Higher alcohol synthesis on magnesium/aluminum mixed oxide supported potassium carbonate promoted molybdenum sulfide
Morrill, Michael R.
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Higher alcohols synthesized via CO hydrogenation reactions have been a topic of intense study both in industry and academia for over thirty years. A variety of transition metals and promoters have been used in catalysts for this reaction. MoS₂, in particular, is popular due to its low cost, resistance to sulfur poisoning, and ability to selectively produce higher alcohols over hydrocarbons. The bulk material has a rich history in hydrodesulfurization reactions (HDS), and as such, a great deal is known about the material's structure and reactivity. However, even with this deep body of knowledge about the bulk catalyst, no one has yet been able to implement an industrially viable variation of the catalyst to make higher alcohols. Supported MoS₂ has also been studied for the same purpose. Generally, supports are employed to improve catalyst productivity per gram of Mo by dispersing the metal and increasing the amount of catalytically active surface area. However, product selectivity may also be influenced by chemical properties of the supports. Specifically, gamma alumina has been shown to raise hydrocarbon formation due to intrinsic surface acidity. The effects of basic supports are reported on the CO hydrogenation reaction are reported. K promoted Mo is supported on two basic materials - commercial sepiolite (Si₁₂Mg₈O₃₀(OH)₄) and hydrotalcite-derived Mg/Al mixed metal oxides (MMO). The catalysts are reacted with syngas, and the resultant product selectivities are compared at isoconversions. Activated carbon supported Mo and bulk MoS₂ are also used as controls. It is shown that MMO provides a unique promotional effect by suppressing methanol formation and favoring higher alcohols. The specific role of MMO in the reaction is investigated by combining it in three different ways with Mo. 1) MMO is impregnated with Mo in the classic fashion. 2) Bare MMO or MMO/K is placed as a secondary bed downstream of the principle catalyst (K promoted Mo supported on MMO). 3) Bare MMO or MMO/K is mixed with the principle catalyst to make a homogeneous bed. It is shown that MMO by itself is somewhat inert in the reaction while MMO/K has some higher alcohol forming activity. More importantly however, it is shown that the MMO:Mo ratio has far greater effects on selectivity than the morphology of MoS₂. There is evidence however that MoS₂ morphology can affect activity. It is hypothesized that a greater degree of stacking in MoS₂ domains leads to reduced activity. The existence of coupling and homologation pathways are investigated by feeding methanol or ethanol into the syngas as it enters the catalyst bed. By comparing changes in the productivity of different higher alcohols with the liquid feed, it is shown that an MMO supported catalyst is much more reactive with methanol and somewhat more reactive with ethanol than its bulk MoS₂ counterpart. It is shown that for both the bulk and supported catalysts, the addition of a Cx alcohol results in the largest increase in Cx+1 products, suggesting that alcohol homologation is in fact the most favored route to higher alcohols by these materials.