemical calculation, and a reaction potential energy surface has been developed. The rate constant of initial Cl atom fission has been calculated by canonical variational transition state theory as k = 1.45 × 1015 exp(-222 ± 9 kJ mol-1/RT) s-1 between 500 and 2000 K. A minimal kinetic model was developed to model the decomposition and major products. Oxidative decomposition was studied in nitrogen with O2 contents of 1, 6, 12, and 20 mol %. Increasing O2 to 6-8% increased the rate of decomposition of HCCP and decreased the yield of 8ClNP. Above 823 K, hexachlorobenzene (HCB) and CO became major products. The oxidative reaction has also been studied quantum chemically. At high O2 content (>∼10%), the rate of decomposition of HCCP declined as did yields of 8ClNP and HCB, but CO yields increased.

Gas Phase Thermal Oxidation of Endosulfan and Formation of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans.

This paper investigates the thermal decomposition of technical endosulfan under oxidative conditions and the subsequent formation of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/F, dioxins), and their precursors. Both quantum chemical calculations and laboratory experiments were employed to investigate the pathways of oxidation of endosulfan. The laboratory scale apparatus used consists of a tubular reactor and product collection system and analysis train. The results report the effect of temperature (523-923 K) and O2 concentration on PCDD/F formation in a N2 bath gas at a residence time of 5 s. The decomposition of endosulfan produces two types of PCDD/F precursors, involving all chlorinated benzenes (CBz) and chlorinated phenols (CPh). Oxidation of endosulfan favors the formation of PCDF over PCDD. Octachlorodibenzofuran is the most abundant homologue group detected in all experiments. The maximum emission factor for PCDD/F was observed at 923 K and O2 content of 6% and corresponds to 64 ng TEQ-WHO2005 per mg of endosulfan and a total dioxin concentration of 1131 ng/mg of endosulfan.

S-Nitrosation of Aminothiones.

Nitrosation reactions span a diverse range of applications, from biochemistry to industrially important processes. This study examines nitrosation of aminothiones in acidic solutions and re-evaluates currently accepted diffusion limits and the true nature of the nitrosating agent for nitrous acid initiated reactions. Experimental measurements from stopped-flow UV/vis spectrophotometry afforded derivation of equilibrium constants and reaction enthalpies. Apparent Keq corresponds to 559-382 M(-2) for thioacetamide (TA, 15-25 °C) and 12600-5590 M(-2) for thiourea (TU, 15-35 °C), whereas the reaction enthalpies amount to -27.10 ± 0.05 kJ for TA and -29.30 ± 0.05 kJ for TU. Theoretical calculations via a thermochemical cycle agree well with reaction free energies from experiments, with errors of -2-4 kJ using solvation method SMD in conjunction with hybrid meta exchange-correlation functional M05-2X and high-accuracy multistep method CBS-QB3 for gas-phase calculations. The kinetic rates increase with acidity at activation energies of 54.9 (TA) and 66.1 kJ·mol(-1) (TU) for the same temperature range, confirming activation-controlled reactions. At pH 1 and below, the main decomposition pathway for the S-nitroso species leads to formation of nitric oxide.

Thermodynamic stability and structure of cuprous chloride surfaces: a DFT investigation.

Density functional theory together with ab initio atomistic thermodynamics has been utilized to study the structures and stabilities of the low index CuCl surfaces. It is shown that the Cl-terminated structures are more stable than the Cu-terminated configurations, and that the defective CuCl(110)-Cu structure is more stable than the stoichiometric CuCl(110) surface. The equilibrium shape of a cuprous chloride nanostructure terminated by low-index CuCl surfaces has also been predicted using a Wulff construction. It was found that the (110) facets dominate at low chlorine concentration. As the chlorine concentration is increased, however, the contributions of the (100) and (111) facets to the Wulff construction also increase giving the crystal a semi-prism shape. At high chlorine concentration, and close to the rich limit, the (111) facets were found to be the only contributors to the Wulff construction, resulting in prismatic nanocrystals.

