Autoxidation Mechanism Research Articles

Influence of temperature and stabilization on oxygen diffusion limited oxidation profiles of polyamide 6

The oxidative degradation behavior of polymers depends on a combination of chemical and physical factors, with oxygen diffusion being one of the most important, especially when the oxygen consumption rate is larger than its permeability.As a result of diffusion limited oxidation (DLO), at high temperatures the degradation rate of polyamide 6 (PA6) plaques is heterogeneous, with the polymer oxidizing much faster at the surface than in the bulk. Normalized carbonyl index (CI) and UV absorption – depth profiles were found to be mostly degradation time independent, implying equilibrium degradation conditions where oxygen permeability and reaction rates did not change significantly with degradation time. The experimental DLO profiles were described using a basic reactive-diffusion model based on Fickian oxygen diffusion and an oxidation rate being first order in local O2 concentration, as well as by applying an established DLO model based on the basic autoxidation mechanism. Analysis with the second model yielded the best estimation of high temperature oxygen permeability (PO2) data. It also showed some of the limitations in the data analysis when using a simple first order DLO model.It was shown that stabilizers have an influence on the oxidation - depth profiles. Better stabilization results in slower polymer oxidation and the oxidation – depth profiles are therefore less pronounced. At 170 °C it was observed that stabilized plaques (0.5 mm) in the center oxidize faster than unstabilized plaques, which is attributed to the complete consumption of oxygen in the outer layers for the unstabilized plaques. Oxidation rates of differently stabilized samples were also determined by applying the second DLO model.

Manganese and Iron Catalysts in Alkyd Paints and Coatings

Many paint, ink and coating formulations contain alkyd-based resins which cure via autoxidation mechanisms. Whilst cobalt-soaps have been used for many decades, there is a continuing and accelerating desire by paint companies to develop alternatives for the cobalt soaps, due to likely classification as carcinogens under the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) legislation. Alternative driers, for example manganese and iron soaps, have been applied for this purpose. However, relatively poor curing capabilities make it necessary to increase the level of metal salts to such a level that often coloring of the paint formulation occurs. More recent developments include the application of manganese and iron complexes with a variety of organic ligands. This review will discuss the chemistry of alkyd resin curing, the applications and reactions of cobalt-soaps as curing agents, and, subsequently, the paint drying aspects and mechanisms of (model) alkyd curing using manganese and iron catalysts.

Highly Oxidized RO2Radicals and Consecutive Products from the Ozonolysis of Three Sesquiterpenes

The formation of highly oxidized multifunctional organic compounds (HOMs) from the ozonolysis of three sesquiterpenes, α-cedrene, β-caryophyllene, and α-humulene, was investigated for the first time. Sesquiterpenes contribute 2.4% to the global carbon emission of biogenic volatile organic compounds (BVOCs) and can be responsible for up to 70% of the regional BVOC emissions. HOMs were detected with chemical ionization-atmospheric pressure interface-time-of-flight mass spectrometry and nitrate and acetate ionization. Acetate ions were more sensitive toward highly oxidized RO2 radicals containing a single hydroperoxide moiety. Under the chosen reaction conditions, product formation was dominated by highly oxidized RO2 radicals which react with NO, NO2, HO2, and other RO2 radicals under atmospheric conditions. The ozonolysis of sesquiterpenes resulted in molar HOM yields of 0.6% for α-cedrene (acetate), 1.8% for β-caryophyllene (acetate), and 1.4% for α-humulene (nitrate) afflicted with an uncertainty factor of 2. Molar yields of highly oxidized RO2 radicals were identical with HOM yields measuring the corresponding closed-shell products. HOM formation from ozonolysis of α-cedrene was explained by an autoxidation mechanism initiated by ozone attack at the double bond similar to that found in the ozonolysis of cyclohexene and limonene.

INVESTIGATION ON OXIDATION AND THERMAL STABILITY OF JATROPHA OIL

The application of jatropha oil as alternative lubricant basestock has emerged recently. However, the resistance to oxidation and thermal stability of this oil become a concern to its application as lubricant. In this study, the oxidation and thermal degradation of crude jatropha oil (CJO) have been studied. The oxidation of the CJO was investigated by air bubbling method. The viscosity and functional group changes due to oxidation were studied before and after oxidation test. The thermal stability of CJO was investigated by using Thermogravimetric analysis (TGA) method in O2 and N2 gases environment. Jatropha oil was found oxidized by autoxidation mechanism produces high molecular species which capable to increase viscosity of the oil. The TGA results shows that CJO thermally stable in the temperature below 168°C in O2 gas environment and below 225°C in N2 gas environment.

