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.

Facile and Eco-Friendly Approach To Produce Confined Metal Cluster Catalysts.

Zeolite-supported metal nanocluster catalysts have attracted significant attention due to their broad application in heterogeneously catalyzed reactions. The preparation of highly dispersed metal catalysts commonly involves the use of organic compounds and requires the implementation of complicated procedures, which are neither green nor deployable at the large scale. Herein, we present a novel facile method (vacuum-heating) which employs a specific thermal vacuum processing protocol of catalysts to promote the decomposition of metal precursors. The removal of coordinated H2O via vacuum-heating restricts the formation of intermediates (metal-bound OH species), resulting in catalysts with a uniform, metal nanocluster distribution. The structure of the intermediate was determined by in situ Fourier transform infrared, temperature-programmed decomposition, and X-ray absorption spectroscopy (XAS) measurements. This alternative synthesis method is eco-friendly and cost-effective as the procedure occurs in the absence of organic compounds. It can be widely used for the preparation of catalysts from different metal species (Ni, Fe, Cu, Co, Zn) and precursors and is readily scaled-up.

Aqueous carbonation of peridotites for carbon utilisation: a critical review.

Peridotite and serpentinites can be used to sequester CO2 emissions through mineral carbonation. Olivine dissolution rate is directly proportional with temperature, presence of CO2, surface area of mineral particles and presence of ligands and is inversely proportional to pH. Olivine dissolution is better under air flow and increases seven times when rock-inhibiting fungus (Knufia petricola) is used. Olivine dissolution retards as silica layers form during reaction. Sonication, acoustic and concurrent grinding using various grinding medias have been used to artificially break these silica layers and achieve high magnesium extraction. Wet grinding using 50 wt.% ethanol enhanced CO2 uptake of dunite 6.9 times and CO2 uptake of harzburgite by 4.5 times. The best economical process is single-stage concurrent grinding at 130 bar, 185 °C, 15 wt.% solids and 50 wt.% grinding media (zirconia) using 0.64 M NaHCO3. Ratio of grinding media to feed should not be less than 3:1. Yield increases with temperature, pressure, time of reaction, pH and rpm and using additives and grinding media and reducing particle size. This review aims to investigate the progress from 1970s to 2021 on aqueous mineral carbonation of olivine and its naturally available rocks (harzburgite and dunite). This paper comprehensively reviews all aspects of olivine carbonation including olivine dissolution kinetics, effects of grinding and concurrent grinding, thermal activation of olivine feedstock (dunites and harzburgites) as well as chemistry of olivine mineral carbonation. The effects of different reaction parameters on the carbonation yield, role of mineral carbonation accelerators and costs of mineral carbonation process are discussed.

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.

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.

On the Chemistry of Iron Oxide Supported on γ-Alumina and Silica Catalysts.

Catalysts of iron oxide on γ-alumina and silica which were prepared by an incipient wetness impregnation technique have been investigated in an effort to understand how the surface chemical properties are influenced by the nature of the supports. Surprisingly, this is the first study to compare in depth the influence of the supports on physicochemical parameters such as acidity, site nuclearity, and reducibility. In this study, surface characterisation techniques including N2 physisorption at -196 °C, ammonia temperature-programmed desorption, inductively coupled plasma optical emission spectrometry, temperature-programmed reduction with hydrogen, CO-chemisorption, scanning electron microscopy, transmission electron microscopy, and NO adsorption by in situ Fourier transform infrared spectroscopy have been performed to understand the different surface reactions occurring over the two different supports. The aim of this study is to ascertain the primary differences between these two catalysts using several catalyst characterization techniques and correlate their chemical and structural differences to their catalytic activity in the conversion of 2-chlorophenol. The results disclose a higher density of acid sites, a smaller particle size of iron oxide, stabilization of Fe(II) aluminate after reduction on the alumina surface, and finally, the formation of isolated iron cations on the surface of alumina which are notably absent on the silica-supported catalyst.

A mechanistic study of the Knoevenagel condensation reaction: new insights into the influence of acid and base properties of mixed metal oxide catalysts on the catalytic activity.

A Knoevenagel condensation reaction between benzaldehyde and ethyl cyanoacetate was performed in the liquid phase under mild and solventless conditions using a series of catalysts modified by impregnating magnesium and barium cations on different supports (SiO2, ZnO, γ-Al2O3 and Fe2O3). The highest reaction rates and yields (after 6 hours) were observed using ZnO which possessed the highest concentration of acidic and basic sites, as determined by TPD-MS. In situ FTIR experiments show that the adsorption of ethyl cyanoacetate on ZnO results in an increase of hydroxyl intermediate species on the ZnO surface. There is evidence to suggest that the reaction proceeds via an autocatalytic pathway, as an induction period was observed before the observed catalytic activity. It was also found that both surface acidic and basic sites are necessary to obtain high rates of the catalytic reaction. By comparing the catalytic activity with the catalyst surface basicity, we are able to determine the optimum proportion of acid and base sites necessary to obtain the highest reaction rates.

