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1.
The direct functionalization of methane into platform chemicals is arguably one of the holy grails in chemistry. The actual active sites for methane activation are intensively debated. By correlating a wide variety of characterization results with catalytic performance data we have been able to identify mononuclear Fe species as the active site in the Fe/ZSM-5 zeolites for the mild oxidation of methane with H2O2 at 50 °C. The 0.1% Fe/ZSM-5 catalyst with dominant mononuclear Fe species possess an excellent turnover rate (TOR) of 66 molMeOH molFe−1 h−1, approximately 4 times higher compared to the state-of-the-art dimer-containing Fe/ZSM-5 catalysts. Based on a series of advanced in situ spectroscopic studies and 1H- and 13C- nuclear magnetic resonance (NMR), we found that methane activation initially proceeds on the Fe site of mononuclear Fe species. With the aid of adjacent Brønsted acid sites (BAS), methane can be first oxidized to CH3OOH and CH3OH, and then subsequently converted into HOCH2OOH and consecutively into HCOOH. These findings will facilitate the search towards new metal-zeolite combinations for the activation of C–H bonds in various hydrocarbons, for light alkanes and beyond.

The monomeric Fe species in Fe/ZSM-5 have been identified as the intrinsic active sites for the low-temperature methane oxidation.  相似文献   

2.
HPLC along with FT-IR technique was used to study the formation of organic peroxides in the Cl2-ethane-air photoreaction system. Ethyl hydroperoxide (CH3CH21OOH, EHP) and peroxyacetic acid ( CH3C(O)OOH, PAA) were conformed to be the peroxide product in the reaction system. In addition, methyl hydroperoxide (CH3OOH, hydroxymethyl hydroperoxide (HOCH2OOH, HMHP) and two unidentified organic peroxides were detected for the first time. EHP and were the dominant peroxide products. The identification of HMHP showed that Criegee biradical CH2OO may be formed as an intermediate in the oxidation of ethane. Simulation results showed that photooxidation of ethane may make substantial contribution to source of organic peroxides in the atmosphere.  相似文献   

3.
HPLC along with FT-IR technique was used to study the formation of organic peroxides in the Cl2-ethane-air photoreaction system. Ethyl hydroperoxide (CH3CH21OOH, EHP) and peroxyacetic acid ( CH3C(O)OOH, PAA) were conformed to be the peroxide product in the reaction system. In addition, methyl hydroperoxide (CH3OOH, hydroxymethyl hydroperoxide (HOCH2OOH, HMHP) and two unidentified organic peroxides were detected for the first time. EHP and were the dominant peroxide products. The identification of HMHP showed that Criegee biradical CH2OO may be formed as an intermediate in the oxidation of ethane. Simulation results showed that photooxidation of ethane may make substantial contribution to source of organic peroxides in the atmosphere.  相似文献   

4.
The development of photocatalysts is an essential task for clean energy generation and establishing a sustainable society. This paper describes the aggregation-induced photocatalytic activity (AI-PCA) of amphiphilic rhodamines and photocatalytic functions of the supramolecular assemblies. The supramolecular assemblies consisting of amphiphilic rhodamines with octadecyl alkyl chains exhibited significant photocatalytic activity under visible light irradiation in water, while the corresponding monomeric rhodamines did not exhibit photocatalytic activity. The studies on the photocatalytic mechanism by spectroscopic and microscopic analyses clearly demonstrated the AI-PCA of the rhodamines. Moreover, the supramolecular assemblies of the rhodamines exhibited excellent photocatalytic hydrogen evolution rates (up to 5.9 mmol g−1 h−1).

