Synthesis of hematite nanowires using a mesoporous hard template

The influence of the impregnation media (ethanol or water) and the calcination atmosphere (air and NO/He) on the hematite nanowires production embedded on a hard template (MCM-41) was studied. The solids were characterized by X-ray diffraction, Mössbauer spectroscopy and magnetic measurements. The results obtained indicate that the more appropriate conditions for the iron oxide nanowires to get inside the MCM-41 hard template seem to be reached using water as a solvent and air as calcination atmosphere.     

Gold‐Catalyzed 1,2‐/1,2‐Bis‐acetoxy Migration of 1,4‐Bis‐propargyl Acetates: A Mechanistic Study

The late transition metal catalyzed rearrangement of propargyl acetates offers an interesting platform for the development of synthetically useful transformations. We have recently shown that gold complexes can catalyze a highly selective tandem 1,2‐/1,2‐bis‐acetoxy migration in 1,4‐bis‐propargyl acetates to form 2,3‐bis‐acetoxy‐1,3‐dienes. In this way, (1Z,3Z)‐ or (1Z,3E)‐ and (1E,3Z)‐1,3‐dienes could be obtained in a stereocontrolled manner depending on the electronic and steric features of the ancillary ligand bound to gold and the substituents at the propargylic positions. In this work, we report an experimental study on the scope of this transformation, plus a detailed theoretical examination of the reaction mechanism, which has revealed the key features responsible for the reaction stereoselectivity. Synthetic applications towards the one‐pot synthesis of quinoxaline heterocycles and tandem Diels–Alder processes have also been devised.     

Dependence of the Structural Stability of MCM-41 on the Impregnating Iron Solution

Hyperfine Interactions - The MCM-41 mesoporous material was chosen as support of very small iron oxide particles. To study the preparation conditions that allow to preserve the support structure...     

A Modular Class of Fluorescent Difluoroboranes: Synthesis,Structure, Optical Properties,Theoretical Calculations and Applications for Biological Imaging

Ten borylated bipyridines (BOBIPYs) have been synthesized and selected structural modifications have been made that allow useful structure–optical property relationships to be gathered. These systems have been further investigated using DFT calculations and spectroscopic measurements, showing blue to green fluorescence with quantum yields up to 41 %. They allow full mapping of the structure to determine where selected functionalities can be implemented, to tune the optical properties or to incorporate linking groups. The best derivative was thus functionalised with an alkyne linker, which would enable further applications through click chemistry and in this optic, the stability of the fluorophores has been evaluated.     

Iron Uniform-Size Nanoparticles Dispersed on MCM-41 Used as Hydrocarbon Synthesis Catalyst

We have synthesized mesoporous MCM-41 and have used it as a support for iron particles to be employed as a catalyst in the Fischer–Tropsch reaction. The solids were characterized by Mössbauer spectroscopy, X-ray diffraction, BET, TGA, CO chemisorption and volumetric oxidation. Although the catalyst showed a high CO conversion when it was used in the hydrocarbon synthesis from CO and H₂ (14.3% at 1 h of reaction time) mainly methane was formed. The high methane production is likely related to the very small size of the metal particles obtained. We suggest some ways to improve the selectivity.     

Highly anisotropic distribution of iron nanoparticles within MCM-41 Mesoporous Silica

Electron microscopy techniques are used to visualize the spatial distribution of iron nanoparticles inside a mesoporous MCM-41 molecular sieve. Direct observation of the iron oxide nanoparticles by STEM-HAADF imaging reveals a highly non-uniform spatial distribution inside the mesopores. These particles are retained in the pores after a reduction treatment unlike the behavior found in other similar systems. It is found that thermal treatments induce changes in its morphology, creating nanowires from particle strings.     

Cu‐Promoted Sydnone Cycloadditions of Alkynes: Scope and Mechanism Studies

Cu salts have been found to promote the cycloaddition reaction of sydnones and terminal alkynes, providing significant reduction in reaction times. Specifically, the use of Cu(OTf)₂ is found to provide 1,3‐disubstituted pyrazoles, whereas simply switching the promoter system to Cu(OAc)₂ allows the corresponding 1,4‐isomers to be produced. The mechanism of the Cu‐effect in each case has been investigated by experimental and theoretical studies, and they suggest that Cu(OTf)₂ functions by Lewis acid activation of the sydnone, whereas Cu(OAc)₂ promotes formation of reactive Cu^I acetylides.     

Mössbauer spectroscopy of <Superscript>57</Superscript>Fe in <Emphasis Type="Italic">α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> following implantation of <Superscript>57</Superscript>Mn<Superscript>*</Superscript>

Two Fe/MCM-41 systems, one of them sylilated, were obtained to be used as catalysts in Fischer–Tropsch reaction. They have more than 90% of the iron species located inside the support channels, leading to a narrow crystal size distribution accessible to reactive gases. The samples were characterized by X-ray diffraction, atomic absorption spectroscopy, N₂ adsorption, Mössbauer spectroscopy and Fourier transformer infrared spectroscopy. Mössbauer spectroscopy allowed us to demonstrate that the catalytic active species were the same in both catalysts. The only difference between them was the surface hydrophobicity, which decreases the “water gas shift reaction” in the sylilated catalyst. Besides, this solid is more active for hydrocarbon production, with a lower methane yield.     

Loading channels of SiO 2 mesoporous matrices with Fe oxides

After embedding hematite nanowires (15 wt.% Fe) into a MCM-41 hard template, we have explored alternative routes to induce the structural transformations that lead from hematite to maghemite and magnetite embedded nanowires. The impregnation media (ethanol or water) and the calcination atmosphere (air and NO/He) on the hematite nanowires production play a significant role at the time of reducing and re-oxidizing the embedded hematite nanoparticles. The solids were characterized by X-ray diffraction, nitrogen adsorption, and Mössbauer spectroscopy. The results indicate that the effect of the solvent on the structural properties of the iron species is more important than the calcination atmosphere. The best conditions for iron magnetic nanowires not to get outside of the MCM-41 channels over the treatments are reached using water as the solvent and air as the calcination atmosphere. When ethanol is the solvent used over the preparation step, the end iron oxides are in the form of nanotubes spread out on the amorphous silica walls of the matrix.