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

Abstract

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.

Understanding the Deactivation of Ag−ZrO2/SiO2 Catalysts for the Single‐step Conversion of Ethanol to Butenes

“…Over Ag−ZrO 2 /SiO 2 , ethanol was found to proceed along the ETB pathway, as described above, where produced butadiene is subsequently hydrogenated into butenes. Although we note that a minor fraction of n‐butene was also produced from crotanaldehyde reduction to butryaldehyde instead of butadiene [11] …”

“…As shown in Figure 2, the number of accessible Lewis acid sites on the fresh and spent catalysts decreased as coke deposition increased. In our recent work, we reported Raman Spectroscopy of spent catalysts after a shorter time on stream testing of ethanol to butenes indicating the presence of coke, with bands at 1597 and 1360 cm −1 corresponding to the G band and the D band, respectively, of the graphitic carbon species [11] . However, the applied regeneration treatment (500 °C, 5 mol% O 2 /He balance) only partially recovered the Lewis acid site acidity to 36–38 μmol g −1 in comparison to the fresh catalyst (88 μmol g −1 ), even though regeneration removed a majority of deposited coke species from the catalysts, as shown in Figure 2 and Table 1.…”

“…Therefore, promotion of ethanol‐to‐butene conversion in a process similar to the ETB process can be achieved by introducing or leveraging the hydrogenation functionality (e. g., Ag and Cu) in catalysts by adding H 2 as a reactant. This has been demonstrated by our recent work on Ag−ZrO 2 /SiO 2 catalysts [11] . The reaction mechanism for ethanol to butenes was elucidated through operando‐nuclear magnetic resonance investigation coupled with reactivity measurements.…”

“…The 4Ag−4ZrO 2 /SBA‐16 catalyst was efficient for conversion of ethanol to butene‐rich hydrocarbon products. Scheme 2 depicts the major pathways of the relevant desired and side reactions, which was proposed by referring to literatures about ethanol‐to‐butadiene conversion [4f,g,5,13] and our previous study on ethanol‐to‐butene conversion [11] . In the scheme, thick arrows represent the predominant reaction pathways while thin arrows refer to minor or possible pathways.…”

“…The Ni promoted catalyst showed the highest stability because it effectively cracks coke precursors, whereas the Ag and Cu promoted catalysts were less stable but had much higher yields of butadiene when compared to the Ni promoted catalyst. Our previous work [4a,11] investigated the deactivation of Ag−ZrO 2 /SiO 2 catalysts in butadiene production. When the reaction was performed in N 2 , ethanol conversion decreased from 100 % to 80 % within 20 hours on stream because of site blocking by coke deposition, but the butadiene selectivity remained stable at 70 %.…”