- Single Chromium Atoms Supported on Titanium Dioxide Nanoparticles for Synergic Catalytic Methane Conversion under Mild Conditions
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Direct conversion of methane to value-added chemicals with high selectivity under mild conditions remains a great challenge in catalysis. Now, single chromium atoms supported on titanium dioxide nanoparticles are reported as an efficient heterogeneous catalyst for direct methane oxidation to C1 oxygenated products with H2O2 as oxidant under mild conditions. The highest yield for C1 oxygenated products can be reached as 57.9 mol molCr?1 with selectivity of around 93 % at 50 °C for 20 h, which is significantly higher than those of most reported catalysts. The superior catalytic performance can be attributed to the synergistic effect between single Cr atoms and TiO2 support. Combining catalytic kinetics, electron paramagnetic resonance, and control experiment results, the methane conversion mechanism was proposed as a methyl radical pathway to form CH3OH and CH3OOH first, and then the generated CH3OH is further oxidized to HOCH2OOH and HCOOH.
- Shen, Qikai,Cao, Changyan,Huang, Runkun,Zhu, Lei,Zhou, Xin,Zhang, Qinghua,Gu, Lin,Song, Weiguo
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- Aqueous Solubility and Reaction Kinetics of Hydroxymethyl Hydroperoxide
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The aqueous-phase equilibria and kinetics of the formation of hydroxymethyl hydroperoxide (HMP) and of bis(hydroxymethyl) peroxide (BHMP) from formaldehyde and hydrogen peroxide, i.e., H2CO + H2O2 HOCH2O2H (1, -1), and HOCH2O2H + H2CO HOCH2O2CH2OH (2) were studied using an amperometric technique which is highly sensitive and selective for H2O2.The equilibrium constants of reactions 1 and 2 between 5 and 35 deg C were determined to be K1 = 2.35*10-2 exp(2610/T) M-1 and K2 = 1.04*10-3 exp(2780/T) M-1, respectively, both independent of pH between 4.0 and 8.4.The rate coefficients of (1) and (-1) determined at pH 7.07 +/- 0.02 between 5 and 35 deg C are k1 = 6.0*1013 exp(-9450/T) M-1 s-1 and k-1 = 1.0*1015 exp(-11800/T) s-1.Both k1 and k-1 are base-catalyzed and are linearly dependent on pH between 4.0 and 8.2, namely, k-1 = (5.0 +/- 0.3)*10-10/ s-1 at 25.8 +/- 0.1 deg C.The Henry's law constants of HMP and BHMP, determined by measuring their corresponding gas and aqueous concentrations at phase equilibrium, are 5.0+1.6-0.9*105 and 6+3-2*105 M atm-1 at 22.0 +/- 0.1 deg C, and 6.2+1.2-0.9*105 and 20+18-7*105 M atm-1 at 10.0 +/- 0.1 deg C, respectively.The reaction kinetics of HMP with S(IV) was studied by a competition technique using the H2O2-S(IV) reaction as the reference; the reaction is acid-catalyzed, with an effective second-order rate constant of (2.2*107) +/- 15percent M-1 s-1 for the pH range 3-4 at 22.0 +/- 0.1 deg C.These results indicate that gas-phase HMP in the atmosphere is efficiently removed by wet scavenging processes and would be quantitavely detected by peroxide instruments involving gas-liquid scrubbers, provided that the scrubbed HMP is stabilized.The time constant of the dissociation of dissolved HMP to H2O2 is fairly short, being ca. 100 min at pH 5.5, shorter at higher pH.Consequently, HMP is expected to be stable and detected in atmospheric liquid water only at pH 5.5.
- Zhou, Xianliang,Lee, Yin-Nan
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- Identification of organic peroxides in the oxidation of C1-C3 alkanes
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Formation of organic peroxides in oxidation of C1-C3 alkanes initiated by C1 atoms in O2-N2 mixtures at 760 torr and 298 K was studied with FT-IR and HPLC method in laboratory. Methyl hydroperoxide (MHP) and ethyl hydroperoxide (EHP) were the main peroxide products and formed early in the oxidation process. Hydroxymethyl hydroperoxide (HOCH2OOH, HMHP) was identified in the reaction systems. Other two types of peroxides detected were peroxyacetic acid (CH3C(O)OOH, PAA) and dimethyl peroxide (C-H3OOCH3, DMP). In addition, more than seven peroxide products were not identified. The finding of HMHP in the system indicated that Criegee biradical CH2OO probably was an intermediate in the process of oxidation of alkanes. The results implied that oxidation of alkanes may have a significant contribution to organic peroxides in the troposphere.
