Photooxidation Of Alkanes Research Articles

Chemical characterization of organosulfates in secondary organic aerosol derived from the photooxidation of alkanes

Abstract. We report the formation of aliphatic organosulfates (OSs) in secondary organic aerosol (SOA) from the photooxidation of C10–C12 alkanes. The results complement those from our laboratories reporting the formation of OSs and sulfonates from gas-phase oxidation of polycyclic aromatic hydrocarbons (PAHs). Both studies strongly support the formation of OSs from the gas-phase oxidation of anthropogenic precursors, as hypothesized on the basis of recent field studies in which aromatic and aliphatic OSs were detected in fine aerosol collected from several major urban locations. In this study, dodecane, cyclodecane and decalin, considered to be important SOA precursors in urban areas, were photochemically oxidized in an outdoor smog chamber in the presence of either non-acidified or acidified ammonium sulfate seed aerosol. Effects of acidity and relative humidity on OS formation were examined. Aerosols collected from all experiments were characterized by ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS). Most of the OSs identified could be explained by formation of gaseous epoxide precursors with subsequent acid-catalyzed reactive uptake onto sulfate aerosol and/or heterogeneous reactions of hydroperoxides. The OSs identified here were also observed and quantified in fine urban aerosol samples collected in Lahore, Pakistan, and Pasadena, CA, USA. Several OSs identified from the photooxidation of decalin and cyclodecane are isobars of known monoterpene organosulfates, and thus care must be taken in the analysis of alkane-derived organosulfates in urban aerosol.

Solvent-Free Photooxidation of Alkanes by Dioxygen with 2,3-Dichloro-5,6-dicyano-p-benzoquinone via Photoinduced Electron Transfer.

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) which acts as a super photooxidant. Solvent-free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ(⋅-) , as revealed by femtosecond laser-induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ(⋅-) , produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH2 .

Role of ozone in SOA formation from alkane photooxidation

Abstract. Long-chain alkanes, which can be categorized as intermediate volatility organic compounds, are an important source of secondary organic aerosol (SOA). Mechanisms for the gas-phase OH-initiated oxidation of long-chain alkanes have been well documented; particle-phase chemistry, however, has received less attention. The δ-hydroxycarbonyl, which is generated from the isomerization of alkoxy radicals, can undergo heterogeneous cyclization and dehydration to form substituted dihydrofuran. Due to the presence of C=C bonds, the substituted dihydrofuran is predicted to be highly reactive with OH, and even more so with O3 and NO3, thereby opening a reaction pathway that is not usually accessible to alkanes. This work focuses on the role of substituted dihydrofuran formation and its subsequent reaction with OH, and more importantly ozone, in SOA formation from the photooxidation of long-chain alkanes. Experiments were carried out in the Caltech Environmental Chamber using dodecane as a representative alkane to investigate the difference in aerosol composition generated from "OH-oxidation-dominating" vs. "ozonolysis-dominating" environments. A detailed mechanism incorporating the specific gas-phase photochemistry, together with the heterogeneous formation of substituted dihydrofuran and its subsequent gas-phase OH/O3 oxidation, is used to evaluate the importance of this reaction channel in dodecane SOA formation. We conclude that (1) the formation of δ-hydroxycarbonyl and its subsequent heterogeneous conversion to substituted dihydrofuran is significant in the presence of NOx; (2) the ozonolysis of substituted dihydrofuran dominates over the OH-initiated oxidation under conditions prevalent in urban and rural air; and (3) a spectrum of highly oxygenated products with carboxylic acid, ester, and ether functional groups are produced from the substituted dihydrofuran chemistry, thereby affecting the average oxidation state of the SOA.

Selective oxygenation of olefins with hydrogen peroxide catalyzed by iron(II) bipyridine complex included in NaY zeolite under visible light irradiation

The photooxidation of olefins by H 2 O 2 catalyzed by iron(II) bipyridine complex included in NaY zeolite under visible irradiation is realized with high turnover number and high selectivity. • A novel visible light photocatalyst of iron(II) bipyridine complex encapsulated within NaY zeolite was explored. • The photocatalyst for oxygenation of styrene has high turnovers and high selectivity. • The photocatalyst was also effective for oxidation of other olefins. • Photooxygenation of styrene occurs within the supercages of zeolite Y. A novel visible light photocatalyst of iron(II) bipyridine complex encapsulated within NaY zeolite for selective oxygenation of styrene with high turnover (1800) and high benzaldehyde selectivity (86%) using hydrogen peroxide as the oxidant under ambient conditions was explored. This photocatalyst was also effective for the oxidation of other olefins, such as 4-cyanostyrene, 4-methoxystyrene, 3-methylstyrene, 1,1-diphenylethylene, cis -stilbene, trans -stilbene, trans -β-methylstyrene, α-methylstyrene, triphenylethylene, 1-octene, 2-octene, cyclohexene and cyclooctene, into the corresponding aldehydes, ketones or olefin oxides. The study shows that the photooxygenation of styrene occurs within the supercages of zeolite Y, the products benzaldehyde and formaldehyde escape into the solution after reaction, leading to high turnovers. Based on the experimental results, the photooxidation mechanism is also discussed.

