Judith M. Corker

Catalytic Cleavage of the C-H and C-C Bonds of Alkanes by Surface Organometallic Chemistry: An EXAFS and IR Characterization of a Zr-H Catalyst

The catalytic cleavage under hydrogen of the C & singlebond;H and C & singlebond;C bonds of alkanes with conventional catalysts requires high temperatures. Room-temperature hydrogenolysis of simple alkanes is possible on a well-defined and well-characterized zirconium hydride supported on silica obtained by surface organometallic chemistry. The surface structure resulting from hydrogenolysis of (≡SiO)Zr(Np)3 (Np, neopentyl) was determined from the extended x-ray absorption fine structure (EXAFS) and 1H and 29Si solid-state nuclear magnetic resonance and infrared (IR) spectra. A mechanism for the formation of (≡SiO)3Zr-H and (≡SiO)2SiH2 and the resulting low-temperature hydrogenolysis of alkanes is proposed. The mechanism may have implications for the catalytic formation of methane, ethane, and lower alkanes in natural gas.

Study of cloverite molecular sieves modified by grafting organometallic complexes inside the pores: I. Synthesis and characterization of cloverite modified by grafting –SnR3 fragments (R=Me, Et, n-Bu, Cy)

Abstract The reaction of various alkyltin compounds [HSnBu 3 and SnR 4 , R=Me, Et, cyclohexyl (Cy)] with cloverite dehydroxylated at 450°C was studied by various methods such as 1 H , 31 P and 119 Sn MAS NMR, 13 C CP-MAS NMR, infrared spectroscopy, analysis of evolved gases, thermolysis and chemical analysis. All techniques showed that there was a formation of grafted –SnR 3 fragments. However, depending on the size of the organometallic fragment, the grafting reaction did not occur in the same manner: while –SnR 3 (R=Me, Et, n -Bu) fragments were grafted homogeneously over the entire crystal volume, –SnCy 3 fragments were grafted only on the external surface, as previously observed on mordenite. In addition, depending on the size of the R alkyl group, different grafted complex loadings were achieved: for R=Me, up to four –SnMe 3 could be grafted per window aperture, while for R=Et and n -Bu, only one complex could be grafted per window aperture.

In situ energy dispersive EXAFS (EDE) of low loaded Pt(acac)2/HI SiO2 catalyst precursors on a timescale of seconds and below

We demonstrate second and sub-second time resolved energy dispersive EXAFS (EDE) for in situ interrogation of phase changes in a supported metal catalyst precursor [(⩽5 wt%) Pt(acac)2/HI SiO2] during thermal decomposition and reduction; the formation of Pt particles is radically altered by the presence H2, in both kinetic and structural senses.

Sulfur K-edge X-ray absorption spectroscopy study of the reaction of zinc oxide with hydrogen sulfide

We report in situ sulfur K-edge XAS data which show that a small number of very reactive sites are involved in the reaction of zinc oxide with hydrogen sulfide. Such sites react before the majority of the bulk oxide and are capable of forming zinc sulfide rapidly even at ambient temperatures. In situ sulfur K-edge EXAFS also provides evidence for a long sulfur–zinc interaction at 2.71 A during the formation of bulk islands of zinc sulfide on the oxide. Powder diffraction and X-ray photoelectron spectroscopy on ex situ treated hydrogen sulfide absorbents confirm the formation of a twin phase material of crystalline zinc oxide and less crystalline zinc sulfide, both wurtzite polymorph, as suggested by EXAFS.

In situ, time resolved, and simultaneous multi-edge determination of local order change during reduction of supported bimetallic (Pt–Ge) catalyst precursors using energy dispersive EXAFS

In situ, time resolved, simultaneous multi-edge energy dispersive EXAFS (EDE) reveals Ge precursor induced perturbations of the reduction of supported Pt(acac)2 to form Pt particles, and details the subsequent thermally induced formation of PtGe species.

Highly active homogeneous nickel catalysts for alkene dimerisation : crystal structure of [Ni(η3-C3H5)(PPh3)Br] and in situ characterisation of AlEt3-activated [Ni(η3-C3H5)(PPh3)Br] by nuclear magnetic resonance and extended X-ray absorption fine structure spectroscopy

In situ NMR and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used to characterise a highly active propene dimerisation catalyst, prepared by the low-temperature addition of AlEt3 to [Ni(η3-C3H5)(PPh3)Br]. The molecular structure of [Ni(η3-C3H5)(PPh3)Br] has been determined by X-ray crystallography and variable-temperature NMR spectroscopy. Crystal data: monoclinic, space group P21/n(no. 14), a= 7.951(1), b= 14.954(1), c= 16.096(3)A, β= 96.05(1)°, Z= 4, R= 0.045. It has previously been assumed that aluminium interacts with nickel in such catalyst systems via halide-bridging ligands in order to decrease the charge on the nickel, thereby facilitating the co-ordination of electron donors to the nickel centre. EXAFS analysis provided direct structural evidence for a Ni ⋯ Al interaction in solution, but questioned the nature of the bridging ligand: the nickel K-edge EXAFS data of a solution of [Ni(η3-C3H5)(PPh3)Br], AlEt3(Ni:Al = 1 : 5) and propene in toluene at –60 °C, showed that the first co-ordination sphere around nickel comprised 3.9 carbons at 1.93 A and 1.0 phosphorus at 2.19 A, with a more distant aluminium being present at 3.21 A Halide loss from the nickel centre is further confirmed by the bromine K-edge EXAFS data These results implied Ni ⋯ Al interactions via alkyl bridges in the predominant solution species during catalysis. Phosphorus-31 NMR spectroscopy at –60 °C of the activated catalyst however showed at least four different species present. Halide loss from the nickel co-ordination sphere was also observed for the reaction of AlBr3 with [{Ni(η3-C3H5)Br}2] in toluene. The nickel K-edge EXAFS was best fitted by 9.0 carbons at 2.01 A, providing strong evidence for the existence of [Ni(η3-C3H5)(η6-C6H5Me)]+-AlBr4–.

Scanning and energy dispersive EXAFS studies of ethyl transmetallation in an alkene oligomerisation catalyst

Nickel K-edge XAS studies show that there is a rapid alkyl and β-diketonate exchange between [Ni(acac)2] and AlEt2(OEt) with [NiR(alkene)(acac)] as the main species present under catalytic conditions.

In situ sulfur K-edge X-ray absorption spectroscopy of the reaction of zinc oxide with hydrogen sulfide

In situ sulfur K-edge XAS shows that the reaction of ZnO and H2S proceeds via island formation of ZnS.

EXAFS studies of the activation of homogeneous nickel catalysts for propene dimerisation by aluminium reagents

Ni K-edge EXAFS studies on propene dimerisation catalysts, formed by the interaction of solutions of NiCl2(PEt3)2 with Al2Me3Cl3 and AlEt3 at –40 °C, indicate a first coordination sphere of C and P atoms; Ni‥Al shells at ca. 2.97 A suggest Ni–C–Al interactions which probably stabilise the nickel centre.

Liquid-phase Cu(acac)2/ZnEt2 syngas conversion catalysts: investigations by copper and zinc K-edge EXAFS

Cu K-edge EXAFS studies on syngas conversion catalysts, generated by the reaction of Cu(acac)2(acac = acetylacetonato) with ZnEt2 at room temp., reveal the presence small Cu metal particles (ca. 10–25 A) in solution; Zn K-edge studies suggest the formation of oligomeric Zn species with oxygen present in the first coordination sphere.