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Activity of Cs (K)-promoted Cu-MgO in the formation of oxygenates from CH₃OH/CO and CO/H₂ Goodarznia, Shahin

Abstract

The selective synthesis of C₂ oxygenates, especially ethanol, from C₁ species such as CH₃OH and synthesis gas (CO/H₂) is of interest as the demand for clean fuels, including biofuels, increases. However, over alkali-promoted Cu-ZnO catalysts the synthesis of C₂ oxygenates occurs with very low selectivity. Previous mechanistic studies suggest that the basic properties and the Cu properties of these catalysts are critical in determining the C₂ oxygenate selectivity. However, the possible synergistic effect of these catalyst properties on the selectivity of C₂ oxygenates is poorly understood. In the present study, Cu-MgO catalysts were investigated since MgO possesses noticeably higher basic properties compared to ZnO. Furthermore to address the knowledge gap in the literature with respect to a synergistic effect between catalyst basic properties and Cu properties on the synthesis of C₂ oxygenates from CH₃OH/CO, MgO, Cu-MgO and Cs (K)-promoted-Cu-MgO catalysts were prepared, characterized and tested at 101kPa and 498-523K. The catalysts had intrinsic basicities of 3.9 – 17.0 μmol CO₂.m⁻², SACu° of < 3 m².g⁻¹ and SACu₂₊ of < 2 m².g⁻¹. The results showed that methyl formate was the dominant C₂ oxygenate, while selectivity to ethanol and acetic acid was low (< 5 C-atom%). At SA_Cu° (< 2 m².g⁻¹), there was an optimum basicity (9.5 µmol CO₂.m⁻²) at which the selectivity to C₂ species and methyl formate reached a maximum. Also, at approximately constant specific basicity (384.5 – 415.9 µmol CO₂.g⁻¹), an increase in SA_Cu°, led to an increase in methyl formate yield, whereas no correlation between SA_Cu₂₊ and methyl formate yield was observed. The 0.5wt%Cs-40wt%Cu-MgO catalyst showed the highest selectivity towards C₂ oxygenates at 101 kPa and was used for high pressure studies to investigate oxygenates synthesis from CO/H₂ at typical industrial conditions (6000-9000kPa and 558-598K). CH₃OH was the dominant produced oxygenate (>66 C-atom%). The reaction kinetics of CH₃OH was studied. The Cs-Cu-MgO catalyst was noticeably less active for the synthesis of oxygenates, compared to a conventional Cs-Cu-ZnO catalyst, which was caused by lower Cu dispersion and weaker Cu-metal oxide interaction in the Cs-Cu-MgO compared to Cs-Cu-ZnO, as well as poor electronic-conductivity and lack of hydrogenation-activity of MgO compared to ZnO.

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