Catalytic glucose and fructose conversions with TiO₂ and ZrO₂ in water at 473 K: Relationship between reactivity and acid-base property determined by TPD measurement

The effects of TiO₂ (anatase TiO₂ or rutile TiO ₂) and ZrO₂ (monoclinic/tetragonal mixture ZrO ₂) on glucose and fructose reactions were examined in hot-compressed water at 473 K with a batch type reactor. Rutile TiO₂ (r-TiO ₂) is inactive during glucose reactions. A monoclinic/tetragonal mixture of ZrO₂ (m/c-ZrO₂) was the basic catalyst for the reaction at this temperature. Anatase TiO₂ (a-TiO₂) showed both acidity and basicity during the reactions. In order to understand the catalytic activity of a-TiO₂, r-TiO₂, and m/c-ZrO ₂, we measured the acidity and basicity by means of NH₃- and CO₂-TPD, respectively. The TPD analysis showed us that the amount of acid (670 μmol/g) and base (550 μmol/g) sites on m/c-ZrO₂ were the highest among these three catalysts, while the density of acid site (17 μmol/m²) and that of basic site (9 μmol/m²) of the a-TiO₂ were the highest. Such the acidity and basicity analyses suggested that the amount of basicity was the key factor for the isomerization while the density of acidity and basicity was important for the HMF formation from glucose.