Chemical Investigation on the Mechanism and Kinetics of the Atmospheric Degradation Reaction of Trichlorofluoroethene by OH⋠and Its Subsequent Fate in the Presence of O2 /NOx.

The M06-2X/6-311++G(d,p) level of theory was used to examine the degradation of Trichlorofluoroethene (TCFE) initiated by OH⋅ radicals. Additionally, the coupled-cluster single-double with triple perturbative [CCSD(T)] method was employed to refine the single-point energies using the complete basis set extrapolation approach. The results indicated that OH-addition is the dominant pathway. OH⋅ adds to both the C1 and C2 carbons, resulting in the formation of the C(OH)Cl2 -⋅CClF and ⋅CCl2 -C(OH)ClF species. The associated barrier heights were determined to be 1.11 and -0.99 kcal mol-1 , respectively. Furthermore, the energetic and thermodynamic parameters show that pathway 1 exhibits greater exothermicity and exergonicity compared to pathway 2, with differences of 8.11 and 8.21 kcal mol-1 , correspondingly. The primary pathway involves OH addition to the C2 position, with a rate constant of 6.2×10-13 cm3 molecule-1 sec-1 at 298 K. This analysis served to estimate the atmospheric lifetime, along with the photochemical ozone creation potential (POCP) and ozone depletion potential (ODP). It yielded an atmospheric lifetime of 8.49 days, an ODP of 4.8×10-4 , and a POCP value of 2.99, respectively. Radiative forcing efficiencies were also estimated at the M06-2X/6-311++G(d,p) level. Global warming potentials (GWPs) were calculated for 20, 100, and 500 years, resulting in values of 9.61, 2.61, and 0.74, respectively. TCFE is not expected to make a significant contribution to the radiative forcing of climate change. The results obtained from the time-dependent density functional theory (TDDFT) indicated that TCFE and its energized adducts are unable to photolysis under sunlight in the UV and visible spectrum. Secondary reactions involve the [TCFE-OH-O2 ]⋅ peroxy radical, leading subsequently to the [TCFE-OH-O]⋅ alkoxy radical. It was found that the alkoxy radical resulting from the peroxy radical can lead to the formation of phosgene (COCl2 ) and carbonyl chloride fluoride (CClFO), with phosgene being the primary product.

Prediction of Thermogravimetric Data in Bromine Captured from Brominated Flame Retardants (BFRs) in e-Waste Treatment Using Machine Learning Approaches.

The principal objective in the treatment of e-waste is to capture the bromine released from the brominated flame retardants (BFRs) added to the polymeric constituents of printed circuits boards (PCBs) and to produce pure bromine-free hydrocarbons. Metal oxides such as calcium hydroxide (Ca(OH)2) have been shown to exhibit high debromination capacity when added to BFRs in e-waste and capturing the released HBr. Tetrabromobisphenol A (TBBA) is the most commonly utilized model compound as a representative for BFRs. Our coauthors had previously studied the pyrolytic and oxidative decomposition of the TBBA:Ca(OH)2 mixture at four different heating rates, 5, 10, 15, and 20 °C/min, using a thermogravimetric (TGA) analyzer and reported the mass loss data between room temperature and 800 °C. However, in the current work, we applied different machine learning (ML) and chemometric techniques involving regression models to predict the TGA data at different heating rates. The motivation of this work was to reproduce the TGA data with high accuracy in order to eliminate the physical need of the instrument itself, so that this could save significant experimental time involving sample preparation and subsequently minimizing human errors. The novelty of our work lies in the application of ML techniques to predict the TGA data from e-waste pyrolysis since this has not been conducted previously. The significance of our work lies in the fact that e-waste is ever increasing, and predicting the mass loss curves faster will enable better compositional analysis of the e-waste samples in the industry. Three ML models were employed in our work, namely Linear, random forest (RF), and support vector regression (SVR), out of which the RF method exhibited the highest coefficient of determination (R2) of 0.999 and least error of prediction as estimated by the root mean squared error (RMSEP) at all 4 heating rates for both pyrolysis and oxidation conditions. An 80:20 split was used for calibration and validation data sets. Furthermore, for showing versatility and robustness of the best-predicting RF model, it was also trained using all the data points in the lower heating rates of 5 and 10 °C/min and predicted on all the data points for the higher heating rates of 15 and 20 °C/min to again obtain a high R2 of 0.999. The excellent performance of the RF model showed that ML techniques can be used to eliminate the physical use of TGA equipment, thus saving experimental time and potential human errors, and can further be applied in other real-time e-waste recycling scenarios.

