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 Al(3+) , Cr(3+) , and Fe(3+) were each introduced, together with Sc(3+) , 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-Fe2 O3 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-Fe2 O3 nanoparticles were also active Lewis acid species, although less active than Sc(3+) in trimer sites. The incorporation of Fe(3+) into MIL-100(Sc) imparts activity for oxidation catalysis and tandem catalytic processes (Lewis acid+oxidation) that make use of both catalytically active framework Sc(3+) and Fe(3+) . A procedure for using these mixed-metal heterogeneous catalysts has been developed for making ketones from (hetero)aromatics and a hemiacetal.

Mixed-metal metal–organic frameworks

“…This is illustrated in Figure 12 a for partially Fe substituted MIL-100(Sc). 55 Regardless of the absolute determination, in situ or ex situ observation of shifts in the absorption edge position as a result of chemical reactions and thermal treatments can readily be interpreted in terms of an increase or decrease of the average oxidation state. Walton et al used this effect to monitor the Fe 3+ to Fe 2+ reduction in MIL-53(Fe) upon insertion of Li + (Figure 12 b).…”

Complete multinuclear solid‐state NMR of metal‐organic frameworks: The case of α‐Mg‐formate

“…There have been many recent reports that include the use of metal SSNMR to characterize MOFs . Although metal centers are ubiquitous within MOFs and should serve as a convenient spectroscopic handle for SSNMR characterization, many MOFs feature multiple non‐equivalent metal sites that reside in similar chemical environments, which give rise to overlapping signals and pose a severe obstacle to metal SSNMR characterization.…”

Metal‐organic Frameworks Incorporating Multiple Metal Elements

“…The ability to combine multiple metal cations allows the modification of the catalytic properties of a certain MOF type. Thus, the catalytic activity of a series of solid solution MIL‐100 (Sc, M, with M=Al, Cr, Fe) materials in Lewis acid catalysed Friedel‐Craft reactions was found to be related to the amount of scandium . In another example of multi‐metal MOFs based on the same trimeric SBU, Deng et al .…”