Zn and Si transport in dual-functioning catalyst for conversion of synthesis gas to dimethyl ether

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Dimethyl ether (DME) is traditionally produced from synthesis gas, i.e., CO, CO2, and H2, with methanol as an intermediate before the final dehydration to DME. However, DME may also be produced in a one-stage synthesis from synthesis gas using a combi-catalyst with both methanol synthesis and methanol dehydration functionalities in a single catalyst. In this work, two combi-catalysts consisting of co-tableted methanol synthesis catalyst (Cu/ZnO/Al2O3) and methanol dehydration catalyst domains (silica-alumina) were tested in synthesis gas as 6 mm in diameter pellets in a single-pellet configuration for 700–850 h at 235–310 °C and 60 bar. The catalyst domain sizes were 50–100 μm in a fine particle catalyst A and 600–1000 μm in a coarse particle catalyst B. Surprisingly, the finer particle combi-catalyst A(fine) showed a markedly faster deactivation of the dehydration activity and slightly faster deactivation of the methanol activity than the coarser combi-catalyst B(coarse). Electron microprobe analysis of the spent catalyst B(coarse) showed that Zn had migrated from the methanol active material and 10 μm into the DME active particles. Similarly, silicon from the silica-alumina methanol dehydration catalyst particles had diffused several hundred µm into the methanol catalyst particles. Based on these experimental results, it is concluded that the methanol dehydration catalyst domains in combi-catalyst A(fine) were poisoned faster than those in combi-catalyst B(coarse) due to the larger contact area between the two catalyst materials, easing the transport of Zn and leading to a faster deactivation. Similarly, silicon migration from the silica-alumina catalyst to the methanol synthesis catalyst might decrease the formation rate of methanol in catalyst A(fine) more than in catalyst B(coarse).