Review of Chemical Reactivity of Singlet Oxygen with Organic Fuels and Contaminants.

Singlet oxygen represents a form of reactive oxygen species (ROS), produced by electronic excitation of molecular triplet oxygen. In general, highly reactive oxygen-bearing molecules remain the backbone of diverse ground-breaking technologies, driving the waves of scientific development in environmental, biotechnology, materials, medical and defence sciences. Singlet oxygen has a relatively high energy of about 94 kJ/mol compared to the ground state molecular O2 and therefore initiates low-temperature oxidation of electron-rich hydrocarbons. Such reactivity of singlet oxygen has inspired a wide array of emerging applications in chemical, biochemical and combustion phenomena. This paper reviews the intrinsic properties of singlet oxygen, emphasising the physical aspects of its natural occurrences, production techniques, as well as chemical reactivity with organic fuels and contaminants. The review assembles critical scientific studies on the implications of singlet oxygen in initiating chemical reactions, identifying, and quantitating the consequential effects on combustion, fire safety, as well as on the low-temperature treatment of organic wastes and contaminants. Moreover, the content of this review appraises computational efforts, such as DFT quantum mechanical modelling, in developing mechanistic (i. e., both thermodynamic and kinetic) insights into the reaction of singlet oxygen with hydrocarbons.

An Insight into Geometries and Catalytic Applications of CeO2 from a DFT Outlook.

Rare earth metal oxides (REMOs) have gained considerable attention in recent years owing to their distinctive properties and potential applications in electronic devices and catalysts. Particularly, cerium dioxide (CeO2), also known as ceria, has emerged as an interesting material in a wide variety of industrial, technological, and medical applications. Ceria can be synthesized with various morphologies, including rods, cubes, wires, tubes, and spheres. This comprehensive review offers valuable perceptions into the crystal structure, fundamental properties, and reaction mechanisms that govern the well-established surface-assisted reactions over ceria. The activity, selectivity, and stability of ceria, either as a stand-alone catalyst or as supports for other metals, are frequently ascribed to its strong interactions with the adsorbates and its facile redox cycle. Doping of ceria with transition metals is a common strategy to modify the characteristics and to fine-tune its reactive properties. DFT-derived chemical mechanisms are surveyed and presented in light of pertinent experimental findings. Finally, the effect of surface termination on catalysis by ceria is also highlighted.

Interfacial and bulk properties of concentrated solutions of ammonium nitrate.

We conducted molecular dynamics (MD) simulations to calculate the density and surface tension of concentrated ammonium nitrate (AN) solutions up to the solubility limit of ammonium nitrate in water, by combining the SPC/E, SPCE/F and TIP4P/2005 water models with OPLS model for ammonium and nitrate ions. This is the first time that the properties of concentrated solutions of nitrates, especially AN, have been studied by molecular dynamics. We effectively account for the polarisation effects by the electronic continuum correction (ECC), practically realised via rescaling of the ionic charges. We found that, the full-charge force field MD simulations overestimate the experimental results, as the ions experience repulsion from the interface and prefer to remain in the subsurface layer and the bulk solution. In contrast, reducing the ionic charges results in the behaviour that fits well with the experimental data. The nitrate anions display a greater propensity for the interface than the ammonium cations. We accurately predict both the density and the rise in the surface tension of concentrated solutions of AN, recommending TIP4P/2005 for water and the scaled-charge OPLS model (OPLS/ECC) for the ions in the solutions. We observe that, the adsorption of anions to the interface accompanies their depletion in the subsurface layer, which is preferentially occupied by cations, resulting in the formation of the electric double layer. We demonstrate the ion deficiency for up to 3 Å below the surface and establish the requirement to include the polarisability effects in the OPLS model for AN. While these results confirmed the findings of the previous studies for dilute solutions, they are new in the solubility limit. Concentrated solutions exhibit a strong effect of the abundance of solute on the coordination numbers of ions and on the degree of ion pairing. Surprisingly, ion pairing decreases significantly at the interface compared with the bulk. The present study identifies OPLS/ECC, along with TIP4P/2005, to yield accurate predictions of physical properties of concentrated AN, with precision required for industrial applications, such as a formulation of emulsion and fuel-oil explosives that now predominate the civilian use of AN. An application of this model will allow one to predict the surface properties of supersaturated solutions of AN which fall outside the capability of the present laboratory experiments but are important industrially.

