Highly efficient isomerization of glucose to fructose over a novel aluminum doped graphitic carbon nitride bifunctional catalyst

Glucose isomerization to fructose is a crucial step for the efficient production of fuel and valuable chemicals from renewable carbohydrates. Aluminum (Al) was introduced into the structure of a graphitic carbon nitride (g-C3N4) by a simple thermal polycondensation of urea and aluminum chloride, and thereby, provided a series of Al-doped g-C3N4 (xAl-UCN) catalysts for the isomerization reaction. A high fructose yield of 48.29%, comparable to the quantitative yields (ca. 50%) by enzymatic routes, was achieved with the 0.5Al-UCN catalyst. Detailed characterizations of a series of Al-UCN catalysts, with different Al loadings, showed that 1) the Al loading amount has a profound effect on the physical-chemical properties of Al-UCN catalysts; 2) excess Al loading can inhibit the formation of a crystalline g-C3N4 structure; and 3) at lower Al loadings, the Al appears to be doped into the g-C3N4 framework possibly via coordination bonds. The mechanism for glucose isomerization to fructose may involve both Lewis acid and base catalyzed routes, with the coordinated Al species (Al[6]) providing Lewis acidity, and N-containing groups on g-C3N4 providing basicity. The additive effect of this dual-functionality is likely responsible for the high catalytic activity. The 0.5Al-UCN catalyst was readily recycled, demonstrating near-constant activity after 5 cycles of use. Altogether, glucose isomerization to fructose over the readily synthesized and recyclable Al-UCN catalyst could provide a highly-efficient and cost-effective step in lignocellulosic biomass valorization.

Automated summary

In this study, Al-doped g-C3N4 catalysts were successfully synthesized for the isomerization of glucose to fructose, achieving high catalytic activity and fructose yield. Detailed characterizations of the catalysts showed the profound effect of Al loading amount on the physical-chemical properties and revealed the mechanism of the isomerization reaction.