Selective catalytic conversion of bio-ethanol to propene: A review of catalysts and reaction pathways

• New catalytic reaction for selective conversion of ethanol to propene was found.
• The order of catalytic activity was Sc/In2O3 > Y2O3-CeO2 > Ni-M41.
• On Ni-M41, the metathesis reaction of ethene and butenes was a key step.
• On the two oxides, ethanol → acetaldehyde → acetic acid → acetone → propene.
• There were two ways for the hydrogenation of acetone to 2-propanol.

The conversion of ethanol to propene were examined on Ni ion-loaded silica MCM-41(Ni-M41), Sc-modified In2O3 (Sc/In2O3), and a solid solution of Y2O3-CeO2. The propene production activity was in the order, Sc/In2O3 > Y2O3-CeO2 > Ni-M41, while their stability during the reaction was Y2O3-CeO2 ∼ Sc/In2O3 > Ni-M41. The propene yield and durability of Sc/In2O3 were greatly improved by addition of water and hydrogen in the reactant stream. The reaction mechanism was greatly dependent on the catalyst employed. On Ni-M41, the metathesis reaction of ethene and butenes, produced through dimerization of ethene, was a key step for the propene formation. On the remaining two oxide catalysts, the major pathways were the common: ethanol → acetaldehyde → acetone → propene. The detailed reaction pathways, however, were different from each other. On Sc/In2O3, acetaldehyde was oxidized to acetic acid with water or a surface hydroxyl group and the resulting acetic acid was converted to acetone and carbon dioxide through ketonization. On the other hand, on Y2O3-CeO2, acetaldehyde was converted to ethyl acetate, and then it decomposed to form acetic acid and ethene. Acetic acid was converted to acetone and carbon dioxide in the same manner as that on Sc/In2O3. The by-production of much amounts of ethene was characteristic on Y2O3-CeO2. On the Sc/In2O3 oxide, a hydrogen molecule was active for the hydrogenation of acetone to 2-propanol. In contrast, on the Y2O3-CeO2 oxide, hydrogenation of acetone did not proceed with hydrogen but did with the co-fed ethanol, that is, by the Meerwein–Ponndorf–Verley reduction.

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