Isomerization of glucose at hydrothermal condition with TiO2, ZrO2, CaO-doped ZrO2 or TiO2-doped ZrO2

• Hydrothermal isomerization of glucose into fructose with solid base catalysts.
• Rate constants of the isomerization of glucose into fructose were determined.
• The base/acid ratios of various solid base catalysts were studied.
• Catalyst with a base/acid ratio over 1.0 is preferred for glucose isomerization.

Catalytic activity of TiO2, ZrO2 and ZrO2 solid solutions (with CaO or TiO2) for the isomerization of glucose into fructose under hydrothermal conditions (120–180 °C for 5–15 min reaction time) was evaluated by experimental kinetic studies and surface acidity–basicity measurements. Kinetic studies were conducted with batch reactors heated by microwave. Under hydrothermal conditions regardless of the temperature, fructose yield was always below 10% and fructose selectivity was rapidly decreased with increasing glucose conversion. By adding 10 mM NaOH, fructose yield reached 20% at 160 °C for 5 min. In the presence of TiO2, fructose yield and selectivity were similar (160 °C, 47% of glucose conversion and 14% fructose yield). ZrO2 showed higher catalytic activity (160 °C, 63% glucose conversion and 21% fructose yield) compared with TiO2. The catalytic activity of TiO2 doped ZrO2 was between TiO2 and ZrO2. To increase the basicity of ZrO2, CaO was doped into the ZrO2 matrix. For experiments with the 24 wt% CaO doped ZrO2, fructose selectivity was higher than 70% even at 30% glucose conversion at 160 °C for 15 min. For systematical understanding catalytic activity of the metal oxides used, acidity and basicity of the catalysts were measured by temperature programmed desorption (TPD) using either CO2 or NH3. It was found that basicity on the surface increased with increasing the amount of CaO in the ZrO2 solid solution while acidity increased with amounts of TiO2. For a simple network model of glucose reactions, rate constants were fitted to the data assuming a simple network model and they were completed with the acid-base properties of TiO2, ZrO2 and CaO doped and TiO2 doped ZrO2. As a result, the reactivity was found to be correlated with the ratio of the base to acid sites on the surface and fructose formation was linearly proportional to the base/acid mole ratio on the surface of the catalysts.

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