Accurate rate constants for decomposition of aqueous nitrous acid.

Decomposition of nitrous acid in aqueous solution has been studied by stopped flow spectrophotometry to resolve discrepancies in literature values for the rate constants of the decomposition reactions. Under the conditions employed, the rate-limiting reaction step comprises the hydrolysis of NO(2). A simplified rate law based on the known elementary reaction mechanism provides an excellent fit to the experimental data. The rate constant, 1.34 × 10(-6) M(-1) s(-1), is thought to be of higher accuracy than those in the literature as it does not depend on the rate of parallel reaction pathways or on the rate of interphase mass transfer of gaseous reaction products. The activation energy for the simplified rate law was established to be 107 kJ mol(-1). Quantum chemistry calculations indicate that the majority of the large activation energy results from the endothermic nature of the equilibrium 2HNO(2) ⇆ NO + NO(2) + H(2)O. The rate constant for the reaction between nitrate ions and nitrous acid, which inhibits HNO(2) decomposition, was also determined.

Rate constants for hydrogen abstraction reactions by the hydroperoxyl radical from methanol, ethenol, acetaldehyde, toluene, and phenol.

An important step in the initial oxidation of hydrocarbons at low to intermediate temperatures is the abstraction of H by hydroperoxyl radical (HO(2)). In this study, we calculate energy profiles for the sequence: reactant + HO(2) → [complex of reactants] → transition state → [complex of products] → product + H(2)O(2) for methanol, ethenol (i.e., C(2)H(3)OH), acetaldehyde, toluene, and phenol. Rate constants are provided in the simple Arrhenius form. Reasonable agreement was obtained with the limited literature data available for acetaldehyde and toluene. Addition of HO(2) to the various distinct sites in phenol is investigated. Direct abstraction of the hydroxyl H was found to dominate over HO(2) addition to the ring. The results presented herein should be useful in modeling the lower temperature oxidation of the five compounds considered, especially at low temperature where the HO(2) is expected to exist at reactive levels.

Experimental study of decomposition of aqueous nitrosyl thiocyanate.

This study has examined the kinetics of the decomposition of nitrosyl thiocyanate (ONSCN) by stopped flow UV-vis spectrophotometry, with the reaction products identified and quantified by infrared spectroscopy, membrane inlet mass spectrometry, ion chromatography, and CN(-) ion selective electrode. The reaction results in the formation of nitric oxide and thiocyanogen, the latter decomposing to sulfate and hydrogen cyanide in aqueous solution. The rate of consumption of ONSCN depends strongly on the concentration of SCN(-) ions and is inhibited by nitric oxide. We have developed a reaction mechanism that comprises three parallel pathways for the decomposition of ONSCN. At high thiocyanate concentrations, two reaction pathways operate including a second order reaction to generate NO and (SCN)(2) and a reversible reaction between ONSCN and SCN(-) producing NO and (SCN)(2)(-), with the rate limiting step corresponding to the consumption of (SCN)(2)(-) by reaction with ONSCN. The third reaction pathway, which becomes significant at low thiocyanate concentrations, involves formation of a previously unreported species, ONOSCN, via a reaction between ONSCN and HOSCN, the latter constituting an intermediate in the hydrolysis of (SCN)(2). ONOSCN contributes to the formation of NO via homolysis of the O-NO bond and subsequent dimerization and hydrolysis of OSCN. Fitting the chemical reactions of the model to the experimental measurements, which covered a wide range of reactant concentrations, afforded estimation of all relevant kinetic parameters and provided an excellent match. The reaction mechanism developed in this contribution may be applied to predict the rates of NO formation from ONSCN during the synthesis of azo dyes, the gassing of explosive emulsions, or nitrosation reactions occurring in the human body.

Air pollutants formed in thermal decomposition of folpet fungicide under oxidative conditions.

