Journal of Molecular Catalysis A: Chemical最新文献

The quest of selective catalytic reactions for direct conversion of alkanes into valued products remains to be the most important task objective of modern chemistry and metal complex catalysis. Nowadays it is adopted that the formation of metal–alkyl intermediates (MR) is a necessary condition for activation and functionalization of alkanes on metal complexes but the mechanism of subsequent reactions of metal alkyls remain obscure, so that effective catalytic systems of this kind are still rare and uncommon. Although it is widely adopted that alkane σ-complexes (M·RH) most frequently are primary hydrocarbon intermediates in these processes, low profile in the literature is given to their reactivity and these are often considered simply just as some ‘collision complexes’. Nevertheless, theoretical and experimental studies provide more and more evidence that the СН bonds in such complexes may be markedly weakened and/or polarized, thus opening wide horizons for occurrence of subsequent direct homolytic or heterolytic reactions of alkanes. This review addresses the discussion of new routes for activation and oxygenation of saturated hydrocarbons, including those via alkane σ-complexes, without formation of metal–alkyl intermediates.

AgTaO₃/AgBr heterojunction was constructed for visible light driven photocatalytic purpose in order to investigate the relevance of phase conversion, electronic structure and photocatalytic properties. The result indicated that AgBr grafted on AgTaO₃ to form AgTaO₃/AgBr heterojunction gave intense visible light absorption, which exhibits highly enhanced photocatalytic performance than their individual counterpart. Theoretical and experimental investigation showed that the matched electronic structure between AgTaO₃ and AgBr induced an efficient transfer of photogenerated electrons from AgBr to AgTaO₃, leading to efficient charge separation and the subsequent improved photocatalytic activity. Partial AgBr converted to AgBr/Ag during the photocatalytic process, leading to the construction of ternary AgTaO₃/AgBr/Ag photocatalyst. Because of the surface plasmon resonance effect of Ag, the resulting AgTaO₃/AgBr/Ag exhibited wide range absorption and improved charge separation efficiency, which showed high durability and superior photocatalytic activity toward methyl orange degradation. On the basis of spin resonance measurement and trapping experiment, it is expected that photogenerated electrons, O₂⁻, and OH active species dominate the photodegradation of methyl orange.

Mechanically prepared clinoptilolite nanoparticles (NC) and coupling were used for increasing photocatalytic activity of ZnO and SnO₂. The raw and modified catalysts were characterized by XRD, FTIR, SEM-EDX, X-ray mapping, DRS, electrochemical impedance spectroscopy (EIS) and BET techniques. The calcined catalysts at 600 °C for 2 h showed the best photocatalytic activity in metronidazole (MZ) aqueous solution. Based on the EIS results, this catalyst has the best charge transfer efficiency with respect to other catalysts calcined at lower and higher temperatures. This caused to lower e/h recombination and hence higher photodegradation activity. The mole ratio of ZnO/SnO₂ affects the degradation activity of the catalysts so the best activities were obtained for the ZnO_2.4-SnO_2(2.0)/NC (ZS-NC) and ZnO_3.3-SnO_2(2.0)/NC (Z₂S-NC) catalysts at pH 3. Cyclic voltammograms of the modified carbon paste electrodes with ZS-NC and Z₂S-NC showed increased peak current in phosphate buffer which confirm formation of the ZnO and SnO₂ semiconductors inside NC. Also, peak current dependence of the modified electrode to MZ concentration confirmed that the degradation extent of MZ can be estimated by electrochemical methods.

The one-pot myrtenol amination was studied over Au (3 wt.%)/ZrO₂ catalyst under mixed N₂/H₂ atmosphere (9 bar). The effect of hydrogen addition was explored with the aim to increase selectivity to the target amines. Hydrogen addition timing depending on myrtenol conversion and hydrogenation temperature affected selectivity to the reaction products. Hydrogen addition (1 bar) after almost complete myrtenol conversion at 100 °C increased the yield to amine up to 68% preserving CC bond in the initial myrtenol structure. Hydrogen addition at 180 °C irrespective of the myrtenol conversion level provoked reduction of both CC and CN bonds with formation of two diastereomers (yield up to 93%), with trans-isomer formation being preferred when hydrogen was added at almost complete myrtenol conversion. It was shown, that in the presence of a gold catalyst controlled hydrogenation of competitive CC and CN groups can be performed during one-pot alcohol amination by regulation of the reaction conditions.

