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1.
Hao Zhang Aleksei Bolshakov Raghavendra Meena Gustavo A. Garcia A. Iulian Dugulan Alexander Parastaev Guanna Li Emiel J. M. Hensen Nikolay Kosinov 《Angewandte Chemie (International ed. in English)》2023,62(32):e202306196
Non-oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas-phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X-ray absorption spectroscopy demonstrates that the isolated Fe3+ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe2+ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3-butadiene, 2-butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small-pore [Fe]CHA, where ethylene and ethane are the only gas-phase products. 相似文献
2.
Jörg Wolfram Anselm Fischer Andreas Brenig Dr. Daniel Klose Prof. Dr. Jeroen Anton van Bokhoven Dr. Vitaly L. Sushkevich Prof. Dr. Gunnar Jeschke 《Angewandte Chemie (International ed. in English)》2023,62(34):e202303574
Cu-exchanged mordenite (MOR) is a promising material for partial CH4 oxidation. The structural diversity of Cu species within MOR makes it difficult to identify the active Cu sites and to determine their redox and kinetic properties. In this study, the Cu speciation in Cu-MOR materials with different Cu loadings has been determined using operando electron paramagnetic resonance (EPR) and operando ultraviolet-visible (UV/Vis) spectroscopy as well as in situ photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy. A novel pathway for CH4 oxidation involving paired [CuOH]+ and bare Cu2+ species has been identified. The reduction of bare Cu2+ ions facilitated by adjacent [CuOH]+ demonstrates that the frequently reported assumption of redox-inert Cu2+ centers does not generally apply. The measured site-specific reaction kinetics show that dimeric Cu species exhibit a faster reaction rate and a higher apparent activation energy than monomeric Cu2+ active sites highlighting their difference in the CH4 oxidation potential. 相似文献
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The triflate and hexafluorophosphate salts of [Cu2(mu-dtbpm)2]2+ (1(2+)) [dtbpm = bis(di-tert-butylphosphino)methane, tBu2PCH2PtBu2] and of [Cu3(mu 3-F)(mu-dtbpm)3]2+ (2(2+)) were synthesized and characterized. Coordination of solvent or counterions to 1(2+) is observed neither in solution nor in the solid state. The two copper(I) centers in 1(2+) indicate weak d10-d10 closed-shell interactions. 1(2+) reacts slowly with PF6- anions in acetone or KF in CH2Cl2 to yield the mu 3-fluorido complex 2(2+) with idealized D3 symmetry, containing a trigonal planar Cu3F core, as shown by single-crystal X-ray diffraction. Distinct structural differences are observed compared to monocationic bicapped, trinuclear copper(I) dppm halide complexes [dppm = bis(diphenylphosphino)methane, Ph2PCH2PPh2]. The average Cu-Cu, Cu-F, and Cu-P distances and the P-Cu-P' angle in 2(2+) are 3.85, 2.22, and 2.28 A and 144.3 degrees, respectively. The P2Cu units are twisted out of the Cu3F plane by an average angle of 18.4 degrees. DFT calculations (BPW91/LANL2DZ) for the model [Cu3(mu 3-F)(mu-dhpm)3]2+ (dhpm = diphosphinomethane, H2PCH2PH2) are used to explain the formation, structure, and bonding pattern of 2(2+). 相似文献
6.
《International journal of quantum chemistry》2018,118(10)
Triplet state mechanism of [2 + 2] photocycloaddition forming a cyclobutane ring from two ethylenes is investigated in the context of photocatalysis. High‐level ab initio calculations are combined with ab initio adiabatic molecular dynamics and ab initio metadynamics for rare events modeling. In a photocatalytic scheme, a reactant reaches the triplet state either via intersystem crossing (ISC) or triplet sensitization. The model system adopts a biradical structure, which represents energy intersection with the ground state. The system either completes cyclization or undergoes fragmentation into two olefinic units. The potential and free energy surfaces of the cyclobutane/ethylenes system are mapped with multireference approaches describing possible reaction pathways. To obtain a full picture of a double bond photoreactivity, ab initio adiabatic dynamical calculations were used to estimate reaction yields and to model the effects of excess energy. The potential use of density functional theory based approaches for [2 + 2] photocycloaddition was investigated for future simulations and design of realistic photocatalytic systems. Dynamical aspects of [2 + 2] photocycloaddition via a triplet state manifold are investigated by combining ab initio multireference methods and ab initio molecular dynamics and metadynamics. The reaction pathways are studied for a model system of two ethylenes forming a cyclobutane ring to provide a basis for further studies on design of photocatalytic systems. 相似文献
7.
