The effect of Mg location on Co-Mg-Ru/gamma-Al2O3 Fischer-Tropsch catalysts

The effectiveness of Mg as a promoter of Co-Ru/γ-Al(2)O(3) Fischer–Tropsch catalysts depends on how and when the Mg is added. When the Mg is impregnated into the support before the Co and Ru addition, some Mg is incorporated into the support in the form of Mg(x)Al(2)O(3+x) if the material is calcined at 550°C or 800°C after the impregnation, while the remainder is present as amorphous MgO/MgCO(3) phases. After subsequent Co-Ru impregnation Mg(x)Co(3−x)O(4) is formed which decomposes on reduction, leading to Co(0) particles intimately mixed with Mg, as shown by high-resolution transmission electron microscopy. The process of impregnating Co into an Mg-modified support results in dissolution of the amorphous Mg, and it is this Mg which is then incorporated into Mg(x)Co(3−x)O(4). Acid washing or higher temperature calcination after Mg impregnation can remove most of this amorphous Mg, resulting in lower values of x in Mg(x)Co(3−x)O(4). Catalytic testing of these materials reveals that Mg incorporation into the Co oxide phase is severely detrimental to the site-time yield, while Mg incorporation into the support may provide some enhancement of activity at high temperature

Novel cobalt zinc oxide Fischer-Tropsch catalysts synthesised using supercritical anti-solvent precipitation

Cobalt zinc oxide catalysts have been prepared by anti-solvent precipitation in supercritical CO2 and investigated for CO hydrogenation. Here we show how the textural and catalytic properties of the catalyst can be tailored by the addition of water to the initial solution of cobalt and zinc acetates in methanol. Characterization of the catalysts by powder X-ray diffraction, infra-red and Raman spectroscopy showed that in the absence of water a high surface area mixed acetate was produced which upon calcination formed wurtzite type Zn1−xCoxO and spinel type ZnxCo3−xO4. The addition of 5 vol.% water resulted in a phase separated Co3O4/ZnO catalyst and enhanced active cobalt surface area as a result of disruption of the solvent/CO2 phase equilibrium during precipitation

Simple method to synthesize high surface area magnesium oxide and its use as a heterogeneous base catalyst

High surface area magnesium oxide (∼250–300 m2 g−1) was synthesized via the thermal decomposition of different precursors including (MgCO3)4Mg(OH)2, Mg(OH)2, MgCO3 and MgC2O4. The high surface area MgO displayed exceptionally high catalytic activity for the liquid phase Meerwein–Ponndorf–Verley (MPV) reaction of benzaldehyde with different alcohols. The effect of the calcination temperature and precursor source on the catalytic activity and morphology of MgO has been investigated in detail. It was found that the optimum calcination temperature was 450 °C leading to a high surface area material containing no MgCO3 which is the key to obtaining high catalytic activity of MgO for the MPV reaction. The high area MgO catalyst could be reused without loss of catalyst activity. The simplicity of the synthesis method gives this methodology widespread applicability in countless applications that currently use homogeneous base catalysts or stoichiometric bases as reagents

Enhanced selective glycerol oxidation in multiphase structured reactors

Gold catalysis has potential for multiphase oxidation using molecular oxygen but hitherto has been evaluated only in batch autoclaves. The use of continuous flow structured reactors for the oxidation of glycerol under mild conditions using a gold/carbon catalyst is demonstrated. Both monolith and meso-scale structured downflow slurry bubble column designs lead to a significant enhancement in the reaction rate over autoclave studies which is an order of magnitude greater for the gold/carbon coated monolith, and two orders of magnitude for the meso-scale structured downflow slurry bubble column reactor. A change in the reaction products is observed between structured monolith reactor and meso-structured slurry bubble column reactor. The monolith and autoclave data show very high selectivity to glyceric acid, whereas the thin channel slurry bubble column yields approximately equal quantities of dihydroxyacetone and glyceric acid under similar conditions. Since the same batch of catalyst is used for both the autoclave and the meso-scale structured downflow slurry bubble column reactor, the difference in selectivity cannot be attributed to a particle size effect. The difference in selectivity is attributed to enhanced interaction between bubbles and particles from structuring gas-liquid-solid reacting flows in thin channels or capillaries. \ua9 2008 Elsevier B.V. All rights reserved

Ceria prepared using supercritical antisolvent precipitation: a green support for gold-palladium nanoparticles for the selective catalytic oxidation of alcohols

