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; H2BPEB = 1,4-bis(1H-pyrazol-4-ylethynyl)benzene; H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene], possessing square planar, potentially accessible metal sites, were preliminarily tested as catalysts in the base-free selective oxidation of hydroxymethylfurfural to 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 O2, water as solvent). Upon flowing a model probe, in situ DRIFT and FT-IR spectroscopy were employed to rationalize the different performances of Ni(BPEB) and Ni3(BTP)2 in terms of adsorbate-adsorbent interactions: Not only hydrogen bonds are at work between the hydroxyl functionality of the probe and the pore walls of the MOF, but also and more importantly, bands 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.