Fructose production via extraction-assisted isomerization of glucose catalyzed by phosphates
Fructose presents a highly attractive substrate for future biorefineries for production of biofuels as well as bulk and fine chemicals. Starting from cellulose, fructose is available via hydrolysis to glucose followed by isomerization. Despite commercial production of high-fructose corn syrup, efficient strategies for a combined chemo-catalytic isomerization and fructose separation are missing. This contribution describes a three-step approach combining the isomerization of glucose into fructose with recovery of the product. The first step covers isomerization of glucose into fructose using water as a solvent and soluble Na2HPO4 + NaH2PO4 as a catalyst. Na2HPO4 + NaH2PO4 exhibits catalytic activity under mild reaction conditions at pH 7.5. The specific-base-catalyzed isomerization proceeds via the formation of an enediol anion. The yield of fructose obtained using phosphates under batch conditions reaches 30%. The second step relates to recovery of fructose by selective anionic extraction due to complexation with ortho-hydroxymethyl phenylboronic acid (HMPBA). Under optimized conditions, up to 72% of fructose can be extracted from a glucose–fructose mixture with 76% selectivity. Fructose is extracted as a mixture of complexes with HMPBA exhibiting 1-to-1 stoichiometry. Formulae of two complexes were proposed, namely β-fructofuranoside esterified at C2 and C3 positions and an ester of β-fructopyranoside at C1 and C2. After extraction, the aqueous phase containing phosphate and the remaining glucose can be recycled. In the third step, fructose can be back-extracted from the organic phase with an acidic solution. This extraction-assisted isomerization strategy significantly improves the yield of fructose reaching up to 51%.