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

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.