Selective liquid phase hydrogenation of citral on Au/Fe2O3 catalysts

Photochemical reduction of nanocrystalline maghemite to magnetite

We present a method for thephotochemical conversion of the inverse spinel iron oxides in which the mixed-valent magnetite phase (Fe₃O₄) is accessed from the maghemite phase (γ-Fe₂O₃) via a stable, colloidal nanocrystal-to-nanocrystal transformation. Anaerobic UV-irradiation of colloidal γ-Fe₂O₃ nanocrystals in the presence of ethanol as a sacrificial reductant yields reduction of some Fe³⁺ to Fe²⁺, resulting in a topotactic reduction of γ-Fe₂O₃ to Fe₃O₄. This reduction is evidenced by the emergence of charge-transfer absorption and increased d-spacing in UV-irradiated nanocrystals. Redox titrations reveal that ∼43% of Fe in <d> = 4.8 nm nanocrystals can be reduced with this method and comparison of optical data indicates similar reduction levels in <d> = 7.3 and 9.0 nm nanocrystals. Addition of excess acetaldehyde during photoreduction shows that the extent of reduction is likely pinned by the hydrogenation of acetaldehyde back to ethanol and can be increased with the use of an alkylborohydride sacrificial reductant. Photochemical reduction is accompanied by increased magnetization and emergence of magnetic features characteristic of Fe₃O₄. Overall, this work provides a reversible, post-synthetic strategy to obtain Fe₃O₄ nanocrystals with well-controlled Fe²⁺ compositions.

Selective Liquid Phase Hydrogenation of Benzaldehyde to Benzyl Alcohol Over Alumina Supported Gold

Abstract

We report for the first time 100% benzyl alcohol yield from the liquid phase (T = 353 K, P = 9 bar) hydrogenation of benzaldehyde over Au/Al2O3. Under the same reaction conditions, a benchmark Pt/Al2O3 catalyst promoted the formation of toluene and benzene as hydrogenolysis by-products. Reaction kinetics was subjected to a Hammett treatment and the reaction constant (ρ = 0.9) was found to be consistent with a nucleophilic mechanism. A solvent (alcohol, water and alcohol + water) effect is demonstrated and ascribed to competitive adsorption where solvation by polar (water) facilitates benzaldehyde activation.

Graphic Abstract

The Low Temperature Solvent-Free Aerobic Oxidation of Cyclohexene to Cyclohexane Diol over Highly Active Au/Graphite and Au/Graphene Catalysts

The selectivity and activity of gold-catalysts supported on graphite and graphene have been compared in the oxidation of cyclohexene. These catalysts were prepared via impregnation and sol immobilisation methods, and tested using solventless and radical initiator-free reaction conditions. The selectivity of these catalysts has been directed towards cyclohexene epoxide using WO3 as a co-catalyst and further to cyclohexane diol by the addition of water, achieving a maximum selectivity of 17% to the diol. The sol immobilisation catalysts were more reproducible and far more active, however, selectivity towards the diol was lower than for the impregnation catalyst. The results suggest that formation of cyclohexane diol through solventless oxidation of cyclohexene is limited by a number of factors, such as the formation of an allylic hydroperoxyl species as well as the amount of in situ generated water.

Au/CNTs catalyst for highly selective hydrodeoxygenation of vanillin at the water/oil interface

Au/CNTs assembled at the interfaces of emulsion droplets exhibits good catalytic activity and 100% selectivity for the hydrodeoxygenation of vanillin to p-creosol.

Conversion of 5-hydroxymethylfurfural to a cyclopentanone derivative by ring rearrangement over supported Au nanoparticles

Supported Au nanoparticles showed efficient catalytic performance for the ring rearrangement of 5-hydroxymethylfurfural (HMF) to a cyclopentanone derivative, 3-hydroxymethylcyclopentanone (HCPN).

Theoretical tools for studying gold nanoparticles as catalysts for oxidation and hydrogenation reactions

In this contribution, the ability of small isolated gold NP to dissociate O2 and generate a reactive surface oxide layer, the nature of the new gold active sites generated, and their implication in the mechanism of alcohol oxidation to aldehydes has been analyzed from a theoretical point of view. The nature of the active sites involved in H2 dissociation and the possible ways in which Au/TiO2 catalysts can be modified in order to increase their activity toward hydrogenation of nitroaromatics without modifying their high chemoselectivity is also explored.

Discrete iron complexes for the selective catalytic reduction of aromatic, aliphatic, and α,β-unsaturated aldehydes under water-gas shift conditions

Iron-catalyzed reductions: Selective iron-catalyzed reduction of aldehydes with hydrogen generated in situ by the water-gas shift reaction is presented (see scheme). The generality and selectivity of this mild procedure are demonstrated by the efficient reduction of various aromatic, aliphatic and α,β-unsaturated aldehydes.

