Diane Stirling

The location of nickel oxide and nickel in silica-supported catalysts: two forms of NiO and the assignment of temperature-programmed reduction profiles

Abstract The preparation of nickel catalysts supported on a range of silicas results in the formation of two distinct types of “NiO” which reduce at very different temperatures under temperature-programmed reduction (TPR) conditions. Examination of the effects of pore structure and experiments designed to concentrate the oxides in either the smaller (~9 nm) or the larger pores (15–30 nm) show that the more reducible oxide is located mainly in the small pores and the less reducible oxide in the large pores. The more reducible oxide resembles bulk NiO and has negligible interaction with the silica. The less reducible oxide is either in the form of crystallites so small as to make nucleation of the reduction to metal difficult or as surface nickel silicates or hydroxysilicates. Reoxidation of the reduced catalyst followed by TPR shows that the nickel oxide and nickel crystallites are immobilized in the pores of the silica at temperatures up to 600 °C.

Chiral Schiff base complexes of copper (II), vanadium (IV) and nickel (II) as oxidation catalysts. X-ray crystal structures of [Cu (R-salpn) (OH2)] and [Cu (±-busalcx)]

Abstract Several new complexes of chiral and achiral tetradentate Schiff bases with copper (II), nickel (II) and oxovanadyl (IV) ions have been synthesised and characterised and the structures of [Cu ( R -salpn) (OH 2 )] and [Cu (±-busalcx)] have been elucidated by X-ray analysis. [Cu ( R -salpn) (OH 2 )] is essentially square pyramidal with an unusually long bond [2.494 (4) A] from Cu to the oxygen of the apical water molecule. Molecules are linked by hydrogen bonding between the coordinated water and the salpn oxygens of neighbouring molecules. A long contact between a salpn CH 2 hydrogen and a neighbouring copper atom [H(3B). . .Cu = 2.966 (1) A] might be regarded as a CH···Cu hydrogen bond and completes a pseudo-octahedral coordination about copper. [Cu (±-busalcx)] displays tetrahedrally-distorted planar coordination about the copper atom and has no significant intermolecular contacts. The copper, nickel and vanadyl complexes were screened as homogeneous catalysts for the oxidation by Bu t OOH or H 2 O 2 of PhMeS to PhMeSO. All of them are active catalysts but of the chiral complexes only [Cu ( R , R -busalcx)] produced a reasonable enantiomeric excess (14%). Supporting the complexes on silica improved the enantioselectivity of the less sterically hindered [Cu ( R -salpn) (OH 2 )] ( from 2 to 10%), but diminished that of the more hindered [Cu ( R , R -busalcx)], probably by slowing the reaction to such an extent that the uncatalysed oxidation could compete effectively. Finally [Cu (salen)], [Cu ( R -salpn)] and [Ni ( R -salpn)] were synthesised in the pores of zeolites X and Y. These were shown to be active heterogeneous catalysts for the sulfide to sulfoxide oxidations, but no significant e.e. resulted from the use of the chiral catalysts.

Modified zinc oxide absorbents for low-temperature gas desulfurisation

The hydrogen sulfide absorption capacity of zinc oxide doped with first-row transtion-metal oxides (ca. 5% metal oxide loading) has been determined using a pulse reactor. The doped oxides were prepared either by impregnation of ZnO with the transition-metal nitrates or by comprecipitation of the transition-metal and zinc nitrates with ammonium/sodium carbonate. These absorbent precursors were then calcined to give the mixed oxides.Transmission electron microscopy studies of the impregnated and calcined absorbents revealed that the transition-metal oxides were finely dispersed over the ZnO as γ-Fe2O3, Co3O4 and CuO from the respective nitrate salts of these metals. The basal planes were the predominat exposed faces of the hexagonal ZnO in all the absorbents, irrespective of whether they were prepared by the coprecipitation or impregnation route. CuO and Co3O4 were not seen as separate phases in the respective calcined coprecipitated absorbents, but the particle morphology was noticeably changed after sulfidation and crystalline ZnS was detected by electron diffraction. Oxides prepared by the coprecipitation route had higher surface areas and a greater capacity for H2S removal than their impregnated counterparts. Doping with iron salts had little effect on the H2S uptake of ZnO, irrespective of whether an impregnation or a coprecipitation route had been used, but doping with copper or cobalt salts resulted in a marked enhancement in the H2S uptake in each case.The reaction of the mixed oxides with H2S was restricted to ca. 0.6 monolayers on average, based on calculations from surface areas, and is therefore largely confined to the surface of the oxides. However, the resulting sulfide is not present entirely as an adsorbed layer; some reaction occurs locally in depth to form ZnS crystallites sufficiently large to give an electron diffraction pattern. The main role of the transition-metal oxide is to increase the total surface area available for reaction with H2S.

