Porous MgO-C Composite Catalyst for Glucose Isomerization to Fructose

Objective Glucose isomerization to fructose represents a pivotal step in the synthesis of high-value platform chemicals, such as 5-hydroxymethylfurfural, levulinic acid, and lactic acid. These compounds function as vital precursors for the development of biofuels, bioplastics, and fine chemicals. The industry mainly relies on glucose isomerase to catalyze glucose isomerization into fructose, but its practical application remains constrained by factors such as high costs, difficulties in catalyst recovery, and stringent reaction conditions. Therefore, chemical catalysis has increasingly garnered attention as an alternative approach. Metal oxides, which possess abundant acidic and basic sites on their surfaces, are typically employed for the catalytic isomerization of glucose to fructose. Nevertheless, the limited specific surface area of metal oxides often leads to the concealment of active sites, thereby diminishing their catalytic performance. This study aims to develop an efficient catalytic system by dispersing magnesium oxide (MgO) on a biomass-derived support, thereby enhancing catalytic performance and facilitating the production of high-value sugar-based chemicals.

Methods In this work, the phenolic hydroxyl groups of alkali lignin were utilized to interact with Mg²⁺ via electrostatic and coordination interactions, forming a hydrogel precursor with a three-dimensional pore structure. By calcination, the precursor was further transformed into MgO-C composite catalyst for the glucose isomerization to fructose. The morphology and pore structure of the catalyst were investigated using scanning electron microscopy, transmission electron microscopy, and a specific surface area and porosity analyzer. The chemical composition and structural properties of the catalyst were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. The catalytic performance in glucose isomerization was evaluated through high-performance liquid chromatography. The key reaction parameters, including MgO loading amount, solvent type, glucose concentration, reaction temperature and time, were systematically optimized.

Results Characterization results revealed that the 20 MgO-C composite catalyst featured a three-dimensional hierarchical pore structure, with a typical type-IV N₂ adsorption-desorption isotherm, a specific surface area of 73.73 m²/g, a pore volume of 0.23 cm³/g, and pore sizes predominantly in the meso- to macroporous range. MgO was highly dispersed on the biochar surface, and the MgO-C composite was enriched with defect structures, thereby exposing numerous active sites to efficiently promote the isomerization of glucose to fructose. The strong interaction between MgO and the carbon matrix significantly enhanced the composite’s chemical and thermal stability. Catalytic tests demonstrated that the increase of MgO loading amount improved the number of active sites, thereby enhancing the glucose-to-fructose conversion rate. However, an excess of active sites led to the further degradation of fructose, thereby decreasing its selectivity. Solvent type had a pronounced effect on catalytic efficiency, with methanol showing the highest performance. In addition, increasing reaction temperature and extending reaction time could improve glucose conversion efficiency. However, such conditions also facilitated the further fructose conversion, thereby resulting in the accumulation of the by-products. Through the optimization of the reaction conditions, glucose (75 g/L) could be efficiently converted into fructose over the MgO-C catalyst (40 mg) in methanol at 100 ℃ for 30 min, achieving a high fructose yield of 34.9% with a selectivity of 81.4%.

Conclusions This study successfully demonstrates that MgO can be effectively dispersed through the rational design of a biomass-derived precursor. The resulting MgO-C composite catalyst exhibits a hierarchical pore structure, highly dispersed active sites, and favorable synergistic effect, leading to excellent catalytic performance in glucose isomerization. This strategy provides a promising solution to the common problem of metal oxide aggregation and holds great application potential for the green conversion of sugar-based chemicals.