Simple amphiphilic rhodamines formed supramolecular assemblies and exhibited aggregation-induced photocatalytic activity and hydrogen evolution in water.  相似文献   

5.
Bimolecular nucleophilic substitution (SN2) reactions at carbon center are well known to proceed with the stereospecific Walden-inversion mechanism. Reaction dynamics simulations on a newly developed high-level ab initio analytical potential energy surface for the F + NH2Cl nitrogen-centered SN2 and proton-transfer reactions reveal a hydrogen-bond-formation-induced multiple-inversion mechanism undermining the stereospecificity of the N-centered SN2 channel. Unlike the analogous F + CH3Cl SN2 reaction, F + NH2Cl → Cl + NH2F is indirect, producing a significant amount of NH2F with retention, as well as inverted NH2Cl during the timescale within the unperturbed NH2Cl molecule gets inverted with only low probability, showing the important role of facilitated inversions via an FH…NHCl-like transition state. Proton transfer leading to HF + NHCl is more direct and becomes the dominant product channel at higher collision energies.

Multiple-inversion, the analogue of the double-inversion pathway recently revealed for SN2@C, is the key mechanism in SN2 at N center undermining stereospecificity.  相似文献   

6.
Theoretical investigations on chemical reactions allow us to understand the dynamics of the possible pathways and identify new unexpected routes. Here, we develop a global analytical potential energy surface (PES) for the OH + CH3F reaction in order to perform high-level dynamics simulations. Besides bimolecular nucleophilic substitution (SN2) and proton abstraction, our quasi-classical trajectory computations reveal a novel oxide ion substitution leading to the HF + CH3O products. This exothermic reaction pathway occurs via the CH3OH⋯F deep potential well of the SN2 product channel as a result of a proton abstraction from the hydroxyl group by the fluoride ion. The present detailed dynamics study of the OH + CH3F reaction focusing on the surprising oxide ion substitution demonstrates how incomplete our knowledge is of fundamental chemical reactions.

Reaction dynamics simulations on a high-level ab initio analytical potential energy surface reveal a novel oxide ion substitution channel for the OH + CH3F reaction.  相似文献   

7.
Loading Ag and Co dual cocatalysts on Al-doped SrTiO3 (AgCo/Al-SrTiO3) led to a significantly improved CO-formation rate and extremely high selectivity toward CO evolution (99.8%) using H2O as an electron donor when irradiated with light at wavelengths above 300 nm. Furthermore, the CO-formation rate over AgCo/Al-SrTiO3 (52.7 μmol h−1) was a dozen times higher than that over Ag/Al-SrTiO3 (4.7 μmol h−1). The apparent quantum efficiency for CO evolution over AgCo/Al-SrTiO3 was about 0.03% when photoirradiated at a wavelength at 365 nm, with a CO-evolution selectivity of 98.6% (7.4 μmol h−1). The Ag and Co cocatalysts were found to function as reduction and oxidation sites for promoting the generation of CO and O2, respectively, on the Al-SrTiO3 surface.

Deposition Ag and Co dual cocatalysts onto Al-SrTiO3 significantly improves its activity for photoreduction of CO2 by H2O, with extremely high selectivity to CO evolution (99.8%), in which Ag and Co enable CO2 reduction and H2O oxidation, respectively.  相似文献   

8.
Oxygen vacancies are common to most metal oxides, whether intentionally incorporated or otherwise, and the study of these defects is of increasing interest for solar water splitting. In this work, we examine nanostructured WO3 photoanodes of varying oxygen content to determine how the concentration of bulk oxygen-vacancy states affects the photocatalytic performance for water oxidation. Using transient optical spectroscopy, we follow the charge carrier recombination kinetics in these samples, from picoseconds to seconds, and examine how differing oxygen vacancy concentrations impact upon these kinetics. We find that samples with an intermediate concentration of vacancies (∼2% of oxygen atoms) afford the greatest photoinduced charge carrier densities, and the slowest recombination kinetics across all timescales studied. This increased yield of photogenerated charges correlates with improved photocurrent densities under simulated sunlight, with both greater and lesser oxygen vacancy concentrations resulting in enhanced recombination losses and poorer JV performances. Our conclusion, that an optimal – neither too high nor too low – concentration of oxygen vacancies is required for optimum photoelectrochemical performance, is discussed in terms of the competing beneficial and detrimental impact these defects have on charge separation and transport, as well as the implications held for other highly doped materials for photoelectrochemical water oxidation.

