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Abstract

Oxymethylene ethers (OME) are a class of chain ethers that have been classified as pollutant reducing synthetic diesel additives in the late 1990s. In view of growing efforts to reduce hazardous emissions from the transport sector and to find alternatives to fossil-based fuels, OME have seen a rise in academic and industrial interest. Various synthesis routes that have methanol as a common intermediate have been reported. However, the production remains the main challenge for introduction of OME as a synthetic fuel.
This work explores the gas-phase synthesis of OME from methanol and formaldehyde as an alternative approach to common liquid-phase routes. In particular, the investigation of solid catalysts for this reaction is the focus of the conducted studies.
For this purpose, a versatile test set-up was built and suitable reaction conditions were identified. The comparison of a selection of solid acid catalysts highlighted the activity of zeolites in gas-phase OME synthesis. In a systematic study of a broad range of zeolite catalysts, a relation between catalyst reactivity and silica-to-alumina ratio was established. It could be shown that low amounts of acid sites are favourable for OME selectivity and that strong acid sites are linked to by-product formation. Carbon supported phosphoric acid was furthermore found to be an active catalyst for the gas-phase synthesis of OME with a superior lifetime as compared to benchmark zeolite catalysts. Steady-state conversion and selectivity were comparable at the same loading of active centres. In view of the simple preparation and low cost of H₃PO₄/C, it provides an attractive alternative to zeolites. In a final step, the viability of the gas-phase synthesis of OME from methanol without separation of intermediates was demonstrated.