Mixed-metal MIL-100(Sc,M) (M=Al, Cr, Fe) for Lewis acid catalysis and tandem C−C bond formation and alcohol oxidation

Abstract

The trivalent metal cations Al3+, Cr3+, and Fe3+ were each introduced, together with Sc3+, into MIL-100(Sc,M) solid solutions (M=Al, Cr, Fe) by direct synthesis. The substitution has been confirmed by powder X-ray diffraction (PXRD) and solid-state NMR, UV/Vis, and X-ray absorption (XAS) spectroscopy. Mixed Sc/Fe MIL-100 samples were prepared in which part of the Fe is present as α-Fe2O3 nanoparticles within the mesoporous cages of the MOF, as shown by XAS, TGA, and PXRD. The catalytic activity of the mixed-metal catalysts in Lewis acid catalysed Friedel–Crafts additions increases with the amount of Sc present, with the attenuating effect of the second metal decreasing in the order Al>Fe>Cr. Mixed-metal Sc,Fe materials give acceptable activity: 40 % Fe incorporation only results in a 20 % decrease in activity over the same reaction time and pure product can still be obtained and filtered off after extended reaction times. Supported α-Fe2O3 nanoparticles were also active Lewis acid species, although less active than Sc3+ in trimer sites. The incorporation of Fe3+ into MIL-100(Sc) imparts activity for oxidation catalysis and tandem catalytic processes (Lewis acid+oxidation) that make use of both catalytically active framework Sc3+ and Fe3+. A procedure for using these mixed-metal heterogeneous catalysts has been developed for making ketones from (hetero)aromatics and a hemiacetal.