A medium concentration of oxygen vacancies (VO ≈ 2%) is critical to the performance of WO3 photoanodes for solar water oxidation, enhancing charge separation and reducing recombination across all timescales examined.  相似文献   

9.
Polymeric carbon nitride (PCN) has been widely used as a metal-free photocatalyst for solar hydrogen generation from water. However, rapid charge carrier recombination and sluggish water catalysis kinetics have greatly limited its photocatalytic performance for overall water splitting. Herein, a single-atom Ni terminating agent was introduced to coordinate with the heptazine units of PCN to create new hybrid orbitals. Both theoretical calculation and experimental evidence revealed that the new hybrid orbitals synergistically broadened visible light absorption via a metal-to-ligand charge transfer (MLCT) process, and accelerated the separation and transfer of photoexcited electrons and holes. The obtained single-atom Ni terminated PCN (PCNNi), without an additional cocatalyst loading, realized efficient photocatalytic overall water splitting into easily-separated gas-product H2 and liquid-product H2O2 under visible light, with evolution rates reaching 26.6 and 24.0 μmol g−1 h−1, respectively. It was indicated that single-atom Ni and the neighboring C atom served as water oxidation and reduction active sites, respectively, for overall water splitting via a two-electron reaction pathway.

Single-atom Ni terminating agent is introduced to coordinate with sp2 or sp3 N atoms in the heptazine units of PCN, realizing visible-light photocatalytic overall water splitting to H2O2 and H2 without additional cocatalyst.  相似文献   

10.
The degradation of neurotransmitters is a hallmark feature of Alzheimer''s disease (AD). Copper bound Aβ peptides, invoked to be involved in the pathology of AD, are found to catalyze the oxidation of serotonin (5-HT) by H2O2. A combination of EPR and resonance Raman spectroscopy reveals the formation of a Cu(ii)–OOH species and a dimeric, EPR silent, Cu2O2 bis-μ-oxo species under the reaction conditions. The Cu(ii)–OOH species, which can be selectively formed in the presence of excess H2O2, is the reactive intermediate responsible for 5-HT oxidation. H2O2 produced by the reaction of O2 with reduced Cu(i)–Aβ species can also oxidize 5-HT. Both these pathways are physiologically relevant and may be involved in the observed decay of neurotransmitters as observed in AD patients.

The mononuclear copper hydroperoxo species (Cu(ii)–OOH) of Cu–Aβ is the active oxidant responsible for serotonin oxidation by Cu–Aβ in the presence of physiologically relevant oxidants like O2 and H2O2, which can potentially cause oxidative degradation of neurotransmitters, a marker of Alzheimer''s disease.  相似文献   

11.
Until now the reactions of organic peroxy radicals (RO2) with alkenes in the gas phase have been essentially studied at high temperature (T ≥ 360 K) and in the context of combustion processes, while considered negligible in the Earth''s atmosphere. In this work, the reactions of methyl-, 1-pentyl- and acetylperoxy radicals (CH3O2, C5H11O2, and CH3C(O)O2, respectively) with 2-methyl-2-butene, 2,3-dimethyl-2-butene and for the first time the atmospherically relevant isoprene, α-pinene, and limonene were studied at room temperature (298 ± 5 K). Monitoring directly the radicals with chemical ionization mass spectrometry led to rate coefficients larger than expected from previous combustion studies but following similar trends in terms of alkenes, with (in molecule−1 cm3 s−1) = 10−18 to 10−17 × 2/2 and = 10−14 to 10−13 × 5/5. While these reactions would be negligible for CH3O2 and aliphatic RO2 at room temperature, this might not be the case for acyl-, and perhaps hydroxy-, allyl- and other substituted RO2. Combining our results with the Structure–Activity Relationship (SAR) predicts kII(298 K) ∼10−14 molecule−1 cm3 s−1 for hydroxy- and allyl-RO2 from isoprene oxidation, potentially accounting for up to 14% of their sinks in biogenic-rich regions of the atmosphere and much more in laboratory studies.