- Qi,Zhang,Chen,Shao,Tang,Hu
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- Efficient Synthesis of Monomeric Fe Species in Zeolite ZSM-5 for the Low-Temperature Oxidation of Methane
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Direct oxidation of methane into value-added C1 oxygenated products, such as methanol, is essential and remains a significant challenge in the field of catalysis. In this work, we have prepared Fe/ZSM-5 materials via three different methods and investigated the influence of various preparation methods on the composition and catalytic performance of Fe/ZSM-5 for the low-temperature oxidation of methane. Through a combination of scanning transmission electron microscopy, ultraviolet-visible diffuse reflectance and 57Fe M?ssbauer spectroscopy, we have found that the highest proportion of monomeric Fe species of 71 % could be achieved in the ZSM-5 zeolite by the solid-state ion-exchange method, affording an excellent C1 oxygenates yield of 120 mol/molFe with a C1 oxygenates selectivity of 96 % at 50 °C for 30 min in the aqueous solution of H2O2.
- Yu, Tao,Su, Yang,Wang, Aiqin,Weckhuysen, Bert M.,Luo, Wenhao
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- Formation mechanism of peroxides in reactions of cyclic olefins with ozone in air
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We carried out reactions of methyl-substituted cyclohexenes and α-pinene with ozone in air and elucidated the mechanisms of formation of the minor products (peroxides and formic acid). Peroxyacetic acid was formed only from the cyclohexenes with a methyl group on the double bond, whereas formic acid was produced in higher yields from the cyclohexenes without a methyl group on the double bond. These differences in product yields allowed us to elucidate the mechanism of formation of the products.
- Hatakeyama, Shiro,Sivanesan, Subramanian,Urabe, Taichiro
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- Fe-O Clusters Anchored on Nodes of Metal–Organic Frameworks for Direct Methane Oxidation
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Direct methane oxidation into value-added organic oxygenates with high productivity under mild condition remains a great challenge. We show Fe-O clusters on nodes of metal–organic frameworks (MOFs) with tunable electronic state for direct methane oxidation into C1 organic oxygenates at 50 °C. The Fe-O clusters are grafted onto inorganic Zr6 nodes of UiO-66, while the organic terephthalic acid (H2BDC) ligands of UiO-66 are partially substituted with monocarboxylic modulators of acetic acid (AA) or trifluoroacetic acid (TFA). Experiments and theoretical calculation disclose that the TFA group coordinated with Zr6 node of UiO-66 enhances the oxidation state of adjacent Fe-O cluster due to its electron-withdrawing ability, promotes the activation of C?H bond of methane, and increases its selective conversion, thus leading to the extraordinarily high C1 oxygenate yield of 4799 μmol gcat?1 h?1 with 97.9 % selectivity, circa 8 times higher than those modulated with AA.
- Zhao, Wenshi,Shi, Yanan,Jiang, Yuheng,Zhang, Xiaofei,Long, Chang,An, Pengfei,Zhu, Yanfei,Shao, Shengxian,Yan, Zhuang,Li, Guodong,Tang, Zhiyong
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supporting information
p. 5811 - 5815
(2021/02/06)
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- Room-Temperature Methane Conversion by Graphene-Confined Single Iron Atoms
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Direct conversion of methane to high-value-added chemicals is a major challenge in catalysis, which usually requires high-energy input to overcome the reaction barrier. We report that graphene-confined single Fe atoms can be used as an efficient non-precious catalyst to directly convert methane to C1 oxygenated products at room temperature. A series of graphene-confined 3d transition metals (Mn, Fe, Co, Ni, and Cu) were screened, yet only single Fe atoms could catalyze the methane conversion. Combining in operando time-of-flight mass spectrometry, 13C nuclear magnetic resonance, and density functional theory calculations, we found that methane conversion proceeds on the O–FeN4–O active site along a radical pathway to produce CH3OH and CH3OOH first, and then the generated CH3OH can be further catalyzed to form HOCH2OOH and HCOOH at room temperature. Methane from natural gas and shale gas is one of the most promising feedstocks because of its high reserves and low price. The selective activation and orientable conversion of methane are considered the “holy grail” in catalysis. Because of the highly stable C–H bond, methane conversion usually requires high temperatures to overcome the high reaction barrier. However, the high-temperature reaction is not favorable for industrial application. Despite many efforts to decrease the reaction temperature, it remains a great challenge to promote methane conversion under mild conditions, especially at room temperature. Herein, we report that graphene-confined single Fe atoms can be used as an efficient non-precious catalyst to directly convert methane to high-value-added C1 oxygenated products at room temperature (25°C), which provides a new route to understanding and designing highly efficient non-precious catalysts for methane conversion at room temperature. Graphene-confined single Fe atoms, screened out from a series of 3d transition metals (Mn, Fe, Co, Ni, and Cu), were used as an efficient non-precious catalyst to directly convert methane to C1 oxygenated products at room temperature. The unique O–FeN4–O structure formed in graphene can readily activate the C–H bond of methane along a radical pathway with a low reaction energy barrier.
