Development of new highly active nano gold catalysts for selective oxidation reactions

Gold catalysts have been found to be efficient for many oxidation reactions. The performance of these catalysts depends strongly on the particle size of the Au nanoparticles. However, other factors also have a strong influence on the catalytic activity such as the preparation method and choice of support. The effect of support and the preparation methods has been investigated with regard to the selectivity and the activity of Au catalysts. The disproportionation of benzyl alcohol has been identified as a source of toluene formation in the solvent free oxidation of benzyl alcohol using supported gold palladium catalysts. The disproportionation reaction of benzyl alcohol oxidation has been performed in a conventional glass stirred reactor. Oxidation and disproportionation reactions respond slightly differently to the changes in reaction parameters, such as oxygen concentration and pressure. When MgO supported gold-palladium catalysts were used for this reaction, the toluene selectivity reduced substantially at the cost of conversion. The synthesis of bimetallic nanoalloys is of great practical importance as they exhibit size-dependent functional properties, which can be exploited in fields such as catalysis. Conventional chemical impregnation routes for generating supported bimetallic nanoparticles are facile, but often generate materials having broad particle size distributions, which typically exhibit core–shell morphologies and significant compositional variations from particle-to-particle. More complex sol-immobilisation synthesis techniques offer much better control over particle size distribution, but retain stabilising ligands on the surface, which can be deleterious for catalysis. Here, a convenient excess anion modification and post reduction step prior to the impregnation method has been used, which permits the reproducible preparation of supported bimetallic AuPd nanoparticles with a tight particle size distribution comparable to that found for sol-immobilisation materials, but without the complication of ligands adsorbed on the particle surface. These advantageous features of the modified impregnation materials resulting in higher activity and stability compared to the catalysts prepared using both conventional impregnation and sol-immobilisation methods. Detailed STEM combined with EDX analyses of individual particles have revealed that an increase in anion concentration increases the gold content of individual particles in the resultant catalyst, thus providing a method for controlling/tuning the composition of the nanoalloy particles. The improved activity and stability characteristics of these new catalysts are demonstrated using: (i) the solvent-free aerobic oxidation of benzyl alcohol and (ii) the direct synthesis of hydrogen peroxide as case studies. The study has examined using this modified impregnation catalyst in different Au:Pd ratios for the solvent-free aerobic oxidation of benzyl alcohol. These modified impregnation catalysts have been found to be exceptionally active, mainly with a Aurich composition, when compared to reduced conventional impregnation catalysts. Moreover, these modified impregnation catalysts have been found to be exceptionally active and stable when evaluated for crotyl alcohol in mild conditions when compared to sol-immobilisation methods. This new preparation protocol could be beneficial for both the academic research community and the industrial community, with a very convenient and reproducible methodology for preparing supported Au-Pd nanoalloy catalysts with high activity and stability, without using any ligands or stabilisers, while not compromising their catalytic activity

Hydrodynamic effects on three phase micro-packed bed reactor performance – Gold–palladium catalysed benzyl alcohol oxidation

The hydrodynamics of a three-phase micro-packed bed reactor and its effect on catalysed benzyl alcohol oxidation with pure oxygen were studied in a silicon–glass microstructured reactor. The microreactor was operated at 120 °C and 1 barg and contained a channel with a 300 μm×600 μm cross-section, packed with 1 wt% Au–Pd/TiO2 catalyst, 65 μm in average diameter. Improvements in the conversion of benzyl alcohol and selectivity to benzaldehyde were observed with increasing gas-to-liquid ratio, which coincided with a change in the flow pattern from a liquid-dominated slug to a gas-continuous flow regime. The observed enhancement is attributed to improved external mass transfer, associated with an increase in the gas–liquid interfacial area and reduction in the liquid film thickness that occur with gradual changes in the flow pattern. A maximum selectivity of 93% to benzaldehyde was obtained under partial wetting – which introduced the added benefit of direct gas–solid mass transfer – outperforming the selectivity in a conventional glass stirred reactor. However, this was at the expense of a reduction in the conversion. A response surface model was developed and then used to predict optimal operating conditions for maximum benzaldehyde yield, which were in the gas-continuous flow regime. This corresponded to relatively high gas flow rate in conjunction with moderate liquid flow rate, ensuring sufficient catalyst wetting with a thin film to reduce transport resistance

Solvent-free aerobic oxidation of alcohols using supported gold palladium nanoalloys prepared by a modified impregnation method

