Methanol Oxidation to Formate on ALD-Prepared VO_<i>x</i>/θ-Al₂O₃ Catalysts: A Mechanistic Study

Well-defined supported VO_<i>x</i>/θ-Al₂O₃ catalysts were prepared by atomic layer deposition (ALD) with vanadium coverages of 0.48, 1.20, and 3.40 wt %. In-situ Raman and UV–vis diffuse reflectance spectroscopy confirm that the monovanadate, VO₄, is the predominant vanadium species at low loadings (0.48 and 1.20 wt %), while polyvanadate VO₄ is the predominant vanadium species for the 3.40 wt % VO_<i>x</i>/θ-Al₂O₃ catalyst. In-situ FTIR spectroscopy of methanol oxidation to formate, in the absence of gas-phase oxygen, on the 0.48 wt % VO_<i>x</i>/θ-Al₂O₃, identified two different formates. A comparison of the frequencies for the formates adsorbed on just V₂O₅ and on just θ-Al₂O₃ demonstrates that one of these formates is located on aluminum sites of VO_<i>x</i>/θ-Al₂O₃ while the other is located on vanadium sites. The oxidation state of vanadium for the VO_<i>x</i>/θ-Al₂O₃ catalyst was determined by XPS after different reaction times. On the basis of the time dependence of the formate absorptions and the change in the oxidation state of vanadium in VO_<i>x</i>/θ-Al₂O₃, a mechanism is proposed for methanol oxidation and we discuss the role of the alumina support in the mechanism

Tuning Lewis Acidity of Metal–Organic Frameworks via Perfluorination of Bridging Ligands: Spectroscopic, Theoretical, and Catalytic Studies

The Lewis acidity of metal–organic frameworks (MOFs) has attracted much research interest in recent years. We report here the development of two quantitative methods for determining the Lewis acidity of MOFsbased on electron paramagnetic resonance (EPR) spectroscopy of MOF-bound superoxide (O₂^•–) and fluorescence spectroscopy of MOF-bound <i>N</i>-methylacridone (NMA)and a simple strategy that significantly enhances MOF Lewis acidity through ligand perfluorination. Two new perfluorinated MOFs, Zr₆-fBDC and Zr₆-fBPDC, where H₂fBDC is 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid and H₂fBPDC is 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-biphenyldicarboxylic acid, were shown to be significantly more Lewis acidic than nonsubstituted UiO-66 and UiO-67 as well as the nitrated MOFs Zr₆-BDC-NO₂ and Zr₆-BPDC-(NO₂)₂. Zr₆-fBDC was shown to be a highly active single-site solid Lewis acid catalyst for Diels–Alder and arene C–H iodination reactions. Thus, this work establishes the important role of ligand perfluorination in enhancing MOF Lewis acidity and the potential of designing highly Lewis acidic MOFs for fine chemical synthesis

Tuning Lewis Acidity of Metal–Organic Frameworks via Perfluorination of Bridging Ligands: Spectroscopic, Theoretical, and Catalytic Studies

The Lewis acidity of metal–organic frameworks (MOFs) has attracted much research interest in recent years. We report here the development of two quantitative methods for determining the Lewis acidity of MOFsbased on electron paramagnetic resonance (EPR) spectroscopy of MOF-bound superoxide (O₂^•–) and fluorescence spectroscopy of MOF-bound <i>N</i>-methylacridone (NMA)and a simple strategy that significantly enhances MOF Lewis acidity through ligand perfluorination. Two new perfluorinated MOFs, Zr₆-fBDC and Zr₆-fBPDC, where H₂fBDC is 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid and H₂fBPDC is 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-biphenyldicarboxylic acid, were shown to be significantly more Lewis acidic than nonsubstituted UiO-66 and UiO-67 as well as the nitrated MOFs Zr₆-BDC-NO₂ and Zr₆-BPDC-(NO₂)₂. Zr₆-fBDC was shown to be a highly active single-site solid Lewis acid catalyst for Diels–Alder and arene C–H iodination reactions. Thus, this work establishes the important role of ligand perfluorination in enhancing MOF Lewis acidity and the potential of designing highly Lewis acidic MOFs for fine chemical synthesis

Titanium(III)-Oxo Clusters in a Metal–Organic Framework Support Single-Site Co(II)-Hydride Catalysts for Arene Hydrogenation

Titania (TiO₂) is widely used in the chemical industry as an efficacious catalyst support, benefiting from its unique strong metal–support interaction. Many proposals have been made to rationalize this effect at the macroscopic level, yet the underlying molecular mechanism is not understood due to the presence of multiple catalytic species on the TiO₂ surface. This challenge can be addressed with metal–organic frameworks (MOFs) featuring well-defined metal oxo/hydroxo clusters for supporting single-site catalysts. Herein we report that the Ti₈(μ₂-O)₈(μ₂-OH)₄ node of the Ti-BDC MOF (MIL-125) provides a single-site model of the classical TiO₂ support to enable Co^II-hydride-catalyzed arene hydrogenation. The catalytic activity of the supported Co^II-hydride is strongly dependent on the reduction of the Ti-oxo cluster, definitively proving the pivotal role of Ti^III in the performance of the supported catalyst. This work thus provides a molecularly precise model of Ti-oxo clusters for understating the strong metal–support interaction of TiO₂-supported heterogeneous catalysts