[reaction: see text] Ab initio and density functional studies (DFT) on cycloaddition reactions of 1,3-diazabuta-1,3-dienes with ketenes are reported. The vinylic (C=C) and the carbonyl (C=O) units of the ketenes are found to participate in concerted asynchronous [4 + 2] cycloaddition reactions. The transition states (3t, 4t, and 7t) for these paths have been located on the PE surface at the correlated levels of ab initio calculations. A reasonable mechanism for the formation of [4 + 2] and [2 + 2] adducts is presented. 相似文献
8.
To investigate the solvation structure of the Cu(II) ion in liquid ammonia, ab initio quantum-mechanical/molecular-mechanical (QM/MM) molecular dynamics (MD) simulations were carried out at Hartree Fock (HF) and hybrid density functional theory (B3 LYP) levels. A sixfold-coordinated species was found to be predominant in the HF case whereas five- and sixfold-coordinated complexes were obtained in a ratio 2:1 from the B3 LYP simulation. In contrast to hydrated Cu(II), which exhibits a typical Jahn-Teller distortion, the geometrical arrangement of ligand molecules in the case of ammonia can be described as a [2 + 4] ([2 + 3]) configuration with 4 (3) elongated copper-nitrogen bonds. First shell solvent exchange reactions at picosecond rate took place in both HF and B3 LYP simulations, again in contrast to the more stable sixfold-coordinated hydrate. NH3 ligands apparently lead to strongly accelerated dynamics of the Cu(II) solvate due to the "inverse" [2 + 4] structure with its larger number of elongated copper-ligand bonds. Several dynamical properties, such as mean ligand residence times or ion-ligand stretching frequencies, prove the high lability of the solvated complex. 相似文献
9.
García B Ibeas S Leal JM Senent ML Niño A Muñoz-Caro C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(14):2644-2652
The mechanism of the protonation of acetohydroxamic acid is investigated comparing experimental results and ab initio calculations. Experimentally, the UV spectral curves were recorded at different temperatures, at constant dioxane/water concentration, and at very high concentrations of strong mineral acids. The process is followed by monitoring the changes in the UV curves with increasing acid concentration. The molecular structures and the solvation energies were calculated with the RHF, B3LYP, and MP2 methods. The solvent is treated as a continuum of uniform dielectric constant. The isolated molecule of acetohydroxamic acid exhibits two protonation sites, the carbonyl oxygen and the nitrogen atom. In dioxane/water mixture, the RHF calculations predict the existence of a third cation of low stability, where the proton is bonded to the OH oxygen. With the MP2 ab initio calculations, the free energies of the formation processes in solution of the two most stable cations, CH3COH-NHOH+ (O3H+) and CH3CO-NH2OH+ have been evaluated to be -160.2 kcalmol(-1) and -157.6 kcal mol(-1). The carbonyl site is the most active center in solution and in the gas phase. The carbonyl site is also the most active center in the UV measurements. Experimentally, the ionization constant was found to be pK(O3H+) = -2.21 at 298.15 K, after the elimination of the medium effects using the Cox-Yates equation for hight acidity levels. Experiments and ab initio calculations indicate that K(O3H+) decreases with the temperature. 相似文献
10.
