Free radicals as intermediates in catalytic oxidation of light alkanes: new opportunities

The analysis of catalytic partial oxidation of light alkanes indicates that processes involving this group proceed via the formation and consecutive transformations of free radicals. Depending on the properties of the catalytic system and reaction conditions the same primary radical can give different final products, olefins, oxygenated organic substances and carbon oxides. An approach to design a complex catalytic system for efficient alkane partial oxidation based on separation of zones where free radicals are formed and where they are transformed into desired products is suggested and examples of its implementation are presented.     

Identification of oxidation products and free radicals of tryptophan by mass spectrometry

New oxidation products and free radicals derived from tryptophan (Trp) oxidation under Fenton reaction conditions were identified using mass spectrometry. After the oxidation of tryptophan using hydrogen peroxide and iron (II) system (Fenton reaction), mono- and dihydoxy tryptophans and N-formylkynurenine were identified using electrospray mass spectrometry (ES-MS) and ES-MS/MS. Besides these products, new products resulting from the reaction of tryptophan and oxidized tryptophan and 3-methyl indole derivatives were also identified. The 3-methyl indole derivatives resulted, most probably, from the oxidation process and not from in-source processes. A dimer formed by cross-linking between two Trp radicals (Trp-Trp), similar to the previously described tyrosine dimer was observed, as well as the corresponding monohydroxy-dimer (Trp-Trp-OH). Tandem mass spectrometry was used to identify the structures of these new oxidation products. Free radicals derived from tryptophan oxidation under Fenton reaction were detected using as spin trap the DMPO. The free radical species originated during the oxidation reaction formed stable adducts with the spin trap, and these adducts were identified by ES-MS. New adducts of oxidized tryptophan radicals, namely monohydroxy-tryptophan and dihydroxy-Trp dimer radicals, with one and two DMPO spin trap molecules where identified. Tandem mass spectrometry was used to confirm the proposed structure of the observed adducts.     

The effect of 1,3-diphenylisobenzofuran on the photo-oxidative degradation of cis-1,4-polybutadiene

1,3-Diphenylisobenzofuran (DPBF) is easily photo-oxidized by two mechanisms viz free radical oxidation and singlet oxygen oxidation. The final products of DPBF oxidation by these two mechanisms are the same. Using light in the range 280–480 nm, DPBF is an effective sensitizer of photooxidative degradation of polybutadiene in the solid and in solution. In a system with methylene blue (MB) in methanol-benzene solution (0.5:9.5) where free radicals from MB and ¹O₂ are formed during irradiation with visible light, DPBF is oxidized by both ¹O₂ and free radical mechanisms. DPBF cannot stop free radical degradation of PB initiated by MB radicals in MB-methanol-benzene solution. These results show that the DPBF is an ineffective stabilizer for polydienes against ¹O₂ and free radical oxidation. It rather acts as a sensitizer for photo-oxidation of polydienes.     

Low-Temperature Oxidation of Phenylalkenes with tert-Butyl Hydroperoxide in the Presence of Aluminum and Titanium tert-Butylates

tert-Butyl hydroperoxide in the presence of aluminum and titanium tert-butylates oxidizes phenylalkenes to carbonyl compounds, as well as unsaturated alcohols and their epoxidation products; the process involves free radicals. Organometallic peroxides take an active part in the formation of secondary oxidation products.     

Free radical formation during ethylbenzene oxidation catalyzed by cetyltrimethylammonium bromide

The dimer of 2,2"-bis[2-(p-dimethylaminophenyl)indane-1,3-dione] and -tocopherol were used as free radical acceptors in a study of the kinetic regularities of free radical formation during ethylbenzene oxidation by molecular oxygen at 60 °C. The reaction initiated by the -phenylethyl hydroperoxide—cetyltrimethylammonium bromide (CTAB) catalytic system, which generates free radicals. The oxidation kinetics was analyzed using computer simulation.     

