Daniele Olivier

Dimerization of olefins with nickel-surface complexes in X-type zeolite or on silica

EPR spectroscopy shows that monomeric paramagnetic Ni(I) species, dispersed in X-type zeolite or on SiO2, can reversibly bind ligands (L) such as CO, C2H4 and C3H6 to lead to various Ni(L)+n species, depending on the pressure. Under low pressure (<40 torr), tetrahedral Ni(L)+ and square planar Ni(L)+2 entities are obtained. At higher pressure, Ni(I) has a trigonal bipyramid structure and forms Ni(L)+3 and Ni(L)+4 complexes. This stereochemistry implies in each case a given number of surface O2−s acting as ligand. The 17- and 19-electron Ni(I) complexes easily undergo monosubstitution reactions in their coordination sphere, while addition reactions are observed for 17-electron complexes. At 298 K the supported Ni(I) ions are active and selective for ethylene and propylene dimerization. After a pretreatment at 973 K, the silica-supported catalyst leads to trimerization together with dimerization. Via poisoning experiments, it is shown that the case of two olefin molecules bonded to each Ni(I) favours dimerization.

Dimerization of ethylene into 1-butene over supported tailor-made nickel catalysts

Abstract Four, two, or one L = PEt 3 ligands are selectively introduced into the coordination sphere of silica-supported Ni(I) ions. Reflectance and EPR spectra are given. The five-coordinate species Ni I L 4 (O s ) are inactive in C 2 H 4 dimerization. The square planar Ni I L 2 (O 5 ) 2 and the unsaturated Ni I L(O s ) 2 species are active, stable and selective at 40 °C. The best results are observed with two PEt 3 ligands: at a conversion of 23%, a selectivity of 95% for 1-butene is obtained. The turnover rates ( v ) are about 0.1 s −1 . From EPR studies a reaction mechanism is proposed. An intermediate metallacyclopentane is characterized. It is shown that the presence of basic phosphine ligands increases the electron density on nickel and allows an easier desorption of the primary reaction products.

Characterization by UV-vis-NIR reflectance spectroscopy of the exchange sites of nickel on silica

Abstract The coordination of Ni2+ ions in Ni/SiO2 catalysts has been studied by UV-vis-NIR reflectance spectroscopy at various stages of the preparation, drying, and calcination followed by rehydration. The catalysts were prepared by competitive cation exchange from an ammoniacal nickel solution. The nickel is first exchanged as a hexammine complex on the silica surface via a purely electrostatic mechanism at pH 9.8 or 10.4. Upon washing with water, substitution reactions occur in the coordination sphere of Ni2+ ions, which remain in octahedral symmetry and become grafted to the silica surface via ligand displacement with surface oxygen. The so formed [Ni(≡SiO)2L4] surface complexes contain ligans L from the solution (H2O or NH3) and ≡SiO mononegative ligands from the support forming ionic bonds with Ni2+. Distortion of the grafted nickel from octahedral symmetry increases when H2O substitutes for NH3 ligands. This is attributed to hydrogen bonds between the ligands L and the unexchanged ≡SiO− surface groups. From these results, a model of the exchange sites and the exchange mechanism are proposed.

Preparation and characterization of silica-supported propylene dimerization catalysts: Ni(I) trialkylphosphine complexes

Trialkylphosphines (TAP) are able to reduce silica-supported unsaturated Ni(II) cations selectively into Ni(I) at room temperature. This chemical reduction is very convenient because it leads to high amounts of Ni(I) while affording the required ligands to build the propylene oligomerization active entity. The highest reduction degree (60%) is achieved with low nickel content exchanged silicas (τ < 2% (w/w)) mainly constituted of isolated three-coordinate Ni(II) cations before reduction. In high nickel content samples, the presence of Ni(II) paired through an oxygen bridge seems to limit reduction by TAP as well as photoreduction. The preparation of NiI(PR3)n (n = 1,2, 3, 4 when R = CH3 or C5H5, and n = 1, 2, 3 when R = c-C6H11or C6H5) is reported. EPR and reflectance spectroscopy parameters of the complexes are given as well as some molecular models. From the IR study, two types of hydrogen bond between the trialkylphosphine and the surface hydroxyl groups are evidenced: (i) one is reversibly formed between the phosphorus doublet of TAP adsorbed on the support and a hydrogen atom of a surface OH group; only P(c-C6H11)3 is basic enough to lead to phosphonium ions. (ii) the other occurs between the alkyl groups of the TAP bonded to the Ni(I) and the surface hydroxyl groups, and could thus explain the distorted symmetry of the unsaturated complexes.

