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.