Cobalt-zinc oxide absorbents for low temperature gas desulfurisation

The hydrogen sulfide absorption capacity of a series of cobalt-zinc oxides with nominal Co/Zn atomic ratios of 0/100, 10/90, 20/80, 30/70, 40/60, 50/50, 70/30, 90/10 and 100/0 was determined using a continuous flow absorption apparatus. The reaction of the mixed oxides with H 2 S amounted to ca. 3 monolayers, and is therefore largely confined to the surface of the oxides. The sulfur uptake was found to be proportional to the surface area of the oxides with a Co/Zn ratio ≤40/60, indicating that lattice diffusion played a major role in the rate determining step, and that the main function of the cobalt was to increase the surface area. At high cobalt concentrations, the sulfur uptake increased more than proportionately with surface area and the reaction was virtually stoichiometric for the oxide with a Co/Zn ratio of 100/0. This was associated with a change in the oxide structure from a bulk biphasic ZnO and Co 3 O 4 absorbent with a ZnCo 2 O 4 surface spinel at Co/Zn ratios ≤30 to a monophasic zincian or pure Co 3 O 4 structure at higher cobalt loadings. Analysis of the sulfided mixed oxides showed that microcrystalline membraneous sheets containing cobalt, zinc and sulfur developed on sulfiding. XPS studies of the sulfided oxides indicated that H 2 S reduced the surface spinel found at Co/Zn ratios ≤30/70 and the zincian/pure Co 3 O 4 found at higher cobalt concentrations to CoO and ZnO prior to the formation of their sulfides. The results are interpreted in terms of a surface reconstruction occurring during sulfiding.

Chiral phosphinoylalcohol complexes of monooxobis(peroxo)molybdenum(VI) and their use as asymmetric oxidants

Abstract Complexes of the type [MoO(O 2 ) 2 (L*)(ROH)], where L*=chiral β-phosphinoylalcohol, have been synthesised and used as stoichiometric oxidants for a number of unfunctionalised alkenes. In all of the complexes the chiral auxiliary is bound through the phosphinoyl oxygen as a monodentate ligand. The coordination about the metal atom in these pseudo-pentagonal bipyramidal molecules is completed by a solvent molecule (ethanol/water) lying opposite the axial MoO bond. Oxidation of small-chain non-functionalised alkenes occurs in variable yield to give epoxides with an enantiomeric excess of up to 40%. These compounds also behave as catalysts for the Bu t OOH oxidations of alkenes, but with similar modest enantioselectivities. The modest enantioselectivities are explained on the basis of the mode of coordination of the chiral ligand, and it is argued that there may be inherent limits in the use of these systems in asymmetric oxidations.

Characterisation of cobalt–zinc hydroxycarbonates and their products of decomposition

A series of cobalt–zinc hydroxycarbonate precursors with nominal Co/Zn atomic ratios of 0/100, 10/90, 20/80, 30/70, 40/60, 50/50, 70/30, 90/10 and 100/0 have been synthesized from their mixed metal nitrates and ammonium carbonate by a coprecipitation route. X-Ray and electron diffraction studies of the precursors revealed that hydrozincite, Zn 5 (CO 3 ) 2 (OH) 6 , was the major phase at Co/Zn ratios≤30/70 and spherocobaltite, CoCO 3 , predominated at Co/Zn ratios of 50/50 to 90/10. The Co/Zn 100/0 precursor formed only the metastable basic carbonate Co(CO 3 ) 0.5 (OH) 1.0 0.1H 2 O. UV–VIS–NIR diffuse reflectance spectroscopy revealed that the cobalt was present in the 2+ oxidation state in an octahedral environment in all the precursors. Decomposition of the Co/Zn precursors at 350 °C resulted in the formation of ZnO as the major phase at low Co loadings and Co 3 O 4 as the major phase at high loadings. The highest surface areas were attained from the decomposition of basic cobalt carbonate or spherocobaltite containing little or no zinc in solid solution. XPS studies of the oxides revealed that only Co 3+ and Zn 2+ ions were present at the surface at Co/Zn ratios≤30/70 indicating the presence of the ‘surface spinel’, ZnCo 2 O 4 . Co 2+ was detected at higher Co loadings.