Gas-Phase Ozonolysis of Cycloalkenes: Formation of Highly Oxidized RO2 Radicals and Their Reactions with NO, NO2, SO2, and Other RO2 Radicals.

The gas-phase reaction of ozone with C5-C8 cycloalkenes has been investigated in a free-jet flow system at atmospheric pressure and a temperature of 297 ± 1 K. Highly oxidized RO2 radicals bearing at least 5 O atoms in the molecule and their subsequent reaction products were detected in most cases by means of nitrate-CI-APi-TOF mass spectrometry. Starting from a Criegee intermediate after splitting-off an OH-radical, the formation of these RO2 radicals can be explained via an autoxidation mechanism, meaning RO2 isomerization (ROO → QOOH) and subsequently O2 addition (QOOH + O2 → R'OO). Time-dependent RO2 radical measurements concerning the ozonolysis of cyclohexene indicate rate coefficients of the intramolecular H-shifts, ROO → QOOH, higher than 1 s(-1). The total molar yield of highly oxidized products (predominantly RO2 radicals) from C5-C8 cycloalkenes in air is 4.8-6.0% affected with a calibration uncertainty by a factor of about two. For the most abundant RO2 radical from cyclohexene ozonolysis, O,O-C6H7(OOH)2O2 ("O,O" stands for two O atoms arising from the ozone attack), the determination of the rate coefficients of the reaction with NO2, NO, and SO2 yielded (1.6 ± 0.5) × 10(-12), (3.4 ± 0.9) × 10(-11), and <10(-14) cm(3) molecule(-1) s(-1), respectively. The reaction of highly oxidized RO2 radicals with other peroxy radicals (R'O2) leads to detectable accretion products, RO2 + R'O2 → ROOR' + O2, which allows to acquire information on peroxy radicals not directly measurable with the nitrate ionization technique applied here. Additional experiments using acetate as the charger ion confirm conclusively the existence of highly oxidized RO2 radicals and closed-shell products. Other reaction products, detectable with this ionization technique, give a deeper insight in the reaction mechanism of cyclohexene ozonolysis.

Aerobic Asymmetric Dehydrogenative Cross-Coupling between Two C(sp3)-H Groups Catalyzed by a Chiral-at-Metal Rhodium Complex.

A sustainable C-C bond formation is merged with the catalytic asymmetric generation of one or two stereocenters. The introduced catalytic asymmetric cross-coupling of two C(sp3)-H groups with molecular oxygen as the oxidant profits from the oxidative robustness of a chiral-at-metal rhodium(III) catalyst and exploits an autoxidation mechanism or visible-light photosensitized oxidation. In the latter case, the catalyst serves a dual function, namely as a chiral Lewis acid for catalyzing enantioselective enolate chemistry and at the same time as a visible-light-driven photoredox catalyst.

Aerobic Asymmetric Dehydrogenative Cross‐Coupling between Two CH Groups Catalyzed by a Chiral‐at‐Metal Rhodium Complex

AbstractA sustainable CC bond formation is merged with the catalytic asymmetric generation of one or two stereocenters. The introduced catalytic asymmetric cross‐coupling of two CH groups with molecular oxygen as the oxidant profits from the oxidative robustness of a chiral‐at‐metal rhodium(III) catalyst and exploits an autoxidation mechanism or visible‐light photosensitized oxidation. In the latter case, the catalyst serves a dual function, namely as a chiral Lewis acid for catalyzing enantioselective enolate chemistry and at the same time as a visible‐light‐driven photoredox catalyst.

Effects of chemical complexity on the autoxidation mechanisms of endocyclic alkene ozonolysis products: from methylcyclohexenes toward understanding α-pinene.

Formation of highly oxidized, multifunctional products in the ozonolysis of three endocyclic alkenes, 1- methylcyclohexene, 4-methylcyclohexene, and α-pinene, was investigated using a chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer with a nitrate ion (NO3(-)) based ionization scheme. The experiments were performed in borosilicate glass flow tube reactors at room temperature (T = 293 ± 3 K) and at ambient pressure. An ensemble of oxidized monomer and dimer products was detected, with elemental compositions obtained from the high-resolution mass spectra. The monomer product distributions have O/C ratios from 0.8 to 1.6 and can be explained with an autocatalytic oxidation mechanism (=autoxidation) where the oxygen-centered peroxy radical (RO2) intermediates internally rearrange by intramolecular hydrogen shift reactions, enabling more oxygen molecules to attach to the carbon backbone. Dimer distributions are proposed to form by homogeneous peroxy radical recombination and cross combination reactions. These conclusions were supported by experiments where H atoms were exchanged to D atoms by addition of D2O to the carrier gas flow. Methylcyclohexenes were observed to autoxidize in accordance with our previous work on cyclohexene, whereas in α-pinene ozonolysis different mechanistic steps are needed to explain the products observed.

