Showing papers on "Chemical decomposition published in 2007"

Chemical Recycling of Poly(ethylene terephthalate)

Abstract: This paper covers the recent research carried out by the authors on the chemical recycling of poly(ethylene terephthalate) (PET) taken from post-consumer soft-drink bottles. The chemical recycling techniques used are critically reviewed and the authors' contribution is highlighted. Hydrolysis in either an alkaline or acid environment was employed in order to recover pure terephthalic acid monomer that could be repolymerized to form the polymer again. Alkaline hydrolysis was carried out in either an aqueous NaOH solution or in a non-aqueous solution of KOH in methyl cellosolve. A phase-transfer catalyst was introduced in alkaline hydrolysis, in order that the reaction takes place at atmospheric pressure and in mild experimental conditions. The reaction kinetics were thoroughly investigated, both experimentally and theoretically, using a simple, yet precise, kinetic model. Moreover, glycolysis was examined as an effective way for the production of secondary value-added materials. The glycolysated PET products (oligomers) can be used as raw materials for the production of either unsaturated polyester resins (UPR) or methacrylated oligoesters (MO). UPR can subsequently be cured with styrene in ambient temperature to produce alkyd resins used as enamel paints or coatings. MO are potential monomers that can be cured either by UV irradiation or temperature to produce formulations used as coatings for wood surfaces, paints, or other applications. Thus, recycling of PET does not only serve as a partial solution to the solid-waste problem, but also contributes to the conservation of raw petrochemical products and energy.

Kinetics of the Decomposition of Fructose Catalyzed by Hydrochloric Acid in Subcritical Water: Formation of 5-Hydroxymethylfurfural, Levulinic, and Formic Acids

Abstract: This paper deals with the dehydration of fructose (F) to 5-hydroxymethylfurfural (HMF) and the rehydration of HMF to levulinic acid (LA) along with formic acid (FA) in subcritical water (sub-CW) in the presence of HCl as catalyst. The experimental conditions were 483-543 K, 4-15 MPa, and residence times of 0.5-300 s. The pressure (in the range of 4-15 MPa) did not affect the decomposition reactions. This differed from other operating variables. The main products were found to be HMF, 2-furaldehyde (2FA), LA, FA, and soluble polymers. The HMF, LA, and FA were also individually subjected for decomposition at the same condition. We found that, during the sub-CW reaction, byproducts (soluble polymers) were produced not only from F but also from HMF. However, we distinguished between two different polymers (i.e., produced from the decomposition of F or HMF) in a proposed reaction model. The kinetics of the sub-CW reaction was developed by considering not only F, HMF, LA, and FA but also soluble polymers. The kinetic constants obtained from the proposed reaction pathway showed good agreement with experimental results.

Thermal decomposition of natural dolomite

Abstract: Thermal decomposition behaviour of dolomite sample has been studied by thermogravimetric (TG) measurements. Differential thermal analysis (DTA) curve of dolomite shows two peaks at 777·8°C and 834°C. The two endothermic peaks observed in dolomite are essentially due to decarbonation of dolomite and calcite, respectively. The TG data of the decomposition steps have also been analysed using various differential, difference-differential and integral methods, viz. Freeman-Carroll, Horowitz-Metzger, Coats-Redfern methods. Values of activation entropy, Arrhenius factor, and order of reaction have been approximated and compared. Measured activation energies vary between 97 and 147 kJ mol−1. The large fluctuation in activation energy is attributed to the presence of impurities such as SiO2, Al2O3, Fe2O3, Cl− etc in the samples. FTIR and XRD analyses confirm the decomposition reaction. SEM observation of the heat-treated samples at 950°C shows cluster of grains, indicating the structural transformation.

Enthalpies of formation, bond dissociation energies and reaction paths for the decomposition of model biofuels: ethyl propanoate and methyl butanoate.

