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International audienc

Palladium nanoparticles have been immobilized into an amino-functionalized metal–organic framework (MOF), MIL-101Cr-NH 2 , to form Pd@MIL-101Cr-NH 2 . Four materials with different loadings of palladium have been prepared (denoted as 4-, 8-, 12-, and 16 wt %Pd@MIL-101Cr-NH 2 ). The effects of catalyst loading and the size and distribution of the Pd nanoparticles on the catalytic performance have been studied. The catalysts were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD), N 2 -sorption isotherms, elemental analysis, and thermogravimetric analysis (TGA). To better characterize the palladium nanoparticles and their distribution in MIL-101Cr-NH 2 , electron tomography was employed to reconstruct the 3D volume of 8 wt %Pd@MIL-101Cr-NH 2 particles. The pair distribution functions (PDFs) of the samples were extracted from total scattering experiments using high-energy X-rays (60 keV). The catalytic activity of the four MOF materials with different loadings of palladium nanoparticles was studied in the Suzuki–Miyaura cross-coupling reaction. The best catalytic performance was obtained with the MOF that contained 8 wt % palladium nanoparticles. The metallic palladium nanoparticles were homogeneously distributed, with an average size of 2.6 nm. Excellent yields were obtained for a wide scope of substrates under remarkably mild conditions (water, aerobic conditions, room temperature, catalyst loading as low as 0.15 mol %). The material can be recycled at least 10 times without alteration of its catalytic properties.

Two families of metal−organic frameworks (MOFs), MIL-88 and MIL-101 built by trinuclear transition metal (TM) clusters (TM = Cr, Fe, or Sc), have been known for several years, but their syntheses are often reported separately. In fact, these MOFs are polymorphs, or framework isomers: they are assembled from the same metal secondary building units and organic linkers, but the connectivity of these components differs. Here we report for the first time the synthesis of the vanadium MOF MIL-88B(V) and compare its synthesis parameters to those of MIL-47(V) and the recently reported MIL-101(V). The properties of MIL-88B(V) and MIL-101(V) are remarkably different. MIL-88B(V) can "breathe" and is responsive to different solvents, while MIL-101(V) is rigid and contains mesoporous cages. MIL-101(V) exhibits the highest specific surface area among vanadium MOFs discovered so far. In addition, both MIL-88B(V) and MIL-101(V) transform to MIL-47 at higher temperatures. We have also identified the key synthesis parameters that control the formation of MIL-88B(V), MIL-101(V), and MIL-47: temperature, time, and pH. This relates to the rate of reaction between the metal and linkers, which has been monitored by ex situ X-ray powder diffraction and V K-edge X-ray absorption spectroscopy during MOF synthesis. It is therefore important to fully study the synthesis conditions to improve our understanding of framework isomerism in MOFs.

No abstract

This review deals with our long-range goal of determining why the Prussian blue pigments, typically either the "soluble" KFe[Fe(CN)]·xHO or the alternative "insoluble" Fe[Fe(CN)]·xHO compounds, used by artists from shortly after the discovery of Prussian blue in 1704 and well into the early twentieth century, often fade when exposed to light. In order to achieve this goal it was decided that first, for comparison purposes, we had to prepare and fully characterize Prussian blues prepared by various, often commercially successful, synthetic methods. The characterization has employed a large variety of modern methods to determine both the stoichiometry of the Prussian blues and the arrangement of the voids found in the latter "insoluble" Prussian blues. The refinement of synchrotron radiation derived X-ray powder diffraction data obtained for a formally soluble and an insoluble Prussian blue required refinement in the Pm3[combining macron]m space group and lead to the K[FeFe(CN)]·{1.9 OH + 7.0HO}, 1, and FeFe(CN)·11.0HO, 2, stoichiometries. The former compound, 1, exhibits an apparently random iron(ii) long-range void arrangement, whereas 2 exhibits a more non-random long-range arrangement, however, a pair distribution function analysis indicates a short-range ordering of the voids in both compounds. After further detailed characterization of many Prussian blue samples, painted samples on linen canvas, were subjected to accelerated light exposure for up to 800 hours either as pure Prussian blues or mixed with (PbCO)Pb(OH), ZnO or TiO, the white pigments often used by artists to lighten the intense Prussian blue colour. The results indicate that the first two of these white pigments play a significant role in the fading of the colour of Prussian blues. In order to achieve our long-range goal, several Prussian blue samples were prepared from "ancient" recipes published in 1758 and 1779. These so-called "ancient" samples, painted in a dark and a pale blue shade, were also subjected to accelerated light exposure. The colorimetric results, in conjunction with X-ray powder diffraction refinements, pair distribution analysis and Mössbauer spectral results, indicate that, depending on the exact method of ancient preparation, the Prussian blue pigments were sometimes badly contaminated with alumina hydrate and/or ferrihydrite, a contamination which leads to extensive fading or decolourization of the Prussian blue pigments. The presence of ferrihydrite was subsequently confirmed in the study of a surface paint fragment from an eighteenth-century polychrome sculpture.

Prussian blue, a hydrated iron(III) hexacyanoferrate(II) complex, is a synthetic pigment discovered in Berlin in 1704. Because of both its highly intense color and its low cost, Prussian blue was widely used as a pigment in paintings until the 1970s. The early preparative methods were rapidly recognized as a contributory factor in the fading of the pigment, a fading already known by the mid-eighteenth century. Herein two typical eighteenth-century empirical recipes have been reproduced and the resulting pigment analyzed to better understand the reasons for this fading. X-ray absorption and Mössbauer spectroscopy indicated that the early syntheses lead to Prussian blue together with variable amounts of an undesirable iron(III) product. Pair distribution functional analysis confirmed the presence of nanocrystalline ferrihydrite, Fe10O14(OH)2, and also identified the presence of alumina hydrate, Al10O14(OH)2, with a particle size of ∼15 Å. Paint layers prepared from these pigments subjected to accelerated light exposure showed a tendency to turn green, a tendency that was often reported in eighteenth- and nineteenth-century books. The presence of particles of hydrous iron(III) oxides was also observed in a genuine eighteenth-century Prussian blue sample obtained from a polychrome sculpture.

The fading of modern laboratory-synthesized and commercial Prussian blue, iron(III) hexacyanoferrate (II), based pigments in a linseed oil medium during exposure to light has been investigated. The Prussian blue pigments were painted with linseed oil, as a pure pigment and mixed with white lead, (PbCO 3 ) 2 Pb (OH) 2 , zinc white, ZnO, or titanium white, TiO 2 , pigment. The samples were subjected to accelerated ageing for 800 h and the light fastness of the Prussian blue pigment was evaluated by reference to blue wool standards. Pure Prussian blue is extremely light fast whilst it strongly fades when mixed with a white pigment, especially with lead white or zinc oxide. The painted samples were studied by UV-visible, iron K-edge X-ray absorption, iron-57 transmission M€ ossbauer, and attenuated total reflectance infrared spectroscopy. X-ray absorption results reveal a decrease in the iron coordination number in aged samples in the presence of white pigment. The M€ ossbauer spectra of the pure Prussian blue and the unaged and aged mixtures of Prussian blue and lead white or zinc oxide at 1 : 100 and 1 : 10 dilution ratios, respectively, indicate the presence of iron(II) and iron(III) in a ratio close to one as expected for the bulk stoichiometric KFe