Journal of Molecular Catalysis A: Chemical最新文献

The coupling of organosilanes and different olefins was performed efficiently in the presence of anionic complexes, [CA]₂[PdX₄] and [CA]₂[Pd₂X₆], where CA = imidazolium or pyridinium cation. The reaction proceeds according to a Pd(II)-mediated pathway, and Cu(OAc)₂ acts as the re-oxidant of Pd(0) formed during the catalytic process. High product yields were obtained for differently substituted olefins at 80 °C in 4 h. Styrylsilanes reacted in the same conditions giving unsymmetrical 1,3-dienes.

Extensive research has been devoted to the assignment of IR bands of CO adsorbed on Pt nanoparticles, which are widely used in heterogeneous and electrocatalysis (e.g. fuel cells). In contrast to single crystal studies, the assignment of CO adsorption to the nanoparticle structure is still controversial. Here we present a case study where we assign CO adsorption bands to the structure of Platinum nanoparticles with a given size distribution. Using a special diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell allows to achieve high quality data under in-situ conditions. Temperature dependent CO adsorption spectra are resolved into three bands which depend on the applied flow and pretreatment conditions. Our calculations using Density Functional Theory (DFT) can mimic the experimental findings and link these bands to the particle structure. By explicitly calculating the IR spectra of CO/Pt nanoparticles of different sizes we show that the IR bands are due to a combined size and site effect. For fully covered small nanoparticles the IR bands are attributed to all binding sites. For larger nanoparticles the dominant contribution is related to {111} facets but the other bands are still site independent. Here we provide a tool to assign CO adsorption bands on Platinum nanoparticles with a given size distribution. This can be related to the structure–acitvity relationship which is required for a tailored catalyst design.

A novel catalytic reaction of alcohol etherification in the system ROH − PtCl₄²⁻ ‐ PtCl₆²⁻ was found. Methanol easily transforms into dimethyl ether in the presence of catalytic amounts of Pt^II chloro complexes at 70 °C. Under the same conditions reaction of ethanol affords diethyl ether (catalytic) and π-ethylene Pt^II complex (stoichiometric). The reactions are accompanied by multiple H/D exchange, which is indicative of intermediacy of corresponding alkyl platinum derivatives. The plausible reaction mechanism involves oxidative addition of alcohol forming intermediate alkyl platinum(IV) derivative followed by decomposition of it via reductive elimination step under the action of alcohol giving the ether and regenerating catalyst. In the case of ethyl alcohol reaction, β-hydrogen abstraction from the intermediate Pt-ethyl species yields π-ethylene platinum(II) complex. Although it seems that the reaction does not involve the initial breaking of CH bonds of an alcohol, this system can be regarded as a model for studying of some peculiarities of Shilov chemistry, in particular, of isotope scrambling mechanisms in Shilov alkane activation.

In contrast to reactions of dimethyl and diethyl ethers formation, tert-butyl ethers formation in CD₃OH/t-BuOH medium is catalyzed by Pt^IV chloro complexes also and is not accompanied by isotope scrambling. These observations argue against intermediacy of alkyl platinum derivatives suggesting that acid-catalyzed mechanism operates in tert-butyl alcohol etherification.

DFT modeling of the active centers formation in the catalytic system LTiCl₂-Bu₂Mg-Et₂AlCl for alkene polymerization, where L is a bidentate ligand of saligenin type, suggests the three-step mechanism of this process. This mechanism includes the addition of the most probable alkylating agent, RMg(μ-Cl)₂AlR₂ or RMg(μ-Cl)₂MgR (R = alkyl), to LTiCl₂ with the formation of the trinuclear heterocomplex with the alkyl bridge bond Mg–C–Ti, followed by its two-step isomerization into the active center via the β-agostic intermediate. The free energy changes at the stages of the addition and isomerization are negative; the maximal energy barrier on the reaction pathway is small. In the Mg-free system LTiCl₂-AlR₃, the isomerization proceeds through the single energy barrier with a significantly higher amplitude. This could be the reason for a relatively high activity of the Mg-containing system and inactivity of the Mg-free system.

Linalool and nerol, bio-renewable terpenic alkenyl alcohols found in many essential oils, were selectively oxidized by molecular oxygen in the presence of the chloride-free Pd(OAc)₂/p-benzoquinone catalytic system. An efficient dioxygen-coupled catalytic turnover was achieved in the absence of auxiliary electron-transfer mediators under 5–10 atm of oxygen pressure. In both substrates, only one of two olefinic bonds was involved in the interaction with palladium, whereas the other one remained intact. Primary allylic acetates were formed as major reaction products: 8-linalyl acetate and 8-neryl acetate from linalool and nerol, respectively. In the case of linalool, the intramolecular cyclization product (known as herboxide), also resulted from the oxidation of the internal double bond, was also formed in significant amounts. All monoterpenic compounds obtained in the present work are natural products found in exotic plants or grape wines and are potentially useful as fragrance ingredients due to their pleasant scents.

