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Ceri Hammond,
Nikolaos Dimitratos,
Robert L. Jenkins,
Jose A. Lopez-Sanchez,
Simon A. Kondrat,
Mohd H. ab Rahim,
Michael M. Forde,
Adam Thetford,
Stuart H. Taylor,
Henk Hagen,
Eric E. Stangland,
Joo H. Kang,
Jacob M. Moulijn, David J. Willock,
Graham J. Hutchings
ACS Catalysis 02/2013; 3(4):689-699.
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Mohd Hasbi Ab Rahim,
Michael M Forde,
Robert L Jenkins,
Ceri Hammond,
Qian He,
Nikolaos Dimitratos,
Jose Antonio Lopez-Sanchez,
Albert F Carley,
Stuart H Taylor, David J Willock,
Damien M Murphy,
Christopher J Kiely,
Graham J Hutchings
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ABSTRACT: Direct and selective: Supported gold-palladium nanoparticles are active for the oxidation of methane, giving a high selectivity for the formation of methyl hydroperoxide and methanol, using hydrogen peroxide as the oxidant. The optimal methanol selectivity is achieved by performing the reaction in the presence of hydrogen peroxide that has been generated in situ from hydrogen and oxygen.
Angewandte Chemie International Edition 12/2012; · 13.45 Impact Factor
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Ceri Hammond,
Robert L Jenkins,
Nikolaos Dimitratos,
Jose Antonio Lopez-Sanchez,
Mohd Hasbi Ab Rahim,
Michael M Forde,
Adam Thetford,
Damien M Murphy,
Henk Hagen,
Eric E Stangland,
Jacob M Moulijn,
Stuart H Taylor, David J Willock,
Graham J Hutchings
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ABSTRACT: The partial oxidation of methane to methanol presents one of the most challenging targets in catalysis. Although this is the focus of much research, until recently, approaches had proceeded at low catalytic rates (<10 h(-1) ), not resulted in a closed catalytic cycle, or were unable to produce methanol with a reasonable selectivity. Recent research has demonstrated, however, that a system composed of an iron- and copper-containing zeolite is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14 000 h(-1) by using H(2) O(2) as terminal oxidant. However, the precise roles of the catalyst and the full mechanistic cycle remain unclear. We hereby report a systematic study of the kinetic parameters and mechanistic features of the process, and present a reaction network consisting of the activation of methane, the formation of an activated hydroperoxy species, and the by-production of hydroxyl radicals. The catalytic system in question results in a low-energy methane activation route, and allows selective C(1) -oxidation to proceed under intrinsically mild reaction conditions.
Chemistry 11/2012; · 5.93 Impact Factor
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ABSTRACT: Ab initio and density functional theory (DFT) calculations on some model systems are presented to assess the extent to which intermolecular hydrogen bonding can affect the planarity of amide groups. Formamide and urea are examined as archetypes of planar and non-planar amides, respectively. DFT optimisations suggest that appropriately disposed hydrogen-bond donor or acceptor molecules can induce non-planarity in formamide, with OCNH dihedral angles deviating by up to ca. 20° from planarity. Ab initio energy calculations demonstrate that the energy required to deform an amide molecule from the preferred geometry of the isolated molecule is more than compensated by the stabilisation due to hydrogen bonding. Similarly, the NH(2) group in urea can be made effectively planar by the presence of appropriately positioned hydrogen-bond acceptors, whereas hydrogen-bond donors increase the non-planarity of the NH(2) group. Small clusters (a dimer, two trimers and a pentamer) extracted from the crystal structure of urea indicate that the crystal field acts to force planarity of the urea molecule; however, the interaction with nearest neighbours alone is insufficient to induce the molecule to become completely planar, and longer-range effects are required. Finally, the potential for intermolecular hydrogen bonding to induce non-planarity in a model of a peptide is explored. Inter alia, the insights obtained in the present work on the extent to which the geometry of amide groups may be deformed under the influence of intermolecular hydrogen bonding provide structural guidelines that can assist the interpretation of the geometries of such groups in structure determination from powder X-ray diffraction data.
Physical Chemistry Chemical Physics 07/2012; 14(34):11944-52. · 3.57 Impact Factor
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ABSTRACT: The activation of CH4 has been probed by studying CH4 oxidation at ambient pressure over gallium- and zinc-based catalysts prepared by precipitation and modified with Au and Pt.
