Improving the sensitivity of magnetic resonance imaging (MRI), a powerful non‐invasive medical imaging technique, requires the development of novel contrast agents with a higher efficiency than gadolinium chelates such as DTPA:Gd (DTPA: diethylenetriaminepentaacetic acid) that are currently used for clinical diagnosis. To achieve this objective, the strategy that we have explored involves the use of gold nanoparticles as carriers for gadolinium chelates. These nanoparticles are obtained by reducing a gold salt in the presence of a dithiolated derivative of DTPA. Characterization of these particles by transmission electron microscopy (TEM), X‐ray diffraction (XRD), thermogravimetric analysis (TGA), colorimetric titration, and X‐ray photoelectron spectroscopy (XPS) reveals the presence of a multilayered shell containing about 150 ligands on 2–2.5 nm sized particles. These particles exhibit a high relaxivity (r1 = 585 mM–1 s–1 as compared to 3.0 mM–1 s–1 for DTPA:Gd), rendering them very attractive as contrast agents for MRI.
A safe, straightforward, and atom economic approach for the oxidation of aliphatic aldehydes to the corresponding carboxylic acids within a continuous flow reactor is reported. Typically, the reaction is performed at room temperature using 5 bar of oxygen in PFA tubing and does require neither additional catalysts nor radical initiators except for those already contained in the starting materials. In some cases, a catalytic amount of a Mn(II) catalyst is added. Such a flow process may prove to be a valuable alternative to traditionally catalyzed aerobic processes.
The reduction mechanism of tertiary phosphine oxides by Ti(OiPr)4/hydrosiloxane was studied. Strong improvement was achieved using a drying agent. ESR spectra of the reaction mixture give evidence for a single electronic transfer (SET) mechanism.
π-Acceptor molecules covalently attached on hydrophilic support were used to selectively remove neutral nitrogen heterocyclic compounds from diesel feed by a charge transfer mechanism. Functionalized hydrophilic polymers can effectively adsorb nitrogen heterocyclic compounds with a high selectivity toward sulfur heterocyclic compounds from model and real feed. The results showed that charge transfer processes coupled with an ion-exchange process to selectively remove basic nitrogen compounds are efficient enough to produce denitrogenated feed in high yield. This study was conducted to determine whether trends in hydrodesulfurization (HDS) activity with lower sulfur content were mainly the result of lower reactivity of hindered sulfur compounds or due to nitrogen species inhibition. The inhibition effects of nitrogen compounds on HDS at conditions commonly used in the hydrotreatment of gas oil feedstocks has been experimentally determined. This study suggests that the selective removal of nitrogen compounds from gas oil strongly enhanced the deep desulfurization.
The
mechanism of acceptorless dehydrogenative coupling reaction
(ADC) of alcohols to esters catalyzed by aliphatic pincer PHNP ruthenium complexes was experimentally studied. Relevant
intermediate species involved in the catalytic cycle were isolated
and structurally characterized by single-crystal X-ray diffraction
studies, and their reactivity (including toward substrates related
to the catalytic process) was probed. VT NMR studies unveiled several
chemical exchanges connecting the Ru amido hydride, the Ru alkoxide,
and the alcohol substrate. Under catalytic conditions, in situ IR
spectroscopy monitoring demonstrated the production of ester via aldehyde
as intermediate. A Tishchenko-like pathway is proposed as the main
path for the production of ester from aldehyde, involving alkoxide
and hemiacetaloxide Ru species (the latter being identified in the
reaction mixture by NMR). Catalytic system deactivation under base-free
conditions was found to be related to water traces in the reaction
medium (either as impurity or derived from aldol reactions) that lead
to the formation of catalytically inactive acetato Ru complexes. These
react with alkali metal alkoxides to afford catalytically active Ru
species. In line with this observation, running the ADC reaction in
the presence of water scavengers or alkoxides allows maintaining sustained
catalytic activity.
The hydroformylation of olefins (oxo synthesis) is the most important process for the production of higher aldehydes (.C 4 ). The liquid phase oxidation of the latter to carboxylic acids by molecular oxygen or air has been known for more than 150 years and is an industrially important process. However, in the recent literature, several different oxidizing reagents and catalytic processes have been reported for this oxidation but most of them have limitations as they use environmentally unacceptable reagents or unnecessarily sophisticated conditions. Herein, we re-evaluated the air oxidation of aldehydes. We show that under mild conditions (air or oxygen and non-optimized stirring), reactions are transfer limited and thus catalyst has no effect on reaction rate. Using efficient stirring (self-suction turbine), uncatalysed air oxidation of 0.8 M aldehyde is possible in 50 min at room temperature whereas less than 10 min was necessary with 10 ppm Mn(II). Thus, recommendations for avoiding common pitfalls that may rise during the evaluation of new catalysts are made.
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