(338d) Isomerization of Glucose to Fructose with Brønsted Base Catalysts

The isomerization of glucose to fructose represents a key intermediate step in the synthesis of biofuels and platform chemicals from renewables. Immobilized xylose isomerase, the current industrial standard for the production of high fructose corn syrup (HFCS), isomerizes glucose to fructose with 42% yield, thus approaching thermodynamic equilibrium. While efficient, this biocatalyst is expensive and requires to accurately control pH, temperature, and feedstock purity to avoid irreversible deactivation. Significant efforts have been devoted to the design of more robust chemical catalysts that could replace the enzyme. Glucose isomerization is catalyzed both by Lewis acids (intramolecular hydride shift) and Brønsted bases (proton transfer). The Lewis acid catalyzed route utilizing Sn-Beta zeolite was found to produce fructose in high yields (31%) with good selectivity (57%). In contrast, the base catalyzed isomerization is typically not considered a viable route for the production of fructose, likely due to reportedly low yields (<10%).

We demonstrate in this work that several commercial organic Brønsted bases, especially amines, are excellent glucose isomerization catalysts [1]. The initial screening performed under homogeneous conditions revealed that the undesired Maillard reaction contributes to byproducts formation for primary and secondary amines but not for tertiary amines. Trietlylamine (TEA) was found to be particularly promising: similar fructose selectivity (64%) and yield (32%) as Sn-Beta were obtained, which opens new perspectives for the base-catalyzed pathway. In situ ¹H NMR, isotopic studies, and reaction kinetics revealed all the steps involved in the reaction pathway [2]. Rate constants obtained from glucose conversion traces are independent of [glucose] and increase with pH. In contrast, analogous studies with fructose showed that byproducts result from thermal and alkaline degradation of fructose. Our results point to unimolecular proton transfer from the anionic acyclic form of the sugars as the rate determining step in isomerization. The obtained results served to design original heterogeneous amine-functionalized nanocarbons catalysts from bottom up.

[1]  Liu, C.; Carraher, J. M.; Swedberg, J. L.; Herndon, C. R.; Fleitman, C. N.; Tessonnier, J.-P.; Selective Base-Catalyzed Isomerization of Glucose to Fructose, ACS Catal. 2014, 4, 4295-4298.  http://dx.doi.org/10.1021/cs501197w

[2]  Carraher, J. M.; Fleitman, C. N.; Tessonnier, J.-P.; Kinetic and Mechanistic Study of Glucose Isomerization Using Homogeneous Organic Brønsted Base Catalysts in Water, ACS Catal. 2015, 5, 3162-3173.  http://dx.doi.org/10.1021/acscatal.5b00316