Temperature effect on the rate of formation of free radicals in CTAB-catalyzed decomposition of hydroperoxides

The temperature effect on the rate of the decomposition of hydroperoxides and the rate of the formation of free radicals in the oxidation of ethylbenzene with molecular oxygen in the presence of -phenylethyl hydroperoxide—cetyltrimethylammonium bromide (CTAB) as a catalytic system for free radical generation was studied by kinetic methods (from the oxygen consumption and hydroperoxide decomposition rates) and the inhibition method involving different acceptors of free radicals.     

Reaction mechanism and modeling study for the oxidation by SO2 of o‐xylene and p‐xylene in Claus process

Claus process, comprising of a furnace and a catalytic unit, is used to produce sulfur from H₂S. The aromatic contaminants (benzene, toluene, and xylenes) in H₂S feed form soot, and clog and deactivate the catalysts. Xylenes are known to be the most damaging ones. Therefore, there is a need to oxidize them in the furnace to enhance catalyst life. This article presents a kinetics study on the oxidation of o‐ and p‐xylene radicals by SO₂ (an oxidant that is already present in the furnace) using density functional theory and a composite method. The mechanism begins with H‐abstraction from xylenes to form xylyl radicals, followed by exothermic addition of SO₂ to them. The breakage of O S bond in the xylyl‐SO₂ adducts leads to the loss of SO molecule, while the remaining O atom on them helps in their oxidation. The isomerization study shows that less‐stable dimethylphenyl radicals have a high tendency to isomerize to resonantly stabilized methylbenzyl radicals. However, methylbenzyl radicals have lower reactivity toward SO₂ than dimethylphenyl radicals. The reaction rate constants were found using transition state theory. The reactor simulations reveal that p‐xylene has lower reactivity toward SO₂ than o‐xylene, and CO, SO, and CHO are the main by‐products of oxidation.     

Electro-oxidation of o-aminophenol studied by cyclic voltammetry and surface enhanced Raman scattering (SERS)

Cyclic voltammetry and surface enhanced Raman scattering (SERS) spectra were used over a wide pH range to examine the products of o-aminophenol oxidation on a roughened silver electrode. The results of the study indicated that at least two oxidation products are formed at the stationary potential of the electrode. The major product in alkaline and neutral media was identified as 2,2′-dihydroxyazobenzene, a linear dimer formed by N---N coupling of o-aminophenol cation radicals. In acidic solutions the cyclic dimer 3-aminophenoxazone formed by C---N coupling of o-aminophenol cation radicals dominates on the silver electrode.     

Radiation-induced oxidation of solid poly(ethylene oxide). II. Mechanism

The mechanism of γ-initiated oxidation of solid poly(ethylene oxide) (PEO) has been investigated, and an overall reaction scheme has been developed which accounts for most of the experimental observations. Data are correlated on the basis that the oxidation process is the sum of two reactions that are first-order and half-order in rate of initiation. They provide evidence that a significant fraction of the interactions of α-alkoxyalkylperoxy radicals is nonterminating at ambient temperature and yield free alkoxy radicals that are very subject to β scission. The unimolecular decomposition of secondary peroxy radicals, which has been invoked previously for the oxidation of PEO in solution, is not needed to explain the products of the oxidation of PEO in the solid phase. Approximately 90% of the total oxygen consumption has been accounted for by the observed products of oxidation. The radiochemical yield for backbone radical formation in irradiated PEO was estimated to be 6.5 ± 1.5.     

γ射线辐射医用级超高分子量聚乙烯自由基的演变

采用γ射线对医用级超高分子量聚乙烯(UHMWPE)进行辐照处理, 利用电子自旋共振波谱仪(ESR)研究了辐照诱导自由基的种类及其在氩气和不同氧分压下的衰减行为. 在氩气中, 辐射诱导UHMWPE主要产生烷基自由基和烯丙基自由基, 总的辐射化学产额约为0.48/100 eV. 室温下烷基自由基的稳定性差, 其寿命仅有 1 d左右. 在含氧气氛中, 自由基主要通过氧化反应而衰减, 其衰减速率随氧分压的增加而增加, 半衰期则由1×10⁵ Pa氩气中的224.0 h降至5×10⁵ Pa O₂气中的1.8 h. 根据此结果推算, 室温下陷落在晶区的自由基迁移至微晶表面的速率非常快, 仅需小时量级.     