Richard A. J. O’Hair Michelle L. Styles Gavin E. Reid 《Journal of the American Society for Mass Spectrometry》1998,9(12):1275-1284
The gas phase fragmentation reactions of protonated cysteine and cysteine-containing peptides have been studied using a combination of collisional activation in a tandem mass spectrometer and ab initio calculations [at the MP2(FC)/6-31G*//HF/6-31G* level of theory]. There are two major competing dissociation pathways for protonated cysteine involving: (i) loss of ammonia, and (ii) loss of the elements of [CH2O2]. MS/MS, MS/MS of selected ions formed by collisional activation in the electrospray ionization source as well as ab initio calculations have been carried out to determine the mechanisms of these reactions. The ab initio results reveal that the most stable [M + H − NH3]+ isomer is an episulfonium ion (A), whereas the most stable [M + H − CH2O2]+ isomer is an immonium ion (B). The effect of the position of the cysteine residue on the fragmentation reactions of the [M + H]+ ions of all the possible simple dipeptide and tripeptide methyl esters containing one cysteine (where all other residues are glycine) has also been investigated. When cysteine is at the N-terminal position, NH3 loss is observed, although the relative abundance of the resultant [M + H − NH3]+ ion decreases with increasing peptide size. In contrast, when cysteine is at any other position, water loss is observed. The proposed mechanism for loss of H2O is in competition with those channels leading to the formation of structurally relevant sequence ions. 相似文献
11.
The metal ion binding sites of human islet amyloid polypeptide (hIAPP) have been investigated to explain the biological activity difference in the fibril formation process. The structures of [hIAPP...Cu (or Al)](n+) and [hIAPP17-30...Cu]2+ complex were investigated by electrospray ionization-mass spectrometry (ESI-MS). The fragmentation patterns of [hIAPP...Cu [or Al)](n+) and [hIAPP17-30...Cu]2+ complex were analyzed by tandem mass spectrometry (MS/MS) and multi-stage mass spectrometry (MS3) spectra. The [hIAPP+Cu+H]3+, [hIAPP+Al+H]4+ and [hIAPP17-30+Cu]2+ complexes were observed in MS spectra. The Cu binding site of hIAPP is suggested to be the N22-F-G-A-I26 part for the [hIAPP+Cu+H]3+ gas-phase complex. The original hIAPP conformation was supposed to be changed by the interaction between the Cu ion and the N22-F-G-A-I26 part in the [hIAPP+Cu+H]3+ gas-phase complex. 相似文献
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Maity D Manna AK Karthigeyan D Kundu TK Pati SK Govindaraju T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(40):11152-11161
We present novel Schiff base ligands julolidine-carbonohydrazone 1 and julolidine-thiocarbonohydrazone 2 for selective detection of Cu(2+) in aqueous medium. The planar julolidine-based ligands can sense Cu(2+) colorimetrically with characteristic absorbance in the near-infrared (NIR, 700-1000 nm) region. Employing molecular probes 1 and 2 for detection of Cu(2+) not only allowed detection by the naked eye, but also detection of varying micromolar concentrations of Cu(2+) due to the appearance of distinct coloration. Moreover, Cu(2+) selectively quenches the fluorescence of julolidine-thiocarbonohydrazone 2 among all other metal ions, which increases the sensitivity of the probe. Furthermore, quenched fluorescence of the ligand 2 in the presence of Cu(2+) was restored by adjusting the complexation ability of the ligand. Hence, by treatment with ethylenediaminetetraacetic acid (EDTA), thus enabling reversibility and dual-check signaling, julolidine-thiocarbonohydrazone (2) can be used as a fluorescent molecular probe for the sensitive detection of Cu(2+) in biological systems. The ligands 1 and 2 can be utilized to monitor Cu(2+) in aqueous solution over a wide pH range. We have investigated the structural, electronic, and optical properties of the ligands using ab initio density functional theory (DFT) combined with time-dependent density functional theory (TDDFT) calculations. The observed absorption band in the NIR region is attributed to the formation of a charge-transfer complex between Cu(2+) and the ligand. The fluorescence-quenching behavior can be accounted for primarily due to the excited-state ligand 2 to metal (Cu(2+)) charge-transfer (LMCT) processes. Thus, experimentally observed characteristic NIR and fluorescence optical responses of the ligands upon binding to Cu(2+) are well supported by the theoretical calculations. Subsequently, we have employed julolidine-thiocarbonohydrazone 2 for reversible fluorescence sensing of intracellular Cu(2+) in cultured HEK293T cells. 相似文献
14.
