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

Formaldehyde (HCHO) reactions in supercritical water (673K and 25-40MPa) with and without acid and base catalysts (homogeneous: H₂SO₄ and NaOH, and heterogeneous: CeO₂, MoO₃, TiO₂, and ZrO₂) were conducted by use of batch reactors. Cannizzaro reaction (2HCHO+H₂O → CH₃OH+HCOOH) and self-decomposition of HCHO (HCHO → CO+H₂) 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 CH₃OH to that of CO (at 15min) 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: CeO₂ > ZrO₂ > MoO₃ > TiO₂ (rutile) > TiO₂ (anatase). At the reaction condition, CeO₂ and ZrO₂ were base catalysts, and MoO₃ and TiO₂ 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 CH₃OH/CO ratio at 15min of reaction time.