Single-Step Conversion of Ethanol to n-Butene over Ag-ZrO2/SiO2 Catalysts

Ethanol is a promising platform molecule for production of a variety of fuels and chemicals. Of particular interest is the production of middle distillate fuels (i.e., jet and diesel blendstock) from renewable ethanol feedstock. State-of-the-art alcohol-to-jet technology requires multiple process steps based on the catalytic dehydration of ethanol to ethylene, followed by a multistep oligomerization including n-butene formation and then hydrotreatment and distillation. Here we report that, over Ag-ZrO2/SBA-16 with balanced metal and Lewis acid sites, ethanol is directly converted to n-butene (1- and 2-butene mixtures) with an exceptional butene-rich olefin selectivity of 88% at 99% conversion. The need for the ethanol dehydration to ethylene step is eliminated. Thus, it offers the potential for a reduction in the number of required processing units versus conventional alcohol-to-jet technology. We also found that the C4 product distribution, n-butene and/or 1,3-butadiene, can be tailored on this catalyst by tuning the hydrogen feed partial pressure and other process/catalyst parameters. With sufficient hydrogen partial pressure, 1,3-butadiene is completely and selectively hydrogenated to form n-butene. The reaction mechanism was elucidated through operando-nuclear magnetic resonance investigations coupled with reactivity measurements. Ethanol is first dehydrogenated to acetaldehyde over the metallic Ag, then acetaldehyde is converted to crotonaldehyde over the acid sites of ZrO2/SiO2 via aldol condensation followed by dehydration. This is followed by a Meerwein–Ponndorf–Verley reduction of crotonaldehyde to butadiene intermediate that is hydrogenated into n-butene over metallic Ag and ZrO2. A minor fraction of n-butene is also produced from crotonaldehyde reduction to butyraldehyde instead of butadiene. Isotopically labeled ethanol NMR experiments demonstrated that ethanol, rather than H2, is the source of H for the hydrogenation of crotonaldehyde to butyraldehyde. Combined experimental-computational investigation reveals how changes in silver and zirconium composition and the silver oxidation state affects reactivity under controlled hydrogen partial pressures and after prolonged run times. Finally, catalyst effectiveness also was demonstrated when using wet ethanol feed, thus highlighting process flexibility in terms of feedstock purity requirements.