High Yield of L‐Sorbose via D‐Glucose Isomerization in Ethanol over a Bifunctional Titanium‐Boron‐Beta Zeolite

Abstract D‐Glucose‐to‐L‐sorbose isomerization on Lewis acidic zeolite is a highly attractive avenue for sorbose production. But the L‐sorbose yield is limited by the competing D‐glucose‐to‐D‐fructose isomerization and reaction equilibrium. In this work, it is suggested that ethanol directs the glucose conformation for selective D‐glucose‐to‐L‐sorbose isomerization. It also reacts with the produced L‐sorbose to form ethyl‐sorboside, which allows the D‐glucose‐to‐L‐sorbose isomerization to proceed beyond the thermodynamic equilibrium limit. It is shown that a bifunctional zeolite Beta containing framework titanium (Ti) and boron (B) is a selective catalyst for this tandem reaction: Lewis acidic framework Ti catalyzes the D‐glucose‐to‐L‐sorbose isomerization via an intramolecular 5,1‐hydride shift process as confirmed by isotopic tracing experiments followed by 13 C‐NMR, while weak Brønsted acid framework B selectively promotes the sorbose ketalization with ethanol. A remarkably high yield of L‐sorbose with a high fraction of sugar (>95 %: 27 % unreacted glucose, 11.4 % fructose, 57 % sorbose) was obtained after the mixture produced in ethanol was hydrolyzed.