This contribution studies the decomposition of folpet fungicide under oxidative conditions and compares the product species with those of captan fungicide, which is structurally related to folpet. Toxic products arising from folpet comprised carbon disulfide (highest emission factor of 4.9 mg g(-1) folpet), thiophosgene (14.4), phosgene (34.1), hydrogen cyanide (2.6), tetrachloroethylene (111), hexachloroethane (167), and benzonitrile (4.5). Owing to their related molecular structures, folpet emitted similar products to captan but at different yields, under the same experimental conditions. It appears that the availability of easily abstractable H atoms, in the structure of captan but not in that of folpet, defines the product distribution. In conjunction with the quantum chemical calculations, these experimental measurements afford an enhanced explanation of the formation pathways of hazardous decomposition products of these two structurally related fungicides.

An equilibrium ab initio atomistic thermodynamics study of chlorine adsorption on the Cu(001) surface.

The effect of chlorine (Cl) chemisorption on the energetics and atomic structure of the Cu(001) surface over a wide range of chlorine pressures and temperatures has been studied using equilibrium ab initio atomistic thermodynamics to elucidate the formation of cuprous chloride (CuCl) as part of the Deacon reaction on copper metal. The calculated surface free energies show that the 1/2 monolayer (ML) c(2 × 2)-Cl phase with chlorine atoms adsorbed at the hollow sites is the most stable structure for a wide range of Cl chemical potential, in agreement with experimental observations. It is also found that at very low pressure and exposure, but elevated temperature, the 1/9 ML and 1/4 ML phases become the most stable. By contrast, a high coverage of Cl does not lead to thermodynamically stable geometries. The subsurface adsorption of Cl atoms, however, dramatically increases the stability of the 1 ML and 2 ML adsorption configurations providing a possible pathway for the formation of the bulk-chloride surface phases in the kinetic regime.

Theoretical study of unimolecular decomposition of catechol.

This study develops the reaction pathway map for the unimolecular decomposition of catechol, a model compound for various structural entities present in biomass, coal, and wood. Reaction rate constants at the high-pressure limit are calculated for the various possible initiation channels. It is found that catechol decomposition is initiated dominantly via hydroxyl H migration to a neighboring ortho carbon bearing an H atom. We identify the direct formation of o-benzoquinone to be unimportant at all temperatures, consistent with the absence of this species from experimental measurements. At temperatures higher than 1000 K, water elimination through concerted expulsion of a hydroxyl OH together with an ortho H becomes the most significant channel. Rice-Ramsperger-Kassel-Marcus simulations are performed to establish the branching ratio between these two important channels as a function of temperature and pressure. All unimolecular routes to the reported major experimental products (CO, 1,3-C(4)H(6) and cyclo-C(5)H(6)) are shown to incur large activation barriers. The results presented herein should be instrumental in gaining a better understanding of the decomposition behavior of catechol-related compounds.

Theoretical study of the ammonia-hypochlorous acid reaction mechanism.

A mechanism for the oxidation of ammonia by hypochlorous acid to form nitrogen gas has been developed at the B3LYP/6-31G(d,p) level of theory using the Gaussian 03 software package. The formation of NH(2)Cl, NHCl(2), and NCl(3) was studied in the gas phase, with explicit water molecules included to examine the transition state energy in aqueous solution. The inclusion of explicit water molecules in the transition state dramatically reduced the reaction barrier in reactions involving transfer of a hydrogen atom between molecules, effects that were not taken into account through use of a solvation model alone. Three mechanisms were identified for the decomposition of chloramine species to form N(2), involving the combination of two chloramine species to form hydrazine, dichlorohydrazine and tetrachlorohydrazine intermediates. The highest barrier in each pathway was found to be the formation of the hydrazine derivative.

Thermochemical properties and decomposition pathways of three isomeric semiquinone radicals.

