Abstract:
The aim of this work was to prepare and test bi-functional direct DME synthesis catalysts, including commercial ones, involving methanol synthesis and methanol dehydration functions. Catalyst beds were prepared by physical mixing of methanol synthesis catalysts (Cu/ZnO/Al2O3, commercial Cu–Zn based HiFUEL R120) with methanol dehydration catalysts (δ-Al2O3, MCM-41 type SiO2/Al2O3, zeolite, phosphotungstic acid hydrate and 20wt%CeO2/δ-Al2O3) except a case in which the bed was composed of only of 2wt%Pd-20wt%CeO2/ γ-Al2O3, which included synthesis and dehydration functions. The Autoclave Engineers' BTRS-Jr-PC reaction system that can handle pressures up to 100 bar and temperatures up to 650 0C was used in the experiments. Parametric studies were first conducted on the bed configuration characterized by physical mixing of commercial Cu–Zn based catalyst, denoted as HiFUEL R120, with δ-Al2O3, methanol dehydration catalyst. The reaction temperature and pressure, feed composition, metal to acid catalyst weight ratio (=mass of methanol synthesis catalyst/mass of methanol dehydration catalyst) were the parameters used in activity and selectivity tests of this catalyst. In addition, the bi-functional catalyst prepared by physical mixing of an in-house, co-precipitated CuO/ZnO/Al2O3 catalyst, denoted as CZA, with δ-Al2O3 was also tested in the temperature range of 250-300 0C. Subsequent experiments were performed at 34 bar and 275 0C for a feed composition of CO/H2/N2=32/64/4. Highest DME selectivity (49.1%) was observed over the bed configuration involving the physical mixture of HiFUEL R120 with δ-Al2O3 catalysts operating at 34 bar and at 275 0C with a molar feed composition of CO/H2/N2=48/48/4. CZA/ δ-Al2O3 catalyst showed lower catalytic activity in comparison with HiFUEL R120/ δ-Al2O3 catalyst. Methane and CO2 formation were observed significantly over SiO2/Al2O3, zeolite, PTA and 2wt%Pd-20wt%CeO2/ γ-Al2O3 catalysts. HiFUEL R120/ zeolite showed the lowest DME selectivity. The modified δ-Al2O3 with cerium oxide showed lower catalytic activity than unmodified one.