Photoinduced charge separation enhanced by the confinement of electron donor and acceptor at different surfaces of porous TiO2 nanotubes and its application in olefin oxidation

A novel photocatalyst, TiO2 nanotubes with the confinement of the electron donor and acceptor at the inner and outer surface, respectively, was fabricated by a universal method. The as-designed nanostructure afforded a void space separation state of electron donors and acceptors, which remarkably enhanced the photoinduced charge separation, resulting in the enhancement of the catalytic activity for the photooxidation of olefins with O2.

ChemInform Abstract: Metal-Catalyzed Organic Photoreactions. Chemo- and Regioselectivities in the CuCl2-Induced Photooxidation of Olefins.

Abstract Copper(II) chloride induced the chemo and regioselective chlorohydroperoxidation, as well as the site-selective chlorination of olefins, under photooxygenation conditions.

Photooxygenation of alkenes by molecular oxygen in the presence of porphyrins and chlorin sensitizers under visible light irradiation

Photooxidation of alkenes by molecular oxygen and visible light in the presence of tetraphenylporphyrin (H2TPP) , tetramesitylporphyrin (H2TMP) , tetrakis pentafluorophenylporphyrin (H2TPFPP) and tetrakis(2,3-dimetoxyphenyl)porphyrin T(2,3-OMeP)P and metalloporphyrins such as ClFeTPP , ClMnTMP , ClMnTPP , ClMnTPFPP , ClCoTPP and ZnTPP has been performed. Photooxidation of alkenes with tetraphenylchlorin (H2TPC) as the sensitizer with visible light has also been studied. The conversion rates for alkene oxidation were in the order of free-base porphyrins > chlorin > metalloporphyrins. In the presence of NaN3/Na2SO3 the yield of oxidation products and conversion percentage is considerably reduced, confirming involvement of 1O2 in the mechanism.

ChemInform Abstract: Remarkable Product Selectivity in Photochemical Reactions Within Supramolecular Systems

Supramolecular photochemistry is concerned with systems where non-covalent interactions become significant or dominate in determining the chemistry of guest/host systems. The supramolecular photochemistry of phenyl phenylacetates, photocycloaddition of diaryl compounds and the photooxidation of alkenes included in zeolites and Nafion membranes is the subject of this report.

γ-Alumina-supported [60]fullerene catalysts: Synthesis, properties and applications in the photooxidation of alkenes

Immobilization of [60]fullerene onto γ-Al 2O 3 surface provides new heterogeneous photocatalysts for the oxidation of organic compounds under oxygen atmosphere. These catalysts have been prepared by simple or successive incipient wetness impregnation (using an organic solvent) followed by air-heating at 180 °C. In the C 60/Al 2O 3 system, C 60 loading was varied in the range of 1–4% (w/w). Several experimental techniques including BET, XRD, DRS, TGA, microelectrophoresis, photoluminescence and kinetic extraction, have been used to characterize these catalytic materials. It was found that the quite high surface exposed by the supported C 60 increases with the amount of the supported C 60, while the dispersion of the supported C 60 decreases. The quite stable supported [60]fullerene phase is comprised from C 60 clusters, small and large aggregates. This non-uniform size distribution is reflected to a non-uniform distribution concerning the ‘supported phase–support’ interactions. These interactions decrease with the amount of the supported C 60. The photocatalysts prepared may be safely used up to 200 °C. Above this temperature the supported C 60 is sublimated/combusted in air. The photocatalytic activity of the so-obtained catalytic systems has been evaluated in terms of substrate conversion in the singlet oxygen ‘ene’ reaction of alkenes. The photooxygenation of 2-methyl-2-heptene has been examined as a probe reaction. It was found that the catalytic activity increases with the increasing amount of the supported C 60 up to the value of 3% (w/w) and then decreases. The intrinsic activity expressed as TON or TOF decreased monotonically with C 60. In all cases, however, the photocatalytic activity of the Al 2O 3-supported C 60 catalysts was substantially increased compared to the unsupported C 60 precursor, exhibiting quantitative conversion yields after short reaction times. The catalytic behavior was attributed to the aforementioned opposite trends which follow the surface exposed by the supported C 60 on one hand and the ‘supported C 60–support’ interactions and the C 60 dispersion on the other hand. The easy separation of these solid catalysts from the reaction mixture, the high activity and stability as well as the retained activity in subsequent catalytic cycles, make these supported catalysts suitable for a small-scale synthesis.