The reactions of organic peroxy radicals with alkenes, overlooked until now, could be more significant than expected for some RO2 in the atmosphere.  相似文献   

12.
A terminal FeIIIOH complex, [FeIII(L)(OH)]2− (1), has been synthesized and structurally characterized (H4L = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene). The oxidation reaction of 1 with one equiv. of tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBAH) or ceric ammonium nitrate (CAN) in acetonitrile at −45 °C results in the formation of a FeIIIOH ligand radical complex, [FeIII(L˙)(OH)] (2), which is hereby characterized by UV-visible, 1H nuclear magnetic resonance, electron paramagnetic resonance, and X-ray absorption spectroscopy techniques. The reaction of 2 with a triphenylcarbon radical further gives triphenylmethanol and mimics the so-called oxygen rebound step of Cpd II of cytochrome P450. Furthermore, the reaction of 2 was explored with different 4-substituted-2,6-di-tert-butylphenols. Based on kinetic analysis, a hydrogen atom transfer (HAT) mechanism has been established. A pKa value of 19.3 and a BDFE value of 78.2 kcal/mol have been estimated for complex 2.

One-electron oxidation of an FeIII–OH complex (1) results in the formation of a FeIII–OH ligand radical complex (2). Its reaction with (C6H5)3C˙ results in the formation of (C6H5)3COH, which is a functional mimic of compound II of cytochrome P450.  相似文献   

13.
We report that Au atoms within van der Waals complexes serve as catalysts for the first time. This was observed in ionization-induced chemistry of 1,6-hexanediol–Au and 1,8-octanediol–Au complexes formed in superfluid helium nanodroplets, where the addition of Au atom(s) made C2H4+ the sole prominent product in dissociative reactions. Density functional theory (DFT) calculations showed that the Au atom significantly strengthens all of the C–C bonds and weakens the C–O bonds in the meantime, making the C–C bonds stronger than the two C–O bonds in the ionized complexes. This leads to a preferential cleavage of the C–O bonds and thus a strong catalytic effect of the Au atoms in the reactions.

Single Au atoms within van der Waals complexes are found to serve as catalysts in ionisation-induced chemistry for the first time.  相似文献   

14.
Biradicaloid compounds with an open-shell ground state have been the subject of intense research in the past decade. Although diindenoacenes are one of the most developed families, only a few examples have been reported as active layers in organic field-effect transistors (OFETs) with a charge mobility of around 10−3 cm2 V−1 s−1 due to a steric disadvantage of the mesityl group to kinetically stabilize compounds. Herein, we disclose our efforts to improve the charge transport of the diindenoacene family based on hexahydro-diindenopyrene (HDIP) derivatives with different annelation modes for which the most reactive position has been functionalized with (triisopropylsilyl)ethynyl (TIPS) groups. All the HDIP derivatives show remarkably higher stability than that of TIPS-pentacene, enduring for 2 days to more than 30 days, which depends on the oxidation potential, the contribution of the singlet biradical form in the ground state and the annelation mode. The annelation mode affects not only the band gap and the biradical character (y0) but also the value of the singlet–triplet energy gap (ΔES–T) that does not follow the reverse trend of y0. A method based on comparison between experimental and theoretical bond lengths has been disclosed to estimate y0 and shows that y0 computed at the projected unrestricted Hartree–Fock (PUHF) level is the most relevant among those reported by all other methods. Thanks to their high stability, thin-film OFETs were successfully fabricated. Well balanced ambipolar transport was obtained in the order of 10−3 cm2 V−1 s−1 in the bottom-gate/top-contact configuration, and unipolar transport in the top-gate/bottom-contact configuration was obtained in the order of 10−1 cm2 V−1 s−1 which is the highest value obtained for biradical compounds with a diindenoacene skeleton.