The unmodified gallium- and zinc-based catalysts were both active for CH4 oxidation. Modification of these catalysts by the addition of Au and Pt, alone and in combination significantly increased
the rate of CH4 oxidation. The 1% Pt/Ga2O3 catalyst was the most active of the gallium-based systems. The addition of Au to ZnO markedly increased the activity compared
with unmodified ZnO, whilst for Au in combination with Pt the activity was further enhanced due to a synergistic effect of
the metals.
Research on Chemical Intermediates 05/2012; 29(7):911-920. · 0.70 Impact Factor
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Ceri Hammond,
Michael M Forde,
Mohd Hasbi Ab Rahim,
Adam Thetford,
Qian He,
Robert L Jenkins,
Nikolaos Dimitratos,
Jose A Lopez-Sanchez,
Nicholas F Dummer,
Damien M Murphy,
Albert F Carley,
Stuart H Taylor, David J Willock,
Eric E Stangland,
Joo Kang,
Henk Hagen,
Christopher J Kiely,
Graham J Hutchings
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ABSTRACT: Iron copper zeolite (Fe-Cu-ZSM-5) with aqueous hydrogen peroxide is active for the selective oxidation of methane to methanol. Iron is involved in the activation of the carbon hydrogen bond, while copper allows methanol to form as the major product. The catalyst is stable, re-usable and activates methane giving >90 % methanol selectivity and 10 % conversion in a closed catalytic cycle.
Angewandte Chemie International Edition 04/2012; 51(21):5129-33. · 13.45 Impact Factor
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ABSTRACT: Single enantiomers of R/S-methylbenzylamine (MBA) were found to selectively form adducts with two chiral Cu-salen complexes, [Cu(II)(1)] (H(2)1 = N,N'-bis(3,5-ditert-butylsalicylidene)-1,2-diaminocyclohexane) and [Cu(II)(2)] (H(2)2 = N,N'-bis-salicylidene-1,2-cyclohexanediamino). The axial g/A spin Hamiltonian parameters of the Cu-MBA adducts were typical of 5-coordinate species. Enantiomer discrimination in the MBA binding was directly evidenced by W-band CW EPR, revealing an 86 ± 5% preference for formation of the R,R-[Cu(1)] + S-MBA adducts compared to R,R-[Cu(1)] + R-MBA; this was reduced to a 57 ± 5% preference for R,R-[Cu(2)] + S-MBA following removal of the tert-butyl groups. The structure of these diastereomeric adducts was further probed by different hyperfine techniques (ENDOR and HYSCORE), although no structural differences were detected between these adducts using these techniques. The diastereomeric adducts were found to possess lower symmetry, as evidenced by rhombic g tensors and inequivalent H(imine) couplings. This was caused by the selective binding mode of MBA onto one side of the chiral Cu(II) complex. DFT calculations were performed on the R,R-[Cu(1)] + S-MBA and R,R-[Cu(1)] + R-MBA adducts. A distinct difference in orientation and binding mode of the MBA was identified in both adducts, confirming the experimental results. The preferred heterochiral R,R-[Cu(1)] + S-MBA adduct was found to be 5 kJ mol(-1) lower in energy compared to the homochiral adduct. A delicate balance of steric repulsion between the α-proton (attached to the asymmetric carbon atom) of MBA and the methine proton (attached to the asymmetric carbon atom) of [Cu(1)] was crucial in the stereoselective binding.
Inorganic Chemistry 06/2011; 50(15):6944-55. · 4.60 Impact Factor
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ABSTRACT: The oxidation of CO by Au/Fe(2)O(3) and Au/ZnO catalysts is compared in the very early stages of the reaction using a temporal analysis of products (TAP) reactor. For Au/Fe(2)O(3) pre-dosing the catalyst with (18)O labelled water gives an unexpected evolution order for the labelled CO(2) product with the C(18)O(2) emerging first, whereas no temporal differentiation is found for Au/ZnO. High pressure XPS experiments are then used to show that CO bond cleavage does occur for model catalysts consisting of Au particles deposited on iron oxide films but not when deposited on ZnO films. DFT calculations, show that this observation requires carbon monoxide to dissociate in such a way that cleavage of the CO bond occurs along with dynamically co-adsorbed oxygen so that the overall process of Au oxidation and CO dissociation is energetically favourable. Our results show that for Au/Fe(2)O(3) there is a pathway for CO oxidation that involves atomic C and O surface species which operates along side the bicarbonate mechanism that is widely discussed in the literature. However, this minor pathway is absent for Au/ZnO.
