Construction of Lewis-Brønsted bifunctional carbonaceous acidic catalyst for efficient transformation of glucose into 5-HMF

Here, a bifunctional carbonaceous solid acid (LDMC-Al0.2-S0.5-300) with both Lewis and Brønsted acid sites (LDMC-Al0.2-S0.5-300) was synthesized by using alkali lignin as carbon source, KCl as salt template, Al(NO3)3·9H2O and p-toluenesulfonic acid as catalytic site precursors, respectively. The as-synthesized LDMC-Al0.2-S0.5-300 catalyst possessed rich oxygen-containing functional groups, high L/B acid sites ratio, appropriate specific surface area and hierarchical porous structure, and could achieve the selective conversion of glucose to 5-HMF with a 61.1 % yield in the aqNaCl-MIBK biphasic solvent system (1:3, v/v) at 170 °C for 1.0 h. The excellent catalytic performance mainly attributed to the strong synergistic effect between the L and B acid sites and the rapid extraction of the generated 5-HMF from aqNaCl to MIBK. In addition, the established reaction kinetic model could accurately predict the concentrations of glucose, fructose, 5-HMFaqNaCl and 5-HMFMIBK, in agreement with the experimental data, indicating its reasonableness. Meanwhile, the activation energies of isomerization and dehydration over LDMC-Al0.2-S0.5-300 catalyst were lower than those of other catalytic systems, which greatly decreased the energy barrier of glucose conversion to 5-HMF, thereby realizing its efficient transformation under relatively mild reaction conditions. Overall, an efficient heterogeneous catalytic system was established in this work, demonstrating great potential in the direct transformation of renewable carbohydrates into platform chemicals.