Comparative investigation of homogeneous and heterogeneous Brønsted base catalysts for the isomerization of glucose to fructose in aqueous media

Glucose isomerization to fructose via Brønsted base catalysis is sparking increasing interest due to the importance of fructose as a feedstock molecule for the production of renewable chemicals. The work reported here stems from an interest in understanding the key parameters governing glucose isomerization in addition to solution basicity, and in identifying catalyst features that are key to fructose selectivity. A wide range of homogeneous and heterogeneous Brønsted bases were investigated under identical reaction conditions to enable their cross comparison. The results showed that glucose conversion was positively correlated with the basicity of the solution, although exceptional cases were observed owing to the specific structure of that catalyst. Interactions between the cations associated with metal hydroxide catalysts and carbohydrates affected the isomerization of glucose. Likewise, cations formed through protonation of organic catalysts in water (e.g. amines) were shown to alter the glucose isomerization process and impact both conversion and selectivity under iso-pH conditions. Principal component analysis revealed that the catalytic patterns were dependent on the catalyst nature and structure. Overall, meglumine showed a superior catalytic performance compared to other homogeneous bases with a yield of 35% fructose and approximately 80% selectivity. These results obtained under identical experimental conditions will help to identify promising catalyst structures for future heterogeneous catalyst design.