Mixed Co–Zn–Al oxides as absorbents for low-temperature gas desulfurisation

High-surface-area zinc–cobalt–aluminium oxides have been prepared from coprecipitated hydroxycarbonate precursors. The oxides were tested for their H2S uptake in a microreactor at 303 K. The precursors all adopted a hydrotalcite-type structure, whilst the oxides were also all single-phase materials. Their structures were either based on that of ZnO or Co3O4, which suggested the presence of solid solutions with all the ions dissolved into one phase. A comparison of the H2S uptake of these materials with that of the Co–Zn oxides suggested that the presence of aluminium ions in the oxide gave rise to an increase in their surface areas, but that the modified compounds did not absorb significantly more H2S. Indeed, the more Co-rich cobalt–zinc–aluminium oxides absorbed less H2S than Co–Zn oxides prepared in a similar way.

Mixed cobalt–iron oxide absorbents for low-temperature gas desulfurisation

High surface area cobalt–iron oxides (with Co ∶ Fe ratios of [3 ∶ 1], [1 ∶ 1] and [1 ∶ 2]) have been prepared by the calcination of predominantly single-phase, coprecipitated hydrotalcite-type precursors prepared from mixed-Co2+–Fe3+ aqueous solutions. The oxide structures were all spinels; for the Co–Fe [3 ∶ 1] oxide, this was a Co3O4-type lattice, for the Co–Fe [1 ∶ 2] sample, it was a Fe3O4-type lattice and for Co–Fe [1 ∶ 1] an equal mix of the two was observed. The H2S uptake of all the oxides was tested at 303 K and the data correlated with sorbent preparation and characterisation. The Co–Fe [1 ∶ 1] oxide absorbed the most H2S before breakthrough and further studies into the preparation of this oxide showed that nitrate-free precursors produced at pH 7 and then aged for 30 minutes or more gave rise to the best oxide sorbents after calcination at 623 K rather than 723 K. These observations have been rationalised on the basis that these oxides had lower density and a greater mix of Co2+/3+ and Fe2+/3+ leading to more distortion of the parent oxide lattice. Increased H2S uptake was ascribed to more rapid ion diffusion through the sorbent lattice leading to enhanced replenishment of surface oxide.

Characterisation of catalysts and their precursors prepared from supported palladium phosphine complexes

Abstract Hydrogenation catalysts prepared from the silica-supported binuclear palladium complexes [Pd2X4(PR3)2] (X=Cl, Br or I, and R=Me, Et and, for X=Br only, Pri and Bu) and mononuclear complexes [PdX2(PR3)2] (X=Cl, Br or I, and R=Me, Et) have been examined throughout their life cycle (induction period, selectively active catalyst and deactivation) by XPS, solid state 31 P NMR spectroscopy and transmission electron microscopy, and their performance and activities compared to a simple Pd/SiO2 catalyst. The binuclear chloro complexes partially react with the silica surface producing some Pd(0) species. The others retain their integrity until contacted by the substrate and hydrogen. The selective solution hydrogenation of cinnamaldehyde to hydrocinnamaldehyde appears to take place at oxidised palladium sites, most likely dipalladium(I) monophosphine species. The formation of phenylpropanol by some of the catalysts, on the other hand, occurs at Pd(0) sites. The induction period before the catalysts become active results from an autocatalytic process during which the Pd(II) precursors are converted to the active, Pd(I) or Pd(0) species. The catalysts are finally deactivated by a combination of continued conversion of the active species to a new, phosphorus-poisoned Pd(0) material and the restructuring of a hydrocarbonaceous overlayer.

Structural and morphological studies of iron sulfide

Treatment of high surface area feroxyhyte (δ′-FeOOH) with hydrogen sulfide, at room temperature, yielded mackinawite (FeS) and amorphous sulfur. On exposure to air, the sulfided material underwent an exothermic reaction during which crystalline sulfur and an amorphous hydrated iron oxide species were formed. On leaving this material in air for a period of days crystalline goethite (α-FeOOH) was formed. These chemical and structural changes were accompanied by alterations in the morphologies of the materials, all of which occurred at, or close to, room temperature. These differences are important for two reasons. First, they are of relevance to the sulfiding of metal oxides and secondly they involve high-melting-point ionic lattices undergoing complete reorganisation at close to ambient temperature.