Acidity and basicity of metal oxide catalysts for formaldehyde reaction in supercritical water at 673 K

• Published in: Applied catalysis. A, General Vol. 245; no. 2; pp. 333 - 341
• Main Authors: Watanabe, Masaru, Osada, Mitsumasa, Inomata, Hiroshi, Arai, Kunio, Kruse, Andrea
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 10.06.2003; Elsevier
• Subjects: Catalysis; Catalytic reactions; CeO 2; Chemistry; Exact sciences and technology; Formaldehyde; General and physical chemistry; Hydroxyl ion; Metal oxide catalyst; MoO 3; Solid acid and base; Supercritical water; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; TiO 2; ZrO 2; Hydroxyl ion; CeO 2; MoO 3; ZrO 2; Metal oxide catalyst; Supercritical water; Solid acid and base; TiO 2; Formaldehyde; MoO3; Base catalysis; Acidity; Acid catalysis; Aldehyde; Experimental study; Heterogeneous catalysis; ZrO2; Transition metal Oxides; Cannizzaro reaction; Homogeneous catalysis; Supercritical solvent; CeO2; Alkalinity; Aqueous solution; Hydroxyl; Lanthanide Oxides; TiO2
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/S0926-860X(02)00656-7

Formaldehyde (HCHO) reactions in supercritical water (673 K and 25–40 MPa) with and without acid and base catalysts (homogeneous: H 2SO 4 and NaOH, and heterogeneous: CeO 2, MoO 3, TiO 2, and ZrO 2) were conducted by use of batch reactors. Cannizzaro reaction (2HCHO+H 2O→CH 3OH+HCOOH) and self-decomposition of HCHO (HCHO→CO+H 2) were found to be primary reactions for all the cases and the contribution of each reaction depended on the condition. In the case of the homogeneous systems, Cannizzaro reaction became dominant with increasing bulk hydroxyl ion (OH −). The simple network model can well express the experimental results in the homogeneous conditions. We correlated the ratio of the yield of CH 3OH to that of CO (at 15 min) against bulk OH − in the homogeneous system. For elucidating acidity and basicity of metal oxide catalysts on HCHO reaction in supercritical water, OH − concentration on the metal oxide surface was calculated by use of the above correlation and the following order was found: CeO 2>ZrO 2>MoO 3>TiO 2 (rutile)>TiO 2 (anatase). At the reaction condition, CeO 2 and ZrO 2 were base catalysts, and MoO 3 and TiO 2 were acid catalysts. The experimental results with the metal oxides can be expressed by the model that was developed under homogeneous systems, with the values of the OH − concentrations that were calculated from the correlation about the CH 3OH/CO ratio at 15 min of reaction time.