Conversions of some small organic compounds with metal oxides in supercritical water at 673 K

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Abstract
Reactions of formaldehyde (HCHO), acetic acid (CH3COOH), 2-propanol (2-PrOH), and glucose with some metal oxides (CeO2, MoO3, TiO2, and ZrO2) were conducted in supercritical water at 673 K and 25–35 MPa, using batch reactors. For the reactions of HCHO, CeO2 and ZrO2 showed basicity, on the other hand, MoO3 and TiO2 were acid catalysts. ZrO2 catalyst promoted bimolecular decarboxylation of CH3COOH to form acetone, which indicates that both acid and base sites exist on the surface of ZrO2 in supercritical water. Dehydration of 2-PrOH with formation of propylene was promoted by acid catalyst (H2SO4), while its dehydrogenation with formation of acetone was catalyzed by alkali (NaOH). All the metal oxides that were used in this study promoted dehydration of 2-PrOH; namely there are mainly acidic sites for 2-PrOH reactions on the surface of all the metal oxides under the conditions used. Among the metal oxides, ZrO2 and TiO2 (rutile) enhanced the formation of acetone in the case of 2-PrOH reaction. This means there are also basic sites for 2-PrOH on the ZrO2 and TiO2 (rutile). In supercritical water at 673 K and 15 min, H2 yield from glucose in the acidic atmosphere (namely in the presence of H2SO4) is lower than that in the absence of additive whereas, on the other hand, the H2 yield in the presence of NaOH is twice as much as that in the absence of the additive. With CeO2 and ZrO2, the H2 yield from glucose was almost twice as high as that without catalyst. By adding MoO3 and TiO2, the amount of H2 formation was suppressed. Through this study, we can show the generality of acidity and basicity of the metal oxides for organic reactions in SCW.