Semiquinones are persistent free environmental radicals formed as important initial products from the decomposition of dihydroxylated benzene isomers. This study develops detailed decomposition pathways for the thermal decomposition of the three isomeric semiquinone radicals. Branching ratios based on the calculated high-pressure limit reaction rate constants predict that p-benzoquinone is a major product from the unimolecular decomposition of the p-semiquinone radical, while the formation of o-benzoquinone from the o-semiquinone radical corresponds to a minor channel. This finding is consistent with the absence of o-benzoquinone from the thermal degradation of the 1,2-dihydroxybenzene isomer and the abundance of p-benzoquinone from the thermal decomposition of 1,4-dihydroxybenzene. Ring contraction/CO elimination is shown to be the dominant sink pathway for the o-semiquinone and m-semiquinone radicals. Thermochemical properties, in terms of enthalpies of formation, entropies, and heat capacities for dihydroxylated benzene isomers, semiquinone radicals, and benzoquinones, are evaluated by quantum chemical calculations. Values of the enthalpies of formation calculated by the B3LYP/GTLarge method show good agreement with those obtained at the G3B3 level of theory.

Theoretical study on the thermodynamic properties and self-decomposition of methylbenzenediol isomers.

Alkylated hydroxylated aromatics are major constituents of various types of fuels, including biomass and low-rank coal. In this study, thermochemical parameters are obtained for the various isomeric forms of methylbenzenediol isomers in terms of their enthalpies of formation, entropies, and heat capacities. Isodesmic work reactions are used in quantum chemical computations of the reaction enthalpies for O-H and H2C-H bond fissions and the formation of phenoxy- and benzyl-type radicals. A reaction potential energy on the singlet-state surface surface is mapped out for the unimolecular decomposition of the 3-methylbenzene-1,2-diol isomer. According to the calculated high pressure-limit reaction rate constants, concerted hydrogen molecule elimination from the methyl group and the hydroxyl group, in addition to intermolecular H migration from the hydroxyl group, dominates the unimolecular decomposition at low to intermediate temperatures (T ≤ 1200 K). At higher temperatures, O-H bond fission and concerted water elimination are expected to become the sole decomposition pathways.

Products and mechanism of thermal decomposition of chlorpyrifos under inert and oxidative conditions.

Chlorpyrifos (CPF) is a widely used pesticide; however, limited experimental work has been completed on its thermal decomposition. CPF is known to decompose into 3,5,6-trichloro-2-pyridinol (TCpyol) together with ethylene and HOPOS. Under oxidative conditions TCpyol can decompose into the dioxin-like 2,3,7,8-tetrachloro-[1,4]-dioxinodipyridine (TCDDPy). With CPF on the cusp of being banned in several jurisdictions worldwide, the question might arise as to how to safely eliminate large stockpiles of this pesticide. Thermal methods such as incineration or thermal desorption of pesticide-contaminated soils are often employed. To assess the safety of thermal methods, information about the toxicants arising from thermal treatment is essential. The present flow reactor study reports the products detected under inert and oxidative conditions from the decomposition of CPF representative of thermal treatments and of wildfires in CPF-contaminated vegetation. Ethylene and TCpyol are the initial products formed at temperatures between 550 and 650 °C, although the detection of HOPOS as a reaction product has proven to be elusive. During pyrolysis of CPF in an inert gas, the dominant sulfur-containing product detected from CPF is carbon disulfide. Quantum chemical analysis reveals that ethylene and HOPOS undergo a facile reaction to form thiirane (c-C2H4S) which subsequently undergoes ring opening reactions to form precursors of CS2. At elevated temperatures (>650 °C), TCpyol undergoes both decarbonylation and dehydroxylation reactions together with decomposition of its primary product, TCpyol. A substantial number of toxicants is observed, including HCN and several nitriles, including cyanogen. No CS2 is observed under oxidative conditions - sulfur dioxide is the fate of S in oxidation of CPF, and quantum chemical studies show that SO2 formation is initiated by the reaction between HOPOS and O2. The range of toxicants produced in thermal decomposition of CPF is summarised.