Photodecomposition properties of brominated flame retardants (BFRs).

This study investigates the geometric and electronic properties of selected BFRs in their ground (S0) and first singlet excited (S1) states deploying methods of the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). We estimate the effect of the S0→ S1 transition on the elongations of the C-Br bond, identify the frontier molecular orbitals involved in the excitation process and compute partial atomic charges for the most photoreactive bromine atoms. The bromine atom attached to an ortho position in HBB (with regard to C-C bond; 2,2',4,4',6,6'-hexabromobiphenyl), TBBA (with respect to the hydroxyl group; 2,2',6,6'-tetrabromobisphenol A), HBDE and BTBPE (in reference to C-O linkage; 2,2',4,4',6,6'-hexabromodiphenylether and 1,2-bis(2,4,6-tribromophenoxy)ethane, respectively) bears the highest positive atomic charge. This suggests that, these positions undergo reductive debromination reactions to produce lower brominated molecules. Debromination reactions ensue primarily in the aromatic compounds substituted with the highest number of bromine atoms owing to the largest stretching of the C-Br bond in the first excited state. The analysis of the frontier molecular orbitals indicates that, excitations of BFRs proceed via π→π*, or π→σ* or n→σ* electronic transitions. The orbital analysis reveals that, the HOMO-LUMO energy gap (EH-L) for all investigated bromine-substituted aromatic molecules falls lower (1.85-4.91 eV) than for their non-brominated analogues (3.39-8.07 eV), in both aqueous and gaseous media. The excitation energies correlate with the EH-L values. The excitation energies and EH-L values display a linear negative correlation with the number of bromine atoms attached to the molecule. Spectral analysis of the gaseous-phase systems reveals that, the highly brominated aromatics endure lower excitation energies and exhibit red shifts of their absorption bands in comparison to their lower brominated congeners. We attained a satisfactory agreement between the experimentally measured absorption peak (λmax) and the theoretically predicted oscillator strength (λmax) for the UV-Vis spectra. This study further confirms that, halogenated aromatics only absorb light in the UV spectral region and that effective photodegradation of these pollutants requires the presence of photocatalysts.

Destruction of dioxin and furan pollutants via electrophilic attack of singlet oxygen.

Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) remain of particular concern owing to their extensive toxicity towards health and accumulation in the environment. Atmospheric oxidation (by ambient oxygen molecules) of this class of persistent environmental pollutants has little to no kinetic feasibility due to very sizable activation energies in the entrance channel. The current control measures involve energy-intensive source incineration of contaminated materials at high temperatures as high as 850 °C. This study finds an alternative low-energy approach of destroying dioxin-like compounds, proposing that advanced oxidation by highly reactive singlet oxygen (O21Δg, originated from chemical, surface-mediated and photochemical processes) can initiate low-temperature remediation of these pollutants. This contribution completes the first milestone in mapping out the mechanisms of the electrophilic addition of singlet oxygen to unsubstituted and chlorinated dibenzo-p-dioxin (DBD) and dibenzofuran (DBF) structures, according to density functional theory DFT-B3LYP method in conjunction with the 6-311+g(d,p) basis set, as well as energy refinements based on the approximate spin-projection scheme. The [2+2]-cycloaddition mechanism appears dominant for singlet oxidation of dibenzo-p-dioxin with a fitted rate constant of k(T) = 5.01 × 10-14 exp(-98000/RT). On the other hand, the addition of singlet oxygen to the aromatic ring of dibenzofuran primarily transpires via [4+2]-cycloaddition channel with a fitted rate constant of k(T) = 2.16 × 10-13 exp(-119000/RT). The results suggest that application of singlet oxygen can reduce the energy cost of recycling halogenated and flame retarded materials.

Recycling of zincite (ZnO) via uptake of hydrogen halides.

