Origins of Unusual Alcohol Selectivities over Mixed MgAl Oxide-Supported K/MoS₂ Catalysts for Higher Alcohol Synthesis from Syngas

Abstract

A series of MoS2 catalysts supported on Mg/Al hydrotalcite-derived mixed-metal oxide (MMO) supports promoted with K2CO3 is used for alcohol synthesis via CO hydrogenation. Alcohol selectivities are found to vary greatly when the Mo is loaded on the support at 5 wt % compared with 15 wt % Mo samples, all with a Mo/K atomic ratio of 1:1. The most striking difference between the catalysts is the comparatively low methanol and high C3+ alcohol selectivities and productivities achieved with the 5% Mo catalyst. This catalyst also produces more ethane than the 15% Mo catalyst, which is shown to be associated with ethanol dehydration and hydrogenation over residual acid sites on this catalyst with lower K content. A series of catalysts with common composition (5% Mo and 3% K supported on MMO) prepared in different manners all yield similar catalytic selectivities, thus showing that selectivity is predominately controlled by the MMO-to-Mo ratio rather than the synthesis method. When the Mo loading is the same, catalytic higher alcohol productivity shows some correlation to the degree of stacking of the MoS2 layers, as assessed via X-ray diffraction and scanning transmission electron microscopy. Control reactions in which K loading is increased or the positioning of the MMO in the catalyst bed is changed via creation of multiple or mixed catalyst beds show that Mo/K/MMO domains play a significant role in alcohol-forming reactions. Higher alcohol-forming pathways are proposed to occur via CO insertion pathways or via coupling of adsorbed reaction intermediates at or near MoS2 domains. No evidence is observed for significant alcohol-coupling pathways by adsorption of alcohols over downstream, bare MMO supports. Nitrogen physisorption, XRD, Raman, UV–vis DRS, STEM, and XANES are used to characterize the catalysts, demonstrating that the degree of stacking of the MoS2 domains differs significantly between the low (5% Mo) and high (15% Mo) loading catalysts.