Free-radical generation mechanism and antimalarial activity of cyclohexyl endoperoxides

A kinetic analysis has been carried out for a cascade of intramolecular oxidation reactions of free radicals generated in the redox reactions of substituted cyclohexyl endoperoxides (15 compounds) with the Fe²⁺ ion. Each radical conversion reaction has been characterized by its enthalpy, activation energy, and rate constant. Kinetic characteristics have been calculated by the intersecting parabolas method. Depending on their structure, cyclohexyl endoperoxides generate one to three radicals. There is a linear empirical correlation between the number of radicals generated by a peroxide and its molar antimalarial activity (IC ₅₀/M, where M is the molar mass of the peroxide). The peroxides that generate no more than one radical show no antimalarial activity.     

The Atmospheric Oxidation Mechanism of Benzyl Alcohol Initiated by OH Radicals: The Addition Channels

Benzyl alcohol (BA) is present in indoor atmospheres, where it reacts with OH radicals and undergoes further oxidation. A theoretical study is carried out to elucidate the reaction mechanism and to identify the main products of the oxidation of BA that is initiated by OH radicals. The reaction is found to proceed by H‐abstraction from the CH₂ group (25 %) and addition to the ipso (60 %) and ortho (15 %) positions of the aromatic ring. The BA–OH adducts react further with O₂ via the bicyclic radical intermediates—the same way as for benzene—forming mainly 3‐hydroxy‐2‐oxopropanal and butenedial. If NO_x is low, the bicyclic peroxy radicals undergo intramolecular H‐migration, forming products containing OH, OOH, and CH₂OH/CHO functional groups, and contribute to secondary organic aerosol (SOA) formation.     

EPR Spectroscopic Studies of Lipoxygenases

Polyunsaturated fatty acids are sources of diverse natural, and chemically designed products. The enzyme lipoxygenase selectively oxidizes fatty acid acyl chains using controlled free radical chemistry; the products are regio‐ and stereo‐chemically unique hydroperoxides. A conserved structural fold of ≈600 amino acids harbors a long and narrow substrate channel and a well‐shielded catalytic iron. Oxygen, a co‐substrate, is blocked from the active site until a hydrogen atom is abstracted from substrate bis‐allylic carbon, in a non‐heme iron redox cycle. EPR spectroscopy of ferric intermediates in lipoxygenase catalysis reveals changes in the metal coordination and leads to a proposal on the nature of the reactive intermediate. Remarkably, free radicals are so well controlled in lipoxygenase chemistry that spin label technology can be applied as well. The current level of understanding of steps in lipoxygenase catalysis, from the EPR perspective, will be reviewed.     

Hydrocarbon Oxidation by a Porphyrin-π-Cation Radical Complex

Heme and chlorin π-cation radical oxidants are widely implicated in biological and synthetic oxidation catalysis. Little insight into the role of π-cation radicals in proton coupled electron transfer (PCET) oxidation is available. We prepared a Ni^II-porphyrin-π-cation complex ([Ni^II(P⋅⁺)]) and found it to be capable of the oxidation of a variety of simple hydrocarbon substrates. Interestingly, some of the products were hydroxylated, with ([Ni^II(P⋅⁺)]) working in concert with atmospheric O₂ to yield hydroxylated hydrocarbons. Kinetic data suggested that the porphyrin-π-cation radical species oxidised substrates through a concerted PCET mechanism, where the porphyrin-π-cation radical accepted the electron, and the proton was transferred to a free anion. Our findings highlight the potential role of π-cation radicals as hydrocarbon activators, demonstrating that porphyrin ligand non-innocence could be a readily manipulated resource for oxidation catalyst development.     