Hydrogen halides (HCl/HBr) represent major halogen fragments from the thermal decomposition of halogen laden materials, most notably PVC and brominated flame retardants (BFRs). Co-pyrolysis of halogen-containing solid waste with metal oxides is currently deployed as a mainstream strategy to treat halogen content as well as to recycle the valuable metallic fraction embedded in electric arc furnace dust (EAFD) and e-waste. However, designing an industrial-scale recycling facility necessitates accurate knowledge on mechanistic and thermo-kinetic parameters dictating the interaction between metal oxides and hydrogen halides. In this contribution, we investigate chemical interplay between HCl/HBr and zincite surfaces as a representative model for structures of zinc oxides in EAFD by using different sets of functionals, unit cell size and energy cut-off. In the first elementary step, dissociative adsorption of the HCl/HBr molecules affords oxyhalide structures (Cl/Br-Zn, H-O) via modest activation barriers. Conversion of the oxyhalide structure into zinc halides occurs through two subsequent steps, further dissociative adsorption of HCl/Br over the same surface Zn atom as well as the release of a H2O molecule. Evaporation (or desorption of zinc halide molecules) signifies a bottleneck for the overall halogenation of ZnO. Our simplified kinetic model on the HCl + ZnO system concurs very well with experimentally reported TGA weight loss profiles on two grounds: accumulation of oxyhalides until ∼700 K and desorption of ZnCl2 at higher temperatures. The thermo-kinetic and mechanistic aspects reported herein could be useful in the pursuit of a design of a large-scale catalytic upgrading unit that operates to extract valuable zinc loads from EAFD.

Reaction of phenol with singlet oxygen.

Photo-degradation of organic pollutants plays an important role in their removal from the environment. This study provides an experimental and theoretical account of the reaction of singlet oxygen O2(1Δg) with the biodegradable-resistant species of phenol in an aqueous medium. The experiments combine customised LED-photoreactors, high-performance liquid chromatography (HPLC), and electron paramagnetic resonance (EPR) imaging, employing rose bengal as a sensitiser. Guided by density functional theory (DFT) calculations at the M062X level, we report the mechanism of the reaction and its kinetic model. Addition of O2(1Δg) to the phenol molecule branches into two competitive 1,4-cycloaddition and ortho ene-type routes, yielding 2,3-dioxabicyclo[2.2.2]octa-5,7-dien-1-ol (i.e., 1,4-endoperoxide 1-hydroxy-2,5-cyclohexadiene) and 2-hydroperoxycyclohexa-3,5-dien-1-one, respectively. Unimolecular rearrangements of the 1,4-endoperoxide proceed in a facile exothermic reaction to form the only experimentally detected product, para-benzoquinone. EPR revealed the nature of the oxidation intermediates and corroborated the appearance of O2(1Δg) as the only active radical participating in the photosensitised reaction. Additional experiments excluded the formation of hydroxyl (HO˙), hydroperoxyl (HO2˙), and phenoxy intermediates. We detected for the first time the para-semibenzoquinone anion (PSBQ), supporting the reaction pathway leading to the formation of para-benzoquinone. Our experiments and the water-solvation model result in the overall reaction rates of kr-solvation = 1.21 × 104 M-1 s-1 and kr = 1.14 × 104 M-1 s-1, respectively. These results have practical application to quantify the degradation of phenol in wastewater treatment.

Study of structural properties and defects of Ni-doped SnO2 nanorods as ethanol gas sensors.

An ethanol gas sensor with enhanced sensor response was fabricated using Ni-doped SnO2 nanorods, synthesized via a simple hydrothermal method. It was found that the response (R = R 0/R g) of a 5.0 mol% Ni-doped SnO2 (5.0Ni:SnO2) nanorod sensor was 1.4 × 104 for 1000 ppm C2H5OH gas, which is about 13 times higher than that of pure SnO2 nanorods, (1.1 × 103) at the operating temperature of 450 °C. Moreover, for 50 ppm C2H5OH gas, the 5.0Ni:SnO2 nanorod sensor still recorded a significant response reading, namely 2.0 × 103 with a response time of 30 s and recovery time of 10 min. To investigate the effect of Ni dopant (0.5-5.0 mol%) on SnO2 nanorods, structural characterizations were demonstrated using field emission scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, x-ray diffraction (XRD) analysis, x-ray photoelectron spectroscopy and an ultraviolet-visible spectrometer. XRD results confirmed that all the samples consisted of tetragonal-shaped rutile SnO2 nanorods. It was found that the average diameter and length of the nanorods formed in 5.0Ni:SnO2 were four times smaller (∼6 and ∼35 nm, respectively) than those of the nanorods formed in pure SnO2 (∼25 and 150 nm). Interestingly, both samples had the same aspect ratio, ∼6. It is proposed that the high response of the 5.0Ni:SnO2 nanorod sensor can be attributed to the particle size, which causes an increase in the thickness of the charge depletion layer, and the presence of oxygen vacancies within the matrix of SnO2 nanorods.