The synthesis of stable, supported, bimetallic nanoalloys with controlled size, morphology and composition is of great practical importance. Compared to their monometallic analogues, such materials exhibit remarkable enhancement in functional properties, which can be exploited in various fields including catalysis. Recently, we have reported a simple excess anion modification of the impregnation method to prepare supported gold–palladium catalysts which gives very good control over the particle sizes and the composition without using any stabilizer ligands in the preparation. Here, we report the results from a comparative study of using this modified impregnation catalyst for the solvent-free aerobic oxidation of alcohols in two different reactors: a glass stirred reactor and a micro packed bed reactor under batch and continuous mode respectively. These modified impregnation catalysts are exceptionally active and more importantly, when tested in a micro packed bed reactor under flow conditions, are observed to be stable for several days without any sign of deactivation in contrast to the same catalyst prepared by the sol immobilization method in the presence of stabilizer ligands which showed a 3–5% decrease in conversion over 10–12 h

Base-free glucose oxidation using air with supported gold catalysts

We report the selective oxidation of glucose to gluconic acid under mild conditions and show that if a basic support is used then the reaction can be carried out without the addition of sacrificial base or pH control. The use of sol-immobilisation prepared catalysts supported on magnesium oxide facilitates the use of ambient air as an oxidant source. These mild conditions resulted in an excellent selectivity towards gluconic acid. Different heat treatments result in an improvement in the activity of the catalyst, these improvements are discussed in terms of XRD, DRIFTD and TEM analysis of the catalysts, despite significant particle growth and phase segregation occurring during the thermal treatments

Continuous Heterogeneously Catalyzed Oxidation of Benzyl Alcohol Using a Tube-in-Tube Membrane Microreactor

© 2015 American Chemical Society. A Teflon AF-2400 tube-in-tube microreactor is investigated for the continuous, solvent-free, catalytic oxidation of benzyl alcohol with oxygen. The semipermeable Teflon AF-2400 tube acts as the interface between the gaseous oxidant and the liquid substrate. Because of the inherent safety of this contacting method, the use of pure oxygen is possible. The semipermeable tube was packed with 1 wt % Au-Pd/TiO2 catalyst particles and placed inside a PTFE tube to provide an annular region which was pressurized with pure oxygen. This design allowed continuous penetration of oxygen through the inner tube during the reaction, resulting in higher oxygen concentration in the catalyst bed and significantly improved conversion compared to a reactor operating with an oxygen presaturated feed. The amount of oxygen available for reaction in the tube-in-tube microreactor was 2 orders of magnitude higher than that in a nonpermeable reactor with oxygen presaturated feed. The semipermeable tube reactor performance in terms of both conversion and selectivity was enhanced by increasing the gas pressure, the catalyst contact time and by dilution of the catalyst. The highest conversion of benzyl alcohol obtained for the range of conditions investigated was 44.1%, with 73.0% selectivity to benzaldehyde, at 120 °C; catalyst contact time, 115 gcat·s/galcohol; and catalyst dilution factor, 4.status: publishe

Continuous heterogeneously catalyzed oxidation of benzyl alcohol using a tube-in-tube membrane microreactor

A Teflon AF-2400 tube-in-tube microreactor is investigated for the continuous, solvent-free, catalytic oxidation of benzyl alcohol with oxygen. The semipermeable Teflon AF-2400 tube acts as the interface between the gaseous oxidant and the liquid substrate. Because of the inherent safety of this contacting method, the use of pure oxygen is possible. The semipermeable tube was packed with 1 wt % Au–Pd/TiO2 catalyst particles and placed inside a PTFE tube to provide an annular region which was pressurized with pure oxygen. This design allowed continuous penetration of oxygen through the inner tube during the reaction, resulting in higher oxygen concentration in the catalyst bed and significantly improved conversion compared to a reactor operating with an oxygen presaturated feed. The amount of oxygen available for reaction in the tube-in-tube microreactor was 2 orders of magnitude higher than that in a nonpermeable reactor with oxygen presaturated feed. The semipermeable tube reactor performance in terms of both conversion and selectivity was enhanced by increasing the gas pressure, the catalyst contact time and by dilution of the catalyst. The highest conversion of benzyl alcohol obtained for the range of conditions investigated was 44.1%, with 73.0% selectivity to benzaldehyde, at 120 °C; catalyst contact time, 115 gcat·s/galcohol; and catalyst dilution factor, 4