Photooxidation of Alkanes Using W10O32 4- on Organomodified SBA-15

Photooxidation of Alkanes Using W₁₀O₃₂ ^4- on Organomodified SBA-15

In Situ Spectroscopic, Electrochemical, and Theoretical Studies of the Photoinduced Host−Guest Electron Transfer that Precedes Unusual Host-Mediated Alkane Photooxidation

In the unusual photooxidation of alkanes within the cavity of a self-assembled M(6)L(4) cage complex (1), the proposed reaction mechanism involves the generation of the host anion radical and guest cation radical via guest-to-host photoinduced electron transfer (PET). In the present study, the postulated host anion radical was elucidated by in situ IR spectroscopy. The difference IR spectrum before and after the irradiation of the clathrate complex 1a [symbol: see text] (2)(4) (2 = adamantane) features a dominant negative peak at approximately 1500 cm(-1), which is attributed to the decrease in the CN stretching vibration and thus indicative of the structural change in the triazine core of the cage. Electrochemical measurements and theoretical calculations show the remarkably great electron-accepting ability of the triazine core. These results strongly support the proposal of guest-to-host PET.

Photooxidation of Olefins under Oxygen in Platinum(II) Complex-Loaded Mesoporous Molecular Sieves

Cyclometalated platinum(II) complex has been successfully incorporated into the (3-aminopropyl) triethoxysilane-modified channels of ordered mesoporous silica SBA-15 that has large pore hexagonal channels. Studies on the (1)O(2) generation conclusively demonstrates that the olefins in the nano-channels of SBA-15 can be enriched 8 times higher than those in the homogeneous solution as the diffusion quantum yield of singlet oxygen ((1)O(2)) is assumed to be unit. The platinum(II) complex loaded in the channel of SBA-15 is stable, and the photosensitized oxidation occurs efficiently. No obvious degradation and leaching of photosensitizers is observed even after 10 runs. Only a simple filtration is needed for the recycled use of the expensive noble metal catalysts. This versatile system is a good example of photochemical reactions occurring in the mesoporous silica molecular sieve. SBA-15 not only provides a support for the photosensitizer, but also acts as a nano-reactor to facilitate the photooxidation.

Confined Space-Controlled Olefin−Oxygen Charge Transfer in Zeolites

Density functional theory (DFT) calculations on the initial charge-transfer step in the photo-oxidation of alkenes in cationic zeolites are presented. The model system used represents a part of the Y-zeolite supercage containing two II sites with alkali-earth cations and coadsorbed 2,3-dimethylbutene-2 (DMB) and O(2) on them. It is found that the electrostatic field of the zeolite cavity plays only a minor role for the stabilization of a charge-transfer state, whereas the relative orientation and the distance between the DMB and O(2) molecules are the most important factors. On the basis of these results, the photo-oxidation considered is due to a confinement effect in which the reagents are oriented in a suitable "pre-transition state" configuration.

Influence of aromatic brominated flame retardants on alkane photo-oxidation: A model and polymer study

Photochemical decomposition of decabromodiphenyl ether ( I) induced by 300–400 nm light is a radical process leading to the formation of hydrobromic acid (HBr) provided there is a hydrogen donor (RH) in the system. The stepwise debromination of the aromatic ring takes place giving rise to lower brominated diphenyl ethers with a number of by-products. The hydrogen donor is converted to a corresponding R–Br derivative in this process. Free HBr is prone to photolytic decomposition, particularly at more advanced stages of conversion of ( I). As shown in the polypropylene case, the radicals thus formed can actively participate in the initiation step of polyolefin chain oxidation. In the presence of HALS the HBr formation through photolysis of ( I) is enhanced while the formation of R–Br is totally suppressed. The rate of disappearance of ( I) is, however, not influenced by the HALS. The enhancement of HBr formation by HALS is thus attributed to a deeper stepwise debromination of ( I) allowing release of more than one bromine radical per molecule of ( I). The detrimental effect of brominated diphenyl ethers on the polymer UV resistance thus stems from three main reasons: HBr formation which ruins the stabiliser performance through the formation of inactive aminium salt with HALS; photolysis of HBr as well as of ( I) that enhance the photoinitiation of alkane polymer photo-degradation.

