Elucidating the catalytic role of lithium (Li) in the glucose-to-fructose isomerization over Li-C3N4 catalyst at 60 °C in water

Abstract

Fully utilizing renewable biomass energy is important for saving energy, reducing carbon emissions, and mitigating climate change. As the main hydrolysate of cellulose, a primary component of lignocellulose, glucose could be employed as a starting material to prepare some other functional derivatives for improving the value of biomass resources. The isomerization of glucose to produce fructose is an important intermediate process during numerous high-value-added chemical preparations. Therefore, the development of efficient and selective catalysts for glucose isomerization is of great significance. Currently, glucose isomerase catalysts are limited by the harsh conditions required for microbial activity, which restricts further improvements in fructose yield. Additionally, heterogeneous Brønsted-base and Lewis-acid catalysts commonly employed in chemical isomerization methods often lead to the formation of undesirable by-products, resulting in reduced selectivity toward fructose. This study has demonstrated that lithium-loaded heterogeneous catalysts possess excellent isomerization capabilities under mild conditions. A highly efficient Li-C₃N₄ catalyst was developed, achieving a fructose selectivity of 99.9% and a yield of 42.6% at 60 °C within 1.0 h—comparable to the performance of the enzymatic method. Characterization using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), proton nuclear magnetic resonance (¹H NMR), and inductively coupled plasma (ICP) analyses confirmed that lithium was stably incorporated into the g-C₃N₄ framework through the formation of Li−N bonds. Further investigations using CO₂ temperature-programmed desorption (CO₂-TPD), in situ Fourier-transform infrared spectroscopy (FT-IR) and ⁷Li magic angle spinning nuclear magnetic resonance (⁷Li MAS NMR) indicated that the isomerization proceeded via a base-catalyzed mechanism. The Li species were found to interact with hydroxyl groups generated through hydrolysis and simultaneously coordinated with nitrogen atoms in the C₃N₄ matrix, resulting in the formation of Li-N₆-H₂O active sites. These active sites facilitated the deprotonation of glucose to form an enolate intermediate, followed by a proton transfer step that generated fructose. This mechanism not only improved the efficiency of fructose production but also provided valuable insight into the catalytic role of lithium within the isomerization process.