Abstract: The complete basis set method CBS-QB3 has been used to study the thermochemistry and kinetics of the esters ethyl propanoate (EP) and methyl butanoate (MB) to evaluate initiation reactions and intermediate products from unimolecular decomposition reactions. Using isodesmic and isogeitonic equations and atomization energies, we have estimated chemically accurate enthalpies of formation and bond dissociation energies for the esters and species derived from them. In addition it is shown that controversial literature values may be resolved by adopting, for the acetate radical, CH3C(O)Ȯ, (298.15K) = −197.8 kJ mol-1 and for the trans-hydrocarboxyl radical, Ċ(O)OH, −181.6 ± 2.9 kJ mol-1. For EP, the lowest energy decomposition path encounters an energy barrier of ∼210 kJ mol-1 (∼50 kcal mol-1), which proceeds through a six-membered ring transition state (retro-ene reaction) via transfer of the primary methyl H atom from the ethyl group to the carbonyl oxygen, while cleaving the carbon−ether oxygen to form ethene...

Chemical Durability Studies of Perfluorinated Sulfonic Acid Polymers and Model Compounds under Mimic Fuel Cell Conditions

Abstract: Peroxide radical treatments of a perfluorinated ionomer used in polymer electrolyte membrane (PEM) fuel cells and its small molecule analogues were carried out, along with analysis of the resultant products. Molecules containing terminal carboxylic acids degraded at least 1 order of magnitude faster than noncarboxylate materials; all of the systems did show peroxide-initiated degradation nonetheless. Product analysis suggests that terminal carboxylic acids react according to a sequential chain shortening, consistent with previous studies. Cleavage of side chains from both polymer and model compounds was also observed to be important and in fact may be the dominate pathway in low carboxyl content commercial PEM membranes, based on the following comparison of reactivity and concentration. The relative reactivities of carboxylic chain ends and ether linkages is approximately 500, as calculated using model compounds fluoride generation rates. Commercial perfluorosulfonic acid (PFSA) products contain minimal c...

Computational study on the kinetics and mechanisms for the unimolecular decomposition of formic and oxalic acids.

Abstract: The kinetics and mechanisms for the unimolecular decomposition reactions of formic acid and oxalic acid have been studied computationally by the high-level G2M(CC1) method and microcanonical RRKM theory. There are two reaction pathways in the decomposition of formic acid: The dehydration process starting from the Z conformer is found to be the dominant, whereas the decarboxylation reaction starting from the E conformer is less competitive. The predicted rate constants for the dehydration and decarboxylation reactions are in good agreement with the experimental data. The calculated CO/CO2 ratio, 13.6−13.9 between 1300 and 2000 K, is in close agreement with the ratio of 10 measured experimentally by Hsu et al. (In The 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1983; p 89). For oxalic acid, its isomer with two intramolecular hydrogen bonds is the most stable structure, similar to earlier reports. Two primary decomposition channels of oxalic acid producing CO2 + HO...

Unimolecular thermal fragmentation of ortho-benzyne.

Abstract: The ortho-benzyne diradical, o-C6H4 has been produced with a supersonic nozzle and its subsequent thermal decomposition has been studied. As the temperature of the nozzle is increased, the benzyne molecule fragments: o-C6H4+Δ→ products. The thermal dissociation products were identified by three experimental methods: (i) time-of-flight photoionization mass spectrometry, (ii) matrix-isolation Fourier transform infrared absorption spectroscopy, and (iii) chemical ionization mass spectrometry. At the threshold dissociation temperature, o-benzyne cleanly decomposes into acetylene and diacetylene via an apparent retro-Diels-Alder process: o-C6H4+Δ→HCCH+HCC–CCH. The experimental ΔrxnH298(o-C6H4→HCCH+HCC–CCH) is found to be 57±3kcalmol−1. Further experiments with the substituted benzyne, 3,6-(CH3)2-o-C6H2, are consistent with a retro-Diels-Alder fragmentation. But at higher nozzle temperatures, the cracking pattern becomes more complicated. To interpret these experiments, the retro-Diels-Alder fragmentation...