The role of peroxyl radicals in polyester degradation--a mass spectrometric product and kinetic study using the distonic radical ion approach.

Mass spectrometric techniques were used to obtain detailed insight into the reactions of peroxyl radicals with model systems of (damaged) polyesters. Using a distonic radical ion approach, it was shown that N-methylpyridinium peroxyl radical cations, Pyr(+)OO˙, do not react with non-activated C-H bonds typically present in polyesters that resist degradation. Structural damage in the polymer, for example small amounts of alkene moieties formed during the manufacturing process, is required to enable reaction with Pyr(+)OO˙, which proceeds with high preference through addition to the π system rather than via allylic hydrogen atom abstraction (kadd/kHAT > 20 for internal alkenes). This is due to the very fast and strongly exothermic subsequent fragmentation of the peroxyl-alkene radical adduct to epoxides and highly reactive Pyr(+)O˙, which both could promote further degradation of the polymer through non-radical and radical pathways. This work provides essential experimental support that the basic autoxidation mechanism is a too simplistic model to rationalize radical mediated degradation of polymers under ambient conditions.

ChemInform Abstract: Sulfuryl Chloride as an Efficient Initiator for the Metal‐Free Aerobic Cross‐Dehydrogenative Coupling Reaction of Tertiary Amines.

A metal-free cross-dehydrogenative (CDC) reaction of tertiary amines was developed using a catalytic amount of sulfuryl chloride (SO2Cl2) under mild aerobic conditions. On the basis of the nature of SO2Cl2, it was assumed that the reagent acts as a radical initiator to induce the metal-free CDC reaction via a radical-initiated autoxidation mechanism.

XPS and DFT Studies on the Autoxidation Process of Cu Sheet at Room Temperature

To better understand the autoxidation mechanism of Cu-based catalysts, we studied the oxidation of Cu sheet exposed to ultrahigh vacuum and air at ambient temperature using X-ray photoelectron spectroscopy (XPS) and density functional theory. Six main stages of Cu autoxidation are revealed: (1) dissociative adsorption of O2, (2) coexistence of nondissociative and dissociative H2O adsorption, (3) diffusion of O and OH into Cu bulk, (4) formation of metastable Cu2O layer, (5) further oxidation and formation of metastable Cu(OH)2 and CuO layer, and (6) transformation phase of the metastable Cu(OH)2 to CuO. The formation of Cu(OH)2 depends on the relative humidity of air and the concentration of adsorbed OH of the Cu sheet. On the basis of these results, we propose that the preservation time of the Cu-based catalysts should be decreased or the catalysts should be preserved in a high vacuum and at low relative humidity. Considering the time of the sample preparation before ex situ XPS analysis and other surfac...

Sulfuryl chloride as an efficient initiator for the metal-free aerobic cross-dehydrogenative coupling reaction of tertiary amines.

A metal-free cross-dehydrogenative (CDC) reaction of tertiary amines was developed using a catalytic amount of sulfuryl chloride (SO2Cl2) under mild aerobic conditions. On the basis of the nature of SO2Cl2, it was assumed that the reagent acts as a radical initiator to induce the metal-free CDC reaction via a radical-initiated autoxidation mechanism.

Evidence for the presence of alternative mechanisms in the oxidation of cyclohexanone to adipic acid with oxygen, catalysed by Keggin polyoxometalates

This report deals with the results of a study on the oxidation of cyclohexanone to adipic acid with air, catalysed by Keggin-type polyoxometalates of composition H 3+ x PMo 12− x V x O 40 ( x = 1 and 2), which was carried out in a semi-continuous stirred-tank reactor. It was found that when conducted in the presence of a water-only solvent, the reaction proceeds with a redox mechanism, in which the step of reoxidation of the reduced POM by oxygen is rate limiting. When, however, the reaction was carried out with an acetic acid co-solvent, a radical-chain autoxidation mechanism prevailed, especially when very low amounts of catalyst were used. Autoxidation overlapped with the redox mechanism when the catalyst-to-cyclohexanone ratio was increased. Moreover, the composition of the polyoxometalate, that is, the number of V atoms per Keggin unit, affected the relative importance of the two mechanisms. The selectivity to adipic acid achieved was a function of the reaction mechanism, but also was affected by cyclohexanone conversion, due to the presence of a complex reaction network.

