Effects of water density on acid-catalytic properties of TiO2 and WO3/TiO2 in supercritical water

• Effects of water density on solid acidity in supercritical water were investigated.
• TiO2 showed Lewis acidity at low water density.
• Brönsted acidity of TiO2 increased with increasing water density.
• WO3/TiO2 showed Brönsted acidity regardless of water density.

The effects of water density on the acid-catalytic properties of TiO2 and WO3/TiO2 catalysts in supercritical water at 400 °C were investigated by using the kinetic analysis of the dehydration reaction of glycerol. The reaction selectivity of TiO2 and WO3/TiO2 catalysts and the apparent-reaction orders for water indicated that the acid-catalytic properties of these two catalysts show different dependence on water density. In the reaction using TiO2, the contribution of Lewis acid sites in TiO2 was large at a low water density, while the contribution of Brönsted acid sites in TiO2 increased with increasing water density. On the other hand, the reaction using WO3/TiO2 was mainly catalyzed by Brönsted acid sites in WO3/TiO2 even at a low water density, and the nature of Lewis/Brönsted acid sites in WO3/TiO2 was not influenced by the water density.

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