- Cui, Xiaoju,Li, Haobo,Wang, Yan,Hu, Yuanli,Hua, Lei,Li, Haiyang,Han, Xiuwen,Liu, Qingfei,Yang, Fan,He, Limin,Chen, Xiaoqi,Li, Qingyun,Xiao, Jianping,Deng, Dehui,Bao, Xinhe
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supporting information
p. 1902 - 1910
(2018/06/20)
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- Oligomerization reaction of the Criegee intermediate leads to secondary organic aerosol formation in ethylene ozonolysis
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Ethylene ozonolysis was investigated in laboratory experiments using a Teflon bag reactor. A negative ion chemical ionization mass spectrometer (NI-CIMS) using SO2Cl- and Cl- as reagent ions was used for product analysis. In addition to the expected gas-phase products, such as formic acid and hydroperoxymethyl formate, oligomeric hydroperoxides composed of the Criegee intermediate (CH2OO) as a chain unit were observed. Furthermore, we observed secondary organic aerosol (SOA) formation from the ethylene ozonolysis, and the particle-phase products were also analyzed by NI-CIMS. The CH2OO oligomers were also observed as particle-phase components, suggesting that the oligomeric hydroperoxides formed in the gas phase partition into the particle phase. By adding methanol as a stabilized Criegee intermediate scavenger, both the gas-phase oligomer formation and SOA formation were strongly suppressed. This indicates that CH2OO plays a critical role in the formation of oligomeric hydroperoxides followed by SOA formation in ethylene ozonolysis. A new formation mechanism for the oligomeric hydroperoxides, which includes sequential addition of CH2OO to hydroperoxides, is proposed.
- Sakamoto, Yosuke,Inomata, Satoshi,Hirokawa, Jun
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p. 12912 - 12921
(2014/01/06)
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- On the Mechanism of Methanol Oxidation Sensitized by Anthraquinone
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Chemical Kinetics / Elementary Reactions / Oxidation / Photochemistry /Radicals The mechanism of photosensitized oxidation of methanol in acetonitrile was investigated in the presence of anthraquinone.Among the main stable products formaldehyde, formic acid, hydrogenperoxide and hydroxymethylhydroperoxide has been found.Their accumulation was measured in the range of 30-60 deg C.Specific experiments were carried out to prove the existence or negligibility of reactions proposed in the literature.The computer simulation based on literature data and on our results combined with estimation of certain rate coefficients as parameters resulted in good agreement with experimental product accumulation curves.The estimated value of monomolecular decomposition of hydroxymethylperoxyl radical (k325 deg C ca. 3 s-1) is in favour of the lowest values that can be found in the literature.In the overall termination process the Russell-type termination step generally accepted in reactions between peroxyl radicals has decreasing importance with increasing temperature.
- Kuti, Zs.,Gal, D.
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p. 1843 - 1847
(2007/10/02)
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- Rate Constants for Reduction of substituted Methylperoxyl Radicals by Ascorbate Ions and N,N,N',N'-tetramethyl-p-phenylenediamine
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Absolute rate constants (k) for reduction of substituted methylperoxyl radicals by ascorbate ions and by TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) in aqueous solutions have been determined by pulse radiolysis.The rate constants vary from 1E6 to 1E9 M-1 s-1, increasing as the electron-withdraving capacity of the substituent on the peroxyl group increases.Linear correlations are observed between log k and the Taft substituents ?* for a wide variety of substituents, but not all substituents fit the same line.In the case of ascorbate as reductant, the points for peroxyl radicals that contain halogens on the α-carbon lie on a different line (ρ*=0.41) than that for the other substituents (ρ*=1.25).In the case of TMPD there are alsotwo families of peroxyl radicals: Those comprimising the electron-donating groups Me through t-Bu (ρ=5.6) and those containing electron-withdrawing substituents (ρ*=0.64).
- Neta, P.,Huie, R. E.,Mosseri, S.,Shastri, L. V.,Mittal, J. P.,et al.
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p. 4099 - 4104
(2007/10/02)
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