Iron copper zeolite (Fe‐Cu‐ZSM‐5) with aqueous hydrogen peroxide is active for the selective oxidation of methane to methanol. Iron is involved in the activation of the carbon–hydrogen bond, while copper allows methanol to form as the major product. The catalyst is stable, re‐usable and activates methane giving >90 % methanol selectivity and 10 % conversion in a closed catalytic cycle (see scheme).

Ein Eisen‐Kupfer‐Zeolith (Fe‐Cu‐ZSM‐5) katalysiert die selektive Oxidation von Methan zu Methanol mit wässrigem Wasserstoffperoxid. Das Eisen aktiviert die Kohlenstoff‐Wasserstoff‐Bindung, während das Kupfer dafür sorgt, dass Methanol als Hauptprodukt gebildet wird. Der Katalysator ist stabil und wiederverwendbar und aktiviert Methan mit >90 % Selektivität und 10 % Umsatz in einem geschlossenen Katalysezyklus (siehe Schema).

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.

Recent advances in the oxidation of alcohols to methyl esters using metal nanoparticles have paved the way for more environmentally benign processes, operating at lower reaction temperatures with high product selectivity. Here, we demonstrate the use of bimetallic 1 wt % Au–Pd/TiO2 catalysts that achieve high activity for the oxidation of methanol to methyl formate at low temperature. The application of a water extraction treatment to retain size-stabilized Au–Pd nanoparticles, in contrast to a more standard thermal treatment, provides the most active catalyst for this reaction. Using in situ DRIFTS, we demonstrate that in situ activation during methanol oxidation enhances the catalytic activity at low temperature and that this is a long-lived effect. Surface adsorbates, particularly formate species, build up on the catalyst surface during the reaction and are proven vital to enhancing the catalytic effect.

The selective oxidation of methane to methanol, using H2O2, under mild reaction conditions was studied using bimetallic 1 wt. % AuPd/TiO2 prepared by stabiliser-free sol-immobilisation.The as-prepared catalysts exhibited low, unselective oxidation activity and deleterious H2O2 decomposition, which was ascribed to the small mean particle size of the supported AuPd nanoparticles. Heat treatments were employed to facilitate particle size growth, yielding an improvement in the catalyst turn-over-frequency and decreasing the H2O2 decomposition rate. The effect of support phase was studied by preparing a range of AuPd catalysts supported on rutile TiO2. The low surface area rutile TiO2 yielded catalysts with effective oxygenate production, but poor H2O2 utilisation. The influence of the rutile-TiO2 support was investigated further by producing catalysts with a lower metal loading to maintain a consistent metal loading per m 2 to the 1 wt.% AuPd/ P25 TiO2 catalyst. When calcined at 800 °C the 0.13 wt.% AuPd catalyst demonstrated significantly improved turn-over frequency of 103 h -1 . In contrast, the turn-over frequency was found to be ca. 2 h -1 for the rutile-supported 1 wt. % AuPd catalyst calcined at 800 °C. The catalysts were probed by electron microscopy and XPS to understand the influence of particle size and oxidation state on the utilisation of H2O2 and oxygenate productivity. This work shows that the key to highly active catalysts involves the prevention of deleterious H2O2 decomposition and this can be achieved through carefully controlling the nanoparticle size, metal loading and metal oxidation state.

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The direct transformation of methane to methanol remains a significant challenge for operation at a larger scale. Central to this challenge is the low reactivity of methane at conditions that can facilitate product recovery. This review discusses the issue through examination of several promising routes to methanol and an evaluation of performance targets that are required to develop the process at scale. We explore the methods currently used, the emergence of active heterogeneous catalysts and their design and reaction mechanisms and provide a critical perspective on future operation. Initial experiments are discussed where identification of gas phase radical chemistry limited further development by this approach. Subsequently, a new class of catalytic materials based on natural systems such as iron or copper containing zeolites were explored at milder conditions. The key issues of these technologies are low methane conversion and often significant overoxidation of products. Despite this, interest remains high in this reaction and the wider appeal of an effective route to key products from C–H activation, particularly with the need to transition to net carbon zero with new routes from renewable methane sources is exciting.

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