In our previous study, we applied Pd@SiO₂ core-shell catalysts to hydrogen peroxide synthesis and obtained a higher yield of hydrogen peroxide than that obtained with the use of general supported-catalysts (Pd/SiO₂). As an extension of the previous study on Pd@SiO₂ catalysts, the effects of the core-shell thickness on the hydrogen peroxide synthesis reaction were examined in this study. A shell below a certain thickness in the core-shell structure of the Pd nanocatalyst results in a decrease in the catalytic activity. Overall, a volcano curve is observed for the hydrogen peroxide production rate as a function of the shell thickness. Through N₂-adsorption and desorption, TEM, CO-chemisorption, and XRD analyses, we identify the causes for the improved direct hydrogen peroxide synthesis yields and later optimize the shell thickness for the efficient utilization of Pd.

CH bond catalytic activation/functionalization has been matter of great interest, since these transformations could establish new grounds from the synthetic perspective, and could serve to propose alternative and greener methods for the production of organic molecules. In this sense, the development of efficient catalytic systems for CH bond activation becomes of great importance, and understanding the principles that govern such transformations, either stoichiometric or catalytic, is essential. To study such processes from the mechanistic or catalytic point of view requires of the design of ligands and catalysts able of performing CH bond activation. (P,N)-chelates have arisen as alternative ligands to develop new catalytic systems, due to their combination of different donor atoms, and the ability to tune their steric and electronic characteristics and to create potential vacant sites at the metal center due to hemilability.

This mini review summarizes the advances from 2005 to 2015 in the field of CH bond activation at transition metal centers from groups 8–10 bearing (P,N)-chelating ligands. Even when the examples of inter- or intramolecular CH bond activation using transition metal complexes with the (P,N) ligands described herein do not engage yet in catalytic applications, their understanding becomes of great importance and sets the basis for future developments in this field.

A series of new rhodium (I) complexes supported by bidentate nitrogen-donor ligands with varying electronic and steric properties were synthesized in situ and evaluated for catalytic arene C–H/D activation. In trifluoroacetic acid (HTFA), these complexes are proposed to mediate H/D exchange of arene C–H/D bonds by an electrophilic aromatic substitution mechanism that involves Rh-mediated activation of HTFA (or DTFA). DFT calculations support the proposed pathway for the H/D exchange reactions.

A new hybrid Fe₃O₄@mSiO₂@Cu₄ material was constructed by loading a bio-inspired tetracopper(II) coordination compound [Cu₄(μ₄-O){N(CH₂CH₂O)₃}₄(BOH)₄][BF₄]₂ (Cu₄) onto the Fe₃O₄@mSiO₂ core-shell nanoparticles (NPs) composed of a magnetite (Fe₃O₄) core and mesoporous silica (mSiO₂) shell with perpendicularly aligned channels. The obtained Fe₃O₄@mSiO₂@Cu₄ magnetic nanoparticles were characterized by transmission electron microscopy (TEM), FT-IR spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and field-dependent magnetization. This hybrid material acts as a magnetically recoverable C−H functionalization nanocatalyst, namely for the mild oxidation, by t-butyl hydroperoxide at 50–70 °C in acetonitrile medium, of cycloalkanes (cyclopentane, cyclohexane, cycloheptane, and cyclooctane) to the corresponding alcohols and ketones (with up to ∼15% yields based on cycloalkane; TON 335). A related oxidation process using cyclohexanol as a more reactive substrate leads to the formation of cyclohexanone in up to ∼25% yield (TON 570). The Fe₃O₄@mSiO₂@Cu₄ nanocatalyst can be recycled five times without an appreciable loss of activity. The bond-, regio-, and stereoselectivity parameters were investigated in the oxidation of different alkane substrates (n-hexane, n-heptane, n-octane, methylcyclohexane, adamantane, cis- and trans-1,2-demethylcyclohexane), and the obtained results were compared with the homogeneous systems based on the Cu₄ catalyst. In particular, the high bond selectivity parameters detected in the oxidation of methylcyclohexane (1°:2°:3° of 1:8:142) and adamantane (2°:3° of 1:21) catalyzed by Fe₃O₄@mSiO₂@Cu₄ suggest that the reactions possibly occur in hydrophobic pockets of the nanocatalyst.

We report the iridium-catalysed desilylative acylation of styryl and dienyl silanes by acid anhydrides to afford (E)-α,β-unsaturated ketones. The [{Ir(μ-Cl)(cod)}₂] catalyst is the first non-rhodium complex successfully applied for this type of transformation. Stoichiometric reaction of [{Ir(μ-Cl)(cod)}₂] with (E)-trimethyl(4-chlorostyryl)silane was carried out to gain insight into the reaction mechanism.