Ammonia-treated metal oxides as base catalysts for selective isomerization of glucose in water

Abstract

Various metal oxides (MgO, Al2O3, SiO2, TiO2, ZrO2, Nb2O5, and CeO2) were treated with NH3 for incorporating nitrogen into the structure of metal oxides. The pristine and NH3-treated metal oxides were used as solid base catalysts for isomerization of glucose to f7ructose in water. SiO2 and Nb2O5 showed a large increment of nitrogen content by the treatment with NH3. FT-IR study on NH3-treated SiO2 revealed that NH3 reacted with terminal silanol groups and siloxane bonds to form Si-NH2 and Si-NH-Si groups. Meanwhile, nitrogen was incorporated into bulk crystal of Nb2O5, causing amorphization of the crystal. Of all the samples, pristine and NH3-treated MgO showed highest catalytic activity for the isomerization of glucose. However, the selectivity to fructose was low due to subsequent reactions of formed fructose. Catalytic activity of Al2O3 and SiO2 was increased by the treatment with NH3, while that of the other metal oxides was not affected. Particularly, the catalytic activity of SiO2 emerged after the treatment with NH3 and was enhanced by increasing the temperature for the treatment up to 800 °C. While fructose was consumed by subsequent reactions over MgO, decreasing the selectivity, such reactions of fructose were not noticeable over NH3-treated SiO2. Consequently, NH3-treated SiO2 showed higher selectivity to fructose and gave higher carbon balance than MgO.