60]Fullerene Supported on Silica and γ-Alumina Sensitized Photooxidation of Olefins: Chemical Evidence for Singlet Oxygen and Electron Transfer Mechanism

Fullerene C 6 0 supported on silica and γ-alumina (2% w/w C 6 0 /SiO 2 and C 6 0 /Al 2 O 3 ) sensitizes the photooxidation of alkenes via singlet oxygen and/or electron transfer mechanism, depending on the solvent and the substrate.

Remarkable product selectivity in photochemical reactions within supramolecular systems

Supramolecular photochemistry is concerned with systems where non-covalent interactions become significant or dominate in determining the chemistry of guest/host systems. The supramolecular photochemistry of phenyl phenylacetates, photocycloaddition of diaryl compounds and the photooxidation of alkenes included in zeolites and Nafion membranes is the subject of this report.

On the nature of the charge-transfer transient in photocatalysis by the decatungstate anion

The decatungstate anion [W 10O 32] 4− is an efficient catalyst in the photooxidation of alkanes. Laser flash photolysis studies suggested that after photoexcitation the decatungstate anion relaxes to an intermediate with an electronic distribution similar to that of the singly reduced cluster. Using vibronic theory of mixed-valence compounds, we study possible electronic distributions of decatungstate with one excess electron in the LUMO. Under some conditions this electron can be delocalized over a part of equatorial tungsten centres. In some cases this delocalized electronic distribution can coexist with the electron localization at one tungsten site. The intermediate form in the photocatalytic reaction could be assigned to the partly delocalized state which can be attained from an initially localized photoexcited state.

Photoinduced oxidation of hydrocarbons with cobalt(III)-alkylperoxy complexes

A set of cobalt(III)-alkylperoxy complexes, [Co(BPI)(OAc)(OO 1Bu)] ( 1), [Co(Me-BPI)(OAc)(OO 1Bu)] ( 2) and [Co(Cl-BPI)-(OAc)(OO 1Bu)] ( 3) where BPI is 1,3-bis(2′-pyridylimino) isoindoline, promotes photooxidation of alkanes at room temperature. Irradiation causes homoylysis of the CoO bond of the CoOO 1Bu moieties in these complexes and produces 1BuOO radicals in the reaction mixture. Both 1BuOO and 1BuO (derived from 1BuOO) are responsible for the oxidation of the CH bonds.

Laser flash photolysis study of the mechanism of photooxidation ofalkanes catalysed by decatungstate anion

Generated during light excitation, the O(2p) → W(5d) ligand-to-metal charge transfer (LMCT) excited state of decatungstate anion (W10O324-) relaxes for t < 30 ps to a transient X which persists for 5 ns. In the nanosecond time domain, it reacts with all components of the photocatalytic system (substrate, organic counter-ion and solvent) to give a one-electron reduced decatungstate species HW10O324- and an organic radical R˙. The latter is quenched by dioxygen at a near diffusion-controlled rate, forming a peroxyl radical ROO˙. In the solvent cage, the peroxyl radical ROO˙ reoxidizes HW10O324- to give the hydroperoxide ROOH and W10O324-, thus closing the catalytic cycle. The catalyst reoxidation is the rate-limiting step of the alkane oxidation.

Photo-assisted oxidation of 2-methylpropane over Rb+-modified V2O5/SiO2. Formation of 2-methylpropane-2-ol from 2-methylpropane and O2

The reactivity of the surface species interacting with Rb ions in the photo-oxidation of 2-methylpropane at room temperature has been investigated to clarify the mechanism of photo-oxidation of alkanes. (Propane is oxidized into oxygenates over photo-irradiated V2O5/SiO2. The addition of alkali metal ions to V2O5/SiO2 greatly enhances the catalytic activity as well as the selectivity to propanone. This is due to the formation of new kinds of surface-active species by interaction between vanadium ions and alkali metal ions. The interaction affects the exciting wavelength of vanadate effective for the oxidation, λ > 400 nm.) The activity of the species is considerably high. Propanone and C4-compounds were formed. The most striking point is the formation of 2-methylpropan-2-ol over the Rb+-modified catalyst. The reaction path of the formation of 2-methylpropan-2-ol has been studied by analysing the products of the reactions with various sole reactants. The results suggest that the formation of 2-methylpropan-2-ol is not formed via 2-methylpropene. A tracer study with 18O2 showed that oxygen atoms incorporated into 2-methylpropan-2-ol were from gaseous oxygen, whereas propanone contained 16O which originated from the lattice oxygen in the catalyst. It is likely that 2-methylpropan-2-ol is formed by mono-oxygenation as found in the enzyme system from 2-methylpropane and gaseous oxygen.