Thermal Reduction of NOx with Recycled Plastics.

This study develops technology for mitigation of NOx formed in thermal processes using recycled plastics such as polyethylene (PE). Experiments involve sample characterization, and thermogravimetric decomposition of PE under controlled atmospheres, with NOx concentration relevant to industrial applications. TGA-Fourier transform infrared (FTIR) spectroscopy and NOx chemiluminescence serve to obtain the removal efficiency of NOx by fragments of pyrolyzing PE. Typical NOx removal efficiency amounts to 80%. We apply the isoconversional method to derive the kinetic parameters, and observe an increasing dependency of activation energy on the reaction progress. The activation energies of the process span 135 kJ/mol to 226 kJ/mol, and 188 kJ/mol to 268 kJ/mol, for neat and recycled PE, respectively, and the so-called compensation effect accounts for the natural logarithmic pre-exponential ln (A/min-1) factors of ca. 19-35 and 28-41, in the same order, depending on the PE conversion in the experimental interval of between 5 and 95%. The observed delay in thermal events of recycled PE reflects different types of PE in the plastic, as measurements of intrinsic viscosity indicate that, the recycled PE comprises longer linear chains. The present evaluation of isoconversional activation energies affords accurate kinetic modeling of both isothermal and nonisothermal decomposition of PE in NOx-doped atmosphere. Subsequent investigations will focus on the effect of mass transfer and the presence of oxygen, as reburning of NOx in large-scale combustors take place at higher temperatures than those included in the current study.

Understanding the adsorptive interactions of arsenate-iron nanoparticles with curved fullerene-like sheets in activated carbon using a quantum mechanics/molecular mechanics computational approach.

The prevalence of global arsenic groundwater contamination has driven widespread research on developing effective treatment systems including adsorption using various sorbents. The uptake of arsenic-based contaminants onto established sorbents such as activated carbon (AC) can be effectively enhanced via immobilization/impregnation of iron-based elements on the porous AC surface. Recent suggestions that AC pores structurally consist of an eclectic mix of curved fullerene-like sheets may affect the arsenic adsorption dynamics within the AC pores and is further complicated by the presence of nano-sized iron-based elements. We have therefore, attempted to shed light on the adsorptive interactions of arsenate-iron nanoparticles with curved fullerene-like sheets by using hybridized quantum mechanics/molecular mechanics (QMMM) calculations and microscopy characterization. It is found that, subsequent to optimization, chemisorption between HAsO42- and the AC carbon sheet (endothermic process) is virtually non-existent - this observation is supported by experimental results. Conversely, the incorporation of iron nanoparticles (FeNPs) into the AC carbon sheet greatly facilitates chemisorption of HAsO42-. Our calculation implies that iron carbide is formed at the junction between the iron and the AC interface and this tightly chemosorbed layer prevents detachment of the FeNPs on the AC surface. Other aspects including electronic structure/properties, carbon arrangement defects and rate of adsorptive interaction, which are determined using the Climbing-Image NEB method, are also discussed.

Reaction of Aniline with Singlet Oxygen (O21Δg).

Dissolved organic matter (DOM) acts as an effective photochemical sensitizer that produces the singlet delta state of molecular oxygen (O21Δg), a powerful oxidizer that removes aniline from aqueous solutions. However, the exact mode of this reaction, the p- to o-iminobenzoquinone ratio, and the selectivity of one over the other remain largely speculative. This contribution resolves these uncertainties. We report, for the first time, a comprehensive mechanistic and kinetic account of the oxidation of aniline with the singlet delta oxygen using B3LYP and M06 functionals in both gas and aqueous phases. Reaction mechanisms have been mapped out at E, H, and G scales. The 1,4-cycloaddition of O21Δg to aniline forms a 1,4-peroxide intermediate (M1), which isomerizes via a closed-shell mechanism to generate a p-iminobenzoquinone molecule. On the other hand, the O21Δg ene-type reaction forms an o-iminobenzoquinone product when the hydroperoxyl bond breaks, splitting hydroxyl from the 1,2-hydroperoxide (M3) moiety. The gas-phase model predicts the formation of both p- and o-iminobenzoquinones. In the latter model, the M1 adduct displays the selectivity of up to 96%. A water-solvation model predicts that M1 decomposes further, forming only p-iminobenzoquinone with a rate constant of k = 1.85 × 109 (L/(mol s)) at T = 313 K. These results corroborate the recent experimental findings of product concentration profile in which p-iminobenzoquinonine represents the only detected product.

