Maggy Kermarec

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.

EPR study of the stability and the role of the O2− species on La2O3 in the oxidative coupling of methane

Abstract The paramagnetic superoxide ion detectable by EPR spectroscopy, can be generated on fully decarbonated La2O3 by oxygen adsorption. After pretreatment in vacuo at 650°C, oxygen may be admitted at two different temperatures: i) at room temperature: all the O2− ions are located on the La2O3 surface; ii) at 650 °C: 85% of the O2− species are on the surface and 15% are between the (LaO)22+ layers of La2O3. The O2− radicals are no longer observed by EPR after the oxidative coupling of methane reaction. The study of their stability in the presence of the reactant gases and oxidation products of the reaction, shows that the main reason for the disappearance of the O2− species is that they are unstable under the low oxygen partial pressure of the gas feed (15 Torr). It appears unlikely that the O2− species are the active sites for the activation of methane.

Coordination chemistry involving oxide catalysts

Abstract The application of various spectroscopic techniques to investigate the coordination of transition metal ions is reviewed using examples which include the preparation of highly dispersed catalysts, the investigation of the coordination sphere of supported transition metal ions on oxide surfaces, the coordination sphere of catalytically active sites and the migration of transition metal ions into the bulk of oxides during catalyst pretreatment. The features of coordination chemistry for supported transition metal ions appear to be very similar to their solution analogues and the same concepts can thus be applied.

EPR study of the stability of the O−2 species on La2O3 and of their role in the oxidative coupling of methane.

Abstract The O−2 superoxide species can be generated on fully decarbonated La2O3 in two different ways. They can be obtained by oxygen adsorption at room temperature after vacuum treatment at 650°C, in which case all the O−2 species are located on the La2O3 surface. They can also be formed by oxygen treatment at 650°C and in that case 15% of the O−2 species are located between the (LaO)2n+2n sheets of La2O3. The adsorption sites are probably vacancies in the coordination sphere of La3+. The O−2 radicals are not observed by EPR after the reaction of oxidative coupling of methane. The study of O−2 stability in the presence of the different reactant gases and products of the reaction shows that the main reason for the non-observation of the O−2 species is that they are unstable under the low oxygen partial pressure of the reactant gas feed (15 Torr). In consequence, the O−2 species are probably not the sites directly responsible for the hydrogen abstraction of methane in this reaction. The possible nature of the active sites is discussed.

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.

Temperature and conversion dependence of selectivities in the oxidative coupling of methane on La2O3 catalysts

Abstract The C 2 + selectivity has been found to be highly dependent on reaction conditions such as conversion of reactants and temperature. For all the La 2 O 3 catalysts investigated, the C 2 + selectivity decreases linearly with methane conversion at a given temperature. The C 2 + selectivity, plotted as a function of temperature and measured at isoconversion of methane, presents a maximum in the range 850–900°C. The best C 2 + yield is obtained for total conversion of oxygen at a temperature slightly higher than 850°C.

Silica-supported nickel(I)—benzene complexes resulting from acetylene cyclotrimerization

Abstract Two silica-supported Ni(I)—benzene complexes (I and II) have been prepared through acetylene cyclotrimerization. They have been characterized by electron paramagnetic resonance, infrared and diffuse reflectance spectroscopy. The following structures consistent with the spectroscopic data are proposed for these π complexes: I is a pseudotetrahedral complex where Ni(I) is bound to one benzene ligand and three surface oxygen atoms, whereas II is a distorted square planar complex where Ni(I) is bound to two benzene ligands and two surface oxygen atoms. In both complexes, benzene is acting as an η2 ligand.

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.

An i.r. study of carbon monoxide fixation by Ni+ ions obtained by photoreduction of a nickel-exchanged silica

It is shown by i.r. spectroscopy, using mixtures of 12CO and 13CO, that isolated Ni+ ions produced by u.v. irradiation in hydrogen at 77 K of an Ni-exchanged silica can form mono- or di-carbonyl adducts in the 10–5–5 Torr range.