IL/MIL-101-NH2 heterogeneous catalyst prepared by acid-base self-assembly for hydrodeoxygenation of 2,5-tetrahydrofurandicarboxylic acid to adipic acid

Bio-based route for preparing adipic acid (AA) from 5-hydroxymethylfurfural is the direction for achieving green and sustainable AA production. In light of the challenges, such as corrosion and difficulties in recycling of I^- containing homogeneous catalyst, large amount, high cost, and significant mass transfer resistance of ionic liquid (IL) catalyst, encountered in the current hydrodeoxygenation of 2,5-tetrahydrofurandicarboxylic acid (THFDCA) to AA process. Herein, 1-sulfobutyl-3-methylimidazolium iodide IL ([HSO₃-b-mim]·I) was used to functionalize MIL-101-NH₂ through acid-base self-assembly approach, hence, a heterogeneous catalyst, IL/MIL-101-NH₂ was designed and applied in the hydrodeoxygenation of THFDCA to bio-based AA for the first time. When reacted at 180 °C for 2 h, the THFDCA conversion and AA yield were 100 % and 99.9 %, respectively. Moreover, IL/MIL-101-NH₂ exhibited remarkable stability and could be reused multiple times with minimal loss in activity. XRD, SEM, FT-IR, BET, XPS, and TG results revealed that the functionalization process involved the reaction between HSO₃ in the IL and NH₂ in MIL-101-NH₂, forming a salt that chemically bonded the IL to MIL-101-NH₂, thereby enhancing stability. The excellent hydrodeoxygenation activity was attributed to the synergistic effect of the Lewis acid site (Cr³⁺) and the Lewis base nucleophilic site (I^-) within IL/MIL-101-NH₂.