Cobalt and mixed iron-cobalt spinel oxides Co_xFe_(3-x)O₄ supported on SiO₂ ‘IMP-Co_x-500′ and their bulk analogues ‘CP-Co_x-500′ were prepared via impregnation and co-precipitation methods respectively and calcined at 500 °C. Thermogravimetric analysis ‘ATG/DTG’, X-ray Diffraction method ‘XRD’, Texture measurements ‘BET/BJH’ and Temperature Programmed Reduction technique ‘TPR’ were used for their characterization. Their catalytic behavior in ethanol combustion was investigated. Results showed that nanostructured oxide materials with spinel structure were obtained with a good dispersion of the active phase at the surface of silica in the case of IMP-Co_x-500 compounds. The reduction behavior of the prepared samples is highly dependent on the oxide composition and interaction with the support; which is directly related to the catalytic behavior. The use of supported catalysts improves the catalytic performance and leads the reaction towards a total oxidation by decreasing greatly the formation of partial oxidation products. The obtained values of activation energies are relatively low in comparison with the literature data. A compensation effect was found by carrying out ethanol combustion through the spinel systems considered in this work.

The present work highlights the application of solid acid catalyst to produce alkyl levulinate from levulinic acid in continuous mode under vapor phase conditions. In this context, tungsten oxide incorporated SBA-16 catalysts were prepared by one pot direct synthesis method and evaluated for the titled reaction. Under optimized reaction conditions, 3 wt% WO₃-SBA-16 catalyst delivered complete conversion of levulinic acid with 95% selectivity towards ethyl levulinate. The synthesized catalysts were characterized to know the physico-chemical features by various techniques, namely, X-ray diffraction, N₂ physisorption, temperature programmed reduction of hydrogen (H₂-TPR), temperature programmed desorption of ammonia (NH₃-TPD), DR-UV–vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The characterization results suggest that, the superior catalytic activity can be ascribed due to the enhanced acidity of SBA-16 obtained through incorporation of tungsten oxide and easy of accessibility for the dispersed active sites through uniform pore channels. The constant catalytic activity in 10 h time on study shows the sturdiness of the catalyst and the spent catalyst can be regenerated several times. Moreover, various alkyl levulinates (methyl, n-propyl, and n-butyl) were synthesized with more than 90% selectivity over this catalyst.

The structure of alumina-supported copper-nickel catalysts prepared by impregnation method was studied by using a combination of various characterization techniques including XRD, N₂-sorption, TEM, H₂-TPR, NH₃-TPD, XANES/EXAFS and CHNS methods. A series of characterizations revealed that dispersion of copper increases with an increase in nickel loading on ɣ-Al₂O₃ due to strong interaction between nickel and copper oxide particles. This also led to the formation of mixed copper-nickel oxides in calcined catalysts at the highest loading of nickel and copper (i.e. Cu/Ni = 1). In catalytic activity, both the monometallic Cu/ɣ-Al₂O₃ and Ni/ɣ-Al₂O₃ showed lower activity and selectivity towards hydrogenation of both Furfural and 5-Hydroxymethylfurfural to 2-methylfuran (2-MF) and 2, 5-dimethylfuran (DMF), respectively. However, with increase in nickel loading, the activity and the selectivity of Cu/ɣ-Al₂O₃ drastically increased for both the cases and Cu-Ni/ɣ-Al₂O₃ (Cu/Ni = 1) showed the highest catalytic activity. Furthermore, combination of Cu/Ni ratios and temperature plays a significant role in the product distribution, as in the case of furfural hydrogenation, at a lower temperature, furfuryl alcohol (FOL) appears as the main product while at a higher temperature, 2-methylfuran (2-MF) is found to be the dominant product over Cu-Ni/ɣ-Al₂O₃ (Cu/Ni = 1) catalysts. Similarly, 2,5-bishydroxymethylfuran (BHF) is found to be the major product at a lower temperature and 2,5-dimethylfuran (DMF) is selectively produced at a higher temperature in the HMF hydrogenation. Furthermore, reaction pathways are discussed for both the reactions.

A novel polyethylene-supported Fe/ionic liquid complex (PEt@Fe/IL) is prepared, characterized and its catalytic performance is investigated in the synthesis of 3,4-dihydropyrimidinones. The chemical properties and thermal stability of the PEt@Fe/IL material were studied by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX) analysis. The morphology of the material was obtained using scanning electron microscopy (SEM). The PEt@Fe/IL material was then successfully applied in the Biginelli condensation of aldehydes, urea and alkylacetoacetates to prepare a set of different 3,4-dihydropyrimidinone derivatives in high to excellent yields. The PEt@Fe/IL was recovered and reused several times without significant decrease in efficiency. The other advantages of this novel catalytic system include excellent yield, short reaction time and solvent-free conditions.

A simple and reliable methodology is described for preparing the first heterogeneous NNN-pincer-copper hybrid catalyst with a high control over surface composition. The strategy relies on the covalently bonding of 2-aminopyridine to cyanuric chloride-functionalized magnetic nanoparticles followed by complexation with CuI. These claims are confirmed by different characterization methods such as SEM, TEM, FT-IR, TGA, ICP, XRD, and elemental analysis. The finely engineered supported catalyst is employed in the aerobic oxidative coupling of terminal alkynes and click reaction using only 0.38 and 0.04 mol% catalyst, respectively. All reactions perform under solvent-free condition or green solvent H₂O. Also, the catalyst is readily recovered and reused for up to 8 and 6 subsequent runs in click and homocoupling reactions without significant loss of activity or leaching.