A first-principles density functional study of chlorophenol adsorption on Cu2O(110):CuO.

First-principles density functional theory and a periodic-slab model have been employed to explore the adsorption of a two-chlorophenol molecule on a Cu(2)O(110) surface containing surface Cu-O bonds, namely, the Cu(2)O(110):CuO surface. The two-chlorophenol molecule is found to interact very weakly with the Cu(2)O(110):CuO surface, forming several vertical and flat orientations. These weakly bound states tend to result from interaction between the phenolic hydrogen and an oxygen surface atom. The formation of a two-chlorophenoxy moiety and an isolated hydrogen on the Cu(2)O(110):CuO surface from a vacuum two-chlorophenol molecule is determined to have an endothermicity of 8.2 kcal/mol (0.37 eV). The energy required to form a two-chlorophenoxy radical in the gas phase is also found to be much smaller when assisted by the Cu(2)O(110):CuO surface than direct breaking of the hydroxyl bond of a free two-chlorophenol molecule. The calculated binding energy of a two-chlorophenoxy radical adsorbed directly onto the Cu(2)O(110):CuO surface is -12.5 kcal/mol (0.54 eV). The Cu(2)O(110):CuO and Cu(100) surfaces are found to have similar energy barriers for forming a surface-bound two-chlorophenoxy moiety from the adsorption of a two-chlorophenol molecule.

Thermal oxidation of dieldrin and concomitant formation of toxic products including polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F).

This paper examines the gas phase thermal decomposition of dieldrin and associated formation of toxic combustion products including polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F). Volatile Organic Carbon (VOC) analysis revealed the formation of pentachlorostyrene (PCS), hexachlorostyrene (HCS) and polychlorinated naphthalene as toxic combustion products generated during the combustion of dieldrin. The thermal pyrolysis of dieldrin resulted in the formation of chlorinated benzenes and chlorinated phenols, which are known PCDD/F precursors. The formation of PCDD/F commenced around 823 K @ 5s residence time and results indicate a preference for the formation of PCDF over PCDD under all experimental conditions studied. Subsequent experiments, to examine the yield of PCDD/F as a function of temperature, reveal the progressive chlorination of PCDD/F with temperatures up to 923 K. Octachlorodibenzofuran (OCDF) was the major dioxin congener detected in the oxidation of dieldrin. The highest toxicity factor for dioxin formation was recorded at 923 K with a 6% O2 content in the feed gas and corresponds to 6.24 ng TEQ WHO 2005/mg of dieldrin and total PCDD/F concentration of 96.8 ng/mg of dieldrin.

Quantum chemical and kinetic study of formation of 2-chlorophenoxy radical from 2-chlorophenol: unimolecular decomposition and bimolecular reactions with H, OH, Cl, and O2.

This study investigates the kinetic parameters of the formation of the chlorophenoxy radical from the 2-chlorophenol molecule, a key precursor to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCCD/F), in unimolecular and bimolecular reactions in the gas phase. The study develops the reaction potential energy surface for the unimolecular decomposition of 2-chlorophenol. The migration of the phenolic hydrogen to the ortho-C bearing the hydrogen atom produces 2-chlorocyclohexa-2,4-dienone through an activation barrier of 73.6 kcal/mol (0 K). This route holds more importance than the direct fission of Cl or the phenolic H. Reaction rate constants for the bimolecular reactions, 2-chlorophenol + X --> X-H + 2-chlorophenoxy (X = H, OH, Cl, O2) are calculated and compared with the available experimental kinetics for the analogous reactions of X with phenol. OH reaction with 2-chlorophenol produces 2-chlorophenoxy by direct abstraction rather than through addition and subsequent water elimination. The results of the present study will find applications in the construction of detailed kinetic models describing the formation of PCDD/F in the gas phase.