Hydrogen Abstraction from Hydrocarbons by NH2.

This contribution investigates thermokinetic parameters of bimolecular gas-phase reactions involving the amine (NH2) radical and a large number of saturated and unsaturated hydrocarbons. These reactions play an important role in combustion and pyrolysis of nitrogen-rich fuels, most notably biomass. Computations performed at the CBS-QB3 level and based on the conventional transition-state theory yield potential-energy surfaces and reaction rate constants, accounting for tunnelling effects and the presence of hindered rotors. In an analogy to other H abstraction systems, we demonstrate only a small influence of variational effects on the rate constants for selected reaction. The studied reactions cover the abstraction of hydrogen atoms by the NH2 radical from the C-H bonds in C1-C4 species, and four C5 hydrocarbons of 2-methylbutane, 2-methyl-1-butene, 3-methyl-1-butene, 3-methyl-2-butene, and 3-methyl-1-butyne. For the abstraction of H from methane, in the temperature windows 300-500 and 1600-2000 K, the calculated reaction rate constants concur with the available experimental measurements, i.e., kcalculated/kexperimetal = 0.3-2.5 and 1.1-1.4, and the previous theoretical estimates. Abstraction of H atom from ethane attains the ratio of kcalculated/kexperimetal equal to 0.10-1.2 and 1.3-1.5 over the temperature windows of available experimental measurements, i.e., 300-900 K and 1500-2000 K, respectively. For the remaining alkanes (propane and n-butane), the average kexperimental/kcalculated ratio remains 2.6 and 1.3 over the temperature range of experimental data. Also, comparing the calculated standard enthalpy of reaction (ΔrH°298) with the available experimental measurements for alkanes, we found the mean unsigned error of computations as 3.7 kJ mol-1. This agreement provides an accuracy benchmark of our methodology, affording the estimation of the unreported kinetic parameters for H abstractions from alkenes and alkynes. On the basis of the Evans-Polanyi plots, calculated bond dissociation enthalpies (BDHs) correlate linearly with the standard enthalpy of activation (Δ⧧H°298), allowing estimation of the enthalpy barrier for reaction of NH2 with other hydrocarbons in future work. Finally, we develop six sets of the generalized Arrhenius rate parameters for H abstractions from different C-H bond types. These parameters extend the application of the present results to any noncyclic hydrocarbon interacting with the NH2 radical.

Formation of Environmentally Persistent Free Radicals on α-Al2O3.

Metal oxides exhibit catalytic activity for the formation of environmentally persistent free radicals (EPFRs). Here, we investigate, via first-principles calculations, the activity of alumina α-Al2O3(0001) surface toward formation of phenolic EPFRs, under conditions relevant to cooling down zones of combustion systems. We show that, molecular adsorption of phenol on α-Al2O3(0001) entails binding energies in the range of -202 kJ/mol to -127 kJ/mol. The dehydroxylated alumina catalyzes the conversion of phenol into its phenolate moiety with a modest activation energy of 48 kJ/mol. Kinetic rate parameters, established over the temperature range of 300 to 1000 K, confirm the formation of the phenolate as the preferred pathways for the adsorption of phenol on alumina surfaces, corroborating the role of particulate matter in the cooling down zone of combustion systems in the generation of EFPRs.

Formation of PCDD/Fs in Oxidation of 2-Chlorophenol on Neat Silica Surface.

