Laura Prati

Covalent Triazine Framework as Catalytic Support for Liquid Phase Reaction

An important goal in the preparation of highly active supported metal particles is the enhancement of the metal support interaction, providing a more stable catalyst, especially for liquid phase reactions as the leaching and reconstruction of the active phase causes deactivation. In this work, a covalent triazine framework (CTF) as support for Pd nanoparticles is compared to activated carbon (AC), the typical support used in liquid phase reactions. The results indicate that the presence of the N-heterocyclic moieties on the surface of the frameworks is beneficial for improving the stability of Pd nanoparticles during the liquid phase glycerol oxidation. Pd/CTF showed better activity and in particular better stability when compared to Pd supported on activated carbon (AC).

Metal sols as a useful tool for heterogeneous gold catalyst preparation: reinvestigation of a liquid phase oxidation

Differently stabilised metal sols have been used as precursors in the preparation of heterogeneous gold catalysts for liquid phase oxidation in water solution. The methodology of sols generation appears to be fundamental to obtaining nanoparticles; the support, instead, plays an important role in maintaining particle dimension and morphology. Three different materials (γ-Al2O3, SiO2 and activated carbon) have been used as the supporting agents for different gold sols that were obtained by reducing HAuCl4 with NaBH4 in the presence of polyvinylalcohol (PVA) or polyvinylpirrolidone (PVP) and with the tetrakis(hydroxymethyl)phosphonium chloride (THPC)/NaOH system. During the immobilisation step, the maintenance of the particle dimension observed in solution depends on both the support and the type of sol. The gold particle mean size of the colloidal suspension is more easily maintained on oxidic supports than on carbon, the latter apparently needing both steric and polar stabilisation of the gold particle. Comparison of Au/γ-Al2O3 and Au/C catalyst activity in the liquid phase oxidation of ethylene glycol to glycolate highlighted the peculiarity of gold on carbon catalysts; in fact, the normally observed trend of reactivity is partially reversed, medium sized gold particle being the most active.

New gold catalysts for liquid phase oxidation

A pre-requisite for good catalytic activity with supported gold is that the metal is present as nanoparticles, but no suitable method for depositing gold on carbon as nanoparticles has previously been reported. The deposition precipitation method which has been used successfully for catalyst preparation with other supports, produces large aggregates with carbon. It is now shown, however, that pre-reduced gold sol is a satisfactory method for preparing gold on carbon catalysts as long as an appropriate choice of sol is made. In this paper the activity of gold on carbon catalysts prepared in a number of different ways is compared in a standardized liquid phase organic oxidation reaction. The activity for Au/C catalysts prepared by the sol method can show a two-fold increase compared with a similar catalyst prepared by deposition precipitation.

Selective Oxidation of Glycerol under Acidic Conditions Using Gold Catalysts

H-mordenite-supported PtAu nanoparticles are highly active and selective in the oxidation of glycerol under acidic conditions, which allows the direct preparation of free acids (see picture). The high selectivity for C{sub 3} compounds results from the negligible formation of H{sub 2}O{sub 2}, in contrast to PtAu nanoparticles supported on activated carbon.

Application of gold catalysts to selective liquid phase oxidation

Abstract New applications of gold catalysts for selective oxidation of organic molecules are reported. All reactions investigated were performed using molecular oxygen in aqueous solution under mild conditions. Polyhydroxylated aliphatic molecules can be oxidised to monocarboxylates with high selectivity towards the primary alcoholic group in the presence of alkali, whereas the phenyl group enhances the reactivity of a benzylic alcoholic group as it limits the selectivity to mandelate starting from phenyl-1,2-ethanediol. α- and β-aminoalcohols react slowly with oxygen in the absence and quickly in the presence of alkali to produce the corresponding aminoacid derivatives. Aliphatic aldehydes and glucose are easily oxidised to free carboxylic acid. A comparison of gold catalysts and conventional Pd and Pt monometallic, bimetallic and tricomponent catalysts has, in some cases, been done.

Selective liquid phase oxidation using gold catalysts

Au/C and Au/oxide (Al2O3, TiO2) have been compared in the liquid phase oxidation of glycols and a different trend in reactivity revealed. On the oxides the activity of supported gold increases by decreasing particle size, whereas on carbon maximum activity is achieved with gold particle mean diameter around 7–8 nm. XPS revealed that in the latter case activity depends not only on the size of the gold particle but also on its surface concentration.

Pd‐modified Au on Carbon as an Effective and Durable Catalyst for the Direct Oxidation of HMF to 2,5‐Furandicarboxylic Acid

Mixed noblility: We show that the modification of a gold/carbon catalyst with platinum or palladium produces stable and recyclable catalysts for the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA): the support and nanoparticle chemistry directly mediate the selective oxidation of terminal hydroxyl groups in bio-derived HMF. This finding is a significant advance over current conversion technology because of the technological importance of FDCA.

Effects of gold nanoparticles deposition on the photocatalytic activity of titanium dioxide under visible light

The effects of gold nanoparticles deposited on titanium dioxide on the photocatalytic oxidative degradation of two organic substrates, i.e. formic acid and the azo dye Acid Red 1, and on the parallel O(2) reduction yielding hydrogen peroxide have been investigated under visible light irradiation. The method employed to reduce Au(iii) to metallic gold in the preparation of Au/TiO(2) photocatalysts was found to affect their photoactivity, also by modifying the properties of TiO(2). The presence of gold on TiO(2) facilitates both the electron transfer to O(2) and the mineralization of formic acid, which mainly proceeds through direct interaction with photoproduced valence band holes. The so-formed highly reductant CO(2)*(-) intermediate species may contribute in maintaining gold in metallic form. The controversial results obtained in the photocatalytic degradation of the dye were rationalised by taking into account that with this substrate, which mainly undergoes oxidation through a hydroxyl radical mediated mechanism, the photogenerated holes may partly oxidise gold nanoparticles, which consequently act as recombination centres of photoproduced charge carriers.

Triazine‐Based Polymers as Nanostructured Supports for the Liquid‐Phase Oxidation of Alcohols

A covalent triazine framework (CTF) was used as support for palladium nanoparticles (NPs) and Pd/CTF was applied as the catalyst in the selective oxidation of benzyl alcohol. N groups in the CTF appeared more efficient than those created on carbon nanotubes (CNTs) by NH(3) /high-temperature treatment in stabilizing Pd NPs against growth during the immobilization step. This assured a high metal dispersion, which led to a highly active and stable catalyst in the alcohol oxidation reaction. Indeed, Pd on the CTF was more stable in recycling than Pd on activated carbon (AC) and on nitrogen-doped CNTs, particularly avoiding leaching of Pd NPs. Moreover, Pd on the CTF was less sensitive than Pd on AC to the decrease of reactant concentration. This in turn led to a higher selectivity to benzaldehyde (98 %) with a considerable activity (turnover frequency 1453 h(-1) ).

Gold catalyzed oxidation of aldehydes in liquid phase

Gold on carbon oxidizes aldehydes to carboxylic acids in water solution under mild conditions without loss of activity on recycling, as does not occur for platinum on carbon. Also scarcely soluble aldehydes can be oxidized but the reaction rate is slower. Experiments carried out in an organic solvent such as CCl4 show a speed up of the reaction, in this solvent being recyclable both Au/C and Pt/C. Another explored possibility was represented by carrying out the reaction in the absence of solvent. However, for solid aldehydes like p- and o-OH-PhCHO the water solvent represent the unique alternative.