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Please cite this article in press as: J. Xu, et al., Continuous selective oxidation of methane to methanol over Cu-and Fe-modified ZSM-5 catalysts in a flow reactor, Catal. Today (2015), http://dx. a b s t r a c tThe selective oxidation of methane to methanol is a key challenge in catalysis. Iron and copper modified ZSM-5 catalysts are shown to be effective for this reaction using H 2 O 2 as the oxidant under continuous flow operation. Co-impregnation of ZSM-5 with Fe and Cu by chemical vapour impregnation yielded catalysts that showed high selectivity to methanol (>92% selectivity, 0.5% conversion), as the only product in the liquid phase. The catalysts investigated did not deactivate during continuous reaction, and methanol selectivity remained high. The effect of reaction pressure, temperature, hydrogen peroxide concentration and catalyst mass were investigated. An increase in any of these led to increased methane conversion, with high methanol selectivity (≥73%) maintained throughout. Catalysts were characterised using DR-FTIR, DR-UV-Vis and 27 Al MAS-NMR spectroscopy.

a b s t r a c tThe direct synthesis of hydrogen peroxide using supported gold palladium catalysts prepared by incipient wetness impregnation is described and discussed. The effect of an acid pre-treatment step on the activated carbon support prior to the deposition of the metals, together with the effect of the calcination temperature, has been investigated. The acid pre-treated samples all show superior activity to those materials prepared with the omission of this acid pre-treatment stage. The calcination temperature affects both the re-usability and hydrogenation activity of the catalysts. Detailed characterisation using X-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy is described. The enhanced activity is associated with a higher surface concentration of palladium in the acid pre-treated samples which is principally present as Pd 2+ . Calcination of the catalysts at 400°C is required to achieve re-usable and stable catalysts, and this is associated with the morphology and dispersion of the metal nanoparticles. The surface ratio of Pd 0 /Pd 2+ is found to be an important factor controlling the hydrogenation of hydrogen peroxide, and a series of controlled reduction and re-oxidation of a sample show how the Pd 0 /Pd 2+ surface ratio can influence the relative rates of hydrogen peroxide synthesis and hydrogenation.

The direct synthesis of hydrogen peroxide offers a potentially green route to the production of this important commodity chemical. Early studies showed that Pd is a suitable catalyst, but recent work indicated that the addition of Au enhances the activity and selectivity significantly. The addition of a third metal using impregnation as a facile preparation method was thus investigated. The addition of a small amount of Pt to a CeO2-supported AuPd (weight ratio of 1:1) catalyst significantly enhanced the activity in the direct synthesis of H2O2 and decreased the non-desired over-hydrogenation and decomposition reactions. The addition of Pt to the AuPd nanoparticles influenced the surface composition, thus leading to the marked effects that were observed on the catalytic formation of hydrogen peroxide. In addition, an experimental approach that can help to identify the optimal nominal ternary alloy compositions for this reaction is demonstrated.

We have prepared supported gold, palladium and gold-palladium bimetallic catalysts by the physical mixing of the acetate salts of the metals followed by a simple heat treatment. The use of the acetates as the metal precursor eliminates chloride from the catalyst preparation step. Extensive characterisation shows the formation of bimetallic alloy particles. These catalysts are extremely active for alcohol oxidations and the direct formation of hydrogen peroxide.

Supported nano-alloys have been prepared using the sol-immobilisation method for two bimetallic combinations, namely gold-platinum and palladium-platinum, using activated carbon and titania as supports. Some of the materials were prepared using a method where both metals are simultaneously reduced, thereby leading to homogeneous alloys being formed. In addition, sequential reduction of the metal combinations has also been investigated to facilitate the formation of core-shell structures. The materials have been characterized using X-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy. The supported nanoparticles have been tested for a two selective oxidation reactions, namely the oxidation of toluene and benzyl alcohol using tertiary butyl hydroperoxide at 80 °C, in order to elucidate any potential structure-activity relationships.

The effect of sodium species on the physical and catalytic properties of Cu/ZnO catalysts derived from zincian georgeite has been investigated. Catalysts prepared with <100 ppm to 2.1 wt% Na, using a supercritical CO antisolvent technique, were characterised and tested for the low temperature water-gas shift reaction and also CO hydrogenation to methanol. It was found that zincian georgeite catalyst precursor stability was dependent on the Na concentration, with the 2.1 wt% Na-containing sample uncontrollably ageing to malachite and sodium zinc carbonate. Samples with lower Na contents (<100-2500 ppm) remained as the amorphous zincian georgeite phase, which on calcination and reduction resulted in similar CuO/Cu particle sizes and Cu surface areas. The aged 2.1 wt% Na containing sample, after calcination and reduction, was found to comprise of larger CuO crystallites and a lower Cu surface area. However, calcination of the high Na sample immediately after precipitation (before ageing) resulted in a comparable CuO/Cu particle size to the lower (<100-2500 ppm) Na containing samples, but with a lower Cu surface area, which indicates that Na species block Cu sites. Activity of the catalysts for the water-gas shift reaction and methanol yields in the methanol synthesis reaction correlated with Na content, suggesting that Na directly poisons the catalyst. In situ XRD analysis showed that the ZnO crystallite size and consequently Cu crystallite size increased dramatically in the presence of water in a syn-gas reaction mixture, showing that stabilisation of nanocrystalline ZnO is required. Sodium species have a moderate effect on ZnO and Cu crystallite growth rate, with lower Na content resulting in slightly reduced rates of growth under reaction conditions.

Methane oxidation using N2O was carried out with Fe-MFI zeolite catalysts at 300 °C. Methane conversion over Fe-ZSM-5, Fe-silicalite-1 and Fe-TS-1 indicates that Brønsted acidity is required to support the Fe-based alpha-oxygen active site for the important initial hydrogen abstraction step. Increasing the calcination temperature of Fe-ZSM-5 from 550 to 950 °C showed that the catalyst retained the MFI structure. However, at 950 °C the Brønsted and Lewis acid sites were altered significantly due to the migration of aluminium, which led to a significant decrease in catalytic performance. Over Fe-ZSM-5 the desired partial oxidation product, methanol was observed to undergo a reaction path similar to the methanol to olefin (MTO) process, which predominately produced ethene and subsequently produced coke.Methanol control experiments over Fe-silicalite-1, Fe-ZSM-5, Fe-TS-1 and H-ZSM-5 indicated that with the presence of Brønsted acidity the catalyst were more effective at forming ethene and subsequent aromatic species from DME, which resulted in an increased level of catalyst fouling. The implication of these observations are that the desorption of methanol is crucial to afford high mass balances and selectivity, however, Brønsted acid sites appear to slow this rate. These sites appear to effectively retain methanol and DME under reaction conditions, leading to low mass balances being observed. Our results confirm that to afford efficient and continuous methane oxidation by N2O, the catalytic active site must be extra-framework Fe coordinated to Al.