This contribution studies partial oxidation of 2-chlorophenol on surfaces of neat silica at temperatures of 250, 350, and 400 °C; i.e., temperatures that frequently lead to catalytic formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) from their precursors. We have identified 2,6-dichlorophenol (2,6-DCPh), 2,4-dichlorophenol (2,4-DCPh), and 2,4,6-trichlorophenol (2,4,6-TriCPh), but have detected no chlorinated benzenes (CBzs). The detected chlorinated and nonchlorinated DD/Fs comprise dibenzo-p-dioxin (DD), 1- and 2-monochlorodibenzo-p-dioxin (1-, 2-MCDD), 1,6-, 1,9-, 1,3-dichlorodibenzo-p-dioxin (1,6-, 1,9-, 1,3-DCDD), 4-monochlorodibenzofuran (4-MCDF), and 4,6-dichlorodibenzofuran (4,6-DCDF) at the reaction temperatures of 350 and 400 °C. However, at a lower reaction temperature, 250 °C, we have detected no PCDD/Fs. We have demonstrated that neat silica surfaces catalyze the generation of PCDD/Fs from chlorophenols at the upper range of the catalytic formation temperature of PCDD/F. The present finding proves the generation of PCDD/Fs on particles of fly ash, even in the absence of transition metals.

Thermal Recycling of Brominated Flame Retardants with Fe2O3.

Plastics containing brominated flame retardants (BFRs) constitute the major fraction of nonmetallic content in e-waste. Co-pyrolysis of BFRs with hematite (Fe2O3) represents a viable option for the thermal recycling of BFRs. Consensus of experimental findings confirms the excellent bromine fixation ability of Fe2O3 and the subsequent formation of iron bromides. This contribution provides a comprehensive mechanistic account of the primary reactions between a cluster model of Fe2O3 and major bromine-bearing products from the decomposition of tetrabromobisphenol A (TBBA), the most commonly deployed BFR. We estimate the thermo-kinetic parameters for interactions of Fe2O3 with HBr, brominated alkanes and alkenes, bromobenzene, and bromophenol. Dissociative addition of HBr at a Fe-O bond proceeds through a trivial barrier of 8.2 kcal/mol with fitted parameters in the Arrhenius equation of k(T) = 7.96 × 10(11) exp(-6400/RT) s(-1). The facile and irreversible nature for HBr addition to Fe2O3 accords with the experimentally reported 90% reduction in HBr emission when Fe2O3 interacts with TBBA pyrolysates. A detailed kinetic analysis indicates that, transformation of Fe2O3 into iron bromides and oxybromides occurs via successive addition of HBr to Fe(Br)-O(H) entities. Elimination of a water molecule proceeds through an intramolecular H transfer. A direct elimination one-step mechanism operates in the dehydrohalogenation of bromoethane into ethene over Fe2O3. Dissociative decomposition and direct elimination channels assume comparable reaction rates in formation of acetylene from vinyl bromide. Results from this study provide an atomic-based insight into a promising thermal recycling route of e-waste.

Inhibition and Promotion of Pyrolysis by Hydrogen Sulfide (H2S) and Sulfanyl Radical (SH).

This study resolves the interaction of sulfanyl radical (SH) with aliphatic (C1-C4) hydrocarbons, using CBS-QB3 based calculations. We obtained the C-H dissociation enthalpies and located the weakest link in each hydrocarbon. Subsequent computations revealed that, H abstraction by SH from the weakest C-H sites in alkenes and alkynes, except for ethylene, appears noticeably exothermic. Furthermore, abstraction of H from propene, 1-butene, and iso-butene displays pronounced spontaneity (i.e., ΔrG° < -20 kJ mol-1 between 300-1200 K) due to the relatively weak allylic hydrogen bond. However, an alkyl radical readily abstracts H atom from H2S, with H2S acting as a potent scavenger for alkyl radicals in combustion processes. That is, these reactions proceed in the opposite direction than those involving SH and alkene or alkyne species, exhibiting shallow barriers and strong spontaneity. Our findings demonstrate that the documented inhibition effect of hydrogen sulfide (H2S) on pyrolysis of alkanes does not apply to alkenes and alkynes. During interaction with hydrocarbons, the inhibitive effect of H2S and promoting interaction of SH radical depend on the reversibility of the H abstraction processes. For the three groups of hydrocarbon, Evans-Polanyi plots display linear correlations between the bond dissociation enthalpies of the abstracted hydrogens and the relevant activation energies. In the case of methane, we demonstrated that the reactivity of SH radicals toward abstracting H atoms exceeds that of HO2 but falls below those of OH and NH2 radicals.

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.