EPR study of temperature dependent rotations of polytetrafluoroethylene chains labeled with peroxy radicals

The temperature dependence of the EPR spectra of peroxy chain radicals (R–CF2–CFOO  –CF2–R′) and peroxy propagating radicals (R″CF2–CF2OO ) trapped in polytetrafluorothylene (PTFE) is studied in the range −196 °C, +300 °C, and analyzed in terms of the rotation freedom in the polymer. The peroxy propagating radical undergoes rotation along the polymer chain axis above −120 °C, whereas the peroxy chain radical has the same rotation but only above −20 °C. This difference is explained in terms of a particular mobility of the extremity of the carbon chain. Above −40 °C, the peroxy propagating radical rotates about the C–O bond in addition to the rotation about the chain axis. The additional rotation about the C–O bond is not observed below the decomposition temperature of the peroxy chain radical.

Ferromagnetic resonance study of dispersed nickel particles prepared by reduction of Ni2+-exchanged X zeolites by hydrogen molecules or hydrogen atom beams

Hydrogen atom beams have been used to prepare nickel particles at very low temperatures by reduction of nickel ions exchanged in X-type zeolite. The metal particles formed are characterised using ferromagnetic resonance (f.m.r.) and compared with those obtained by conventional thermal reduction using molecular hydrogen. Studies of the f.m.r. line-shape show that lorentzian and symmetrical lines recorded above 473 K are associated with small, homodispersed metal particles, while heterodispersion results in broadening of the symmetrical lines. Particles highly homodispersed in the zeolite supercages can be obtained if, prior to reduction at 273 K by hydrogen atom beams, a migration of Ni2+ ions is induced by treatment at 373 K in carbon monoxide. In the case of Ni10CaX zeolite, the degree of reduction is 100 % and nickel particles of diameter 10 A are obtained.

An EPR study of the reactivity of peroxy radicals trapped in polytetrafluoroethylene

The effect of CO, C2H4, SO2, and 17O2 on the EPR spectrum of peroxy chain radicals (R–CF2–CFOO.–CF2R′) and of peroxy propagating radicals (R″–CF2OO.) of polytetrafluoroethylene (PTFE) has been investigated. The peroxy chain radicals appear chemically stable up to 100 °C while in the same conditions, the peroxy propagating radicals do react. The transformation of peroxy propagating radicals always takes place through the rupture of the C–O bond. In the reaction with SO2, evidence is presented for the formation of R″–CF2–SO2. radical with gxx=1.999, gyy=2.011, gzz=2.005 at 77 K and g∥=2.007, g⊥=2.002 at 300 K. The temperature dependence of the EPR spectra is analyzed in view of the rotational freedom in the polymer. The R″–CF2–SO2. undergoes rotation around the polymer chain axis. From these data the structure of the radical is discussed.

Characterization of supported NiICO(TAP)n(n < 3) complexes by the CO stretching vibration. Electron-donating effect of trialkylphosphine ligands

The i.r. characterization of supported NiI trialkylphosphine (TAP) complexes NiI(TAP)n(n < 3) is performed using a CO probe molecule. The affinities of NiII and NiI ions for CO are compared. It is evidenced that TAP is a stronger ligand than CO: NiI(TAP)n(n < 3) complexes cannot coordinate more than one CO ligand at low CO pressure, whereas phosphine-free NiI complexes can coordinate two CO molecules. The increase of electron density on the metal ion causes a shift downwards of the νco frequency, proportional to the number and to the basicity of TAP ligands. It also produce a weakening of the M—CO bond, resulting in the easier removal of CO.