Pt nanoparticles entrapped in mesoporous metal-organic frameworks MIL-101 as an efficient catalyst for liquid-phase hydrogenation of benzaldehydes and nitrobenzenes

Metal organic-framework MIL-101 and inorganic mesoporous composites Al₂O₃@SBA-15 supported Pt catalysts, Pt/MIL-101 and Pt/Al₂O₃@SBA-15 catalysts, were prepared and characterized by means of X-ray diffraction (XRD), N₂ adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), CO chemisorption and thermo-gravimetric (TG) analysis. Pt nanoparticles were highly dispersed on both supports. In liquid-phase hydrogenation of nitrobenzene, benzaldehyde and their derivatives, the Pt/MIL-101 catalyst was superior to the Pt/Al₂O₃@SBA-15 catalyst in water. For liquid-phase hydrogenation of nitrobenzene with the Pt/MIL-101 catalyst, owing to high solubility of nitrobenzene in ethanol, the reaction in ethanol went much faster than that in water, furnishing a turnover frequency (TOF) in ethanol up to 18,053 h^-1, almost triple of that obtained in water under similar conditions. The highest TOF of 25,438 h^-1 was obtained in ethanol for hydrogenation of 3-chloro-nitrobenzene with the Pt/MIL-101 catalyst. As for hydrogenation of benzaldehyde series, 2-fluoro-benzaldehyde and 3-fluoro-benzaldehyde gave the highest TOFs of 5146 h^-1 and 3165 h^-1 in water with the Pt/MIL-101 and Pt/Al₂O₃@SBA-15 catalysts, respectively. We deduce that surface property of MIL-101 with high hydrophobicity is helpful to enrich reactants around the Pt/MIL-101 catalyst in water, where nitrobenzene or benzaldehyde and its derivatives have a limited solubility, so that high catalytic performance was achieved with the Pt/MIL-101 catalyst in water. Of particular note is that the Pt/MIL-101 catalyst can be reused at least four times without loss in activity or selectivity.