Fe~(2+)协同热活化过一硫酸氢钾盐诱导自由基脱除NO的研究

在气液撞击流反应器中,研究了Fe~(2+)协同热活化过一硫酸氢钾盐诱导自由基脱除模拟烟气中的NO。考察了主要工艺参数(溶液温度、Fe~(2+)浓度、过一硫酸氢钾盐浓度、溶液pH值、NO入口浓度)对NO脱除效率的影响。分析检测了反应产物和自由基。基于不同系统的对比研究、反应产物检测和活性自由基的捕获,揭示了NO脱除过程的机制和反应路径。结果表明,提高溶液温度、Fe~(2+)浓度和过一硫酸氢钾盐浓度均提高了NO的脱除效率,而提高溶液pH值和NO入口浓度均降低了NO的脱除效率。Fe~(2+)和热对活化过一硫酸氢钾盐产生自由基有显著的协同效应。自由基氧化是NO脱除的主要路径,而过一硫酸氢钾盐直接氧化是次要的脱除路径。Fe~(2+)和热的协同活化体系具有比其他体系高得多的NO脱除率。     

Laser flash photolysis in a O3/Cl2 mixture at 266 nm in a very low-pressure flow system

The laser flash photolysis in a very low-pressure flow system with mass spectrometry detection technique was developed for the study of oxidation reactions of chlorofluorocarbons. In this work, we have studied the UV photolysis of O3 in the presence of Cl2 at room temperature, which presents two catalytic cycles of O3 depletion with efficiencies dependent on the partial pressures in the photoreactor. The ozone dissociation was initiated with fourth harmonic pulses of a Nd:YAG laser. The detection of the reactants and the final and intermediate reaction products was performed with real-time mass spectrometry. The variations of the O3, Cl2 and ClO concentration were measured. The equations system associated to a proposed kinetic scheme was solved numerically and excellent agreement with the experimental results was obtained. The results from this work allowed the determination of the wall loss rates of the O(1D), Cl and ClO radicals.     

The gas-phase decomposition of methylsilane. Part II. Mechanisms of decomposition under static system conditions

The static system pyrolysis of methylsilane (T ～ 700 K, P_T ～ 150 torr), pure and in the presence of ethylene, propylene, and acetylene, has been investigated. It is proposed that in the uninhibited system, the major products (silane and dimethylsilane) are produced by free radical processes, and that the free radicals are formed at the walls from methylsilylenes. In the presence of olefins, the free radicals are trapped to form methylsilane adducts. In acetylene, trapping of methylsilylenes prevents free radical production and eliminates the free radical produced products of the pure and the olefin inhibited systems. Rates of initiation correlate with rates of reactant loss in acetylene inhibited systems, and with rates of hydrogen formation in olefin inhibited systems. Rough estimates of primary dissociation process yields give for the 1,1-H₂ elimination ?_1,1 ? 0.78, for the 1,2-H elimination ?_1,2 ? 0.16, and for the methane elimination ?CH₄ ? 0.06 at 700 K. Deuteration lowers initial step kinetics by about 15%.     

Effect of cationic surfactants on limonene oxidation

Kinetic regularities of free radical formation during limonene oxidation with molecular oxygen in a chlorobenzene solution at 60 °C were studied by the inhibitor method under autooxidation conditions and with additives of surfactants (cetyltrimethylammonium chloride and bromide) using dimer 2,2′-bis[2-(p-dimethylaminophenyl)indane-1,3-dione] and α-tocopherol as acceptors of radicals. In the presence of the surfactants, the rates of radical generation and oxidation increase sharply. The catalytic activity of cetyltrimethylammonium chloride is much higher than that of the corresponding bromide.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2110–2113, October, 2004.     

Synthesis of highly substituted pyrroles via oxidative free radical reactions of β-aminocinnamates

Oxidative free radical reactions of β-aminocinnamates are described. Imine radicals produced by tetra-n-butylammonium cerium(IV) nitrate (TBACN) oxidation of enamines undergo efficient addition to the C-C double bond of β-aminocinnamates. This TBACN mediated free radical reaction between β-aminocinnamates and enamines provides a novel method for the synthesis of highly substituted pyrroles. The direct TBACN oxidation of β-aminocinnamates gave the dimerization products effectively.