Preparation of biotic and abiotic iron oxide nanoparticles (IOnPs) and their properties and applications in heterogeneous catalytic oxidation.

Abstract: Iron oxide nanoparticles (IOnPs) as solid catalyst were prepared using a biotic method, i.e., biomineralization, and abiotic methods, i.e., thermal decomposition and electrochemical methods, for use as solid catalysts in the heterogeneous catalytic ozonation of para-Chlorobenzoic acid (pCBA). It was determined that characteristics of IOnPs, including particle size, morphology, surface area, electrokinetic mobility, basic group content, and chemical composition were significantly influenced by the preparation methods. TEM and FE-SEM analyses showed that the thermal decomposition method produced monodispersed and regularly spherical particles. The smallest iron oxide was also prepared by the thermal decomposition method, whereas the electrochemical method produced the largest iron oxide in terms of mean particle size. The specific surface area was found to be inversely proportional to the mean particle size. In catalytic ozonation at acidic pH levels, it was clearly observed that IOnPs enhanced the degradat...

Thermal Decomposition of Solids and Melts: New Thermochemical Approach to the Mechanism, Kinetics and Methodology

Abstract: Preface.-Abbreviations.-Introduction.-Part I. Thermal Decomposition: Primary Concepts: Mechanism: Congruent Dissociative Vaporization.- Direct Observation of Primary Products of Decomposition by Mass Spectrometric Methods.- Induction, Acceleration and Process Localization.- Nucleus Shape and Position.-Peculiarities of Structure and Composition of Solid Product.-Thermal Stability of Complex Gaseous Molecules.- Kinetics: The Arrhenius Equation.- The Hertz-Langmuir Equation and Langmuir Diffusion Equations.- Modification of the Hertz-Langmuir Equation as Applied to Decomposition Reactions.- Equilibrium Pressure of Products in the Equimolar and Isobaric Modes.- Equations for Calculation of the Absolute Rates of Decomposition.- Interpretation of the Arrhenius Parameters by the Use of the Langmuir Evaporation Equations.- Methodology: The Arrhenius-plots Method.- The Second-law Method.-The Third-law Method.- Approximate Evaluation of the Molar Enthalpies.- Reproducibility of Measurements.- Accuracy.- Measurement Time.- Part II. Interpretation and Quantitative Analysis of the Effects, Phenomena and Regularities Accompanying the Decomposition: Decomposition Conditions and the Molar Enthalpy: Ratio of Initial Decomposition Temperature to the Molar Enthalpy.- Thermal Desorption and Criteria for its Identification.- Impact of the Decomposition Mode and Reaction Stoichiometry on the Molar Enthalpy.- Decomposition of Carbonates in the Presence of CO2.- Decomposition of Crystalline Hydrates in the Presence of H2O.- The Self-cooling Effect: The Model of Temperature Calculation.- The Temperature Distribution in Powder Reactants.- Corrections to the Kinetic Parameters.- Experimental Evaluation of Self-cooling.- The Topley-Smith Effect: Experimental Observations.- Modeling of the Topley-Smith Effect for Li2SO4 .H2O.- Modeling of the Topley-Smith Effect for CaCO3.- Impact of Vapour Condensation on the Enthalpy Change: The Contribution of the Condensation Energy to the Enthalpy Change.- Increase of the Enthalpy Change with Temperature.- Reactant Melting and the Decomposition Enthalpy.- Thermochemical Analysis of the Composition of the Primary Decomposition Products.- Effect of the Crystalline Structure of a Substance on the Composition of the Primary Decomposition Products: Oxides.- Nitrides.- Phosphorous, Arsenic and Antimony.- Evaporation Coefficients.- The Kinetic Compensation Effect.- Conclusions.- Part III. Thermal Decomposition of Individual Substances: Instruments for Thermogravimetric Measurements.- Measurement Conditions and Procedures for Thermogravimetric Studies: Evaluation of the Absolute Rate of Decomposition.- Choice of the Decomposition Temperature.- Choice of the Residual Pressure of Air in the Reactor (equimolar mode).- Decomposition in the Presence of the Gaseous Product (isobaric mode).- Decomposition of Hydrates in Air and Carbonates in Argon and Air.- Evaporation and Decomposition of Some Substances: Non-metals (phosphorus).- Metalloids (arsenic, antimony).- Oxides.- Higher Oxides for IV Group of Metals.- Sulfides, Selenides and Tellurides.- Nitrides.- Azides.- Hydroxides.- Clays.- Hydrates.- Nitrates.- Sulfates.- Carbonates.- Oxalates.- Final conclusions.- Appendices: Fundamental Physical Constants.- Atomic Masses of Elements.- Coefficients of Mutual Diffusion of Gases.- Spectral and Integrated Emittance of Some Metals and Oxides.- Composition of Dry Atmospheric Air.- Saturation Pressure of Water Vapour.- Subject Index.

