Journal of Molecular Catalysis A: Chemical最新文献

Amberlyst A-21 supported CuI was found to be highly efficient novel heterogeneous catalyst for the three-component reaction between ketones, amines and alkynes, commonly called KA²-coupling. This inexpensive, easy-to-prepare, simple and recyclable catalyst has been employed in solventless conditions in the KA² coupling reaction involving both primary and secondary amines, ketones and various alkynes. The secondary and tertiary propargylamine products were obtained in good to excellent results in moderate reaction times.

Conjugated dienes are desirable reagents for several important applications. We report that sterically uncrowded PCP-pincer iridium complexes, including precursors of (^iPr4PCP)Ir and (^Me2tBu2PCP)Ir, catalyze the transfer dehydrogenation of pentane, using high concentrations of t‐butylethylene (TBE) as hydrogen acceptor, to give high yields of 1,3-pentadiene (piperylene). Piperylene yields are ca. 100-fold greater than those obtained with the more widely used di(t‐butyl)phosphino substituted pincer iridium catalysts. This represents, to our knowledge, the first reported high-yield synthesis of dienes via the dehydrogenation of n-alkane using molecular catalysts. To our knowledge, this is the first reported high-yield synthesis of dienes achieved via the dehydrogenation of n-alkane using molecular catalysts.

In the present work, four mononuclear iron(III) complexes containing BMPA (BMPA = bis-(2-pyridylmethyl)amine) and derivative ligands, have been studied as catalyst in toluene oxidation, at 25 °C and 50 °C, using hydrogen peroxide as oxidant and acetonitrile as solvent. All catalysts were able to oxidize toluene with satisfactory yields, producing o-, m-, p-cresols, benzaldehyde and benzyl alcohol, as main products, and traces of 2-methylbenzoquinone and benzoic acid. The catalyst [Fe(BMPA)Cl₃] presented the most promising results, reaching yields up to 30.2% at 50 °C after 24 h. Furthermore, [Fe(BMPA)Cl₃] was applied in the oxidation of other aromatic compounds as benzene, ethylbenzene, cumene, n-propylbenzene, p-xylene and anisole. The reaction with H₂O₂ was monitored by electronic UV–vis spectroscopy in the presence and absence of toluene and its oxidation products, as well as by ESI-(+)-MS/Q-TOF mass spectrometry, in order to provide some information about the reaction mechanism.

The chemisorption and hydrogenating properties of Ni/SiO₂ catalysts prepared by the hydrazine method then calcined at 400 °C with various times were investigated. Metal dispersion and activity in benzene hydrogenation increased with increasing calcination time whereas desorbed amounts of hydrogen significantly decreased. Dilution of a calcined sample by the support led to a sharp increase of both hydrogen storage by the support and catalytic activity. Metal dispersion and hydrogen storage capacity influenced the reaction mechanisms of hydrogenation of benzene which, therefore, is believed to occur on the metal phase or/and on the support by the hydrogen spillover mechanism. The metal active phase would be composed of an ensemble of metallic and oxidized nickel species.

Catalytic systems RhCl₃–KI–NaCl and RhCl₃–Cu(OAc_f)₂–NaCl in aqueous perfluorinated carboxylic acids (CF₃COOH, C₃F₇COOH) are effective in coupled oxidation of alkanes and carbon monoxide with dioxygen. In their presence, predominant is the outer-sphere oxidation of alkanes into respective esters (alcohols) with involvement of peroxo rhodium species as an oxidant (mechanism A). The process occurs partially by the inner-sphere mechanism B involving Rh–alkyl intermediates. Mechanism B is supported by (a) formation of alkyl chlorides, (b) synthesis of acetic acid in conversion of methane, and (c) positional selectivity in oxidation of propane.

Ordered mesoporous MCo₂O₄ (M = Cu, Zn and Ni) spinel catalysts were synthesized via nano-replication method using mesoporous silica KIT-6 as the hard template. They were applied to methane combustion, in comparison with bulk MCo₂O₄ spinel catalysts prepared by co-precipitation method. A combined N₂ adsorption-desorption, XRD and TEM results clearly confirm that mesoporous MCo₂O₄ (m-MCo₂O₄) spinel catalysts contain ordered mesostructure, resulting in the higher BET surface area and pore volume than bulk ones (b-MCo₂O₄). Moreover, the former catalysts demonstrate the better thermal stability as indicated by larger amount of MCo₂O₄ phase and smaller size of crystallite domain after calcination at 550 °C. Therefore, such excellent properties rationalize that the m-MCo₂O₄ spinel catalysts reveal higher catalytic activity for methane combustion than bulk counterparts. When it comes to the catalytic performance of the meso catalysts for methane combustion, the m-CuCo₂O₄ spinel catalyst has superior performance, which is related to the high normalized amount of Co³⁺ cations on the surface, as evidenced by XPS.

Zeolite catalyzed Friedel-Crafts acetylation of 2-methoxynaphthalene to produce 2-methoxy-6-acetylnaphthalene with high selectivity and conversion has been a challenging task, because the obtained compound is a key intermediate for an anti-inflammatory agent, Naproxen. However, no satisfactory results have been obtained with zeolite catalysts, and harmful solvents have been used to gain a high selectivity together with a high conversion. Here, we report the synthesis of 2-methoxy-6-acetylnaphthalene from 2-methoxynaphthalene with a high selectivity and a high conversion under an unprecedented simple reaction system; acetic anhydride as an acetylating agent, acetic acid as a solvent, and proton-type zeolite catalysts with low acidity. Among the examined zeolites, a proton-type H-MOR (SiO₂/Al₂O₃ = 200) with a low acid content shows a conversion of 82% and an 86% selectivity for 2-methoxy-6-acetylnaphthalene. Further, detailed control experiments using H-MOR catalyst in acetic acid solvent were carried out to propose a plausible reaction mechanism.

The oxidation reactions of alkanes with hydrogen peroxide and peracids (peracetic acid (PAA) and m-chloroperoxybenzoic acid (mCPBA)) catalysed by two Fe(II) complexes of pentadentate {N₅}-donor ligands have been investigated. Kinetic isotope effect experiments and the use of other mechanistic probes have also been performed. While the total yields of oxidized products are similar regardless of oxidant (e.g. 30–39% for oxidation of cyclohexane), the observed alcohol/ketone ratios and kinetic isotope effects differ significantly with different oxidants. Catalytic reactions in H₂O₂ medium are consistent with the involvement of hydroxyl radicals in the CH bond cleavage step, and resultant low kinetic isotope effect values. On the other hand, catalytic reactions performed using peracid media indicate the involvement of an oxidant different from the hydroxyl radical. For these reactions, the kinetic isotope effect values are relatively high (within a range of 4.2–5.1) and the C3/C2 selectivity parameters in adamantane oxidation are greater than 11, thereby excluding the presence of hydroxyl radicals in the CH bond cleavage step. A low spin Fe(III)-OOH species has been detected in the H₂O₂-based catalytic system by UV/Vis, mass spectrometry and EPR spectroscopy, while an Fe(IV)-oxo species is postulated to be the active oxidant in the peracid-based catalytic systems. Computational studies on the CH oxidation mechanism reveal that while the hydroxyl radical is mainly responsible for the H-atom abstraction in the H₂O₂-based catalytic system, it is the Fe(IV)-oxo species that abstracts the H-atom from the substrate in the peracid-based catalytic systems, in agreement with the experimental observations.

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.

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.