CeO2 has been prepared from an acetate precursor by an antisolvent precipitation technique using supercritical CO2. The supercritically synthesized ceria support was used to prepare Au–Pd based catalysts for the selective oxidation of alcohols in solvent-free conditions using molecular oxygen as oxidant. The supercritically precipitated catalyst demonstrated high activity for alcohol oxidation, and it was much more active than catalysts prepared using a CeO2 support derived from the acetate through a non-supercritical synthesis route. The bimetallic Au–Pd supported catalyst was considerably more active than monometallic catalysts containing Au and Pd only. HAADF imaging and STEM–XEDS mapping showed that both Au and Pd metallic components were intimately mixed and uniformly highly dispersed over the supercritical nanocrystalline CeO2 support spheres. In contrast, the Au–Pd catalyst on the non-supercritical CeO2 support showed discrete uniform Au–Pd alloy particles with a size range of 50 to 150 nm. The homogeneous alloy particles were Au-rich and Pd-deficient as compared with the preparation ratio and a low number of highly dispersed Pd was also associated with the support. XPS data for the ceria supported catalysts confirmed the differences of metal dispersion and identified that in both cases the surface species present were Au0 and Pd2+. On extended re-use the non-supercritical Au–Pd/CeO2 catalyst showed sequential deactivation. On the contrary, the supercritical Au–Pd/CeO2 catalyst showed a significant increase of activity, and it was only during the third re-use that the activity was marginally lower than the fresh catalyst. After use the spherical morphology of nanocrystalline supercritical CeO2 started to increasingly break down, until the morphology started to resemble the CeO2 prepared by the non-supercritical route. Simultaneously there was an increase of the metal particle size on the supercritically prepared CeO2 support, as discrete Au-rich and Pd-rich bimetallic particles were formed, and there was a decrease of the metal content. XPS confirmed the loss of metal on use and showed that there was reduction of the ceria surface during use. The increase of activity with a corresponding loss of metal components indicates that the fresh catalyst contains Au and Pd in inactive forms, whilst the active sites have extremely high turnover frequencies

Pulsed-Field Gradient NMR Spectroscopic Studies of Alcohols in Supported Gold Catalyststs

We report a pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopic study of the effective diffusivity of alcohols in catalysts comprising gold supported on silica, titania and ceria and gold-palladium alloy nanoparticles supported on titania. These catalysts are shown to be highly active for the selective oxidation of alcohols. However, we observe that molecules possessing hydroxyl functional groups in the 2-position exhibit very low reactivities. To help understand the nature of conversion and selectivity, we observe from traditional catalytic measurements involving gas chromatography of the reaction mixtures, we have studied the effective self-diffusivities, Deff, of 1-, 2-, and 3-octanols and 1,2- and 1,4-butanediols in Au-ceria, Au-silica, Au-titania, and Au-Pd-titania using PFG-NMR spectroscopy. The results show that the octanols diffuse approximately 35 slower on silica supports than on titania. In addition, a marked two-component diffusive behavior is seen for ceria-supported catalysts with the dominant component, for 1-, 2-, and 3-octanols, being close to that of the free bulk liquid, and the slower component being an order of magnitude slower. The values of the 1,2- and 1,4-butanediol self-diffusion coefficients for silica-based gold catalysts are closer to those of the bulk liquid 1,2- and 1,4-butanediols. Au-Pd-titania also showed reduced self-diffusivities when compared with the bulk liquids but were similar to their monometallic counterparts. A new parameter, ?, the PFG-NMR interaction parameter, is introduced and is defined as the ratio of free liquid diffusivity to effective liquid diffusivity within the porous medium and accounts, collectively, for the functional group interaction of the probe molecule with itself and the porous medium. This parameter, along with reference tortuosity values determined by PFG-NMR gives new insight into the dynamics of hydrogen-bonded networks of different functional groups that exist within the porous catalyst matrix. The inhibition effect observed from traditional catalytic activity studies for the oxidation of 2-octanol is considered to result from competitive adsorption of the ketone product. © 2010 American Chemical Society

Solvent-free oxidation of primary carbon-hydrogen bonds in toluene using Au-Pd alloy nanoparticles

Selective oxidation of primary carbon-hydrogen bonds with oxygen is of crucial importance for the sustainable exploitation of available feedstocks. To date, heterogeneous catalysts have either shown low activity and/or selectivity or have required activated oxygen donors. We report here that supported gold-palladium (Au-Pd) nanoparticles on carbon or TiO2are active for the oxidation of the primary carbon-hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions. Differences between the catalytic activity of the Au-Pd nanoparticles on carbon and TiO2supports are rationalized in terms of the particle/support wetting behavior and the availability of exposed corner/edge sites

Preparation of Fischer–Tropsch supported cobalt catalysts using a new gas anti-solvent process

Cobalt and ruthenium-promoted cobalt Fischer–Tropsch catalysts supported on titania have been prepared for the first time by gas anti-solvent precipitation. The use of dense CO2 as an anti-solvent enables the precipitation of cobalt acetate and ruthenium acetylacetonate onto preformed titania. The gas anti-solvent process produces catalysts with the desired 20 wt % cobalt content as precursors, which on calcination give highly dispersed Co3O4. The addition of ruthenium to the gas anti-solvent prepared cobalt catalysts has been investigated by two methods (a) coprecipitation with cobalt acetate and (b) wet impregnation onto a precalcined cobalt titania catalyst, and these resulted in catalysts with distinctly different properties. These catalysts were compared with a standard ruthenium-promoted cobalt catalyst prepared by wet impregnation and were found to be substantially more active for the Fischer–Tropsch reaction