Effect of charged and excited states on the decomposition of 1,1-diamino-2,2-dinitroethylene molecules.

Abstract: The authors have calculated the electronic structure of individual 1,1-diamino-2,2-dinitroethylene molecules (FOX-7) in the gas phase by means of density functional theory with the hybrid B3LYP functional and 6-31+G(d,p) basis set and considered their dissociation pathways. Positively and negatively charged states as well as the lowest excited states of the molecule were simulated. They found that charging and excitation can not only reduce the activation barriers for decomposition reactions but also change the dominating chemistry from endo- to exothermic type. In particular, they found that there are two competing primary initiation mechanisms of FOX-7 decomposition: C-NO2 bond fission and C-NO2 to CONO isomerization. Electronic excitation or charging of FOX-7 disfavors CONO formation and, thus, terminates this channel of decomposition. However, if CONO is formed from the neutral FOX-7 molecule, charge trapping and/or excitation results in spontaneous splitting of an NO group accompanied by the energy release. Intramolecular hydrogen transfer is found to be a rare event in FOX-7 unless free electrons are available in the vicinity of the molecule, in which case HONO formation is a feasible exothermic reaction with a relatively low energy barrier. The effect of charged and excited states on other possible reactions is also studied. Implications of the obtained results to FOX-7 decomposition in condensed state are discussed.

Decomposition reaction of organophosphorus nerve agents on solid surfaces with atmospheric radio frequency plasma generated gaseous species.

Abstract: The decomposition and detoxification of compounds are of great interest in environmental protection and defense-related areas. We report the generation of gaseous excited species by scanning atmospheric radio frequency (rf) plasma and their reactions with two representative organophosphorus nerve agents, paraoxon and parathion, deposited on solid surfaces. The excited gaseous species generated in the Ar and Ar/O2 plasma were identified as atomic oxygen, OH radical, and excited nitrogen molecule from optical emission spectroscopy analysis. The reaction of these species with paraoxon and parathion was monitored with reflection-absorption infrared spectroscopy and compared with the decomposition by UV irradiation and UV/ozone treatments. The decomposition products of the atmospheric rf plasma treatment were similar to those of the UV/ozone treatment. The atomic oxygen and excited OH species generated by the plasma appear to be responsible for the oxidation of paraoxon and parathion. The plasma-induced decomposition process was much faster and more efficient than the UV/ozone process. The complete detoxification of paraoxon and parathion upon a short time exposure to the Ar/O2 plasma was confirmed by the Drosophila melanogaster culture test.

Nonisothermal decomposition kinetics and computational studies on the properties of 2,4,6,8-tetranitro-2,4,6,8-tetraazabicyclo[3,3,1]onan-3,7-dione (TNPDU).

