Adsorbent_Adsorbate Interactions in the Oxidation of HMF Catalyzed by Ni-Based MOFs: A DRIFT and FT-IR Insight

The three Ni-based metal-organic frameworks (MOFs) Ni(BDP), Ni(BPEB) and Ni3(BTP)2 [H2BDP = 1,4-(4-bispyrazolyl)benzene; H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene], possessing square planar, potentially accessible metal sites, were preliminarily tested as catalystsin the base-free selective oxidation of hydroxymethylfurfuralmto 2,5-diformylfuran (DFF). While Ni(BDP) undergoes degradation, Ni3(BTP)2 is the most active of the three MOFs, yielding 27% DFF after 24 h with a selectivity close to 100% under relatively mild reaction conditions (120 °C, 30 bar, water as solvent). Upon flowing a model probe, in situ DRIFT and FT-IR spectroscopy were employed to rationalize the different performances of Ni(BPED) and Ni3(BTP)2 in terms of adsorbate-adsorbent interactions. Not only hydrogen bonds are at work between the hydroxyl functionalityof the probe and the pore walls of the MOF, but also and more importantly, banfs ascribed to Ni-OR stretching are detected, denouncing the insurgence of Ni-probe interactions, The different intensity of these bands in the two cases confirms the different accessibility of the metal centers, as suggested by Crystal structure analysis and catalytic tests.