Physical Chemistry Chemical Physics 02/2011; 13(7):2528-38. · 3.57 Impact Factor
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ABSTRACT: Oxidation chemistry with supported Au nanoparticles as catalysts is an area of intense research. Even so there is still much discussion as to the nature of Au species generated on the complex surfaces of these catalysts and the types of oxygen species that are present. Recent experimental work has highlighted Au bi-layers with dimensions of 0.5 nm supported on iron oxide as a very efficient catalyst system for CO oxidation. This size scale implies clusters containing only 10 Au atoms, making the simulation of the nanoparticles, oxide surface and their interface amenable to perioidic density functional theory calculations. We present simulation results which demonstrate that the dissociation of O2 is energetically favourable at the interface between nanoparticle and oxide, with both surface Fe cations and Au atoms taking part in the adsorption site. Here the barrier to dissociation of O2 is found to be lower than the energy required for molecular desorption which is not the case for isolated Au clusters. This reaction also produces oxidised Au atoms, as confirmed by Bader charge analysis. For isolated clusters we show that such oxidised Au species give rise to empty d-band states, whereas molecular adsorption of O2 does not.
Faraday Discussions 01/2011; 152:135-51; discussion 203-25. · 5.00 Impact Factor
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09/2010; , ISBN: 9780471227618
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ABSTRACT: The mode of chiral interaction between a series of asymmetric epoxides (propylene oxide, butylene oxide, epifluorohydrin and epichlorohydrin) and a chiral vanadyl salen complex, N, N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino-vanadium (iv) oxide, [VO()], was investigated by a range of electron magnetic resonance techniques (EPR, ENDOR, HYSCORE) and DFT. Enantiomer discrimination of the weakly bound epoxides by the vanadyl complex was evident by cw-ENDOR. The origin of this discrimination was attributed to a number of factors including H-bonds, steric properties and electrostatic contributions, which collectively control the outcome of the chiral interaction. DFT revealed the role of a key H-bond, formed between the epoxide oxygen atom (O(epoxide)) and the methine proton (H(exo)) attached to the asymmetric carbon atom of the chiral vanadyl salen complex, thereby providing a direct pathway for stereochemical communication between complex and substrate. These findings reveal the potential importance of weak outer sphere interactions in stereoselectivities of enantioselective homogeneous catalysis.
Physical Chemistry Chemical Physics 09/2009; 11(31):6757-69. · 3.57 Impact Factor
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ABSTRACT: In this chapter we cover the application of quantum chemistry to gain an understanding of the properties of oxide materials commonly used in catalysis. We begin with the background concepts of Hartree Fock and density function theories concentrating on their treatment of the electron-electron interaction through exchange and correlation energies. The emphasis of this contribution is the use of periodic boundary conditions and so there is then a resume of band theory.The first set of examples cover oxide materials for which the oxidation state of the metal centers is easily determined. This includes the relative stability of the phases of alumina and its related hydroxides and a comparison of the surface structures of MgO and AI2O3. The stoichiometric surface of TiO2 is also included here.For reducible oxides, in which electron localisation at transition metal cation sites is possible, we consider the relative merits of hybrid functionals and the DFT+U method drawing on TiO2, MoO3 and CeO2. These methods are shown to be required for a correct description of the defect structures that are known to be important in catalysis.
03/2009: pages 323 - 389; , ISBN: 9783527626113
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ABSTRACT: This critical review covers the application of quantum chemistry to the burgeoning area of the heterogeneous oxidation by Au. We focus on the most established reaction, the oxidation of CO at low temperature. The review begins with an overview of the methods available comparing the treatment of the electron-electron interaction and relativistic effects. The structure of Au particles and their interaction with oxide reviews is then discussed in detail. Calculations of the adsorption and reaction of CO and O2 are then considered and results from isolated and supported Au clusters compared (155 references).
Chemical Society Reviews 10/2008; 37(9):2046-76. · 28.76 Impact Factor
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ABSTRACT: The first row diatomic molecules N2 and CO have played central roles in developing fundamental models of electronic structure, and their interactions with transition metals have been widely investigated. By contrast, the valence isoelectronic molecules EX (E = group 13 element, X = group 17 element) have yet to be isolated under ambient conditions, either as the "free" molecule or as a simple metal complex. Here, we find that iodide abstraction from Cp*Fe(dppe)(GaI2) yields the stable complex [Cp*Fe(dppe)(GaI)]+ which features a terminally bound GaI ligand, characterized by a near linear (but readily deformed) Fe-Ga-I geometry and by very short Fe-Ga and Ga-I distances. Chemical and computational evidence reveal a relatively weak metal-ligand bond similar in strength to that found for dinitrogen analogues.