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.

Towards understanding the improved stability of palladium supported on TS-1 for catalytic combustion.

A novel Pd supported on TS-1 combustion catalyst was synthesized and tested in methane combustion under very lean and under highly humid conditions (<1%). A notable increase in hydrothermal stability was observed over 1900 h time-on-stream experiments, where an almost constant, steady state activity obtaining 90% methane conversion was achieved below 500 °C. Surface oxygen mobility and coverage plays a major role in the activity and stability of the lean methane combustion in the presence of large excess of water vapour. We identified water adsorption and in turn the hydrophobicity of the catalyst support as the major factor influencing the long term stability of combustion catalysts. While Pd/Al2O3 catalyst shows a higher turn-over frequency than that of Pd/TS-1 catalyst, the situation reversed after ca. 1900 h on stream. Two linear regions, with different activation energies in the Arrhenius plot for the equilibrium Pd/TS-1 catalyst, were observed. The conclusions were supported by catalyst characterization using H2-chemisorption, TPD, XPS analyses as well as N2-adsorption-desorption, XRD, SEM, TEM. The hydrophobicity and competitive adsorption of water with oxygen is suggested to influence oxygen surface coverage and in turn the apparent activation energy for the oxidation reaction.

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.

Reaction of carbon tetrachloride with methane in a non-equilibrium plasma at atmospheric pressure, and characterisation of the polymer thus formed.

In this paper we focus on the development of a methodology for treatment of carbon tetrachloride utilising a non-equilibrium plasma operating at atmospheric pressure, which is not singularly aimed at destroying carbon tetrachloride but rather at converting it to a non-hazardous, potentially valuable commodity. This method encompasses the reaction of carbon tetrachloride and methane, with argon as a carrier gas, in a quartz dielectric barrier discharge reactor. The reaction is performed under non-oxidative conditions. Possible pathways for formation of major products based on experimental results and supported by quantum chemical calculations are outlined in the paper. We elucidate important parameters such as carbon tetrachloride conversion, product distribution, mass balance and characterise the chlorinated polymer formed in the process.

In situ XAS and IR studies on Cu:SAPO-5 and Cu:SAPO-11: the contributory role of monomeric linear copper(i) species in the selective catalytic reduction of NOx by propene.

Cu:SAPO-5 and Cu:SAPO-11 were prepared by conventional and hydrothermal ion exchange. Copper incorporation is increased six-fold by hydrothermal ion exchange relative to conventional methods. In all cases, the amount of copper taken up by SAPO-11 is superior to uptake in SAPO-5. Copper is divalent and in tetragonally-distorted octahedral environments in the as-prepared samples independent of the method of incorporation for both systems. The local structures about the metal and the valence states associated with the different steps in the selective catalytic reduction of NO(x) in the presence of propene (SCR-HC) have been investigated using X-ray absorption spectroscopy (XAS). For both the Cu:SAPO-5 and Cu:SAPO-11 systems, heating in helium partially autoreduces copper(ii) to copper(i). Following activation in oxygen, propene causes further reduction to copper(i) in all four samples as shown by the evolution of an intense pre-edge diagnostic feature. XANES analysis reveal this to be characteristic of monomeric linear two coordinate copper(i) species. This is a prime example of a pre-edge peak with such a high intensity being observed in the solid state. This is supported by IR where peaks attributed to bidentate copper were observed for Cu:SAPO-11/HT. For all four samples NO(x) partially reoxidises the copper(i) formed in the helium and propene steps. Ion exchanged Cu:SAPO-5 and Cu:SAPO-11 exhibit low activity in reducing NO(x) by propene in an oxygen rich environment. The role of the copper ion during NO adsorption was studied using in situ infra red spectroscopy. The activity of copper exchanged materials is governed by both the degree of reducibility of copper(ii) and the ease of reversing the valence states with the structural characteristics of the parent materials playing a crucial role.

A glimpse of the inner workings of the templated site.

This is the first direct experimental probe, using EXAFS, of the active site within molecularly imprinted polymers and paves the way to a more detailed understanding of the inner workings of molecular imprinting.

Selective oxidation of benzene to phenol over FeAlPO catalysts using nitrous oxide as oxidant.

A tetrahedrally coordinated iron in framework substituted microporous AlPO-5 catalysts are shown to be active and selective for the hydroxylation of benzene to phenol, using nitrous oxide as the oxidant.