Abstract5‐hydroxylmethylfurfural (HMF) is a bio‐based chemical that can be prepared from natural abundant glucose by using combined Brønsted–Lewis acid catalysts. In this work, Al3+ catalytic site has been grafted on Brønsted metal–organic frameworks (MOFs) to enhance Brønsted–Lewis acidity of MOF catalysts for a one‐pot glucose‐to‐HMF transformation. The uniform porous structure of zirconium‐based MOFs allows the optimization of both acid strength and density of acid sites in MOF‐based catalysts by incorporating the desired amount of Al3+ catalytic sites at the organic linker. Al3+ sites generated via a post‐synthetic modification act as Lewis acid sites located adjacent to the Brønsted sulfonated sites of MOF structure. The local structure of the Al3+ sites incorporated in MOFs has been elucidated by X‐ray absorption near‐edge structure (XANES) combined with density functional theory (DFT) calculations. The cooperative effect from Brønsted and Lewis acids located in close proximity and the high acid density is demonstrated as an important factor to achieve high yield of HMF.
Zirconium-based Metal-organic framework ((Zr6O4(OH)4(1,4-benzenedicarboxylate)6), UiO-66) is one of the most outstanding MOFs for heterogeneous catalysis owning to its high thermal and chemical stabilities. 1,4-benzenedicarboxylate, which is an organic linker of UiO-66, can be functionalized by a sulfonic acid group (UiO-66-SO3H) for several acid-catalyzed reactions. In this work, the structures and acidity of the synthesized UiO-66 and UiO-66-SO3H were characterized using X-ray diffraction, IR-spectroscopy and adsorption experiments. The adsorption of ammonia on UiO-66 type frameworks was studied on the C162H122O67SZr12 cluster model by different DFT functionals including B3LYP, CAM-B3LYP, M06-L, PBE, TPSS and ωB97X-D. The adsorption energies were calculated to be -16.4, -19.2, -18.0, -19.7, -17.4 and -19.3 kcal/mol, respectively, which agreed well with the value from MP2 calculation (-17.9 kcal/mol). This study provides a guideline on the suitable calculation models and yield more insights on the adsorption in acid-functionalized UiO-66 MOF framework.
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