Copper-exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over-oxidation products like CO and CO2. However, mechanistic pathways for methane over-oxidation by copper-oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over-oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over-oxidation by [Cu3O3]2+ active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ-oxo atom of the active site. This μ-(O−CH3) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper-monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper-monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction step must be stabilized away from copper-oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper-oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane-to-methanol conversion. 相似文献
15.
Direct conversion of methane(CH4)to methanol(DMTM)is a promising,but very challenging process for the utilization of abundant CH4 as a low carbon resource.In this context,Cu loaded zeolites,mordenite(MOR)in particular,were recognized as the most effective system to perform DMTM.In this work,different Cu salts were used to exchange with MOR,by which the effect of counter ions on the catalytic performance towards DMTM was investigated.The prepared catalysts were characterized and evaluated systematically.It was found that the counter ions affected the speciation of Cu sites,probably due to their capability in extraction of protons from MOR and the influence on the hydrolysis state of the Cu2+in aqueous solution.These behaviors adjusted the association between Cu2+and the exchangeable protons in MOR.As a result,varied DMTM performance was observed.Among the used Cu salts,Cu(CH3COO)2 exchanged MOR showed the highest performance,achieving stable CH3 OH yield of 117±28μmol/g in 5 consecutive cycles,these values are among the highest for Cu loaded MOR zeolites in open publications. 相似文献
16.
Yong Jun HU Yong CAI Ri Chang LU Nan Quan LOU Xiu Yan WANG* State Key Laboratory of Molecular Reaction Dynamics Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian 《中国化学快报》2001,(3)
Proton transfer process in hydrogen-bonded clusters has attracted great interest of many chemists in physical chemistry and biochemistry1-5. Pyrrole (C4H5N) is one of the building blocks of some important biomolecules6. And pyrrole is a compound of five-membered hetero-cyclic aromatic ring, in which a lone pair of electrons offered by the N atom and the two double bonds form a delocalized big ( bond. In this paper we report on the observations for the cluster system pyrrole-water by use of a… 相似文献
17.
A density functional theory study of molecular and dissociative adsorption of H2 on active sites in mordenite 总被引:1,自引:0,他引:1
Adsorption and chemisorption of H2 in mordenite is studied using ab initio density functional theory (DFT) calculations. The geometries of the adsorption complex, the adsorption energies, stretching frequencies, and the capacity to dissociate the adsorbed molecule are compared for different active sites. The active centers include a Br?nsted acid site, a three-coordinated surface Al site, and Lewis sites formed by extraframework cations: Na+, Cu+, Ag+, Zn2+, Cu2+, Ga3+, and Al3+. Adsorption properties of cations are compared for a location of the cation in the five-membered ring. This location differs from the location in the six-membered ring observed for hydrated cations. The five-membered ring, however, represents a stable location of the bare cation. In this position any cation exhibits higher reactivity compared with the location in the six-membered ring and is well accessible by molecules adsorbed in the main channel of the zeolite. Calculated adsorption energies range from 4 to 87 kJ/mol, depending on electronegativity and ionic radius of the cation and the stability of the cation-zeolite complex. The largest adsorption energy is observed for Cu+ and the lowest for Al3+ integrated into the interstitial site of the zeolite framework. A linear dependence is observed between the stretching frequency and the bond length of the adsorbed H2 molecule. The capacity of the metal-exchanged zeolite to dissociate the H2 molecule does not correlate with the adsorption energy. Dissociation is not possible on single Cu+ cation. The best performance is observed for the Ga3+, Zn2+, and Al3+ extraframework cations, in good agreement with experimental data. 相似文献
18.
