Search results for " Catalysis"

Can Macroscopic Parameters, Such as Conversion and Selectivity, Distinguish between Different Cracking Mechanisms on Acid Catalysts?

1997

n-heptane cracking has been carried out for very short time periods on an established reaction mechanism which includes the following group of stream reactions on different zeolites: Initiation reactions (protolytic cracking), charge isomerization, chain isomerization, hydride transfer, β-scission–alkylation, desorption–adsorption, and hydrogen transfer–cyclization. It has been observed that a given experimental distribution of products can be explained by more than just one reaction scheme. This does not allow one to clearly distinguish the fraction of reactant which disappears via monomolecular initiation reactions with respect to bimolecular hydride transfer reactions.

Acid-mediated reaction of bis(pyridine)iodonium(I) tetrafluoroborate with aromatic compounds. A selective and general iodination method

1993

Reaction of aromatic compounds with bis(pyridine)iodonium(I) tetrafluoroborate (IPy2BF4) in the presence of HBF4 or CF3SO3H in CH2Cl2 at room temperature furnishes monoiodo derivatives with excellent regioselectivity and yields. Use of either acid gives comparable results with activated aromatics, whereas CF3SO3H is much more effective in the iodination of deactivated aromatics. © 1993 American Chemical Society.

Simultaneous production of furfural and levulinic acid from pine sawdust via acid-catalysed mechanical depolymerization and microwave irradiation

2019

In this work pine sawdust was converted into levulinic acid (LA) and furfural. Sawdust was first pre-treated with sulfuric acid-catalysed mechanical depolymerization. The conversion reactions were then performed with microwave heating at 180 °C. To enhance the furfural yield and the efficient separation of furfural and LA, a biphasic water-toluene reaction system was used. The effect of an additional catalyst, AlCl3, on the yield of LA and furfural was also studied. According to the results the pre-treatment method enhanced the yields of LA. In addition, due to the microwave heating the reaction times were short. Additional AlCl3 catalyst enhanced the LA yield, however excellent furfural yi…

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…

ChemInform Abstract: Radical Addition to Iminium Ions and Cationic Heterocycles

2015

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…

Confined But-2-ene catalytic isomerization inside H-ZSM-5 models: A DFT study

2015

The isomerization of cis-but-2-ene to trans-but-2-ene within a 22T H-ZSM-5 zeolite model, also in the presence of two adsorbed Pd atoms, has been studied by DFT calculations. The results obtained allow us to state that the cis/trans but-2-ene isomerization can easily proceed inside unsupported zeolite cavities. In this case, differently than in the gas phase reaction, the trans-but-2-ene is less stable than the cis-but-2-ene, when adsorbed on the zeolite inner surface. Excluding the adsorption-desorption steps, the isomerization process involves two intermediates and three transition states, whose energy content is always very low with respect to that of reagents and intermediate species. T…

Catalytic Enantioselective Aza-Reformatsky Reaction with Cyclic Imines

2016

A catalytic highly enantioselective aza-Reformatsky reaction with cyclic aldimines and ketimines for the synthesis of chiral b-amino esters with good yields and excellent enantioselectivities is reported.Areadily available diaryl prolinol is used as a chiral ligand, ZnMe2 as a zinc source and ethyl iodoacetate as reagent in the presence of air atmosphere. The reaction with cyclic ketimines generates a quaternary stereocenter with excellent levels of enantioselectivity. Furthermore, five-membered N-sulfonyl ketimines were used as electrophiles with good enantiomeric excesses, under the optimized reaction conditions. Moreover, several chemical transformations were performed with the chiral b-…

Mo–V–Nb mixed oxides as catalysts in the selective oxidation of ethane

2003

Abstract Mo–V–Nb–O mixed metal oxides, obtained by heat-treatment in N 2 at 425 °C, have been studied as catalysts in the oxidative dehydrogenation of ethane. They present higher catalytic activity, while maintaining the same selectivity to ethylene, than the corresponding metal oxides calcined under air. Both amorphous and crystalline phases are present on active and selective catalysts. The implications of the presence of these phases as well as their physicochemical characteristics on the nature of active and selective sites are discussed.

Deep oxidation of volatile organic compounds using ordered cobalt oxides prepared by a nanocasting route

2010

Ordered Co3O4 with high surface area (until 173 m2/g) has been successfully obtained through a nanocasting route using mesoporous KIT-6 silica as a hard template and tested in the deep oxidation of a series of representative volatile organic compounds (VOCs): propane as a model of short chain alkane and toluene as a model of monoaromatic hydrocarbon. It has been demonstrated that the catalytic activity for VOC deep oxidation is very elevated and its catalytic stability at moderate temperatures very good. However, the role of the ordered structure in the catalytic performance does not seem to be beneficial. The enhanced catalytic activity has been explained in terms of both the high surface …

Improved (photo)catalytic propene hydration in a gas/solid system by using heteropolyacid/oxide composites: Electron paramagnetic resonance, acidity,…

2017

Binary materials composed of the oxides SiO2, TiO2 and N-doped TiO2 and the Keggin heteropolyacid (PW12) were prepared and physicochemically characterized. They were used as catalysts and photocatalysts for the hydration of propene to 2-propanol. The characterization of the samples, particularly the electron paramagnetic resonance (EPR) spectroscopy results and the acidity properties, were useful to explain the key role played by the PW12 in the composite materials in the thermal and photoassisted catalytic processes. The simultaneous pres-ence of heat and UV light improved the activity of PW12 in the thermal process, and the binary materials showed better (photo)catalytic activities than t…