Insight into Structural and Physicochemical Properties of ZrO₂-SiO₂ Monolithic Catalysts with Hierarchical Pore Structure: Effect of Zirconium Precursor

Zirconia-silica monolithic catalysts with hierarchical micro/macroporous structure were obtained in a sol-gel process combined with phase separation using inorganic salts, i.e., oxychloride, oxynitrate and sulphate, as a zirconium source. It was found that the use of zirconium oxychloride and prehydrolysis of tetraethoxysilane (TEOS) resulted in materials characterized by a well-developed continuous structure of macropores with a diameter of ca. 10 mu m. For zirconium oxynitrate and sulfate modified materials, the prehydrolysis hardly affected the macropore size. The micropores with a diameter of 1.5 nm in the skeleton of all materials provided a large surface area of 550-590 m(2)/g. A high dispersion of zirconia in the silica skeleton in all studied materials was shown. However, the largest surface concentration of Lewis and Br & ouml;nsted acid sites was found in the monolith synthesized with zirconium oxychloride. The monoliths were used as a core for continuous-flow microreactors and high catalytic activity was confirmed in the deacetalization of benzylaldehyde dimethyl acetal. The process was characterized by a high efficiency at low temperature, i.e., 35 degrees C.