Yi HB Diefenbach M Choi YC Lee EC Lee HM Hong BH Kim KS 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(18):4885-4892
To understand the self-assembly process of the transition metal (TM) nanoclusters and nanowires self-synthesized by hydroquinone (HQ) and calix[4]hydroquinone (CHQ) by electrochemical redox processes, we have investigated the binding sites of HQ for the transition-metal cations TM(n+)=Ag(+), Au(+), Pd(2+), Pt(2+), and Hg(2+) and those of quinone (Q) for the reduced neutral metals TM(0), using ab initio calculations. For comparison, TM(0)-HQ and TM(n+)-Q interactions, as well as the cases for Na(+) and Cu(+) (which do not take part in self-synthesis by CHQ) are also included. In general, TM-ligand coordination is controlled by symmetry constraints imposed on the respective orbital interactions. Calculations predict that, due to synergetic interactions, silver and gold are very efficient metals for one-dimensional (1D) nanowire formation in the self-assembly process, platinum and mercury favor both nanowire/nanorod and thin film formation, while palladium favors two-dimensional (2D) thin film formation. 相似文献
19.
Supramolecular calix[4]arene conjugate (L) has been developed as a sensitive and selective sensor for Zn(2+) in HEPES buffer among the 12 metal ion by using fluorescence, absorption and ESI MS and also by visual fluorescent color. The structural, electronic, and emission properties of the calix[4]arene conjugates L and its zinc complex, [ZnL], have been demonstrated using ab initio density functional theory (DFT) combined with time-dependent density functional theory (TDDFT) calculations. The TDDFT calculations reveal the switch on fluorescence behavior of L is mainly due to the utilization of the lone pair of electrons on imine moiety by the Zn(2+). The resultant fluorescent complex, [ZnL], has been used as a secondary sensing chemo-ensemble for the detection of -SH containing molecules by removing Zn(2+) from [ZnL] and forming {Cys/DTT·Zn} adducts as equivalent to those present in metallothioneins. The displacement followed by the release of the coordinated zinc from its Cys/DTT complex by heavy metal ion (viz. Cd(2+) and Hg(2+)), as in the metal detoxification process or by ROS (such as H(2)O(2)) as in the oxidative stress, has been well demonstrated using the conjugate L through the fluorescence intensity retrieval wherein the fluorescence intensity is the same as that observed with [ZnL], which in turn mimics the zinc sensing element (MTF) in biology. 相似文献
20.
<正>Two novel organic-inorganic heteropolyoxometalate compounds [{Cu(1,10- phen)OH}_2]_2[V_2W_4O_19]·6H_2O (1) and [Co(1,10-phen)_3]_2[V_2W_4O_(19)]·5H_2O (2) (1,10-phen = 1,10- phenanthroline) were hydrothermally synthesized and characterized by elemental analysis, IR spectra and single-crystal X-ray diffraction. Compound 1 contains a di-V substituted classical Lindqvist-type polyanion [V_2W_4O_(19)]~(4-), two dinuclear copper cations [{Cu(1,10-phen)OH}_2]~(2+) and six water molecules of crystallization. Two dinuclear copper cations [{Cu(1,10-phen)OH}_2]~(2+) consist of four transition metal coordination cations {Cu(1,10-phen)}~(2+), bridged by four hydroxyl groups. Meanwhile, both two dinuclear cations are grafted on the Lindqvist polyanion [V_2W_4O_(19)]~(4-) through two terminal oxygen (O(9)) and two μ_2-oxygen (O(6)) atoms. Such an unusual linking fashion is unique in the polyoxometalate chemistry. The basic framework of 2 is similar to that of 1 and contains one Lindqvist-type polyanion [V_2W_4O_(19)]~(4-), two six-coordinated cobalt cations [Co(1,10-phen)_3]~(2+) and five free water molecules. 相似文献