Biradicaloid HDIP derivatives show that the ΔES–T gap does not follow the reverse trend of the biradical character but depends more on the delocalization of the radical centres at the outer rings.  相似文献   

15.
Drug-induced liver injury (DILI) is an important cause of potentially fatal liver disease. Herein, we report the development of a molecular probe (LW-OTf) for the detection and imaging of two biomarkers involved in DILI. Initially, primary reactive oxygen species (ROS) superoxide (O2˙) selectively activates a near-infrared fluorescence (NIRF) output by generating fluorophore LW-OH. The C Created by potrace 1.16, written by Peter Selinger 2001-2019 C linker of this hemicyanine fluorophore is subsequently oxidized by reactive nitrogen species (RNS) peroxynitrite (ONOO), resulting in cleavage to release xanthene derivative LW-XTD, detected using two-photon excitation fluorescence (TPEF). An alternative fluorescence pathway can occur through cleavage of LW-OTf by ONOO to non-fluorescent LW-XTD-OTf, which can react further with the second analyte O2˙ to produce the same LW-XTD fluorescent species. By combining NIRF and TPEF, LW-OTf is capable of differential and simultaneous detection of ROS and RNS in DILI using two optically orthogonal channels. Probe LW-OTf could be used to detect O2˙ or O2˙ and ONOO in lysosomes stimulated by 2-methoxyestradiol (2-ME) or 2-ME and SIN-1 respectively. In addition, we were able to monitor the chemoprotective effects of tert-butylhydroxyanisole (BHA) against acetaminophen (APAP) toxicity in living HL-7702 cells. More importantly, TPEF and NIRF imaging confirmed an increase in levels of both O2˙ and ONOO in mouse livers during APAP-induced DILI (confirmed by hematoxylin and eosin (H&E) staining).

Drug-induced liver injury (DILI) is an important cause of potentially fatal liver disease.  相似文献   

16.
Doubly base-stabilised cyano- and isothiocyanatoborylenes of the form LL′BY (L = CAAC = cyclic alkyl(amino)carbene; L′ = NHC = N-heterocyclic carbene; Y = CN, NCS) coordinate to group 6 carbonyl complexes via the terminal donor of the pseudohalide substituent and undergo facile and fully reversible one-electron oxidation to the corresponding boryl radical cations [LL′BY]˙+. Furthermore, calculations show that the borylenes have very similar proton affinities, both to each other and to NHC superbases. However, while the protonation of LL′B(CN) with PhSH yielding [LL′BH(CN)+][PhS] is fully reversible, that of LL′B(NCS) is rendered irreversible by a subsequent B-to-CCAAC hydrogen shift and nucleophilic attack of PhS at boron.

Borylenes of the form (CAAC)(NHC)BY (Y = CN, NCS; CAAC = cyclic alkyl(amino)carbene; NHC = N-heterocyclic carbene) coordinate to group 6 carbonyl complexes via Y, and show reversible boron-centered Brønsted basicity and one-electron oxidation.  相似文献   

17.
The replacement of HgCl2/C with Au/C as a catalyst for acetylene hydrochlorination represents a significant reduction in the environmental impact of this industrial process. Under reaction conditions atomically dispersed cationic Au species are the catalytic active site, representing a large-scale application of heterogeneous single-site catalysts. While the metal nuclearity and oxidation state under operating conditions has been investigated in catalysts prepared from aqua regia and thiosulphate, limited studies have focused on the ligand environment surrounding the metal centre. We now report K-edge soft X-ray absorption spectroscopy of the Cl and S ligand species used to stabilise these isolated cationic Au centres in the harsh reaction conditions. We demonstrate the presence of three distinct Cl species in the materials; inorganic Cl, Au–Cl, and C–Cl and how these species evolve during reaction. Direct evidence of Au–S interactions is confirmed in catalysts prepared using thiosulfate precursors which show high stability towards reduction to inactive metal nanoparticles. This stability was clear during gas switching experiments, where exposure to C2H2 alone did not dramatically alter the Au electronic structure and consequently did not deactivate the thiosulfate catalyst.