Journal of the American Chemical Society 05/2008; 130(16):5449-51. · 9.91 Impact Factor
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Angewandte Chemie International Edition 02/2008; 47(8):1414-6. · 13.45 Impact Factor
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ABSTRACT: Insertion reactions of dicyclohexylcarbodiimide with aminoboranes and with aminoboryl and -borylene transition metal complexes have been examined as potential routes to new boron-containing ligand systems. Reactions with systems containing two-coordinate boron centres are found to be significantly more facile than those with three-coordinate substrates. Thus, reaction of (dicyclohexylamino)boron dichloride () with dicyclohexylcarbodiimide over 36 h at 50 degrees C generates the (structurally authenticated) guanidinate complex Cy(2)NC(NCy)(2)BCl(2) () via insertion into the BN bond. By contrast, the corresponding reaction with the cationic aminoborylene complex [CpFe(CO)(2)(BNCy(2))](+)[BAr(f)(4)](-) () proceeds rapidly at ca.-30 degrees C, via initial insertion into the FeB bond to give [CpFe(CO)(2)C(NCy)(2)BNCy(2)](+)[BAr(f)(4)](-) (). Consistent with related studies, a key factor in facilitating such insertion chemistry is thought to be the formation of an initial donor/acceptor complex between the diimide and the group 13 centre. Thus, DFT studies suggest that [CpFe(CO)(2)B(NCy(2))(CyNCNCy)](+)[BAr(f)(4)](-) is a potential intermediate in the reaction of with CyNCNCy, and that further reaction to give the observed product, , is strongly exergic (-183 kJ mol(-1)). By contrast, DFT calculations for the alternative isomer [CpFe(CO)(2)B(CyN)(2)CNCy(2)](+)[BAr(f)(4)](-) (), formed by BN insertion, suggest that it is 112 kJ mol(-1) less stable than . Such experimental and computational findings imply that under reaction conditions where a suitable isomerisation pathway is available, cationic complexes such as , which contain a four-membered boron-donor heterocycle are likely to be disfavoured with respect to alternative C-bound isomers.
Dalton Transactions 11/2007; · 3.84 Impact Factor
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Angewandte Chemie International Edition 02/2007; 46(12):2043-6. · 13.45 Impact Factor
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Angewandte Chemie International Edition 10/2006; 45(37):6118-22. · 13.45 Impact Factor
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ABSTRACT: The reactivity of several transition metal half sandwich complexes towards an anionic gallium(I) heterocyclic complex, [K(tmeda)][Ga{[N(Ar)C(H)]2}](Ar = C6H3Pri2-2,6), has been investigated. This has led to the anionic half sandwich complexes, [K(tmeda)][(C5H4R)M(CO)n[Ga{[N(Ar)C(H)]2}]](M = V, R = H, n= 3; M = Mn, R = Me, n= 2; M = Co, R = H, n= 1), which crystallographic studies show to form dimers (M = Mn and Co) or a polymer (M = V) through bridging potassium cations. The metal-gallium bond lengths in all complexes are very short which, combined with some spectroscopic evidence, is suggestive of M-Ga pi-bonding. Density functional theory studies of models of all complexes indicate that the level of back-bonding in these complexes is, however, minimal and of a similar order to that seen in analogous complexes incorporating neutral N-heterocyclic carbene ligands. Reactions of the metallocenes, [M(C5H4Me)2](M = V or Cr), with the digallane4, [Ga{[N(Ar)C(H)]2}]2, have afforded the neutral complexes, [M(C5H4Me)2[Ga{[N(Ar)C(H)]2}]], which are thought to be formed via an initial oxidative insertion of the transition metal centre into the Ga-Ga bond of the digallane. X-Ray crystallography shows the complexes to be monomeric. One (M = V) reacts with one equivalent of [K(tmeda)][Ga{[N(Ar)C(H)]2}] to give the crystallographically characterised, anionic bis(gallyl)-complex, [K(tmeda)][V(C5H4Me)2[Ga{[N(Ar)C(H)]2}]2]. For comparison, the reaction of [K(tmeda)][Ga{[N(Ar)C(H)]2}] with [Mn{CH(SiMe3)2}2] was carried out and gave the monomeric, anionic complex, [K(tmeda)][Mn{CH(SiMe3)2}2[Ga{[N(Ar)C(H)]2}]].
Dalton Transactions 08/2006; · 3.84 Impact Factor
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Angewandte Chemie 07/2006; 118(37):6264 - 6268.
