Supported K/MoS2 and K/Mo2C Catalysts for Higher Alcohol Synthesis from Synthesis Gas: Impact of Molybdenum Precursor and Metal Oxide Support on Activity and Selectivity

Abstract

Potassium promoted molybdenum sulfide catalysts are well-known for the conversion of synthesis gas (H2 and CO) to higher alcohols, primarily ethanol and propanol. Basic supports composed of mixed MgAl based oxides from decomposed hydrotalcites are known to yield enhanced higher alcohol selectivities compared to bulk or carbon supported molybdenum sulfide catalysts. In this study, the role of the metal oxide support and the active as well as precursor molybdenum phases on higher alcohol productivity and selectivity are probed. At fixed loadings of potassium (3 wt%) and molybdenum (5 wt%), supported molybdenum sulfide (MoS2) and molybdenum carbide (Mo2C) catalysts are prepared on mixed MgAl oxide, α-alumina, and magnesium oxide supports and evaluated in higher alcohol synthesis. At low conversions, the catalytic results suggest that basic supports provide similar effects as the alkali promoter, helping to produce lower methanol (MeOH) selectivities and higher C2+OH selectivities while shifting overall selectivity from hydrocarbons towards alcohols. Under similar conditions, it is also shown that Mo2C compositions produce more hydrocarbons than MoS2 catalysts, suggesting that higher potassium loadings are needed to fully eliminate acidity in molybdenum carbide phases. Conversion of the Mo2C phase to a MoS2 phase in situ, followed by catalytic testing in syngas hydrogenation shows that similar catalytic selectivities are obtained at similar CO conversions for catalysts with similar overall molybdenum and potassium loadings, regardless of the molybdenum phase in the precatalyst (MoO3 vs. Mo2C). The highest C2+ alcohol selectivities and productivities among the catalysts tested here are obtained on presulfided MoO3 catalysts on both α-alumina and magnesium oxide supports.