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The production of 2,5-furandicarboxylic acid (FDCA) from the selective oxidation of 5-hydroxymethylfurfural (HMF) is a critical step in the production of biopolymers from biomass-derived materials. In this study, we report the catalytic performance of monometallic Au and Pd, and bimetallic AuPd nanoparticles with different Au : Pd molar ratios synthesised under continuous flow conditions using a millifluidic set-up and subsequently deposited onto titanium dioxide as the chosen support. This synthetic technique provided a better control over mean particle size and metal alloy composition, that resulted in higher FDCA yield when the catalysts were compared to similar batch-synthesised materials. A 99% FDCA yield was obtained with the millifluidic-prepared AuPd/TiO2 catalyst (Au : Pd molar composition of 75 : 25) after being calcined and reduced at 200 °C. The heat treatment caused a partial removal of the protective ligand (polyvinyl alcohol) encapsulating the nanoparticles and so induced stronger metal-support interactions. The catalyst reusability was also tested, and showed limited particle sintering after five reaction cycles.

The reactivity of AuPd nanoparticle catalysts prepared by sol immobilisation is often explained by a structure activity relationship based solely on particle size or composition. In this contribution, we compare colloidal AuPd nanoparticles stabilised with polyvinylpyrrolidone (PVP) with the same AuPd nanoparticles supported on TiO2 for the direct synthesis of hydrogen peroxide and methane oxidation to methanol. We show that while the particles have similar rates of H2O2 synthesis, supporting the particles can affect the rates of H2O2 decomposition and hence the effectiveness of the catalyst for reactions which rely on H2O2 as an initiator or oxidant. We demonstrate that the absence of PVP results in high rates of H2O2 decomposition in methane oxidation experiments but this can be minimised by the addition of PVP to the reactor. These results also show that for AuPd alloys, both polymer stabiliser and support effects need to be taken into account when describing the activity of the nanoparticles and the active sites should in fact be thought of as a metal-support-polymer interface with many degrees of freedom.

Conspectus\udMethane represents one of the most abundant carbon sources for fuel or chemical production. However, remote geographical locations and high transportation costs result in a substantial proportion being flared at the source. The selective oxidation of methane to methanol remains a grand challenge for catalytic chemistry due to the large energy barrier for the initial C–H activation and prevention of overoxidation to CO2. Indirect methods such as steam reforming produce CO and H2 chemical building blocks, but they consume large amounts of energy over multistage processes. This makes the development of the low-temperature selective oxidation of methane to methanol highly desirable and explains why it has remained an active area of research over the last 50 years.\ud\udThe thermodynamically favorable oxidation of methane to methanol would ideally use only molecular oxygen. Nature effects this transformation with the enzyme methane monooxygenase (MMO) in aqueous solution at ambient temperature with the addition of 2 equiv of a reducing cofactor. MMO active sites are Fe and Cu oxoclusters, and the incorporation of these metals into zeolitic frameworks can result in biomimetic activity. Most approaches to methane oxidation using metal-doped zeolites use high temperature with oxygen or N2O; however, demonstrations of catalytic cycles without catalyst regeneration cycles are limited. Over the last 10 years, we have developed Fe-Cu-ZSM-5 materials for the selective oxidation of methane to methanol under aqueous conditions at 50 °C using H2O2 as an oxidant (effectively O2 + 2 reducing equiv), which compete with MMO in terms of activity. To date, these materials are among the most active and selective catalysts for methane oxidation under this mild condition, but industrially, H2O2 is an expensive oxidant to use in the production of methanol.\ud\udThis observation of activity under mild conditions led to new approaches to utilize O2 as the oxidant. Supported precious metal nanoparticles have been shown to be active for a range of C–H activation reactions using O2 and H2O2, but the rapid decomposition of H2O2 over metal surfaces limits efficiency. We identified that this decomposition could be minimized by removing the support material and carrying out the reaction with colloidal AuPd nanoparticles. The efficiency of methanol production with H2O2 consumption was increased by 4 orders of magnitude, and crucially it was demonstrated for the first time that molecular O2 could be incorporated into the methanol produced with 91% selectivity. The understanding gained from these two approaches provides valuable insight into possible new routes to selective methane oxidation which will be presented here in the context of our own research in this area.

The synthesis of supported bimetallic nanoparticles with well-defined size and compositional parameters has long been a challenge. Although batch colloidal methods are commonly used to pre-form metal nanoparticles with the desired size-range in solution, inhomogeneous mixing of the reactant solutions often leads to variations in size, structure and composition from batch-to-batch and even particle-to-particle. Here we describe a millifluidic approach for the production of oxide supported monometallic Au and bimetallic AuPd nanoparticles in a continuous fashion. This optimised method enables the production of nanoparticles with smaller mean sizes, tighter particle size distributions and a more uniform particle-to-particle chemical composition as compared to the conventional batch procedure. In addition, we describe a facile procedure to prepare bimetallic Au@Pd core-shell nanoparticles in continuous flow starting from solutions of the metal precursors. Moreover, the relative ease of scalability of this technique makes the proposed methodology appealing not only for small-scale laboratory purposes, but also for the industrial-scale production of supported metal nanoparticles.