Abstract: The thermal decomposition and the nonisothermal kinetics of the thermal decomposition reaction of 2,4,6,8-tetranitro-2,4,6,8-tetraazabicyclo[3,3,1]onan-3,7-dione (TNPDU) were studied under the nonisothermal condition by differential scanning calorimetry (DSC) and thermogravimetry−derivative thermogravimetry (TG−DTG) methods. The kinetic model function in differential form and the value of Ea and A of the decomposition reaction of TNPDU are f(α) = 3(1 − α)[−ln(1 − α)]2/3, 141.72 kJ mol-1, and 1011.99 s-1, respectively. The critical temperature of thermal explosion of the title compound is 232.58 °C. The values of ΔS⧧ , ΔH⧧, and ΔG⧧ of this reaction are −15.50 J mol-1 K-1, 147.65 kJ mol-1, and 155.26 kJ mol-1, respectively. The theoretical investigation on the title compound as a structure unit was carried out by the DFT-B3LYP/6-311++G** method. The IR frequencies and NMR chemical shift were performed and compared with the experimental results. The heat of formation (HOF) for TNPDU was evaluated by designin...

Kinetics of phenylurea herbicides oxidation by Fenton and photo‐Fenton processes

Abstract: The chemical oxidation of four selected phenylurea herbicides (linuron, chlortoluron, diuron, and isoproturon) was studied by means of the Fenton system. The influence of the initial concentrations of hydrogen peroxide and ferrous ions, the pH and the type of buffer (perchloric acid/perchlorate, acetic acid/acetate, or phosphoric acid/phosphate) was established according to the degradation levels obtained. In the kinetic study, the general decomposition reaction was divided into two stages with different reaction rates, which was justified by considering the whole reaction mechanism for this system. In this kinetic study, a competition kinetics model, which used p-chlorobenzoic acid as a reference compound, was applied for the evaluation of the rate constants for each reaction between the herbicides and the hydroxyl radical. The proposed values for these rate constants are: 7.5 × 109 L mol−1 s−1 for chlortoluron, 5.6 × 109 L mol−1 s−1 for linuron, 7.1 × 109 L mol−1 s−1 for diuron and 5.7 × 109 L mol−1 s−1 for isoproturon. Finally, some experiments with the photo-Fenton system reveal increases in the decomposition levels of the herbicides, due to additional generation reactions of hydroxyl radicals. Copyright © 2007 Society of Chemical Industry

Decomposition of aqueous organic compounds in the Atacama Desert and in Martian soils

Abstract: [1] Carbon-13 labeled formate, alanine, and glucose decompose when added in aqueous solution to soils collected from the “Mars-like” Yungay region (S 24° 4′ 9.6″, W 69° 51′) of the Atacama Desert. During the first 5 d of incubation, alanine (5 mM) and glucose (5 mM) solutions decomposed at rates of 0.1 to 0.2 μmol/d, and formate solution (50 mM) decomposed at rates of 0.4 to 1.6 μmol/d. The observation of approximately equal 13CO2 initial production rates by soils treated with D-glucose and L-alanine, compared to soils treated with L-glucose and D-alanine, indicates the presence of one or more nonbiological chemical decomposition mechanisms. An increase in the decomposition rates of D-glucose and L-alanine, compared to L-glucose and D-alanine ∼5 d after the addition of these organics, demonstrates that the soils are also biologically active. When treated with sodium formate solution, tested soils released 13CO2 gas in a manner that reproduces the initial gas release observed in the Mars Viking Labeled Release (LR) experiment. Our results indicate that the 13CO2 produced in Yungay soils is consistent with an initial phase of nonbiological decomposition followed by biological decomposition of added organics. Heat treatment of Yungay soils eliminated all CO2 production, while in the Viking LR experiment, the initial rapid CO2 release was eliminated by heat treatment, but a slower secondary CO2 production was not. Our results indicate that the mechanism for the decomposition of organics in Yungay soils is different from the processes observed in the Viking LR experiment.