Defective MIL-53(Al)-NH2: A bifunctional Lewis-Brønsted acid catalyst for efficient 5-HMF production from glucose

Developing functional catalysts with both Lewis and Brønsted acid sites is an essential task for achieving the direct conversion of glucose to 5-hydroxymethylfurfural (5-HMF), a process that is crucial for the sustainable production of renewable fuels and chemicals. In this work, a mixed-linker strategy is proposed to prepare defective MIL-53(Al)-NH2 by introducing trifluoroacetic acid into the MIL-53(Al)-NH2 synthesis process. The creation of defects facilitates the exposure and accessibility of Al3+ centers, as well as changes the coordination environment within the MIL-53(Al)-NH2 skeleton. Both the number of Lewis and Brønsted acid sites are boosted, which is beneficial for glucose activation and 5-HMF generation. Through optimizing the reaction conditions, a glucose conversion of 98.3% with a 5-HMF yield of 59.3% was achieved by using defective 6d-MIL-53(Al)-NH2 in a biphasic tetrahydrofuran/H2O-NaCl system. Moreover, the 6d-MIL-53(Al)-NH2 catalyst showed excellent recyclability and reusability, with its catalytic efficiency maintained almost unchanged for consecutive 10 cycles. The catalytic mechanism was elucidated and the acidity of the Lewis and Brønsted acid was modulated by defect engineering to achieve exceptional performance.