In situ chlorine and sulphur XAS shows a dynamic ligand environment around cationic Au single-sites during acetylene hydrochlorination.  相似文献   

18.
Dye-sensitised photoanodes modified with a water oxidation catalyst allow for solar-driven O2 evolution in photoelectrochemical cells. However, organic chromophores are generally considered unsuitable to drive the thermodynamically demanding water oxidation reaction, mainly due to their lack of stability upon photoexcitation. Here, the synthesis of a dyad photocatalyst (DPP-Ru) consisting of a diketopyrrolopyrrole chromophore (DPPdye) and ruthenium-based water oxidation catalyst (RuWOC) is described. The DPP-Ru dyad features a cyanoacrylic acid anchoring group for immobilisation on metal oxides, strong absorption in the visible region of the electromagnetic spectrum, and photoinduced hole transfer from the dye to the catalyst unit. Immobilisation of the dyad on a mesoporous TiO2 scaffold was optimised, including the use of a TiCl4 pretreatment method as well as employing chenodeoxycholic acid as a co-adsorbent, and the assembled dyad-sensitised photoanode achieved O2 evolution using visible light (100 mW cm−2, AM 1.5G, λ > 420 nm). An initial photocurrent of 140 μA cm−2 was generated in aqueous electrolyte solution (pH 5.6) under an applied potential of +0.2 V vs. NHE. The production of O2 has been confirmed by controlled potential electrolysis with a faradaic efficiency of 44%. This study demonstrates that metal-free dyes are suitable light absorbers in dyadic systems for the assembly of water oxidising photoanodes.

Construction of a water oxidising photoanode with an organic chromophore-catalyst dyad.  相似文献   

19.
A catalytic enantioselective approach to the Myrioneuron alkaloids (−)-myrifabral A and (−)-myrifabral B is described. The synthesis was enabled by a palladium-catalyzed enantioselective allylic alkylation, that generates the C(10) all-carbon quaternary center. A key N-acyl iminium ion cyclization forged the cyclohexane fused tricyclic core, while vinyl boronate cross metathesis and oxidation afforded the lactol ring of (−)-myrifabral A. Adaptation of previously reported conditions allowed for the conversion of (−)-myrifabral A to (−)-myrifabral B.

A catalytic enantioselective approach to the Myrioneuron alkaloids (−)-myrifabral A and (−)-myrifabral B is described.  相似文献   

20.
Di(2-pyridyl)ketone dimethylplatinum(ii), (dpk)PtII(CH3)2, reacts with CD3OD at 25 °C to undergo complete deuteration of Pt–CH3 fragments in ∼5 h without loss of methane to form (dpk)PtII(CD3)2 in virtually quantitative yield. The deuteration can be reversed by dissolution in CH3OH or CD3OH. Kinetic analysis and isotope effects, together with support from density functional theory calculations indicate a metal–ligand cooperative mechanism wherein DPK enables Pt–CH3 deuteration by allowing non-rate-limiting protonation of PtII by CD3OD. In contrast, other model di(2-pyridyl) ligands enable rate-limiting protonation of PtII, resulting in non-rate-limiting C–H(D) reductive coupling. Owing to its electron-poor nature, following complete deuteration, DPK can be dissociated from the PtII-centre, furnishing [(CD3)2PtII(μ-SMe2)]2 as the perdeutero analogue of [(CH3)2PtII(μ-SMe2)]2, a commonly used PtII-precursor.

Di(2-pyridyl)ketone dimethylplatinum(ii), (dpk)PtII(CH3)2, reacts with CD3OD at 25 °C to undergo complete deuteration of Pt–CH3 fragments in ∼5 h without loss of methane to form (dpk)PtII(CD3)2 in virtually quantitative yield.  相似文献   

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