Autoxidation and Prooxidation Mechanisms of Danshensu

A hydrogen peroxide (H2O2) electrode and electron spin resonance (ESR) methods were used to continuously monitor and analyze the autoxidation and prooxidation of Danshensu in aqueous solutions with various pH values.We found that Danshensu exhibited obvious autoxidative activity in alkalescence buffer solutions (pH=8.5) and the presence of Cu2+ evidently accelerated the generation of H2O2.The primary product of the autoxidation was a superoxide anion radical (O2-·) and then it could be converted into H2O2.The autoxidation of Danshensu was totally inhibited under weak acidic conditions (pH=6.5) and a partial prooxidation of Danshensu occurred in the presence of cupric ion.However, under physiological conditions (pH=7.4), the autoxidation of Danshensu was very slow and its prooxidation, initiated by Cu2+, was very limited.Consequently, the autoxidation and prooxidation of Danshensu cannot be completely ignored when used in clinical practice especially under alkaline body fluid conditions.

Spin Trapping of Au−H Intermediate in the Alcohol Oxidation by Supported and Unsupported Gold Catalysts

Electron paramagnetic resonance (EPR) spectroscopy and spin trapping were used to explore the mechanism of alcohol oxidation over gold catalysts. Reaction of secondary alcohols with supported and unsupported gold catalysts (e.g., Au/CeO(2), polymer-incarcerated Au nanoparticles, PPh(3)-protected Au nanoparticles) in the presence of spin traps led to the formation of a hydrogen spin adduct. Using isotope labeling, we confirmed that the hydrogen in the spin adduct originates from the cleavage of the C-H bond in the alcohol molecule. The formation of the hydrogen spin adduct most likely results from the abstraction of hydrogen from the Au surface by a spin trap. These results thus strongly suggest intermediate formation of Au-H species during alcohol oxidation. The role of oxygen in this mechanism is to restore the catalytic activity rather than oxidize alcohol. This was further confirmed by carrying out gold-catalyzed alcohol oxidation in the absence of oxygen, with nitroxides as hydrogen abstractors. The support (e.g., metal oxides) can activate oxygen and act as an H abstractor from the gold surface and hence lead to a faster recovery of the activity. Peroxyl radicals were also observed during alcohol oxidation, consistent with a free-radical autoxidation mechanism. However, this mechanism is likely to be a minor side reaction, which does not lead to the formation of an appreciable amount of oxidation products.

Reinvestigation of a Ru2-Incorporated Polyoxometalate Dioxygenase Precatalyst, “[WZnRu2III(H2O)(OH)(ZnW9O34)2]11−”: Evidence For Marginal, ≤0.2 Equivalents of Ru Incorporation Plus Faster Catalysis by Physical Mixtures of [RuII(DMSO)4Cl2] and the Parent Polyoxometalate [WZn3(H2O)2(ZnW9O34)2]12−

A 1997 Nature paper (Nature 1997, 388, 353-355) and subsequent 1998 J. Am. Chem. Soc. paper (J. Am. Chem. Soc. 1998, 120, 11969-11976) reported that a putative Ru(2)-substituted polyoxoanion, "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)", (1), is an all inorganic dioxygenase able to incorporate one O(2) into two adamantane CH bonds to yield 2 equiv of 1-adamantanol as the primary product. In a subsequent 2005 Inorg. Chem. publication (Inorg. Chem. 2005, 44, 4175-4188), strong evidence was provided that the putative dioxygenase chemistry is, instead, the result of classic autoxidation catalysis. That research raised the question of whether the reported Ru(2) precatalyst, 1, was pure or even if it contained two Ru atoms, since Ru is known to be difficult to substitute into polyoxoanion structures (Nomiya, K.; Torii, H.; Nomura, K.; Sato, Y. J. Chem. Soc. Dalton Trans. 2001, 1506-1521). After our research group had contact with three other groups who also had difficulties reproducing the reported synthesis and composition of 1, we decided to re-examine 1 in some detail. Herein we provide evidence that the claimed 1 actually appears to be the parent polyoxoanion [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) with small amounts of Ru (</=0.2 atoms) either substituted into the parent complex or present as a small amount of a Ru(n+) impurity, at least in our and two other group's hands. The evidence obtained, on three independent samples prepared from two research groups including ours, includes elemental analysis on the bulk samples, single crystal X-ray diffraction, elemental analysis on single crystals from the same batch used for X-ray diffraction, (183)W NMR, and adamantane oxidation oxygen uptake and product determination studies. Also re-examined herein are the two previously reported crystal structures of 1 that appear to be very similar to the structure of the parent polyoxoanion, [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-). Furthermore, we report that trace Ru alone, in the form of [Ru(DMSO)(4)Cl(2)], or that the parent polyoxoanion [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) alone, are capable of producing the same products. More significantly, a simple physical mixture of [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) plus the average 0.13 equiv of Ru found by analysis added as the [Ru(DMSO)(4)Cl(2)] starting material is a ca. 2-fold kinetically more competent catalyst than is "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)", (1). In short, the evidence is strong that the putative "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)", (1), which underlies the previously reported all-inorganic dioxygenase catalysis claim, is probably not correct. That does not mean that 1 cannot or even does not exist, but just that (a) no reliable synthesis of it exists if it has actually been made before, and (b) that a simple mixture of the [Ru(DMSO)(4)Cl(2)] plus [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) precursors gives about 2-fold faster catalysis of adamantane hydroxylation that occurs by, the evidence suggests, a radical-chain autoxidation mechanism rather than via the previously claimed, novel all-inorganic-based dioxygenase catalysis.

