Search results for " Catalysis"

Selective oxidative activation of isobutane on a novel vanadium-substituted bimodal mesoporous oxide V-UVM-7

2006

Abstract New vanado-silicates with a bimodal mesoporous system (V-UVM-7) and high vanadium contents (Si/V ≥ 7) have been prepared by a modification of the so-called atrane method. Its morphology consists of aggregation of nano-particles with V-MCM-41 structure, creating a second broader porous system in their interparticle space. Its structure allows to incorporate a high V contents into tetrahedral sites of the framework while keeping vanadium dispersion. V-UVM-7 catalysts are very active and selective for the direct transformation of isobutane to isobutene and methacrolein. Total conversion and methacrolein-to-isobutene ratio in the products increased with increasing vanadium contents. Th…

Liquid phase selective oxidation of benzyl alcohol over Pd–Ag catalysts supported on pumice

2001

Abstract Selective oxidation of benzyl alcohol to benzaldehyde was carried out over pumice supported bimetallic and monometallic Pd and Ag catalysts. Preliminary kinetic studies were performed at 333 K in autoclave, at pressure of 2 atm in pure oxygen. Under these conditions, small amounts of benzoic acid were detected with the monometallic Pd pumice being the most active catalyst. The reaction was also carried out under flowing oxygen at atmospheric pressure and at 348 K. Under these conditions, the selectivity to benzaldehyde was 100%. The catalytic activity of the catalysts was measured after different oxidation and reduction treatments at high temperature. In addition, two mechanical mi…

Effect of Gold on the HDS Activity of Supported Palladium Catalysts

2002

Abstract The effect of gold on palladium catalysts supported on amorphous aluminosilicate was investigated in the hydrodesulfurization of thiophene. A series of bimetallic AuxPdy catalysts was prepared by the slow deposition–precipitation method with decomposition of urea. The structural and surface properties of the samples were analyzed by X-ray diffraction and X-ray photoelectron spectroscopy techniques at different stages of the catalyst life. After calcination at 673 K, gold-enriched solid solutions of approximately Au90Pd10 and Au80Pd20 composition were formed for all samples of different Au/Pd ratios, along with free palladium phases. Au80Pd20 was the prevailing phase. Hydrogen treat…

Mo-containing tetragonal tungsten bronzes. The influence of tellurium on catalytic behaviour in selective oxidation of propene

2009

Abstract Te-free and Te-containing multicomponent Mo-based catalysts presenting tetragonal tungsten bronze (TTB) structure have been prepared, characterized and tested for the selective oxidation of propene. The catalysts were prepared hydrothermally and heat treated at 700 °C. The structural characterization performed by means of XRD, FTIR, SAED and HRTEM confirms the incorporation of tellurium to the TTB structural framework and shows that its presence does not modify in essence the crystalline structure. However, the incorporation of Te strongly modifies the catalytic performance, and the highest yield to acrolein has been achieved on catalysts with Te/Mo atomic ratios in bulk of 0.03. T…

Interesterification of rapeseed oil catalysed by a low surface area tin (II) oxide heterogeneous catalyst

2018

Abstract The interesterification of rapeseed oil was performed in a batch reactor using for the first time low surface area massive tin(II) oxide as heterogeneous catalyst and methyl acetate as acyl acceptor. The effect of reaction temperature, methyl acetate to oil molar ratio and catalyst loading on the performances of the process were investigated. Yields in fatty acid methyl esters (FAMEs) and triacetin (TA) up to 90% and 70% respectively, were achieved after 4 h of reaction time at 483 K in the presence of 0.69 mol of SnO per mole of rapeseed oil using a methyl acetate to oil molar ratio of 40. Quite interestingly, the catalyst performances improved when water was added to the reaction…

A Synthetic Model for the [4+2] Cycloaddition in the Biosynthesis of the Brevianamides, Paraherquamides, and Related Compounds

2000

Abstract The reactivity of model systems for the proposed [4+2] cycloaddition in the biosynthesis of the brevianamides, paraherquamides, and marcfortines is explored. The model for the intermolecular reaction reveals that the cycloaddition takes place under mild conditions only if activated, very reactive dienophiles are used. When relatively unreactive dienophiles such as cyclopentene and cyclohexene are used, harsh reaction conditions and/or a Lewis acid catalyst are necessary for the reaction. In contrast, the model for the intramolecular reaction demonstrates that the cycloaddition takes place within a few hours at room temperature, even in the absence of a Lewis acid catalyst. Conclusi…

IONIC LIQUIDS MODIFIED CATALYSTS: SYNTHESIS, IMMOBILIZATION AND USES.

Synthesis, catalytic activity and recovery of Ionic liquid modified and un-modified catalysts have mainly been focused in this thesis. Particularly, our attention has been given to 2,2,6,6-Tetramethyl-piperidine-N-oxyl (TEMPO) catalyst for its unique redox behavior in alcohol oxidation to carbonyl compounds. In order to tag appropriate number of IL moieties, [60]Fullerene as well as benzylic linker units were employed as a molecular support. However, covalently supported catalysts were recovered by a short silica pad while, ionic liquid-tagged TEMPO catalysts were recovered by a non-covalent “Release and Catch” approach using silica grafted polymeric multilayered covalently supported ionic …

Covalently Supported Imidazolium Salts In Catalysis

2020

Synthesis and catalytic applications of supported imidazolium salts covalently linked to the proper support are described. As supports here we report, among others, the use of amorphous and mesoporous silica, polyhedral oligomeric silsesquioxanes, magnetic-based silica, polystyrenes, halloysite nanotubes, carbon nanoforms such as fullerenes, carbon nanotubes, carbon nanohorns, graphene. The catalytic applications have been divided into two main sections, metal-free based and metal-based reactions.

Radical addition to iminium ions and cationic heterocycles

2014

Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ-generated cationic substrates with various types of C-radicals, including alkyl, alkoxyalkyl, trifluoromethyl, aryl, acyl, carbamoyl, and alkoxycarbonyl species. Despite its high reactivity, the strong interaction of the radical’s SOMO with the LUMO of the cation frequently results in a high regioselectivity. Intra- and intermolecular processes such as the Minisci reaction, the Porta react…

Organocatalytic Enantioselective Synthesis of Pyrazoles Bearing a Quaternary Stereocenter

2016

An efficient one-pot asymmetric synthesis of pyrazoles bearing a chiral quaternary stereocenter has been developed. Quinine-derived thiourea catalyzed the enantioselective addition of pyrazolones to isatin-derived ketimines, providing the corresponding acetylated pyrazoles after in situ treatment with Ac2O/Et3N. The corresponding pyrazoles were afforded in high yields and excellent enantioselectivities.