Kinetic study of oxygen adsorption over nanosized Au/γ-Al₂O₃ supported catalysts under selective CO oxidation conditions

O₂ adsorption is a key process for further understanding the mechanism of selective CO oxidation (SCO) on gold catalysts. Rate constants related to the elementary steps of O₂ adsorption, desorption and surface bonding, as well as the respective activation energies, over a nanosized Au/γ-Al₂O₃ catalyst, were determined by Reversed-Flow Inverse Gas Chromatography (RF-IGC). The present study, carried-out in a wide temperature range (50–300 °C), both in excess as well as in the absence of H₂, resulted in mechanistic insights and kinetic as well as energetic comparisons, on the sorption processes of SCO reactants. In the absence of H₂, the rate of O₂ binding, over Au/γ-Al₂O₃, drastically changes with rising temperature, indicating possible O₂ dissociation at elevated temperatures. H₂ facilitates stronger O₂ bonding at higher temperatures, while low temperature binding remains practically unaffected. The lower energy barriers observed, under H₂ rich conditions, can be correlated to O₂ dissociation after hydrogenation. Although, H₂ enhances both selective CO reactant’s desorption, O₂ desorption is more favored than that of CO, in agreement with the well-known mild bonding of SCO reactant’s at lower temperatures. The experimentally observed drastic change in the strength of CO and O₂ binding is consistent both with well-known high activity of SCO at ambient temperatures, as well as with the loss of selectivity at higher temperatures.

Origin of the different activity and selectivity toward hydrogenation of single metal Au and Pt on TiO2 and bimetallic Au-Pt/TiO2 catalysts

To improve the activity of Au/TiO(2) catalysts toward hydrogenation without decreasing chemoselectivity, a number of isolated and supported Au, Pt, and Au-Pt nanoparticles have been investigated by means of density functional theory (DFT) calculations. H(2) dissociation on Pt and Au-Pt nanoparticles is considerably faster than that on Au, because H(2) adsorption on Pt atoms is dissociative and no activation barriers are involved. The high chemoselectivity of Au/TiO(2) catalysts does not exist in Pt/TiO(2) catalysts no matter how small the Pt particles are, but can be preserved in Au-Pt/TiO(2) catalysts if the Au/Pt ratio is high enough as to keep Pt atoms isolated and not at the active metal-support interface.

Binary Au/MWCNT and ternary Au/ZnO/MWCNT nanocomposites: synthesis, characterisation and catalytic performance

Gold nanoparticles of 10-24 and 5-8 nm in size were obtained by chemical citrate reduction and UV photoreduction, respectively, on acid-treated multiwalled carbon nanotubes (MWCNTs) and on ZnO/MWCNT composites. The shape and size of the deposited Au nanoparticles were found to be dependent upon the synthetic method used. Single-crystalline, hexagonal gold particles were produced in the case of UV photoreduction on ZnO/MWCNT, whereas spherical Au particles were deposited on MWCNT when the chemical citrate reduction method was used. In the UV photoreduction route, n-doped ZnO serves as the e(-) donor, whereas the solvent is the hole trap. All materials were fully characterised by UV/Vis spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and BET surface analysis. The catalytic activity of the composites was studied for the selective hydrogenation of alpha,beta-unsaturated carbonyl compound 3,7-dimethyl-2,6-octadienal (citral). The Au/ZnO/MWCNT composite favours the formation of unsaturated alcohols (selectivity=50% at a citral conversion of 20%) due to the presence of single-crystalline, hexagonal gold particles, whereas saturated aldehyde formation is favoured in the case of the Au/MWCNT nanocomposite that contains spherical gold particles.

Sustainable preparation of supported metal nanoparticles and their applications in catalysis

Metal nanoparticles have attracted much attention over the last decade owing to their unique properties as compared to their bulk metal equivalents, including a large surface-to-volume ratio and tunable shapes. To control the properties of nanoparticles with particular respect to shape, size and dispersity is imperative, as these will determine the activity in the desired application. Supported metal nanoparticles are widely employed in catalysis. Recent advances in controlling the shape and size of nanoparticles have opened the possibility to optimise the particle geometry for enhanced catalytic activity, providing the optimum size and surface properties for specific applications. This Review describes the state of the art with respect to the preparation and use of supported metal nanoparticles in catalysis. The main groups of such nanoparticles (noble and transition metal nanoparticles) are highlighted and future prospects are discussed.