Chemical, physical and electrical properties of aged dodecylbenzene 2: thermal ageing of single isomers in air

The linear isomer of dodecylbenzene (DDB), 1-phenyldodecane, was aged at temperatures of 105 and 135 degC in air and the resultant products were analyzed using a range of analytical techniques. On ageing, the 1-phenyldodecane darkened, the acid number, dielectric loss and water content increased and significant oxidation peaks were detected in the infrared spectrum. When aged in the presence of copper, a characteristic peak at 680 nm was also detected by UV/visible spectroscopy but, compared with previous studies of a cable-grade DDB, the strength of this peak was much increased and no appreciable precipitate formation occurred. At the same time, very high values of dielectric loss were recorded. On ageing in the absence of copper, an unusually strong infrared carbonyl band was seen, which correlates well with the detection of dodecanophenone by gas chromatography/mass spectrometry and nuclear magnetic resonance spectroscopy. It was therefore concluded that the ageing process proceeds via the initial production of aromatic ketones, which may then be further oxidized to carboxylic acids. In the presence of copper, these oxidation products are present in lower quantities, most of these oxidation products being combined with the copper present in the oil to give copper carboxylates. The behavior is described in terms of a complex autoxidation mechanism, in which copper acts as both an oxidizing and a reducing agent, depending on its oxidation state and, in particular, promotes elimination via the oxidation of intermediate alkyl radical species to carbocations.

Mechanism of Thermal Toluene Autoxidation

Aerobic oxidation of toluene (PhCH3) is investigated by complementary experimental and theoretical methodologies. Whereas the reaction of the chain-carrying benzylperoxyl radicals with the substrate produces predominantly benzyl hydroperoxide, benzyl alcohol and benzaldehyde originate mainly from subsequent propagation of the hydroperoxide product. Nevertheless, a significant fraction of benzaldehyde is also produced in primary PhCH3 propagation, presumably via proton rather than hydrogen transfer. An equimolar amount of benzyl alcohol, together with benzoic acid, is additionally produced in the tertiary propagation of PhCHO with benzylperoxyl radicals. The "hot" oxy radicals generated in this step can also abstract aromatic hydrogen atoms from PhCH3, and this results in production of cresols, known inhibitors of radical-chain reactions. The very fast benzyl peroxyl-initiated co-oxidation of benzyl alcohol generates HO2* radicals, along with benzaldehyde. This reaction also causes a decrease in the overall oxidation rate, due to the fast chain-terminating reaction of HO2*with the benzylperoxyl radicals, which causes a loss of chain carriers. Moreover, due to the fast equilibrium PhCH2OOH+HO2* right harpoon over left harpoonPhCH2OO* + H2O2, and the much lower reactivity of H2O2 compared to PhCH2OOH, the fast co-oxidation of the alcohol means that HO2* gradually takes over the role of benzylperoxyl as principal chain carrier. This drastically changes the autoxidation mechanism and, among other things, causes a sharp decrease in the hydroperoxide yield.

Solvent-Free Aerobic Oxidation of n-Alkane by Iron(III)-Substituted Polyoxotungstates Immobilized on SBA-15

The tetrairon(III)-substituted polytungstates [Fe(4)(H(2)O)(10)(beta-XW(9)O(33))(2)](n-) (n = 6, X = As(III), Sb(III); n = 4, X = Se(IV), Te(IV)) were immobilized on (3-aminopropyl)triethoxysilane-modified SBA-15 and showed an excellent catalytic performance for solvent-free aerobic oxidation of long-chain n-alkanes using air as the oxidant under ambient conditions through a classical free-radical chain autoxidation mechanism.

Synthetic Uses of Peroxides

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